Everything from the first two lessons transfers, with one substitution: steam is the boiler's evaporation. Water boils off pure and leaves its dissolved solids in the drum, exactly as a tower's vapour leaves them in the basin.

$\text{COC} = \dfrac{\text{TDS}_b}{\text{TDS}_{fw}}$, where $\text{TDS}$ is total dissolved solids in ppm. The subscripts, stated once: b is the boiler water — the drum sample, the concentrated one — and fw is the feedwater, which is fresh makeup blended with whatever condensate came home. Feedwater, not makeup: returned condensate is nearly pure, so a plant with good returns starts from a cleaner feedwater and earns its cycles more easily.

$\%B = \dfrac{100\,\text{TDS}_{fw}}{\text{TDS}_b}$ is the blowdown percentage of feedwater, and it is simply one over the cycles in a percent coat: 20 cycles is 5 % blowdown, 10 cycles is 10 %. Note the difference from the tower — a boiler's percentage is taken against FEEDWATER, which already includes the blowdown, so there is no minus-one correction in it.

$B = \dfrac{S}{\text{COC} - 1}$ is where the correction returns. $S$ is the steam production rate and $B$ the continuous blowdown, both as mass flows in lb/hr — the currency of every North American boiler nameplate, alongside boiler horsepower at 34.5 lb/hr each. Same algebra as the tower, letter for letter, with $S$ where $E$ stood.

What limits the cycles is not scale, as at a tower, but carryover. Concentrate the drum too far and the water foams; foam is carried into the steam; wet steam erodes turbine blades and hammers traps. That is why the TDS limit is a number in a code table and not an opinion — roughly 3500 ppm for a firetube boiler, far less for a high-pressure watertube. And every pound blown down leaves at saturation temperature, so it is fuel going to drain as well as water. That is what a blowdown heat exchanger is for.