Documentation

AIR

When air is the product or used as the service fluid in a heat exchanger it is important to know the basis on which the air flow is measured.
The common bases for measurement are as follows:

Cubic Metres per Hour	[m³/hr]
Cubic Feet per minute	[cfm]
Kg per hour	[kg/hr]
Pounds per hour	[lb/hr]

The commonly used points of reference for the volume measurements are:

[1] Volume measured at ‘Standard conditions’	[St.m³/hr] [scfm]
[2] Volume measured at suction to the compressor	[m³/hr] [cfm]
[3] Volume measured at compressor discharge	[Actual m³/hr] [acfm]

‘Standard conditions’ [1] are defined as being at a temperature of 20ºC at a pressure of 1,013 Bar.Abs. Suction or ‘Actual’ volumes [2] & [3] are measured at the temperature and pressure at the point of measurement.

Thermal design calculations are always carried out using the mass flow of the fluids so when using air [or any other gas] it is important to calculate the mass flow using the densities which apply at the volume conditions specified and the density of the air [or gas] under the entry/exit conditions at the heat exchanger at the working pressure and temperatures.

AIR CONTAMINATION, ACCUMULATION AND VENTING

Oxygen can cause a problem in fired boilers so the boiler operators take care to deoxygenate the feed water supply to the boiler and minimize as far as possible air leakage into the system. Under normal working conditions however it is impossible to make any steam system leak-proof and there will almost inevitably be a small air content in steam flows. Even high pressure systems suffer from air migration into the supply lines through valve stems, seals etc. so the heat exchanger designer must be aware of the potential problems this can cause.

If there is entrained air in the steam flow to a heat exchanger the effect will be to reduce the condensation coefficient by inhibition of the heat flow. In equipment such as vent condensers and autoclaves there will probably be a significant air content unless the vessel is evacuated before the vapour venting process starts.

It is important therefore to consider the need to fit a vent connection in any vapour condenser in an area where an accumulation of a non-condensable gas is possible.

ALPHA VALUE

This the term commonly used for the partial heat transfer coefficients derived for the shell side and the tube side of a shell and tube heat exchanger.

The relative values of ai and ao control the tube wall temperature (cf.) of the heat exchanger and the designer can use this to compute appropriate values when there is a danger of boiling or freezing onto the tube surface.

ANNULAR SPACE HEAT EXCHANGERS

Annular space heat exchangers are a development of monotube heat exchangers which incorporate one or two additional concentric tubes to surround the product being treated with the service fluid on both sides. In order to facilitate this there is normally one Product inlet connection and one Product outlet connection but two Service fluid inlet connections and two Service fluid outlet connections.

The inner tube assembly is normally demountable to allow for inspection and cleaning purposes, the pressure being sealed using a double O ring assembly with a leakage path to atmosphere to indicate seal failure.

The annular space units are particularly useful in processing very viscous products – without particulates – for the food processing industries. In these applications a double bend assembly is sometimes included to maximize the heating capacity of the units.

One very important feature of larger diameter units is a susceptibility to tube collapse when the inner tube(s) are subjected to high pressures so it is vitally important that the working and test pressures are accurately established and a detailed mechanical design carried out in order to determine the minimum thicknesses required for all of the tubes to withstand the external pressures on the inner tubes.

ASME VIII DIVISION 1

ASME VIII Division 1 is the American design code for non-fired pressure vessels for industrial purposes.

It is a well-established and widely accepted design code, particularly in the American dominated industries such as Oil production and processing but is thought by some engineers to be fairly conservative in its approach and that it has fallen behind the European codes such as EN 13445 which have been written using the latest European knowledge and experience.

It is important to note that ASME VIII Div.1 can only be used for pressure vessels (including heat exchangers) which have a shell diameter above (and including) 6 inches (168,3 mm) in diameter. Below this diameter the code is invalid.

These are two very different ways of working with ASME VIII Div.1:

[1] To be FULLY compliant with ASME VIII Div.1 the pressure vessel must not only be mechanically designed using the code formulae, material standards etc. but also MUST be manufactured in a workshop which has been approved by the ASME Inspectorate and authorized to apply the ASME ‘U’ stamp which is the equivalent of the European CE mark in giving a guarantee of quality.

[2] The pressure vessel can be designed mechanically using the ASME VIII Div.1 formulae but manufactured using European standard materials and a non-ASME approved workshop. In this case the ASME ‘U’ CANNOT be applied.

In reality only pressure vessels being exported to certain American States MUST have the ‘U’ stamp, other Nations using ASME VIII Div.1 as a Code do so as a convenience and will often (but not always) waive the need for an approved workshop.

ASME BPE

The ASME BPE standard defines the quality of components needed for Pharmaceutical use. It is obligatory in the US market but the Pharmaceutical Industry as a whole recognizes the standard and will often specify its use.

The two major factors which the standard specifies are:

[1] The surface finish required for wetted surfaces

[2] The manufacturing tolerances that must be adhered to for the components (such as internal diameters of ferrules) including the accuracy of the machining processes and the diametric tolerance.

ATEX

ATEX is a classification system for explosive environments classified according to European Directives 94/9/EC and 99/92/EC.

Unless a shell and tube heat exchanger is likely to accumulate static electricity (which is possible under certain circumstances) there is nothing to do but if static electricity is a possibility then an earthling boss has to be welded somewhere, usually to a foot support or a flange, so that the unit can be electrically earthed.

If air cooled radiators or cooling towers are being supplied then the motors, fans and all controls have be ATEX certified according to the category of the hazard and the corresponding Directive.

ATMOSPHERIC PRESSURE

‘Standard’ atmospheric pressure is normally taken as being 1013 mBar at sea level.

If equipment is working at significant altitude the air Density must be corrected to allow for the reduced pressure as this will reduce the mass flow for a given volume flow. This is particularly important in heat exchangers using atmospheric pressure air as a cooling medium such as Air Blast Radiators and Evaporative Cooling Towers but also affects the volume taken into air compressors.

AZEOTROPE

Also known as a Constant Boiling Mixture, an Azeotrope is a mixture of two or more liquids in a ratio where the composition cannot be changed by distillation. If such a mixture is boiled the vapour will have the same composition as the liquid.

A typical well-known Azeotrope is a mixture of 96% Ethanol with 4% water which boils at 78.2°C a boiling point which is lower than either of its constituents.

It is very important from the heat exchanger designers’ point of view that such fluids are identified correctly and appropriate data obtained before any heat transfer calculations are carried out.

BAFFLES

Baffle plates are used for two reasons in shell and tube heat exchangers:

[1] To direct the fluid flow across the tube bundle in order to increase the fluid velocity, create more turbulence and therefore a higher coefficient of heat transfer.

[2] To support tubes which would otherwise sag under their own weight, taking into account the weight of the fluid that they contain.

Baffle plate minimum and maximum spacing, minimum thicknesses, number and diameter of tie rods and maximum tube hole clearances are specified in TEMA section 5.4.1 and although these are not mandatory they represent wide experience in heat exchanger design and it is usually convenient to follow these guidelines.

It is important to note that the Thermal design programme used to calculate the heat transfer is only valid for a minimum baffle spacing of 50 mm and for maximum baffle spacing equal to the shell pipe inside diameter. Outside these limits the calculations have a greater margin of uncertainty and error.

Although there are several types of segmental baffle plate designs available it is normally only necessary to use two types in small heat exchangers:

[1] Horizontal cut – directing the shell side fluid up and down over the tube bundle – which are used when the shell fluid is liquid or gas

[2] Vertical cut – directing the fluid from side to side – which are normally only used with condensing vapours

With either type the minimum recommended cut is 25% of the baffle diameter and the maximum cut recommended is 45% of the baffle diameter. It is considered good practice to ensure that the line of the baffle cut falls either between two rows of tubes or on the centreline of a row of tubes. For vertical cut baffles the line of the cut should fall in line with the centreline of a column of tubes.

With horizontal cut baffles the first baffle and last baffle must have the cut away part of the baffle plate on the opposite side of the shell to the connection. When the shell side connections are on opposite sides of the shell there will therefore be an even number of baffle plates and when the shell connections are on the same side of the shell there will be an odd number of baffle plates.

With vertical cut baffles it up to the designer which position is used for the first and last baffles but they should follow the same rules as for horizontal cut baffles with respect to the number of baffles (an even number or an odd number) and position of the shell side connections.

The baffle plates are spaced using Tie Rods (cf.) spaced around the baffle to maintain the correct spacing and support the baffle plate. TEMA Section 5.4.71 gives recommended diameters and numbers for the Tie Rods.

BAFFLE DESIGN

The baffle plate design must be decided during the Thermal design phase of the heat exchanger selection as the addition of baffle plates, the % baffle cut used and the distance between baffles will influence the heat transfer on the shell side of the heat exchanger.

With closely spaced baffles the first and last baffle plates should be fixed at about 50 mm away from the connection towards the centre of the heat exchanger. When larger baffle spacing is used it is normally most convenient to have an equal spacing for all of the baffle plates.

A baffle design which is not normally used but can be useful in some design applications in large diameter heat exchangers is the ‘double segmental’ baffle where the total percentage baffle cut is taken by cutting the centre part out of one baffle and then two segments at 180° to each other in the adjacent baffle. Details of this baffle type can be found in TEMA Section 5.4.1. This type of baffle is particularly useful in reducing the shell side pressure loss as the loss will be one half of the loss produced by ‘Single Segmental’ baffles.

BAFFLES WITH STEAM

Only vertical cut baffles acting as tube supports are normally used with condensing vapours.

These direct the condensing vapour from side to side over the tube bundle and it is important to note that if the heat exchanger is mounted horizontally a drainage notch or slot must be cut in the lower part of the baffle plate to allow for free condensate drainage. A total notch area equal to or greater than the outlet condensate connection will normally ensure free drainage.

BAFFLE CUT

This is the percentage of the baffle, expressed as baffle diameter, that is cut away to allow the passage of fluid over the baffle and to direct it up and own (or from side to side) across the tube bundle.

The minimum cut normally used is 25% of the diameter and the recommended maximum is 45% in order to assure adequate support from the baffle.

The Thermal design software calculates the fluid velocity through the cut away portion (called the baffle window) and the resulting pressure loss. If a reduction is pressure loss is required decreasing this velocity by increasing the baffle cut will often give the required result.

BELLOWS

In any heat exchanger a large temperature difference between the tube metal temperature and the shell temperature can produce a significant difference in the expansion (or contraction) of the tubes and the shell of the heat exchanger.

This difference in overall growth can produce unacceptably large stress levels in both tubes and shell unless they are allowed to expand or contract to their natural positions at the working temperatures. One way of achieving this is to use a design where one end of the tube bundle is allowed to expand/contract within the shell (such as the ‘packed floating tubeplate’ or ‘packed floating head’ designs illustrated in TEMA or the XLG MD and BD series units) but a more economical solution is to fit an expansion bellows into the shell which will allow the differences in length to be taken up without producing excessive stress.

There are two types of expansion bellows (also called expansion joints) used in pipework and heat exchanger applications, the thick walled single convolution bellows used in the petrochemical industries and the thin walled multi-convolution bellows used in other industrial applications.

The XLG range of fixed tubeplate units (XLG B series, Multitube, Monotube and Pharmagrade series) use all stainless steel thin walled multi-convolution bellows to minimize the stress levels in tubes and shell.

The design basis used for the thin walled multi-convolution bellows is normally the American Expansion Joint Manufacturers Association (EJMA) code which for the various types of bellows and bellows supports gives formulae for Axial and Angular movements and design stress levels. The EJMA recommendation is that unless otherwise specified by the end user the designer should assume that the minimum life of the bellows (for failure by fatigue) should be 3500 cycles.

If the bellows is expanded/contracted and pressurised/depressurized once per day this represents a design life of over 9½ years but in many applications such as CIP heating systems the temperature and pressure cycling will be much more rapid. It is very important therefore that the working pattern of heat exchangers is established in the initial design phase so that an increased number of cycles can be allowed for.

In reality, thin walled bellows used in relatively small diameter, low pressure and low temperature applications have an almost infinite life as the stress levels under normal working conditions are low but as part of the mechanical design process this must always be checked because large movements and high pressures can limit the number of cycles.

As part of the design process for bellows the designer will obtain the bellows ‘spring rate’ which is the force needed to expand or compress the bellows. This stress will be transmitted to the tubes and shell under working conditions and must be allowed for in the design of both of these components as well as the tubeplates.

If the bellows is purchased in from an outside supplier the spring rate of the bellows must be obtained from the bellows supplier. It is important to note that manufacturing techniques vary and while some suppliers use a few layers of fairly thick metal (typically 1, 2 or 3 sheets of 0,8 or 1,0 mm material) others use a higher number of layers of much thinner material (typically 4 or 6 layers of 0,4 mm material) which produces a much more flexible bellows.

When specifying a bellows assembly therefore the designer must not only give the bellows manufacturer the expected levels of axial and lateral movement but also the working and test pressures and temperatures so that he can take all of these factors into account.

It is preferable to purchase a bellows from an outside supplier with a short section of pipe to match the shell pipe welded to the bellows to enable the bellows to be inserted into the shell using butt welds which can be radiographed if necessary.
BOILING

There are three main types of equipment producing steam:

[1] Fired pressure vessels (usually called boilers) which use a burning fuel to generate medium to high pressure steam

[2] Small capacity electrically heated boilers which use industrial standard heating elements within a pressurised vessel to generate low to medium pressure steam

[3] Unfired pressure vessels (usually called steam generators) which use either a condensing vapour, a hot gas stream (for example internal combustion engine exhaust gas) or a hot liquid such as superheated hot water or hot oil to generate medium to low pressure steam

Type [1] vessels are usually used for central steam raising plant in large installations, type [2] in smaller industrial installations but type [3] can be used in a very wide range of applications. They are typically used in applications involving heat recovery to increase plant efficiency, which require limited quantities of medium to low pressure industrial steam or in applications which require hygienic steam for pharmaceutical or food installations.

