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The first relies on physical or chemical effects to delay deposition of scale on the hot surfaces; the second restricts the concentration to a level at which precipitation will not occur. In both cases, the accumulation of solids is removed by bleeding off water from the circuit to drain, in addition to that which is evaporated
The concentration of solids in the circulating water will increase until the amount carried away by the bleed water compensates for that not carried away in the water vapor. So, if
cm = concentration of solids in make-up water (kg/kg)
cb = concentration of solids in bleed-off (kg/kg)
we = mass flow of water evaporated (kg/s) wm = mass flow of make-up water (kg/ s
Mass of solids entering =mass of solids leaving
cm × wm =cb × (wm – we)
The concentration of mains make-up water, cm, is obtained from the water supply authority. The permissible concentration, cb, will be decided by the method of water treatment or the assumed concentration of untreated water which will prevent precipitation.
