Baffels

CEP February 2012 www.aiche.org/cep 27

Heat Transfer

The first step in specifying a shell-and-tube heat exchanger is selecting the right shell, which was discussed in a previous CEP article (1). The next step

is determining the most effective baffle arrangement. Shell-and-tube heat exchangers employ baffles to trans-port heat to or from tubeside process fluids by directing the shellside fluid flow. The increased structural support that baffles provide is integral to tube stability, as they minimize both tube sagging due to structural weight and vibration due to cyclic flow forces. However, baffles improve heat transfer at the expense of increased total pressure drop. Baffles come in a range of shapes and sizes, the most common of which is the segmental baffle. The Tubular Exchanger Manufacturers Association, Inc. (TEMA) provides design guidelines for segmental baffles. Other, non-TEMA-type baffles include helical, disc-and-donut, and grid baffles. This article summarizes the performance characteristics of the different types of baffles and offers guidance on choosing effective baffles for shell-and-tube heat exchanger design.

Segmental baffle configurations Segmental baffles, often referred to simply as TEMA baffles, are circular plates with one or more segments removed to allow the shellside fluid to flow through an open area, or window. To prevent bundle flow bypass, sealing strips may be placed in notches along the edges of segmental baffles. Baffles may also have holes through which steel tie-rods can pass to provide increased structural support. TEMA baffles can be single- or multi-segmental, or tube support plates. Tube support plates are used in the no-tubes-in-window (NTIW) design to ensure that all baffles support every tube, eliminating tubes with long unsupported spans.

Figure 1 shows the most common types of TEMA baffles. Baffle spacing, cut, and orientation are key characteris-tics of TEMA baffle designs. Single-segmental baffles are used in many industrial heat exchangers because of their suitability for a wide range of applications. They operate well in single-phase processes, and crossflow heat transfer (across the tubes) is greater than the longitudinal heat transfer (through the windows). In addition, they are relatively easy to fabricate, so they are less expensive than other types of baffles. However, single-segmental baffles may not be effec-tive with very viscous fluids, where improperly mixed flow, bypass, and leakage streams reduce the efficiency of heat transfer. Furthermore, this configuration generates an undesir-

Baffles play a crucial role in regulating shellside fluid flow and improving heat transfer between shellside

and tubeside process fluids. Here’s how to choose the correct baffle to meet process requirements.

Salem BouhairieHeat Transfer Research, Inc.

Selecting Baffles for Shell-and-Tube

Heat Exchangers

p Figure 1. In exchangers with TEMA baffle types, smaller windows result in higher pressure drops.

Single-SegmentalHighest Pressure Drop, ΔPS

No-Tubes-In-WindowWide Spacing

Support Plate Baffle

ΔPD ≈ 0.33ΔPS – 0.5ΔPS

Double-Segmental

ΔPT ≈ 0.25ΔPS – 0.33ΔPS

Triple-Segmental

Copyright © 2012 American Institute of Chemical Engineers (AIChE)

28 www.aiche.org/cep February 2012 CEP

Heat Transfer

ably high pressure drop, especially with high-velocity flows. Double-segmental baffles split the flow so that it passes around center baffles and between wing baffles (Figure 2). In general, the center and wing baffles overlap by two to four tube rows. The window flow area outside the center baffle should generally equal the window flow area between the wing baffles. Pressure drop is one-third to one-half that in a shell with single-segmental baffles. However, this results in lower crossflow heat transfer — 60–90% of the heat transfer with single-segmental baffles at the same spacing and cut and the same total flowrate. Triple-segmental baffles have lower longitudinal-flow and crossflow velocities (whereas double-segmental baffles have only lower crossflow velocities) for a given baffle spac-ing. Triple-segmental baffles produce roughly one-fourth to one-third the pressure drop of single-segmental baffles in a comparably sized unit, and have heat-transfer rates that are as much as one-half lower (2). Triple-segmental baffles typically consist of three distinct baffle groups that create the equiva-lent of two double-segmental streams in parallel (Figure 3). No-tubes-in-window (NTIW) configurations provide sup-port for all of the tubes to mitigate tube vibration in the win-dow zone. Tube support plates are placed between widely spaced baffles. Because tubes cannot occupy the window spaces, larger shells are required to accommodate a specified tube count; this can be expensive for units operating at high shellside pressures. The lack of tubes in the window reduces pressure drop, while added support plates enhance crossflow. This results in better conversion of pressure drop to heat transfer than in exchangers with single-segmental baffles. The relative reduction in pressure drop depends on baffle cut, and the relative increase in heat transfer depends on the number of support plates added.

