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APV Heat Transer HandbookA History O Excellence


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For more than 75 years, APV has provided customers worldwidewith the latest advancements in heat exchanger technology. Today,we continue to lead the industry with our world renowned state-o-the-art technology, unsurpassed process knowledge and anunwavering commitment to our customers.

APV has evolved and grown over the years to better meet the changing needs o our

customers and their industries. The irst commercially successul plate-and-rame heat

exchanger was introduced in 1923 by the Aluminum and Vessel Company Ltd., whichbecame known as APV. The irst Paralow Plate Heat Exchanger, constructed o cast

gunmetal plates and enclosed within a crude rame, set the standard or today's computer-

designed thin metal plates.

Our vision or the uture is rooted in a long standing tradition o excellence and commitment

to progress. We strive to oer customers the highest quality products and services today,

tomorrow and beyond.

Stainless Steel

Tie Bar FrameCarbon Steel

Tie Bar Frame


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This handbook describes the paralow and its operating principle,

compares it with conventional tubular exchangers, and outlines the

many types o thermal duties or which it may be used.

CONTENTS

Section 1. The Paralow and its Principle . . . . . . . . . . . . . . . . . . . . . . . . 4

Section 2. Thermal Perormance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Section 3. The Problem o Fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Section 4. Comparing Plate and Tubular Exchangers . . . . . . . . . . . . . 29

Section 5. Recovery o Process Heat . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Section 6. Paralows in Sea Water Cooling Systems . . . . . . . . . . . . . 40

Section 7. Corrosion and Heat Transer . . . . . . . . . . . . . . . . . . . . . . . . 48

Section 8. The Design o Plate Heat Exchangers or Rerigerants . . . 59

For additional inormation speciic to your needs, please contact APV at

800-207-2708 or at [email protected].

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ThE ParaflOw aNd iTS PriNCiPlE

The paralow is the original plate type heat exchanger designed by APV to providemaximum eiciency and cost eectiveness in handling thermal duties whileminimizing maintenance downtime and loor space requirements.

ThE fraME

The paralow plate heatexchanger, as shown in

Figure 1, consists o astationary head and endsupport connected by atop carrying bar and bottomguide rail. These orm a rigidrame, which supports theplates and moveable ollower.In most units, plates aresecurely compressed between

the head and ollower bymeans o tie bars on eitherside o the exchanger. In aew models, central tighteningspindles working against areinorced end support areused or compression. WhenParalows are opened, theollower moves easily along

the top bar with the aid o abearing-supported roller, toallow ull access to eachindividual plate. With the exception o some sanitary models, which are clad withstainless steel, paralow rames are abricated o carbon steel and are inished inchemical resistant epoxy paint. Frame ports accept bushings o stainless steel oralternative metals, which with various types o langed or sanitary connections,orm the inlet and outlet nozzles. By using intermediate connector plates as shown inFigure 2, units can be divided into separate sections to accommodate multiple duties

within a single rame.

ThE PlaTES

The closely spaced metal heat transer plates have troughs or corrugations, whichinduce turbulence to the liquids lowing as a thin stream between the plates (Figure 3).The plates have corner ports, which in the complete plate pack orm a maniold or evenluid distribution to the individual plate passages (Figure 4).

4

TOP CARRYING BAR

BOTTOM

CARRYING BAR

MOVEABLE

FOLLOWER

FOLLOWER

ROLLER

END SUPPORT

PORT

HEAD

PLATE

RACK

TIE BAR

Figure 1


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ThE gaSkETSThe seal between the plates is established by aperipheral gasket which also separates the thruportand low areas with a double barrier. The interspaceis vented to atmosphere to prevent cross-contaminationin the rare event o leakage.

uNiquE iNTErlOCkiNg gaSkETS

As an exclusive eature, paralow heat exchangerplates have interlocking gaskets in which upstandinglugs and scallops are sited intermittently around theoutside edges. These scallops ensure that there areno unsupported portions o the gaskets, and incombination with the patented orm o pressed groove,provide mechanical plate-to-plate support or thesealing system. The upstanding lugs maintain platealignment in the paralow during pack closure andoperation. The groove orm provides 100% peripheralsupport o the gasket, leaving none o the materialexposed to the outside. In addition, the gasket/groovedesign minimizes gasket exposure to the process liquid.

FIRST STAGE

COOLING LIQUID

SECOND STAGE

COOLING LIQUID

Figure 2. Two section paraflow with connector plate.

HOT FLUID OUT

COLDFLUIDIN

COLDFLUIDOUT

HOT FLUID IN

Figure 3. Cutaway of paraflow

plate shows turbulence during

passage of product and

service liquids.

Figure 4. Single pass counter-current flow.

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PlaTE arraNgEMENT

Comparison o paralow plate arrangementto the tube and shell side arrangement in ashell and tube exchanger is charted in Figure

7. Essentially, the number o passes on thetube side o a tubular unit can be comparedwith the number o passes on a plate heatexchanger. The number o tubes per pass alsocan be equated with the number opassages per pass or the paralow. However,the comparison with the shell side usually ismore diicult since with a paralow, the totalnumber o passages available or the low o

one luid must equal those available or theother luid to within 1. The number o crosspasses on a shell, however, can be relatedto the number o plate passes. Since thenumber o passages/pass or a plate isan indication o the low area, this can beequated to the shell diameter. This is not aperect comparison but it does show the relative parameters or each exchanger.

With regard to low patterns, the paralow advantage over shell and tube designs is the

ability to have equal passes on each side in ull counter-current low, thusobtaining maximum utilization o the temperature dierence between the twoluids. This eature is particularly important in heat recovery processes with closetemperature approaches and even in cases with temperature crossovers.

Whenever the thermal duty permits, it is desirable to use single pass, counter-currentlow or an extremely eicient perormance. Since the low is pure counter-low,correction actors required on the LMTD approach unity. Furthermore, with allconnections located at the head, the ollower is easily moved and plates are morereadily accessible.

PlaTE CONSTruCTiON

Depending upon type, some plates employ diagonal low while others are designed orvertical low (Figure 8). Plates are pressed in thicknesses between 0.020 and 0.036inches (0.5 to 0.9 mm), and the degree o mechanical loading is important. The mostsevere case occurs when one process liquid is operating at the highest working

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Figure 5.

OUTSIDE

PLATE

EDGE

HEAT

TRANSFER

SURFACE

PROCESS

SERVICE

PROCESS

Figure 6. Exclusive interlocking gasket.

Figure 7. Pass arrangement comparison:

plate vs. tubular.

Shell and Plate

tube equivalent

Tube No. of passes No. of passes Side

side No. of tubes/pass No. of passages/pass one

Shell No. of cross passes No. of passes Side

side (No. of baffles +1) two

Shell diameter No. of passages/pass


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pressure, and the other is at zero pressure.The maximum pressure dierential is applied

across the plate and results in a considerableunbalanced load that tends to close thetypical 0.1 to 0.2 inch gap.Its essential, thereore, that some orm ointerplate support is provided to maintain thegap and two dierent plate orms do this.

One method is to press pipes into a plate withdeep washboard corrugations toprovide contact points or about every 1 to 3

square inch o heat transer surace (Figure9). Another is the chevron plate o relativelyshallow corrugations with support maintainedby peak/peak contact (Figure 10). Alternateplates are arranged so that corrugations crossto provide a contact point or every 0.2 to 1square inch o area. The plate then can handle a largedierential pressure and the cross pattern orms a tortuous path thatpromotes substantial liquid turbulence, and

thus, a very high heat transer coeicient.

MixiNg aNd variablE lENgThPlaTES

To obtain optimum thermal and pressuredrop perormance while using a minimumnumber o heat exchanger plates, mixingand variable length plates are available or

several APV paralow plate heat exchangermodels. These plates are manuactured tothe standard widths speciied or theparticular heat exchanger involved but areoered in dierent corrugation patterns andplate lengths.

Since each type o plate has its ownpredictable perormance characteristics, it ispossible to calculate heat transer surace,

which more precisely matches the requiredthermal duty without oversizing the exchanger. This results in the use o ewer plates and asmaller, less expensive, exchanger rame.

To achieve mixing, plates which have been pressed with dierent corrugation anglesarecombined within a single heat exchanger rame. This results in low passages that diersigniicantly in their low characteristics, and thus, heat transer capability rom passagescreated by using plates that have the same corrugation pattern.

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Figure 8.

Figure 9. Corregations

pressed into plates are

perpendicular to the

liquid flow.

Figure 10. Troughs are

formed at opposite

angles to the center line

in adjacent plates.

Vertical Flow Diagonal Flow


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For example, a plate pack (Figure 11) o standard plates, which have a typical 50corrugation angle (to horizontal), develops a ixed level o thermal perormance (HTU)

per unit length. As plates o 0 angle (Figure 12) are substituted into the plate packup to a maximum o 5O% o the total number o plates, the thermal perormanceprogressively increases to alevel that typically is twice thato a pack containing only 50angle plates.

Thus, it is possible or a givenplate length to ine tunethe paralow design in a

single or even multiple passarrangement, exactly tothe thermal and pressuredrop requirements o theapplication.

A more recent developmentis plates o ixed width withvariable lengths, which extendthe range o heat transer

perormance in terms o HTU.This is proportional to the eective length o the plate and typically provides a range o3 to 1 rom the longest to the shortest plate in the series. As shown in Figure 13, mixingalso is available in plates o varied lengths and urther increases the perormance rangeo the variable length plate by a actor o approximately two.

8

+

50 =

50

50 +

0 =

Figure 11. Low HTU passage.

Figure 12. High HTU passage.

Figure 13.


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This extreme lexibility o combining mixing and variable length plates allows more dutiesto be handled by a single pass design, maintaining all connections on the stationary

head o the exchanger to simpliy piping and unit maintenance.

