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Another min. flow issue, with VFD

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ChasBean1

Mechanical
Jun 8, 2001
1,163
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
 
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I'd be willing to go as low as 5 gpm before overheating became an issue. However as itsmoked is suggesting, you're going to have other problems if you run it down that low habitually anyway.

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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)
 
That's startup territory ONLY. Besides, your motor won't be loaded, your VFD won't be loaded, so efficiency for both will be crap and you won't get any head out of your pump to speak of, so why bother running down that low at all?

**********************
"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)
 
As a 'rule of thumb', lacking anything more concrete, you can typically vary the min flow proportionally with speed. If the min speed is 25% of full speed, and the min flow at full speed is 10 gpm, I'd expect the min flow at 25% speed to be around 2.5 gpm.

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.
 
That's all used up by the higher load INefficiencies of the motor and VFD. At 10 gpm x 4.5 feet (Isn't that iteself appx = zero) what % of full power load is that? Motor & VFD efficiencies must be.. hummm what 10%, 15%?

**********************
"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)
 
Ok - thanks all for your input. I know there's no concrete answers for exact flow rate to prevent pump damage, just wanted gage your thoughts relative to min. order of magnitudes, etc.
 
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