A waterside economizer is a heat exchange device that sends condenser return water to a exchange heat with cooling tower (or a dry cooler) water, whicht is cooled by ambient air. The return water is used to help a chiller operate more efficiently, or in some cases to meet the entire cooling load.
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Waterside economizers are not as common as airside economizers, and often require reasonably low temperatures to meet the entire cooling load. It is best to consult with your manufacturer, as a waterside economizer can be very effective in the right circumstances