Baffle spacing Baffle spacing is the longitudinal distance between baf-fles. It controls the amount of effective heat transfer derived from the pressure drop within each compartment and affects

the potential for flow-induced vibration. The baffle spac-ing should be set such that the free-flow areas through the windows and across the tube bank are roughly equal. TEMA standards specify that the minimum spacing between segmental baffles should be the larger of one-fifth of the shell inside diameter or 51 mm (3). Spacing that is too small will result in higher pressure drop and poor bundle flow penetration — i.e., it increases the axial flow inertia through the outer leakage areas between the baffle and shell. Small baffle spacing also makes it difficult to mechanically clean the outsides of the tubes. Maximum spacing between segmental baffles (with tubes in window) should equal one-half the maximum unsupported span length. To enhance end-zone flow control and distribution, the baffles near the shell inlet and outlet should be located as close as practical to the shell nozzle. The distance between the first and second baffles should not be less than the central baffle spacing, as shellside flow tends to accelerate in the end zones. The optimum ratio of baffle spacing to shell inside diam-eter that results in the highest conversion of pressure drop to heat transfer is generally between 0.3 and 0.6 (4).

Baffle cut Baffle cut is the ratio of the baffle window height to the shell inside diameter. If the baffle cut is too small, the flow will jet through the window area and flow unevenly through the baffle compartment (Figure 4, left). If the baffle cut is too large, the flow will short-cut close to the baffle edge and avoid cross-mixing within the baffle compartment (Figure 4, right). A baffle cut that is either too large or too small can increase the potential for fouling in the shell. In both cases, recirculation zones of poorly mixed flow cause thermal maldistribution that reduces heat transfer. To divert as much heat-carrying flow across the tube bundle as possible, adjacent baffles should overlap by at least one tube row. This requires a baffle cut that is less than one-half of the shell inside diameter.

p Figure 3. In exchangers with triple-segmental baffles, larger window areas are responsible for lower total pressure drops.

p Figure 2. The window flow areas around the center and wing baffles in a double-segmental baffle arrangement should be roughly equal.

Center Baffle Wing Baffles Center Baffle(First Baffle Group)

Wing Baffles(Third Baffle Group)

Support Plates(Second Baffle Group)



Optimum baffle cuts are typically 25% of the shell inside diameter (Figure 5). However, for a single-segmental baffle configuration with low-pressure gas flows, a 40–45% baffle cut is common to limit pressure drop. For NTIW configurations, a 15% baffle cut is most com-mon. The ratio of the window velocity to crossflow velocity should be less than 3:1 for effective flow distribution.

Baffle orientation The orientation of TEMA baffles is particularly important for horizontal shell-and-tube heat exchangers, especially near the inlet and outlet nozzles. Baffle cuts for segmental baffles may be parallel or perpendicular to the nozzle axis, or inclined, as shown in Figure 6. The best baffle orientation depends on the baffle and shell type. Single-segmental baffles. For single-phase service, single-segmental baffles with a perpendicular baffle-cut orientation in an E- or J-shell are preferred to improve flow distribution in the inlet and outlet regions. With vertical inlet or outlet nozzles, parallel-cut baffles are preferred if the shellside process fluid condenses and needs a means of drainage. Parallel-cut baffles should also be used when the shellside fluid has the potential for particulate fouling, and in multipass F-, G-, or H-type shells to facilitate flow distribu-tion. (For an introduction to shell types, see Ref. 1.) How-ever, parallel-cut baffles have the potential for significant flow and temperature maldistribution in the end zones, which can induce local tube vibration and reduce the effective heat-