There are many methods available for estimating the heat transfer coefficients in boiling applications which depend on tube surface finish, working pressure and temperature, mounting position etc. etc. The heat exchanger designer has to use his expertise to decide which method is most appropriate in any given application.

It is important to note that in boiling applications there are several mechanisms which depend on the above factors and which have an upper limit of applicability. If too much heat per unit area, called Heat Flux (cf.), is transferred to the boiling liquid the mechanism changes from Nucleate Boiling to Film Boiling and Film Boiling has a much lower rate of heat transfer so must be avoided if at all possible.

As the driving force for boiling is temperature difference, limiting the Heat Flux is achieved by limiting the temperature difference between the hot fluid and the saturation temperature of the boiling liquid. Many text books contain graphs showing heat flux against temperature difference for a variety of tube types and liquids and it is advisable for the heat exchanger designer to use these as a guide in order to avoid passing into the Film Boiling regime.

BUBBLE POINT

When heating a liquid consisting of two or more components, the bubble point is the point where first bubble of vapor is formed. Given that vapor will probably have a different composition than the liquid, the bubble point (along with the dew point) at different compositions are essential data when designing distillation systems.
For single component liquids the bubble point and the dew point are the same and are commonly referred to as the boiling point of the liquid.

CIP

CIP or Cleaning in Place is often used to clean heat exchangers.

In the Food and Pharmaceutical Industries it is usually carried out by passing a low strength Acid solution through the heat exchanger on the product circuit followed by an Alkali solution to neutralize the acid and then a clean water rinse to remove any solution residues.

The velocity of the CIP solutions is typically 1.5 m/s in order to achieve efficient cleaning at a temperature of 85°C to 90°C.

CIP cleaning can sometimes be preceded by a Pigging process (cf.) in order to maximize the amount of product removed from the heat exchanger before cleaning starts.

CIP cleaning is often followed by a sterilization process (cf.) in order to ensure than if there are any lingering residues or bacteria then these are neutralized. Sterilization is typically carried out by raising the temperature of all components to at least 140°C and holding it for a short period at that temperature. The process designer should specify the temperature and time for each product being treated.

CLAMPS

In the food and pharmaceutical industries cleanliness and hygiene are vitally important and it is normal for heat exchangers to be routinely cleaned and inspected. To facilitate ease of removal and reconnection it is normal in these industries to use hygienic ferrules which are sealed by an elastomeric gasket, the sealing pressure being provided by a Vee shaped clamp fitted over wedge shaped ferrules.

Various styles of clamp are available commercially, some approved by TÜV and ASME, and the designer must choose the style and type which are appropriate for the application and the working pressures and temperatures specified for the equipment.

Most clamps are fitted with stainless steel wing nuts to tighten the clamp but for high pressure applications the use of a bronze nut is sometimes advisable to allow ease of tightening against the high pressure and prevent the stainless steel to stainless steel seizure which can occur if the threads of the bolt(s) are not lubricated.

Manufacturers such as Stahlcon (Germany) or Advanced Couplings (United Kingdom) have a wide range of approved clamps and publish pressure and temperature capabilities for each type. Alfa Laval also supplies clamps (Trademark ‘Tri-Clover’ clamps) but these should be avoided if possible as they are normally more expensive than other suppliers and have very restricted pressure capabilities.

CLOGGING (ALSO CALLED BLOCKING)

When the working fluids of a heat exchanger contains solid particles there is always a danger of particles settling out of the liquid if the velocity falls below a critical velocity [the settling velocity (cf.)] and causing a build-up of solids on the tube surfaces. If the solids content is high or if the particle size is significant compared to the tube diameter there is an additional danger which is that the solids particles can cause a complete blockage of a tube.

If the heat exchanger is a monotube unit with a single fluid passage the blockage will be noted immediately as the tube side flow will be reduced or at worst stopped completely.

If the unit is a multitube unit however there is a high probability that the blockage will not be noted immediately as the tube side fluid will still flow through the tubes that have not been blocked. In industrial applications this will probably be inconvenient in the short term although more serious in the longer term as it could cause corrosion.

In food based applications however a partial blockage of this sort is far more serious as the blockage of one or two tubes would probably not be noted immediately but it would inhibit CIP (cf.) cleaning and pose a serious threat to hygiene and food safety as bacteria would be able to grow in the blockage and leach out into the product during normal operation.

CONTAMINATION

Any fluid, vapour or gas can be contaminated by unwanted substances which will have an effect on the heat exchanger or its performance.

At worst a contaminating substance could cause corrosion of the metal or deterioration of the gasket material in contact with the carrier fluid and/or have an adverse effect on the heat transfer performance achieved.

Some common contaminants are:

• Air or other non-condensable gases in condensation applications which can seriously inhibit heat transfer and require constant venting to prevent them from masking heat transfer surfaces

• Silt, weed or organisms such fish or shellfish in raw water sources from rivers, canals and the sea which can cause long term fouling and/or corrosion problems

• Metallic contamination such as rust and scale from inadequately cleaned pipework which can cause long term deterioration of stainless steel components

• Silt or chemical contamination in water from cooling towers which can cause long term fouling and corrosion problems

It is essential that the heat exchanger designer has details of any likely contamination when carrying out the thermal design and material selection as additional fouling margins (cf.) may be required and the material selection will affect both the tube wall resistance of the tubes and their corrosion resistance capability.

CONDENSATE REMOVAL

In vapour condensing applications it is vitally important that the condensate removal from the heat exchanger takes place in the most efficient way. Failure to do this could have several undesirable consequences which will affect the performance of the heat exchanger and its service life.

If the heat exchanger has been designed to sub-cool the condensate to below its saturation temperature there will be a condensate level established within the heat exchanger which has been taken into account in the mechanical design. If however condensate backup occurs due to inefficient drainage then there will be differences in metal temperatures which have not been allowed for and which can crack welds and cause other damage.

There is also a risk of live steam impinging onto a cold water surface which will send shock waves through the liquid and which could eventually cause fatigue failures. This is usually accompanied by loud cracking sounds and sometimes pipework vibration.

The responsibility for ensuring efficient condensate drainage lies with the designer of the pipework systems into which the heat exchanger will be installed. He must not only ensure that the pipework and steam trapping devices are appropriately sized but also that the necessary non-return valves, shut off valves etc. are correctly positioned and installed.

CORING

In applications involving fluids having a large viscosity change over the range of operating temperatures there is a potential problem in that the viscosity of the fluid boundary layers will differ significantly from the viscosity of the fluid in the centre of the tube. In cooling applications there is a danger that the viscous drag at the tube wall will slow the fluid flow but that the lower viscosity in the centre of the tube will allow the central ‘core’ to flow at a higher velocity.

This is known as coring and is more likely with large diameter tubes than with small diameter tubes. It can be combatted by fitting twisted tape or other turbulence enhancing inserts into the tubes but this usually has a pressure loss penalty. Corrugated tubes will tend to minimise the possibility of coring with a lower pressure loss penalty as they create a greater turbulence at the tube wall and encourage mixing within the fluid.
CORROSION

XLG standard heat exchangers are manufactured using AISI 304 and/or AISI 316 Stainless Steel for all wetted and non-wetted surfaces. These Austenitic Stainless Steels have good corrosion resistance to most fluids but they are susceptible to corrosion by some chemical substances and some other mechanisms which the heat exchanger designer must take into account.

Stress corrosion cracking is probably the most common cause of corrosion failure in Austenitic Stainless Steels and this is most usually (but not exclusively) caused by Chloride solutions. To suffer from this form of corrosion there must not only be a sufficient percentage of Chlorides in solution in contact with the metal surfaces, which is temperature dependent, and the component must be stressed. It is easily identifiable as small cracks appear at the metal grain boundaries which can be detected by dye penetrant or high magnification visual examination. If it is not possible to remove the source of the Chlorides then a change of material to a more resistant alloy is required.

Another form of corrosion affecting Stainless Steels is Erosion Corrosion which as the name suggests is when the corrosion resistant surfaces of the stainless Steel are eroded by a fluid containing an erosive element (such as corrosion scale from Carbon Steel components or air bubbles) impinging onto the surface at high velocity. This form of corrosion produces horseshoe shaped pits in the metal which can be seen with the human eye.

Austenitic Stainless Steels rely on a hard oxide film forming on the metal surfaces to prevent corrosion of the underlying metal. They are to a certain extent self-healing in that if the oxide film is removed (eroded) it will reform as long as there is oxygen present in the fluid in contact with the metal. Another form of corrosion failure can occur however if the presence of oxygen is inhibited by the formation of blanketing layers of silt or scale on some parts of the heat exchanger components. These can deplete the oxygen in the blanketed parts and thus set up a differential aeration cells which will cause the metal to corrode. An additional form of corrosion sometimes occurs is caused by contamination of the stainless steel surfaces by (often microscopic) particles of carbon steel. This contamination can be from a variety of sources but it is commonly the result of either storing or working (welding, machining, forging etc.) carbon steels in the same area as stainless steel components. It is characterized by dark spots on the surface of the stainless steel with minute surface cracks under the staining. This corrosion can be guarded against most easily be ensuring good separation of the carbon steel and stainless steel working areas but if it is impossible to prevent contamination then the stainless steel surfaces must be pickled (cf.) and passivated (cf.) to protect the surfaces.

CORROSION ALLOWANCE

On metal surfaces subjected to pressure and which may be corroded by the fluid(s) in contact with them it is normal to add an allowance on thickness over and above the thickness required for pressure to allow for the component becoming thinner due to corrosion.

With Carbon Steels this allowance can range from 1.6 mm for low levels of corrosion up to 3.0 mm for highly corrosive conditions.

With the Austenitic Stainless Steels used by XLG a corrosion allowance is not normally required as the components are unlikely to suffer from corrosion but the designer must always be aware of the possibility of contamination from corroding substances and must make the appropriate allowances where necessary.

Note that it is normal to exclude tubes from this allowance; it is assumed that the tube material will always be chosen to ensure resistance against all forms of corrosion failure.

CONNECTIONS

In order for a heat exchanger to work within a system it is necessary to connect it to the other system components with an appropriate type of connection (sometimes called a nozzle or a branch).

The heat exchanger designer has several decisions to make about the connections and he must take into account the requirements of the system designer.

[1] Diameter – the diameter of the connection will determine the velocity of the fluid passing through it to enter either the tubes or the shell. The pressure loss in the fluid will be a function of the change in fluid velocity as it makes this transition and, particularly in vapour condensation applications, this pressure loss must be calculated to determine the effects (if any) on the process.

Similarly the change in velocity as the fluid exits from the tubes or tube bundle into the outlet connection will cause a pressure loss and this must also be calculated.

Pressure losses will often be the determining factor in choosing the appropriate connection diameter but in applications involving condensation the maximum vapour velocity though the connection must be within the limits recommended for the vapour.

[2] Type of fitting – the type of fitting to be used will depend primarily on the requirements of the system designer but whichever type is specified the heat exchanger designer must confirm that it is to be used within its pressure and temperature capabilities and that an appropriate seal or gasket will be used.

These fittings could be PN rated flanges, Hygienic ferrules and clamps, screwed couplings such as RJT or SMS or internally screwed connections such as BSP (British) or NTP (American).

[3] Connections to the shell – there are three main types of weld formation that are commonly used with thin wall stainless steel connections:

Set-on – where the connection pipe is shaped to the curvature of the shell and welded onto a hole in the shell which is equal to the inside diameter of the connection pipe. A full penetration weld is used to attach the pipe to the shell.

Set in – where the connection pipe is shaped to the curvature of the shell and welded into a hole in the shell which is equal to the outside diameter of the connection pipe. The connection pipe is allowed to project through the shell wall and fillet weld is used to attach the pipe to the shell.

Locally formed (shaped) – where the shell is mechanically deformed outwards so that the deformed section has the same outside diameter as the connection pipe. A full penetration butt weld is used to attach the connection pipe to the shell. This type is only recommended when the connection diameter is relatively small compared to the shell diameter and when the respective metal thicknesses are similar. It must be appreciated that there is an area of very high stress in the shell pipe as a result of the deformation and in corrosive conditions this can result if premature failure.

CONCENTRIC REDUCERS

When the incoming and outgoing pipework to the tube side of a heat exchanger differs in size from the shell diameter a transition must be made from one to the other. This can be done by the pipework supplier but is often done by using a header at each end with connections to match the adjacent pipework and which are bolted or welded to the heat exchanger to form the tube side connections.

An alternative method for single pass heat exchangers when the change in diameter is modest is to use a conical reducer which carries a connection to match the adjacent pipework on one end and a connection to match the heat exchanger at the other.

In a concentric reducer these two different size connections are on the same centreline so that the adjacent pipework is in line with the heat exchanger centreline.

It is important to note that when mounted horizontally, heat exchangers fitted with concentric reducers cannot be self-draining on the tube side.

Another important point to note is that if the pipework diameter specified by the system designer results in a high velocity through the pipework there may be a jetting effect across the conical header which can result if differential velocities through the tubes of the heat exchanger.

The effects of this poor distribution of fluid across the surface of the tubeplate can have several detrimental effects.

• Different heat transfer rates across the tube bundle

• Different tube wall temperatures across the tube bundle

• Different tube expansion rates across the tube bundle

• Different stress levels across the tube bundle

The most important of these is probably the different stress levels caused by the previous three factors as these can cause tubeplate distortions leading to cracking of tube end welds.
DENSITY

The density of a substance is the weight per unit volume and it is an important property for the heat exchanger designer as flow rates of fluids or gases are often quoted in terms of the volume flow by the system designer but the heat exchanger designer needs the mass flow.

