Segmental Baffles

The horizontal cut baffles are good for all gas-phase or all liquid-phase service in the shell. However, if dissolved gases in the liquid can be released in the exchanger, this baffling should not be used, or notches should be cut at the top for gas passage. Notches will not serve for any significant gas flow, just for traces of released gas. Liquids should be clean otherwise sediment will collect at the base of every other baffle segment and blank off part of the lower tubes to heat transfer. 

Table 10-7 shows suggested tie rod count and diameter for various sizes of heat exchangers, as recommended by TEMA . Other combinations of tie rod number and diameter with equivalent metal area are permissible however, no fewer than four tie rods, and no diameter less than /g-in., should be used. Any baffle segment requires a minimum of three points of support. 

There are no general, reUable pressure—drop formulas for baffled sheUside flow avaUable in the open Uterature. Therefore, the Donohue correlation for a pair of disk-donut or segmental baffles is presented herein because of its simplicity (15) ... 

Baffle cuts are expressed as the ratio of segment opening height to shell inside diameter. Cross-flow baffles with horizontal cut are shown in Fig. 11-36 7, c, andf. This arrangement is not satisfactoiy for horizontal condensers, since the condensate can be trapped between baf-... 

Maximum shell-side heat-transfer rates in forced convection are apparently obtained by cross-flow of the flmd at right angles to the tubes. In order to maximize this type of flow some heat exchangers are built with segmental-cut baffles and with no tubes in the window (or the baffle cutout). Maximum baffle spacing may thus equal maximum unsupported-tube span, while conventional baffle spacing is hmited to one-h f of this span. 

The maximum baffle spacing for no tubes in the window of single segmental baffles is unhmited when intermediate supports are provided. These are cut on both sides of the baffle and therefore do not affect the flow of the shell-side fluid. Each support engages all the tubes the supports are spaced to provide adequate support for the tubes. 

FIG. 11 -40 Baffle cuts, (a) Baffle cuts for single segmental baffles, (h) Baffle cuts for double segmental baffles, (c) Baffle cuts for triple segmental baffles. 

For counterflow plates, the curves of Fig. 14-25 may be used for open areas of 20 percent or greater. Plates with 15 percent open areas have about 85 percent of the cui ve values, and open areas or less than 15 percent are not recommended. For counterflow-plate columns of the segmental-baffle type, 50 percent cut, allowable values are about 15 percent greater than tnose shown in Fig. 14-25, when vertical spacings of the baffles are equal to the trav spacings shown. 

With segmental baffles, where the shellside fluid flows across the tube bundle between baffles, the following tube patterns are usual ... 

Previous equations determine the pressure drop across the tube bundle. For the additional drop for flow through the free area above, below, or around the segmental baffles use... 

Nq = Number of baffles in series per shell pass Sb = Cross-sectional area for flow around segmental baffle, ft ... 

The baffle patterns in the column can be segmental (simple) up to about 4-ft diameter column, and larger columns can use a disk and donut design as in heat exchangers, or the double segmented or even multi-segmented as in the layouts discussed under bubble caps earlier in this text. 

Figure 10-15. Horizontal cut segmental baffles. (Used by permission B.G.A. Skrotzki, B.G.A. Power, June 1954. McGraw-Hill, Inc. All rights reserved.)...

Although these baffles can be as effective as the segmental ones for single-phase heat transfer, they are not used as often. The fluid must be clean otherwise sediment will deposit behind the doughnut and blank off the heat transfer area. Also, if inert or dissolved gases can be released, they cannot be vented effectively through the top of the dough-... 

Kern s correlation checks well for the data of Short, Bowman, and Tinker for a wide variety of baffle cuts and spacing for segmental baffles with and without leakage as summarized by Donohue. Short s data for disc and doughnut baffles is better calculated by ... 

Figure 10-54. Shell-side heat transfer curve for segmental baffles. (Used by permission Engineering Data Book Section II, 1959. Wolverine Tube, Inc.)...

Apb = pressure drop across window opening of segmental baffles, total for all baffles, psi Ap,. = pressure drop across the bundle in cross-flow, psi s = specific gravity of gas or liquid referenced to water = number of baffles... 

Figure 10-140 is used for determining the friction factor (dimensional) for segmental type baffles. The loss across the tube bundle and through the baffle window is represented in the combined factor, f, which is to be used with the equation for pressure drop. ... 

The baffle used in the preceding equation has 20% segmental cuts. Shell-side cross-flow velocity ... 

Figure 10-148. Shell-side friction factors for bundles with 20%-cut segmental baffles, one seal strip per 10 rows of tubes, and TEMA clearances. These factors can be used for plain or low-finned tubes with the appropriate values of D s or d s. (Source Engineering Data Book, 1960. Wolverine Tube, Inc. Used by permission Kern, D. Q., and Kraus, A. D. External Surface Heat Transfer, p. 511, 1972. McGraw-Hill Book Co., Inc. All rights reserved.)...

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