VFD pumps and efficiency
VFD pumps and efficiency
(OP)
We have been trying to determine the impact using a VFD drive has on the efficiency of large water pumps used by municipalities. Let's say in a typical 110 kW pump driving a single stage double suction split case centrifugal pump, that we add a VSD.
The fixed speed pump was operating at a BEP of 90% at 1470 RPM. If I reduce the RPM to 1200 RPM I know from the affinity laws how to shift the head/flow curve and the power curve. However what happens to the efficiency curve? Does it retain the same shape and peak and just move to the left on the flow axis? Does it move left and drop down on the efficiency axis? Is there a general formula to calculate the new efficiency curve based on the speed ratio of N2 / N1?
The fixed speed pump was operating at a BEP of 90% at 1470 RPM. If I reduce the RPM to 1200 RPM I know from the affinity laws how to shift the head/flow curve and the power curve. However what happens to the efficiency curve? Does it retain the same shape and peak and just move to the left on the flow axis? Does it move left and drop down on the efficiency axis? Is there a general formula to calculate the new efficiency curve based on the speed ratio of N2 / N1?





RE: VFD pumps and efficiency
RE: VFD pumps and efficiency
RE: VFD pumps and efficiency
There are no rules for efficiency change due to speed change - only assumptions based on knowing the particular pump design etc.
RE: VFD pumps and efficiency
However if I am given a pump efficiency curve by a manufacturer, can I not use the same affinity laws to scale the efficiency curve? Has anyone done this from first principles and compared it to measured data? can anyone point me to a manufacturers web site for water pumps where various speed curves are shown for a pump? I have found no end of sites where different impellor curves are shown but none with speed.
In fact given that;
R is impellor radius, d is speed
Capacity varies directly as the speed or diameter:
G2 = G1 (R2/R1) or G2 = G1 (d2/d1)
Head varies as the square of the speed or diameter:
H2 = H1 (R2/R1)^2 or H2 = H1 (d2/d1)^2
Horsepower varies as the cube of the speed or diameter:
P2 = P1 (R2/R1)^3 or P2 = P1 (d2/d1)^3
So the impellor curve relationship appears to be the same as the speed relationship. I have impellor size graphs that clearly show the peak efficiency drops as the impellor gets smaller, so can I make the reasonable extrapolation that EXACTLY the same drop occurs when I vary speed?
RE: VFD pumps and efficiency
The link below is to a scanned copy of the variable speed curve for a 3196 6x8-13 - you can see how the efficiency varies with speed as an example.
RE: VFD pumps and efficiency
Another warning! Some manufacturer's programs do not ask for system curve information, only pump information. That is a signal that they DO NOT take into consideraton the effects of the system curve in their calculations. Therefore, if you have static head, be extremely careful with any results you get from them until you are positively sure that they do consider system curves too.
Other important factors that enter into the calculation of overall efficiency are the efficiencies of both the motor and the VFD when they are operating at reduced load. It is another thing that some pump manufacturer's programs do not consider. Under low power loads, those efficiencies reduce very fast. Be sure they consider those reduced efficiencies before believing any predictions of energy savings at the lower power loads.
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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/