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It is normally installed on return because of the following reasons
.
1. ASHRAE Recommendations.
2. The coil of the unit should be flooded in case the valve is closed.
3. With a control valve on the return side, the water is at a higher pressure through the coil, reducing off gas, thereby lowering dissolved oxygen content. Lower dissolved oxygen extends equipment life by minimizing the air entrapment.
4. If it is a hot system, some manufacturers have temperature limitations on the actuators. If there is a180°F limit on the actuator (common to some actuators), the return side is preferred as it will operate at a cooler temperature.
Traditionally, balance valves are used on all heating and cooling
coils when three-way coil control valves are used. The
advent of variable volume systems with two-way control
valves on these coils creates new considerations for the use of
balance valves.
Well explained by Engr Naweed
Balancing valves are used for setting the scheduled distribution of water flow rate, and the uniform distribution of available pressure from the chilled water pumping system (source) to all the piping networks and all the AC equipment including chillers. The supply of chilled water to the equipment is to the cooling coils for the purpose is self-explanatory. So we need to consider the pressure drop in the cooling coil too when discussing about the chilled water system.
Hence the DRV or the balancing valve should be downstream of any individual equipment or a branch piping. The term double regulating valve (DRV) denotes to regulate water flow rate as well as pressure.
Answer of engineer Naweed Parkar is excellent, and enough for me
