Free Cooling - Refrigerant Migration

One method for reducing the energy consumption of a centrifugal water chiller is to add a refrigerant-migration free cooling cycle. This type of free cooling is based on the principle that refrigerant migrates to the coldest point in a refrigeration circuit.

When water returning from the cooling tower is colder than the chilled water, refrigerant pressure within the condenser is lower than that in the evaporator. This pressure differential drives the refrigerant vapor "boiled off" in the evaporator to the condenser, where it liquifies and flows by gravity back to the evaporator. As long as the proper pressure difference exists between the evaporator and condenser, refrigerant flow and the consequent free cooling continues.

Under favorable conditions, refrigerant-migration free cooling can provide as much as 40 percent of the chiller's design tonnage if the chiller is designed appropriately. Since the chiller and free cooling cycle cannot operate simultaneously, free cooling of this type can only be used when the cooling capacity of the tower water is sufficient to meet the entire building load.

Little, if any, free cooling capacity is available when the ambient wet bulb temperature is above 50°F. Accessories such as chilled water pumps, condenser water pumps and cooling tower fans continue to operate in the conventional manner while the chiller operates in the free cooling mode. The energy cost savings realized from free cooling operation results from the compressor's inactivity during this cycle. The cooling tower must be designed for winter operation.