selection of heatexchanger types

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Process Engineering Guide: GBHE-PEG-HEA-506

Selection of Heat Exchanger Types Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Selection of Heat Exchanger Types

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 BACKGROUND 4 5 FACTORS INFLUENCING SELECTION 4 5.1 Type of Duty 4 5.2 Temperatures and Pressures 4 5.3 Materials of Construction 5 5.4 Fouling 5 5.5 Safety and Reliability 5 5.6 Repairs 5 5.7 Design Methods 6 5.8 Dimensions and Weight 6 5.9 Cost 6 5.10 GBHE Experience 7



6 TYPES OF EXCHANGER 9

6.1 Shell and Tube Exchangers 9 6.2 Cylindrical Graphite Block Heat Exchangers 17 6.3 Cubic Graphite Block Heat Exchangers 19 6.4 Air Cooled Heat Exchangers 20 6.5 Gasketed Plate and Frame 22 6.6 Spiral Plate 24 6.7 Tube in Duct 25 6.8 Plate-fin 27 6.9 Printed Circuit Heat Exchanger (PCHE) 29 6.10 Scraped Surface/Wiped Film Exchangers 31 6.11 Welded or Brazed Plate 32 6.12 Double Pipe 35 6.13 Electric Heaters 37 6.14 Fired Process Heaters 38

TABLE (1) ADVANTAGES AND DISADVANTAGES OF DIFFERENT

SHELL AND TUBE DESIGNS 14 FIGURES 1 ESTIMATED MAIN PLANT ITEM COSTS 7 2 ESTIMATED INSTALLED COSTS 8 3 TEMA HEAT EXCHANGER NOMENCLATURE 11 4 F CORRECTION FACTORS' : TEMA E SHELL WITH EVEN

NUMBER OF PASSE 13 5 SHELL AND TUBE HEAT EXCHANGER HEAD TYPES 15



6 GENERAL ARRANGEMENT OF A CYLINDRICAL GRAPHITE BLOCK HEAT EXCHANGER 17

7 EXPLODED VIEW OF A CUBIC GRAPHITE BLOCK

HEAT EXCHANGER 19

8 TYPICAL AIR COOLED HEAT EXCHANGER 21 9 GENERAL VIEW OF ONE END OF A 3-STREAM

PLATE-FIN HEAT EXCHANGER 27

10 TYPICAL PCHE PLATE 29 11 VICARB COMPABLOC' EXCHANGER 33 12 BROWN FINTUBE' MULTITUBE HEAT EXCHANGER 36

13 FIRED HEATER : SCHEMATICS AND NOMENCLATURE 39 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 42



0 INTRODUCTION/PURPOSE This Guide is one of a series on heat transfer produced for GBH Enterprises. Although the majority of heat exchangers used on chemical plant are shell and tube units, there are many other types available. The engineer may have little knowledge of the attributes of these other types so is not able to make a reasonable selection. This Guide aims to assist in this task. 1 SCOPE This Guide describes the factors which influence the choice of heat exchanger and introduces the various types, giving their advantages and disadvantages. It does not give hard and fast rules which will automatically lead to the selection of the 'best' exchanger for a given duty; often there is no one correct solution. Rather it seeks to give the engineer the information on which a rational decision can be made. 2 FIELD OF APPLICATION This Guide applies to process engineers in GBH Enterprises worldwide, who may be involved in the specification or design of heat exchangers. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: HTFS Heat Transfer and Fluid Flow Service. A cooperative research

organization, based in the U.K., involved in research into the fundamentals of heat transfer and two phase flow and the production of design guides and computer programs for the design of industrial heat exchange equipment.

HTRI Heat Transfer Research Incorporated. A cooperative research

organization, based in the USA, involved in research into heat transfer in industrial sized equipment, and the production of design guides and computer programs for the design of such equipment.



PCHE Printed Circuit Heat Exchanger. A design of compact exchanger manufactured from chemically etched plates which are joined by diffusion bonding.

TEMA The Tubular Exchanger Manufacturers Association. An association

of US manufacturers of shell and tube exchangers whose purpose is to draw up standards for their manufacture. See Ref. [2].

With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 BACKGROUND Increased profits are more likely to come through the development of new products or processes than from the selection of better heat exchangers. Because of this, the order of emphasis in selection is: (a) Safety and reliability. (b) Performance. (c) Cost. Attempts have been made in the past to develop selection methods for heat exchangers, either in the form of flowcharts or scoring techniques. These approaches often implicitly assume that there is a single 'correct' solution for each problem. This is rarely the case. An initial screening can be done to reject designs which are unsuitable for reasons of materials, operating conditions or safety, for example, but the engineer will often be left with a range of designs to consider. The final decision will be based on engineering judgment. Probably 90% or more of heat exchangers in the industry are of the shell and tube type. Although not necessarily the best design, shell and tube exchangers can give satisfactory performance on most duties and their design, operation and maintenance are well understood. As a result they are likely to be the standard against which any alternative will be judged. Some of the hurdles which have to be overcome before a different type of exchanger is installed are given in Ref. [1].



