Why their is less water pressure on the outlet side of ahu when compared to inlet side side ?

its mainly beacuse of pressure drops in AHU, as it consists of number of bends in AHU there is a pressure drop, so while selecting the pump you must be consider the pressure drops too.....

From the pump the water flows through the piping and then enter the terminal units or AHU and there is a pressure drop inside the coil of AHU so the pressure at outlet is dropped by the same amount (approximately) of pressure drop inside the AHU coil.

Hope this will work

Pressure drop exist in the AHU against the cooling coils and the cooling valve,valves and fittings and for the main reason that this is the return line.

deframed question the actuall question must be