Heat Transfer
Selecting Baffles for Shell-and-Tube Heat Excha Exchangers ngers Salem Bouhairie
Heat Transfer Research, Inc.
Baffles play a crucial role in regulating shellside fluid flow and improving heat transfer between shellside and tubeside process fluids. Here’s Here’s how to choose the correct baffle to meet process requirements.
T
he rst step in specifying a shell-and-tube heat exchanger is selecting the right shell, which was discussed in a previous CEP article article (1 (1). The next step is determining the most effective bafe arrangement. Shell-and-tube heat exchangers employ bafes to trans port heat heat to to or from from tubesi tubeside de process process uids by directi directing ng the the shellside uid ow. The increased structural support that bafes bafes provide provide is is integral integral to to tube stability stability,, as they they minimi minimize ze both tube tube saggin sagging g due to structu structural ral weight weight and vibrat vibration ion due due to cyclic ow forces. However, bafes improve heat transfer at the expense of increased total pressure drop. Bafes come in a range of shapes and sizes, the most common of which is the segmental bafe. The Tubular Exchanger Manufacturers Association, Inc. (TEMA) provides design guidelines for segmental bafes. Other, non-TEMAtype bafes include helical, disc-and-donut, and grid bafes. This article summarizes the performance characteristics of the different types of bafes and offers guidance on choosing effective bafes for shell-and-tube heat exchanger design.
Segmental baffle configurations Segmental bafes, often referred to simply as TEMA bafes, bafes, are circular circular plates plates with one or or more more segments segments removed to allow the shellside uid to ow through an open area, or window. To To prevent bundle ow bypass, sealing strips may be placed in notches along the edges of segmental bafes. bafes. Bafes Bafes may may also have holes through through which which steel steel tie-rods can pass to provide increased structural support. TEMA bafes can be single- or multi-segmental, or tube support plates. Tube support plates are used in the no-tubesin-window (NTIW) design to ensure that all bafes support every tube, eliminating tubes with long unsupported spans. Copyright © 2012 American Institute of Chemical Engineers (AIChE)
Figure 1 shows the most common types of TEMA bafes. Bafe spacing, cut, and orientation are key characteristics of TEMA bafe designs. Single-segmental bafes are bafes 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 crossow 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 bafes. However, single-segmental bafes may not be effective with very viscous uids, where improperly mixed ow, bypass, bypass, and and leaka leakage ge strea streams ms reduc reducee the the efcie efciency ncy of heat heat transfer. Furthermore, this conguration generates an undesir-
Single-Segmental Highest Pressure Drop, ΔPS
Double-Segmental ΔPD ≈ 0.33Δ 0.33ΔPS – 0.5Δ 0.5ΔPS
Support Plate
Triple-Segmental 0.25ΔPS – 0.33Δ 0.33ΔPS ΔPT ≈ 0.25Δ
Baffle
No-Tubes-In-Window Wide Spacing
In exchangers with TEMA baffle types, smaller windows result in higher pressure drops. p Figure 1.
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Heat Transfer
Center Baffle
Wing Baffles
The window flow areas around the center and wing baffles in a double-segmental baffle arrangement should be roughly equal.
Center Baffle (First Baffle Group)
Support Plates (Second Baffle Group)
Wing Baffles (Third Baffle Group)
p Figure 2.
In exchangers with triple-segmental baffles, larger window areas are responsible for lower total pressure drops. p Figure 3.
