Thermal design countercurrent flow

Countercurrent flow can always be assumed, if thermally possible, regardless of the range of the temperature cross as long as the temperature approaches (e.g., hot outlet and cold inlet temperatures) are greater than 5°F. Also, only TEMA E and F -type shells should be used for countercurrent flow designs, provided that the number of shell and tube passes are the same. 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

The resin is exposed to several shocks, both thermal and osmotic, in the regeneration sequence. The resins used are quite resistant to these shocks, and in fact replacement rates are only a few percent per year. One notable effect with implications in vessel design is the change in bead density between forms. The sodium form can be 35% less dense than the hydrogen form, and the bed must be free to expand while being converted. Upward flow of the caustic solution is useful from this standpoint, even as it provides a countercurrent regeneration. 

Fig. 12.25. The principal characteristics of such beds include cross flow of solid and drying gas, a solids residence time controllable from seconds to hours, and suitability for any gas temperature. It is necessary that the solids be free-flowing, of a size range 0.1 to 36 mm (59]. Since the mass flow rate of gas for thermal requirements is substantially less than that required for fluidization, the bed is most economically operated at the minimum velocity for fluidization. Multistage, cross-flow operation (fresh air for each stage) is a possibility [2], as is a two-stage countercurrent arrangement, as in Fig. 11.28 (58]. A tentative design procedure has been proposed (40].

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