A useful formula for deriving the density of any gas at STP is the following:

Density = MW/22.4

Where:

Density is in g/L at Standard Temperature and Pressure (STP)
MW = Molecular weight in g/mol.
22.4 = a constant in L/mol.
STP = 0°C and 1013 mBar

[1 g/L = 1 kg/m³]

For gases working at temperatures and pressures different from STP the value must be corrected for both temperature and pressure at the working conditions.

Liquid densities are best derived from weighing a known volume of liquid. As liquid densities change with temperature (but very little with pressure) tests over a range of temperatures should be carried out.

A very useful website giving not only density values but also comprehensive information on most chemical substances is the following:

http://webbook.nist.gov/chemistry/

DESIGN PRESSURE

Under the rules of the Pressure Equipment Directive (cf.) heat exchangers are defined as pressure vessels and must therefore be designed to withstand the highest predictable pressure to which the equipment will be subjected. In the case of vapour condensers this will normally be the relief valve setting + 10% positive pressure but also Full Vacuum (cf.) on the assumption that at some point in its working life the isolating valves will be closed with the heat exchanger full of vapour which will condense to produce vacuum conditions.

It is the system designer who must specify the maximum pressure to be used as it will depend on his pump capabilities, relief valve settings and system losses but the heat exchanger designer has a legal responsibility under the Pressure Vessel Directive to make sure that the equipment is correctly designed and safe to use.

The rule must always be therefore: If the maximum working pressure is not given by the end user on his enquiry YOU MUST ALWAYS ASK WHAT THIS WILL BE.

Note that with steam heated units or condensers steam supplies to heat exchangers are often reduced from boiler pressure with an automatic pressure reduction valve (cf.) and in the case of valve failure the higher steam pressure may reach the heat exchanger. This higher pressure must be used as the design pressure unless a pressure relief device is fitted to limit the pressure to a lower value.

DESIGN TEMPERATURE

Under the rules of the Pressure Equipment Directive (cf.) heat exchangers are defined as pressure vessels and must therefore be designed to withstand the highest predictable temperature to which the equipment will be subjected.

It is the system designer who should specify what this temperature should be but it is normally safe to assume that the maximum temperature specified for the Thermal Design conditions will be the maximum design conditions.

Note that with steam heated units or condensers this may not always be a safe assumption as steam supplies to heat exchangers are often reduced from boiler pressure with a pressure reduction valve (cf.) and in the case of valve failure the higher steam temperature may reach the heat exchanger.

It is the heat exchanger designer who has the legal responsibility under the European Pressure Equipment Directive 97/23/EC to make sure that the equipment is designed to the correct values for both maximum pressure and maximum temperature so if these are not clearly stated you must always ask.

DESIGN FACTORS

There are many factors involved in designing appropriate heat exchangers for any given application, some obvious and some not so obvious.

 The basic heat transfer requirements, the flow rate and inlet and outlet temperatures for both fluids.
 The characteristics of the fluids, specific heat, thermal conductivity etc.
 Pressure loss limits.
 Mounting position.
 Space limitations.
 Transport and offloading limitations.
 International, national or industry design codes to be applied.
 Safety issues.
 Hygienic issues
 Environmental issues, earthquakes, atmospheric pollution

DESIGN CODES

The various mechanical design codes produced by National Authorities are commonly referred to as the ‘Design Codes’. They are mostly National Codes being specific to equipment to be supplied to an end user within that territory but some have achieved International acceptance.

The most commonly specified National Codes are:

 AD 2000 Merkblätter Germany
 AS1210 Australia
 ASME VIII Division 1 United States of America
 PD5500 United Kingdom
 Swedish Pressure Vessel Code Sweden
 Stoomwesen Holland

Some Codes are accepted internationally and these are:

 ASME VIII Division 1 United States of America
 EN13445 European Union

Other Codes specify not only how pressure vessels have to be designed but also specify the manufacturing qualifications and design approval procedures that must be adopted.

Examples of this type are:

 GOST Russian Federation
 Chinese Pressure Vessel code China

Design Codes of another type are also used, particularly in the Food Industry, which specify design features rather than metal thicknesses.

Examples of this type of code are:

 EHEDG (Food Equipment Hygiene) European Union
 3A (Food Equipment Hygiene) USA
 TEMA (Refinery type heat exchangers) USA

DESIGNATIONS

The way of designating heat exchangers is open to manufacturers to choose a method convenient for them but the internationally accepted method is as outline in TEMA section N-1 which recommends that the units should be designated by including the following features:

 Shell nominal diameter
 Nominal tube length (which may vary from the actual tube length)
 Type of unit (fixed tubeplate, monotube etc.) designating the type of headers if appropriate

DEW POINT

The dew point of a vapour is the temperature at which the vapour will condense. With a single component vapour this is the condensation temperature but with multi-component mixtures it is the temperature at which the first component starts to condense.

It is important to note with multi-component condensing fluids that as the individual components reach their dew point (at the partial pressure of the component) the vapour composition and the individual partial pressures will change and there will be a range of temperatures for the condensation process.

For heat exchanger designers this is important as the temperature difference will change during the process and the design must take this change into account.

DRAINAGE

After heat exchangers have been hydraulically tested or when they have to be removed for servicing or repair it is essential that they are drained of any liquids on both the shell side and the tube side. This can be done through the system pipework but it is sometimes necessary for the units to be fitted with specific drain points to ensure safe drainage.

Typically a female screwed connection (BSP or NTP) would be appropriate but the designer must choose not only a size but also an appropriate type taking into account the working conditions, fluid toxicity, fluid volumes etc. The heat exchanger designer should always consider how equipment will be drained during testing in the Works and on site when it has been installed.
DRY BULB TEMPERATURE

Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature.
It is the temperature measured by a regular thermometer exposed to the airstream.

Unlike wet bulb temperature (cf.), dry bulb temperature does not indicate the amount of moisture in the air.

When sizing air blast radiators the Dry Bulb Temperature is normally used to determine the amount of surface area required.

DYE PENETRANT EXAMINATION

Dye penetrant examination is a Non-Destructive Examination method (NDE) used for detecting surface cracks in metals.

Metals such as Carbon Steel suffer from cracking after welding due to weld shrinkage if welding preheat temperatures are not adequate or cooling rates are too rapid. Because of this Carbon Steel welds are routinely examined using Dye Penetrant Examination to detect surface flaws and by using Radiography (cf.) to detect internal flaws.

Stainless Steel does not suffer from the same problem during welding so the only use made by XLG of Dye Penetrant Examinations is during the investigation of cracking due to inappropriate working conditions or stress corrosion cracking (cf.) due to an inappropriate choice of materials.

EARTHING POINTS

When a liquid or gas flows through a heat exchanger it is possible for static electricity to build up on the surfaces of the unit and unless these are safely grounded (earthed) then it can be unpleasant for an operator who touches the surface and more importantly they can be dangerous if there are flammable or explosive substances in the working area.

In applications where there are these risks it is common practice to attach an ‘Earthling Boss’ to a convenient point on the heat exchanger, normally a welded support.

The earthling boss takes the form of a female screwed socket welded to the structure into which a brass or copper rod is screwed and secured. On site, an earthling cable will be attached to this screwed rod and connected to a grounding rod to take any static electricity to earth.

EFFECTIVE MEAN TEMPERATURE DIFFERENCE

The Effective Mean Temperature Difference (EMTD) is the product of the Logarithmic Temperature Difference (cf.) (LMTD) and a correction factor which reduces the LMTD to take into account the negative effects of having more than one tube pass or more than one shell pass.

TEMA (cf.) Section 7 contains charts illustrating various tube and shell pass arrangements which allow the heat exchanger designer to obtain values for F for correcting the LMTD.

Values for P and R must be calculated for the operating temperatures and from the graph illustrating the flow pattern being used a value of F obtained where:

P = [ t2 – t1 ] / [ T1 – t1 ]
R = [ T1 – T2 ] / [ t2 – t1 ]

T1 = Shell side fluid inlet
T2 = Shell side fluid outlet
T1 = Tube side fluid inlet
T2 = Tube side fluid outlet

Examination of the graphs will show that when the value of F is less than 0.75 it becomes increasing difficult to obtain a value with any certainty so it is recommended that heat exchanger configurations which give values below 0.75 should be rejected and the design parameters re-examined.

EFFICIENCY

The efficiency of heat exchangers is calculated by comparing the maximum possible amount of heat transferred to the actual amount transferred. The ratio is always less than 1 (which is a physical impossibility) but the closer it gets to 1, the more efficient a heat exchanger is considered to be.

In heat exchangers where no change of state occurs the efficiency is calculated as follows:

µt = (t2 – t1) / (t3 –t1)

Where:

t1 = Cold fluid inlet temperature
t2 = Cold fluid outlet temperature
t3 = Hot fluid inlet temperature

The highest efficiencies can only be achieved by heat exchangers working in Counterflow conditions (cf.) and in general terms the higher the efficiency the more expensive the heat exchanger will be.

ECCENTRIC REDUCERS

Similar to the concentric reducer (cf.) but with the centre-lines of the two connections offset from one another.

It is important to note that when mounted horizontally with the sides parallel to the heat exchanger centre-line axis in the correct positions the heat exchanger can be made to be largely self-draining and self-venting.

ELASTOMERS

Elastomers are rubber like compounds (a polymer) which generally have a low Young’s Modulus value and a high strain yield when compared to other materials.

They are normally ‘Thermoset’ materials (cf.) which require vulcanization and are used for a variety of types of seals and gaskets.

They are manufactured in a wide range of materials, the ones which are most useful in heat exchanger applications being as follows:

EPDM
Nitrile
Neoprene
Silicone
VITON®

Each of these materials has a specific range of temperature and pressures acceptable for their use as well as a range of chemical resistance. The heat exchanger designer, when using elastomeric seals or gaskets, must choose a material which is acceptable to the end user, which is chemically resistance to the working fluid(s) and will be used within its safe operating temperatures and pressure.

It is normally convenient to specify a hardness value for the elastomer of about 80 Shore which will give an adequate resistance to being over-compressed.

It must be realized – and the end user advised – that all elastomers are sensitive to heat, UV rays and air pollution so spare seals and gaskets must be stored on site in shaded storage areas away from sources of heat in a clean environment. In any case they WILL deteriorate over time and even spares must be periodically examined and renewed if they are showing signs of deterioration.

EPDM

EPDM (Ethylene Propylene Diene Monomer) is an elastomer (cf.) used in a wide range of seals and gaskets.

Its major use is in water or steam based applications and it has a maximum continuous service temperature of +140°C. It is not normally suitable for mineral oil based applications.

Its minimum service temperature is -50°C.

The normal compound is black and therefore it cannot be used in food related or hygienic applications but there are white compounds available which are FDA approved although these are normally more expensive with a longer delivery lead time than the black compounds.

EVAPORATION

See Boiling.

EVAPORATIVE COOLING TOWERS

These are commonly used to provide a source of cooling water for heat exchangers and other process equipment and work by evaporating a small quantity of the warm water which is sprayed into the tower at the top. A cooling/evaporating air stream is passed through the tower in the opposite direction to achieve the cooling effect.

They use atmospheric air as a cooling medium and have the advantage that they use the wet bulb temperature (cf.) to achieve the cooling and in some climates this can be several degrees lower than the dry bulb temperature used in dry air cooled radiators.

There are some major disadvantages when using evaporative towers:

 They lose a significant amount of water due to splashing and evaporation which has to be replaced. In arid areas this may be impossible and in less arid areas may represent a significant and ongoing running cost.

 They contaminate the water with whatever contaminants are in the atmosphere so if they are in a coastal area the water will be contaminated by Sodium Chloride and if they are in a heavily industralised area they will pick any air pollutants that are present, such as oxides of Sulphur, and produce an acidic water stream.

 They will similarly filter out any dust particles or other organic substances in the atmosphere so the water can become very dirty and the system require constant cleaning to remove silt.

There is alternative to this open spray system which minimises the effects described above and that is the ‘Baltimore Tower’ which combines the advantages of a closed circuit radiator with a cooling effect using evaporation and wet bulb air temperature.

This achieved by spraying water onto a closed circuit radiator matrix which maintains a clean water circuit internally. The major disadvantage is that the water losses are significant and require constant replacement.

FACTORY TESTS

Factory tests are normally limited to pressurisation of both fluid circuits (independently) to the Test Pressures (cf.) required by the mechanical design code using fresh clean mains water but other tests are sometimes required in order to prove weld quality or leak tightness.

Testing the internal quality of stainless steel welds is normally carried out using X-Ray techniques (Radiography) but Ultrasonic techniques may also be used. Both of these methods look at the internal structure of the welds to find out if there are any faults as defined by the Welding Codes (cf.). If either of these tests is to be carried out, the design codes allow a higher level of design stress to be used in the design formulae which may result in thinner material sections being required.

The minimum testing for either method is 10% of the total weld length but for critical applications 100% may be required. These are tests that have to be sub-contracted to a specialist test organisation as they require proven expertise and qualified personnel.

If Radiography is to be carried out normal working in the factory must be suspended during the tests for safety reasons as radioactive materials are used.

A leak tightness test is sometimes carried out to ensure that the heat exchanger will not leak from any welds or roller expanded joints and this involves filling the fluid circuit with a low pressure halogen gas and using an electronic ‘sniffer’ to check for leakage. Halogen gases are used because they have the ability to leak through the smallest of cracks and are easily detected using electronic techniques. This is a test best sub-contracted to a specialist test organisation as it requires proven expertise and qualified personnel. A well ventilated workplace is required for this test as the electronic sniffers are very sensitive and a tightly closed environment can result in an accumulation of halogen gas and false positive readings.

Sometimes the customer may also call for thermal performance tests to be carried out but this is normally a very expensive operation as it requires a lot of pipework, pumping for fluids, a heat source, cooling sources, instrumentation etc. so if it is possible a test ‘in situ’ at the heat exchanger destination will be a better and more economical solution.