ably high pressure drop, especially with high-velocity ows. Double-segmental bafes split the ow so that it passes around center bafes and between wing bafes (Figure 2). In general, the center and wing bafes overlap by two to four tube rows. The window ow area outside the center bafe should generally equal the window ow area between the wing bafes. Pressure drop is one-third to one-half that in a shell with single-segmental bafes. However, this results in lower crossow heat transfer — 60–90% of the heat transfer with single-segmental bafes at the same spacing and cut and the same total owrate. Triple-segmental bafes have lower longitudinal-ow and crossow velocities (whereas double-segmental bafes have only lower crossow velocities) for a given bafe spacing. Triple-segmental bafes produce roughly one-fourth to one-third the pressure drop of single-segmental bafes in a comparably sized unit, and have heat-transfer rates that are as much as one-half lower (2). Triple-segmental bafes typically consist of three distinct bafe groups that create the equivalent of two double-segmental streams in parallel (Figure 3). No-tubes-in-window (NTIW) congurations provide sup port for all of the tubes to mitigate tube vibration in the window zone. Tube support plates are placed between widely spaced bafes. Because tubes cannot occupy the window spaces, larger shells are required to accommodate a specied 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 crossow. This results in better conversion of pressure drop to heat transfer than in exchangers with single-segmental bafes. The relative reduction in pressure drop depends on bafe cut, and the relative increase in heat transfer depends on the number of support plates added.
the potential for ow-induced vibration. The bafe spacing should be set such that the free-ow areas through the windows and across the tube bank are roughly equal. TEMA standards specify that the minimum spacing between segmental bafes should be the larger of one-fth of the shell inside diameter or 51 mm (3). Spacing that is too small will result in higher pressure drop and poor bundle ow penetration — i.e., it increases the axial ow inertia through the outer leakage areas between the bafe and shell. Small bafe spacing also makes it difcult to mechanically clean the outsides of the tubes. Maximum spacing between segmental bafes (with tubes in window) should equal one-half the maximum unsupported span length. To enhance end-zone ow control and distribution, the bafes near the shell inlet and outlet should be located as close as practical to the shell nozzle. The distance between the rst and second bafes should not be less than the central bafe spacing, as shellside ow tends to accelerate in the end zones. The optimum ratio of bafe spacing to shell inside diameter that results in the highest conversion of pressure drop to heat transfer is generally between 0.3 and 0.6 (4).
Baffle cut
Bafe spacing is the longitudinal distance between bafes. It controls the amount of effective heat transfer derived from the pressure drop within each compartment and affects
Bafe cut is the ratio of the bafe window height to the shell inside diameter. If the bafe cut is too small, the ow will jet through the window area and ow unevenly through the bafe compartment (Figure 4, left). If the bafe cut is too large, the ow will short-cut close to the bafe edge and avoid cross-mixing within the bafe compartment (Figure 4, right). A bafe 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 ow cause thermal maldistribution that reduces heat transfer. To divert as much heat-carrying ow across the tube bundle as possible, adjacent bafes should overlap by at least one tube row. This requires a bafe cut that is less than one-half of the shell inside diameter.
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Copyright © 2012 American Institute of Chemical Engineers (AIChE)
Baffle spacing
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Nozzle Axis
If the baffle cut is too small (left) or too l arge (right), fouling can occur in the shaded areas due to uneven flow distribution. p Figure 4.
Parallel-Cut Baffle
Perpendicular-Cut Baffle
Inclined-Cut Baffle
Baffle orientation is referenced with respect to the nozzle axis, and can be parallel, perpendicular, or inclined. p Figure 6.
12 m/s (39.4 ft/s) d
Window hw
Baffle Cut = hw / d
p Figure 5.
The optimum baffle cut is 25% of the shell inside diameter.
Optimum bafe cuts are typically 25% of the shell inside diameter (Figure 5). However, for a single-segmental bafe conguration with low-pressure gas ows, a 40–45% bafe cut is common to limit pressure drop. For NTIW congurations, a 15% bafe cut is most common. The ratio of the window velocity to crossow velocity should be less than 3:1 for effective ow distribution.
Parallel
Perpendicular
p Figure 7. Although
preferred for certain shellside fluids, parallel-cut single-segmental baffles can cause uneven flow in the inlet and outlet regions.
