Heat Transfer

40 www.aiche.org/cep April 2007 CEP

Recognizing the need for a more effective procedurefor designing heat exchangers, Heat TransferResearch, Inc. (HTRI), a global research and

development consortium, organized the Exchanger DesignMargins Task Force (EDMTF) in 2005. Twenty-five com-panies — engineering contractors, heat exchanger fabrica-tors and processors (box, p. TK) — are currently repre-sented on the EDMTF. Its goal is to establish a consensusfor the definition and application of design margins. Thisconsensus is needed because design margins have tradi-tionally been concealed in fouling factors instead of beingexplicitly designated, thereby resulting in inconsistentmargin application and ambiguous design comparisons.

Defining “design margin”Heat exchanger design margin is defined as any heat

transfer area exceeding what is required by a clean heatexchanger to satisfy a specified duty, as defined by Eqs. 1–3:

In order of magnitude: Uclean ≥ Uactual ≥ Urequired. Modern heat exchanger design software calculates the

clean overall heat-transfer coefficient incrementally by:

where the subscript j denotes the variable value at a spe-cific increment, and Uclean,j, based on the clean outsidearea, excluding fouling resistances, is given by:

The logarithmic mean area for increment j is:

The actual overall heat-transfer coefficient is calculatedincrementally from:

For increment j, Uactual,j, the actual overall heat-transfercoefficient based on the clean outside area, including foul-ing resistances, is therefore:

Improving Heat Exchanger

DesignsChristopher A. BennettR. Stanley KistlerThomas G. LestinaHeat Transfer Research, Inc.

David C. KingBP p.l.c.

This article defines and explains the factors that affect heat exchangerdesign margins. With the proper application of design margins,

capital costs can be lowered and plant operation improved.

% Excess Area from Fouling = 100U

Uclean

actuaal

actual

requi

U

U

−⎛

⎝⎜⎞

⎠⎟( )

=

1 1

100% Overdesignrred

cleaU

−⎛

⎝⎜

⎞

⎠⎟ ( )

=

1 2

100% Total Excess Area nn

requiredU−

⎛

⎝⎜

⎞

⎠⎟ ( )1 3

UA

U Acleano total

clean j o jj

n

= ( )=∑

14

1,, ,

UA

U Aactualo total

actual j o jj

n

= ( )=∑

17

1,, ,

AA A

A

A

lm jo j i j

o j

i j

,, ,

,

,

ln

=−

⎛

⎝⎜

⎞

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( )6

1 1 1

U h

A

A

x

k

A

Aclean j o j

o j

lm j

w j

m j

o j

i j, ,

,

,

,

,

,

,

= + +hhi j,

5( )

Reprinted with permission from CEP (Chemical Engineering Progress), April 2007.Copyright © 2007 American Institute of Chemical Engineers (AIChE).

CEP April 2007 www.aiche.org/cep 41

The overall heat-transfer coefficient required by a spe-cific heat exchanger to satisfy the process conditions is:

The effective mean temperature difference (EMTD) is:

The logarithmic mean temperature difference overincrement j (LMTDj) is computed by applying the standardLMTD formula to the inlet and outlet temperatures of anincrement. The average total fouling resistance, based onthe clean outside area, is:

Reasons for adding a design marginMargins are added to heat exchangers during the design

stage to account for fouling, uncertainties in heat transfermethods and fluid properties, variable process or ambientconditions, lessons learned from previous experience, andrisks associated with an exchanger that does not meet theprocess requirements.

Fouling. Fouling is defined as a conductive resistancethat accumulates on the heat-transfer surface. It can lead toan unacceptable pressure drop.

A common misconception is that heat exchangersalways foul. A few streams that usually do not foul arelisted in Table 1a. Other streams, such as boiler feedwaterand cooling water, can be maintained relatively clean withproper attention. However, some process streams, such asthose listed in Table 1b, can foul heavily.

During heat exchanger design, fouling is traditionally

handled by adding fouling resistances, common-ly known as fouling factors, to the overall heat-

transfer resistance, as shown in Eq. 8. These fouling resist-ances can be obtained from multiple sources, includingcompany experience and the Tubular ExchangerManufacturers Association (TEMA) standards (1).

Fouling factors have become quite controversial inrecent years for many reasons. Published fouling resistanc-es often do not reflect true performance; for some servic-es, they are too high, while for others, they are too low.Fouling factors are static values, but some fouling mecha-nisms are dynamic. Temperature and velocity can greatlyinfluence fouling, but published fouling factors accountfor these effects in a limited manner, at best. Fouling fac-tors often implicitly account for uncertainty in the heattransfer methods, which can result in the duplication ofuncertainty effects.

