I Compactos

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Vishwas V. Wadekar,HTFS, AEA Technology Hyprotech

AChEsGuide

toCHEs

variety of heat exchangers canbe employed to heat or cool process streams.More often than not, though, shell-and-tube ex-changers are selected for most chemical processindustries (CPI) applications.

However, this situation is gradually chang-ing, and compact heat exchangers are now gain-ing increased attention as viable cost-effectivealternatives. Several factors are responsible forthis change:

The advantages of CHEs are becoming in-creasingly apparent in their original fields of ap-plication, such as refrigeration and air condi-tioning, cryogenics, food processing, etc.

In recent years, new CHEs have been intro-duced, including some specifically for high-temperature, high-pressure applications in theCPI.

Software tools for the selection and designof CHEs are now available from independentsources.

There is increased awareness about CHEsthrough specialist conferences and studygroups.

In many retrofit applications, equipmentwith increased throughput yet occupying lessfloor space is required, forcing engineers tolook for alternatives to conventional shell-and-tube exchangers.

Offshore applications, where incentives are

much greater for weight- and space-savingequipment, have become test beds for new CHEapplications, highlighting the practicality andadvantages of some of the CHEs.

Of course, compact heat exchangers do havea number of real (and some perceived) limita-tions and disadvantages. Generally, though, thecost and energy saving benefits offered byCHEs over the conventional shell-and-tube heatexchanger make it imperative that they be con-sidered as a serious alternative.

This article gives a broad overview of com-pact heat exchangers. It provides some back-ground on the thermal benefits of CHEs, theconcepts of thermal effectiveness and tempera-ture approach, and the degree of compactnessof an exchanger, and it describes the differenttypes of CHEs. Finally, it offers guidelines forselecting an appropriate CHE for a particularapplication.

Thermal benefits of CHEsTo understand some of the advantages of

compact heat exchangers, lets start with thebasic question for the overall heat transferredwithin a heat exchanger:

Q = UAFtTlm (1)

Due to their inherently complex, often tortu-

Compact heat exchangers(CHEs) offer high

heat-transfer coefficientsand large surface areas with

a small footprint, makingthem a cost-effective

alternative to shell-and-tubeexchangers in many

applications.

CEP December 2000 www.aiche.org/cep/ 39

A

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40 www.aiche.org/cep/ December 2000 CEP

Compact Heat Exchangers

ous and noncircular flow passagestructure, CHEs tend to have higherheat-transfer coefficients for both thehot and the cold streams. This in-creases the CHEs overall heat-trans-fer coefficient, U.

Due to the higher area density(heat-transfer area per unit volume ofthe exchanger), the incremental costof incorporating a larger heat-transferarea is generally less for CHEs thanfor shell-and-tube exchangers. Thismeans that the value of the heat-transfer area, A, in Eq. 1 is likely tobe higher for CHEs. Some CHEs,such as plate-fin exchangers, containextended surfaces or secondary heat-transfer area, which further increasesthe total effective heat-transfer areasignificantly.

In Eq. 1, Ft is a correction factorfor the log mean temperature differ-ence, Tlm, to account for the depar-ture from pure countercurrent flow.Thus, if the streams within a heatexchanger are flowing in a truecountercurrent manner, Ft = 1. Com-pact heat exchangers can generallybe configured as essentially purecountercurrent flow devices, with Ftnearly approaching the value ofunity.

In view of the high values of theoverall heat-transfer coefficient andthe heat-transfer area, coupled withthe value of Ft close to unity, Eq. 1can be interpreted in two ways. For agiven mean temperature difference,the heat duty that could be achievedin a compact heat exchanger will behigher. Alternatively, for given heatduty, a smaller mean temperature dif-ference will be required.

Thermal effectiveness and temperature approach

These two terms are often usedin connection with heat exchangers.Because they characterize the ther-mal performance of an exchanger,they are especially relevant and fre-quently used in quantifying thethermal benefits of compact heatexchangers.

Thermal effectiveness is a ratio ofthe actual heat transferred in the ex-changer to the thermodynamic maxi-mum. If a two-stream heat exchangeris handling streams with equal ther-mal capacity, mcp (flow rate timesheat capacity) [i.e., (mcp)Stream 1 =(mcp)Stream 2], then the thermal effec-tiveness, , is simply given by theratio of the actual temperature changefor a stream to the maximum possibletemperature change. For the exampledepicted in Figure 1, the temperaturechange for Stream 1 is (T1,out T1,in).If the heat exchanger had an infinitearea, the outlet temperature of Stream1 would be equal to the inlet tempera-ture of Stream 2. The maximum pos-sible temperature change for Stream1 is, therefore, (T1,in T2,in). Thus, thethermal effectiveness will be given by

(2)The temperature approach is the

minimum difference between the

local stream temperatures in the ex-changer. For the unit shown in Figure1, it remains the same everywherethroughout the exchanger because thetwo stream temperature profiles areparallel to each other.

