Evaporator Pressure-Regulating Valves Operation
For any pressure sensitive valve to modulate to a more closed or open position, a change in operating pressure is required. The unit change in the valve stroke for a given change in the operating pressure is called the valve gradient. Every valve has a specific gradient designed into it for the best possible operation. Valve sensitivity and the valve’s capacity rating are functions of the valve gradient. Thus, a relatively sensitive valve is needed when a great change in the evaporating temperature cannot be tolerated. Therefore, the valves have nominal ratings based on the 8-psi evaporator-pressure change, rather than a full stroke.
Evaporator pressure-regulator valves respond only to variations in their inlet pressure (evaporator pressure). Thus, the designation for evaporator pressure-regulating valves is ORI (opens on the rise of the inlet pressure) (see Fig. 11-29).
Pressure at the outlet is exerted on the underside of the bellows and on top of the seat disc. The effective area of the bellows is equal to the area of the port. Thus, the outlet pressure cancels out and the inlet pressure acting on the bottom of the seat disc opposes the adjustable spring force. These two forces are the operating forces of the ORIT. (The “T” added to the valve designation indicates an access valve on the inlet connection.) When the evaporator load changes, the ORIT opens or closes in response to the change in evaporator pressure. An increase in inlet pressure above the valve setting tends to open the valves. If the load drops, less refrigerant is boiled off in the evaporator and evaporator pressure will decrease. The decrease in evaporator pressure tends to move the ORIT to a more closed position. This, in turn, keeps the evaporator pressure up. The result is that the evaporator pressure changes as the load changes. The operation of a valve of this type is improved by an antichatter device built into the valve. Without this device, the OBIT would be susceptible to compressor pulsations that can reduce the life of a bellows. This antichatter feature allows the ORIT to function at low load conditions without chattering or other operating difficulties.