This Guide assists in the selection process by discussing the various types of exchanger using a range of factors which should be considered before finalizing the choice. For some types, more detailed discussions are given to assist in choosing between options within the general type. 5 FACTORS INFLUENCING SELECTION 5.1 Type of Duty Duties can be classified as: (a) Heating of liquids. (b) Evaporation and boiling. (c) Heating of gases and vapors. (d) Cooling of gases and vapors. (e) Condensation. (f) Cooling of liquids. (g) Heating and cooling of slurries. Certain types of equipment are less suitable than others for some of these duties. For example plate and frame exchangers are not particularly suitable for handling gases. Often an exchanger is required to perform more than one of these duties for the hot or cold fluid, or both. An example is the production of superheated vapor from subcooled liquid. Sometimes it may be possible to do this in one exchanger. At other times separate exchangers may be required for each stage; different types of exchanger may be appropriate for the differing duties. 5.2 Temperatures and Pressures Mechanical and materials constraints may limit the operating pressures and temperatures that can be handled by the equipment. 5.3 Materials of Construction Materials of construction are selected for their combination of corrosion resistance and mechanical strength. For the case of a heat exchanger, there is the additional requirement of reasonable thermal conductivity, although with the exception of plastics, this is not usually of great importance.



For most types of exchanger design, the choice of material is not an overriding process consideration, although it may influence the economics. Even if the exchanger cannot be fabricated in the cheapest suitable material, there is usually a more expensive choice which may still give an acceptable overall cost. Thus plate and frame exchangers, which use thin sheets of material with no corrosion allowance, are not fabricated in carbon steel, but the item cost of a stainless steel plate and frame unit is often comparable with that of a carbon steel shell and tube exchanger designed for the same duty, and the installed cost is usually lower than that for the shell and tube unit. However, certain types of exchanger may only be available in a limited range of materials because of the methods of fabrication. An example of this is the plate-fin exchanger. Although this is available in stainless steel in small sizes, developed for the aerospace industries, larger units have until recently only been fabricated in aluminium, using either salt bath or vacuum brazing. A number of companies have announced a new range of units in stainless steel and nickel alloys, but, like the aluminium units, these are of brazed construction, which will limit their application in corrosive environments. 5.4 Fouling Many fluids handled on chemical plants have a tendency to foul the heat transfer surfaces. Fouling is more likely in some types of exchanger than others. For instance, particulate fouling tends to occur in zones where the velocity is low, such as round the baffles in shell and tube exchangers. Fouling may reach an acceptable asymptotic value, but often it is desirable to be able to clean the heat transfer surfaces. Some designs can readily be cleaned mechanically. For others this is not possible and chemical cleaning may be necessary. If the very small passages in a PCHE are blocked, even chemical cleaning may not be successful. 5.5 Safety and Reliability Safety and reliability is the most important factor to be considered in selection. On nonhazardous duties it may be acceptable to have some risk of equipment failure, which would not be acceptable for hazardous duties. For example, with a plate and frame heat exchanger, there is always a possibility of gasket failure, although with correct installation and maintenance this should be low. Such a failure may be an acceptable inconvenience for an exchanger handling water; it would be unacceptable if the fluid were liquid chlorine.



5.6 Repairs Corrosion or fatigue may put a limit on the acceptable working life of an exchanger. Unforeseen plant upsets may lead to premature failure. Some designs of exchanger can be repaired in an acceptable and economic fashion. For example, in many types of shell and tube exchanger individual tubes which have failed may be replaced, or if this is not possible, plugged with little deterioration in overall performance. Other designs of exchanger may not facilitate repairs, and a replacement unit may be required. The likelihood and consequences of failure should be assessed at the selection stage. 5.7 Design Methods See GBHE-PEG-HEA-502 for information on computer programs for the thermal design of heat exchangers. Well established and proven methods exist for the thermal and mechanical design of shell and tube exchangers. This is not so for many of the other types, particularly those which can be classified as 'proprietary' designs. These often use correlations for the thermal design which have not been published, and for some types the mechanical design does not conform to any recognized code. When purchasing such units, the engineer has to rely to some extent on the manufacturer. It is true that manufacturers will generally offer a thermal and mechanical guarantee, but their liabilities only cover replacing the unit if it fails to perform. The consequential loss to a company may far outweigh the value of the item. Where the mechanical design does not conform to an established code, the GBH Enterprises mechanical engineer may insist on the unit or parts of a prototype being pressure tested to a much higher pressure than would be normal for an established design. 5.8 Dimensions and Weight The installed cost of an item is significantly greater than the item cost, typically by a factor of about 3. This covers the need to provide pipework, instruments, foundations, lagging etc. Many of these are related to either the dimensions or the weight of the item, or both. Considerable savings can often be made in the total cost of an exchanger by the use of a compact design, even if the item cost exceeds that of a more conventional exchanger. An extreme case of this is equipment to be used offshore; the cost of providing 1 m2 of platform space was quoted in 2009 at about $180,000 !