Nomenclature

Ai,j = inside area of increment j, m2

Alm,j = logarithmic mean area of increment j, m2

Ao,j = outside area of increment j, m2

Ao,total = total outside heat-transfer area, m2

EMTD = effective mean temperature difference, Khi,j = inside heat-transfer coefficient of increment j,

W/m2-Kho,j = outside heat-transfer coefficient of increment j,

W/m2-Kj = counting variable, dimensionlesskm,j = metal thermal conductivity of increment j, W/m-KLMTD = logarithmic mean temperature difference, KLMTDj = logarithmic mean temperature difference of

increment j, Kn = number of increments, dimensionlessQj = calculated duty of increment j, WQspecified = specified duty, WQtotal = total calculated duty, WRf = total fouling resistance, m2-K/WRfi,j = inside fouling resistance of increment j, m2-K/WRfo,j = outside fouling resistance of increment j, m2-K/WUactual = overall heat-transfer coefficient, based on outside

area, including fouling resistance, W/m2-KUactual,j = overall heat-transfer coefficient, based on outside

area, including fouling resistance, of increment j,W/m2-K

Uclean = overall heat-transfer coefficient, based on outsidearea, excluding fouling resistance, W/m2-K

Uclean,j = overall heat-transfer coefficient, based on outsidearea, excluding fouling resistance, of increment j,W/m2-K

Urequired = overall heat-transfer coefficient, based on outsidearea, needed by a specific design to satisfyprocess specifications, W/m2-K

xw,j = wall thickness of increment j, m

UQ

A EMTDrequiredspecified

o total

= ( ),

9

1 110

1EMTD Q

Q

LMTDtotal

j

jj

n

= ( )=∑

Rn

RA

ARf fo j

o j

i jfi j

j

n

= +⎛

⎝⎜

⎞

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=∑

111

1,

,

,,

1 1

U hR

A

A

x

k

A

actual j o jfo j

o j

lm j

w j

m j

o

, ,,

,

,

,

,

= + + + ,,

,,

,

, ,

j

i jfi j

o j

i j i jAR

A

A h+ ( )1

8

Table 1. Fouling tendencies of common streams.

a. Streams that Typically Do Not FoulRefrigerantsDemineralized WaterNon-Polymerizing (Olefin-Free) Condensing GasesLiquid Natural Gas (LNG)

b. Streams that Typically Foul HeavilyCrude Oil Crude Oil Distillation OverheadAminesHydrogen Fluoride (HF)Coal GasificationImproperly Maintained Cooling Water

42 www.aiche.org/cep April 2007 CEP

The main reason fouling factors are so controversial,however, is that they can result in significant overdesign,resulting in the specification of an expensive heatexchanger with unnecessary area. For many applications,fouling factors should not contribute more than 20%excess area to the heat exchanger design.

Uncertainty. There are uncertainties associated with boththe fluid properties (2) and the methods used to determineheat-transfer coefficients (3). These uncertainties propagatethrough the computations (4) and result in a calculatedoverdesign that can deviate from the true value. This devia-tion will result in a heat exchanger that performs more orless efficiently than the computations say it should.

Literature values for heat-transfer coefficient uncertain-ties are given in Table 2 (4). Fluid property uncertaintiesvary dramatically depending upon the predictive methodused, and the reader is encouraged to peruse the literaturefor values (2). Uncertainty propagation is highly depend-ent on the heat transfer methods utilized, as well as therelative thermal resistances.

Variable process conditions. Process conditions canvary due to day-to-day changes in process operations andturn-up and turn-down conditions. Turn-up is particularlycritical at present, as existing plants are pushed harder togenerate additional revenue. Turn-up can result in exceed-ing the erosion velocity for the fluid/metal combinationand presents the potential for vibration damage to tubulardesigns. Because turn-down often results in lower veloci-ties, it can cause the exchanger to foul.

Variable ambient conditions. When one of the heatexchanger streams is influenced by ambient conditions, itresults in a variable EMTD, which affects the unit’s per-formance, particularly in such devices as air coolers andonce-through cooling water exchangers. Process reduc-tions occur when design temperature limits are exceeded.

Previous experience. One of the most frequently citedreasons for adding margin to a heat exchanger design is thatis how it has always been done. As the example coveredlater will demonstrate, this is not always the best practice.

