Two major categories of evaporators used in industrial refrigeration practice are air coils and liquid chillers. A typical air coil is shown in Fig. 6.2 and several types of liquid chillers are shown in Fig. 6.3. In the air coil, refrigerant flows through the tubes and air passes over the outside of the tubes. For effective heat transfer, fins are fastened to the outside of the tubes and the air flows between the fins.
The liquid-chilling evaporators in Figs. 6.3a and 6.3b are of the shell-and tube design, while Figs. 6.3c is a plate-type chiller. In Fig. 6.3a the refrigerant boils in the shell while the liquid flows through the tubes. In Fig. 6.3b the roles of the tubes and shell are reversed. The plate-type evaporator in Fig. 6.3c is growing in market share and is an adaptation of the plate-type heat exchanger used for many years in the food industry. Some of its popularity is attributable to its compactness and also that the refrigerant charge is less than in a shell and-
tube evaporator. The reduced charge characteristic is attractive both when ammonia and the new chlorine-free refrigerants are used. A low-charge ammonia system is desirable for safety reasons, and a low-charge chlorine-free system minimizes the cost of these expensive refrigerants.