Exchangers that contain moreheat-transfer area, provide high over-all heat-transfer coefficients, andhave pure countercurrent flow tend tohave a higher thermal effectiveness.This is illustrated in Figure 2, whichplots thermal effectiveness againstthe maximum number of transferunits (NTUmax).

Cmin is the minimum of(mcp)Stream 1 and (mcp)Stream 2. Notethat for a given position along thex-axis, the countercurrent flow ar-rangement provides the maximumthermal effectiveness, followed bycrossflow, and then cocurrent flow.The curves approach different limit-ing values of thermal effectivenessasymptotically 0.5 and 1.0 forcocurrent and countercurrent flow,respectively, with an intermediatevalue for crossflow.

For any given flow arrangement,the thermal effectiveness rises withan increase in the overall heat-trans-fer coefficient and heat-transfer area,although the rate of increase slowsdown asymptotically. It should benoted that exchangers with higherthermal effectiveness result in closertemperature approaches.

=T1,out T1,inT1,in T2,in

Figure 1. Schematic diagram of streamtemperatures in a two-stream exchanger.

T1, in

T1, out

T2, out

T2, in

Figure 2. Thermal effectivenessvs. number of transferunits.

00

0.2

0.4

0.6

0.8

1.0

Cocurrent

Countercurrent

Crossflow

1 2 3 4 5

NTUmax = UA/Cmin

Ther

mal

Effe

ctiv

enes

s


As mentioned earlier, compactheat exchangers offer high overallheat-transfer coefficients and heat-transfer areas. Hence, they can oper-ate at a high thermal effectiveness,making them especially suitable forclose temperature approach duties.Again, many CHEs can be config-ured as nearly ideal countercurrentflow devices. Thus, they fall on orvery near the high thermal effective-ness curve for countercurrent flow inFigure 2.

The flow passages of compactheat exchangers offer another advan-tage. The flow velocities of thestreams tend to be more uniformacross the flow width thereby mini-mizing the stagnant or low-velocityzones within the exchanger. Becausesuch zones are more susceptible tofouling, their elimination means thatCHEs have less propensity to foul.Although compact exchangers areless likely to foul on this basis, thepossibility of blockage of the smallflow channels by suspended particlesneeds to be taken into account fornot-so-clean fluids. In many cases,this calls for the installation ofstrainers before the streams enter theexchanger.

Degree of compactnessHeat exchangers can be classified in

a variety of ways. One way that is espe-cially relevant to compact heat exchang-ers is based on two closely related pa-rameters the flow channel size andthe heat-transfer area density. Normally,the smaller the flow channel size in theexchanger, the higher the area density.

Figure 3 compares several broadcategories of heat exchangers. Shell-and-tube exchangers use plain tubesthat are typically 10 to 30 mm in di-ameter, which translates to area den-sities of about 100 m2/m3. Plate-typeexchangers (e.g., plate-and-frame ex-changers) generally have 5-mm to 8-mm channels and area densities morethan 200 m2/m3. Plate-fin exchang-ers, the category to which car radia-tors belong, have channel sizes ofabout 2 mm and area densities be-tween 800 and 1,500 m2/m3. Special-ity heat exchangers, which includethe printed circuit heat exchanger,have channels with hydraulic diame-ters of roughly 1 to 2 mm and areadensities of over 2,000 m2/m3. Thehuman lung, with flow passages of0.2 mm equivalent diameter and areadensities of more than 10,000 m2/m3,is shown for comparison.

Plate heat exchangerIn the broadest sense, this category

includes all heat exchangers that useplates in their construction. Examplesare the various types of exchangerscontaining cross-corrugated channels,spiral plate heat exchangers, andsome proprietary welded exchangers.

Gasketted plate-and-frameheat exchanger

This exchanger, referred to as aplate-and-frame heat exchanger orsimply a plate heat exchanger, con-sists of a pack of plates held togetherin a frame. Figure 4 shows an explod-ed view of the assembly of a plateheat exchanger. More details of con-struction are available from a numberof sources (e.g., Ref. 1).

As shown in Figure 4, the twostreams flow in alternate channels be-tween plates, entering and leaving viaports in the corners of the plates.Each plate has a gasket around theedge and around the ports. The gas-kets around the plate edge define theflow paths and are arranged to makethe two streams flow in alternate platepassages.

The exchanger can be completelydismantled for cleaning. This is themain reason for its widespread use inthe food industry and other cleanapplications.

