Flooded evaporators, first described in Sec. 6.7, are equipped with a surge drum at a level slightly above the evaporator coils. Either a horizontal or vertical surge drum is possible, as shown in Fig. 10.14, with the choice usually dependent upon the space restrictions. The surge drum serves the purpose of liquid storage and liquid/vapor separation. A need for surge volume arises when the evaporator is subjected to a sudden heavy heat load, in which case the rate of boiling abruptly increases and the vapor thus developed pushes liquid out of the evaporator. In the case of air coils during hot-gas defrost the surge drum provides storage during the transient conditions.
The principles for disengaging vapor from liquid are the same ones applicable to low-pressure receivers, as discussed in Secs. 10.4 and 10.5. Usually the manufacturer of the coil provides the surge drum as part of the package and thereby makes the decision on the dimensions of the surge drum. As a suggestion of the order of magnitude of the volume of the surge drum, one recommendation for finned evaporator coils is that horizontal surge drums have a free volume as great as the internal volume of the evaporator coils. The free volume is defined as the vapor space from the controlled level to the level at which liquid would carry out of the vessel.
One manufacturer offers. guidelines for dimensions of horizontal surge drums, such as shown in Fig. 10.14a, as expressed in Table 10.3. For vertical surge drums, as shown in Fig. 10.14b, the permitted refrigeration capacity of the coil is about twice that shown in Table 10.3 for a given vessel size and evaporating temperature.