PlaTE SizE aNd fraMECaPaCiTy

Paralow plates are available with aneective heat transer area rom0.65 t2 to 50 t2 and up to 600 o anyone size can be contained in a single

standard rame. The largest paralowcan provide in excess o 30,000 t2o surace area. Flow ports are sizedin proportion to the plate area andcontrol the maximum permissible liquidthroughput (Figure 14). Flow capacityo the individual Paralow, based ona maximum port velocity o 20 ps,ranges rom 25 GPM in the TR1 to

11,000 GPM in the Model SR23AO.This velocity is at irst sight somewhathigh compared to conventionalpipework practice. However, the highluid velocity is very localized in theexchanger and progressively is reducedas distribution into the low passages occurs rom the port maniold. I pipe runs arelong, it is not uncommon to see reducers itted in the piping at the inlet and exitconnections o high throughput machines.

PlaTE MaTErialS

Paralow plates may be pressed rom 304 or 316 stainless steels, Avesta 254SM0or 904L, nickel 200, Hastelloy B-2, C2000, C-276 or G-3, Incoloy 825, Inconel 625,Monel 400, titanium or titanium-palladium as required to provide suitable corrosionresistance to the streams being handled.

gaSkET MaTErialS

As detailed in Figure 15, various gasket materials are available as standard, which haschemical and temperature resistance, coupled with excellent sealing properties. Thesequalities are achieved by speciically compounding and molding the elastomers or longterm perormance in the APV paralow.

Since the temperatures shown are not absolute, gasket material selection must takeinto consideration the chemical composition o the streams involved, as well as theoperating cycles.

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Figure 14. Throughput vs. port diameter at 14 ft/sec.


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ThErMal PErfOrMaNCE

The paralow plate heat exchanger is used most extensively in liquid-liquid duties under

turbulent low conditions. In addition, it is particularly eective or laminar low heattranser and is used in condensing, gas cooling and evaporating applications.

For plate heat transer in turbulent low, thermal perormance can best be exempliied bya Dittus Boelter type equation:

NU = (C) (Re)n (Pr)m (/w)x where

Nu Nusselt number hDe/k

Re Reynolds number vDe/

Pr Prandtl number Cp/k

De Equivalent diameter,(2x average plate gap)

(/w) Sieder Tate correction Factor

and reported values o the constant and exponents are

C = 0.l5 to 0.40 m= 0.30 to 0.45

n = 0.65 to 0.85 x = 0.05 to 0.20

Typical velocities in plate heat exchangers or water-like luids in turbulent low are 1to 3 t/s (0.3 to 0.9 m/s), but true velocities in certain regions will be higher by a actor

o up to our due to the eect o the corrugations. All heat transer and pressure droprelationships are, however, based on either a velocity calculated rom the average plategap or on the low rate per passage.

Figure 16 illustrates the eect o velocity or water at 60F on heat transer coeicients.This graph also plots pressure drop against velocity under the same conditions. The ilmcoeicients are very high and can be obtained or a moderate pressure drop.

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Gasket material Approximate maximum Applicationoperating temperature

Nitrile 300F 148C General aqueous service, aliphatic hydrocarbons

EPDM 320F 160C High temperature resistance for a wide range

of chemicals and steam

Paraprene (Polychloroprene/chloroprene) 194F 90C Refrigerants:Ammonia (R717)

Freon (13, 22, R134a)

Paradur (Fluoroelastomer) 400F 205C Mineral oil, fuels, vegetable and animal oils

Paraflor (Fluoroelastomer) 400F 205C Steam, sulfuric acids

Paramine 400F 205C Rich amine service, hut amine with

welded pairs, acids, alkalis, sour gas

Figure 15. Paraflow gasket guide.


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One particularly important eatureo the paralow is that the turbulence

induced by the troughs reduces theReynolds number at which the lowbecomes laminar. I the characteristiclength dimension in the Reynoldsnumber is taken at twice the averagegap between plates, the Re number atwhich the low becomes laminar variesrom about 100 to 400 according tothe type o plate.

To achieve these high coeicients, it isnecessary to expend energy. With theplate unit, the riction actors normallyencountered are in the range o 10

to 400 times those inside a tube or the same Reynolds number. However, nominalvelocities are low and plate lengths do not exceed 7.5 t so that the term (V2)L/(2g)in the pressure drop equation is much smaller than one normally would encounter intubulars. In addition, single pass operation will achieve many duties so that the pressuredrop is eiciently used and not wasted on losses due to low direction changes.

The riction actor is correlated with the equations:

= B/(Re)y p = LrV2/2g d

where y varies rom 0.1 to 0.4 according to the plate and B is a constant characteristico the plate.

I the overall heat transer equation Q = U A T is used to calculate the heat duty,it is necessary to know the overall coeicient U, the surace area A and the meantemperature dierence T.

The overall coeicient U can be calculated rom

1/U = rh + rc + rdh + rdc

The values o rh and rc (the ilm resistances or the hot and cold luids, respectively)can be calculated rom the Dittus Boelter equations previously described, and the wallmetal resistance (rw) can be calculated rom the average metal thickness and thermalconductivity. The ouling resistances o the hot and cold luids (rdh and rdc) oten arebased on experience.

The value taken or A is the developed area ater pressing. That is the total areaavailable or heat transer, and due to the corrugations, will be greater than the

projected area o the plate, i.e., 1.81 t2

vs. 1.45 t2

or an HX plate.The value o T is calculated rom the logarithmic mean temperature dierencemultiplied by a correction actor. With single pass operation, this actor is about 1,except or plate packs o less than 20 when the end eect has a signiicant bearingon the calculation. This is due to the act that the passage at either end o the platepack only transers heat rom one side, and thereore, the heat load is reduced.

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FLOW RATE/PASSAGE LBS/ HR

PRESSURELOSSPSI

FILMCOEFFICIENTBTUHRF/FT

4000

3000

2000

1000

14

12

10

8

6

4

2

00

0 1000 2000 3000 4000 5000 6000 7000 8000

Figure 16.


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When the plate unit is arranged or multiple pass use, a urther correction actor mustbe applied. Even when two passes are countercurrent to two other passes, at leastone o them must experience co-current low. This correction actor is shown in Figure17 against a number o heat transer units (HTU = temperature rise o the processluid divided by the mean temperature dierence). As indicated, whenever unequal

passes are used, the correction actor calls or a considerable increase in area. Thisis particularly important when unequal low conditions are handled. I high and lowlow rates are to be handled, the necessary velocities must be maintained with the lowluid low rate by using an increased number o passes. Although the plate unit is mosteicient when the low ratio between two luids is in the range o 0.7 to 1.4, other ratioscan be handled with unequal passes. This is done, however, at the expense o theLMTD actor.

lESS PlaTE fOuliNg

The question o how to speciy a ouling resistance or a plate heat exchanger isdiicult to resolve. Manuacturers generally speciy 5% excess HTU or low oulingduties, 10% or moderate ouling, and 15 to 20% excess or high ouling. The allowedexcess surace almost always is suicient even though in many cases it representsa low absolute value o ouling in terms o (Btu/hr t2 F)-1. I a high oulingresistance is speciied, extra plates have to be added, usually in parallel. This resultsin lower velocities, more extreme temperatures during startup, and the probabilityo higher ouling rates. Because o the narrow plate gaps, and in particular because

o the small entrance and exit low areas, ouling in a plate usually causes moreproblems by the increase in pressure drop and/or the lowering o low rates thanby causing large reductions in heat transer perormance. Increasing the number oplates does not increase the gap or throat area, and a plate unit sometimes can oulmore quickly when over suraced.

Unless the customer has considerable experience with both his/her process and theplate heat exchanger, he/she should allow the equipment manuacturer to advise onouling and the minimum velocity at which the exchanger should operate.

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NUMBER OF HEAT TRANSFER UNITS (HTU),

LMTDCORRECTIONFACTOR

1.0

0.9

0.8

0.7

0.6

0.5 0 1 2 3 4 5 6 7 8 9 10 11

2/1

3/1

2/2

1/1

3/3

4/4

Figure 17. LMTD correction factor.


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laMiNar flOw

The other area suitable or the plate heat exchanger is that o laminar low heat transer.It has been pointed out already that the paralow can save surace by handling airlyviscous luids in turbulent low because the critical Reynolds number is low. Once theviscosity exceeds 20 to 50 cP, however, most plate heat exchanger designs all into theviscous low range. Considering only Newtonian luids, since most chemical duties allinto this category, in laminar ducted low the low can be said to be one o three types:1. ully developed velocity and temperature proiles (i.e., the limiting Nusselt case), 2.ully developed velocity proile with developing temperature proile (i.e., the thermalentrance region), or 3. the simultaneous development o the velocity and temperatureproiles.

The irst type is o interest only when considering luids o low Prandtl number, andthis does not usually exist with normal plate heat exchanger applications. The thirdis relevant only or luids such as gases which have a Prandtl number o about one.Thereore, consider type two.

As a rough guide or plate heat exchangers, the rate o the hydrodynamic entrancelength to the corresponding thermal entrance length is given by

Lth/Lhyd = 1.7 Pr

Plate heat transer or laminar low ollows the Dittus Boelter equation in this orm.

Nu = c (Re Pr De/L) 1/3 (/w)x

where L = nominal plate lengthc = constant or each plate (usually in the range 1.86 to 4.50)x = exponent varying rom 0.1 to 0.2 depending upon plate type

For pressure loss, the riction actor can be taken as = a/Re where a is a constantcharacteristic o the plate.

It can be seen that or heat transer, the plate heat exchanger is ideal because the valueo d is small and the ilm coeicients are proportional to d-2/3. Unortunately, however,

the pressure loss is proportional to (d)-4 and the pressure drop is sacriiced to achievethe heat transer.

