One method of head-pressure control during low ambient conditions is to use a mechanical hold-back or flooding valve located at the outlet of the condenser to hold back or flood liquid refrigerant in the condenser (see Figure 1). This valve is often referred to as an open on rise of inlet (ORI) valve because it will start to throttle open on a rise in condenser pressure.
This pressure-actuated holdback valve is installed at the condensed outlet. The valve will throttle shut when the condenser pressure reaches a preset minimum pressure in a cold ambient condition. This throttling action will backup liquid refrigerant in the bottom of the condenser, which causes a flooded condition. The condenser now has a smaller internal volume, which is what is needed for colder ambient condition. The condenser pressure will now rise, giving sufficient liquid line pressures to feed the expansion valve.