Variable Primary Chilled Water Distribution

[HVACHawaii]

Can anyone tell me what the preferred configuration for pumps in a variable primary chilled water distribtuion system? Is it better to have pumps dedicated to each chiller or is it better to have the pumps with a common heaher requiring that control valves be provided at each chiller? Please explain your answer.

I have asked this question to many people and seem to get a different answer each time.

Any help would be appreciated.

 

[BronYrAur]

I don't know which is "better", but if you just have 1 pump for each chiller, you lose redundancy. Strictly from the standpoint of having backup, I would header them.

 

[HVACHawaii]

You have redundancy because there would be two chillers. In a headered configuration, if one pump failed you would still only be able to operate one chiller unless you have a stand-by pump.

 

[SAK9]

No major issues with both the systems.

In a headered configuration,assume that the first chiller is in operation.Before you start the second chiller,you need to open the motorised valve on it.This will divert part of the flow from the first chiller causing it to trip on low flow.By incorporating a time delay on the low flow cut out(normally a differential pressure switch),this can be avoided.

 

[marcoh]

Either system works but a pump dedicated to each chiller is simpler and most systems I have seen have used this approach.

If redundancy is required there should be a redundant chiller/pump. With a dedicated pump to each chiller, if the chiller or pump fails then both will be out. And with motorised valves there is an additional item to potentially fail?

 

[RossABQ]

In a headered configuration,assume that the first chiller is in operation.Before you start the second chiller,you need to open the motorised valve on it.This will divert part of the flow from the first chiller causing it to trip on low flow.By incorporating a time delay on the low flow cut out(normally a differential pressure switch),this can be avoided.

If this is a variable primary flow system, there would presumably be controls to prevent this scenario. Your description is valid for most constant primary flow systems.

No info is given on the chiller size, but most 100 - 200 ton aircooled chiller units typically used for office/commercial HVAC will tolerate a reduction in primary flow down to 50%. The problem is that the #1 chiller (already running) will then start to unload based on LWT. To prevent this the #2 pump should start just before the #2 chiller valve is opened and the chiller started (in a headered configuration). Both pumps are presumably under VFD control for variable primary flow operation.