Waterside economiser. A plate exchanger between the cooling-tower loop and the chilled-water loop: when the tower makes water colder than the chilled-water return, the exchanger carries the load and the chiller idles — free cooling on the water side, the way the airside economiser is free cooling on the air side. A building run gives a plant with one a free cooling stage ahead of its chiller stage. See the plate exchanger.
A plate heat exchanger between the cooling-tower loop and the chilled-water loop. When the tower makes water colder than the chilled-water return — a winter or a dry night — the exchanger carries the load and the chiller idles; in between, the integrated arrangement puts the plate ahead of the evaporator so it pre-cools the return and the chiller trims what is left. The changeover valves keep it out of circuit when the tower water is warmer, or it would heat the chilled return instead.
It earns its keep on loads that persist in cold weather — data halls, laboratories, process cooling — and less on buildings whose handlers already free-cool on outside air, since those take the load away exactly when the tower is cold. HVAC Builder's plate exchanger kind carries the rated point a datasheet prints and an economiser flag; a plant holding a leaving-water setpoint with one gets a free-cooling stage ahead of its chiller stage, cycling and staging its tower fans on leaving-water temperature.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year