Staged Evaporation

a method for increasing the purity of the steam produced in drum-type steam boilers by artificially distributing salts and other impurities throughout the boiler water. The method consists in increasing the impurity concentration in the part of the boiler from which blowdown is conducted and reducing the concentration in the part in which most of the steam is produced and from which the steam passes into the steam superheater.

In staged evaporation, the water compartment of the boiler is divided into several sections by partitions. The feedwater is continuously supplied to the first-stage section; because of the difference in levels between adjacent sections, the boiler water flows through the openings in the partition into the second-stage sections and becomes the feedwater for these sections. The salt content of the boiler water increases in each successive stage of evaporation, and there is continuous boiler blowdown from the final stage. Two-stage or three-stage evaporation (Figure 1) is usually used, and the salt sections sometimes function as exhaust cyclones.

Staged evaporation was proposed in 1937 in the USSR by Professor E. I. Romm. The operation of boilers equipped with staged-evaporation devices has shown a significant long-term improvement in the quality of the steam produced.