How much water flow at low speed?

[chem geek]

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