5.9 Cost As indicated in 5.8, it is the total installed cost which is of ultimate concern. In addition, there may be extra running costs associated with the exchanger, such as power requirements for pumping. Some information on costs, including installation costs, of certain types of exchanger OGJ Nelson Cost Index. Data are available for: (a) Shell and tube exchangers with a variety of materials of construction,

including graphite tubes. (b) Air cooled heat exchangers with a variety of tube materials. (c) Plate heat exchangers with stainless steel, titanium or Hastelloy plates. (d) Cubic graphite block exchangers with phenolic resin impregnation. (e) Cylindrical 'polyblock' graphite exchangers with phenolic resin and PTFE

impregnation. However, the Nelson Index data has to be used with care, as the cost information is not always up to date; occasionally some serious discrepancies can occur. More reliable information, including data on types of exchanger not covered by Nelson Index, can be obtained by consulting manufacturers.( Ultimately, the only totally reliable data for main plant item costs are manufacturers' quotations.) The estimation of installed cost can be even more difficult, especially if the object is to get a reasonable estimate of the difference in total cost between two different types of exchanger for the same duty. Figure 1 shows some data for main plant item costs obtained from NELSON Index and are for illustrative purposes only. Figure 2 shows the estimated installed costs for the same data. Estimates of cost are frequently given on the basis of exchanger area. This can be misleading when comparing different types unless the different heat transfer coefficients of different designs are taken into account. What really matters is the cost for the required duty. Some so-called 'Compact' exchangers often score by giving higher heat transfer coefficients than the equivalent shell and tube units for the same pressure drop, rather than by packing more surface into a given volume.



5.10 GBH Enterprises Where information is available, examples of the use of different types of exchanger are given. FIGURE 1 ESTIMATED MAIN PLANT ITEM COSTS

(Data are for illustrative purposes only!)



FIGURE 2 ESTIMATED INSTALLED COSTS (Data are for illustrative purposes only!)



6 TYPES OF EXCHANGER 6.1 Shell and Tube Exchangers For additional information on shell and tube exchangers and their applications, see GBHE-PEG-HEA-507, GBHE-PEG-HEA-508, GBHE-PEG-HEA-515, GBHE-PEG-HEA-516 and Ref. [2]. (a) Type of duty

Suitable for all types of duty; single phase gas and liquid, boiling, condensation. Slurries should not be handled on the shell side because of the risk of deposit build-up in the dead zones. Materials which become viscous on cooling, if cooled on the shell side, may give problems with severe bypassing.

(b) Operating limitations

Can be designed for almost any combination of temperature and pressure. (c) Materials of construction

Can be fabricated in most materials. The tubes are generally metallic, but specialist manufacturers offer units with tubes of graphite, plastic or silicon carbide.

(d) Fouling

Can operate reasonably on moderately fouling duties. Can usually be cleaned mechanically on the tube side. Prone to sedimentation fouling on the shell side, especially in the dead zones around baffles, but can be designed for mechanical cleaning on shell side if a removable bundle is used.

(e) Safety and reliability

Generally good. Areas to watch are tube-tubesheet joints; corrosion, especially in the dead zones around baffles; tube vibration. Good design should avoid these problems.



(f) Inspection and repairs With a removable bundle, all parts can be inspected visually. With a fixed tubesheet design, only the tube side can be inspected visually, but techniques such as ultrasonic thickness measurements can be useful to give a measure of the condition of the tubes. Except for U-tube designs, replacement of individual tubes is possible.

(g) Dimensions and weight

Shell and tube exchangers have a low surface/volume ratio. The basic surface/volume ratio of the bundle is typically 50 to 120 m2/m3; the overall figure is reduced further by the volume of the headers, allowance for the flanges etc., and also, if a removable bundle is used, by the space required for bundle removal. They are thus relatively large and heavy. Units have been fabricated with over 5000 m2 of heat transfer surface, with diameters of over 4 m and lengths of over 20 m.

(h) Design methods

Well established computer-based methods are available for the thermal design and rating of shell and tube exchangers (see GBHE-PEG-HEA-502). The mechanical design is covered by established codes such as BS 5500, ASME Boiler and Pressure Vessel Code: Section VIII: Division I, etc.

(j) Cost

See Figures 1 and 2. (k) GBH Enterprises

Shell and tubes exchangers are widely recommended for most client plants, and if properly designed and operated give years of trouble free performance.

6.1.1 Types of Shell and Tube Exchanger

Shell and tube exchangers are usually classified with reference to the TEMA designations, with a three letter code describing front end head type, shell type and rear end head type. See Ref. [2] for more information.



Figure 3, which shows the various types diagrammatically, is taken from Ref. [2].

Note: Type P is sometimes referred to as Outside packed stuffing box, type S as Split ring floating head and type W as Outside packed lantern ring.

6.1.1.1 Shell Types

6.1.1.1.1 Single pass

The simplest form of shell and tube exchanger is the single pass fixed tubesheet design, or TEMA type -EL, -EM or -EN.

The tube to tubesheet joints on a shell and tube exchanger are subject to forces due to differential expansion between the shell and tubes. If these forces would be too high for a simple fixed tubesheet design, some provision has to be made to reduce them. This is done either by providing a bellows in the shell or using one of the other rear end head types shown in Figure 3.

The use of bellows should be avoided where possible. Not only are bellows expensive, sometimes adding significantly to the cost of the exchanger, but they represent a weak point in the shell. They are sensitive to corrosion, overload (including an excessive number of cycles) and poor welding. They provide a stagnant region where, because solids can settle out or conditions become modified for other reasons, corrosion is more likely. Repair is often impossible and replacement can require major dismantling of the exchanger.

When there is a hazardous fluid on the shell side of an exchanger, bellows pose a particular hazard, as failure could lead to a major release. If it is impossible to avoid their use, special care should be taken in design and fabrication, and regular inspections will be necessary during operation. Consult a vessels engineer for advice.

The cost of bellows increases rapidly with pressure to an upper practical limit of about 35 bar.