transfer rate in the inlet and outlet baffle spaces. Figure 7, obtained via computational fluid dynamics (CFD) modeling, illustrates this phenomenon. Double-segmental baffles. To distribute flow effectively in the inlet region with double-segmental baffles, a center baffle with a parallel-cut orientation is generally selected as the first baffle. The parallel baffle cut reduces the accumula-tion of deposits from high-fouling shellside fluids. It is good practice to locate the first baffle under the nozzle, where high flowrates can cause tube vibration. The first baffle is often shaped like a T to provide intermediate tube support where bundle entrance velocities have high kinetic energy (Figure 8, top). If perpendicular-cut double-segmental baffles are used with single inlet and outlet nozzles, thermally ineffective areas will form in the end zones (Figure 8, middle). Better end-zone distribution can be achieved with two inlet and two outlet nozzles, plus wing baffles in the end zones to maintain flow symmetry upon entry and exit (Figure 8,

p Figure 4. If the baffle cut is too small (left) or too large (right), fouling can occur in the shaded areas due to uneven flow distribution.

p Figure 5. The optimum baffle cut is 25% of the shell inside diameter.

hw

Baffle Cut = hw/d

d

p Figure 6. Baffle orientation is referenced with respect to the nozzle axis, and can be parallel, perpendicular, or inclined.

Parallel-CutBaffle

Perpendicular-CutBaffle

Nozzle Axis

Inclined-CutBaffle

p Figure 7. Although preferred for certain shellside fluids, parallel-cut single-segmental baffles can cause uneven flow in the inlet and outlet regions.

Window

12 m/s(39.4 ft/s)

Parallel Perpendicular



Heat Transfer

bottom). In effect, the performance of perpendicular-cut double-segmental baffles depends on the number of nozzles. Triple-segmental baffles. The triple-segmental baffle set shown in Figure 3 has five different components and is one of several possible arrangements. Other designs, which are not discussed here, have six pieces or three pieces. The permutations complicate the determination of baffle orienta-tion, particularly in the inlet and outlet zones. Orientation of triple-segemental baffles has not been studied extensively, and general guidelines have not been developed.

Non-TEMA baffle types Most non-TEMA-type baffles usually produce lower pressure drops and have better flow and heat-transfer distri-butions. The improvements stem from the baffles generating crossflow through swirling, maximizing longitudinal flow, or increasing symmetrical flow and heat-transfer distribu-tion. Helical, disc-and-donut, and grid baffles are the most common non-TEMA-type baffles.

Helical baffles Helical baffles promote swirling flow, which helps to alleviate bypass and stagnant flow areas that can occur with conventional segmental baffles. They are effective for low- to high-viscosity fluids, and they are commonly used in oil-refinery and refrigeration applications. Heat exchangers with helical baffles may experience less shellside fouling than exchangers with segmental baffles. Helical baffles are subject to bundle-to-shell bypass at very high mass flow-rates. Unlike segmental baffles, helical baffles do not seem

to benefit from added seal strips to block bypass, as the strips only increase pressure drop. Helical baffles can be continuous spiral assemblies. However, these are not common because they are difficult (and expensive) to fabricate. Instead, most helical-baffled heat exchangers use baffles that are inclined at an angle from a transverse plane perpendicular to the shell axis (Figures 9 and 10). These quadrant baffles (each of which occupies one-fourth of the shell cross-section) touch each other at crossover points that define a cross-fraction. The baffle cross-fraction is the ratio of the distance from the center of the shell to the crossover point divided by the shell radius (Figure 10). Helical baffles can cross near their midpoint (a cross-fraction of 50%), tip-to-tip (a cross-fraction of 100%), or at a point

within this range. Depending on user and fabricator prefer-ences, the cross-fraction selected may range from 20% to 100%. Reducing the cross-fraction (increasing the overlap) enhances tube support and protects against vibration, but at the expense of increased pressure drop. In general, helical-

p Figure 8. Double-segmental baffle configurations should use a T-shaped first plate with a parallel-cut orientation. Perpendicular-cut orientation should be used only with double inlet and outlet nozzles.

