Chemical-Consuming Loss Rate

This is the most commercially important flow on a cooling tower and it is not the makeup rate. Pure water vapour leaves the stack carrying no phosphonate, no azole, no polymer and no dissolved solids whatsoever — that is precisely why the basin concentrates and why cycles of concentration exists as a concept. Inhibitor can only leave dissolved in liquid water, so it leaves through the bleed valve and over the drift eliminators, and nowhere else: L = B + D. On a 750 gpm tower at four cycles that is 2.5 gpm of bleed plus 0.0375 gpm of drift, 2.5375 gpm — and that flow, times the year, times the residual you have promised to maintain, is the whole annual chemical budget.

Size a contract on makeup instead and you will be wrong by a factor of about four. The same tower's makeup is 10.04 gpm against 2.54 gpm of liquid loss, so the makeup basis predicts around 1996 kg of inhibitor a year where the true design maximum is 505 kg — 3.956 times the real consumption, because evaporation is the largest single loss and it consumes nothing. Quote that and you lose the tender to whoever did the balance properly; buy that much product and four fifths of it ages out in the store. The error runs the other way too: a pump set proportional to makeup rather than to bleed overfeeds by the same factor until someone throttles it and stops trusting the numbers. And do not drop the drift term because it is small — 0.0375 gpm is 1.5% of the liquid loss here and it leaves at full basin strength, while a tower with damaged eliminators can throw ten times its rated drift and quietly become the dominant chemical loss.