Boiler Cycles of Concentration

A boiler is an evaporator with a pressure gauge on it: steam leaves as essentially pure water and every dissolved solid stays in the drum, so boiler water concentrates exactly the way a cooling tower does. Feedwater at 125 ppm TDS producing 2500 ppm in the drum is running at 20 cycles. Because the boiler drum is small relative to the throughput, that concentration builds in hours, not days.

What caps the cycles here is not scale but carryover. Above roughly 3000–3500 ppm TDS in a low-pressure firetube drum, the surface tension changes and the water begins to foam and prime, throwing slugs of dissolved solids into the steam header, where they bake onto superheater tubes, cut control valve seats and wreck turbine blades. ASME's boiler water guidelines tighten sharply with pressure: several thousand ppm is tolerated below 300 psi, but only a few hundred at 900 psi and single digits in a modern utility unit. The practical fix for high cycles is not chemistry, it is a better feedwater: softening, dealkalisation or reverse osmosis lowers the numerator and the denominator at once.