FERRULES

In the food and pharmaceutical industries cleanliness and hygiene are vitally important and it is normal for heat exchangers to be routinely cleaned and inspected. To facilitate ease of removal and reconnection it is normal in these industries to use hygienic ferrules which are sealed by an elastomeric gasket, the sealing pressure being provided by a shaped clamp fitted over wedge shaped ferrules.

Various standards are available commercially, some approved by TÜV and ASME, and the designer must choose the style and type which are appropriate for the application and the working pressures and temperatures specified for the equipment.

Manufacturers such as Stahlcon (Germany) or Advanced Couplings (United Kingdom) have a wide range of approved ferrules. Alfa Laval also supplies ferrules (Trademark ‘Tri-Clover’) but these should be avoided if possible as they are normally more expensive than other suppliers.

FLANGES

Most industrial systems use bolted flanges to connect pipework and the remaining system components together. There are many different types and styles of flanges available to the designers and they will normally be specified by the system designer or end user to match the other components making up the system.

Two of the commonly used flange styles have either:

[1] Completely flat gasket surfaces which require full face gaskets

or

[2] A shallow raised portion forming a reduced diameter gasket surface – a raised face

Type [1] are normally the cheapest to purchase as they are less complicated to produce but type [2] have the advantage that the sealing pressure on the gasket for a given bolt load is increased, making them more secure in high pressure applications.

Some flange standards which are commonly used are:

 ANSI B16,5 United States of America
 BS 4504 United Kingdom
 DIN Germany
 ISO Europe in general

If the use of elastomeric gaskets is proposed it is advisable to machine the gasket contract surfaces of the flange with a series of shallow concentric grooves to act a key for the gasket. Failure to do this may, under high pressure conditions, allow the gasket to be forced out of the flange assembly. When the system is pressurised this is obviously a dangerous occurrence and must be avoided.

EN 1092-1 Paragraph 5.7.2 gives details of the machining recommended for various types of flange facings.

FLANGE RATING

Each flange standard which specifies the flange diameter, thickness, the diameter, number and P.C.D. of the bolts and if appropriate the diameter of the raised face also specifies the maximum pressure and temperature conditions for which the flange may be used.

If the flange is to be used safely it is essential that the flange standards are consulted to ensure a correctly rated flange is used and that the bolting standards are adhered to.

FLASH COOLING

Also known as vacuum cooling flash cooling is a technique that combines liquid cooling with a concentration process.

By feeding a pressurised hot liquid hot liquid into a chamber that is under a lower pressure or vacuum vapour will be emitted until the liquid reaches thermal equilibrium. This causes an instantaneous reduction of the temperature in the liquid, and the concentration of dry solids increases.

The concentrated liquid is discharged by means of pumping.

Depending on the initial temperature, the pressure or level of vacuum maintained within the vessel, the final temperature and the properties of the liquid, the concentration can be increased by between 1% and 10% of dry solids, and the temperature can be lowered towards 0°C.

The selection of final temperature can be made independently of the process, but the more cooling the greater will be the increase in concentration.

FLASH STEAM

When a liquid is introduced into a vessel or tank operating at a pressure which has a saturation temperature lower than the actual liquid temperature (for example water at 120°C introduced into a tank at atmospheric pressure) flash steam will be produced by the excess energy.

In the above example water at 120°C [saturation pressure 1,98 Bar.Abs] has an enthalpy equal to 503,813 kJ/kg whilst at atmospheric pressure and 100°C the water has an enthalpy equal to 419,098 kJ/kg.

When it loses pressure therefore there is excess enthalpy equal to [503,813 – 419,098] = 84,715 kJ/kg.

The Latent Heat (cf.) of steam at atmospheric pressure is 2256.66 kJ/kg so the excess enthalpy in the water will produce [84,715/2256.66] = 0 0.0375 kg of steam per kg of condensate when the pressure is reduced. It should be borne in mind that this is the MASS of steam and because of its low density its volume will be 62.8 Litres per kg of condensate.

This is particularly important when pressurised condensate at the steam saturation temperature flows through a steam trap as the trap will have a loss of pressure through the valve. If this pressure loss is significant then a significant quantity of flash steam will be produced.

The best way of preventing this flash steam is to cool the condensate within the heat exchanger to a temperature less than the saturation temperature on the low pressure side of the valve. If this is done in a controlled manner it will also increase the efficiency of the heat exchanger.

FLUID DATA

Fluid data can be divided into two main categories:

[1] Data required for safety purposes and categorization under the rules of the Pressure Equipment Directive (cf.) which classify fluids (including liquids, gases and vapours) as being either:

Category 1 – Dangerous if they are:

 Flammable
 Toxic
 Corrosive

Category 2 – Not Dangerous for everything else

Category A if they are a Gas or a Vapour whose partial pressure at the maximum allowable working temperature (cf.) is more than or equal to 0,5 Bar above normal atmospheric pressure.

Category B if they are a Liquid whose partial pressure at the maximum allowable working temperature (cf.) is not more than or equal to 0,5 Bar above normal atmospheric pressure.

So water at 15.0 Bar and 150°C would be classified as A(2) for the purposes of PED.

[2] Data required which will enable the heat exchanger designer to:

[a] Select appropriate materials for both the wetted and non-wetted components

This requires detailed knowledge of the chemistry of the working fluids and any contaminants that might be present. As material selection is a complex process it is advisable that for all but well-known common fluids, gases and vapours advice should be sought from specialist metallurgists.

[b] Carry out the thermal design of the heat exchanger

This requires the following physical and transport data for the working fluid at a range of temperatures (and if necessary pressures) covering the working temperature range.

 Density
 Specific Heat
 Thermal Conductivity
 Viscosity
 Flow Behaviour Index (cf.) if the fluid is Non-Newtonian (cf.)
 Latent Heat if the fluid is a condensing vapour or an evaporating liquid

It must be stressed that any heat exchanger design is only as reliable as the fluid data used to produce the design. There is an old saying amongst computer programmers which applies equally to heat exchangers: “If you put rubbish in you get rubbish out”.

FOULING

There are several different types of fouling which can be broadly summarised as follows:

Deposition fouling – where the fluid stream carries solid particle such as silt in river water which, because of a low velocity in the tubes, falls under the action of gravity onto the tube surfaces.

Chemical fouling – where a chemical change within the fluid occurs during heating or cooling and allows one of the constituents to deposit out onto the tube surface. A common example of this type of fouling is hard water scaling which occurs when water containing dissolved Calcium Chloride is heated. Heating causes the solubility of the Calcium Chloride to fall and it is deposited onto the heating surfaces.

Corrosion fouling – where one of the operating fluids is corrosive to the tube material and causes a film of corrosion products to form on the tube surfaces.

Biological fouling – where biological agents such as algae or shellfish are contained within the circulating fluid (usually cooling water) and deposit out onto the surfaces of the heat exchanger.

FOULING TREATMENTS

A variety of methods have to be used to combat the fouling mechanisms summarised above:

Deposition fouling – filtration of the silt laden water or passing it through settling tanks prior to use can minimise the level of deposits. Once they have formed however mechanical cleaning methods such as wire brushing or high pressure water jetting have to be used to remove the deposits.

Chemical fouling – as this is entirely dependent on the process little can be done to minimise or prevent this type of fouling. Because many different factors including the process fluids and base materials etc. are involved it is recommended that advice from a specialist industrial cleaning company is sought in order to identify the best means of removal.

Corrosion fouling – this can only be combatted by choosing the correct materials to be in contact with the operating fluids. If the corrosion is severe there is a risk of tube failure so a complete re-tube of affected equipment may be required with – if possible – a more corrosion resistant material being used as a replacement.

Biological fouling – this can be combatted in two ways. The initial choice of materials can inhibit biological fouling as certain copper based materials are poisonous to various biological organisms and can be successfully used in applications where sea water is used as a coolant. Treatment of the cooling water is also useful particularly in recirculating systems such as cooling towers where dosing the water with anti-fouling agents can minimise contamination.

FOULING FACTOR

In many heat transfer applications the working fluids carry with them substances which will either deposit out onto heat transfer surfaces or cause corrosion of these surfaces.

When this happens, an additional resistance to heat flow is added and the performance of the heat exchanger (its efficiency) will be affected.

There are two ways used to allow for this build-up of on the heat transfer surfaces and these are:

[1] To allow additional surface area to compensate for a reduction in the heat transfer coefficient because of the additional dirt (or fouling) layer

[2] To introduce a factor into the heat transfer coefficient calculations to artificially reduce the coefficient.

This additional factor (or factors if both fluid streams are affected) is called the ‘fouling factor’ and its value has to be determined by using experience of similar applications and a knowledge of the working fluids and their environment.

TEMA has a list of recommended fouling factors in section 10 but as these are for Refinery applications they are normally considered to be too severe for normal industrial use. Experience is the best guide to what values should be used.

FATIGUE

This is defined as progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the UTS limit and may also be below the Yield Stress limit for the material.

Fatigue occurs when a component is subjected to repeated loading and unloading and in the case of heat exchangers this can be thermal loading (heating and cooling) or pressure loading (pressurisation and depressurization).

Fatigue will cause cracking in the most highly stressed areas of a component and the susceptibility of a component is affected by its shape. Sharp corners and square holes should be avoided wherever possible as these can lead to elevated levels of stress because of stress raisers (cf.) so, as rounded corners and round holes are less susceptible they should always be used.

The European Code EN13445 gives two methods of checking the fatigue level in a pressure vessel, a simplified method (UNE-EN-13445-3 Section 17) and a more rigorous routine (UNE-EN-13445-3 Section 18).

The problem with both of these methods however is that they require detailed information about the working cycles of the vessel which the heat exchanger designer is unlikely to have at the contract design stage. A more generalized approach is therefore necessary in applications where the designer suspects that fatigue may be a significant factor during the working life of a unit.

Applications where fatigue is likely to occur are those where the unit is not working continuously such as CIP heating systems where the units are required to be on standby for periods and are then suddenly switched to full load so the designer must take this into account in the design of the unit.

As a ‘rule of thumb’ the maximum stress imposed on any component within the heat exchanger should be limited to 25% of the maximum allowable stress level. If this rule is followed, by increasing section thicknesses if necessary and by using adequately radiused corners etc. the likelihood of a fatigue failure will be minimized.

To be absolutely certain however the analysis recommended by EN 13445 must be carried out and the detailed information required for this analysis requested from the end user.

GASKETS

Many different types of gaskets are used for sealing flanged connections against leakage and it is important that the material and type of gasket are chosen to suit the application.

There are two basic forms; ring gaskets which match the inner and outer diameter of the flange raised face (where fitted) and full face gaskets which match the inner and outer diameter of the whole flange.

Full face gaskets are often easier to fit as they carry bolt holes which can be used to position and secure the gasket while the flanges are being attached.

If soft (elastomeric) gaskets of either type are used it is advisable to ensure that the gasket surfaces of the flanges are machined with concentric grooves to act as a restraint to stop the gasket being extruded under high pressure.

EN 1092-1:2007 gives dimensions for this type of flange face preparation.

Gasket materials will vary with the application as they have to be chemically resistant to the working fluid and sufficiently strong to withstand the working pressure and temperature.

Typical gasket types are as follows:

 EPDM or other elastomeric materials (cf.) for low pressure, low temperature applications
 Non-asbestos fibre gaskets (such as Klingersil C-4430) used for a wide range of applications involving medium pressure steam, hot or cold potable water applications and oil and other hydrocarbon applications
 Reinforced gaskets with PTFE outer coating for corrosive applications
 Spiral wound gaskets which alternate a metallic spiral with a fibre filling and inner and outer reinforcement rings used for a wide range of industrial and petrochemical applications

Selection of appropriate gasket materials is crucial to the safe performance of heat exchangers. The material must not only be chemically resistant to the fluids being used but also have sufficient strength at the working temperatures to resist the forces imposed by the bolting (compressive strength) and working pressures (tensile strength).

As the range of applications for heat exchangers is very wide it is important that the designer uses the facilities provided by the gasket and seal manufacturers to select an appropriate material.

Some useful data on material selection can be obtained from the following web sites:

www.klinger.co.uk
www.dupontelastomers.com
www.parker.com

GRIT FINISH

When a specification is given for the surface finish required for the wetted surfaces of a heat exchanger it can use a variety of ways to describe the level of finish.

The modern way is to specify the RMS Peak value in microns but the old way of giving the information was to give a ‘grit finish’ measurement and this is still sometimes used.

There are unfortunately two standards for Grit finishes and you must know which standard is being used as they vary:

American standard United Kingdom standard Ra
Grit finishes Grit finishes µm
– 120 3.0
– 180 2.0
80 – 1.65
– 240 1.5
– 320 0.75
180 – 0.62
240 – 0.45
– 500 0.4
320 – 0.25

It is important that only the Ra value is quoted to sub-contractors and customers on drawings and other documentation as this is the only internationally accepted standard recognized by all.

GREASE

It is good practice to use an appropriate grease to lubricate bolt threads, help in securing gaskets to flange faces etc.

Most of the bolting used by XLG is Stainless Steel and copper containing grease is often used to prevent binding of the threads during assembly. For securing O rings or ring gaskets petroleum jelly is normally used to hold the seal or gasket in position.

There is one application however where grease should never be used and that is in any heat exchanger involving Oxygen as a working fluid. There is a strong likelihood that the Oxygen will immediately ignite the hydrocarbons in the grease causing a potentially very dangerous situation.

In ALL applications involving Oxygen therefore all components must be thoroughly degreased before delivery to the client.

GROOVES IN TUBE HOLES

When tubes are roller expanded into tubeplates (for additional security, for pharmaceutical units with double tubeplates or when the tube and tubeplate materials cannot be welded together) it is advisable to anchor the tubes into the tubeplates by means of shallow grooves machined into the tubeplate.

