Reprinted from CHEMICAL ENGINEERING PROGRESS, July 1992

Plate exchangersoffer high

heat transfer,compactness,

and cleanability.This overviewexplains what

these devices areand how to applythem to chemicalprocess industries

applications.

APV. Initially a number of cast gun metalplates enclosed in a frame similar to a filterpress, the plate-and-frame heat exchangerevolved into its current design with theintroduction in the 1930s of plates pressedin thin-gauge stainless steel.

As shown in Figure 1, today's plate-and-

Follower RollerTop Carrying Bar\.

decades, process heat transferhas been performed in the familiar

heat exchangers.Today, however, an increasing

number of heat transfer operations arebeing performed in plate-and-frame heatexchangers (often called simply plateexchangers).

Plate-and-frame heat exchang-ers are primarily applied to liquid-liquid heat-transfer duties. Theyare, however, increasingly beingused in condensing and boilingapplications, where their compactsize and thinner material require-ments for wetted parts offer advan-tages over other types of heat-transfer equipment.

Commercial development of theplate-and-frame heat exchangerwas prompted by two basic processobjectives -high rates of heattransfer at a low temperature differ-ential and cleanability with fullaccess to both sides of the heat-transfer surface. Both featuresremain important to users of heat-transfer equipment.

"0I:

W

I/"Tie Bar

James A. Carlson,APV Crepaco, Inc.

Bottom ICarrying Bar

Exchanger designThe original idea for the plate-

and-frame heat exchanger waspatented over a century ago, andthe first commercially successfuldesign was introduced in 1923 bythe Aluminium Plant and VesselCompany Ltd., now known as

Plate Pack/

Moveable

Follower

.Figure 1. A plate-and-frame heat exchanger, as the nameimplies, consists of a series of corrugated plates withina structural frame.

26 .JULY 1992 .CHEMICAL ENGINEERING PROGRESS

.Figure 2. The process and servicejluidjlow countercurrently between the plates.

frame heat exchanger consists of a framethat carries a series of closely spacedmetal plates that have been pressed with acorrugated trough or pattern. The plates,which are clamped between a fixed headand movable follower, have corner ponsto permit the passage of process and ser-vice liquids, with elastomeric gasketsaround the pons and plate edges to pre-vent leakage. The plates are grouped intopasses within the heat exchanger, and theproduct and service fluids flow counter-current to each other between the parallelpassages in each pass, as illustrated inFigure 2. The gasketing in the through-pon area of the plate provides a doubleseal between the fluid streams and pre-vents intermixing, as depicted in Figure3. The space between the seals is ventedto the atmosphere so that in the event ofleakage of either liquid there is an theescape path for the fluid as well as a visu-al indication of the leak.

A recent development is welded platepairs. As the name implies, pairs of platesare resistance seam or laser weldedtogether. This forms a metal seal on oneflow channel, and the seal between pairsis provided by an elastomer gasket.

Plate heat exchangers in a variety ofmetals and gasketing materials are avail-able to handle design pressures to 350psi. Units presently are available withtotal heat-transfer surface areas up to20,000 ft2 and with pons large enough tohandle flows up to 18,000 gal/min.

With the corrugated plate patternsinducing liquid turbulence at Reynolds

numbers as low as 150, overallheat-transfer coefficients as highas 1,500 Btu/h-ft2_0F may beachieved with a 20-psi pressuredrop. In contrast, conventionalexchangers require tube sideReynolds numbers of 2,000 orgreater to achieve turbulent flow.

The frame, The frame, whichconsists of the head, follower, topand bottom carrying bars, tie barsand nuts, and end support column,forms a rigid structure to hold theplates in alignment and maintainproper gasket compression. Abolted-frame construction is animportant consideration for instal-lation in areas with restrictedaccess and for future expansion ofthe unit in the field.

The head (or fixed cover) formsthe stationary end of the frame andgenerally contains all four prod-uct/service-media connections forsingle-pass design. The follower(or movable cover) forms themovable end, which compressesthe plate assembly (called the platepack) against the head. The fol-lower is suspended from the topcarrying bar and is guided by thebottom carrying bar. Liquid con-nections can be fitted to the fol-lower for multipass plate arrange-ment. Connections can be flangednozzles, threaded pipe, grooved. Figure 3. The gasketed plate provides apipe, or studded for direct flange double boundary between the fluids and isconnection. The carrying bars vented to the atmosphere.

