AHU Control Valve Location

[biswaji]

It is the standard engineering practice that control (2 way modulating) valve is installed in the return pipe of an Air Handler Unit (AHU). What is the reason behind choosing return line over supply line? Apreciate your response.

 

[quark]

Biswaji!

For proper heat transfer the entire coil should be filled up at all conditions. If you put a control valve in the supply line, this will keep coil partially filled and this may even create airlock. But there are some control valves which are being used in supply line. They are 3-way valves and are called diverging valves (If in return line they are called mixing valves)

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[KenRad]

I don't know why others do it, but I like having the chilled water control valve on the return side of the coil simply because it is relatively difficult to achieve a good insulation vapor barrier on a valve. In many cases, the return line temperature is above dewpoint, in which case the valve won't sweat even with a poor insulation job.

In a closed loop system with a pressurized return line, the coil should stay full at all times, regardless of whether the valve is placed in the supply or return.

 

[235zzzo]

Hello!

Good post of yours, Quark, about the 2-ways/gates! That's correct! I tested a better thermal response!
About the 3-ways/gates, the first point remains valid as well. Let me add another strong reason, which turns to be the place of the control valve, that is on the return side, just because it is always easier in terms of Hydraulics, to join two mass-flow rates than to diverting them in almost control/modulating functions. That is, the valve-authority is maximal for this flow-configuration/arrange. That's why the most convenient flow sense is expressed/indicated on valve hydraulic body according.
zzzo

 

[ChasBean1]

I like all above answers, especially KenRad's about it being less likely to sweat - never really considered that before but it's a good thought.

My thoughts are,

1) Possible fluid energy losses in throttling at the control valve, mainly in the form of pressure. Might result in small temperature changes, can reduce overall coil efficiency (this one might be a reach).

2) Lower pressure in the coil with valve on the supply line could be more likely to cause gases to come out of solution throughout twists and turns inside the coil, reduce coil efficiency. Could increase the likelihood of hammer.

3) The Bible (ASHRAE Applications, Chapter 45) draws it that way...