From these correlations, it is possible to calculate the ilm heat transer coeicient andthe pressure loss or laminar low. This coeicient combined with the metal coeicientand the calculated coeicient or the service luid, together with the ouling resistance,are then used to produce the overall coeicient. As with turbulent low, an allowancehas to be made to use the LMTD to allow or either end eect correction or small platepacks and/or concurrency caused by having concurrent low in some passages. Thisis particularly important or laminar low since these exchangers usually have more than

one pass.

bEyONd liquid/liquid

Over many years, APV has built up considerable experience in the design and use oparalow plate heat exchangers or process applications that all outside the normalturbulent liquid low that is common in chemical operations. The paralow, or example,can be used in laminar low duties, or the evaporation o luids with relatively high

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viscosities, or cooling various gases, and or condensing applications where pressuredrop parameters are not overly restrictive.

CONdENSiNg

One o the most important heattranser processes within the CPIis the condensation o vapors a duty which oten is carried out onthe shell side o a tubular exchangerbut is entirely easible in the plate

type unit. Generally speaking, thedetermining actor is pressure drop.

For those condensing duties wherepermissible pressure loss is lessthan one PSI, there is no doubtbut that the tubular unit is mosteicient. Under such pressure dropconditions, only a portion o thelength o a paralow plate would be

used and substantial surace areawould be wasted. However, whenless restrictive pressure drops areavailable the plate heat exchangerbecomes an excellent condensersince very high heat transercoeicients are obtained, and thecondensation can be carried out ina single pass across the plate.

PrESSurE drOP OfCONdENSiNg vaPOrS

The pressure drop o condensing steam inthe passages o plate heat exchangershas been experimentally investigated or a series o dierent paralow plates. Asindicated in Figure 18 which provides data or a typical unit, the drop obtained isplotted against steam low rate per passage or a number o inlet steam pressures.

It is interesting to note that or a set steam low rate and a given duty, the steampressure drop is higher when the liquid and steam are in countercurrent ratherthan co-current low. This is due to dierences in temperature proile.

Figure 19 shows that or equal duties and lows, the temperature dierence orcountercurrent low is lower at the steam inlet than at the outlet with most o the steamcondensation taking place in the lower hal o the plate. The reverse holds true orco-current low. In this case, most o the steam condenses in the top hal o the plate,

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STEAM

STEAM

PROCESS

PROCESS

COUNTERCURRENT

CO-CURRENT

TEMPERATURE

PLATE LENGTH

Figure 19. Temperature profile during

condensation of steam.

4

3

2

1

00 50 100 150 200 250 300 350 400 450

5 psi

10 psi

15 psi

STEAM SIDE PRESSURE FOR R5

psi

STEAM CONDENSATION RATE LB/HR PER PASSAGE

P

6

5

Figure 18. Steam side pressure drop for R5.


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the mean vapor velocity is lower, and a reduction in pressure drop o between 10% to40% occurs. This dierence in pressure drop becomes lower or duties where the inalapproach temperature between the steam and process luid becomes larger.

The pressure drop o condensing steam thereore is a unction o steam low rate,pressure and temperature dierence. Since the steam pressure drop aects thesaturation temperature o the steam, the mean temperature dierence, in turn, becomes

a unction o steam pressure drop. This is particularly important when vacuum steam isbeing used since small changes in steam pressure can cause signiicant changes in thetemperature.

By using an APV computer program and a Martinelli Lockhart approach to the problem,it has been possible to correlate the pressure loss to a high degree o accuracy.Figure 20 cites a typical perormance o a steam heated Series R4 paralow. From thisexperimental run during which the exchanger was equipped with only a small numbero plates, it can be seen that or a 4 to 5 PSI pressure drop and above, the plate iscompletely used. Below that igure, however, there is insuicient pressure drop available

to ully use the entire plate and part o the surace, thereore, is looded to reduce thepressure loss. At a 1 PSI allowable pressure drop, only 60% o the plate is used orheat transer, which is not particularly economic.

This example, however, well illustrates the application o a plate heat exchanger tocondensing duties. I suicient pressure loss is available, then the plate type unit is agood condenser. The overall coeicient o 770 Btu/hr t2 F or 4.5 PSI pressureloss is much higher than a coeicient o 450 to 500 Btu/hr t2 F, which could beexpected in a tubular exchanger or this type o duty. However, the tubular design orshell side condensation would be less dependent on available pressure loss and or a

1 PSI drop, a 450 to 500 Btu overall coeicient still could be obtained. With the plate,the calculated coeicient at this pressure is 746 Btu, but the eective coeicient basedon total area is only 60% o that igure or 445 Btu/hr t2F.

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Water low rate 16,000 lbs/hr.

Inlet water temperature 216F.

Inlet steam temperature 250F.Total number o plates -7.

Available pressure loss PSI 1 2 3 4

Total duty BTU/hr 207,000 256,000 320,000 333,000

Fraction of plate flooded 40 30 4 0

Effective overall heat transfer coefficient, clean 445 520 725 770

Pressure loss PSI 1 2 4 4.5

Figure 20. Steam heating in an R4 Paraflow.


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gaS COOliNg

Plate heat exchangers also are used or gas cooling with units in service or coolingmoist air, hydrogen, and chlorine. The problems are similar to those o steam heatingsince the gas velocity changes along the length o the plate due either to condensationor pressure luctuations. Designs usually are restricted by pressure drop, so machineswith low pressure drop plates are recommended. A typical, allowable, pressure losswould be 0.5 PSI with rather low gas velocities giving overall heat transer coeicientsin the region o 50 Btu/hr t F.

EvaPOraTiNg

The plate heat exchanger also can be used or evaporation o highly viscous luids whenas a paravap the evaporation occurs in the plate or as a paralash the liquid lashesater leaving the plate. Applications generally have been restricted to the soap and oodindustries. The advantage o these units is their ability to concentrate viscous luids oup to 5,000 centipoise.

CONCluSiON

It has been shown, thereore, that the plate heat exchanger is a relatively simple

machine on which to carry out a thermal design. Unlike the shell side o a tubularexchanger where predicting perormance depends on bale/shell leakage, bale/tubeleakage and leakage around the bundle, it is not possible to have bypass streams on aparalow. The only major problem is that the pressure loss through the ports can causeunequal distribution in the plate pack. This is overcome by limiting the port velocity andby using a port pressure loss correlation in the design to allow or the eect o unequaldistribution.

The low in a plate also is ar more uniorm than on the shell side. Furthermore, thereis no problem over calculation o heat transer in the window, across the bundle or o

allowing or dead spots (as is the case with tubular exchangers). As a result, theprediction o perormance is simple and very reliable once the initial correlations havebeen established.

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ThE PrOblEM Of fOuliNg

In view o its complexity, variability and the need to carry out experimental work on along-term basis under actual operating conditions, ouling remains a somewhatneglected issue among the technical aspects o heat transer. Still, the importanceo careully predicting ouling resistance in both tubular and plate heat exchangercalculations cannot be overstressed. This is well illustrated by the ollowing examples.Overall coeicient 1,000 Btu/hr t2 F.

Note that or a typical water/water duty in a plate heat exchanger, it would benecessary to double the size o the unit i a ouling actor o 0.0005 was used oneach side o the plate (i.e. a total ouling o 0.001).

Although ouling is o great importance, there is relatively little accurate data availableand the rather conservative igures quoted in Kern (Process Heat Transer) are used alltoo requently. It also may be said that many o the high ouling resistances quoted havebeen obtained rom poorly operated plants. I a clean exchanger, or example, is startedand run at the designed inlet water temperature, it will exceed its duty. To overcome

this, plant personnel tends to turn down the cooling water low rate and thereby reduceturbulence in the exchanger. This encourages ouling and even though the water lowrate eventually is turned up to design, the damage will have been done. It is probablethat i the design low rate had been maintained rom the onset, the ultimate oulingresistance would have been lower. A similar eect can happen i the cooling water inlettemperature alls below the design igure and the low rate is again turned down.

Six TyPES Of fOuliNg

Generally speaking, the types o ouling experienced in most CPI operations can bedivided into six airly distinct categories. First is crystallization the most common typeo ouling which occurs in many process streams, particularly cooling tower water.Frequently superimposed with crystallization is sedimentation which usually is causedby deposits o particulate matter such as clay, sand or rust. From chemical reaction andpolymerization oten comes a build-up o organic products and polymers. The suracetemperature and presence o reactants, particularly oxygen, can have a very signiicanteect.

17

a tpc te/te-t esn b r405 po, te/te t

Clean oveall coecent Oveall coecent500 Btu/hr ft2 F 1,000 Btu/hr ft2 F

Snle pass pessue loss 9 PSigfouln dty % Exta fouln dty % Extaesstance coecent suace esstance coecent suacehr ft2 F/Btu Btu/hr ft2 F eque hr ft2 F/Btu Btu/hr ft2 F eque

.0002 455 10 .0002 833 20

.0005 400 25 .0005 666 50

.001 333 50 .001 500 100

.002 250 100 .002 333 200


18/64

Coking occurs on high temperaturesuraces and is the result o

hydrocarbon deposits. Organic materialgrowth usually is superimposed withcrystallization and sedimentation, andis common to sea water systems.Corrosion o the heat transer suraceproduces an added thermal resistance,as well as a surace roughness.

In the design o the plate heatexchanger, ouling due to coking is

o no signiicance since the unitcannot be used at such hightemperatures. Corrosion also is irrelevantsince the metals used in these units arenoncorrosive. The other our types oouling, however, are most important. Withcertain luids such as cooling tower water, ouling can result rom a combination ocrystallization, sedimentation and organic material growth.

a fuNCTiON Of TiME

From Figure 21, it is apparent that the ouling process is time dependent with zeroouling initially. The ouling then builds up quite rapidly and in most cases, levels o ata certain time to an asymtotic value as represented by curve A. At this point, the rateo deposition is equal to that o removal. Not all ouling levels o, however, and curve Bshows that at a certain time the exchanger would have to be taken o line or cleaning.It should be noted that a paralow is a particularly useul exchanger or this type o dutybecause o the ease o access to the plates and the simplicity o cleaning.

