Why we connect the chilled water supply pipe to the FCU or AHU from bottom and the return pipe from top?

Answer 1

When system is out of service, the air will trap in top of the coil. When system starts after a while (like in morning for commercial buildings), the supply water must enter from the lower part of the coil to run and compressed the air at the top of the coil to active the relief valve on top of the coil to release the air, then the control valve on the return pipe at the upper level of the coil will open shortly after releasing the trapped air in the coil (by programing the control system) and system will be in efficient mode. If supply water enters from the top or upper level of the coil, the air will run to the system through the return pipe at the lower level of the coil and makes system less efficient.

Answer 2

Density of chill water is more than the hot water, as the chill water enters the heat exchanger (or FCU & AHU) and exchange the heat, the temperature of the chill water will rise and it becomes less dense, which will effect the efficiency of system.

Answer 3

as explained others, the heat exchanger inlet connected to bottom water flow throught bottom even air is trapped which will be release at the top of the coil or out let of coil in top, the hot air trap /in water get release on the top of the system. hence to avoid air trap and for effective efficiency of heat exchange alway in let of chilled water pipe connection to FCU/AHU will be always bottom.