When the ambient air temperature drops to a set temperature, a modulating valve allows all or part of the chilled water to by-pass an existing chiller and run through the free cooling system, which uses less power and uses the lower ambient air temperature to cool the water in the system.
There are three ways to use free cooling:
One limitations of free cooling is freezing can be difficult to avoid once the cooling tower water temperature gets below 39 °F.
Another limitation of free cooling is the temperature difference across the heat exchanger. A heat exchanger that has a very low temperature difference across can become economically unrealistic. The economics of the heat exchanger allow for a minimum free cooling water temperature of about 41 °F.