In the case o crystallization and suspended solid ouling, the process usually is o thetype A. However, when the ouling is o the crystallization type with a pure compoundcrystallizing out, the ouling approaches type B and the equipment must be cleanedat requent intervals. In one particularly severe ouling application, three Series HMBparalows are on a 4 1/2 hour cycle and the units are cleaned in place or 1 1/2 hoursin each cycle.

Biological growth can present a potentially hazardous ouling since it can provide amore sticky type surace with which to bond other oulants. In many cases, however,treatment o the luid can reduce the amount o biological growth. The use ogermicides or poisons to kill bacteria can help.

18

A

B

BUILD UP OF FOULING RESISTANCE

TIME

FOULINGRESISTANCE

Figure 21.


19/64

lOwEr rESiSTaNCE

It generally is considered that resistance due to ouling is lower with paralow plate heatexchangers than with tubular units. This is the result o ive paralow advantages:

1. There is a high degree o turbulence which increases the rate o oulant removaland results in a lower asymtotic value o ouling resistance.

2. The velocity proile across a plate is good. There are no zones o low velocitycompared with certain areas on the shell side o tubular exchangers.

3. Corrosion is maintained at an absolute minimum.

4. A smooth heat transer surace can be obtained.

5. In certain cooling dutiesusing water to cool organics,the very high waterilmcoeicient maintains amoderately low metalsurace temperature whichhelps prevent crystallizationgrowth o the inverselysoluble compounds in thecooling water.

The most important o these isturbulence HTRI (Heat TranserResearch Incorporated). It hasshown that or tubular heatexchangers ouling is a unctiono low velocity and riction.Although low velocities are low withthe plate heat exchanger, riction

actors are very high and this resultsin lower ouling resistance. The eecto velocity and turbulence is plotted inFigure 22.

Marriot o Ala Laval has produced atable showing values o ouling or anumber o plate heat exchanger duties(see right).

These probably represent about one

hal to one ith o the igures usedor tubulars as quoted in Kern but itmust be noted that the Kern iguresprobably are conservative, even ortubular exchangers.

APV, meanwhile, has carried out testwork which tends to conirm thatouling varies or dierent plates

19

FOULINGRESISTANCE

TUBULAR

PH E2 FT/SEC

5 FT/SEC

8 FT/SEC

TIME

Figure 22.

fluid fouling resistance

(hr ft2 F/Btu)

wate emnealze o stlle 0.00005

Soft 0.00010

ha 0.00025

Cooln toe (teate) 0.00020

Sea (coastal) o estuay 0.00025

Sea (ocean) 0.00015

rve, canal 0.00025

Enne jacket 0.00030

Oils, lubricating 0.00010 to 0.00025

Oils, vegetable 0.00010 to 0.00030

Solvents, organic 0.00005 to 0.00015

Steam 0.00005

Pocess lu, eneal 0.00005 to 0.00030


20/64

with the more turbulent type o plate providing the lower ouling resistances. In testingan R405 heating a multi-component aqueous solution containing inverse solubility

salts (i.e., salts whose solubility in water decreases with increasing temperature), it waslearned that the rate o ouling in the paralow was substantially less than that insidethe tubes o a tubular exchanger. The tubular unit had to be cleaned every three or ourdays while the paralow required cleaning about once a month.

Additional tests on cooling water ouling were sponsored by APV at Heat TranserResearch, Inc. (HTRI) with the test luid being a typical treated cooling tower water.Results o these experiments are described in the ollowing section.

ObjECTivESIn many streams ouling is an unavoidable by-product o the heat transer process.Fouling deposits can assume numerous types such as crystallization, sedimentation,corrosion, and polymerization. Systematic research on ouling is relatively recent andextremely limited (Res. 1, 2, 3) and almost exclusively concentrated on water -the most common luid with ouling tendencies. The ew research data which existusually are proprietary or obtained rom qualitative observations in plants. However,it generally is recognized that the prime variables aecting ouling buildup are lowvelocity, surace temperature and surace material. In the case o water, the water quality

and treatment must also be considered.The importance o ouling on the design o heat exchangers can be seen rom the rateequation

1/U = 1/h1 + 1/h2 + Rwhere

U = overall heat transer coeicient (Btu/hr t2 F)

h1, h2 = ilm coeicients o the two heat transerring luids (Btu/hr t2 F)

R = ouling resistance (hr t2F/Btu)

It is obvious rom the equation that the higher the ilm coeicients, the greater eect theouling resistance will have on the overall coeicient, and thereore, on the size o theexchanger.

In tubular heat exchangers, waterside heat transer coeicients in the order omagnitude o 1,000 Btu/hr t F are quite common. In plate exchangers, thecoeicients are substantially higher, typically around 2,000 Btu/hr t2 F. Assumingthat both types o equipment operate with water-water systems, overall cleancoeicients o 500 and 1,000 Btu/hr t F, respectively, are obtained. Using atypical ouling resistance o 0.001 hr t F/Btu (equal to a coeicient o 1,000

Btu/hr t F), the inclusion o the ouling will cause the tubular exchanger size toincrease by a actor o our while or the plate exchanger, the corresponding actoris seven.

This example clearly demonstrates the crucial importance o ouling, especially in plateexchangers. Yet, ouling resistances, which are unrealistically high, oten arespeciied and invariably have been based on experiences derived rom tubular equipment.

20


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The common source o water ouling resistances is TEMA4 which recommends valueso R spanning a tremendous range between 0.0015 and 0.005 hr t2 F/Btu.

Flow velocity, as mentioned earlier, is a crucial operating parameter which inluencesthe ouling behavior. For low inside o tubes, the deinition o low velocity and thevelocity proile is straightorward. But in plate exchangers, low velocity is characterizedby constant luctuations as the luid passes over the corrugations. It is postulatedthat this induces turbulence which is superimposed on the low velocity as a actorwhich diminishes ouling tendencies. This has been observed qualitatively in practicalapplications and conirmed by unpublished APV research.

TEST aPParaTuS aNd CONdiTiONSThe plate heat exchanger (PHE) tested was an APV Model 405 using APV Type R40stainless steel plates, 45 inches high and 18 inches wide. The plate heat transerarea was 4 sq t with a nominal gap between plates o 0.12 inch. Due to the platecorrugations, the maximum gap was 0.24 inches. Seven plates were used creatingthree countercurrent passages each o the cooling water and the heating medium.A schematic diagram o the installation is shown in Figure 23.

The PHE was mounted on the HTRI Shellside Fouling Research Unit (SFRU) togetherwith two small stainless steel shell and tube exchangers. The PHE was heated using

hot steam condensate and the ouling was determined rom the degradation o theoverall heat transer coeicient. Simultaneously, tests also were run on an HTRI PortableFouling Research Unit (PFRU) which uses electrically heated rods with the coolingwater lowing in an annulus. The SFRU and PFRU are described in a paper by Fischeret al (reerence 5).

The ouling tests were conducted at a major petrochemical plant in the Houston, Texasarea. The test units were installed near the cooling tower basin o the plant. The coolingtowers operating characteristics are summarized in Table 1. The 140,000 GPM systemhad two 1,600 GPM sidestream ilters. Filter backwash accounted or most o theblowdown. The makeup water or the cooling water system came rom several sourcesand was clariied with alum outside the plant. The water treatment used is summarizedbelow:

Chromate-zinc based inhibitor or corrosion control (20.25 ppm chromate)

Organic phosphonate and polymer combination as dispersant (2 ppmorganic phosphonate)

Polyphosphate as anodic passivator (5 to 6 ppm total inorganic phosphate)

Chlorine or biological control (continuous eeding o 386 lbs/day)

Biocide or biological control (15 ppm biocide once/week)

Suluric acid or pH control (pH o 6 to 6.5)

21


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22

COOLING WATER

HOT WATER

Figure 23. The APV plate heat exchanger with three parallel countercurrent parallelpassages for each of the cooling water and hot water.

Table 1. Cooling water system characteristics.

Circulation rate 140,000 GPM

Temperature difference across tower 23.4F

Number of sidestream filters 2

Treatment 10 Cycle concentration Unclarified San Jacinto

River 1,000 GPM

Blowdown from coolers Clarified San Jacinto River

and sidestream filters 2,400 GPM

Lissie Sand Well475 GPM

Typical composition

Total hardness as CaCO3, ppm 520 48

Calcium as CaCO3, ppm 420 40

Magnesium as CaCO3, ppm 120 8

Methyl Orange Alkalinity as CaCO3, ppm 20 34

Sulfate as SO4, ppm 1,600 34

Chloride as CI, ppm 800 48

Silica as SiO2, ppm 150 18

Total inorganic phosphate as PO4, ppm 10

Orthophosphate as PO4, ppm 7

pH 6 7.5Specific conductance, micromhos, 18C 5,000 300

Chromate as CrO4, ppm 25

Chromium as Cr, ppm 0.5

Soluble zinc as Zn, ppm 2.5

Total iron as Fe, ppm 0.8

Suspended solids, ppm 100 10

Tower and circulation system Description

Water description Cooling tower water Makeup water

Cooson te om con stee copon tests


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TEST PrOCEdurE

The velocity or the PHE is deined by V= w/rAc

where V = velocityw = mass low rater = luid densityAc = cross-sectional low area

The cross-sectional low area is based on the compressed gasket spacing. In otherwords, the low area is computed as i there was no corrugation but smooth platesinstead. The surace temperature is deined as the luid-deposit interace temperature.Since the PHE was heated by constant temperature steam condensate, the metal

temperature remained constant and the surace temperature decreased as ouling builtup. In addition, due to the counterlow nature o the PHE, the surace temperature wasnot constant rom inlet to outlet. The surace temperature reported here is the initialsurace temperature at the midpoint o the plate.

