The flash-tank/desuperheater is the vessel in a two-stage system operating at the intermediate pressure that provides removal of flash gas in direct cooling of liquid (Fig. 3.2) or subcooling liquid with a heat exchanger (Fig. 3.5). The other purpose of the vessel is to desuperheat discharge vapor from the low-stage compressor by bubbling it through the liquid in the vessel (Fig. 3.9). Section 10.1 indicated that vessels in industrial refrigeration systems have either or both the function of liquid storage and/or separation of liquid from vapor. Usually the flash-tank/desuperheater must be sized only for separation of liquid and vapor. The exception to that limited role is when other streams in addition to the liquid supply and discharge vapor from the low-stage compressor flow into the vessel. If there are intermediate-temperature evaporators discharging into the vessel, for example, surge volume should be provided to allow for the rush of liquid coming from the evaporators during defrost.
Desuperheating of vapor, which is a unique function of this vessel, is not required in other vessels of the system. One method of desuperheating is to bubble the vapor through the liquid, a process that is effective, but requires two simultaneous functions that are somewhat conflicting. One process is agitation for good desuperheating and the other is an orderly flow pattern for good liquid/vapor separation. Some vessel designers choose a configuration like that shown in Fig. 10.12a which injects the vapor below a perforated plate to spread out the vapor bubbles, thus providing more intimate contact of vapor and liquid and avoiding a geyser which would make the liquid and vapor separation more difficult. Still another feature used by some designers is the installation of a plate against which the vapor is directed. The purpose of this plate is to prevent the vapor bubbles from blowing to the liquid outlet and being drawn out of the vessel. This precaution would be especially appropriate if the vessel is part of a liquid recirculation systems where vapor bubbles in the liquid could vapor- bind the pump.
The other concept of desuperheating, as shown in Fig. 10.12b, is to spray liquid into the incoming vapor. The diagram shows a control valve regulating the liquid flow, but, as was discussed in Chap. 9, the bypass from a centrifugal pump may serve as the liquid source.