While accounting for the performance of previouslydesigned units is important, it should not be the only fac-

tor considered. The heat exchanger designer should initial-ly consult the company’s design recommendations.Fouling factors and overdesign should then be assigned asthe field experience and anticipated turn-up dictate.

Risk. Heat exchangers are often intentionally oversizedbecause of perceived risk. Because certain heat exchangersare particularly essential to operations, the designer willintentionally overdesign a unit to ensure that it will satisfythe duty no matter what occurs during operation. What thedesigner needs to realize, however, is that excessiveoverdesign can actually cause fouling and other problemswith the exchanger and plant operations.

Problems with excessive design marginThe excessive use of design margin has several draw-

backs. Clearly, superfluous heat-transfer area translates direct-ly to unnecessary capital cost. Needless heat-transfer areaalso results in a larger, heavier exchanger; weight and foot-print are very important considerations for offshore applica-tions. Worst of all, excessive design margin can also result inaccelerated fouling — becoming a self-fulfilling prophesy.

Designers often incorporate excess margin by increasingthe shell diameter. This increases the cross-sectional areaavailable for flow, resulting in lower shellside velocities for

Heat Transfer

Table 2. Uncertainties in single-phase heat-transfercoefficient as a function of geometry (4).

Exchanger Uncertainty inGeometry Heat-Transfer Coefficient

Shell-and-TubeTubeside ±10%Shellside ±20%–50%

Plate-and-Frame ±10%–30%

Plate-Fin ±20%

Basic Design Algorithm

Detailed design algorithms for fouling mitigation and excessmargin reduction have been published elsewhere (5) and willnot be reiterated here. Instead, the following is the mostbasic of design algorithms.

1. Check company experience with the heat exchanger tobe designed.

2. Decide on fouling factors.a. If a stream is determined to be non-fouling, do not

use a fouling factor for that stream in Eq. 8.b. If a stream is known to foul, use a fouling factor in

Eq. 8 according to the company’s best practices.3. Place the most heavily fouling stream on the tubeside

to facilitate cleaning, if necessary, and to avoid the areas oflow velocity that occur on the shellside.

4. Design for high velocities within erosion and vibrationlimits. If possible,

a. Tubeside velocity should be ≥ 2 m/s.b. Shellside B-stream (the main crossflow stream

through the bundle) (6) velocity should be ≥ 0.6 m/s. Exceptions to this general high-velocity rule for fouling

mitigation include corrosion, geothermal brines, and slurriesthat present an erosion limit. Note the importance of metalselection on corrosion and erosion.

5. Keep overdesign (Eq. 2) between 0 and 20% whereindustry experience permits. Consider larger overdesigns fortubeside laminar flow, mist flow boiling, and shellside mixturecondensation in deep gravity flow.

CEP April 2007 www.aiche.org/cep 43

a given flowrate. Furthermore,the number of tubes increases,which reduces tubeside veloci-ty. Lower velocities oftenincrease the rate of fouling.Over-performance caused byexcess heat-transfer area canalso accelerate fouling becausethe process stream temperaturechange will be greater thandesired, requiring the flowrateof the utility stream to bereduced or other measures to betaken. Turn-down results in lowervelocities, which can initiate oraccelerate fouling.

ExampleTo illustrate the significant

impact that excess margin can haveon heat exchanger design, consider agas compression process of an off-shore facility as depicted schemati-cally in Figure 1. Production gas firstenters a partial condenser, where theheavy ends are condensed and the lightends cooled. The condensate is subsequent-ly removed in a separator. Then the gas iscompressed and enters another partial con-denser, where the remaining heavy ends arecondensed. Effluent from the second partialcondenser enters another separator and thenproceeds through a triethylene glycol (TEG)contactor to strip water from the processstream. The process stream proceedsthrough the final separator, is compressedagain, and is cooled in a final heat exchang-er before being metered and exported.

The final heat exchanger in the train isthe focus of this discussion because experi-ence has revealed that neither stream foulsunder normal operating conditions. Theshellside fluid is chlorine-treated once-through seawater and the tubeside fluid wasmodeled as supercritical methane.

Three configurations for this final heatexchanger were analyzed, and the salientdetails are presented in Table 3. The basecase is the one-shell-pass, four-tube-pass (1-4) CEU TEMA type that is currently inservice (illustrated in Figure 2). The cen-

� Figure 1. Gas compression process at an offshore facility analyzed in the example.

ProductionGas

CW

CW

Triethylene GlycolContactor

Compressor

Gas/LiquidSeparator

MeteringPackage

CompressorGas/LiquidSeparator

Gas/LiquidSeparator

HeatExchanger

CoolingWater(CW)

HeatExchanger

HeatExchanger

GasExport

Table 3. Salient details of the heat exchanger designs.

