Assumeing a constant pump speed, when the
thermostat closes and head pressure goes up, power required to drive the
waterpump does NOT go up. It actually goes DOWN. Reason: There
is less mass being accelerated (energy) at lower flow rates. In the
extream example (zero flow) the same water in the pump housing is being
spun around at a constant velocity which requires no energy. Of course
there are losses in the pump so the energy consumed is not zero.
This argument applies ONLY to centrifugal pumps (of
which automotive waterpumps are an example) and not positive displacement
types (like oil pumps).
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Tracy, I agree with you with respect to the amount
of energy being used to pump water, however the frictional losses in the pump
increase with rpm and decreasing flow. The point I was trying to make
(apparently poorly) is that the power dissipated in the pump can rise rapidly
with RPM. The energy into the pump is turned into heat energy, and although this
effect is small at lower RPM, it rises rather quickly with RPM.
A water brake dynomometer is just a pump that is
dumping its energy into raising the temperature of the water.
Bill Schertz