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Hi Tracy,
Here is a URL on theory of Centrifugal Pump operation (with drawings and
diagrams yet).
http://wrc.chinalake.navy.mil/warfighter_enc/SHIPS/shipeng/pumps/pump.htm
If I am interpreting what I quickly read, there is a pressure head and a
velocity head. If the thermostat is open it looks like the velocity head
absorbs the power, if the thermostat is closed it looks like the power is
absorbed by maintaining a Pressure head. Here are some extracts:
One of the most widely used non-positive displacement pumps is the
centrifugal pump. It utilizes the throwing force of a rapidly revolving
impeller. Liquid supplied to the center of the impeller or "eye" of the
impeller discharges at the outer rim. In traveling this distance, the water
or other fluid has acquired considerable velocity. A volute or a series of
diffusing passages slow down the fluid. As the velocity of the liquid
decreases, its pressure increases (velocity head decreases, pressure head
increases).
Pressure head (PH), also known as pump head, is the pressure produced at the
discharge side of a pump. This pressure head must be great enough to
overcome the head losses acting against it if the pump is going to move the
fluid. Pressure head is inversely proportional to velocity head.
Figure 2
Velocity head (VH) is the energy transferred to the fluid to impart a
velocity upon it. Flow rate is a measure of velocity head (measured in
gallons per minute or GPM). As stated above, velocity head and pressure head
are inversely proportional to each other, meaning that as velocity head
(flow rate) increases, pressure head (discharge pressure) decreases. Figure
2 shows this relationship for a typical centrifugal pump. Notice that when
there is no flow through a pump, its pressure head is at its maximum value.
This is the shut off head discussed earlier.
So between Pressure Head and Velocity head it looks like the pump energy is
absorbed by one or the other or some balance between the two.
This figure would seem to imply that regardless of whether the water is
flowing or not, there is kinetic energy being imparted by the pump to the
water. If the thermostat is open this presumbaly turns into Velocity Head,
if the thermostat is closed appearent this energy becomes Pressure Head.
At least that is how it appears to me.
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
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