If bellows are specified for an exchanger which is lagged and out of doors, there is a possibility of rainwater corrosion unless steps are taken to prevent water from getting below the lagging.



As a rough guide, bellows are likely to be required for a fixed tube sheet design when the temperature difference exceeds about 50C if the shell is hotter than the tubes, and 30C if the tubes are hotter than the shell. These figures are only a guide; the actual stresses will be calculated by the mechanical designer, who will determine whether bellows are required.



FIGURE 3 TEMA HEAT EXCHANGER NOMENCLATURE



6.1.1.1.2 Multiple tube side passes

The U-tube exchanger obviously has to have more than a single pass on the tube side. Multiple pass exchangers are also used with the other types of rear end head for one of two reasons:

(a) Where a single pass exchanger would be of an excessive length.

(b) Where the tube side flow rate is much less than the shell side. The

use of multiple passes allows the designer to achieve a reasonable tube side velocity and hence heat transfer coefficient without an excessive shell side pressure drop.

Unless one or both of the fluids is essentially isothermal, for example a single component fluid either boiling or condensing, there is a penalty to pay for multi-pass design. Because some of the tube side passes are in co-current flow to the shell side fluid, the effectiveness of the exchanger is less than that of a pure counter-current design with the same area and heat transfer coefficient. This is often accounted for in single phase flow by the 'F' correction factor to the log mean temperature difference. Refs. [3] or [4] show how to calculate this correction.

Some sources recommend the avoidance of designs where the 'F' correction factor falls below some arbitrary value, often 0.75. This can be misleading and potentially dangerous, as can be seen from Figure 4, which shows the value of the 'F' factor for a TEMA E shell with an even number of passes. The parameters P and R are functions of the terminal temperatures as shown in the Figure. The regions to avoid are where small changes in P or R result in large changes in F, because this implies the exchanger performance is very sensitive to small changes in temperature. This corresponds to regions where the curves become steep. It can be seen that although F = 0.75 may be acceptable for R = 1.0, for R = 10 even temperatures which result in F = 0.9 are unsafe.

In extreme cases, a duty which can be performed in a counter-current design cannot be achieved in a mixed flow design regardless of length, and multiple shells in series are required.



The use of the two pass TEMA F shell (see Figure 3) in theory can get round this problem for an exchanger with two tube side passes, by maintaining pure counter-current flow. However, the performance of the F shell design in practice is rarely as good as theory. Leakage of fluid and thermal leakage across the longitudinal baffle can seriously affect the performance. Sealing devices to prevent the physical leakage are often damaged if the exchanger is dismantled. Because of this, the GBH Enterprises strongly discourage the use of F shells, unless the longitudinal baffle is welded to the shell. If this is done, it prevents bundle removal but still does not overcome thermal leakage. For special duties designs have been produced with a double longitudinal baffle with an insulating air gap between the baffles, but such solutions require careful consideration of the mechanical design; Vessels Section should be consulted. Multiple shells in series may prove more economic.

A 4-pass F shell design can be fabricated with a welded longitudinal baffle and removable bundle. This is equivalent to two 2-pass E shells in series, and has a 30 to 40% cost advantage over the separate shells. A check for the effect of thermal leakage should be performed.

The use of multiple tube passes other than two pass U-tubes for tube side condensation or boiling can present design problems because of phase separation in the headers. This results in different flowrates and compositions in different tubes of the same pass, which cannot be allowed for with the design programs used. Condensers can be designed in which the condensate is removed at the end of each pass (inter-pass luting). This gets round the design problem, in that each tube of a given pass will have the same composition. However, the flowrate through each pass will be different, so the normal programs cannot be used directly; each pass will require to be designed in isolation and the results merged together. Moreover, there may be problems with flashing in the pipework where the condensate streams from the various passes are mixed.

The split flow arrangements of types G and H are usually only found in horizontal thermosyphon reboilers. Here, some leakage around the longitudinal baffle will have only a minor effect on the performance.

The type J shell, with one shell side inlet and two outlets, is used for cases where there is a low allowable pressure drop, since half the flow is flowing through half the length of the shell. It is effectively two E shells back-to-back.



Note: With a single pass on the tube side, half the exchanger will be in co-current flow and half in counter-current, so its performance will not be as good as a pure counter-current exchanger.

The kettle type K is a special case of the shell and tube exchanger with an oversized shell to allow for vapor disengagement when used as a boiler. The bundle can be a fixed tubesheet design, in which case the shell will taper at both ends rather than just the one as shown in Figure 3. More usually, the bundle will be a type U or T (see 6.1.1.2). See GBHE-PEG-HEA-507 for more information on reboilers.

FIGURE 4 F CORRECTION FACTORS' : TEMA E SHELL WITH EVEN

NUMBER OF PASSES



6.1.1.2 Head Types

Rear end head types P, S, T, U and W offer different approaches to the problem of differential expansion. Table 1 summarizes some of the advantages and disadvantages of the different types. It is worth noting that the pull-through floating head type T requires a large clearance around the bundle to permit withdrawal. This can lead to excessive bypassing of the shell side fluid, with a resulting poor performance. The split ring floating head design type S is preferred in this respect, but is more expensive.



