The nonadjustable ORO head pressure-control valve and the ORD pressure-differential valve offer the most economical system of refrigerant side head-pressure control. Just as the ORI/ORD system simplified this type of control, the ORO/ORD system offers the capability of locating the condenser and receiver on the same elevation (see Fig. 11-36). By making these two valves available either separately or brazed together, there is added flexibility in the piping layout. The operation of the ORO/ORD system is such that a nearly constant receiver pressure is maintained for normal operation. As the temperature of the ORO element decreases, the pressure setting decreases accordingly. However, by running the bypassed hot gas through the ORO the element temperature is adequately maintained so the ORO/ORD system functions well to ambient temperatures of ?40°F (?40°C) and below. This third connection on the ORO also eliminates the need for a tee connection in the liquid-drain line.
Note that in Fig. 11-36 the ORO is located in the liquid-drain line between the condenser and the receiver, while the ORD is located in a hot-gas line bypassing the condenser. Other than the fact that the ORO operates in response to its outlet pressure (receiver pressure), the ORO/ORD operates in the same basic manner as the ORI/ORD system previously explained.
