The low-temperature liquid recirculating system illustrated in Fig. 10-22 uses several controls. During the cooling cycle, Number 1 pilot valve is opened and Number 2 pilot valve is closed, holding the gas-powered solenoid valve wide open. This allows flow of liquid through the energized liquid-solenoid valve from the recirculator and then through the circuit of the unit. The in-line check valve installed between the drain-pan coil header and suction line prevents drainage of liquid into the drain-pan coil.
For defrost, the liquid solenoid valve is closed. The Number 1 pilot solenoid is de-energized. The Number 2 solenoid is opened, closing the gas-powered solenoid valve tightly. The hot-gas solenoid is energized. This allows distribution of the hot gas through the drain-pan coils, the in-line check valve, the top of the suction header, and the coil. The gas comes out the bottom of the liquid header.
Check valve A prevents the flow of the high-pressure gas in the liquid line. Therefore, the gas is relieved through the safety-valve relief regulator (B). This is set to maintain pressure in the evaporator to promote rapid, efficient defrost.