Primary/Secondary flow issues?

[MarauderX]

Will there ever be a problem with primary pumps overwelming the secondary pumps? The only condition I can think of is that the bypass isn't large enough and there is little demand on the secondary.

 

[pretendfarmer]

MarauderX,

In almost 30 years of dealing with primary-secondary I have never experienced a case where the primary pumps force flow into the secondary. However,I can see that happening if the bridge piping is undersized. I hope this helps.

 

[DrRTU]

I believe this can happen when you have a chiller / fan coil system in which the chiller pump(s) are fixed volume i.e. no VFDs and the loads are fan coils on 2 way control valves connected to the secondary pump on a VFD. If most of the 2 ways valves close down and the VFD slows the secondary pump to minimum, then the chiller pump can exceed the house pump flow rates and you will have reverse flow in the close spaced tee by-pass line.

 

[cry22]

1. Normally, the secondary pump, is sized for two chillers (hypothetical 2-chiller plant configuration) while the primary pumps are sized only one pump per chiller.
2. 2-way valves and probably about 10% of flow on 3-way for each zone or floor for pressure relief at very low loads, OR
3. A prerssure relief valve could be installed at the secondary discharge to relieve water when load is less than the lowest secondary pump VFD speed.
4. Chiller staging should be set properly. If your load is less than say one shiller in a 2-chiller plant configuration, you should disable lag chiller when load is equal to Tonnage of one chiller +/- 10% (adjustable) - Tonnage is calculated with secondary water flow and delta T- for a period of 10 minutes (adjustable).
5. your by-pass should be sized to handle flow of one chiller minus lowest expected load (there are some diferent opinions on this particular point) and I would like to know from other engineers' opinions here on how to properly size the de-coupler..