Conditions o operation were selected so that all the SFRU test exchangers operatedroughly at the same nominal low velocity and surace temperature. During theoperation, weekly water samples were taken or chemical analysis. At the end o eachtest, deposits were photographed and sampled or chemical analysis.The exchanger plates then were cleaned and the unit reassembled and new test

conditions established.

TEST aNd rESulT dESCriPTiON

Five test series were perormed testing several velocities and surace temperatures.Typical ouling time histories are shown in Figure 24 along with the operatingconditions. Notice that the PHE ouling resistances establish a stable asymtoticvalue ater about 600 to 900 hrs. o operation. The results o the other tests are shownin Figure 25 only as the values o the asymtotic ouling resistances, as these are the

data required or design.The chemical analysis o the ouling deposit was made ater each test and the resultsare summarized in Table 2. The primary elements ound were phosphorus, zinc, andchrome rom the water treatment, and silicon rom suspended solids in the water.

The picture o the plates o the PHE shown in Figure 23 at the termination o Test 2indicates that ouling only occurred in the upper third o the plates. This is the regionnear the hot water inlet and cold water outlet, a region o high surace temperature.Figure 24 shows a surace temperature proile or Test 2 conditions o 2.8 t/secand an inlet bulk temperature o 90F. From the water chemistry parameters, the

saturation temperature or calcium phosphate above which precipitation is expectedwas calculated to be 163F. The shaded region on Figure 26 indicates the expectedprecipitation region and corresponds closely to the ouled region seen in Figure 23.

The surace temperatures during some o the tests were higher than those experiencedin normal PHE operation. This is the result o the setup condition criteria that the threeexchangers on the SFRU operate at the same midpoint surace temperature. Sincethe ratio o low rate-to-surace area or the shell side o the shell and tube exchangersis twice that or the PHE, the cooling water had a longer residence time in the PHE

23


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24

FOULINGRESISTANCE,HRFTF/BTU

FOULINGRESISTANCE,MC/W

1.5 FT/SEC

2.8 FT/SEC

0.0010

0.0005

00 600 1200 1800 2400

0

0.0001

TIME, HR

INITIAL SURFACE TEMPERATURE = 140 F

VELOCITY, M/S

VELOCITY, FT/SEC

ASYMPTOTICFOULING

RESISTANCE

HRFTF/BTU

ASYMPTOTICFOULING

RESISTANCE

MC/W

MIDPOINTSURFACE TEMPERATURE

.003

.002

.001

00 1 2 3

0

0 0.25 0.50 0.75

0.0004

0.0002

142F(61C)

130F(54C)

116F(47C)

Figure 25. Asymtotic fouling resistance versus velocity

with surface temperature as a parameter.

Figure 24. Typical PHE fouling curves.

Loss on ignition 25 25 17 21

Phosphorus 19 24 13 12 22

Aluminum 2 2 8 6 4

Silicon 1 4 18 16 13

Calcium 4 8 3 15 9Chrome 5 13 15 15 9

Iron 2 7 4 5 13

Zinc 2 20 12 13 7

Sodium 1 1 1

Magnesium 1 1

Fouling deposit analysis, percent

Series 2 Series 3 Series 4 Series 5 Series 6

Table 2. Deposit analysis.


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than in the tubular exchanger. As a result, the lower low rate-to-surace area ratioyields a larger temperature rise o the cooling water. The result is the steeper surace

temperature proile shown in Figure 26. Although co-current operation was investigatedas a correction or this problem, the required higher hot water low rate was notavailable. Consequently, the PHE data in some o these tests was slightly penalizeddue to part o the surace being above the critical temperature or tricalcium phosphateprecipitation, a condition which normally would not be encountered in industrialoperations.

It is diicult to make meaningul comparisons between the ouling tendencies odierent types o heat exchangers. Unless careully done, the results are

25

TRI CALCIUM PHOSPHATE

PRECIPITATION TEMPERATURE

AVERAGESURFACETEMPERATURE145 F (63 C)

TUBULA

REXC

HANG

ERS

PLATEEX

CHA

NGER

SURFACETEMPERATURE,F

SURFACETEMPERATURE,C

INLET OUTLET

180

190

170

160

150

140

130

120

110

100

90

80

70

60

50

40

Figure 26. Surface temperature profile with indicated calcium phosphate precipitation.

Overall heat transer Coeicient

Fouling coefficient

Operating Clean TEMA Present studyvelocity 0.002 hr ft2 F/Btu 0.0005 hr ft2 F/Btu

(both sides) (0.00035 m2 C/W) (0.00009 m2 C/W)

1.5 ft/sec 1078 Btu/hr ft2 F 341 Btu/hr ft2 F 700 Btu/ hr ft2 F

0.45 m/s 6121 W/m2 C 1936 W/m2 C 3975 W/m2 C

Table 3. Effects of fouling resistance on PHE performance.


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misleading. This is particularly true when attempting to compare the ouling experiencein the PHE to that o tubeside operation as in the PFRU. The geometries, surace area,

and operational characteristics are very dierent even though the cooling water may bethe same. Such comparisons oten are attempted using TEMA4-recommendedouling resistances.

TEMA recommends a ouling resistance o 0.002 hr t2 F/Btu or the watersystem used. However, the maximum ouling resistance measured in the PHE wasless than 0.0005 hr t2 CF/Btu only 25% o the TEMA recommendation. Thisconirms the earlier assumption that applying TEMA-recommended ouling resistancesor a shell and tube exchanger to the PHE seriously handicaps the perormanceratings. Because o the inherently high heat transer coeicients, the eects oouling resistances are more pronounced. Consider a PHE operating at 1.5 t/sec. FromTable 3, the ouled overall coeicient using a TEMA ouling resistance o 0.002 hr t2 F/Btu is about one hal o the overall coeicient using the measured oulingresistance o this investigation. This example illustrates the need or caution in usingTEMA ouling recommendations or equipment other than shell and tube exchangers.Figure 27 compares the perormance o the PHE with typical tubeside data and with theTEMA-recommended ouling resistance.

26

Figure 27. Comparison of tubeside and PHE fouling.

FOULING RESISTANCE, HR

INITIAL SURFACE TEMPERATURE = 140F

TEMA RECOMMENDED

TUBESIDE, 4.4 FT/SEC (0.45 M/S)

PHE, 1.5 FT/SEC (0.45 M/S)

0.0004

0.0002

6000

0.0010

0 0

0.0020

0.0030

1,200 1,800 2,400

ASYMPTOMATICFOULING

RESISTANCE,

HRF

T2

F/BTU

ASYMPTOMATICFOULING

RESISTANCE,

M2C/W

FOULING DEPOSIT ANALYSIS, PERCENT

Plate Shellside

Loss on ignition 17 24

Phosphorus 12 9

Aluminum 6 7

Silicon 16 33

Calcium 15 7Chrome 15 9

Iron 5 4

Zinc 13 8

Table 4. Comparison of deposit analyses from plate exchanger and shellside test

exchanger for Series 5.


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A direct comparison o the PHE and the shellside exchangers can be made throughthe deposit analyses. A sample analysis is given in Table 4. The amounts o phosphates,

calcium, chrome, and zinc are somewhat higher or the PHE. The operation o the PHEat a higher surace temperature than the shellside units resulted in more crystallizationouling. However, the amount o silicon or the shellside unit is much higher than or thePHE. The high silicon concentration is due to sedimentation in the bale-shell cornerso the bundle, while the high turbulence promoted by the corrugated plates in the PHEminimizes sedimentation ouling. In heavily sediment-laden waters, the PHE would beespecially superior.

CONCluSiONS

Although the tests were perormed on one water system only, experience with tubulardata indicates that the overall trends generally are valid, i.e., plate exchangers shouldbe designed to substantially lower values o ouling than would be used on tubularequipment or the same stream. However, the comparison is not always that straightorward, as low velocity itsel is not a valid criterion. The wall shear stress in a PHEoperating at 2.8 t/sec. is equivalent to that in a tubeside exchanger operating at8.2 t/sec. On the basis o typical industrial velocities, it was ound that the PHE at1.5 t/sec. ouls about one hal as much as a tubeside exchanger operating at

5.9 t/sec. Furthermore, because o the high heat transer coeicients typical to thePHE, surace temperature may dier rom those in tubular equipment and should becareully watched.

The turbulence inducing corrugation pattern prevalent in heat exchanger platesproduces very high local velocities. This results in high riction actors, and thereore,high shearing. This high shear, in turn, results in less ouling in plate exchangers thanin tubular units. For cooling water duties, Heat Transer Research Incorporated (HTRI)has shown that the plate exchanger ouls at a much lower rate than either the tubesideor shellside o tubulars. TEMA recommendations or ouling, thereore, should never be

used or plate units since they probably are ive times the value ound in practice. Inparticular, the plate exchanger is ar less susceptible to ouling with heavy sedimentedwaters. This is contrary to popular belie. It must be noted, however, that all particulatematter must be signiicantly smaller than the plate gap and generally particles over 0.1inch in diameter cannot be handled in any plate heat exchanger.

It has been shown that quoting a high ouling resistance can negate a plate heatexchanger design by adding large amounts o surace and thereby overriding thebeneits o the high coeicients. Thereore, ouling design resistance should be chosenwith care, keeping in mind that with a paralow unit it always is possible to add or

subtract surace to meet exact ouling conditions.As a general policy, HTRI as the project operator disclaims responsibility or anycalculation or design resulting rom the use o the data presented.