TEMA Section 5-7.24 gives details for the grooves but in general terms, if the tubeplate thickness is greater than 25.4 mm TEMA requires two grooves. For tubeplates equal to or less than 25.4 mm a single groove is permissible.

Each groove should be 0.4 mm deep and have a width of 3.2 mm.

It has been shown by testing that a tube to tubeplate roller expanded joint with grooves requires a significantly greater force to pull the tube out of the tubeplate than a joint without grooves so in applications where there is a significant differential expansion resulting in a high tube end load the design will be more secure.

HEAT FLUX

Heat flux is the amount of heat per unit area passing through the heat transfer surface into an evaporating liquid.

In processes involving evaporation there are two important modes involved.

 Film boiling is where the amount of heat per unit area – the heat flux – is so high that the evaporating surfaces are blanketed with a film of evaporated vapour. The vapour film inhibits the evaporation process, the so called ‘Leidenfrost effect’, which results in a very inefficient heat exchanger.

 Nucleate boiling is the most efficient process where the vapour is produced in small bubbles within the liquid. These bubbles rise to the surface where they escape and form a vapour cloud above the liquid.

The type of boiling mechanism which will result with a given fluid depends not only on the fluid characteristics but more importantly on the temperature difference between the tube wall and the liquid in contact with it. If the difference is too high film boiling will be produced and the overall heat transfer coefficient reduced.

For each fluid there will be a characteristic ‘boiling curve’ of temperature difference plotted against the heat flux produced and the heat exchanger designers task is to maximize the heat flux but keep it below the critical heat flux when film boiling starts.

HOLDING TUBES

In food processing applications many liquid or puree products have to be pasteurized to allow them to be stored for an acceptable period after processing and for general hygiene reasons.

Each product is different so the pasteurization temperature and the period of time at which that temperature has to be maintained will also be different and when the heat exchanger designer sizes his equipment he will be advised by the process designer (usually the end user) the time at which the pasteurization temperature must be held before the product is cooled for onward processing or packing.

The holding time (cf.) is a function of the volume of product flowing and the inside diameter of the tubes through which it flows. If the product is being processed in monotube type heat exchangers then it is normally convenient (for cleaning purposes) to use the same diameter holding tubes as fitted in the heat exchanger which will enable the operators to use a pigging system (cf.) for product retrieval and cleaning.

HOLDING TIME

This is the length of time required for a product to be held at the pasteurizing temperature in order to kill the maximum quantity of pathogens.

This time must be determined by the process designer from his knowledge of the product and the pathogens likely to be present. He will determine the holding time needed to reduce the number of viable pathogens to the number allowed under food hygiene legislation for the product.

Holding times are normally specified as minimum times so the heat exchanger designer, when incorporating holding tubes (cf.) into his system should always err on the safe side and make sure that the time is exceeded slightly at the maximum product flow rate specified.

HYDROGEN

There are a variety of applications which use Hydrogen as a working fluid.

Hydrogen has a very high specific heat (cf.) so the thermal design will be special.

Importantly from the safety point of view the designer has to bear in mind that because the Hydrogen molecule is very small the gas is able to leak through the slightest gap in gaskets, weld fissures etc. and that it is extremely dangerous if this happens because Hydrogen is extremely flammable and when ignited burns with an invisible flame which will seriously injure anyone in the vicinity.

If dealing with Hydrogen as a working fluid the designer MUST take account of these dangers and refer to specialist gasket suppliers to assist in gasket selection and also ensure that the construction of the unit is monitored closely with appropriate non-destructive examination of the welded areas and pressure and leak tests rigorously applied.

IMPINGEMENT PLATES

In shell and tube heat exchangers there is a potential problem area directly under the inlet and outlet connections on the shell side of the heat exchanger. It is common practice to match these connections to the system designers pipework sizes but this may lead to a higher than desirable fluid velocity through the connections.

An additional factor is the nature of the fluid flowing through the connections. If it is a clean liquid without solids or entrained air then there should be no problem but if it is possible for the fluid to contain solids or entrained gases – or in the cases of gases and vapours of containing liquid droplets – then a high velocity through the connections can cause erosion problems of the area of the tubes directly under the connections. In order to provide protection against this causing damage to the tubes an impingement plate is fitted in the shell space directly under the connections in order to absorb some of the damaging effects.

Recommendations for sizing impingement plates and the methods of assessing if they are needed are contained in TEMA Section 5 RCB-4.6 and it is recommended that this section is studied if it thought that there may be a problem with a specific application. It should be borne in mind that tube side connections can also – under certain circumstances – lead to tube side erosion and TEMA Section 5 RCB-4.63 gives recommendations for protection.

INCLINATION

Heat exchangers used in some applications in the Food Industry or Pharmaceutical Industry have to be self-draining to enable the units to be cleaned or serviced with minimum product loss.

This is only possible with single pass heat exchangers and can be achieved by mounting the unit at an angle of 2º to 5º from the horizontal axis providing appropriate inlet and headers are fitted. With multiple units however this can be more complicated. If the units are mounted in a single column this can be achieved by fitting eccentric headers and mounting the units so they incline in alternate directions but this can result in a very high heat exchanger assembly.

It must be borne in mind that if corrugated tubes are used drainage will not be 100% even with inclined units as a small quantity of fluid will be trapped in the corrugation troughs even at quite steep angles of inclination. If complete drainage is essential with corrugated tubes the heat exchanger must be mounted vertically.

INSTALLATION

Installation of heat exchangers is normally fairly straightforward and consists of connecting the pipework to the nozzles. More detailed general instructions are given in the Installation and Maintenance Manual provided with the heat exchanger together with more specific instructions if appropriate.

The responsibility for installation is normally carried by a third party company and they must ensure that the units are installed in a correct manner taking into account the instructions provided.

They must be made aware of the possible need for expansion devices to be incorporated into adjacent pipework and for one end of the heat exchanger to be left with the ability to expand/contract under working temperatures. A 6.0 mm long heat exchanger using dry saturated steam at 10.0 Bar(g) will expand by approximately 14 mm and allowance for this must be made in the pipework design and supports otherwise damaging stresses in the shell, tubes and tubeplates of the heat exchanger could result.

ISO 9001

ISO 9001 is the International Standard for Managing Quality.

It is intended to give the essential requirements for a Quality Management System focused on the processes required by any organisation to ensure the satisfaction of their clients and compliance of their products with current legislation.

The XLG Quality Manual is written to satisfy the requirements of this Standard in the context of designing and manufacturing heat exchangers and pressure vessels.

K VALUE

This is usually the term commonly used for the overall heat transfer coefficient of a heat exchanger.

It is sometimes described as the overall ‘U’ value but whichever designation is used it is calculated in the same way, by combining the partial heat transfer coefficients (cf.) with the tube wall resistance and any fouling factors:

K =1 / [ (1/ai) + (1/ao) + Rw + FFi + FFo ]

Where:

ai = Tube side partial heat transfer coefficient

ao = Shell side partial heat transfer coefficient

Rw = Tube wall resistance

FFi = Tube side fouling factor

FFo = Shell side fouling factor

LATENT HEAT OF EVAPORATION

This is the isothermal (constant temperature) heat input required to achieve a change of state from liquid to vapour at the same temperature.

At atmospheric temperature water boils at 100ºC and the heat required to change its state is 2256.66 kJ/kg.

At 10 Bar(g) this falls to 1999,67 kJ/kg so it is important when designing equipment involving either evaporation (cf.) or condensation (cf.) processes that the correct pressure and temperature for the process is obtained.

LENGTH BETWEEN TUBEPLATES

When computing the surface area fitted in a heat exchanger only the area between the inner surfaces of the tubeplates can be used and this is computed using the length of the tube(s) between tubeplates.

It must be borne in mind that it is common practice to define the length of heat exchangers using a ‘nominal’ length which is not necessarily the true tube length. With corrugated tubes particularly there is tube shrinkage because of the corrugating process and wastage at each end because of the need to ensure that the tube is an exact length and has square ends and both of these result is a shortened tube.

A nominal 6000 mm tube for example will have an actual cut length of 5908 mm and the length used for heat transfer will be [5908 – (2 x Tubeplate thickness)].

If double tubeplates with a separation air gap are fitted then the distance between the inner surfaces of the innermost tubeplates must be used and the surface area will be reduced accordingly.

LIGAMENT EFFICIENCY

This represents a measure of the weakening effects of the tube holes in the tubeplates and is calculated as follows for triangular or rotated triangular tube patterns for use in calculating tubeplate thicknesses:

μ = [ p – d ] / p

Where:

P = tube pitch

d = Tube hole diameter

It should be noted that if a tube pitch less than 1.2 x tube outside diameter is used the tubeplate is inherently weak and suitable only for low pressures. For roller expanded joints and high pressures 1.25 x tube outside diameter is preferred.

 

At 1.2 x OD pitch the ligament efficiency would be:

Tube OD 18.0 mm

Tube pitch 21.6 mm

μ = [21.6 – 18.0 ] / 21,6 = 0,1667

 

At 1.25 x OD pitch the ligament efficiency would be:

Tube OD 18.0 mm

Tube pitch 22.5 mm

μ = [22.5 – 18.0 ] / 22,5 = 0,2

LIVE STEAM

This term applies to any steam source that is fed directly from a fired boiler.

It should be borne in mind that live steam is often fed to user equipment through a pressure reduction valve so unless the system is fitted with pressure relief valves set to operate at the lower maximum allowable pressure (cf.) of the user equipment the equipment must be designed for full boiler steam pressure and temperature.

LMTD

The LMTD is used in heat exchanger design and is the Logarithmic Mean Temperature Difference between the two fluid streams.

It is calculated from:

LMTD = [ ΔTA – ΔTB ] / [ ln (ΔTa/ΔTB) ]

Where:

ΔTA = is the temperature difference at one end (A) of the heat exchanger

ΔTB = is the temperature difference at the other end (B) of the heat exchanger

For single pass heat exchangers operating with single phase fluids the LMTD can be used directly in the heat transfer calculation of surface area required but if multi-pass construction is contemplated, with more than one tube side pass or more than one shell side pass, a correction factor (F) must be applied to give the Effective Mean Temperature Difference (cf.)

MARGIN

When designing heat exchangers there are often many unknown factors because of uncertainties about precise fluid data, temperatures etc.

In addition to the unknowns the designer can never be sure that the system as built will perform as designed, that pumps will deliver the correct flow rates, other process equipment will perform as designed etc.

Because of the unknown parameters and the performance variations it is normal for the heat exchanger designer to allow additional surface over and above the surface area required for the heat transfer design performance.

This is the surface margin and is normally quoted as a percentage of the fitted surface area to give an indication of the system variations it can accommodate.

For water based applications this is typically 10 to 15% of the surface area fitted and for oil based applications 5 to 10% of the surface area fitted but these must be assessed on an individual basis for all applications.

MAXIMUM ALLOWABLE PRESSURE

Under the rules of the European Pressure Equipment Directive this is the design pressure for the equipment and represents the highest pressure that the end user is permitted to apply to the equipment.

MAXIMUM ALLOWABLE TEMPERATURE

Under the rules of the European Pressure Equipment Directive this is the design temperature for the equipment and represents the highest temperature that the end user is permitted to apply to the equipment.

MONOTUBE HEAT EXCHANGER

The XLG MD series and M series heat exchangers which have two concentric tubes connected so that the product (usually) flows through the inner tube and the service fluid through the annular space between the two tubes.

The designs can be fixed tubeplate or demountable depending on the application.

One very important design consideration with monotube heat exchangers is the effect of the shell side design pressure acting on the outside of the inner tube. With tubes above 88,9 mm the ability of the tube to withstand external pressure decreases rapidly so each case must be checked according to EN13445 (cf.) to ensure that the inner tube will not collapse.

MULTITUBE HEAT EXCHANGERS

The XLG BD, D and Pharmaceutical series units are multitube heat exchangers which have several small diameter tubes within a larger pipe forming the shell.

Tube inner size can vary from a normal minimum of 12.0 mm outside diameter up to a normal maximum of 42 mm outside diameter but special designs with larger or smaller tubes can be manufactured depending on the material combination.

The designs can be fixed tubeplate or demountable depending on the application.

NAMEPLATE

Under the rules of the European Pressure Equipment Directive 97/23/EC (cf.) a nameplate carrying the CE mark and giving the following information for all fluid circuits MUST be permanently fixed to pressure equipment:

 Maximum allowable pressures
 Maximum allowable temperatures
 Contained fluid volumes
 Fluid categories (under 97/23/EC)
 Test pressures
 The Name and contact details (address, telephone number etc.) of the manufacturer
 Model designation
 Intended use (heat exchanger or containment vessel)
 A unique serial number
 The registration number of the Notified Body (for Category II vessels and higher)
 The year of manufacture
 The inspection date
 The corrosion allowances
 The CE mark in accordance with Directive 97/23/EC Annex VI

If the equipment cannot be CE marked (See Directive 97/23/EC Article 3 Paragraph 3) the nameplate must carry all of the details given above but the CE mark CANNOT be included.

It should be noted that it is an offence under the European Pressure Directive to remove or deface a nameplate fixed to pressure equipment. If the design conditions for the equipment are amended or corrected for any reason then it must be under the supervision of the original manufacturer. If the equipment is Category II or higher a Notified Body must recheck the designs/equipment before the changes take place. If the equipment has been exported to another European country the manufacturer must make arrangements for a Notified Body authorized to work within the country of use to oversee the changes and substitution of the nameplate.

It should also be noted that if a heat exchanger is built up from several units in series/parallel then the equipment must be categorized, tested and inspected using the contained volumes of the entire unit in which case a single nameplate can be attached.