Figure 5 shows a typical chevronpattern, which forms the cross-corru-gated passages in the plate heat ex-changer with chevron patterns of theconsecutive plates pointing in oppo-site directions. The plates are normal-ly made of stainless steel; they arealso available in other higher alloysand metals (such as titanium) for spe-cial duties. Plates can be from 0.2 mto over 3 m long, with widths typical-ly 20% to 40% of their length. Theplate thickness is usually in the rangeof 0.4 to 0.9 mm, and the plate spac-ing varies between 2.5 and 5 mm, ex-cept for special wide-gap platessometimes used for viscous or fibrousmaterials. The hydraulic diameter forflow between plates is approximatelytwice the plate spacing.

Figure 3. Flow channel size and heat-transfer area density for various types of heat exchangers.

100

60 10 1

Human Lungs

Specialty

Plate-Fin

Plate

Shell-and-Tube

0.1

1,000 10,000

Area Density, m2/m3

Hydraulic Diameter, mm

Operating pressures up to 20 barare standard, and somewhat higherpressures can be achieved usingheavy-duty frames. The gaskets, em-ployed to seal the flow passages, usu-ally limit the operating temperaturerange, with a lower limit of 25Cand an upper limit of 160C to180C, depending on the specific gas-ket material.

The main advantage of this type ofexchanger is that it can be opened,providing complete accessibility tothe heat-transfer surface. This alsogives the flexibility of adding or re-moving some plates to accommodatechanges in the heat duty.

The main limitation of the plate-and-frame heat exchanger is that theprocess fluids must be compatiblewith the gasket material. The gasket-ted construction also makes theseunits unsuitable for refinery applica-tions where prolonged resistance tofire may be required. Partially weldedplate-and-frame exchangers (dis-cussed later) allow the user to balancethe advantages of flexibility and ac-cessibility arising from the gaskettedconstruction against the higher tem-perature and pressure operation witha wider range of fluid types offeredby the welded construction. (Fullywelded exchangers can operate at

even higher temperatures if flexibilityand accessibility are not necessary.)

For single-phase liquid duties in-volving moderate temperatures andpressures, plate-and-frame exchangerscan be a cost-effective alternative to theconventional shell-and-tube exchanger.

Flow passage structure in plate exchangers

Plate heat exchangers have cor-rugated plates. The corrugationsprovide both support against in-ternal pressures and heat-transferenhancement.

The most common type of platehas crossed corrugations, that is, thecorrugation patterns in adjacent platesare at an angle to each other, giving alattice of support points where theytouch and a complex flow channelshape between the plates. The corru-gations are usually formed aschevrons. There may be a singlechevron pattern, as in Figure 5, ormultiple rows of chevrons across theplate width. Other variants have thechevron pattern running along thelength rather than width of the plate.In all cases, however, the local flowgeometry has the same cross-corru-gated structure.

For the cross-corrugated platesformed from the chevron pattern,

chevron angle is an important designvariable. The chevron angle is theangle of the corrugations with respectto a horizontal line, designated as inFigure 5. A plate with a low chevronangle offers a high heat-transfer coef-ficient and high pressure drop, where-as a plate with a high chevron anglehas lower heat transfer and lowerpressure drop. The low- and high-chevron angle plates can also be re-ferred to as hard and soft plates, re-spectively, reflecting the resistancethat they present to a flowing fluid.

For single-phase duties, reliableinformation is generally available onthe effect of chevron angle on heattransfer and pressure drop (for exam-ple, Ref. 2). Therefore, selecting softor hard plates (or a combination) tomatch specific pressure drop andheat-transfer requirements is relative-ly straightforward.

In addition to the main chevronpattern, the pattern on the distributionregions of the plates is also importantand plays a significant role in uniformdistribution of a stream in a givenplate channel (34).Partially welded plate heat exchanger

This variant of the plate-and-frameheat exchanger attempts to combinesome of the advantages of gaskettedand welded construction. This designis useful when a suitable gasket mate-



Figure 5. Typical chevron pattern on a plate

Soft Plate

Hard Plate

Figure 4. Exploded view of a plate-and-frame heat exchanger. Courtesy of Alfa Laval Thermal Inc.

rial cannot be found because of thechemical aggressivenes of one of thefluids.

Pairs of plates are welded togetheraround the edges to form gasket-freechannels through which the aggres-sive fluid can flow. Gaskets are usedbetween the welded pairs for the lessaggressive fluid. Such a heat ex-changer is referred to as a welded-pair plate exchanger (Figure 6).

The aggressive fluid, while flow-ing through the ports, does come incontact with the circular port gasketsmounted on the gasketted side of theplates. Because these gaskets are cir-cular and therefore easy to seal, andare relatively small, they can be madefrom a less flexible but more chemi-cally resistant material, such as poly-tetrafluoroethylene (PTFE, or Teflon).

Welded-pair plate exchangers havethe same operating temperature andpressure limits as the fully gaskettedplate-frame exchangers. Advantagesof accessibility and flexibility also re-main the same except for the accessto the welded side of the plates.