CHEMICAL ENGINEERING PROGRESS. JULY 1992 .27

HEAT TRANSFER

locate and guide the platesand follower. Tie bars bearagainst opposite sides of thehead and follower to com-press the plate pack. The car-rying bars and tie bars aresized to accommodate thenumber of plates currentlyrequired. and frequentlyfuture expansion as well.

The platesThe plate pack -the

heart of a plate-and-frameheat exchanger -is com-pressed between the head andfollower to form the separateflow paths for the processand service fluids.

The corrugated plates aretypically formed from metal0.5-0.9 mm {0.020-0.036in.) thick. Proper plate designand material thickness aredetermined by the manufac-turer so that the plate packcan withstand the full designpressure.

Plate corrugation can beof many types. One patternknown as washboard corru-gation is illustrated in Figure4. This design consists oftroughs perpendicular to thedirection of the liquid flow.These troughs mate withthose of the adjacent plateb~t arthe kept apart by raised. Figure 4. Plates having a washboard corrugationrips .at contact. correspond- experience a ribbon flow path.mg pomts on adjacent plates,thus forming flow channels. !This results i~ a ribbon flow path. variable length options for a givenThe gap formmg the flow channel model. Flow channet gaps ontypically ranges from 0.150 in. to chevron plates typically range from0.390 in. depending on plate design. 0.0 0 in. to 0.250 in.

The most widely used plate pat- 0 provide a quantitative compar-tern is the chevron. This design is iso of these two basic corrugationbased on corrugations formed at an pat ems with each other and withangle to the liquid flow which, in tub lar exchangers, the term temper-combination with plates of opposite atu ratio, TR, is defined as:or different angles, make contact at ithe corrugation cross-over points to tR = LlTm/LMTDpermit flow between them. This pat-

Xtern forms a flow path broken into wh re LlT m is the larger temperaturemany high-turbulence helical cha ge of the two fluids involvedstreams. Figure 5 illustrates 50-deg. and LMTD is the log mean tempera-and O-deg. chevron angles (as mea- turel difference in the exchanger. (TRsured from horizontal) as well as the is a~so known as heat-transfer units,

HTU, or number of thermalunits, NTU.) For simplicity,only countercurrent flow will bediscussed here.

A single-pass conventionaltubular exchanger in countercur-rent flow service has a practicallength limit. This translates to amaximum TR capability ofabout 0.3 per shell. To meet agreater TR duty would, there-fore, require multiple shells inseries.

The washboard type of platecorrugation typically provides awider plate gap than the chevronstyle and has a TR capabilityranging from 0.6 to 2.0 per pass.In this case, TR capabilityincreases with decreasing flowgap, decreasing corrugationpitch, and increasing platelength. Washboard plates havefewer plate-to-plate contactpoints and, thus, require thickerand more costly plate materialto handle a given pressure com-pared to chevron plates.However, the wider gap andreduced number of contactpoints is often an advantagewhen handling debris-ladenstreams and slurries.

The chevron type of platecorrugation has a TR capabilityranging from 1.0 to 6.0 per pass;depending on plate gap, corru-gation angle, and length. Here,too, the TR capability increaseswith decreasing flow gap andwith increasing plate length, aswell as with decreasing corruga-

tion angle (as measured from hori-zontal). Combining different corru-gation angles with variable lengthplates allows the thermal and pres-sure-drop performance to be opti-mized to the required duty.

Most applications with clean flu-ids use chevron plates with flowchannel gaps in the lower half of therange (0.080 in. to 0.160 in.). At theupper end of the range (near 0.250in.), modem wide-gap chevron platesoften replace the traditional wash-board style plates, because chevronplates can achieve greater mechani-cal strength with reduced plate thick-ness and, in turn, a lower cost.

28 .JULY 1992 .CHEMICAL ENGINEERING PROGRESS

Either type of plate-and-frameexchanger has a higher TR capabilityper pass than a shell-and-tubeexchanger. Thus, for a given duty(TR requirement), the plate exchang-er's fewer passes means less overallpressure loss due to reduced entranceand exit .losses. Furthermore, plate-and-frame exchangers can also incor-porate multiple passes in full coun-tercurrent flow in a single frame.