NATURAL FREQUENCY

In shell and tube heat exchangers comparatively small diameter tubes are used with long tube lengths. Because of this the tubes have a significant unsupported length (between alternate baffles (cf.) or support plates) and if subjected to disturbing forces can often vibrate.

Every tube or rod has an inherent Natural Frequency (or fundamental frequency) which is the frequency it will oscillate once it has been set in motion if there is no outside interference.

For tubes with one end fixed into the tubeplate and the other end free (assuming that there is a clearance between the tube and the tube hole in the baffle plate) the fundamental natural frequency is calculated as follows – taken from TEMA Section 6 – all dimensions in Imperial units:

fn = 10,838 x [(A * C) / (L2)) x (((E x I) / wo)0,5]

Where:

fn = the fundamental natural frequency
A = tube stress multiplier taken from TEMA Paragraph V-6.1
C = the factor from TEMA Table V-6.3 for one end fixed and one free
L = the unsupported tube length
E = Young’s modulus for the tube
I = the Moment of Inertia for the tube [(P/64) x ((od4)-(id4))]
wo = weight of the tube per unit length including the weight of the contained fluid

NEOPRENE

Neoprene is a synthetic rubber used in a wide variety of forms for different applications.

It has excellent compression set (cf.) resistance and has a low gas permeability making a useful material for O rings and seals.

Neoprene can be used in a temperature range of -20°C to + 95°C continuous exposure but can withstand up to 200°C for short times.

NITRILE

Nitrile rubber (also known as Buna-N or NBR) is a synthetic rubber used in a wide variety of forms for different applications.

It has excellent resistance to oils and is therefore very useful for O rings and seals in oil based heat exchanger applications.

Nitrile can be used in a temperature range of -30°C to + 120°C continuous exposure but can withstand up to 200°C for short times.

NON-CONDENSABLE

In some condensation applications or applications using a vapour as a heating medium it is sometimes possible for the vapour stream to be contaminated by a non-condensable gas. In vent condensers this may be air or whatever gas is being used to blanket the liquid in the vessel and in steam condensers usually air.

Boiler systems are usually treated to minimise the oxygen content of the boiler water but in complex systems there will frequently be air leakage into the system through leaking valve or pump seals.

Whenever contaminating gas is present the condensation coefficients will be reduced and the heat exchanger designer must take this into account by computing a correction factor based on the relative quantities of gas and vapour and the relative heat loads resulting.

NON-NEWTONIAN

A non-Newtonian fluid is a fluid whose properties differ in any way from a Newtonian fluid. Most commonly the viscosity values of non-Newtonian fluids are dependent on the shear rate.

In practical terms this means that when fluid data is entered into the heat transfer equations for non-Newtonian fluids both a viscosity and the associated shear rate must be entered so that the software can compute the apparent viscosity based on the velocity of the fluid through the heat exchanger tubes.

If the heat exchanger designer is presented with a non-Newtonian fluid he must obtain (either from the customer or from standard data) values for the viscosity and associated shear rates across the operating temperature range.

NOTIFIED BODY

Under the European Pressure Equipment Directive for pressure vessels (Directive 97/23/EC) the safety of each pressure vessel (including heat exchangers) has be assessed to determine the risk posed by the vessel. For vessels posing a high safety risk, principally those having a very large volume or handling dangerous fluids (cf. Fluid Data) a high degree of design and manufacturing oversight by a suitably qualified Third Party Inspectorate is required.

The Third Part Inspectorates are examined by the European Union to confirm their expertise in carrying out this work and those who are approved can be appointed as Notified Bodies within the terms of specific Directives. When appointed as a Notified Body they are allowed to carry out the design scrutiny and manufacturing oversight required within the Directive.

It is important to note that Notified Bodies are required under the Directive to have knowledge of the manufacturers of the equipment and the Quality Management System that they have in place and that they are acting on behalf of the European Union to ensure that the Directive requirements are met in full. Because of this the European Pressure Directive states clearly that it is the Manufacturer who has the responsibility for appointing the Notified Body to carry out his inspections etc. The customer or end user has no right to appoint a specific Notified Body. If a customer wishes to appoint an Independent Third Party Inspectorate he has the right to do so, at his cost, but this Inspectorate CANNOT act as the Notified Body unless the manufacturer agrees.

NOZZLES

This is the American term for connections or branch connections (cf.).

An important consideration in the mechanical design of a heat exchanger is what ‘nozzle loads’ will be imposed on the heat exchanger by connecting pipework.

These loads are fully defined in TEMA section 10.6 and represent the forces imposed by pipework misalignment on the heat exchanger connections.

It is important to note that unless the end user advises that there are specific nozzle loads to be imposed on the heat exchanger the designer should always assume these to be zero.

On thin wall stainless steel vessels it is important that the pipework is designed and supported correctly to avoid imposed loads as it is very easy to cause distortion (and therefore highly stressed areas) in the shell pipe.

If nozzle loads are advised then EN 13445-3 Section 16 should be used to compute the effects on the stress levels within the heat exchanger components.

N.T.U.

The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the LMTD (cf.). In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used; but when these temperatures are not available The NTU or The Effectiveness method is used.
To define the effectiveness of a heat exchanger we need to find the maximum possible heat transfer that can be hypothetically achieved in a counter-flow heat exchanger of infinite length. Therefore one fluid will experience the maximum possible temperature difference, which is the temperature difference between the inlet temperature of the hot stream and the inlet temperature of the cold stream. The method proceeds by calculating the heat capacity rates (i.e. mass flow rate multiplied by specific heat) and for the hot and cold fluids respectively, and denoting the smaller one as . The reason for selecting smaller heat capacity rate is to include maximum feasible heat transfer among the working fluids during calculation.

A quantity

is then found, where is the maximum heat that could be transferred between the fluids. According to the above equation, to experience the maximum heat transfer the heat capacity should be minimized since we are using the maximum possible temperature difference. This justifies the use of in the equation.
The effectiveness (E) is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate:

where

Effectiveness is dimensionless quantity between 0 and 1. If we know E for a particular heat exchanger, and we know the inlet conditions of the two flow streams we can calculate the amount of heat being transferred between the fluids by

For any heat exchanger it can be shown that

For a given geometry, can be calculated using correlations in terms of the ‘heat capacity ratio’

The number of transfer units, is

Where is the overall heat transfer coefficient and is the heat transfer area.

OPERATING PRESSURE

Under the rules of the European Pressure Equipment Directive (cf.) the designer of the pressure vessel or heat exchanger must establish, to the best of his ability, the maximum operating pressure for the equipment.

As this will depend on many factors outside his direct control it is essential that end user or system designer gives this information in a clear and unequivocal manner. Failure to do so could result in equipment which is dangerous to use.

If this value is not clearly defined then the heat exchanger designer must state in a clear and unequivocal way what values he has used in preparing his design.

The maximum operating pressures must also be clearly marked on the heat exchanger nameplate so that the user of the equipment knows what the pressure limits are.

The maximum operating pressure will normally be calculated from the lift pressure of a safety relief valve or bursting disc rupture pressure +10% which represents the accumulation (cf.) of pressure through the valve or disc when it has operated.

ORIFICE PLATES

Orifice plates are used to regulate or measure the flow rate through a pipe by introducing a fixed and known resistance into the pipe (the orifice) and measuring the pressure loss across it.

For (incompressible) liquid flows through an orifice plate:

m = r x Q = C x A2 x [ 2 x r x (P1 – P2) ]^0,5

Where:

Q = Volumetric flow rate in m³/s
m = Mass flow rate in kg/s
C = Orifice flow coefficient – generally taken as 0.62 for approximations
A1 = Cross sectional area of the pipe in m²
A2 = Cross section area of the orifice in m²
r = Fluid density in kg/m³
P1 = Upstream pressure in Pa
P2 = Downstream pressure in Pa

Compressible fluids such as gases and vapours are calculated differently and reference should be made to standard literature for these equations which need the following additional data:

R = Universal gas constant = 8,3145 J/mol.k
K = Ratio of the Specific Heats [ Cp/Cv ]
M = Gas or Vapour molecular mass in kg/mol (also known as the Molecular weight)
T = Upstream gas temperature in degrees Kelvin (ºC + 273)
P1 = Upstream gas pressure in Pa
P2 = Downstream gas pressure in Pa
Z = gas compressibility factor at T1 and P1

PACKING

Heat exchangers should always be protected against damage during transport to site.

Foe very large equipment or multiple units mounted in framework dedicated lorry transport is normally advisable to avoid multiple loading and offloading operations in which case minimal protection is normally required.

With small units however it is advisable to always use appropriate transport packaging. For shipments by road within Europe a sturdy open wooden crate is normally suitable, dimensioned to make sure that the unit cannot move during loading, transport and offloading.

For shipments by sea to more distant destinations it is advisable to use a full case, dimensioned to make sure the unit cannot move during loading, transport and offloading, and fully lined with waterproof paper to minimise the possibility of damage by sea water. It should be borne in mind that many overseas destinations do not have sophisticated offloading and transport infrastructures and packing cases will be subjected to very rough handling. The packing case manufacturer should be notified of the possibility of rough handling and if possible confirmation sought from the transport Company of the offloading and transport facilities within the destination Country.

It must be noted that many destinations involving sea transport require certificated proof and markings on ALL timber used in manufacturing the case or crate to confirm that the timber has been subjected to an approved chemical treatment to kill all insects and larvae that may be present in the timber.

All packing cases or crates should be marked with the appropriate internationally recognised symbols for lifting points, protection against wet conditions, mounting position etc.

ISO 780 and ASTM D5445 show the commonly used symbols that should be used when appropriate.

Some applications require heat exchangers to be shipped pressurised with an inert gas (usually Nitrogen) in order to prevent contamination. In this case special transport markings (and pressure gauges) are required.

Advice should be sought from specialist shipping agents to ensure that current legislation is followed.
PARTICULATES

Usually encountered in food related applications this is the term used to refer to solid pieces of the product within a liquid stream. For example when treating purees it is quite common to find that the puree will contain pieces of the fruit which has been pureed.

It is important that the heat exchanger designer is aware of the presence of particulates as the minimum tube inside diameter must (as a general rule) be at least three times the maximum particle section in order to prevent blockage (cf. clogging) of the heat transfer tubes.

PASSIVATION

Passivation is the process of returning Stainless Steel components to a corrosion resistant condition.

The passivation processes required during the manufacture of XLG heat exchangers are normally confined to repairing the weld affected areas to return them to their original state and a combined pickle and passivation acid gel is used to achieve this. After the gel has been in contact with the weld affected areas for the time recommended by the paste suppliers it is washed off with clean mains water.

If the stainless steel components have been contaminated by exposure to Carbon Steel residues a more comprehensive process is used.

The components are firstly cleaned thoroughly to remove all oil and grease contamination and then the components completely immersed in a passivating acid bath. The precise acid solution mix and strength should be discussed with the Stainless Steel suppliers’ metallurgists as the grade of stainless steel involved will determine these parameters.

A useful and more comprehensive guide to passivating Stainless Steel components will be found at:

http://www.mmsonline.com/articles/how-to-passivate-stainless-steel-parts

% DISSOLVED SOLIDS

Total Dissolved Solids (often abbreviated TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular (colloidal solution) suspended form.
Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two microns. Total dissolved solids are normally discussed only for freshwater systems, as salinity comprises some of the ions constituting the definition of TDS.
Although TDS is not generally considered a primary pollutant (e.g. it is not deemed to be associated with health effects) it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants.

% SUSPENDED SOLIDS

Total suspended solids is a water quality measurement usually abbreviated TSS. It is listed as a conventional pollutant and was at one time called non-filterable residue (NFR), a term that refers to the dry-weight of particles trapped by a filter, typically of a specified pore size.

PICKLING

Pickling stainless steels is a cleaning procedure used prior to passivation (cf.) to ensure that the stainless steel surfaces are free of all contaminants such as oil or grease, iron particles etc. after all of the machining and welding processes have been completed. Various proprietary brands of pickling paste are available for use on small areas, welds etc. all of which are usually a combination of acids.

Some of these pastes also contain the passivation solutions to give both pickling and passivation in one step.

PIGGING

In systems using monotube heat exchangers to process food industry products it is desirable to recover as much product from the system before any CIP (cf.) cleaning operations take place.

With low viscosity products such as clear juices and other liquids it may be as simple as draining the product through appropriate discharge valves but with more viscous products such as purees or honey draining the complete system would take too much time for efficient use of the plant.

It is common practice with these more viscous products to use a system known as ‘pigging’ which is to force the product out of the system using either water flowing through the system or solid ‘pigs’ which are flexible elastomeric shapes designed to match the internal diameter of the pipework and heat exchangers.

With water pigging there has to be a system for diverting the product before the water behind it enters the processing system. There is inevitably a product loss is doing this as the system designer has to assume a worst case scenario and waste product rather than risk water contaminating the product.

With solid pigs product wastage tends to be lower but the system pipework and heat exchanger internal pipe diameters must be an exact match and any bends sized to allow the pig to pass through. The pigs are flexible but they do have a limited capacity to bend.

The pig supplier must be consulted during the initial design stages in order to find what his standard diameters are and what minimum bend radius they are designed to pass through.

PRESSURE EQUIPMENT DIRECTIVE (PED) 97/23/EC

Any pressure equipment sold within the European Union MUST be assessed and categorized in accordance with the European Pressure equipment Directive 97/23/EC to determine the level of design scrutiny and manufacturing oversight required.

The assessment procedure for heat exchangers is straightforward and involves calculating the volumes of each the fluid circuits and multiplying these volumes by the maximum allowable working pressure for the fluid circuit involved. Using the Fluid Category of each of the working fluids (cf. Fluid Data) the relevant Conformity Assessment tables in Annex II of the Pressure Equipment Directive is selected and the value of Pressure x Volume in Bar. Litres used in this graph to categorize the equipment. The heat exchanger category must be the highest of the individual fluid circuit categories obtained.

