Chilled Water Coil

[fayazdin1]

Dear friends,
I have a very simple chilled water system with the following characteristics
• Warehouse application
• Two chillers with constant flow pumps
• Four AHUs for with 3-way control valves
Sequence: When space temperature reaches a setpoint, flow through AHU coil stops and chilled water is bypassed.
Question: Why do I need balancing valve in bypass line while it is only ON/OFF 3-way control system. I have a balancing valve in return line downstream of 3-way valve

Fayaz

 

[KiwiMace]

The balancing valves in your bypass lines serve to represent the same pressure drop as the AHU coil when the valve is in bypass. Without these valves you would see a large change in pressure in the system as the 3 way valves opened and closed. Big step variations in pressure could destabalise the control.

The return line balancing valves trim the flow to each AHU depending on how far each is from the pump.

 

[Waramanga]

Hey Kiwimace, I looked this up not so long ago and I was surprised that the DRV in the bypass did not actually make much difference to the operation of the system, I think it was an ASHRAE article, I will have to track it down again. I came onto a job towards the end and noticed that they didnt have any balancing valve in the bypass. I spoke to some plumbers and they said sometimes they put them in and sometimes they dont depending on the designer. I dug around and found some authoriative source which indicated they were nice but made not a heap of difference....I will have to find it again!

 

[KiwiMace]

Hmm.. this would come down to valve authority. I'd guess that if you have 20+ coils it probably would't make a difference, but if you have four, I would err safe side & stick them in - that's 25% or more of your flow taking a short cut. Also if the distribution dp far outweighs the coil dp, it might not matter so much either. I would be interested to read the article though - note the month/year if you find it.

 

[Drazen]

waramanga, did you check pumps in your examined systems? i would say that it is difference between variable flow and constant flow system rather than designer's preferences. balance valves are more or less disappearing in variable flow systems.