In a plate-and-frame exchanger,the geometry of the flow channel isidentical on both sides of the platesand is formed to a close and consis-tent tolerance. Thermal and pressure-drop predictions, therefore, can bemade much more accurately than fortubular exchangers. In the 1atter,practical manufacturing toleranceson the shell side allow flow to bypassthrough the baffle tube holes andaround the baffle perimeter. Thesebypass streams reduce the film coef-ficient (and pressure drop) and alsothe temperature correction over thelength of the unit, thus requiringadditional heat-transfer surface tocorrect for these deficiencies.

Furthermore, in a plate exchanger thegeometric similarity of the flowchannels between the plates andequ~l access to both sides of theplates eliminates the tubularexchanger dilemma of determiningwhich fluid should be the shell sidefluid and which the tube side fluid toallow the unit to be cleaned.

Materials of construction

To provide corrosion resistance toa wide range of process and servicestreams, plates are available in suchmaterials as stainless steel, titanium,nickel, and nickel alloys (such asHastelloys, Incoloy, Inconel, andMonel).

CHEMICAL ENGINEERING PROGRESS. JULY 1992 .29

HEAT TRANSFER

Gaskets can be of variousmaterials, depending on the tem-perature and corrosivity of thefluids being handled. Table I listssome of the most common gasketmaterials and their applications.

For welded-plate-pair units,the number of gaskets is reducedby half. This limits gasket expo-sure to aggressive chemicals andhigh temperatures and, wherenecessary, allows the cost-effec-tive use of elastomers having bet-ter chemical resistance.

Advantages ofplate exchangers

The gasketed plate exchangeroffers six key advantages:

High heat transfer. Filmcoefficients three to five timeshigher and the lower thermalresistance of the plates (due tothinner material) combine to pro-vide high heat-transfer ratescompared to tubular or spiral-plate designs. In combination withfully countercurrent arrangement ofthe plates, this allows heat recoveryor regeneration of 90-95% in manyprocesses.

Compactness. As a direct resultof its high heat-transfer capability,the plate heat exchanger may be

or plant expansions or modifications.Welded plate exchangers do not

enjoy the same degree of accessibili-ty and flexibility as gasketed plateexchangers. Welded plate pair unitscan only be disassembled and recon-figured in pairs.

Economy. The high heat-transfercapability reduces surface arearequirements and, thus, the initialcost of the equipment. Additionalcost savings carry through as theresult of reduced installation space,ease of maintenance, high energyrecovery, and reduced service fluidrequirements.

installed in one-fourth to one-tenththe floor space required by otherty~es of heat-transfer equipment,often performing at a higher heatload.

Cleanability. The efficient use ofheat-transfer surface eliminates areasof little or no flow, thus preventingthe buildup of dirt or debris. ThisalsO allows tor effective "cleaning inplace" (CIP) to remove chemicalfilm or scaling deposits.

Accessibility. The gasketed plateexchanger provides full access toboth sides of the heat-transfer sur-face for inspection, maintenance, andcleaning in cases where the unit hasbe~n allowed to foul beyond thecapabilities of chemical cleaning.This access is readily accomplishedwithin the installed space of the unit.-

Flexibility. Many processes arenot at their optimum as designed andreq~ire equipment changes and mod-ifications after startup to achievema\l(imum throughput. This fine-tun-ing: is readily achieved with the plateheat exchanger by adding, removing,or rearranging plates as required tomeet actual process conditions.Similar adjustments can also bemade to accommodate future process

Exchanger sizingWhile generic programs for sizing

plate-and-frame heat exchangers areavailable through membership inprofessional groups involved in heat-transfer research, thermal sizing orrating is usually best done on a case-by-case basis by the manufacturer,who has full knowledge of the para-meters for the company's specificplates. To work most effectively withthe equipment manufacturer, the usershould provide the information out-lined in Table 2. mD

30 .JULY 1992 .CHEMICAL ENGINEERING PROGRESS

.CHEMICAL ENGINEERING PROGRESS. JULY 1992 .31