Baffle orientation The orientation of TEMA bafes is particularly important for horizontal shell-and-tube heat exchangers, especially near the inlet and outlet nozzles. Bafe cuts for segmental bafes may be parallel or perpendicular to the nozzle axis, or inclined, as shown in Figure 6. The best bafe orientation depends on the bafe and shell type. Single-segmental bafes. For single-phase service, single-segmental bafes with a perpendicular bafe-cut orientation in an E- or J-shell are preferred to improve ow distribution in the inlet and outlet regions. With vertical inlet or outlet nozzles, parallel-cut bafes are preferred if the shellside process uid condenses and needs a means of drainage. Parallel-cut bafes should also be used when the shellside uid has the potential for particulate fouling, and in multipass F-, G-, or H-type shells to facilitate ow distribution. (For an introduction to shell types, see Ref. 1.) However, parallel-cut bafes have the potential for signicant ow and temperature maldistribution in the end zones, which can induce local tube vibration and reduce the effective heat-
Copyright © 2012 American Institute of Chemical Engineers (AIChE)
transfer rate in the inlet and outlet bafe spaces. Figure 7, obtained via computational uid dynamics (CFD) modeling, illustrates this phenomenon. Double-segmental bafes. To distribute ow effectively in the inlet region with double-segmental bafes, a center bafe with a parallel-cut orientation is generally selected as the rst bafe. The parallel bafe cut reduces the accumulation of deposits from high-fouling shellside uids. It is good practice to locate the rst bafe under the nozzle, where high owrates can cause tube vibration. The rst bafe 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 bafes 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 bafes in the end zones to maintain ow symmetry upon entry and exit (Figure 8,
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Heat Transfer
to benet from added seal strips to block bypass, as the strips only increase pressure drop. Helical bafes can be continuous spiral assemblies. However, these are not common Intermediate Vibration Support because they are difcult (and expensive) to Perpendicular Cut (End View) fabricate. Instead, most helical-bafed heat exchangers use bafes that are inclined at an angle from a transverse plane perpendicular Ineffective Region to the shell axis (Figures 9 and 10). These quadrant bafes (each of which occupies Perpendicular Cut (End View) one-fourth of the shell cross-section) touch each other at crossover points that dene a cross-fraction. The bafe 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). p Figure 8. Double-segmental baffle configurations should use a T-shaped first plate with a Helical bafes can cross near their parallel-cut orientation. Perpendicular-cut orientation should be used only with double i nlet midpoint (a cross-fraction of 50%), tip-toand outlet nozzles. tip (a cross-fraction of 100%), or at a point bottom). In effect, the performance of perpendicular-cut within this range. Depending on user and fabricator preferdouble-segmental bafes depends on the number ences, the cross-fraction selected may range from 20% to of nozzles. 100%. Reducing the cross-fraction (increasing the overlap) Triple-segmental bafes. The triple-segmental bafe enhances tube support and protects against vibration, but at set shown in Figure 3 has ve different components and is the expense of increased pressure drop. In general, helicalone of several possible arrangements. Other designs, which are not discussed here, have six pieces or three pieces. The permutations complicate the determination of bafe orientation, particularly in the inlet and outlet zones. Orientation of triple-segemental bafes has not been studied extensively, and general guidelines have not been developed. T-Baffle (End View)
Parallel Cut (End View)
Non-TEMA baffle types Most non-TEMA-type bafes usually produce lower pressure drops and have better ow and heat-transfer distri butions. The improvements stem from the bafes generating crossow through swirling, maximizing longitudinal ow, or increasing symmetrical ow and heat-transfer distribution. Helical, disc-and-donut, and grid bafes are the most common non-TEMA-type bafes.
Helical baffles promote swirling, which helps to reduce bypass and stagnant flow. p Figure 9.
r
B
Baffle Crossover Points
B
r co
C
C
Helical baffles
Ø S
Helical bafes promote swirling ow, which helps to alleviate bypass and stagnant ow areas that can occur with conventional segmental bafes. They are effective for lowto high-viscosity uids, and they are commonly used in oil-renery and refrigeration applications. Heat exchangers with helical bafes may experience less shellside fouling than exchangers with segmental bafes. Helical bafes are subject to bundle-to-shell bypass at very high mass owrates. Unlike segmental bafes, helical bafes do not seem 30
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A
A
D
D
(End View)
(Elevation View)
Cross-fraction = r co / r r = shell radius r co = radial distance from shell axis to baffle crossover point Ø S= baffle angle relative to transverse plane through shell
Helical quadrant baffles touch at crossover points, which define the cross-fraction. p Figure 10.