Completely welded plate heat exchanger

Recently, a fully welded plate-pack construction has been intro-duced in the market. In this arrange-ment, the plate pack is welded fully

and is completely free of gaskets. Theplate pack is held within a frame in aconventional manner. Ducts of thesame material as the plates are weld-ed to the plate pack at the port holesand carry fluids to and from theflanges attached to the frame and theplate pack, eliminating the need for agasket between the front plate and thehead plate of the frame.

The welded construction allowsthe exchanger to operate at tempera-tures up to 350C and pressures up to40 bar. However, because it is weld-ed, the plate pack cannot be openedfor cleaning and plates cannot beadded or removed from it.

Brazed plate heat exchangerThis design (Figure 7) has a plate

structure similar to that of the con-ventional plate-and-frame heat ex-changer, but the plate pack is brazedtogether using copper as the brazingmaterial. Plates are made from stain-less steel or higher alloys. Brazingeliminates the need for both a frameand gaskets.

Brazing also increases the operat-ing temperature and pressure rangeconsiderably. The exchanger can op-erate from 195C to 200C at pres-sures up to 30 bar.

Plate lengths are usually 1 m orless, although larger units with longer

plates are continually becomingavailable. The exchangers overallsize is still relatively small comparedto the large plate-and-frame units.

These exchangers are now widelyused in the refrigeration industry forsingle-phase and two-phase duties.They are probably the cheapest stain-less steel exchangers available on themarket today. They should be usedonly for relatively clean fluids be-cause of their small passages and in-accessibility of the heat-transfer sur-face for mechanical cleaning.

More recently, nickel brazed plateheat exchangers have been introducedto the market. They are particularlyuseful for duties involving ammoniaas a working fluid where copperbrazed heat exchangers cannot beused.

Plate-and-shell heat exchanger

An interesting variant of the plateexchanger is the plate-and-shell heatexchanger (Figure 8). It consists of astack of welded circular cross-corru-gated plates fitted into a cylindricalshell. The stack is formed by weldingthe plates alternately around the portsand around the outer periphery. Onestream flows through the plate pairsand the other between the alternateplate gaps.

The plates are made of stainlesssteel and higher alloys. Plate diame-


Figure 6. Partially welded, or welded-pair, plate heat exchanger. Courtesy of APV Heat ExchangerProduct Group.

EndplateGasket

EndplatePair

SealplatePair

Process

Service

FlowplateGasket

FlowplatePair

FlowplatePair

FlowplateGasket

FlowplateGasket

WeldedSeal

WeldedSeal

WeldedSeal

WeldedSeal

Head

Figure 7. Brazed plate heat exchanger. Courtesy of Alfa Laval Thermal Inc.

ters range from 200 to 1,000 mm.Standard designs can accommodateheat-transfer areas from 0.5 to 500 m2in a single unit. These units can oper-ate in the temperature range of200C to 600C and at pressures upto 40 bar.

It is claimed that plate-and-shellexchangers can handle duties involv-ing thermal cycling, because the platepack is able to expand and contractwithin the shell. These exchangershave been used in single-phase andtwo-phase duties in refrigeration andother industries.

Plate-fin heat exchangerThe conventional brazed alu-

minum plate-fin heat exchangers areused extensively in cryogenic appli-cations, such as air separation andethylene plants. However, becausethey are made from aluminum, theycannot be used for higher temperatureapplications. Their derivatives madeof stainless steel and titanium havemore potential applications in theCPI.

Brazed aluminum plate-fin heat exchanger

A typical brazed aluminum ex-changer handling multiple streams isillustrated in Figure 9. It consists ofalternating layers of plates (referredto as parting sheets) and corrugatedfins. Flow passages are formed be-tween the consecutive parting sheets,with the sealing provided by the sidebars along the edges. The partingsheets and fins provide the primaryand secondary surface for heat-trans-fer, respectively. In addition to pro-

viding the secondary area for heattransfer, the brazed fins hold the heatexchanger together. In most plate-finexchangers, the effective length ofthe block consists of finning laid par-allel to the block axis, to give truecounterflow heat exchange among thestreams.

At the end of the exchanger, padsof finning are laid at an angle andserve as distributors. These distributethe flow coming from the headers andnozzles into the main heat-transferpassages or collect the flow comingfrom the passages and direct it intothe headers and nozzles. The headersand nozzles are welded onto the out-side of the block.

Within the plate-fin core, eachstream flows in a number of layers,each of which is divided into numer-ous parallel, nearly rectangular sub-channels by the fins. Fin heights andfin frequencies determine the size ofthese subchannels. Fin heights aretypically between 5 and 9 mm, whilefin frequencies, in the main heat-transfer region, are typically 590 to787 fins/m (15 to 20 fins/in.). Theequivalent hydraulic diameters ofthese subchannels are, thus, only afew millimeters. These small pas-sages result in heat-transfer area den-sities of about 800 to 1,500 m2/m3.Such high area density, coupled withthe aluminum construction, meansthat for a given heat-transfer area, theexchangers are smaller and lighterthan any other exchanger type.