27


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rEfErENCES

1 Taborek, J.; Aoki, T.; Ritter, RB.; Palen, J.W.; and Knudsen, J.G.; Fouling The MajorUnresolved Problem in Heat Transer, Parts l and II, CEP, 68, Nos. 2 and 7 (1972)

2 Suitor, J.W.; Marner, W.J.; and Ritter, RB.; The History and Status o Research inFouling o Heat Exchangers in Cooling Water Service, Paper No. 76-CSME/CSChE 19 presented at the 16th National Heat Transer Conerence, St. Louis, Missouri,August 8 to 11, 1976

3 Morse, Robert W. and Knudsen, James G., Eect o Alkalinity on the Scaling oSimulated Cooling Tower Water, Paper No. 76-CSME/CSChE 24 presented atthe 16th National Heat Transer Conerence, St. Louis, Missouri, August 8 to 11,

19764 Standards o Tubular Exchanger Manuacturers Association, 5th Ed. New York. 1968

5 Fisher, P.; Suitor, J.W.; and Ritter, RB.; Fouling Measurement Techniques andApparatus, CEP,71. No.7,66 (1975)

6 Green, J. and Holmes, J.A., Calculation o the pH o Saturation o TricalciumPhosphate, J. Amer., Water Works Assn., 39, No. 11 (1947)

28


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COMPariNg PlaTE aNd TubularExChaNgErS

In orming a comparison between plate and tubular heat exchangers, there are a numbero guidelines which will generally assist in the selection o the optimum exchanger orany application. In summary, these are:

1. For liquid/liquid duties, the plate heat exchanger usually has a higher overall heattranser coeicient and oten the required pressure loss will be no higher.

2. The eective mean temperature dierence will usually be higher with the plate heatexchanger.

3. Although the tube is the best shape o low conduit or withstanding pressure, itis entirely the wrong shape or optimum heat transer perormance since it has thesmallest surace area per unit o cross sectional low area.

4. Because o the restrictions in the low area o the ports on plate units, it is usuallydiicult, unless a moderate pressure loss is available, to produce economic designswhen it is necessary to handle large quantities o low-density luids such as vaporsand gases.

5. A plate heat exchanger will usually occupy considerably less loor space than a

tubular or the same duty.6. From a mechanical viewpoint, the plate passage is not the optimum, and gasketed

plate units are not made or operating pressures much in excess o 300 PSIG.

7. For most materials o construction, sheet metal or plates is less expensive per unitarea than tube o the same thickness.

8. When materials other than mild steel are required, the plate will usually be moreeconomical than the tube or the application.

9. When mild steel construction is acceptable and when a close temperature approach

is not required, the tubular heat exchanger will oten be the most economic solutionsince the plate heat exchanger is rarely made in mild steel.

10. Plate heat exchangers are limited by the necessity that the gasket be elastomeric.Even compressed asbestos iber gaskets contain about 6% rubber. The maximumoperating temperature thereore is usually limited to 500F.

hEaT TraNSfEr COEffiCiENTS

Higher overall heat transer coeicients are obtained with the plate heat exchanger

compared with a tubular or a similar loss o pressure because the shellside o atubular exchanger is basically a poor design rom a thermal point o view. Considerablepressure drop is used without much beneit in heat transer due to the turbulence in theseparated region at the rear o the tube. Additionally, large areas o tubes even in a welldesigned tubular unit are partially bypassed by liquid. Thus, low heat transer areas arecreated.

Bypassing in a plate exchanger is less o a problem and more use is made o the low

29


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separation, which occurs over the plate troughs, since the reattachment point on theplate gives rise to an area o very high heat transer.

For most duties, the luids have to make ewer passes across the plates than wouldbe required through tubes or in passes across the shell. Since a plate unit can carryout the duty with one pass or both luids in many cases, the reduction in the number orequired passes means less pressure lost due to entrance and exit losses, andconsequently, more eective use o the pressure.

MEaN TEMPEraTurE diffErENCE

A urther advantage o the plate heat exchanger is that the eective mean temperaturedierence is usually higher than with the tubular unit. Since the tubular is always amixture o cross and contralow in multi-pass arrangements, substantial correctionactors have to be applied to the log mean temperature dierence. In the plate heatexchanger where both luids take the same number o passes through the unit, the LMTDcorrection actor is usually in excess o 0.95. As is illustrated in Figure 28, this actor isparticularly important when a close or relatively close temperature approach is required.

In practice, it is probable that the sea water low rate would have been increased toreduce the number o shells in series i a tubular had to be designed or this duty. Whilethis would reduce the cost o the tubular unit, it would result in increased operating costs.

30

Figure 28. DUTY: Demineralized water/sea water. To cool 864,000 lb/hr of water from

107F to 82.4F using 773,000 lbs/hr of 72F sea water.

Figure 29. Case studies of plate heat exchangers and tubular designs all tubular designs

were carried out with the aid of the HTRI program ST3.

Size 3 shells in series: R10 Paraflow with 313 plates arranged for 3 process20 tubes, 1 shellside pass, 4 tube passes, passes and 3 service passes, 3630 sq ft of area2238 tubes, 12,100 sq ft area

Overall 270 Btu/hr ft2 F 700 Btu/hr ft2 Fcoefficient

MTD 7.72F 9.0F

Pressure loss 10 PSI/19 PSI 10.8 PSI/8.8 PSI

Tubular Plate

Case Fluid A Fluid B Duty Tubular design Plate designstudy lbs/hr lbs/hr F pressure loss pressure loss

A B ft2 A B ft2 Type

A 13,380 53,383 320 A 120 .42 3.05 301 .3 2.2 92 HX

hydrocarbon water 120 B 92

B 864,000 77,300 107 A 82.4 19 19 12,100 10.8 8.8 3,630 R10

water sea water 99.1 B 71.6

C 17,500 194,000 140 A 104 2.9 4.5 1,830 3.0 3.7 445 R5

solvent water 95 B 79

D 148,650 472,500 222 A 100 10.8 8.0 1,500 4.5 10 380 R4

desalter salt water 106 B 68

effluent


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By comparison, one cubic oot o tubular exchanger o equilateral triangular pitch witha tube pitch/tube diameter ratio o 1.5 has a surace area o 10 t 2 or 2 in OD tubes

or 40 t2

or 1/2 GD tubes. The average contained liquid is proportionately 0.27 gal/t2 down to 0.07 gal/t2 o heat exchanger area with no allowance or the headers. I theheat transer coeicient ratio between plate/tubular is conservatively taken as two, theplate exchanger volume to meet a given duty varies rom 1/10th to 1/5th o that o thetubular. For a lower tube pitch/tube diameter ratio o 1.25, the comparison becomes1/7th to 1/4th. These acts demonstrate why the paralow plate heat exchanger isreerred to as compact.

ThErMal liMiTaTiONS

While it would appear ohand that the plate heat exchanger always provides a betterperormance at usually a lower price than the tubular exchanger, consideration must begiven to the thermal as well as mechanical limitations o the plate type machine. Theseusually are based on allowable pressure loss.

For single phase liquid/liquid duties, the plate heat exchanger can be designed ormoderately low pressure loss. However, i the pressure loss across any plate passagewhich has liquid lowing downward is lower than the available liquid static head, theplate will not run ull and perormance thereore will be reduced. This is termed low

plate rate. Use o a plate below the minimum plate rate is inadvisable since it causes awastage o surace area and results in unreliable operation. It is, however, possible tounction below the minimum plate rate in a single pass arrangement by making sure thatthe low plate rate is operated with a climbing liquid low.

These problems are not quite so severe with a tubular exchanger, and thereore,operation at a moderately lower available pressure loss is possible.

CONCluSiON

To summarize, the gasketed plate heat exchanger generally will be the most economicalheat exchanger or liquid/liquid duties providing the material o construction is not mildsteel and providing the operating temperatures and pressures are below 500F and300 PSIG respectively. For other types o duties such as gas cooling, condensation orboiling, the plate heat exchanger can be a very economical type o unit i thepressure loss allowed is suicient to utilize the very high heat transer perormancecharacteristics o the plate.

32

Figure 31. Volumetric comparison, plate vs. tube.

Plate Tube

Ratio Heat transfer Liquid containment perHeat transfer Liquid contained area per sq ft of heat transferPlate area per cubic per sq ft of Tube tube pitch cubic foot of area (average opitch foot of exchanger heat transfer area dia. tube dia. exchanger of both sides)

1/4 50 ft2 0.06 gal 2 1.5 10 ft2 0.32 gal

1/8 100 ft2 0.03 gal 1/2 1.5 40 ft2 0.085 gal


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rECOvEry Of PrOCESS hEaT

Since it is quite clear that never again will energy be as inexpensive as in the past,it thereore is necessary to conserve this natural resource to recover more o theprocess heat that currently is dissipated to waterways and the atmosphere. Some othis heat can be recovered with the aid o high eiciency heat exchangers, which caneconomically operate with a close temperature approach at relatively low pumpingpowers. One type o unit that is particularly suited or this duty is the APV paralow plateheat exchanger. For many applications this equipment can transer heat with almost truecountercurrent low, coupled with high coeicients, to provide eicient and inexpensive

heat transer.Unortunately, the plate heat exchanger has been considered by many chemicalengineers to be suitable only or hygienic heat transer duties. This, o course, is notso. Nowadays, many more plate-type units are sold or chemical and industrial usethan are sold or hygienic applications. A urther mistake is the claim that the plate heatexchanger can be used only or duties when the volumetric lows o the two luids aresimilar. Again, this is not true, although it must be stated that the plate heat exchanger isat its most eicient when lows are similar.

PlaTE Or Tubular?

Since plate and tubular heat exchangers are the most widely used types o heattranser equipment, it is helpul to draw a brie comparison o their respective heatrecovery capabilities or the energy-conscious plant manager.

While the plate heat exchanger does have mechanical limitations with regards towithstanding high operating pressures above 300 PSIG, it is thermally more eicientthan shell and tube units, especially or liquid/liquid duties. In many waste heatrecovery applications, however, both pressure and temperature generally are moderate.

