How much water flow at low speed?

I built a pool this spring with a Jandy 2 speed pump. It is a big pump -- 2.5 HP and at high speed it would shoot water 20-25 feet in the air out the fountain if you directed all the flow there. If you did the same on low speed, it would go up about a foot. (Obviously this is not how I normally use the pump, but it somewhat of a basis for measurement for comparison).

My understanding is that low speed drives the pump at half power. Is it correct to expect that you would get roughly half the water coming out (or a little more, because your flow resistance would be lower)?

Thanks-

Jason

Low speed is half the GPM not half the power so yes you should get roughly half the flow rate.

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

Mark was just making an estimate based on typical system curves being quadratic (i.e. a parabola) or approximately where "Head = constant * GPM^2". In that situation, and with pump affinity equations, cutting the RPM in half roughly cuts the flow rate (GPM) in half.

Actual system curves typically have a power that is somewhat less than "2" (often around 1.85) so the head doesn't drop quite as fast and that means the flow rate in the low-speed mode is somewhat higher than half the flow rate in the high-speed mode.

As Mark pointed out, energy consumption is not half. In fact, it's cut by much more than that assuming similar efficiencies at the two speeds. The input energy (e.g. Watts) is some constant (usually 2 or more) times the output energy since energy efficiency is around 50%. The output energy is a constant times the Head times the GPM. Since the GPM is roughly cut in half, the head is cut by one-fourth so the energy is cut roughly by one-eighth. If I use the more typical 1.85 factor, then the energy is lower at low speed by somewhat less than one-seventh as compared to high speed -- possibly around one-sixth.

Richard

Quote:

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Originally Posted by Poconos

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

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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.

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

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.