Copyright © 2012 American Institute of Chemical Engineers (AIChE)
Spiral Crossflow
Bypass Flow Outside Tube Bundle Radially Expanding Crossflow around Disc Baffle
Longitudinal Flow within Tube Bundle
Y
Radially Contracting Crossflow through Donut Baffle
X Z
Disc-and-donut baffles distribute flow in a radially symmetric manner. p Figure 12.
Flow Angle
Z
Z
Baffle Angle
The flow through a helical-baffled exchanger includes spiral crossflow, longitudinal flow, and bypass flow. p Figure 11.
bafed exchangers have less potential for tube vibration because the tubes are well supported by the quadrant bafes. Helical bafes come in single- or double-helix congurations. Many industrial applications use 12-deg. to 15-deg. angled bafes. Larger bafe angles result in lower pressure drop and increased longitudinal ow relative to crossow. Some studies have reported that bafe angles of 25 deg. to 40 deg. produce optimal conversion of pressure drop to heat transfer (5). The quadrant bafes induce ow that combines spiral crossow, longitudinal ow, and bypass ow (Figure 11). In reality, this ow is far from an ideal helix — it undergoes fewer revolutions than the number of bafe revolutions through the length of the exchanger. The design of these bafes requires knowledge of both the bafe angle and the actual ow angle (i.e., the direction of the resulting vector of the three principal ow velocity components (x, y, and z, or axial, radial, and tangential), relative to a plane transverse to the exchanger axis).
Disc-and-donut baffles Disc-and-donut bafes generate radially symmetric ow in both the crossow and longitudinal ow directions — the ow expands around the disc bafe and contracts through the donut bafe (Figure 12). A step change in both pressure drop and temperature occurs between consecutive pairs of disc bafes and donut bafes. The main thermally effective crossow stream can occupy up to 80% of a bafe compartment, minimizing the bypass ow around the outer tubes (6). The driving forces for bypass and leakage streams in an exchanger with disc-
Copyright © 2012 American Institute of Chemical Engineers (AIChE)
and-donut bafes are smaller than those in exchangers with segmental bafes. Disc-and-donut bafes are often installed in NTIW arrangements. Radial tube layouts are preferred over conventional triangular or square layouts, because the resulting radial ow distribution produces uniform heat transfer throughout the tube bundle cross-section. Disc-and-donut bafes are most effective in shellside vapor environments, and are commonly selected for gas-gas applications where vibration can be a problem.
Grid baffles Grid bafes are metal lattices that generate primarily longitudinal ow. They produce low pressure drops, which results in high heat-transfer-to-pressure-drop ratios and protects against tube vibration. In addition, shellside ow distribution is uniform, which is particularly important for shellside vaporization because it eliminates vapor pockets that can cause pitting of tubes and bafes (7). The most common generic grid bafe designs are rodtype bafes and strip bafes. Each grid type has a characteristic bafe ow-contraction ratio, which is dened as the free ow area through the bafe divided by the free ow area through the bundle between bafes. 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 bafes act as strainers on the bundle free-ow area, locally contracting and accelerating the heat-carrying ow longitudinally 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 contraction ratio. Rod-type bafes are used in such applications as overhead condensers, gas coolers and heaters, feed and efuent 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
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Heat Transfer
Y X
Y Z
X
p Figure 13.
