difference between 3 way and 4 way chilled water valves

One way to control the capacity of a chilled-water air-cooling unit would be to use a simple 2-way valve to throttle the water flow. However, if the valve closes all the way, the pump would then be discharging into a closed head. But if you use a three-way valve, you can blend chilled and unchilled water while maintaining full pump flow. I have done a number of glycol-cooled fruit warehouses in this manner, but have never had occasion to use a four-way valve in such a chiller application.

A three-way valve can be mixing (inlet ports A and B merge to outlet port C), or diverting (inlet port A branches to outlet ports B and C).

A four-way valve (such as a hydraulic directional valve) typically has ports identified as P, A, B, and T. P is pressure, A and B go to a cylinder or motor, and T returns to the tank. In one position P is connected to A, and B to T. In the other position, P connects to B, A to T. This reverses the flow to the cylinder or motor.