TABLE 1 ADVANTAGES AND DISADVANTAGES OF DIFFERENT SHELL AND TUBE DESIGNS

Increasing cost



The method of sealing of the type P and W rear end heads is prone to leakage. These designs should be avoided for high pressure or hazardous duties. Their use is normally restricted to cooling water duties. Single pass floating head designs of type S or T usually also involve a sealed gland as in Figure 5 (a). An alternative to this where hazardous materials are involved is the use of an internal bellows as shown in Figure 5 (b). Although this bellows may fail, unlike a full shell bellows it is relatively cheap, can be replaced relatively easily if necessary, and any leakage arising from failure will be contained within the shell. The last point assumes that the shell and tube side fluids are compatible. This type of bellows is especially relevant to high pressure interchangers.

FIGURE 5 SHELL AND TUBE HEAT EXCHANGER HEAD TYPES



Head types A, C, L, N and P have removable covers, which enables access to the tube side without having to disconnect the tube side pipework. This can be an advantage if frequent mechanical cleaning of the tube side is required. However, the design does involve extra flanges and gaskets, with potential for leaks, and will generally be more expensive than the integral cover of types B and M.

Types C and N are used to reduce the number of main flanges on the exchanger, particularly when handling hazardous materials and/or subject to severe mechanical duties (e.g. high temperature cyclic duties). They have the disadvantage that it is difficult to access the tubesheet face for repairs, particularly for replacement of the outer tubes; it may be necessary to specify a larger than normal bundle-to-shell clearance for this purpose, which may lead to excessive bundle bypassing. As the clearance is only required on the tube side, this can be avoided by using a larger diameter for the header than the main shell, as shown in Figure 5 (c).

. 6.1.2 Fluid Allocation

The choice as to which fluid to put on the shell side and which on the tube side is influenced by several factors. Often these are in conflict, with some factors suggesting the hot fluid on the shell side and some on the tube side. In this case some compromise is required and it may be necessary to perform designs for both alternatives, and select the cheaper.



6.1.2.1 Materials of Construction

If one of the fluids requires the use of a corrosion resistant, and hence probably expensive, material of construction, whereas the other can be contained in a cheaper material, it is usually cheaper to put the corrosive fluid on the tube side. This will require the tubes and headers to be made of the corrosion resistant material, whereas the shell and baffles can be in the cheaper material. The tubesheet will either be made of the expensive material in solid form or from cheaper material such as carbon steel clad with the more expensive material on the header side, whichever is cheaper.

If the corrosive material is placed on the shell side, only the headers can be made of the cheaper material. Cladding the shell side of the tubesheet is not usually a practical proposition. It may be economic to consider lining the shell with a corrosion resistant material.

6.1.2.2 Fouling

Fouling is generally less of a problem on the tube side, and cleaning of the tube side is easier than the shell side. If it is required to clean the shell side mechanically, it will probably be necessary to use a square tube pitch, which implies a larger shell for the same surface area.

6.1.2.3 Pressures

It is usually preferable to put the fluid which requires the higher design pressure on the tube side as the required shell thickness is then less. This becomes more important at higher pressures. 6.1.2.4 Flowrates

It is usually easier to produce a reasonable design for exchangers where the fluid with the higher volumetric flowrate is on the shell side. For this reason, gases are usually handled on the shell side.

6.1.2.5 Pressure Drops

For a given flowrate and film coefficient, the pressure drop of the shell side fluid is generally less than the tube side.



6.1.2.6 Enhancement of Heat Transfer

When the shell side heat transfer coefficient is controlling, the effective coefficient can sometimes be enhanced by the use of low fin tubing with relatively small pressure drop penalty. Tube side enhancement devices, such as twisted tapes or looped wire inserts, usually impose a larger penalty on pressure drop, but can be very useful when dealing with high viscosity fluids or laminar flow applications.

6.1.2.7 Cooling of Viscous Fluids

If a viscous fluid is cooled on the shell side of an exchanger, the fluid in the bundle becomes colder than that in the bypass lanes, and consequently more viscous. This can result in a significant increase in bypassing, possibly leading to the situation where essentially all the flow is bypassing the bundle. The computer programs used for design are not at present capable of modeling this phenomenon.

6.2 Cylindrical Graphite Block Heat Exchangers These units are sometimes known as 'polyblock' exchangers. See Figure 6. For general information on graphite heat exchangers.



FIGURE 6 GENERAL ARRANGEMENT OF A CYLINDRICAL GRAPHITE BLOCK HEAT EXCHANGER



(a) Type of duty

Suitable for all types of duty; single phase gas and liquid, boiling, condensation.


The design pressure is normally limited to 5 to 6 bar on the process side and 8 bar on the service side, although some units can be designed for up to 16 bar operation. The maximum operating temperature depends on the type of impregnation used: 165 to 185C for phenolic resin, 230C for PTFE, 400C for carbon. The use in high temperatures, particularly where there is a risk of thermal shock, needs careful design; the manufacturers should be consulted for advice.

(c) Materials of construction

Graphite has very good chemical resistance to a wide range of corrosive chemicals. The corrosion resistance of the graphite used in exchangers is generally limited by that of the impregnant used. PTFE or carbon impregnation offer better corrosion resistance, but tend to be mechanically weaker. If both fluids are corrosive it is possible to line the shell with, for example, PTFE.

(d) Fouling

Mechanical cleaning is possible, but there is a risk of damage to the blocks.