The overall size of these exchang-ers can be up to 1.2 m wide, 1.2 mdeep (the stack height), and 6.2 mlong. They are used for single-phase

and two-phase duties involving boil-ing and condensation. In low-temper-ature cryogenic applications, theyprovide the benefit of a multistreamcapability, ensuring that all the coldstreams produced in a process areused to cool the incoming warmstreams. They can operate at a ther-mal effectiveness up to 98% and areable to handle temperature approach-es down to less than 2C. In cryo-genic duties where economics aredominated by the cost of energy re-quired to generate the low tempera-tures, such close temperature ap-proach is of vital importance.

Brazed aluminum exchangers canbe used for streams at pressures up to100 bar and generally within a tem-perature range of 269C to 100C;with appropriate alloys for the head-ers and nozzles, they can be used attemperatures up to 200C. However,the maximum operating temperaturefor aluminum alloys decreases rapid-ly with increasing pressure.

Four basic fin geometries (Figure10) are used in plate-fin exchangers.All manufacturers make plain, perfo-rated, and serrated (offset strip) fins.Some make wavy fins; others preferserrated fins with a long serrationlength.

The perforations provide a smallenhancement over plain fins for im-proved single-phase performance.Perforated fins are often used forboiling. The perforations help toequalize flows among the subchan-nels, mitigating against local block-age or pressure fluctuations arisingfrom the evaporation process.

Serrated fins significantly increase



Figure 8. Plate-and-shell heat exchanger.

both heat transfer and pressure dropover plain fins. They are used for sin-gle-phase gas duties, where the increasein heat-transfer coefficient is most de-sirable. Sometimes, they are also usedfor boiling duties because they arethought to aid the onset of boiling.

Plain fins find applications in con-densation and single-phase duties,where lower pressure drop character-istics may be more important. Forserrated fins, the standard length ofthe serrations is 3 mm (q in.). Alonger length (12 or 15 mm) resultsin a fin whose performance is be-tween that of perforated fins and stan-dard serrated fins.

Stainless steel plate-fin heat exchanger

Plate-fin heat exchangers can bemanufactured of materials other thanaluminum so that they can be operat-ed at higher temperatures and pres-sures. Stainless steel exchangers havebeen used for some time in vehicleand aerospace applications, mainlyfor single-phase duties. These aretypically small exchangers blockswith sides less than 0.3 m. Somemanufacturers, however, can supplylarger brazed stainless-steel plate-finunits (up to 0.6 m by 0.6 m by 1.5 mlong) for CPI applications.

Brazed stainless steel exchangersare geometrically similar to brazedaluminum plate-fin exchangers, butthey normally have lower fin heights(less than 5 mm high) because of therelatively poor thermal conductivityof stainless steel. They generally em-ploy plain fins, because other fintypes are difficult to manufacture instainless steel. Copper is used as thebraze metal for stainless steel ex-changers.

The effect of the braze on processfluids has sometimes been of concernto potential users. Therefore, somemanufacturers are trying to developdiffusion bonding techniques forstainless steel plate-fin exchangers toavoid problems associated with thecopper braze.


Figure 9.Brazed aluminumplate-fin heatexchanger.

HeatTransfer Fin

DistributorFin

Inlet

Outlet

Cap Sheet

Parting Sheet

SupportPlate

WearPlate

Header

Nozzle

SpacerBar

Figure 10. Plain, serrated, perforated, and wavy fins.

Plain Fins Serrated (Offset) Fins

Perforated Fins Wavy (Herringbone) Fins

Diffusion-bonded titaniumplate-fin heat exchanger

Another development in the manu-facture of plate-fin heat exchangerscapable of high-pressure, high-tem-perature operation is the applicationof superplastic forming and diffusionbonding technology (which was orig-inally developed for titanium turbineblades) (5). The manufacturing tech-nique is illustrated in Figure 11.

Three sheets of titanium are diffu-sion bonded at selected positionsusing a bond inhibitor. These threesheets are then expanded superplasti-cally in a closed die at elevated tem-peratures by pressurizing the unbond-ed regions between the plates. Thisforms a single element equivalent to asingle layer of plate-fin geometry,where the middle sheet forms thesubchannels (i.e., the secondary sur-face). The subchannels, however, aretrapezoidal rather than rectangular,and somewhat larger than the sub-channels in aluminum plate-fin ex-changers. The heat exchanger core isassembled by diffusion bonding theseelements together.

The typical height of the trape-zoidal subchannels is 2 to 5 mm.They are made as wavy rather thanstraight subchannels. Different wavyfrequencies are offered to accommo-date a range of pressure drop andheat-transfer characteristics.

