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To World Pumps: Response to Akseli Savolainen Article "Driving towards a better future" in the April 2005 issue of World Pumps makes the same mistake as many other articles about variable speed drives. In this article the author makes reference to controlling a pump with a control valve as being similar to driving a car with one foot on the brake and one on the gas. This statement should be used as an indicator that this person does not understand the basic principle of centrifugal pumps. A car engine is a piston type engine which is more akin to a positive displacement or PD pump than a centrifugal pump. With PD pumps and car engines, controlling the RPM has a direct effect on horse power and output. With PD pumps or car engines, power required to move the vehicle or displace fluid at the TDH required can be accomplished at very low RPM. With centrifugal pumps, TDH is reduced by the square of the RPM. This limits the minimum speed or RPM that will produce enough head to buck the static pressure. Statements about running a pump at half speed only using one-eighth of the horse power, simply confuse readers into believing this is possible. Unless the pump was absurdly oversized to start with, very little reduction in RPM is possible if the TDH is to be maintained. This can be verified by using proper pump curves. However, the pump curves used in Figure 2 and Figure 3 of Akseli’s article are a gross misrepresentation of a comparison between throttling valves and variable speed pumps. Figure 2 falsely shows the horse power increase as the pump is throttled from a Q of 10 to a Q of 7. This would be true of PD pumps and car engines but, with centrifugal pumps the Q or flow rate is directly proportional to the power required and inversely proportional to the back pressure. Therefore, an actual pump curve would show that as flow rate decreases, the power required also decreases. In Figure 3 of this article you can clearly see that the head required, which is H 10, has been reduced to 7 with this reduction of RPM. Only if the system piping where engineered too small to start with, would a reduction of flow from Q10 to Q7 cause a reduction of head from H10 to H7. Otherwise the head required would stay at H10, as the high rise building or the mountain we are pumping over cannot change its height from H10 to H7 simply because the flow required is reduced. Maintaining an H of 10 while reducing the flow from Q10 to Q7, this pump could only be slowed by 9%. This would only make a reduction in horse power of 25% not 58% as stated in this article. The same 25% reduction in horse power could easily be met by a pump running at full speed and simply restricted with a throttling valve. Most pumping systems require the same head at low flow as they require at high flow. This makes the energy calculations in this article completely invalid. This makes remarks about the reduction of CO2 emissions from variable speed drives also absurd. Thank You Cary Austin |
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