The Categories and their meanings are as follows:

 Equipment which cannot be CE marked because of the size and/or design conditions
 CE marked Category I which requires final inspection by the manufacturer only
 CE marked Category II which requires final inspection by a Notified Body (cf.)
 CE marked Category III which requires design scrutiny and final inspection by a Notified Body
 CE marked Category IV which requires design scrutiny and final inspection by a Notified Body

Irrespective of the category into which the equipment falls the manufacturer has a legal obligation to maintain a Technical File (cf.) which contains the technical documentation, calculations, drawings etc. used in the manufacture of the equipment. The Directive states that the manufacturer must be able to compile and provide the Technical File to a Notified Body if required to de so ‘within a reasonable time’ and the records must be kept for a minimum period of 10 years.

When a unit has to CE marked under the Directive a nameplate carrying the CE mark must be attached in a permanent manner to the equipment. It is an offence to remove or deface a CE marked nameplate and if it ever has to be renewed (as it has been damaged accidentally or the design conditions changed) this must be organized by the manufacturer involving (if necessary) a qualified Notified Body.

For more detailed information visit:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1997L0023:20031120:en:PDF
QUALITY CONTROL

The Quality Control procedures followed by XLG are documented in the Quality Manual (cf.) and individual procedures referenced within the Manual.

As the manufacturing processes are normally sub-contracted to specialist companies the Quality Control excercised by XLG is by documenting the manufacturing standards and tests by means of drawings and/or manufacturing specifications sent to the third party company.

Pressure tests and other Non-Destructive Examinations (cf.) are normally witnessed by XLG personnel to ensure compliance with the design requirements.

QUALITY STANDARD

This defines the design and/or manufacturing standards with which the heat exchanger must comply.

There are two sources for these Quality Standards:

 National or International codes – compliance with these is normally obligatory.

 Quality Standards defined by individual Companies to define the standards that they require for specific contracts or projects – compliance with these is a commercial decision

Whenever specific Quality Standards are included in an enquiry or contract the equipment must comply with the standards defined.

QUALITY MANAGEMENT SYSTEM

In order to manufacture heat exchangers operating under pressure the manufacturer must have a documented Quality Management System.

ISO 9001 is the International Standard for Quality Management Systems and this lays down in very general terms the procedures which the manufacturers must follow.

The XLG Quality Manual defines the system used to ensure compliance with this standard and also the legal requirements of the Pressure Equipment Directive (cf.) and other legislation currently in force.

The Quality Manual defines:

How XLG ensures that the requisites of the client are correctly documented and followed

 How XLG ensures that the design processes are carried out in a controlled and documented manner at all stages

 How XLG ensures that the manufacturing processes are carried out in accordance with their documented requirements

 How XLG ensures that the equipment is tested correctly and these tests correctly documented

 How XLG ensures adequate protection for transport and information for installation of the equipment

 How XLG keeps appropriate records to enable an adequate after sales service to be provided

REBOILERS

Reboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation.
Proper reboiler operation is vital to effective distillation. In a typical classical distillation column, all of the vapor driving the separation comes from the reboiler. The reboiler receives a liquid stream from the column bottom and may partially or completely vaporize that stream. Steam usually provides the heat required for the vaporization.
The most critical element of reboiler design is the selection of the proper type of reboiler for a specific service. Most reboilers are of the shell and tube heat exchanger type and normally steam is used as the heat source in such reboilers. However, other heat transfer fluids like hot oil or Dowtherm (TM) may be used

Commonly used heat exchanger type reboilers are:

Kettle reboilers

A typical steam-heated kettle reboiler for distillation towers

Thermosyphon reboilers

These do not require pumping of the liquid into the reboiler, natural circulation is obtained by using the density difference between the reboiler inlet column bottoms liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid head to deliver the liquid into the reboiler. Thermosyphon reboilers (also known as calandrias) are more complex than kettle reboilers and require more attention from the plant operators.
There are many types of thermosyphon reboilers. They may be vertical or horizontal and they may also be once-through or recirculating. Some fluids being reboiled may be temperature-sensitive and, for example, subject to polymerization by contact with high temperature heat transfer tube walls. In such cases, it is best to have a high liquid recirculation rate to avoid having high tube wall temperatures which would cause polymerization and, hence, fouling of the tubes.
Relative volatility of feed to reboiler must be considered before designing thermosyphon reboilers. The recirculation rate and pressure profile of the thermosyphon loop have to be calculated by balancing the driving pressure against the system pressure losses.

A typical steam-heated forced circulation reboiler for distillation towers

Forced circulation reboiler

This type of reboiler uses a pump to circulate the liquid through the reboiler.
It should be noted steam is not the only heat source that can be used. Any fluid stream at a high enough temperature could be used for any of the many shell and tube heat exchanger reboiler types

RESIDENCE TIME

This is the term used for the time that the product remains within the heat exchanger or holding tubes (cf.) which will be specified by the food technologists to achieve the level of bacteriological kill needed to achieve an appropriate shelf life.

RELIEF VALVES

If there is a possibility that the pressure within a system could rise to a level in excess of the design pressure then a pressure relieving device must, under the European Pressure Directive (cf.) rules, be fitted.

It is sometimes convenient for the device, which can be either a valve or a bursting disc, to be mounted on or adjacent to the heat exchanger.

If there is danger that the heat exchanger isolation valves could be closed on one fluid circuit while the other circuit was flowing then this can cause an over pressurisation of the closed circuit and a pressure relieving device MUST be fitted between the heat exchanger isolation valves.

TLV France has a wide range of pressure relief valves available and reference should be made to them for sizing an appropriate valve.

SEALING SYSTEMS

Shell and tube heat exchangers use a variety of sealing systems to achieve pressure soundness.

In the XLG MD, triple tube and BD units a series of double O ring seals form the sealing system to maintain pressure soundness of the product and/or service fluid circuits.

Food related applications commonly use hygienic ferrules and clamps with hygienic seals.

Industrial applications use a variety of O ring, square section or flat gaskets depending on the style of heat exchanger being used and the service requirements of pressure, temperature and contact fluids.

SCALING

In water sources that contain hardness salts, most commonly Calcium Chloride, the salt loses solubility as the water temperature rises and will tend to deposit out on heating surfaces in the form of hardness scale.

This layer will build progressively and inhibit the flow of heat causing performance problems in the heat exchanger. The scale must therefore be removed periodically to return the heat exchanger to optimum performance.

If the scaling takes place on the tube side of a heat exchanger it can be removed mechanically using wire brushes or a high pressure water jet but if it is on the shell side surfaces chemical removal is usually more effective.

It is advisable to take advice from a specialist cleaning company before scaling is attempted as they will be able to assess the scaling agents involved and recommend the best cleaning method taking into account the site conditions materials of construction etc.

SILICONE

Silicone is an elastomer (cf.) used in a wide range of seals and gaskets.

Its major use is in food based applications and can be supplied with FDA approval certification. It has a maximum continuous service temperature of +200°C and is suitable for most oil based applications.

Its minimum service temperature is -40°C.

It is not suitable for applications requiring high tensile strength, tear strength or resistance to abrasion.

SHELL

This is the term typically used to describe the pressure envelope surrounding the inner tube(s) of a shell and tube heat exchanger.

XLG use a variety of diameters of shell, usually based on either DIN standard or ASTM standard pipes. The surface finish is as welded matt finish for Industrial applications and polished (mirror finish) for Food related or Pharmaceutical applications.

In fixed tubeplate designs of shell and tube heat exchangers an expansion bellows (cf.) is usually fitted in order to absorb the differential expansion between the shell and inner tube(s).

SPECIFIC HEAT

This is the parameter used to describe the amount of energy required to raise a given mass of a substance (solid, liquid or gas) through 1 degree.

The internationally agreed SI system units are J/g.K.
That is the number of Joules required to raise 1 kg of matter through 1ºK.
For water at 20ºC this is 4.1841 J/g.K at 1.0 Bar(abs)

The conversion factors to other units systems are:

 SI to Metric = [ SI / 4,1868 ] kcal/kg.ºC
 SI to Imperial = [ SI / [ 4,1868 ] Btu/lb.ºF

STERILISATION

See Pasteurisation

SUPERHEATED STEAM

When water boils to produce steam the temperature of the vapour is pressure dependent, referred to as the ‘saturation temperature’.

When it is necessary to transport the steam round a system there are inevitably system losses due to thermal radiation and heat conduction to colder surfaces and these losses cause a small amount of steam to condense which is then carried round the system as liquid water. This can cause problems in user equipment, including heat exchangers’ as the liquid could be travelling at high velocity and cause erosion of surfaces.

A common solution to this problem in central steam raising plant is to superheat the steam that is raise the temperature of the vapour to form a dry gas so that small losses in temperature will not cause the vapour to condense within the piping system.

From the heat exchanger designers perspective this has the benefit that there is a reduced likelihood of liquid water causing erosion damage but the disadvantage that the initial calculations for the steam must assume that the superheated steam must be cooled as a gas before condensation can start.

In reality however there is a way around this disadvantage by ensuring that the temperature of the tube wall adjacent to the steam inlet is below the saturation temperature of the steam which causes the steam to condense immediately, the superheat being removed through the layer of condensate formed on the tube surfaces.

Steam consumptions must be calculated however using first the gas phase specific heat and temperature drop to achieve the saturation temperature and then the latent heat corresponding to the working pressure.

SURFACE TENSION

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.
It is important for the heat transfer designer as it is a factor used in calculating boiling coefficients in evaporators and reboilers.

Data on surface tension values can be obtained from standard literature, a useful source being the following:

http://webbook.nist.gov/chemistry/

SUPPORT PLATES

In shell and tube heat exchangers each tube diameter has a recommended unsupported tube length (cf.). In order to achieve an economic design it is sometimes necessary to use tube lengths which exceed these recommended maximum lengths so the tubes must be supported at intermediate lengths in order to minimise the possibility of tube vibration causing failure.

The necessary support is often provided by support plates which are essentially baffle plates with a large cut out (usually a maximum of 45% of the baffle diameter) mounted at intervals along the length of the tube bundle.

SWAN NECK

When it is required for condensing vapours to be cooled below their saturation temperature (sub-cooled) in order to prevent vapour flashing off when the pressure is reduced through the condensate drainage system it is sometimes convenient to both condense and subcool within the same heat exchanger.

In order to achieve this, a liquid level has to be established within the heat exchanger (which is preferable mounted vertically to minimise temperature distortions within the unit) and this can be achieved in one of two ways:

• A liquid level control system can be mounted on connections fitted onto the shell pipe with the control throttling valve on the outlet connection set to control at a level which will achieve both condensation and subcooling

• By using a swan neck condensate outlet connection which requires no controls but which will ensure condensate cooling to a preset level

The swan neck is formed by mounting the condensate outlet pipe so that it is fed from the bottom of the unit (on a vertically mounted unit adjacent to the lower tubeplate) and climbs back up the shell pipe to the required level. This pipe is then turned so the connection is horizontal away from the heat exchanger shell. The top if this horizontal section is then vented back to the vapour space above the liquid level so that the pressure on both sides of the condensate level (within the pipe and within the shell) is equal.

This equalisation of pressure will ensure that the condensate cannot be forced out of the heat exchanger under pressure before it has risen to the level established by the pipework.

As it requires no controls the swan neck type of system is simple and maintenance free, the only disadvantage being that the level of subcooling is predetermined and cannot be adjusted later.

TEMA

TEMA is the shorthand name for the ‘Standards of the Tubular Exchanger Manufacturers Association’ based in New York in the USA and which are published in book form to assist users, engineers and designers to specify and install heat exchangers.

The standards are essentially aimed at the petrochemical industry, the chemical industry and heavy general process engineering industry and describe what are considered to be the best practices for manufacturing safe and reliable heat exchangers.

Although many of the design examples are based on large diameter units it does contain some useful reference data which can be applied to smaller units (cf. Natural Frequency) and recommendations which can be applied to many different units.

If referencing TEMA the user must be aware that it is based entirely on American standards and dimensions are all based on Imperial units.

TEST PRESSURE

In order to test the strength and leak tightness of heat exchangers (and most other pressure vessels) it is normal to pressurise the unit to a pressure which is above the Design Pressure (cf.).

The mechanical design codes describe the method which must be used to work out what the test pressure should be and these usually take into account the relative material strengths at atmospheric temperature and design temperature in order to subject the material to a realistic test.

The European design code EN13445-5 defines the test pressure as:

Not less than the higher of either:

[1] Pt = 1.25 x Ps x (fa/ft)

or

[2] Pt = 1.43 x Ps

where

Ps = Maximum admissible working pressure
Pt = Test pressure to be applied
fa = nominal allowable stress for the weakest component at the test temperature
ft = nominal allowable stress at the maximum admissible working temperature

It is normal to apply the test pressures independently (with the other fluid circuit empty and depressurised) for a time of 30 minutes or for the length of time required by any Third Party Inspectorate (cf.) responsible for final inspection on the heat exchanger.

THERMOSYPHON

A thermosyphon is a method of passive heat exchange based on natural convection, which circulates liquid without the necessity of a mechanical pump. This circulation can either be open-loop, as when liquid in a holding tank is passed in one direction via a heated transfer tube mounted at the bottom of the tank to a distribution point or it can be a vertical closed-loop circuit with return to the original vessel. Its intended purpose is to simplify the pumping of liquid and/or heat transfer, by avoiding the cost and complexity of a conventional liquid pump.

It is commonly used by XLG in the design of steam generators where a natural thermosyphon effect is used to circulate the liquid being evaporated through the tube side of a vertically mounted heat exchanger.

TIE RODS

When baffles are fitted to direct the shell side fluid flow or to support the tubes they are usually tied together by a framework which is attached at one end to a tubeplate.