In terms of general heat transferand pressure drop performance, theseexchangers are similar to aluminumplate-fin exchangers, offering thesame advantage of high thermal ef-fectiveness. The use of titanium cou-pled with the metallurgical benefits ofthe manufacturing technology allowthem to operate at temperaturesabove 550C and at pressures above200 bar. The other main advantage ofthis type of exchanger is that titaniumwhich is a highly corrosion-resistantmaterial, and no other metal is in-volved as a braze.

All the existing applications ofthese exchangers are for single-phaseduties (6).

Printed-circuit heat exchanger

The printed-circuit heat exchang-er is manufactured by diffusionbonding technology. The termprinted circuit is used becausesemicircular flow passages arechemically etched onto flat plates,which resemble printed circuitboards (Figure 12). The plates arethen stacked and diffusion bondedtogether to produce an exchanger ca-pable of operating at pressures up to1,000 bar and temperatures up to900C. The exchangers can be man-ufactured of either stainless steel orvarious higher alloys.

The flow passages in a printed-cir-cuit heat exchanger are normally be-tween 0.5 and 2.0 mm deep, and thecross-section approximates a semicir-cle. Zigzag, as well as other more-complicated patterns, can be etched.Various combinations of crossflowand counterflow can be employed inthe exchanger as required.

Welded compact heat exchanger

Plate-and-frame exchangers with



Figure 11. Steps in manufacturing anelement for a diffusion-bonded titanium plate-fin exchanger.

After Bonding

After Superplastic Forming

After Ironing

Literature Cited1. Hewitt, G. F., G. L. Shires, and T. R.

Bott, Process Heat Transfer, CRCPress, London (1994).

2. Heavner, R. L., H. Kumar, and A. S.Wanniarachchi, Performance of an In-dustrial Plate Heat Exchanger: Effect ofChevron Angle, AIChE Symposium Se-ries, Vol 89, AIChE, New York, pp.262267 (1993).

3. Kumar, H., M. F. Edwards, P. R. Davi-son, D. O. Jackson, and P. J. Heggs,The Importance of Corner Header Dis-tributor Designs in Plate Heat Exchang-ers, Proceedings of the 10th Interna-tional Heat Transfer Conference,Brighton, U.K., published by IChemE,Rugby, U.K., Industrial Session, Paper1/2-CHE-5, pp. 8186 (1994).

4. Haseler, L. E., V. V. Wadekar, and R.H. Clarke, Flow Distribution Effects ina Plate Frame Heat Exchanger, 3rdU.K. National Heat Transfer Conference,published by IChemE, Rugby, U.K.,IChemE Symposium Series 129, Vol. 1,pp. 361367, (1992).

5. Adderley, C., and J. O. Fowler, TheUse of a Novel Manufacturing Processfor High Performance Titanium Plate-Fin Heat Exchanger, Chapter 17, HeatExchange Engineering, Vol. 2, E. A.Foumeny and P. J. Heggs, eds., EllisHorwood, Chichester, U.K. (1991).

6. Haseler, L. E., and D. Butterworth,Boiling in Compact Heat Exchangers/In-dustrial Practice and Problems, KeynotePaper IV, International Conference onConvective Flow Boiling, Banff, Canada,published by Taylor & Francis, Philadel-phia, PA, pp. 5770 (1995).

7. Guide to Compact Heat Exchangers,Prepared for the Energy Efficiency Of-fice by Energy Technology Support Unit(ETSU), Harwell, U.K. (1994).

8. Oswald, J. I., D. A. Dawson, and L. A.Clawley, A New Durable Gas TurbineRecuperator, ASME Gas Turbine Con-ference, Indianapolis, IN, ASME 99-GT-369, ASME, New York (1999).

9. Ramshaw, C., Intensified Heat Trans-fer: The Way Ahead?, Chapter 15,Heat Exchange Engineering, Vol. 2, E.A. Foumeny and P. J. Heggs, eds., EllisHorwood, Chichester, U.K. (1991).

10. Ferrato, M., and B. Thonon, A CompactCeramic Plate-Fin Heat Exchanger for GasTurbine Heat Recovery, in Compact HeatExchangers for the Process Industry, R. K.Shah, ed., Begell House Inc., Wallingford,U.K. and New York, pp. 195199 (1997).

fully welded plate packs were dis-cussed earlier. There are also othertypes of proprietary welded designs.

In one, large plates up to 10 m longand 1.5 m wide are welded togetherand the plate pack is contained within

a cylindrical shell. This arrangementcan operate at pressures up to 300 barand temperatures ranging from200C to 700C. Because of thelarge plate size, the heat-transfer areaof a single unit can be as high as10,000 m2. A typical application forthis type of exchanger is feed effluentduty in a catalytic reforming plant.

All welded exchangers are moreexpensive than the gasketted plate heatexchanger. But, the use of large plateshelps reduce the cost differential.

