Chilled water system bypass

[ssn61]

In order to keep the minimum flow through a chiller (or a boiler), where is a good location to put a bypass in a variable primary pump system?

 

[KiwiMace]

After your primary pump. You may want to put it at the end of line to remove stagnant water, but technically it can be anywhere. Use of a bypass runs the risk of returning water too cold to the chiller and the resulting short cycling, so consider a buffer tank in your system to mitigate the impact on your customer side.

 

[DRWeig]

Hi ssn61,

Listen to Kiwi, good answer.

Also, please avoid posting the same thing in more than one forum. Multiple threads on the same topic makes for a confusing situation.

This is the most appropriate place for your question, I think.

Take a read through the forum policies when you have a chance, the link is below my signature.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[ssn61]

Thanks Kiwi, can you elaborate on the required buffer tank please?

 

[KiwiMace]

When you run with a very low load, and with a lot of bypass, your chiller gets a low return temp and will cut the compressor out and in to prevent freezing the evaporator, which makes the temp cycle. This is also an issue when the chiller is oversized.

This might or not matter depending on your application. If you have one relatively small 24hr critical load then the cycling of temp might be detrimental to control. If you have a few zones of comfort cooling on night setback then no one may care.

The buffer tank adds capacity to the hydronic loop which stores and releases energy, evens out the temp variation and reduces the cycling on the compressor.

 

[cdxx139]

The chiller mnaufacturer will have a minimum water volume/ton requirement. When you do your expansion tank calulcation and get the volume of the system, see if you meet the minimum volume required for the chiller mnaufacturer. If you dont meet the minimum, then the difference is the smal;lest size your buffer tank should be (Bigger is always better).

Another option to a buffer tank is:

Oversize your air seperator
OR
Oversize your mains

to meet the minimum volume requirement needed

knowledge is power

 

[Drazen]

some schools advocate putting it long way downstream, as once some coil opens there will be no immediately available cold water so undesirable cool-down periods can compromise comfort, depending on length of lines and load pattern - longer lines with zones that have quickly variable loads are the worst case.

 

[SAK9]

ssn61:
The best place to have the minimum chiller bypass valve is the plant room itself between the supply and return headers.I have not seen it being located anywhere in VPF systems.This valve operates with a high differential pressure and therefore can be noisy.The buffer tank is provided for an entirely different reason and is no substitute for the bypass valve in VPF systems.

 

[urgross]

I finished a VPF last year. Can't disagree with SAK9, that's exactly where the bypass went. Another consideration for the location is cost.