Gas and liquid leg shutoff
Figure 10-18 illustrates a flooded floor unit suitable for operation down to ?70°F (?57°C). The gas-pressure-powered valve used in this circuit has a solenoid pilot operator. This provides positive action with gas or liquid loads at high or low temperatures and pressures.
To defrost a group of evaporators without affecting the temperatures of the common surge drum, the gas-powered valve is used at each end of the evaporator. A reseating safety valve is a relief regulator. It controls the defrost pressure to the relief-line accumulator. A check valve prevents backflow into the relief line. The in-line check valve prevents crossover between adjacent evaporators.
At high temperatures [above ?25°F (?31°C)], use of the gas-powered check valve in place of the gas-powered solenoid valve is recommended.
Gas leg shutoff
The system shown in Fig. 10-19 is similar to that shown in Fig. 10-18. However, a single gas-type, pressure-powered valve is used. Overpressure at the surge drum is relieved by valve B, a defrost-relief regulator. This is normally wide open. It becomes a regulating valve when its solenoid is de-energized during defrost.
Defrost gas flows through the hot-gas solenoid when energized. It then flows through the glove valve and the in-line check valve to force the evaporator fluid into the surge drum.
An optional hot-gas thermostat bulb may be used to sense heating of the bottom of the evaporator. Thus, it can act as a backup for the timed defrost cycle.
