By Norm Hall | October 4, 2010
|Printer Friendly (PDF)
R. L. Deppmann Company Monday Morning Minutes addressed corrections to system GPM (gallons per minute) and system friction head during the last three weeks. Today we look at the pump selection. Centrifugal pump curves and the brake horsepower required changes depending on the fluid. In HVAC design we normally assume water is the pumped fluid. There are many times when the assumption is water, since there is no fluid mentioned on the schedule, yet we find a glycol makeup unit attached to the system. The design engineer should schedule the fluid type and temperature range expected as well as the flow rate and pump head required.
The curves below show some of the effects of different fluids. The first curve is a Bell and Gossett 1510 pump for water. The impeller is 10.125” and the brake horsepower is about 15HP. The second curve uses 50% Ethylene Glycol/Water. The impeller remains 10.125” but the horsepower required goes up to 17.11. Notice the pump curve. The blue line is the normal flow/head curve and the green line is corrected for the fluid. They are the same. In fact, the only effect that makes a difference is the horsepower. The third curve shows the effect of pumping a Quench Oil, not your typical HVAC fluid! Notice the horsepower is now 24.31 HP. Also notice the impeller diameter must be greater due to flow/head corrections. Click the images below to study the curves.
REMEMBER: Correct the system flow and head for the fluid being used and then schedule the fluid type and temperature range. Let Deppmann or the B&G selection software make the corrections to the curve.
* Trademark of The Dow Chemical Company
Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.