The Tie Rods for the horizontal spacers for the baffle plates and may be welded to each baffle or carry a series of spacer tubes

TUBES

This is the general term used for the (usually) small diameter pipes used in shell and tube heat exchangers. The essential difference between a pipe and a tube is that the pipes have greater wall thicknesses.

XLG tubes are usually corrugated to enhance the heat transfer capabilities of the tubes and are supplied in a variety of Stainless Steels, Duplex alloys and higher grade materials such as Titanium, Incoloy® etc.

TUBE HOLE GROOVES

When tubes are roller expanded into tubeplates (for additional security, for pharmaceutical units with double tubeplates or when the tube and tubeplate materials cannot be welded together) it is advisable to anchor the tubes into the tubeplates by means of shallow grooves machined into the tubeplate.

TEMA Section 5-7.24 gives details for the groove but in general terms, if the tubeplate thickness is 25.4 mm or more two grooves are required. For tubeplates equal to or less than 25.4 mm a single groove is permissible.

Each of the grooves should be 0.4 mm deep and have a width of 3.2 mm.

It has been shown by testing that a tube to tubeplate roller expanded joint with grooves requires a significantly greater force to pull the tube out of the tubeplate than a joint without grooves.

TUBE WALL TEMPERATURE

In a great number of applications it is desirable to know the metal temperature in contact with the fluids to ensure that they will not freeze or burn onto the surface or suffer a change of state.

The tube wall temperature in shell and tube heat exchangers can be calculated as follows.

[1] If the hot fluid is on the tube side:

tw = tc – [( ho / [ hi + ho ]) x [ Tc – tc ]]

Where:

tw = tube wall temperature
tc = cold fluid temperature
ho = shell side partial heat transfer coefficient
hi = tube side partial heat transfer coefficient referred to the outside surface
Tc = hot fluid temperature

[2] If the hot fluid is on the shell side:

tw = tc + [( ho / [ hi + ho ]) x [ Tc – tc ]]

Where:

tw = tube wall temperature
tc = cold fluid temperature
ho = shell side partial heat transfer coefficient
hi = tube side partial heat transfer coefficient referred to the outside surface
Tc = hot fluid temperature

TUBEPLATES

Tubeplates perform several different functions within a heat exchanger and the specific design details for each one will be determined by the function it has to perform.

In XLG multitube B series units the tubeplates are extended to form the inter-connection to the tube side pipework and as such must not only be sized for withstanding both the tube side and shell side working pressures but must also conform to the chosen flange standard.

Where the tube side interconnection is not incorporated into the tubeplate the thickness will be determined from the mechanical design calculations using the tube side and shell side design pressures and (where applicable) the addition pressure caused by the differential expansion between the tubes and the shell.

TUBE SIZES

Tube sizes vary according to the manufacturer and application. In deciding which tube size is most appropriate in specific applications the factors which are taken into account must be as follows:

 Material to be used for maximizing corrosion resistance and thermal conductivity.
 Presence of particulates – the tube internal diameter must be at least 3 times the maximum particulate section.
 Cleaning requirements – if a pigging system is to be used for clearing product from the tube(s) there will be a minimum inside diameter specified by the system designer.
 Heat exchanger type – monotube, multitude, triple tube etc.
 Size availability for the chosen material.

TUBE SPACING

Following the recommendations of TEMA Section 5 C-2.5 XLG multitube heat exchangers normally use a tube spacing of between 1.2 and 1.25 x the tube outside diameter. For performance reasons this is sometimes increased to give a greater flow area on the shell side and therefore a lower pressure loss characteristic.
TUBE PATTERNS

The tube patterns used in XLG multitube heat exchangers are also taken from TEMA Section 5 RCB 2.4, the most commonly used being the 30º triangular pattern which gives the greatest tube density within a given configuration.

TUBE VIBRATION

Because the tubes in a shell and tube heat exchanger have fluids passing over them continuously they are prone to vibrate because of their large length/diameter ratio. It is essential that the vibration characteristics of individual applications are check to ensure that adequate supports are provided and that vibration at the tube natural frequencies are avoided.

Calculation of tube vibration is a complex process but TEMA provides a ‘rule of thumb’ chart for recommended maximum unsupported tube lengths (Table RCB-4-52) and in Section 6 ‘Flow Induced Vibration gives a method of carrying out a vibration analysis for specific units.

THERMAL FLUIDS

In applications requiring a high temperature fluid heating source there are various options available to the designer. High pressure, high temperature steam sources are often used but have the disadvantage all of that the system components have to be designed to withstand high pressures at high temperatures and this often limits the materials available.

An alternative to steam are Thermal Fluids which can be heated to high temperatures using either electric or fired heaters and can operate at low pressures, simplifying the mechanical design. They can equally work at very low temperatures with a wide variety of oils available, each for a specific temperature range.

Most are synthetic fluids blended to give the range of operating temperatures required with little or no corrosion or product breakdown due to high temperature or waxing due to very low temperatures.

For detailed information on specific fluids and their operating characteristics refer to the web sites of the major manufacturers such as: Mobil, Shell, BP, Santos, Exxon.
The system designer will normally specify the type and trade name of the fluid to be used as process requirements and heating facilities will decide which is most suitable.
THERMAL CONDUCTIVITY

Thermal conductivity – normally denoted k – is the property of a material’s ability to conduct heat and appears primarily in Fourier’s Law for heat conduction.
Heat transfer across materials of high thermal conductivity occurs at a higher rate than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat exchanger applications and materials of low thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature dependent and the reciprocal of thermal conductivity is thermal resistivity.

Stainless steel is normally used by XLG for both tubes and the pressure containment and although this material has a relatively high thermal resistance – low thermal conductivity – its strength and corrosion resistance allow thinner material sections to be used which limits the overall thermal resistance.

TYPES OF SHELL AND TUBE HEAT EXCHANGERS

There are many different types of shell and tube heat exchanger produced, each designed to meet a specific set of design criteria and application needs.

The principal types of unit produced by XLG are as follows but it should be borne in mind that variants of these types are always available to meet specific application needs. Most designs can be produced to meet pharmaceutical industry requirements or EHEDG and 3-A standards.

 Straight tube multitube units – fixed tube type (B series) or demountable type (BD series)
 U tube multitube units – normally demountable (BU series)
 Straight tube monotube units – fixed tube type (M series) or demountable type (MD series)
 Straight tube triple tube units – fixed tube type
 Kettle type units – demountable U tube units

U VALUES

See K values

UNDER DESIGN

This is the term used when a heat exchanger is unable to meet the thermal performance for which it was designed. This may be caused by an underestimation of the design requirements, system deficiencies causing inadequate flow rates or temperature profiles or an overestimation of the heat transfer coefficients which would be achieved. In applications involving fluids with little or no reliable transport or physical data available it is essential that an adequate design margin is incorporated to allow for unknown factors.

‘U’ STAMP

See ASME VIII Division 1

UNSUPPORTED TUBE LENGTH

TEMA (cf.) contains recommendations for the maximum lengths of tubes of a range diameters which should be allowed to be unsupported.

There are two main reasons for these recommendations:

 The tubes will sag under their own weight if the unsupported span is too long and may collide with one another under flow conditions
 Longer tubes would have a low natural frequency (cf.) which will make them susceptible to vibration and consequent damage

The recommendations in TEMA are all based on Imperial size tubes but the metric tube lengths can be obtained by interpolation.

The TEMA recommendations are as follows:

 6,4 mm maximum unsupported span 660 mm
 9,5 mm maximum unsupported span 889 mm
 12,7 mm maximum unsupported span 1118 mm
 15,9 mm maximum unsupported span 1321 mm
 19,05 mm maximum unsupported span 1524 mm
 22,2 mm maximum unsupported span 1763 mm
 25,4 mm maximum unsupported span 1880 mm
 31,8 mm maximum unsupported span 2235 mm
 38,1 mm maximum unsupported span 2540 mm
 50,8 mm maximum unsupported span 3175 mm
 63,5 mm maximum unsupported span 3175 mm
 76,2 mm maximum unsupported span 3175 mm

VISCOSITY

Viscosity describes a fluid’s internal resistance to flow and may be thought of as a measure of fluid friction and which is temperature dependent.

It is important to establish the viscosity of any fluid at the operating temperature(s) being used in heat transfer as it is a fundamental factor in establishing the Reynolds number used in determining the heat transfer coefficient.

There are two distinct types of viscosity measurement:

 Dynamic Viscosity (unit Pascal-Seconds or Poise)

 Kinematic Viscosity (unit Stokes)

A useful conversion from Kinematic Viscosity to a Dynamic Viscosity as carried out as follows:

Dynamic Viscosity = (Kinematic Viscosity / 1000) x Density

Where:

Dynamic Viscosity is in cP (mPa.s)

Kinematic Viscosity is in cSt (Stokes/100)

Density is in kg/m³

In heat transfer calculations it is normal to use the Dynamic Viscosity.

VITON®

VITON is the trade name (manufactured by DuPont) of a range of very useful elastomers well known for their excellent heat resistance (400°F/200°C) and excellent resistance to aggressive fuels and chemicals and has worldwide ISO 9000 and ISO/TS 16949 registration.

Details of the performance characteristics of this material which can be used for both O ring seals and flat gaskets can be found on:
www.dupontelastomers.com/products/viton
WATER

Cooling Water sources for heat exchanger use will vary with the installation but they can be broadly classified as follows:

• Raw water sources; it is important to monitor the chemical composition of the water source to ensure that it is suitable for use with the AISI 300 series stainless steels, particularly the quantity of chlorides. The water should be filtered through mesh filters to ensure that any solids carried through to the heat exchanger will not cause blockage. The user should bear in mind that there are environmental controls in force in most countries which limit the temperature of raw water sources returned to the environment.

• Fresh water sources; these are normally taken from mains supplied drinking water systems. The chloride level in these sources is normally low but frequently they will have high Carbonate levels which will result in Hard Water Scale formation when heated. The user must have facilities for Hard Water Scale removal (normally using chemical methods) if it is intended to use for cooling services where temperatures are elevated.

• Chilled water sources; these are closed systems which can normally be assumed to be clean non-fouling supplies. It is unlikely that the working temperatures will cause Carbonate scale precipitation so little fouling is likely over extended periods.

General requirements:

Water Quality is an important factor in determining heat exchanger service life and the user will maximise the life of the heat exchanger if water quality is monitored to ensure that the water source remains within specification.

• Regular chemical analysis should be used to determine the chloride and carbonate levels and appropriate actions taken if the quantities found rise above acceptable levels. Appropriate specialist advice should be sought to confirm that any water source is suitable for use with AISI 304/316 materials at the temperature levels likely to be experienced in service.

• Manual shut off valves should be installed before and after the heat exchanger to ensure safety during servicing activities.

• If the unit is using water as a cooling medium, a safety relief device should be installed between the manual shut off valves and the heat exchanger, sized to ensure a pressure accumulation no greater than 10% of the maximum allowable working pressure marked on the nameplate of the heat exchanger under all foreseeable conditions. This is especially important when the fluid being cooled is at high temperature which could cause a rapid rise in pressure within the cooling system in the event of a failure in the supply.

• The user must ensure that the system and heat exchanger are fully vented of air before being put into service, especially at first start up and after servicing operations.

WATER HAMMER

Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). Water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It’s also called hydraulic shock.
This pressure wave can cause major problems, from noise and vibration to pipe collapse. It is possible to reduce the effects of the water hammer pulses with accumulators and other features.

WET BULB TEMPERATURE

Atmospheric air normally contains a small quantity of moisture.

The wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one’s skin is wet and is exposed to moving air. Unlike dry bulb temperature (cf.) wet bulb temperature is an indication of the amount of moisture in the air.

Wet-bulb temperature can have several technical meanings:

 Thermodynamic wet-bulb temperature: the temperature that a volume of air would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the volume of air.

 The temperature read from a wet bulb thermometer

 Adiabatic wet-bulb temperature: the temperature a volume of air would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process.

For the heat transfer designer it is important to know the wet bulb temperature when using an evaporative cooling tower as a source of cooling water as these towers use the wet bulb temperature as a reference to determine the size of tower required and the predicted water evaporation losses that will have to be replaced.
X-RAY

X-rays are used to view a material such as the construction material of a heat exchanger in order to check for the presence of discontinuities in welded areas as well as in the parent material. By using the physical properties of the ray an image can be developed which displays areas of different density and composition.
A heterogeneous beam of X-rays is produced by an X-ray generator and is projected towards the area under examination. According to the density and composition of the different areas of the object a proportion of X-rays are absorbed by the object. The X-rays that pass through are then captured behind the object by a detector (film sensitive to X-rays or a digital detector) which gives a 2D representation of all the structures superimposed on each other.

YOUNG’S MODULUS

Young’s modulus, also known as the tensile modulus, is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke’s Law holds. In solid mechanics, the slope of the stress-strain curve at any point is called the tangent modulus. The tangent modulus of the initial, linear portion of a stress-strain curve is called Young’s modulus. It can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material. In anisotropic materials, Young’s modulus may have different values depending on the direction of the applied force with respect to the material’s structure.
It is also commonly called the elastic modulus or modulus of elasticity, because Young’s modulus is the most common elastic modulus used, but there are other elastic moduli measured, too, such as the bulk modulus and the shear modulus.
It is an essential property of the materials used in the construction of pressure vessels and heat exchangers and is used in the determination of the maximum stress levels within a pressure retaining structure.
ZINC

In applications using sea water as a cooling medium it is sometimes necessary to provide anti-corrosion protection to the heat transfer surfaces. Where the tube material chosen is non-ferrous (brass or copper nickel) protection is often provided by zinc pads fitted in the sea water circulation system which will corrode preferentially to protect the more noble surfaces.

If this system is used it is essential that the zinc pads are regularly checked and replaced as necessary as once depleted the heat transfer surfaces will be vulnerable to corrosion.