Parallel Cut (End View) T-Baffle (End View)

Intermediate Vibration Support

Ineffective Region

Perpendicular Cut (End View)

Perpendicular Cut (End View)

p Figure 9. Helical baffles promote swirling, which helps to reduce bypass and stagnant flow.

p Figure 10. Helical quadrant baffles touch at crossover points, which define the cross-fraction.

BBrco

D

ØS

D

A A

CC

(End View) (Elevation View)

Baffle CrossoverPoints

Cross-fraction = rco/rr = shell radius rco = radial distance from shell axis to baffle crossover pointØS= baffle angle relative to transverse plane through shell

r



baffled exchangers have less potential for tube vibration because the tubes are well supported by the quadrant baffles. Helical baffles come in single- or double-helix configu-rations. Many industrial applications use 12-deg. to 15-deg. angled baffles. Larger baffle angles result in lower pressure drop and increased longitudinal flow relative to crossflow. Some studies have reported that baffle angles of 25 deg. to 40 deg. produce optimal conversion of pressure drop to heat transfer (5). The quadrant baffles induce flow that combines spiral crossflow, longitudinal flow, and bypass flow (Figure 11). In reality, this flow is far from an ideal helix — it undergoes fewer revolutions than the number of baffle revolutions through the length of the exchanger. The design of these baffles requires knowledge of both the baffle angle and the actual flow angle (i.e., the direction of the resulting vector of the three principal flow velocity components (x, y, and z, or axial, radial, and tangential), relative to a plane trans-verse to the exchanger axis).

Disc-and-donut baffles Disc-and-donut baffles generate radially symmetric flow in both the crossflow and longitudinal flow directions — the flow expands around the disc baffle and contracts through the donut baffle (Figure 12). A step change in both pressure drop and temperature occurs between consecutive pairs of disc baffles and donut baffles. The main thermally effective crossflow stream can occupy up to 80% of a baffle compartment, minimizing the bypass flow around the outer tubes (6). The driving forces for bypass and leakage streams in an exchanger with disc-

and-donut baffles are smaller than those in exchangers with segmental baffles. Disc-and-donut baffles are often installed in NTIW arrangements. Radial tube layouts are preferred over con-ventional triangular or square layouts, because the result-ing radial flow distribution produces uniform heat transfer throughout the tube bundle cross-section. Disc-and-donut baffles are most effective in shellside vapor environments, and are commonly selected for gas-gas applications where vibration can be a problem.

Grid baffles Grid baffles are metal lattices that generate primarily longitudinal flow. They produce low pressure drops, which results in high heat-transfer-to-pressure-drop ratios and protects against tube vibration. In addition, shellside flow distribution is uniform, which is particularly important for shellside vaporization because it eliminates vapor pockets that can cause pitting of tubes and baffles (7). The most common generic grid baffle designs are rod-type baffles and strip baffles. Each grid type has a charac-teristic baffle flow-contraction ratio, which is defined as the free flow area through the baffle divided by the free flow area through the bundle between baffles. This parameter ranges from zero to one, with practical values of about 0.2 for high contraction and 0.7 for low contraction. The grid baffles act as strainers on the bundle free-flow area, locally contracting and accelerating the heat-carrying flow longitu-dinally along the tubes. The higher the contraction (i.e., the lower the contraction ratio), the higher the pressure drop. An exchanger design with a low contraction ratio, therefore, requires more pumping power than one with a high contrac-tion ratio. Rod-type baffles are used in such applications as over-head condensers, gas coolers and heaters, feed and effluent exchangers, and kettle reboilers. They consist of rods laid out in a grid pattern that provide a supporting structure for the heat exchanger tubes and basic structural rigidity. The

p Figure 12. Disc-and-donut baffles distribute flow in a radially symmetric manner.

p Figure 11. The flow through a helical-baffled exchanger includes spiral crossflow, longitudinal flow, and bypass flow.

