Heat exchangers, baffles transfer rates

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. 

Figure 7.3 compares calculated overall heat-transfer coefficients for several reactor heat exchangers, using water for both the jacket and reactor fluid. The figure shows that the highest heat-transfer coefficient is obtained with internal coils and the lowest with the simple jacket (called the conventional jacket in Figure 7.3) without a spiral baffle or agitation. It is assumed that the flow rate for the internal coil is the coil flow rate and not the jacket flow rate, as plotted in Figure 7.3. Heat-transfer coefficients for the half-pipe coil, agitated, and baffled jackets are conparable.

Example 15J. A tubular exchanger 35 in. (889 mm) ID contains eight hundred and twenty-eight -in. (19-mm) OD tubes 12 ft (3.66 mm) long on a 1-in. (25-mm) square pitch. Standard 25 percent baffles are spaced 12 in. (305 mm) apart. Liquid benzene at an average bulk temperature of 60°F (. d C) is being heated in the shell side of the exchanger at the rate of 100,000 Ib/h (45,360 kg/h). If the outside surfaces of the tubes are at 140°F (60°C), estimate the individual heat-transfer coefficient of the benzene. 

Alternately, the cell may be designed similarly to a shell-and-tube heat exchanger, with flow inside the tubes and on the outside or shell side. The shell-side flow may be strictly parallel to the tubes or also across the tubes, or tube bundle, and directed by the use of baffles and baffle cuts. Such a layout is illustrated in Figure 6.3, with more information about the intricacies provided by Hoffman. There is an analogy with the treatment of absorbers, strippers, and distillation columns as a continuum, described in terms of the rate of mass transfer."... 

In general, helical baffle tube bundles are most cost-effectively used as replacement bundles in existing shells. However, for this to actually improve heat-transfer rates, the controlling resistance to heat transfer must be on the shell side and not the tube side of the heat exchanger. Basically this means that the shell-side flow must suffer from some combination of ... 

The closer the baffles are spaced the greater the turbulence and heat transfer. The pressure drop increases at close spacings but not in proportion to the increased transfer rate. However, spacings of less than 6 in. are seldom used because of the difficulty encountered in cleaning the outside of the tubes. Exchangers in use today have spacings of 2 to... 

Example. Given Shell exit temperature, Tout = 100 F tube fluid temperature, t, at reversal of tubes = 71° F overall heat transfer coefficient, C/, multiplied by the surface area of the exchanger per baffle section, Ag (sq.ft.) or UAg = 2,000 shell flow rate, W, (lb./hr.) multiplied by the specific heat of the sheU fluid, C, (Btu/lb. °F) or WC = 10,000. Ratio of stream heat capacities, JR, equals the tube fluid flow rate, w (Ibs./hr.) times the specific heat of the tube fluid, c, (Btu/lb. °F) divided by WC equals one, or i = wcjWC = 1 magnitude of the by-pass stream expressed as a fraction of total flow, I = 0.6. 

Another example from oil refineries is in crude preheat service with vacuum resid again on the shell side (with the helical baffles). Once again, similar results are seen as described above, with less fouling, reduced rate of increase in pressure drop, and better maintenance of heat-transfer coefficient as compared to the conventional shell-and-tube exchanger design. Our more direct experience of this comes from current practice in the United States, but we have also seen evidence of similar applications in Australia, as discussed in a recent article on the subject of crude preheat exchanger train redesign. ... 

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