Some of the proprietary exchangertypes and their pressure and tempera-ture limits, along with examples oftheir applications, are described inRef. 7.

Spiral recuperatorA new recuperator has been devel-

oped to withstand thermal cycling(8). Unlike existing recuperators, it ismade from two continuous sheets ofmetal wound into a spiral with a cor-rugated sheet providing finned chan-nels for the hot gas stream (Figure13). Air enters the top and flowsdown, while the gas enters at the bot-tom and flows upward.

An unusual feature of the spiral re-cuperator is that the fins on the gasside of the matrix are not physically

attached to the pressure retainingsheets. Instead, the high pressure onthe air side maintains the contact be-tween the gas-side fins and the adja-cent sheet.

This exchanger is not yet beingmanufactured on a commercial scale.But when it is, it is likely to be cost-effective because it can be manufac-tured by a continuous process.

Nonmetallic exchangersCompact heat exchangers can also

be fabricated of nonmetallic materialsof construction, such as graphite,polymer films, and ceramics, for spe-cialized applications.

Graphite is used in making platesfor the conventional plate-and-frameheat exchanger. With special gasketsmade from carbon fibers, these ex-changers are used for highly corro-sive fluids such as acid and salt solu-tions in the mineral processing in-dustry. Graphite is also used as amaterial of construction for carbonblock exchangers, where circularpassages are machined in a solid car-bon block, typically in a crossflowarrangement.

A detailed discussion of ceramicand polymer film heat exchangers isgiven by Ramshaw (9). More recent-ly, Ferrato and Thonon (10) have in-vestigated the use of ceramic plate-finheat exchangers for high-temperatureapplications.

SelectionChoosing an appropriate compact

heat exchanger for a given duty is acomplex process. However, a prelimi-nary selection procedure can be com-pared to a simple two-stage separa-tion process that applies a coarse fil-ter followed by a fine filter.

In this case, we are separating thevarious types of CHEs into suitableand unsuitable designs using techni-cal criteria as the filters. Thecoarse filter makes a preliminarycut by rejecting the obviously unsuit-able types and leaving behind thosethat are capable of performing thespecified duty. The fine filter then


Figure 13.Construction of aspiral recuperator.

Air In

Air OutGas In

Gas Out

Figure 12. Printed-circuit heat exchanger.Courtesy of Heatric.

further narrows the choice based onheat-transfer area and exchanger cost.

Step 1: The coarse filterBased on considerations of operat-

ing temperature, pressure, and fluidcompatibility, the exchangers thatcannot be used for a given duty canbe rejected. Other factors, such asmechanical or chemical cleaning ofthe heat-transfer surface, multistreamcapabilities, and so on, can also betaken into account.

Table 1 can be used to apply thiscoarse filter to the CHEs coveredhere. This involves considering thefollowing:

1. Maximum pressure. ManyCHEs can be employed only up tomoderate pressures, and these willbe ruled out for higher-pressureservices.

2. Temperature range. Differentexchangers have different tempera-

tures ranges, so some exchangertypes can be ruled out on this basis.

3. Fluid compatibility. Compatibili-ty refers to that between the fluid andthe materials of construction for theheat exchanger. Gasketted exchangers,for example, may be excluded if thereis a problem of compatibility betweenthe fluid and the gasket material.

4. Other issues. This could includesuch factors as the consequences ofleakage of one stream into another.For example, if there is a likelihoodof a violent chemical reaction, a dou-ble-wall type heat exchanger shouldbe considered. Another factor is tem-perature cross i.e., where the outlettemperature of the hot stream is high-er than the inlet temperature of thecold stream. If there is a temperaturecross, then only exchangers that canbe configured as countercurrent de-vices can be used.

As a result of this filtering, one or

more exchangers could be left as vi-able. Note that Table 1 is by nomeans exhaustive and could be sup-plemented with relevant data frommanufacturers, especially for the pro-prietary exchanger types.

Step 2: The fine filterAll of the exchangers identified in

Step 1 as capable of performing theduty need to be investigated further inStep 2 to narrow down the choice.This involves approximating the heat-transfer area and cost for each ex-changer. Based on these two parame-ters, a final selection can be made.

To determine the heat-transferarea, Eq. 1 can be rearranged:

A = 1/U (Q/T) (3)

In principle, the heat-transfer areacan be multiplied by cost per unitarea to obtain the cost of the ex-



Table 1. A preliminary selection guide to compact heat exchangers.

