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Old 01/10/2008, 08:37 PM   #33
jdieck
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Join Date: Dec 2002
Location: Maumee, OH
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Quote:
Originally posted by NanoReefWanabe
i am curious why they say that valving the pump will save on hydro though...i thought increasing head pressure would increase power consumption which in this case would be wasted as heat...i could see if you actually changed the power to the pump to slow it down then i could see how that would reduce flow and save power...
The change in power consumption on a centrifugal pump is proportional to the change in flow and also to the change in head.

The actual formula is:
BHP = (Q x H x SG)/(3960 x % EF)
where
BHP = Break Horse Power
Q = Flow in gal per minute
H = Pressure head in feet
SG = Specific Gravity (1.0264 for salt water at 35 PPT salinity)
3960 is conversion factor foot-pounds for horsepower and weight of 1 gal of water (33,000 / 8.33)
% Eff is the efficiency of the pump

So as you can see for a given required flow increasing the head will require an increase in power and for a given head increasing the flow will require an increase in power.
When you install a valve to restrict the output of a pump you do so by increasing the head so to keep the same flow more power will be required but because you do not change the motor size the flow will drop and the drop in flow makes the power drop. It just happens that the drop in flow reduces the power more than the increase of head increases it so the net effect is an overall drop in power.

Take a look at this chart.
In indicates a typical performance curve for a centrifugal pump and charts the head vs flow (which is the one we usually get from manufacturers); the BHP vs flow (That we should get from manufacturers but our aquarium pump manufacturers do not usually provide it, I think they are afraid we find out about power consumption claims being manipulated) and finally efficiency vs flow.



Note that all centrifugal pumps are usually designed to operate optimally at certain point where a combination of flow and head will intercept with the maximum efficiency; either before or beyond that point the efficiency will drop but power will drop for before the point and will increase beyond that point.

So note that in this example a 10% increase in head (from 100 to 110%) will drop the flow from 100 to about 65%, the power from 100 to 80% and the efficiency also from 100 to 80% of the maximum efficiency.

On the other hand, an increase in flow from 100 to 120% by reducing the head from 100 to 78% will increase power consumption by about 8% (100 to 108%) but efficiency will drop from 100 to about 88%.

Hope this explanation helps in better understanding the behaviour of centrifugal pumps.


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