-
To join an online discussion about this articlewith the author
and other readers, go to theProcessCity Discussion Room for CEP
articlesat www.processcity.com/cep.
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
Compact Heat ExchangersCopyright 2000 American Institute of
Chemical Engineers.All rights reserved.Copying and downloading
permittedwith restrictions.
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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
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CEP December 2000 www.aiche.org/cep/ 41
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
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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-
Compact Heat Exchangers
42 www.aiche.org/cep/ December 2000 CEP
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-
CEP December 2000 www.aiche.org/cep/ 43
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.
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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
Compact Heat Exchangers
44 www.aiche.org/cep/ December 2000 CEP
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.
CEP December 2000 www.aiche.org/cep/ 45
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
Compact Heat Exchangers
46 www.aiche.org/cep/ December 2000 CEP
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
CEP December 2000 www.aiche.org/cep/ 47
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-
Compact Heat Exchangers
48 www.aiche.org/cep/ December 2000 CEP
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
CEP December 2000 www.aiche.org/cep/ 49
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).