A Practical Manual On Sea Water Distillation
With A Description Of The Necessary Machinery For The Process

MULTIPLE DISTILLATION. 209 being used to heat the boiler feed, it may be sent into the coils of the first evaporator, giving it a direct multiple effect. If this arrangement is made, a reducing valve must be placed on the boiler primary steam to the first evaporator, or the back pressure on the pump exhaust will stop the pump. Or, (2) this exhaust steam after passing into the tubes of the first heater may be made to continue on into the evaporator coils of the first evaporator, the water formed by condensation in the heater tubes going along with it. This has the ad- vantage of profitably using any uncondensed exhaust steam that has passed through the tubes of the first . heater unused—i.e., unconverted into water—and, there- fore, still having some latent heat to dispose of. (3) Or the pump exhaust may be led direct into the coils of the second evaporator, along with the secondary steam from the first evaporator. This has its merits, but it also lias a rather disturbing effect on the ratios of pressure and production in the various stages of distillation. It, however, gives to the pump exhaust multiple effect, but its effect is one grade lower than that of the boiler steam. Note.—Perhaps the most convenient way (although not quite so economical) is for the pump exhaust to heat the boiler feed, as first suggested. Construction, of Heater. 87. Ilie construction of a heater has already been ex- plained in pp. 115-122, and the construction of the heaters for multiple distilling apparatus is substantially the same, but it must be borne in mind that the feed-waters respec- tively passing through their heater shells are at a higher pressure than in the case of double distiller on a steam ship, where the feed-water may enter the evaporator 14