The plate type unit is an excellent choice since its thermal perormance advantagebecomes very signiicant or low temperature approach duties. Higher, overall, heattranser coeicients are obtained with the plate unit or a similar loss o pressurebecause the shellside o a tubular basically is a poor design rom a thermal point oview. A certain pressure drop is used without much beneit in heat transer on theshellside due to the low reversing direction ater each cross pass. In addition, evenin a well designed tubular heat exchanger, large areas o tubes are partially bypassedby liquid and areas o low heat transer are thus created. Conversely, bypassing o theheat transer area is ar less o a problem in a plate unit. The pressure loss is used more

eiciently in producing heat transer since the luid lows at low velocity, but with highturbulence, in thin streams between the plates.

For most duties, the luids also have to make ewer passes across the plates than wouldbe required either through tubes or in passes across the shell. In many cases, the plateheat exchanger can carry out the duty with one pass or both luids. Since there areewer passes, less pressure is lost due to entrance and exit losses, and the pressure isused more eectively.

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A urther advantage o the plate heat exchanger is that the eective mean temperaturedierence usually is higher than with the tubular. Since in multi-pass arrangements the

tubular is always a mixture o cross- and contra-low, substantial correction actors haveto be applied to the log mean temperature dierence. In the plate unit or applicationswhere both luids take the same number o passes through the exchanger, the LMTDcorrection actor approaches unity. This is particularly important when a close or evenrelatively close temperature approach is required.

ThErMal PErfOrMaNCE daTa

Although the plate heat exchanger now is widely used throughout industry, precise

thermal perormance characteristics are proprietary, and thus, unavailable. It is possible,however, to size a unit approximately or turbulent low liquid/liquid duties by use ogeneralized correlations which apply to a typical plate heat exchanger.The basis o this method is to calculate the heat exchanger area required or a givenduty by assuming that all the available pressure loss is consumed and that any size unitis available to provide this surace area.

For a typical plate heat exchanger, the heat transer can be predicted in turbulent lowby the ollowing equation (1):

(1) hDe/k = 0.28(GDe/)0.65 (C/k)0.4

(2) = 2.5(GDe/)-0.3

(3) P = 2G2L/grDe

The pressure loss can be predicted rom equations (2) and (3). Obviously, equation(1) cannot accurately represent the perormance o the many dierent types o plateheat exchangers that are manuactured. However, plates that have higher or lower heattranser perormance than given in equation (1) usually will give correspondingly higheror lower riction actors in equation (2). Experience indicates that the relationship orpressure loss and heat transer is reasonably consistent or well designed plates. In

the Appendix, equations (A1) (A3) are urther developed to show that it is possibleor a given duty and allowable pressure loss to predict the required surace area. Thistechnique has been used or a number o years to provide an approximate starting pointor design purposes and has given answers to 20%. For accurate designs, however, itis necessary to consult the manuacturer.

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hEaT rECOvEry duTiES

In any heat recovery application, it is always necessary to consider the savings inthe cost o heat against the cost o the heat exchanger and the pumping o luids. Eachcase must be treated individually since costs or heat, electricity, pumps, etc. will varyrom location to location.

One point is obvious. Any increase in heat recovery, and thus heat load, results in adecrease in LMTD and considering a constant heat transer coeicient, subsequentlyin the cost o the exchanger. This eect is tabulated in Figure 32. Because the cost oan exchanger increases considerably or relatively small gains in recovered heat abovethe 90% level, such applications even with the plate heat exchanger must be closely

studied to veriy economic gain. The economic break-even point is ar lower or a tubularexchanger. Situations where it is advantageous to go above 90% recovery usuallyinvolve duties where higher heat recovery reduces subsequent heating or cooling o theprocess stream. High steam or rerigeration costs, thereore, justiy these higher heatrecoveries.

As shown in Figure 33, the cost o increasing heat recovery rom 85% to 9O% at aconstant pressure loss o 12 lbs/in2 is $2600. From a practical standpoint, goingrom 90% to 95% requires a signiicantly higher pressure loss and nearly doubles theexchanger cost. However, even with this 95% heat recovery and assuming steam costs

at $6.00/1,000 lbs, payback on the plate heat exchanger would take 530 hours.Thus, the plate heat exchanger provides a most economic solution or recovering heat.This degree o heat recovery cannot be economically achieved in a tubular exchangersince the presence o crosslow and multipass on the tube side causes the LMTDcorrection actors to become very small, or alternately, requires more than one shell inseries. This is shown in the Figure 34 comparison.

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Figure 32. Effect of percentage heat recovery on PHE cost.

Heat 12 lb/in2 Pressure loss 25 lb/in2 Pressure loss

recovered Temp. Actual Area Actual Area(Btu/h) (%) (F) LMTD Factor MTD (t2) Price LMTD Factor MTD (t2) Price

3,600,000 60 200140 40 0.985 39.4 85 5,500 40 0.985 39.4 74 5,400

1601004,500,000 75 200125 25 0.975 24.4 197 6,450 25 0.980 24.5 172 6,250

175100

5,100,000 85 200115 15 0.965 14.4 425 8,400 15 0.965 14.4 362 7,850

185100

5,400,000 90 200110 10 0.95 9.5 739 11,000 10 0.92 9.2 629 10,100

190100

5,700,000 95 200105 5 5 0.83 4.16 1580 18,100

195100

hot liquid 60,000 lb/h of water at 200Fcold liquid 60,000 lb/h of water at 60F


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As detailed, this example illustrates that the plate heat exchanger has considerablethermal eiciency, and thereore, a price advantage over the tubular exchanger or aheat recovery o 70%. Since the overall heat transer coeicient and the eective meantemperature dierence both are much higher or the plate unit, reduced surace areais needed. Furthermore, because o crosslow temperature dierence problems in thetubular, three shells in series were needed to handle the duty within the surace areaquoted. Using only two shells would have resulted in a urther 40% increase in suracearea.

The small size o the plate heat exchanger also results in a saving o space and a lowerliquid hold up. For this type o heat recovery duty, a stainless steel plate heat exchangeralmost always will be less expensive than a mild steel tubular unit. Although the tubular

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Figure 33. Two shrouded paraflow units provide an optimum return on investment with

energy and cost saving regeneration of 88% and 81%.

Plate Tubular

Heat transfer area 4,820 13,100 ft2

Heat transfer coefficient (clean) 734 386 Btu/hr ft2 F

Heat transfer coefficient (dirty) 641 271 Btu/hr ft2 F

Effective mean temperature difference 10.0 8.7 F

Pressure drop: hot fluid/cold fluid 4.6/4.7 15/15 lb/in2

Fouling resistance: hot fluid/cold fluid 0.0001/0.0001 0.0005/0.0005 (Btu/hr ft2 F)-1

Pass arrangement 1/1 4 tube side baffled

3 shells in series

Approximate price $95,000 $135,000Stainless Steel Mild Steel

Duty: To heat 1,300,000 lb/h of water from 73F to 97F using 1,300,000 lb/h of water at 107F. Available pressure loss 15 lb/in2 for

both streams. Heat recovery = 70.5%. Number of heat transfer units (HTU) = 2.4.

This example demonstrates that quite high heat transfer coefficients can be obtained from a PHE with only a moderate pressure loss.

Figure 34. Heat recovery comparison between plate and tubular heat exchangers.


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exchanger physically will be capable o withstanding higher temperatures and pressures,there is a considerable (and or the most part unnecessary) penalty to pay or these

eatures both in price and size. For heat recovery duties in excess o 70%, the plate heatexchanger will become increasingly more economic than the tubular.

TyPiCal aPPliCaTiONS

One o the more common uses o regeneration is ound in many o the nations brewerieswhere it is necessary to cool huge amounts o hot wort beore it is discharged toermentation tanks. Typical in scope is an operation where 850 barrels per hour o wort(220,000 lbs/hr) are cooled rom 200F to 50F by means o 242,000 lbs/hr(R = 1.1:1) o water entering at 35F and being heated to 165F. The result: approximately33,000,000 Btu/hr are saved, there is an excellent water balance or use throughout thebrewery, and no water is discharged to the sewer.

For a dairy, regeneration usually involves the transer o heat rom pasteurized milk tocooler raw milk entering the system. Ater initially heating 100,000 lbs/hr o milk rom40F to 170F by high temperature, short time pasteurization, 90% regeneration permitscooling o the milk back down to 40F with a savings o 11,700,000 Btu/hr. Only 10%o the total heat or cooling must be supplied and no cooling medium such as city wateris used and discarded.

Chemically, there are many and varied regenerative applications. For desalination, APVhas supplied a number o paralows which are virtually perpetual motion machines.These units achieve 95% regeneration in heating 87,000 lbs/hr (175 GPM) rom 70Fto 197F while cooling a secondary stream lowing at 78,000 lbs/hr rom 214F to73F. Savings in the Btu load in this case are 11,050,000 per hour.

While low rates in hot oil applications are quite low in comparison, temperatures arevery high. It is possible to cool 400 GPH o vegetable oil rom 446F to 266F whileheating 400 GPH o oil rom 200F to 400F with 80% regeneration.

And in the production o caustic soda where very corrosive product streams are

encountered, paralows with nickel plates are being used to cool 10,000 lbs/hr o 72% NaOHrom 292F to 210F while heating 14,000 lbs/hr o 50% NaOH rom 120F to 169F.

dOllar SaviNgS

To examine a hypothetical case o Paralow regeneration rom the viewpoint o eiciencyand dollar savings, consider the ollowing process duty:

dt

Heat 100 GPM o luid #1 rom 40F to 190F while cooling 100 GPM o luid #2 rom

190F to 40F.Under ordinary conditions, steam required to heat luid #1 would be in the nature o7,500 lbs/hr with an equivalent cooling requirement or the second stream o 625 tons orerigeration. Using 90% regeneration, however, the energy needs are drastically reduced.

