Quote:
Originally posted by 8-Ball
By adding, do you mean that I can develop a curve by considering each discrete flow rate in turn and adding the corresponding pressure drops for all of the components.
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Some handy, "good enough" techniques:
Most devices have a PQ curve of the form:
dP= Q^2 * Rf
where
dP is the pressure differential across the device
Q is the the flowrate through the device
Rf is what I call the flow resistance
Based on Bill's data, waterblocks generally match the equation very well, radiators are more aberrant but 'close enough' for meaningful quicky checks.
You can calculate Rf for a device by picking a point towards the right end of a device's PQ curve and solving the equation. A quicky shortcut for determining Rf, is to just take the pressure drop at 10 lpm and divide it by 100.
So, find Rf for the devices in the loop. Then add them up for a combined Rf, or:
dP-system = Q^2 * ( Rf-block + Rf-rad + Rf-tubing... )
Plug the equation into Excel and graph. Voila! System PQ curve.
As myv65 pointed out, the nature of centrifugal pump PQ curves tends to minimize the effect of errors, so although this is a somewhat sloppy technique, you still can get results accurate enough to do meaningful comparisons fairly quickly.