BFU with In-service Fouling

Parameter CEU Factors BFU

Heat-Transfer Area, m2 187 154 93.5

Tube Material Titanium Titanium Titanium

Relative Cost 1.5 1.3 1

Estimated Weight, kg 5,700 5,350 5,000

Estimated Footprint, m × m 0.81 × 7.0 0.78 × 5.9 0.78 × 4.6

Total Fouling Resistance, m2-K/W 0.000429 0.000429 0

Uclean, W/m2-K 1,540 1,510 1,510

Uactual, W/m2-K 928 917 1,510

Urequired, W/m2-K 748 757 1,250

Overdesign, % 24.1 21.1 21.0

Excess Area From Fouling, % 65.9 64.9 0

Total Excess Area, % 106 99.8 21.0

Qspecified, MW 4.29 4.29 4.29

EMTD, °C 30.7 36.8 36.8

B-Stream (6) Fraction 0.382 0.685 0.691

Tube-Side Velocity, m/s 2.95 2.88 2.89

B-Stream (6) Velocity, m/s 0.86 1.10 1.05

� Figure 2. One-shell-pass, four-tube-pass (1-4) CEU heat exchanger in service at the exampleoffshore facility.

44 www.aiche.org/cep April 2007 CEP

tral baffle spacing was 22.6% of the shell inside diameter,resulting in a low B-stream (6) fraction and inefficientheat transfer. Individual fouling factors of 0.000176 m2-K/W were used for both streams (which combined via Eq.11 to yield a total fouling resistance of 0.000429 m2-K/W). These specifications resulted in a heat-transfer arearequirement of 187 m2 and a total excess area of 106%.

The second heat exchanger design investigated was atwo-shell-pass, four-tube-pass (2-4) BFU TEMA type(Figure 3), with the same fouling factors (0.000176 m2-K/W) for both streams as in the CEU exchanger. A lower-cost front head (TEMA Type B) was used because foulingwill not be a problem, thereby negating the need for easyaccess to the tubesheet. An F-shell was selected to reducethe exchanger footprint and weight via increased EMTDacross the exchanger. U-tubes were chosen to prevent ther-mal expansion problems resulting from the large terminaltemperature difference of this exchanger. Titanium wasutilized to avoid corrosion problems.

This BFU configuration reduced the heat-transfer arearequirement by 18%, with the exchanger cost dropping con-comitantly. The total excess area isstill quite high at 99.8%, and thetotal excess area is not simply thesum of the overdesign and theexcess area from fouling factors;this demonstrates the compoundingof fouling factors in the overdesign.

The third exchanger design con-sidered was an identical BFUexcept that no fouling resistancewas used in Eq. 8. An equivalentoverdesign of 21% was achievedby shortening the tubes. Changingthe tube length is normally themost economical approach for

adjusting heat transfer area. Uclean, duty, EMTD, and thevelocities were effectively the same between the two BFUdesigns, confirming the comparability of this approach.

Comparing the BFU design with no fouling factorswith the other two designs reveals striking differences. Forexample, the heat transfer area is reduced to only 93.5 m2,resulting in an exchanger that is 23% less expensive thanthe BFU with fouling factors and 33% less expensive thanthe in-service CEU. Because no fouling factors were used,the overdesign and total excess area are identical at 21%, areasonable value that gives flexibility to the process. Theweight and footprint of this exchanger are also less thanthe other designs, which is an important consideration forthis offshore application. Because the in-service exchangerdoes not foul and this design has similar temperatures,velocities, and metallurgy, we are confident in theviability of this low-cost design.

Heat Transfer

Alfa Laval Lund ABAPV North America, Inc.BASF AktiengesellschaftBechtel Ltd.BP p.l.c.Celanese Ltd.Chevron Energy Technology Co.ConocoPhillips Co.Eastman Chemical Co.Ecodyne MRM, Inc.E.I. du Pont de Nemours & Company, Inc.ExxonMobil Research and Engineering Co.High Performance Tube, Inc.

Joseph Oat Corp.Kellogg Brown & Root, Inc.Koch Heat Transfer Company, L.P.Mitsubishi Chemical Engineering Corp.Nooter/Eriksen, Inc.Reliance Engineering Associates (P) Ltd.Shell Canada Ltd.Shell Global Solutions International B.V.Shell Global Solutions (U.S.), Inc.Statoil ASATechnipToyo Engineering Corp.