At this point in the process, luid #2 in a conventional system has been cooled to only90F by means o 85F city water and will require supplemental rerigeration or inalcooling to 40F. At the same time, luid #2 in an APV regenerative system has beencooled to 55F by means o 90% regeneration and must be cooled urther to 40F.

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dEfiNiTiON

The number o heat transer units (HTU) is deined as the temperature rise o luid one divided bythe mean temperature dierence. A urther term or the HTU is the temperature ratio (TR).

i.e. HTU = t1 t2/ MTD

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Fluid #1 Conventional 90% RegenerationHeat 100 GPM Heat 100 GPM40F190F 40F175F

by cooling fluid #2from 190F 55F

Steam 50,000 x 150 = 7,500 lbs/hr 50,000 x 15 = 750 lbs/hrrequired 1,000 1,000

to heat to 190F supplemental heat to raise temp to 190F

Assuming averagesteam cost: $8.00 per 1,000 lbs.steam cost/hr 7.5 x $8.00 = $60.00/hr 0.75 x $8.00 = $6.00/hrannual water cost $60 x 24 x 365 = $525,600 $6 x 24 x 365 = $52,560

flu #2 Conventonal 90% reeneaton Cool 100 GPM Cool 100 GPM190F 40F 190F 55F

by heating fluid #1from 40F 175F

Equivalent refrigeration 7,5000,000 = 625 tons 750,000 x 15 = 62.5 tonsrequired 12,000 12,000

to cool to 40F supplemental cooling to lower temp to 40F

Using available 85F 200 GPM to cool Nonecity water 190F90F

200 x 60 = 12,000 GPH None

Assuming averagewater cost: $0.07 per 1,000 galwater cost/hr 12 x 0.07 = $0.84/hr Noneannual water cost $0.84 x 24 x 365 = $7,360 None

duty Cool om Cool om90F to 40F 55F to 40F

Supplemental 50,000 x 50 = 208.3 tons 50,000 x 15 = 62.5 tonsrefrigeration required 12,000 12,000

Power required 208.3 x 1.2 x 0.746 = 186 KWH 62.5 x 1.2 x 0.746 = 56 KWH

Assuming average electrical cost 0.08/KWH

Annual supplemental 0.08 x 186 x 24 x 365 = $130,350 0.08 x 56 x 24 x 365 = $39,245refrigeration cost

Steam $525,600 $52,560 $473,040

Cooling Water $7,360 None $7,360

Refrigeration $130,350 $39,245 $91,105

$663,310 $91,805 $564,145annual savings

Onal Cost reeneatve reeneatve System Cost Savns

Note: Capital cost o a Paralow plate heat exchanger or the above duty would be approximately $30,000.


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aPPENdix

For heat transer in a typical PHE with turbulent low one can write the heat transerperormance in terms o a dimensionless Dittus Boelter type equation:

(A1) hDe/k = 0.28(GDe/)0.65 (Cr/k)0.4 (/w)0.14

For applications in turbulent low it is usually suiciently accurate to omit the SiederTate viscosity ratio and thereore the equation reduces to:

(A2) hDe/k = 0.28(GDe/)0.65 (Cr/k)0.4

The pressure drop can be predicted in a similar exchanger by equations (A3) and (A4).

(A3) F = 2.5(GDe/)-0.3

(A4)

P = 2G2L/grDeTo solve the above equations or a particular duty it is necessary to know G, L, De andit is shown below how these can be eliminated to produce a general equation.

For any plate:

(A5) G = m/A

where m is the total mass lowrate and A the total low area.

De is deined by APV as our times the low area in a plate divided by the wettedperimeter. Since the plate gap is small compared with the width then:

(A6) De = 4 A/wetted perimeterwhere h1 and h2 are calculated rom an empirical modiication o the constantsin equation (A8). The powers in equation (A8) are not modiied.

Since As = L x wetted perimeter where As is the total surace area:

(A7) L = As/4 A De

and it is possible to eliminate L rom equation (A4).

Similarly, by substituting G using equation (A5) in equations (A2), (A3) and (A4) andthen by rearranging the equations to eliminate A it is possible to arrive at equation

(A8) or the ilm heat transer coeicient.(A8) H = J2/J10.241 (m/As)0.0241 De-0.28

where J1 = 0.250.3/2grP

and J2 = 0.28(Cr/k)0.4 k-0.65

That is, the ilm heat transer coeicient is expressed only in terms o the surace areaand equivalent diameter. A computer program solves equation (A8) using a constantvalue o De and using an empirical actor Z to account or port pressure loss and otherdeviations rom equations (A2) to (A4). The equation is solved using the HTU approach

where: HTU = t1 t2 /MTD = UAs/m1c1

That is, the number o heat transer units (HTU) is the temperation rise o luiddivided by the mean temperature dierence.

U is deined as:

1/U = 1/h1 + 1/h2 + metal resistance

Note that h1 and h2 are calculated rom an empirical modiication o the constantsin equation (A8). The powers in equation (A8) are not modiied.

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ParaflOwS iN SEa waTEr COOliNg SySTEMS

The use o sea water as a cooling medium is ar rom new. It has been commonpractice with powered ships or many decades. It is a result o technical experiencesrom on board ships that many o the materials used in the handling o sea water andthe practices related to its use have been developed. On shore, the extensive use osea water is a somewhat later development, although it now is common practice atcoastal sites and becoming even more so with the increasing shortage o good qualityindustrial cooling water. The rapid development o the global oshore industry and oamphibious chemical, steel and power plants will lead to an even more intensive use o

this medium.

baSiC ESSENTialS Of a SEa waTEr COOlEr

Sea water is a very complex liquid. It can vary in cleanliness and salinity. In addition,it is an excellent nutrient medium supporting many orms o animal and plant lie.Regardless o variations, two common potential problems can arise in all coolingoperations. Corrosion and ouling, particularly biological ouling, aect both downtimeand maintenance costs. Thereore, it is essential that cooling equipment, which is

intended to employ sea water, should embody a number o basic eatures. Coolersshould be constructed rom corrosion-resistant materials. They should minimize deadareas, i.e., regions o low velocity which could permit organisms to settle and develop,or silt to deposit. They should also be designed to discourage surace ouling.

The traditional materials or handling sea water are copper alloys, such as aluminumbrass, and the cupronickels speciically developed or marine use. Even with marineapplications, there are disadvantages to these materials, principally that o beingerosion-prone. This limits the upper level o velocities that can be used in the design othe cooling equipment. Shipboard applications o sea water are relatively simple in that

the medium to be cooled usually is non-corrosive, being either a lubricating oil or cleanwater used in the jackets and pistons o the ships prime mover.

For other industries, particularly chemical, the cooling problem is more complex.It is requently ound that a more sophisticated material has to be employed, not onlyto combat the corrosivity o the sea water but also that o the medium which is beingcooled. The latter may be any o the wide range o process streams handled by amodern chemical plant. Typical o these materials are titanium, titanium/palladium alloy,Incoloy 825 and the various Hastelloys. O these, the one which appears to be gainingthe widest prominence is commercially pure titanium although this is by no means a

panacea and other materials oten are needed. Unortunately, metals or alloys which cansuccessully cope with this dual corrosion situation generally tend to be expensive. Thelist o attributes required o cooling equipment using sea water should include a highrate o heat transer in order to minimize the extent o usage o these high-cost alloys.

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One class o heat exchanger, the plate and rame type, in large measure possesses theserequirements since it is designed to avoid pockets and crevices. This automatically ulills

one o the criteria or its use with sea water and other biomedia.

PlaTE hEaT ExChaNgEr

In operation, the plate heat exchanger pack is tightly clamped between the head andollower, but the unit is easily dismantled or cleaning or maintenance. It also has theurther advantage o lexibility because a heat transer surace can either be added to ordeleted rom the original coniguration.

When the plate pack is assembled the geometry o the low channel on both sides o

the plates is identical. Thermal and pressure drop predictions can be made much moreaccurately, than or instance, in the case o shell and tube (S and T) heat exchangers.In tubular exchangers the shellside geometry, in particular, is so complex that largemargins o saety have to be allowed or the ilm coeicient on this side o the unit.Furthermore, geometrical similarity o the plate heat exchanger passages and equalaccess to both sides o the plates removes the tubular exchanger dilemma o whichshall be the shellside luid and which the tube side.

Without doubt, the most important eature o plate units in the context o this paper is

that the overall heat transer coeicient achieved on liquid-to-liquid heat transer dutiesis o two to three times that which can be achieved even in a well-designed tubularexchanger. The criterion o economy in the use o costly constructional materials isreadily met. Typical overall design coeicients or a sweet water/sea water PHE wouldbe in the range o 600 to 800 Btu/hr t2 F. Table 5 compares areas and pressuredrops or S and T, and PHE designs or typical industrial duties two o which are or asea water cooling unction. The S and T designs were prepared with the assistance othe HTRI (Heat Transer Research Inc.) S.T.3 program.

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Table 5. Comparison of areas and pressure drops (S and T versus PHE).

1 1

1 13,404 53,495 320A 120 301 0.4 3 93 0.3 2.2hydrocarbon water 120B 92

2 865,810 774,594 107A 82 12,110 19 19 3,633 11 8.7water sea water 99B 72

3 17,537 19,445 140A 104 1,830 2.9 44 446 2.9 3.6solvent water 95B 79

4 198,966 473,498 222A 100 1,502 10.9 8 380 4.4 10.2desalter effluent sea water 106B 68

Flow rate - lbs/hr Temperature Surace area (t2) and P (PSI)proile S&T PHE

Case Fluid Fluid F t 2 PSI PSI t2 PSI PSIA B A B A B


42/

apv plate hx

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