Q:In a chilled water system, when installing FCU or AHU, where exactly we are supposed to fix this valve, on the supply line or return line.Has it to be a 3 way diverting valve or 3 way mixing valve. Please describe how does it function, and how its modulation is controlled, via thermostat or what.Should it be normally open type or normally closed type.Thanks

A:For 3way diverting valves, most of the engineers prefer to install at the exit position of the unit, because the temperature control is more accurate at this point. 

The 3way valves are diverting when the intention of the valve is to maintain the temperature of the air to the correct setting as the room thermostat calls for for it. The final temperature of the water is governed by the desired air temperature, the modulation of the valve is to allow raise in the temperature of the water entering the coil so the temperature of the air is also modulated as it meets the room demands. The use of diverting 3way valves is usually for closed circuits like FCU or AHU chilled water systems, there is a supply entering and a return exiting, and the unit with valves in between. 
By the other hand the intention of the mixing valve is to maintain control over the final water temperature, there is two supplies with different temperature and one exit at the desired water temperature, the use of 3way mixing valves is usually for open circuits. Here there is also modulation of flow as the temperature of the supplies may vary in order to maintain a fixed exit water temperature.