Rod-type baffles support the tubes and provide structural
rigidity. Segmental Baffle
2 Rod-Type Baffles per Baffle Space
Rod-type baffles may be combined with segmental baffles for better tube support. p Figure 14.
longitudinal ow friction effectively generates heat transfer, especially in exchangers with long tubes. Rod-type bafes are made by welding round rods to a supporting ring (Figure 13), which also serves as a seal to prevent leakage ows. The rods are often located after every second tube row, with consecutive bafes 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 bafe). Triangulargrid bafes permit higher tube densities, produce higher turbulence, and generate higher heat transfer. However, mechanical cleaning of the tubes is more difcult due to limited access lanes. Therefore, triangular layouts are appropriate 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 bafes in place and maintain the proper bafe spacing. Rods with as small a diameter as possible are preferred, to permit a higher tube density and hence higher heat-transfer rate. The bafe contraction ratio for rod-type bafes is about 0.55–0.65 (7). Some industrial applications benet from combining rod-type bafes with segmental bafes. Figure 14 shows two rod-type bafes tted within the space between single-segmental bafes. This bafe combination provides increased tube support for vibration protection, without 32
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Z
Strip baffles have lower baffle contraction ratios and higher pressure drops than rod-type baffles. p Figure 15.
increasing pressure drop signicantly. Strip bafes (Figure 15) are grid bafes formed from at 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 bafes. Strip bafes 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 uidelastic instability, so tube vibration potential is minimized. With a bafe contraction ratio of approximately 0.2–0.25, strip bafes produce higher pressure drops per bafe than rod-type bafes (7).
Closing thoughts Bafing is the most crucial shellside consideration in shell-and-tube heat exchanger design, because bafes regulate shellside uid ow and improve heat transfer while offering signicant tube support. Although TEMA bafes are easier to fabricate, they usually have higher pressure drops than non-TEMA-type bafes. It is equally important to consider how bafe selection affects other shellside parameters, such as tube pitch ratio, tube layout pattern, tube size, shell type, and shell diameter. A basic understanding of the various bafe types and their advantages and disadvantages (Table 1) is essential to choosing an effective CEP bafe conguration. 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 investigations 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.
Copyright © 2012 American Institute of Chemical Engineers (AIChE)
Table 1. Each baffle type has advantages and disadvantages that make it suitable for different applications. Baffle Type
Single-Segmental s e fl f a B Double-Segmental e p y T Triple-Segmental A M E T No-Tubes-in-Window
Advantages
Disadvantages/Limitations
Highest potential heat-transfer rates
Highest potential pressure drop
Easiest to fabricate
Cannot be used for very viscous fluids
Least expensive Lower pressure drop than with single-segmental baffles
Lower heat-transfer rates than with singlesegmental baffles
Lower pressure drop than with double-segmental baffles
Lower heat-transfer rates than with doublesegmental baffles
All tubes are supported, eliminating tube vibration
Requires a smaller tube bundle and/or larger shell; a larger shell makes this configuration more expensive
Configuration
Higher conversion of pressure drop to shellside heat transfer than single-segmental baffles
Helical
Less shellside fouling Moderate heat-transfer rates and pressure drops Minimizes or eliminates areas of stagnant flow Minimizes or eliminates tube vibration
Disc-and-Donut s e fl f a B e p y T A M E T n o N
Radially symmetric flow distribution
Significant bundle-to-shell bypass at high mass flowrates More expensive than traditional double-segmental baffles
Minimizes bypass flow Same pressure drop as with double-segmental baffles, with better heat transfer Well suited for gas-gas applications
Grid
Difficult fabrication, design methods are not standardized
Preferred radial tube layout requires a lesscommon fabrication method than triangular and square layouts In a r adial 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
Provides tube support Uniform flow distribution
Relatively low heat-transfer rates, unless the tubes are long
Relatively low pressure drops
Specific tube layouts are required
High conversion ratio of pressure drop to heat transfer
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 Bafing Bafe 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 Bafes,” 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 Bafes,” AIChE Spring Meeting, New Orleans, LA (2004).
7.
Taborek, J., “Longitudinal Flow in Tube Bundles with Grid Bafes,” in “Heat Exchanger Design Handbook — Part 3, Thermal and Hydraulic Design of Heat Exchangers, Section 3.3.12,” Begell House, New York, NY (1998).
Copyright © 2012 American Institute of Chemical Engineers (AIChE)
Further Reading Bell, 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, Wolverine 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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