The following valves and controls are used in the hot-gas defrost systems of ammonia-type evaporators:
Hot-gas or pilot-solenoid valve. The valve is a 1/8 in. ported solenoid valve. It is a direct-operated valve suitable as a liquid, suction, hotgas, or pilot valve at pressures to 300 lb.
Suction-, liquid-, or gas-solenoid valve. The suction-solenoid valve is a one-piston, pilot-operated valve suitable for suction-, liquid-, or gas-lines at pressures to 300 lb. It is available with a 9/16 in. or 3/4 in. port.
Pilot-operated solenoid valve. The valve is a one-piston, pilot-operated solenoid valve used as a positive stop valve for applications above ?30°F (?34°C) on gas or liquid.
Pilot-operated two-piston valve. The solenoid valve is a rugged, pilotoperated, two-piston valve with spring return for positive closing under the most adverse conditions. It is used for compressor unloader, suction, liquid, and hot-gas applications.
Gas-powered solenoid valve. The gas-powered solenoid valve is a power-piston type of valve that uses high pressure to force the valve open through the control of pilot valves. Because of the high power available to open these valves, heavy springs may be used to close the valves positively at temperatures down to ?90°F (?68°C).
Dual-pressure regulator valve. The dual-pressure regulator valve is designed to operate at two predetermined pressures without resetting or adjustment. By merely opening and closing a pilot solenoid, either the low- or high-pressure setting is maintained.
Reseating safety valve. The reseating safety valve is generally used as a relief regulator to maintain a predetermined system pressure. The pressure maintained by the valve is adjustable manually. Back-pressure regulator arranged for full capacity. The back-pressure regulator is normally used where pressure control of the evaporator is not required—as in a direct expansion system. A pilot solenoid is energized, allowing pressure to bypass the sensing chamber of the regulator holding the valve wide open. De-energizing the pilot valve allows the valve to revert to its function as a back-pressure regulator maintaining a preset pressure upstream of the valve. The valve performs both as a suction solenoid and as a relief regulator.
Differential relief valve. The differential relief valve is a modulating regulator for liquid or gas use. It will maintain a constant preset pressure differential between the upstream and downstream side of a regulator.
Reverse-acting pressure regulator. The reverse-acting pressure regulator is used to maintain a constant predetermined pressure downstream of the valve. When complete shutoff of the regulator is required, a pilot valve is installed in the upstream feeder line. When the solenoid valve is closed, the regulator closes tightly. When the solenoid valve is open, the regulator is free to operate as the pressure demands. With the solenoid installed as described above, this becomes a combination reverse-acting regulator and stop valve.
Gas-powered check valve. The gas-powered check valve is held in a normally open position by a strong spring. Gas pressure applied at the top of the valve closes the valve positively against the high system pressures. A manual opening stem is standard.
Check valve. The check valve is a spring-loaded positive check valve with manual opening stem. It is used to prevent backup of relatively high pressure into lower-pressure lines.
In-line check valve. The in-line check valve is used in multiple-branch liquid lines fed by a single solenoid valve. This check valve prevents circulation between evaporators during refrigeration. The in-line check valve is also used between drain pans and evaporators to prevent frosting of the drain pan during refrigeration.
These valves and controls are necessary. They cause defrosting operations to take place in large evaporators used for commercial jobs. Some manufacturing operations also call for large-capacity refrigeration equipment.