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Understanding NPSH Part 2
Thu, March 03, 2019
Last time, we saw that as we supply more pressure to the pump suction than what the pump manufacturer requires, we are OK. But that isn’t the whole story: most pumps in HVAC operate over a range of flows, meaning they will change flow rates based on loads or control strategies. So, what happens as flow rates change? The NPSHr curve can be described in three sections. The curve representing the lower flow rates is almost linear, with a very small increase as flows increase. As flow continues to increase, the curve begins to resemble a squared function, where a small increase in flow will have a more significant impact on the NPSHr. And at some point, the curve becomes almost vertical. It is usually best to operate within the flatter portions of the NPSHr curve. That brings us back to where the pump is operating. Pumps operating as close to their best efficiency points are more stable and will be able to handle higher flow rates with less of an impact due to NPSHr. Pumps operating near the end of their curves or at high speed may not be able to satisfy NPSHr if allowed to run out on their curves. Since NPSHr is based on the geometry of the pump itself, the size of the impeller eye can lead to a higher or lower NPSHr value. If the eye is smaller, NPSHr will likely be higher than for the same flow rate with a larger impeller eye. That’s one reason to be cautious in selecting pumps close to the end of their published curves. Also, higher RPM pumps will have a higher NPSHr because they are chosen to be physically smaller (with smaller impeller eyes) than pumps with lower speeds. Where NPSH may be a concern, such as cooling tower applications, select your pumps carefully. This is one reason why Bell & Gossett designed their pumps with a larger “efficiency island” (more about that later). Next time, we look at why a published NPSHr curve still allows for cavitation, and how adding air can actually help in certain situations.
Tags: bell & gossett , Bornquist , Education , engineering , HVAC , motors , NPSH , pump curves , pumps
Last time, we saw that as we supply more pressure to the pump suction than what the pump manufacturer requires, we are OK. But that isn’t the whole story: most pumps in HVAC operate over a range of flows, meaning they will change flow rates based on loads or control strategies. So, what happens as flow rates change? The NPSHr curve can be described in three sections. The curve representing the lower flow rates is almost linear, with a very small increase as flows increase. As flow continues to increase, the curve begins to resemble a squared function, where a small increase in flow will have a more significant impact on the NPSHr. And at some point, the curve becomes almost vertical. It is usually best to operate within the flatter portions of the NPSHr curve. That brings us back to where the pump is operating. Pumps operating as close to their best efficiency points are more stable and will be able to handle higher flow rates with less of an impact due to NPSHr. Pumps operating near the end of their curves or at high speed may not be able to satisfy NPSHr if allowed to run out on their curves. Since NPSHr is based on the geometry of the pump itself, the size of the impeller eye can lead to a higher or lower NPSHr value. If the eye is smaller, NPSHr will likely be higher than for the same flow rate with a larger impeller eye. That’s one reason to be cautious in selecting pumps close to the end of their published curves. Also, higher RPM pumps will have a higher NPSHr because they are chosen to be physically smaller (with smaller impeller eyes) than pumps with lower speeds. Where NPSH may be a concern, such as cooling tower applications, select your pumps carefully. This is one reason why Bell & Gossett designed their pumps with a larger “efficiency island” (more about that later). Next time, we look at why a published NPSHr curve still allows for cavitation, and how adding air can actually help in certain situations.
Tags: bell & gossett , Bornquist , Education , engineering , HVAC , motors , NPSH , pump curves , pumps
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