In fan coil units, why we put water inlet in the bottom of coil and the water outlet (return) in the above of coil ?

It is not very necessary but it is positioned like this because to make most of the moisture as low as possible in order to remove water droplets from air. As water enters the coil it is at lowest temp so most of the moisture is removed by most cooled coil

Hope this works

The reason behind that is the difference in the density between (chilled-in) and (chilled-out) water due to the Temperature difference of them,as a consequent,they put the water out which has less weight in the top of the coil.

Because this path from bottom to top consumes much time than from top to bottom; thereby reulting in effective heat exchange between the two media, chilled water that takes up the heat & air that evetually gives desired cooling.

not only in FCU but also in AHU,FAHU and HRU

it make it easer to guide all the air to the top(if any) and not to obstruct the natural flow of the air were it will produce turbulance in water. beside that this way is matching the standards of heat exchanger.

To have maximum Energy exchange

To let max air (If present) purge out of system

Universally applied for all Energy exchange systems.

Water enters from down going to  up to insure full wet for whole  internal cross section area of coil pipe in order to fill pipe with water completly  without leaving any empty position inside the pipe that water can not reach.

In & out connections either be up & down. actually it depends upon direction of Flow from AHU fan. CHW return always be from Fan  Air Flow direction being logic that  the Air Temperature should decrease gradually from higher Temperature i.e CHW return  to Lower Temperature i.e CHW Supply to avoid Turbulence. Therefore Air Flow from FAN must touch first CHW return pipe either it is up or down,it only depends upon Coil manufacturing.

It is very simple that when water  travel from the bottom of the coil it reaches to the above outlet it exchange

the heat and condense the moisture.

Because of two reasone 

1. Enhance initial heat exchange through laminar flow 
2. Avoid bare heat transfer areas through pushing bubles away from the FCU Coils

thanks everyone i really appreciate your valuable reply, it is very important and useful for me,thank you again 

The theory behind counter-flow, which explains why this is the case, is to keep the highest temperature difference across the coil while the air is going through it.