X-Mozilla-Keys:
Fred wrote:
My thoughts on the questions raised:
1) The pump performance for an incompressible fluid depends only on the
pressure rise across the pump, not the inlet pressure, as long as the inlet
is not cavitating. Cavitation is unlikely as long as the inlet is not
restricted and the water temperature is low (near ambient) since cavitation
is a boiling phenomenon, and you need very low pressures (a couple of inches
of mercury) to boil water at 60-70F. Suck out of a bucket with a water level
that is at or above the pump inlet to help avoid cavitation, but mainly to
get the pump primed so it will start. Control flow with a valve at the pump
outlet, not the inlet.
2) Flow meters are devilish things, and a venturi flow meter provided a
pressure drop signal that is proportional to the square of the velocity.
Thus it has limited flow range. A 3 to 1 change in flow yields a 9 to one
change in the pressure signal. At the low end it is difficult to read
accurately. Similarly, the pressure drop through the flow meter (if it is a
venturi) varies as the flow rate squared, so you get a lot of pressure drop
at high flows. In my opinion, it is not worth the effort to build a flow
meter since you need to run it into a bucket anyway to calibrate it.
3) Nothing wrong with garbage cans and scales or garbage cans marked off with
an ink felt pen showing how much water is contained as determined by adding
water via a well known measurement device, like a quart measuring cup. See
note on calibrating flow meters above. In grad school we always had to
calibrate flow meters by flowing into a bucket and a large scale. Failure to
do so could cut your lab report grade several points. Amazing the errors one
could find....
4) Avoid putting a right angle fitting in front of the pump inlet.
Centrifugal inlets do not like angled inlets as they can cause complicated
secondary flows that screw up flow in the impeller and thus affect
performance.
5) It takes a LOT of mechanical energy to heat a volume of water so if you
recirculate, you need not worry about heating. One horsepower hour is 2550
Btu. So let's say you have 30 gallons of water, or about 250 pounds (8.33
pounds per gallon). Putting one horsepower into pump for one hour will
create 2550 Btu of dissipation in friction that heats water and raises the
water temperature 10 degrees F. Not worth worrying about. No cooler needed.
Hope this helps.
Fred Moreno
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