Doing a Pump-Down in Compression Chillers 

  1. With most compression chillers, doing a pump-down is necessary during the shutdown. This is intended to prevent the abrupt intrusion of liquid refrigerant from the evaporator to the compressor during startup, which damages the device.

  2. Pump-down means the compressor does not immediately shut down after the thermostat commands the electric valve to close the liquid circuit (and the compressors to shut down), but with the continued collection of liquid refrigerant in the condensers via the discharge circuit, due to the liquid circuit being closed, the evaporator pressure is reduced and the compressor shuts down when the pressure decreases below the point at which the Low-Pressure Controller (LPC) is adjusted.

  3. In this case, almost the entire liquid refrigerant in the evaporator is transported to the condenser through the compressor of the compression chiller, therefore, the evaporator is free of liquid refrigerant that can rapidly enter the compressors via suction, damaging the compressor valves during startup.

  4. Due to the discharge pressure difference in systems with water-cooled (cooling towers) and air-cooled condensers, adjusting the High Pressure Controller (HPC) is naturally different in the two types of chillers as the refrigerant line is adjusted at 250 psi in water-cooled systems and at 350 psi in air-cooled ones.