Condenser Water Flow Rate

A cooling tower has to dump more heat than the building put into the chilled water, because the compressor's own electrical input ends up in the refrigerant too. The heat rejection factor accounts for that: about 1.25 for a typical electric chiller (12,000 BTU/hr of cooling plus roughly 3,000 BTU/hr of compressor work = 15,000 BTU/hr rejected per ton), 1.15–1.20 for a very efficient machine, and 1.7–1.8 for an absorption chiller, which is why absorption towers are so much larger.

Run one ton through the arithmetic at a 10 °F tower range: 15,000 ÷ (500 × 10) = 3.0 gpm — and there is the industry's favourite rule, 3 gpm per ton, standing on ASHRAE's 85 °F/95 °F condenser design. Widen the range to 15 °F and you need only 2 gpm per ton, which saves pump horsepower but costs tower approach; that trade-off is the whole subject of variable-flow condenser design. On the water-treatment side, ΔT and flow set the cycles of concentration you can hold: every 10 °F of range evaporates roughly 1 % of the circulating flow, so a 300 gpm tower at 10 °F range boils off about 3 gpm and must be bled and inhibited accordingly, or the tubes scale and the approach quietly walks away from you.