How much water flow at low speed?

[mas985]

Two speed pumps run at typically 3450 and 1725 RPM on the motor. This doesn't necessarily mean 1/2 the gpm flow rate. The actual ratio 1/2 to full speed would depend on the centrifugal pump design and I have no idea how to calculate that and even if I did at one time in fluid dynamics class, it would have been long, long forgotten. It would be true in positive displacement pumps that are of a cylinder/piston or gear design. The actual flow and power consumption characteristics should be specified at each speed by the manufacturer.
Al

Actually, it is pretty close to 1/2 and it is based upon hydraulics equations. Approximation? Yes, but it is pretty close. These equations are generally refered to as pump affinity equations and can be read about here:

http://en.wikipedia.org/wiki/Affinity_laws

or more detailed here:

http://www.pumped101.com/pumpintro.pdf


and many other sites as well.

[jasontx]

Thanks for all the responses. While there seems to be some discussion regarding the actual GPM I should be expecting and what the electricity savings would be -- it does seem to confirm that what I am seeing is not right.

From information on Jandy's reference sheet (http://www.jandy-downloads.com/pdfs/pumprefguide.pdf), my SHPM 2.5-2 should provide roughly 160 GPM at 40' of head (high speed using 11 amps), and 80 GPM at 10' of head (low speed using 4 amps). I have no idea what my head is, but those numbers seem like they might be consistent with each other for comparison (and follow Richard's comment). If the 4 amps vs. 11 amps (from the Jandy chart) seem right, it isn't quite the one-sixth Richard was talking about, but could still provide some energy savings to run twice as long on low speed. 27% savings from current usage I guess ((11 amps - 2 times as long * 4 amps) / 11 amps).

Jason

[mas985]

That is correct and prettyr typical of two speed pumps. The energy consumption is usually about 1/3 of high speed but you have to run twice as long so the effective energy consumption is 2/3 or about 1/3 savings. Different pumps will vary slight about this number but they are all pretty close.

By the way, if you look at any two speed pump head curve, you can actually prove the pump affinity equations. The example you gave has 1/2 the GPM at 1/4 the head which is exactly what the affinity equations predict. The entire low speed curve is simply a scaled version of the high speed curve.