why is motorized valve located in the outlet line of fan coil unit

i want to know ,why not in the supply line,is this so that line will not become empty of chilled water.

I am assuming this is a 2-port valve which just opens and closes the water flow? Generally the supply to a chilled water fan coil is at the bottom of the coil and exits at the top of the coil. This is to drive any air out of the system. With your pumps circulating water around the system, when the valve is closed, water won't circulate through the fan coil unit. It is a basic system. If all your fan coil units only have 2-port valves, you must have a pump by-pass circuit somewhere, otherwise there may be occasions when all the valves are closed and the pump pushes against a blockage! Hope this helps.

It doesn't really make any difference where the valve is situated, the effect is the same.

Maybe in that way you have more certain that the air trapped can be expel from the coil, even when the fan coil is stopped. Also when the fan coil is working due to the pressure drop in the valve the coil will be "more filled" with brine (with less air) which could increase a little the heat convective coefficient on the brine side.

Of course I believe there are many situation which the effect can be no noteciable.

Another reason for control valves to be placed on outlet is some older coils have air purgers which work beter under more pressure. As for heating and cond water valves it reduces any air buble sizes and increases heat tranfer. The old pressure/temperature/volume relationship.

Where would a water flow control valve be best positioned?

Exit of HE, or inlet & why?

I was always told the control valves on water coils go on the exit to prevent from air binding the coil.

There is no difference in placing the control valve at inlet or out let of FCU.If you place at outlet side the coil is filled with water always and it can give some heat transfer even when the valve is closed.Also airvent is located on top outlet of FCU which interm assist in purging.

because of the arrangement flow that needed by the fcu, when motorized valve located in the outlet fcu, any movement of mv (like open or close) will cause influence flow of water (gpm). for example if mv has signal from thermostat or room temp sensor that order it to open only 25%, the speed of water flow in coil fcu will decrease because the mv that located in outlet fcu only open in 25 % and then cause of the heat transfer process because the reduce water flow that contact by air. it will different if mv located in inlet fcu