Companies Represented on the Exchanger Design Margins Task Force

Literature Cited

1. Tubular Exchanger Manufacturers Association,“Standards of the Tubular Exchanger ManufacturersAssociation,” 8th ed., TEMA, New York (1999).

2. Reid, R. C., J. M. Prausnitz and B. E. Poling, “TheProperties of Gases and Liquids,” 4th ed., McGraw-Hill,New York (1987).

3. Lestina, T., and K. Bell, “Thermal Performance Testing ofIndustrial Heat Exchangers,” Advances in Heat Transfer, 35,pp. 1–55 (2001).

4. American Society of Mechanical Engineers, “Single-PhaseHeat Exchangers,” ASME Performance Test Code 12.5,ASME, New York (2001).

5. Nesta, J., and C. A. Bennett, “Reduce Fouling in Shell-and-Tube Heat Exchangers,” Hydrocarbon Processing, 83 (7),pp. 77–82 (2004).

6. Palen, J. W., and J. Taborek, “Solution of Shellside FlowPressure Drop and Heat Transfer by Stream AnalysisMethod,” Chem. Eng. Progress Symposium Series, 65 (92),pp. 53–63 (1969).

� Figure 3. Two-shell-pass, four-tube-pass (2-4) BFU heatexchanger studied in the example.

CEP

CEP April 2007 www.aiche.org/cep 45

CHRISTOPHER A. BENNETT is a researcher specializing in fouling at HeatTransfer Research, Inc. (HTRI; 150 Venture Dr., College Station, TX,77845; Phone: (979) 690-5069; Fax: (979) 690-3250; E-mail:cab@htri.net). He holds a BS in chemical engineering from the Univ. ofToledo and an MS and PhD in chemical engineering from the Univ. ofMichigan. His diverse research experience in wet chemistry, surfacechemistry, materials science, mathematical modeling and aquaticbiology has proven very useful to understanding heat exchangerfouling. Bennett co-chairs the HTRI Exchanger Design Margins TaskForce and chairs the HTRI Crude Oil Fouling Task Force.

R. STANLEY KISTLER is vice president, research and softwaredevelopment at HTRI (Phone: (979) 690-5070; Fax: (979) 690-3250; E-mail: rsk@htri.net). He obtained his undergraduate and master’sdegrees, as well as his PhD in chemical engineering with an emphasison boiling, from the Univ. of Missouri-Rolla. Since joining HTRI in 1973,he has primarily focused on software development. He has alsoconducted experimental research on shellside single-phase flow.Kistler has helped develop many HTRI workshops and has taughtdozens of courses and workshops around the world. He also serves asa guest lecturer for academic courses and has been involved in variousengineering events in academia. An AIChE Fellow, Kistler is past chairof AIChE’s Heat Transfer and Energy Conversion division, and haschaired numerous sessions at National Heat Transfer Conferences. Heco-chairs the HTRI Exchanger Design Margins Task Force.

THOMAS G. LESTINA, P.E., vice president, engineering services at HTRI(Phone: (979) 690-5063; Fax: (979) 690-3250; E-mail: tgl@htri.net),has 20 years of engineering project management experience. As theperson responsible for HTRI training, he develops, customizes andteaches at HTRI events and member companies. In addition, he hasdeveloped and taught the course, Heat Exchanger Design andOperation for ASME/AIChE. He also manages HTRI’s growing contractservices and technical support. Prior to joining HTRI, he worked as alead engineer for MPR Associates, Inc. A licensed ProfessionalEngineer in Texas, Lestina earned a BS in mechanical engineering fromUnion College (Schenectady, NY) and an MS in mechanical engineeringfrom Rensselaer Polytechnic Institute (Troy, NY). He is a member ofASME and serves as chair of the technical committee for the ASMEPerformance Test Code 12.5, Single Phase Heat Exchangers.

DAVID C. KING is a senior heat transfer consultant for BP p.l.c (ChertseyRd., Sunbury-on-Thames, Middlesex, TW16 7LN, UK; Phone: +44-1932-775621; Fax: +44-1932-738414; E-mail: dave.king@uk.bp.com). With 32years of experience working in the refining, petrochemical, andexploration and production sectors, he is currently responsible fordeveloping and leading a global heat exchange community and forproviding leadership in heat transfer to the exploration and productionsector. He developed the case for establishing HTRI’s ExchangerDesign Margin Task Force and has actively participated in task forceactivities. He holds a BSc Honors in fuel and combustion engineeringfrom Leeds Univ.