When refrigeration systems are exposed to low ambient conditions, the condensing pressure (head pressure) will fall. If the head pressure gets too low, the metering device will not have enough pressure drop across it to operate properly. The metering device will underfeed the evaporator and a low pressure will result in the evaporator. This can cause refrigeration systems to short cycle from the low-pressure control, opening prematurely. Also, once the refrigeration system is off, it may never turn back on again because the evaporator pressure may never reach the cut-in pressure of the low-pressure control. Condensing units that are exposed to low ambient conditions must have some sort of low ambient control installed on them to prevent their head pressures from falling too low.
Low ambient control (Figure 1) is a style of head-pressure control valve for low ambient conditions that has a round pressurized dome at its top that is pressure charged. This valve is often referred to as a low ambient control (LAC) valve. It is used in a refrigeration system. The dome charge of the LAC is independent of the refrigerant charge in the actual refrigeration system. The dome charge will expand and contract in volume and act on an internal diaphragm as the outside ambient changes. This happens because the entire valve is located in the condensing unit, which is exposed to an outside ambient temperature.