Another min. flow issue, with VFD
Another min. flow issue, with VFD
(OP)
I have a question and would like your opinion about minimum pump flow. Application is a relatively small heating hot water system and variable frequency drive pump, running on loop DP controls. The pump is a very common vertical inline centrifugal. Pump duty point is 40 gpm at 72 ft.
Asked the mfr. what minimum flow would be required with the pump operating at minimum speed (25%) in order to size an end-of-loop bypass. The manufacturer came back with a 10 gpm limit for this model pump operating at full speed.
I reiterated the question: what would we need to flow with the pump at MINIMUM speed, not full speed. (under this condition the pump transfers 1.6% of its full speed energy to the fluid). Manufacturer came back with 10 gpm again, no matter what.
This is resulting in a change order for some fancy minimum flow controls. I think my manufacturer rep. (mfr. will remain unnamed) is all wet. To me, you need just enough flow to displace the volume of water in the pump to keep it from overheating. I'd think a 1 gpm end of loop bypass would be plenty, 2 gpm more than enough.
Thoughts? Maybe I'M all wet?
Thanks, CB
Asked the mfr. what minimum flow would be required with the pump operating at minimum speed (25%) in order to size an end-of-loop bypass. The manufacturer came back with a 10 gpm limit for this model pump operating at full speed.
I reiterated the question: what would we need to flow with the pump at MINIMUM speed, not full speed. (under this condition the pump transfers 1.6% of its full speed energy to the fluid). Manufacturer came back with 10 gpm again, no matter what.
This is resulting in a change order for some fancy minimum flow controls. I think my manufacturer rep. (mfr. will remain unnamed) is all wet. To me, you need just enough flow to displace the volume of water in the pump to keep it from overheating. I'd think a 1 gpm end of loop bypass would be plenty, 2 gpm more than enough.
Thoughts? Maybe I'M all wet?
Thanks, CB





RE: Another min. flow issue, with VFD
Maybe ask him what the expected sub 10gpm damage would end up being.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Another min. flow issue, with VFD
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Another min. flow issue, with VFD
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Another min. flow issue, with VFD
Remember that your head at 25% speed will be 1/16 of the full speed head - ie, 4.5 ft in your example...this may well be below any practical operating point of the pump.
RE: Another min. flow issue, with VFD
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Another min. flow issue, with VFD