X

Z

Y

Spiral Crossflow

Longitudinal Flowwithin Tube Bundle

Bypass FlowOutside Tube Bundle

Flow Angle

Baffle Angle

ZZ

Radially ContractingCrossflow through

Donut BaffleRadially ExpandingCrossflow around

Disc Baffle



Heat Transfer

longitudinal flow friction effectively generates heat transfer, especially in exchangers with long tubes. Rod-type baffles are made by welding round rods to a supporting ring (Figure 13), which also serves as a seal to prevent leakage flows. The rods are often located after every second tube row, with consecutive baffles assembled at 90-deg. angles. Thus, they are generally limited to square tube layouts. It is possible to corrugate the rods to support a triangular tube layout (known as a triangular-grid baffle). Triangular-grid baffles permit higher tube densities, produce higher turbulence, and generate higher heat transfer. However, mechanical cleaning of the tubes is more difficult due to lim-ited access lanes. Therefore, triangular layouts are appropri-ate for shellside services that use chemical cleaning. In the bundle shown in Figure 13, four longitudinal tie bars are placed around the supporting ring to hold the baffles in place and maintain the proper baffle spacing. Rods with as small a diameter as possible are preferred, to permit a higher tube density and hence higher heat-transfer rate. The baffle contraction ratio for rod-type baffles is about 0.55–0.65 (7). Some industrial applications benefit from combin-ing rod-type baffles with segmental baffles. Figure 14 shows two rod-type baffles fitted within the space between single-segmental baffles. This baffle combination provides increased tube support for vibration protection, without

increasing pressure drop significantly. Strip baffles (Figure 15) are grid baffles formed from flat strips that are placed in a crisscross pattern, with a strip after every tube row in both directions. The overall structure is welded to a ring for rigidity and ease of assembly. The strips are notched to lock the tubes in place. Figure 15 shows square-layout strip baffles. Strip baffles can also accommodate 30-deg. triangular layouts. For a given tube pitch ratio (i.e., the spacing between tubes divided by the tube outside diameter), 30-deg. layouts have the highest critical velocities prior to fluidelastic instability, so tube vibration potential is minimized. With a baffle contraction ratio of approximately 0.2–0.25, strip baffles produce higher pressure drops per baffle than rod-type baffles (7).

Closing thoughts Baffling is the most crucial shellside consideration in shell-and-tube heat exchanger design, because baffles regulate shellside fluid flow and improve heat transfer while offering significant tube support. Although TEMA baffles are easier to fabricate, they usually have higher pressure drops than non-TEMA-type baffles. It is equally important to consider how baffle selection affects other shellside parameters, such as tube pitch ratio, tube layout pattern, tube size, shell type, and shell diameter. A basic understand-ing of the various baffle types and their advantages and disadvantages (Table 1) is essential to choosing an effective baffle configuration.

p Figure 13. Rod-type baffles support the tubes and provide structural rigidity.

X Z

Y

p Figure 14. Rod-type baffles may be combined with segmental baffles for better tube support.

Segmental Baffle

2 Rod-Type Baffles per Baffle Space

p Figure 15. Strip baffles have lower baffle contraction ratios and higher pressure drops than rod-type baffles.

X Z

Y

SAlEM BouHAiriE is a research engineer at Heat Transfer Research, Inc. (HTRI) (Email: [email protected]), where he conducts computational fluid dynamics (CFD) simulations and physical experiments for projects and contracts. He teaches workshops on heat exchanger vibration analysis and conducts webinars on heat exchanger design. Prior to joining HTRI, he worked at Northwest Hydraulic Consultants in Edmonton, Alberta, Canada, where he conducted hydraulic structure modeling investiga-tions and river hydrology assessments. He has delivered presentations on his work in Canada, the U.S., Brazil, Thailand, and Korea, and has published research in the Journal of Fluid Mechanics and the Journal of Hydro-environment Research. Bouhairie earned his BEng, MEng, and PhD in civil engineering from McGill Univ. in Montreal, Quebec, Canada.

CEP



Table 1. Each baffle type has advantages and disadvantages that make it suitable for different applications.

