First of all, let’s start off with talking about what is an economizer. An economizer is a mode that uses the evaporative cooling capacity of a cooling tower to produce chilled water and can be used instead of the mechanical cooling (Chiller/SWUD’s) during the winter months. Essentially you are taking the cold outside water from the cooling tower, running it through the building loop where the heat exchange takes place, and then the cycle repeats itself by rejecting the heat to the cold outside air. You are bypassing the mechanical cooling functions (chiller and compressor) while using this economizer mode because the outside temperature is so low.
Why and when should you use an economizer? It all depends on what the temperature/humidity is outside and what is the load of the building. When the ambient outside temperature is low enough, a water economizer can reduce compressor run hours and energy use. This saves money for the building through decreasing electrical utility consumption as well as extending the life of your mechanical equipment