Flow rate is in units of volume fluid moved per unit time. Head is usually used when referring to total pumping head, which is a measure of the total pressure that a pump must develop to produce a certain water velocity. Total head is the sum of the pressure from height differences, the pressure from friction losses in the loop, and the velocity head. You see "head loss" used over "pressure drop" by engineers and mfgrs because the numbers for head loss are not dependent on the identity of the fluid being pumped. For our purposes, pressure drop and head loss are interchangable I think.
So a pump's performance is quantified by a P-Q curve where the pump's total head is plotted vs. flow rate to yield how pump performance decreases as they are faced with more restrictive (higher head loss) discharge situations.
To further complicate things, flow rate (Q) is not what really is relevant to cooling; fluid velocity (Q/area) is what carries through in all heat transfer and turbulence calculations.
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