Graphite is a brittle material, which can be damaged by mechanical or thermal shock. Experience has been mixed, some plants having many years of satisfactory performance, whereas others have had repeated failures. Some chemicals may leach the impregnants from the graphite, resulting in the graphite becoming porous over a period of time.

(f) Repairs

The units can be dismantled for inspection. If the graphite is damaged, the damaged blocks have to be replaced, but undamaged ones can be reused.




The surface/volume ratio of the blocks is typically 30 to 70 m 2 /m3 , which makes this design even bulkier than a shell and tube exchanger. Block sizes range from 200 to 1800 mm in diameter, and heights from 100 to 700 mm, depending on diameter. Exchangers are built up from multiple blocks, up to 25 being possible in one shell, depending on block size. Total heat transfer areas approaching 1000 m2 can be obtained with exchangers using the largest blocks. The units are usually mounted vertically, but some recent designs allow horizontal mounting.

(h) Design methods

The performance of a cylindrical graphite block exchanger can usually be simulated using methods developed for shell and tube units, with some adjustment for the 'shell side' coefficient. The mechanical design of the steel shell, which forms the ultimate pressure envelope, is covered by established codes such as BS 5500.

(j) Cost

See Figures 1 and 2. (k) GBH Enterprises experience

Contact Us. 6.3 Cubic Graphite Block Heat Exchangers For general information on graphite heat exchangers . An exploded view of a typical cubic graphite block exchanger is shown in Figure 7.



FIGURE 7 EXPLODED VIEW OF A CUBIC GRAPHITE BLOCK HEAT EXCHANGER

(a) Type of duty

Used on all types of duty. Common in the pharmaceuticals industry, where the relatively small maximum size is not a problem, and the resistance of graphite to a wide variety of chemicals is an advantage in multi-purpose plants.


The design pressure is normally limited to about 6 bar. The maximum operating temperatures are as for cylindrical graphite units.


As for cylindrical graphite units.



(d) Fouling

As for cylindrical graphite units. (e) Safety and reliability

Generally as for cylindrical graphite units. However, unlike the cylindrical units, there is no outer steel shell to act as a containment in the event of a major failure of the graphite.

(f) Repairs

Damaged blocks cannot be repaired and have to be replaced. (g) Dimensions and weight

Units are available with up to 90 m2 of heat transfer surface. The surface to volume ratio is relatively low, typically 50 m2/m3.

(h) Design methods

The thermal performance of cubic graphite block exchangers can be estimated with methods developed for shell and tube units, but with some difficulty. The mechanical design is proprietary.

(j) Cost

See Figures 1 and 2. (k) GBH Enterprises experience

Contact us.



6.4 Air Cooled Heat Exchangers A typical air cooled process heat exchanger is shown in Figure 8. For more detailed information on air cooled heat exchangers and a discussion on the relative merits of air and water cooling see GBHE-PEG-HEA-513. (a) Type of duty

Cooling of gases and liquids, condensation. (b) Operating limitations

Air cooled exchangers tend to become uneconomic compared with water cooled units when the required exit temperature is less than 20 to 30C above ambient. There is no practical pressure limitation.


Tubes and headers can be fabricated in most materials. Fins are usually aluminium, although galvanized steel is sometimes used in corrosive environments, but with a considerable loss in fin efficiency.

(d) Fouling

With the correct choice of header box design, easy access to the tube side for mechanical cleaning is possible. See GBHE-PEG-HEA-513 for more information on fouling and cleaning of the air side.



FIGURE 8 TYPICAL AIR COOLED HEAT EXCHANGER

Notes: (1) The supports for the fan and motor have been omitted for clarity. (2) One fan and plenum have been omitted to show the tubing. (e) Safety and reliability

Serious corrosion of aluminium fins has occurred in the past on sites with a polluted atmosphere, but current hygiene standards have largely eliminated this problem. The major problems are around air side cleaning and fan drives. Noise emitted from the fans and drives can be a problem.

(f) Repairs

Individual tubes can be plugged. Replacement of individual tubes is not generally possible without major dismantling. Complete re-tubing is possible.




Typical air cooled heat exchangers using 1" tubes on a 2 to 2.5" pitch have a low process (inside) surface/volume ratio of about 30 to 35 m2/m3. Moreover, as the thickness of the bundle is usually low, the plot area requirements are large. Large units are built up from individual bundles with multiple fans. There is thus no limit to the ultimate size of an installation. However, there may be distribution problems with the manifold of large installations. See GBHE-PEG-HEA-513 for more details.

(h) Design methods

It is usual to ask a manufacturer to perform thermal design. Commercially available computer programs exist, for checking designs (see PEG.HEA.005), which experience suggests are reasonably accurate.

(j) Cost

The capital cost of air cooled heat exchangers is high, principally because of the high installation costs. In addition, there is a continued running cost for the fans. However, this has to be compared with the capital and operating cost of cooling tower installations. An optimization of capital against running cost should be performed at the design stage. See Appendix A of PEG.HEA.203 for a preliminary costing method for air cooled heat exchangers.

(k) GBH Enterprises experience

Contact us. 6.5 Gasketed Plate and Frame (a) Type of duty

Where suitable, the plate and frame exchanger is likely to be the most economic choice. Very good for liquids. Their use for condensing and boiling duties is low at present, but the manufacturers are moving into these applications. Not well suited for gases. Not very suitable for liquids where the flowrates of the two streams differ widely. For an economic design, a reasonable allowable pressure drop (say > 0.25 bar) is generally desirable, but they should not be ruled out for lower pressure drop cases.