Baffle Type Advantages Disadvantages/Limitations

TEM

A-T

ype

Baf

fles

Single-Segmental Highest potential heat-transfer rates

Easiest to fabricate

Least expensive

Highest potential pressure drop

Cannot be used for very viscous fluids

Double-Segmental Lower pressure drop than with single-segmental baffles

Lower heat-transfer rates than with single- segmental baffles

Triple-Segmental Lower pressure drop than with double-segmental baffles

Lower heat-transfer rates than with double- segmental baffles

No-Tubes-in-Window Configuration

All tubes are supported, eliminating tube vibration

Higher conversion of pressure drop to shellside heat transfer than single-segmental baffles

Requires a smaller tube bundle and/or larger shell; a larger shell makes this configuration more expensive

Non

-TE

MA

-Typ

e B

affle

s

Helical Less shellside fouling

Moderate heat-transfer rates and pressure drops

Minimizes or eliminates areas of stagnant flow

Minimizes or eliminates tube vibration

Difficult fabrication, design methods are not standardized

Significant bundle-to-shell bypass at high mass flowrates

Disc-and-Donut Radially symmetric flow distribution

Minimizes bypass flow

Same pressure drop as with double-segmental baffles, with better heat transfer

Well suited for gas-gas applications

More expensive than traditional double-segmental baffles

Preferred radial tube layout requires a less- common fabrication method than triangular and square layouts

In a radial tube layout, the angular gaps between tubes near the shell are larger than those between tubes near the center; this requires the addition of an improvised, nonradial (e.g., triangular or rotated square) layout between the radial tube rows

Grid Provides tube support

Uniform flow distribution

Relatively low pressure drops

High conversion ratio of pressure drop to heat transfer

Relatively low heat-transfer rates, unless the tubes are long

Specific tube layouts are required

Literature Cited 1. Lestina, T. G., “Selecting a Heat Exchanger Shell,” Chem. Eng.

Progress, 107 (6), pp. 34–38 (June 2011).2. Green, D. W., and R. H. Perry, “Heat-Transfer Equipment” in

“Perry’s Chemical Engineers’ Handbook,” 8th ed., McGraw-Hill, New York, NY (2008).

3. Tubular Exchanger Manufacturers Association, “Standards of the Tubular Exchanger Manufacturers Association,” 9th ed., TEMA, New York, NY (2007).

4. Mukerjee, R., “Don’t Let Baffling Baffle You,” Chem. Eng. Progress, 92 (4), pp. 72–79 (Apr. 1996).

5. Lutcha, J., and J. Nemcansky, “Performance Improvement of Tubular Heat Exchangers by Helical Baffles,” Trans. IChemE., 68 (Part A), pp. 263–270 (1990).

6. Taborek, J., “Pressure Drop to Heat Transfer Conversion in Shell-and-Tube Heat Exchangers with Disk-and-Donut Baffles,” AIChE Spring Meeting, New Orleans, LA (2004).

7. Taborek, J., “Longitudinal Flow in Tube Bundles with Grid Baffles,” in “Heat Exchanger Design Handbook — Part 3, Ther-mal and Hydraulic Design of Heat Exchangers, Section 3.3.12,” Begell House, New York, NY (1998).

Further ReadingBell, K. J., and A. C. Mueller, “Wolverine Engineering Data Book

II,” available at www.wlv.com/products/databook/databook.pdf, Wolverine Tube, Inc., Decatur, AL (2001).

Hewitt, G. F., et al., “Process Heat Transfer,” CRC Press, Boca Raton, FL (1994).

Hewitt, G. F., ed., “Heat Exchanger Design Handbook,” Begell House, New York, NY (1998).

Kakac, S., and H. Liu, “Heat Exchangers: Selection, Rating, and Thermal Design,” 2nd ed., CRC Press, Boca Raton, FL (2002).

Kern, D., “Process Heat Transfer,” McGraw-Hill, New York, NY (1950).

Rohsenow, W., et al., “Handbook of Heat Transfer,” 3rd ed., McGraw-Hill, New York, NY (1998).

Serth, R. W., “Process Heat Transfer: Principles and Applications,” Elsevier, New York, NY (2007).

Thome, J. R., “Wolverine Engineering Data Book III,” available at www.wlv.com/products/databook/db3/DataBookIII.pdf, Wolver-ine Tube, Inc., Decatur, AL (2004–2010).

Webb, R., “Principles of Enhanced Heat Transfer,” 2nd ed., John Wiley & Sons, Inc., Hoboken, NJ (2005).


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