Baffle spacing

It is assumed that process conditions and physical properties are known and the following are known or specified tube outside diameter D, tube geometrical arrangement (unit cell), shell inside diameter D shell outer tube limit baffle cut 4, baffle spacing and number of sealing strips N,. The effective tube length between tube sheets L may be either specified or calculated after the heat-transfer coefficient has been determined. If additional specific information (e.g., tube-baffle clearance) is available, the exact values (instead of estimates) of certain parameters may be used in the calculation with some improvement in accuracy. To complete the rating, it is necessary to know also the tube material and wall thickness or inside diameter. 

Minimum baffle spacing is generally one-fifth of the shell diameter and not less than 50.8 mm (2 in). Maximum baffle spacing is hm-ited by the requirement to provide adequate support for the tubes. The maximum unsupported tube span in inches equals 74 dP (where d is the outside tube diameter in inches). The unsupported tube span is reduced by about 12 percent for aluminum, copper, and their alloys. 

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. 

When combating erosion-corrosion by changing tube metallurgy, caution must be exercised to ensure appropriate tube and baffle spacing so that vibration-associated cracking problems are not introduced. 

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

In some instances the baffle spacing must be rearranged to allow for a nozzle or coupling connection, ft is important that changes in baffle location be reviewed, as performance or pressure drop can be seriously affected. This is of extreme importance in vacuum units. Baffle orientation is sometimes misinterpreted by the fabricator, and this can cause serious problems where liquid drainage is concerned, or the revised vapor flow path can allow for bypassing the tube surface. 

Dp = equivalent tube diameter, ft dp = equivalent tube diameter, in. a, = flow area across the tube bundle, fF B = baffle spacing, in. c = specific heat of fluid, Btu/lb (°F) p. = viscosity at the caloric temperature, Ib/ft (hr) py, = viscosity at the tube wall temperature, Ib/ft (hr)... 

Cross-flow area for Figure 10-54 is based upon the maximum flow area at the nearest tube row to the centerline of the shell. The length of the flow area is the baffle spacing. 

Compare values calculated in steps 10 and 11. If the calculated unit is too small, re-assume a new larger unit for step 3 or try closer baffle spacing in step 7 but do not get baffles closer than V5 the shell l.D. 

Try to obtain a better coefficient by closer baffling, check extreme of 2-in. baffle spacing. 

Calculate the shell-side dry-gas film coefficient, hg or h, for outside tube conditions. Assume a baffle spacing or about equal to one shell diameter. Use the shell-side method described in Equation 10-48 and Figure 10-54. This is necessary for inlet conditions and then must be checked and recalculated if sufficient change occurs in the mass flow rate, G, to yield a change in hg. 

From equation 10-48 for use with Figure 10-54, assume 18-in. baffle spacing ... 

B is held to a 2-in. minimum or V5 shell diameter (I.D.) and is 26 in. maximum for -in. tubes and 30 in. for 1-in. tubes. Refer to TEMA for tube support and baffle spacing recommendations. 

Gentry, G. G. and W. M. Small, RODbaffle Exchanger Thermal-Hydraulic Predictive Models Over Expanded Baffle-Spacing and Reynolds No. Ranges, National Heat Transfer Gonference, Boulder, GO., Aug. 5-8, (1985). 

TWo tubular heat exchangers are available each with a 0.44 m i.d. shell fitted with 166 tubes, 19.0 mm o.d. and 15.0 mm i.d., each 5.0 m long. The tubes are arranged in two passes on 25 mm square pitch with a baffle spacing of 150 mm. There are two passes on the shell side and operation is to be countercurrent. With benzene passing through the tubes, the anticipated film coefficient on the tube side is 1000 W/m2K. 

Area for flow = shell diameter x baffle spacing x clearance/pitch 0.44 x 0.150 x 0.006... 

This value is very high and thought should be given to increasing the baffle spacing. If this is doubled, this will reduce the pressure drop by approximately (1/2)2 = 1/4 and ... 

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