Their use is normally limited by availability of suitable gaskets. Historically, the normal maximum design pressure was 10 bar, but more recent designs have raised this to 25 bar. The normal operating temperature range with elastomeric gaskets is minus 25 to plus 160C; in special circumstances it can be as high as 250C.


Plates can be made from any material which can be pressed, but are not available in carbon steel. Alfa-Laval and Vicarb also offer plates in graphite. Gaskets are normally elastomeric, with Viton as the most resistant readily available. Selection of a suitable gasket material is often more of a problem than plate material. Some applications using CAF (compressed asbestos fibre) gaskets are known, but these frequently pose problems with achieving and maintaining a seal.

(d) Fouling

Intrinsically less prone to many types of fouling than are shell and tube units. Typical design fouling resistances should be to of those for shell and tube units. There are no dead spaces where deposits may accumulate. However, fibrous solids tend to collect at the contact points between successive plates, and build up rapidly. Either the tube side of a shell and tube unit or a spiral plate exchanger is better for fibrous materials. Can be easily disassembled for cleaning if required.


The plates are very thin, typically under 1 mm, so no significant corrosion can be tolerated. The weak points are usually the gaskets, although with proper assembly these should give no problems within their design limit.

Note: The gasket design prevents cross contamination of the fluids even if the gasket should leak; all leaks are to atmosphere. Cross contamination can only occur from corrosion failure of the plates. It is recommended practice to surround the plate stack with a thin carbon steel shroud, open at the base, to deflect any leaks to ground and avoid spraying personnel. Not recommended for toxic or flammable liquefied gases.



(f) Repairs

Units can be readily disassembled for inspection and repair, giving complete access to all the heat transfer surface. Individual plates can readily be replaced. Re-gasketing may be required from time to time. Modern plate designs normally have some form of snap-on gasket attachment, which makes re-gasketing easy. Older designs require the gaskets to be glued to the plates; removal of the old gasket and cleaning of the locating channels for the replacement gaskets can be difficult. Many manufacturers offer a re-gasketing service. In some cases, a spare plate stack may be advantageous, allowing rapid turn-round.


Typical surface/volume ratios for plate heat exchangers are 50 to 120 m2 /m3. This at first sight seems low, considering that plate exchangers are considered to be compact. However, the volume here is the total volume occupied by the plate exchanger, including the header plates, frame and allowance for disassembly. The heat transfer coefficients obtainable within a plate exchanger are generally higher than those for a shell and tube unit with the same pressure drop, a factor of three not being unreasonable for a liquid-liquid duty. Thus the plate exchanger is considerably more compact than the shell and tube for a given duty.

(h) Design methods

Manufacturers have their own thermal design methods, which are specific to the detailed dimensions of their plate designs. Computer programs are commercially available to do preliminary estimations (see GBHE-PEG-HEA-513 05), but should not be relied on for final design. Because of the ease with which the size of a plate and frame exchanger can be modified, by the addition or subtraction of plates, reliance on manufacturers presents no major problem. Mechanical design is proprietary.

(j) Cost

Very competitive, especially in exotic materials of construction. Typically, a stainless steel plate and frame exchanger is comparable in item cost with a carbon steel shell and tube unit for the same duty; the installation costs of the plate and frame will be less. The use of expensive gasket materials, such as Viton, can have a significant effect on the cost;



In one instance it almost doubled the price of an exchanger with titanium plates. The costs of spare gaskets should be included in any cost comparison.

6.6 Spiral Plate (a) Type of duty

Can be used for any type of duty; condensing, boiling or single phase, including slurries.


The normal maximum design pressure is 15 bar. There are no particular limits on design temperature, but avoid excessive temperature cycling.


Can be fabricated in any material. (d) Fouling

The smooth flow path with absence of dead zones makes the spiral exchanger particularly suited to handling slurries, including fibrous materials. Other forms of fouling, such as scaling, may be expected to be comparable with the tube side of shell and tube exchangers, although the manufacturers claim that the curved flowpath produces a scrubbing action that keeps the surfaces clean, enabling lower fouling resistances to be used.

Normal designs, with spiral flow both sides, give access to both sides for mechanical cleaning. The spiral flow/cross flow variant can only be cleaned mechanically on the cross flow side.


Unlike the gasketed plate and frame exchanger, the spiral plate only has two main gaskets. Moreover, these are arranged between conventional flanges which can be bolted up to a high degree, and use conventional gasket materials. The unit should thus be as reliable as a shell and tube exchanger, and should be suitable for handling toxic or flammable liquefied gases.



(f) Repairs

If the heat transfer surface fails through local corrosion, unless this occurs at the edge of the plates, it is almost impossible to repair the exchanger. The main block of the exchanger will have to be replaced, although it may be possible to reuse the cover plates.


Spiral exchangers are comparable in volume to shell and tube units with the same surface. However, they are generally more squat in shape, and require less space for servicing. The standard design, with spiral flow on both sides, operates in pure counter-current flow, so will have a higher effectiveness than a shell and tube unit with mixed flow. Their squat shape and circular cross section makes them suitable for direct mounting on distillation columns as condensers or reboilers. See GBHE-PEG-HEA-516 for more information on the use of spiral exchangers as refluxing condensers.

