Constant Primary-Variable Secondary Loop CWS Problem
Constant Primary-Variable Secondary Loop CWS Problem
(OP)
System Description:
2 Trane Centravac 145 Ton chillers in parallel each with a constant flow 290 gpm at 35 ft. head (5 hp) centrifugal chilled water primary pump. 2 variable speed (Danfoss VLT6000 Drives)secondary chilled water pumps (B&G 5x5x9.5 in-line centrigugal)in parallel (1 on standby)with a max. flow of 540 gpm at 45 ft. head (10 hp). Two main loops are 6" diam pipe, and common piping is 6" diam with check valve.
Problem:
When system sees low load and secondary variable flow pumps are operating at low flow, chillers trip out due to low flow conditions (from constant flow primary pumps).
Question:
Does anyone have any idea why the constant flow primary pumps would be operating at lower flow conditions than when the system operates at full load? Can anyone suggest a good reference for design and trouble shooting Constant Primary-Variable Secondary pumping systems?
2 Trane Centravac 145 Ton chillers in parallel each with a constant flow 290 gpm at 35 ft. head (5 hp) centrifugal chilled water primary pump. 2 variable speed (Danfoss VLT6000 Drives)secondary chilled water pumps (B&G 5x5x9.5 in-line centrigugal)in parallel (1 on standby)with a max. flow of 540 gpm at 45 ft. head (10 hp). Two main loops are 6" diam pipe, and common piping is 6" diam with check valve.
Problem:
When system sees low load and secondary variable flow pumps are operating at low flow, chillers trip out due to low flow conditions (from constant flow primary pumps).
Question:
Does anyone have any idea why the constant flow primary pumps would be operating at lower flow conditions than when the system operates at full load? Can anyone suggest a good reference for design and trouble shooting Constant Primary-Variable Secondary pumping systems?





RE: Constant Primary-Variable Secondary Loop CWS Problem
More seriously, you obviously don't have a primary-secondary system, otherwise there would be no effect from one ciruit to the other. Study your schematic.
RE: Constant Primary-Variable Secondary Loop CWS Problem
Thanks for the reply.
RE: Constant Primary-Variable Secondary Loop CWS Problem
RE: Constant Primary-Variable Secondary Loop CWS Problem
RE: Constant Primary-Variable Secondary Loop CWS Problem
RE: Constant Primary-Variable Secondary Loop CWS Problem
RE: Constant Primary-Variable Secondary Loop CWS Problem
What type of control valves are using at your load/process side.Is there any by pass line in primary circuit.If so, how do you control the bypass flow and what is your differential pressure in primary circuit.
RE: Constant Primary-Variable Secondary Loop CWS Problem
It is my opinion that DougMB came the closest in nailing this problem. I would bet a dollar to a doughnut that the trip is as likely low temperature and low flow. The chiller control safeties just may not be indicating which was first. Still, the check valve in the primary circuit may cause this anomaly...hence low flow and low temp cutouts.
This sounds like a common control problem when there is no automated control to command lead-lag chiller operation. Depending on the amount that these chillers can effectively un-load, they would likely fail on refrigeration safeties before they fail on flow alarms. However, with the check valve in the primary circuit....I must question the Cv and the actual pressure drop on this. (See my PS on this matter)
It is more likely that one chiller at low load would be capable of handling the low flow required by the VFD secondary system. This sounds like a classic control mis-match that cannot control design rate vs. low demand. Refrigeration machine Mfgs are not know for allowing this level of flexibility given the safeties that are build into their products.
Would recommend that a sequence that measures the chiller delta t vs. the secondary chilled water pump VFD operation. As the flow rate drops, a chiller can be alarmed to be turned off if the pressure differential of the system is being maintained. (We do not recommend the chillers be fully automated for Stop/Start functions) A bell alarm would work well here.
Recommend that you verify the actual percentage that each chiller can unload to. Without knowing the specifics, I would expect not less than 30% unless special provisions have been provided in this selection. Coupled with the chiller flow (Check Valve PD) and time-out for restart in low temperature conditions, these are your likely culprits.
PS: Contact Bell & Gossett and they will confirm that the use of check valves in a primary and secondary systems are NOT recommended. They cause more problems than they solve. Likely one of the contributing factors here.
RE: Constant Primary-Variable Secondary Loop CWS Problem
RE: Constant Primary-Variable Secondary Loop CWS Problem
RE: Constant Primary-Variable Secondary Loop CWS Problem
Could you let us know how your chiller system is going on?
Thank you.