Partially Diffusion-Plate-and- Welded BondedFrame Plate-and- Brazed Plate and- Brazed Titanium Printed(Gasketed) Frame Plate Shell Plate-Fin Plate-Fin Circuit

Compactness (m2/m3) Up to 200 Up to 200 Up to 200 8001,500 700800 >2,000

Stream Types Liquid-Liquid Liquid-Liquid Liquid-Liquid Liquids Liquid-Liquid Liquid-Liquid Liquid-LiquidGas-Liquid Gas-Liquid Two-phase Gas-Liquid Gas-Liquid Gas-LiquidTwo-Phase Two-Phase Two-Phase Two-Phase Two-Phase

Materials Frame: Frame: Stainless Stainless Aluminum, Titanium S/S,Carbon Steel Carbon Steel Steel Steel, Stainless Nickel,Plates: Plates: Titanium Steel, TitaniumStainless Steel, Stainless Steel, Nickel Alloy InconelTitanium, Incoloy, IncoloyIncoloy, Hastelloy,Hastelloy,Graphite

Temperature Range (C) 35 to +180 35 to +180 195 to +200 200 to +600 269 to +100

changer. However, for some exchang-ers, especially those containing ex-tended surfaces, it may be difficult todefine the heat-transfer area. For thisreason, Hewitt et al. (1) proposedcost factors (C) based on Q/T.

Table 2 presents typical data forthe overall heat-transfer coefficientand the cost factor at Q/T = 5,000W/K for shell-and-tube exchangershandling a variety of streams. (Com-plete tables for shell-and-tube andplate-and-frame heat exchangers aregiven in Ref. 1.) The steps involvedin the application of this fine filtercan be illustrated as follows.

1. Calculate the heat duty, Q, froma heat balance.

2. Estimate the mean tempera-ture difference, T, between thestreams, using a correction factor(Ft) if necessary.

3. Calculate the ratio Q/T. Notethat the ratio may be different for dif-

ferent heat exchangers and flow con-figurations if the value of the correc-tion factor is different.

4. Obtain values of C and U fromtables such as Table 2 (which isadapted from Ref. 1) and using loga-rithmic interpolation if necessary.Logarithmic interpolation should beused to interpolate for in-betweenvalues of Q/T.

5. Calculate the cost of the heatexchanger by multiplying C andQ/T.

6. Calculate the area of the heatexchanger using Eq. 3.

If there is one heat exchanger orheat exchanger flow configurationthat is significantly better (by a factorof 1.5 or so), then this type warrants adetailed design and cost estimation. Ifthere are several exchangers withcomparable costs, then all of themneed to be investigated in detail.

It should be noted that extensive

tables of information giving C values,as well as software for selection ofheat exchangers, is available fromcommercial sources. CEP


V. V. WADEKAR is Research Manager at HTFS,AEA Technology Hyprotech, Harwell, U.K.(Phone: +44-1235-434249; Fax: +44-1235-831981; E-mail:[email protected]). In addition to leading his research team atHarwell, he chairs the HTFS IndustrialReview Panel on compact heat exchangers.He has authored or coauthored a number oftechnical and research papers in the area ofcompact heat exchangers, multiphase flowheat and mass transfer, and boiling heattransfer. He has lectured internationally andpresented numerous training courses relatedto compact and other exchanger types.Recently, he has started teaching a shortcourse on compact heat exchangers at theAIChE Spring National Meeting. He obtainedhis BChemEng and PhD degrees fromBombay Univ. Dept. of Chemical Technology.He is a member of the Heat Transfer Society,U.K., and of AIChE.

Table 2. Typical heat-transfer coefficient (U) and cost factor (C) data for a shell-and-tube heat exchanger with Q/DT = 5,000 W/K.

Hot-Side FluidLow- High- Condensing

Low- High- Viscosity Viscosity HydrocarbonPressure Pressure Process Organic Organic Condensing Condensing With

Cold-Side Fluid Parameter* Gas Gas Water Liquid Liquie Steam Hydrocarbon Inert Gas

Low-Pressure U 55 93 102 99 63 107 100 86Gas (1 bar) C 2.13 1.88 1.71 1.76 2.24 1.62 1.74 1.82

High-Pressure U 93 300 429 375 120 530 388 240Gas (20 bar) C 1.88 1.20 0.95 1.08 1.68 0.99 1.05 1.16

Treated U 105 484 938 720 142 1,607 764 345Cooling Water C 1.65 1.08 0.81 1.07 1.41 0.48 1.01 1.17

Low-Viscosity U 99 375 600 500 130 818 524 286Organic Liquid C 1.76 1.08 0.87 1.05 1.55 0.93 1.01 1.26High-Viscosity U 68 138 161 153 82 173 155 336Organic Liquid C 2.07 1.46 1.25 1.32 1.91 1.16 1.30 1.62Boiling U 105 467 875 677 140 1,432 722 336Water C 1.65 1.13 0.87 0.78 1.44 0.54 1.05 1.20

Boiling U 99 375 600 500 130 818 524 286Organic Liquid C 1.76 1.08 0.87 1.05 1.55 0.93 1.01 1.26

* Units for U are W/m2K, units for C are $/WK.

Source: Adapted from (1).

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