(h) Design methods

Spiral plate exchangers are normally considered as proprietary items and are designed by the manufacturers. Commercially available programs can be used to rate a spiral plate exchanger used as a thermosyphon reboiler, but checking calculations for all other duties have to be done by hand. The recommended approach is to use the 'hydraulic mean diameter' concept combined with conventional correlations for heat transfer and pressure drop in pipes. It is debatable whether any enhancement to allow for the curved flow path should be included. Such enhancement occurs in flow in curved tubes due to the secondary circulation set up in the flow. Secondary circulation in the flat rectangular passages of a spiral plate exchanger is likely to be much less.

(j) Cost

The costs of spiral exchangers are generally between those of shell and tube and gasketed plate types.


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6.7 Tube in Duct

These units usually consist of a rectangular duct across which are arranged several rows of tubes in a direction normal to the flow in the duct. As the fluid in the duct is normally a gas, the tubes are usually finned to counter the poor gas-side coefficient. For low to medium temperature operation spiral wound finned tubing similar to that used for air cooled heat exchangers is generally used. For high temperature duties stud fins are often used because of their greater robustness. The number of rows of tubes is determined by the duty, and can range from a single row to more than 30 rows. For duties where a gas is heated by steam and more than one pressure level is available, it is common practice to use low pressure steam in the first banks and higher pressure in later ones.

(a) Type of duty

Normally used for the heating or cooling of low pressure gases in the duct. Examples include the preheating of combustion air for furnaces, heat recovery from furnace combustion gases, e.g. furnace convection banks, and the heating of air for driers.


Because the ducts are generally of rectangular construction, and large dimension, it is difficult to design them to withstand any significant pressure differential with the atmosphere. Typical pressure limits are 0.1 to 0.2 bar. Temperature limits are governed by the choice of materials. There are no practical pressure limitations on the tube side.


Ducts can be made from any suitable material; lining may be used if the gas is corrosive. Tube material is governed by the nature of the tube side fluid. Fins will normally be of aluminium for non-corrosive duties up to medium temperatures, say gas temperatures up to 150 to 200C. At higher temperatures carbon steel fins are often used, and for corrosive gases more exotic materials will be necessary.



Note: The efficiency of fins made from these materials will be lower than that for aluminium fins of the same dimension.

(d) Fouling

Finned surfaces are always prone to fouling by dusts. If the gas is dirty, it will be necessary to clean the outside of the tubes. Some designs of tube in duct exchanger allow individual banks of one or two rows of tubes to be withdrawn from the duct for cleaning. In other cases, access for cleaning lances may be provided. In the convective sections of boiler plant permanent cleaning nozzles are sometimes employed (so-called soot blasters). For air heaters, some form of dust filter is often provided in the duct before the tube banks.


No information.

(f) Repairs

Maintenance is easier if the exchanger is designed so that individual banks can readily be removed. If severe corrosion is expected, the holding of a spare bank could be a worthwhile investment. Depending on the method of fabrication, replacement of an individual failed tube within a bank may not be easy without a major dismantling of the bank.


No data.

(h) Design methods

The calculation of the outside heat transfer coefficient and pressure drop is a relatively simple procedure. Standard correlations have been developed for cross flow over rectangular banks of plain and finned tubes; the recommended methods are as given in the HTFS Handbook (Ref. [5]). These methods are provided in commercially available programs.



The tube side coefficient and pressure drop can be calculated by the appropriate established methods for in-tube flow. There is as yet no general purpose program which will combine the tube side and outside calculations. Most commercially available programs assume constant gas properties through the bundle. If the gas temperature varies significantly it is necessary to perform several runs to determine behavior through the bundle, and combine these results with tube side values by hand.

(j) Cost

No data.


Contact Us 6.8 Plate-fin For general information on plate-fin exchangers see Refs. [6] and [7]. Figure 9 shows the constructional details of a typical plate-fin exchanger.



FIGURE 9 GENERAL VIEW OF ONE END OF A 3-STREAM PLATE-FIN HEAT EXCHANGER



(a) Type of duty

Widely used in the cryogenics field. Becoming used in petrochemical applications. Can handle liquids, gases, condensing and boiling duties. Multi-stream duties are normal; up to at least 11 separate streams can be handled in one exchanger. A long thermal length and good counter-current flow makes close approach temperatures possible; units have been designed for cryogenic service with temperature differences of less than 2C.


The use of aluminium limits the upper temperature of such units to about 65 to 90C. There is no lower temperature limit, units being used on liquid helium production. Typical upper pressure limits are 70 bar.

Some manufacturers have announced units in brazed stainless steel or nickel alloys which can operate at up to 650C; the pressure limits of these units are stated to be over 100 bar.


Although large plate-fin exchangers are no longer limited to aluminium, they are generally fabricated by brazing. This introduces other materials, which gives rise to the possibility of dissimilar metal (galvanic) corrosion. A European manufacturer is currently attempting to develop an all stainless steel plate-fin exchanger, without brazing metal. Rolls-Royce and Associates, in conjunction with Alfa-Laval have developed an exchanger of plate-fin type in titanium, using diffusion bonding and super-plastic deformation techniques.

(d) Fouling

Not suitable for fouling duties; mechanical cleaning is not possible, and the small passages can readily become blocked.


No information. (f) Repairs

No information.

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