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

THE EVAPORATOR. 69 is a much more simple and convenient form of working distilling apparatus. 7. The difficulty attending the use of exhaust steam for working the evaporator is its very unsteady pressure. It is not much used in ships of the mercantile marine for this very reason, and live boiler steam is the usual steam (primary) supplied to the evaporator. Economy of Using Exhaust Steam. 8. The pressure of the primary steam supplied to the inside of the coil thus varies according to circumstances. Where there is plenty of steam available, of a low pressure, say from the exhaust steam pipe on a steam ship, it is evidently better to use the heat in this exhaust steam than to take live steam from a boiler working at a high pressure. Therefore, where there is exhaust steam not exceeding, say, 25 lbs. to the square inch, and there is ample space for the corresponding size of evaporator (which we shall see requires to be made larger), and arrangements are made to compensate for the varying pressure of the exhaust steam, it is more economical to use such exhaust steam. Economy Generally. 9. The economy of working distilling apparatus (or rather the evaporator) is the proportion that exists be- tween the amount of secondary, or gained, steam that is produced to the consumption of primary steam, weight for weight. By consumption of primary steam is meant the weight of primary steam that is re-converted into water in the operation of giving its latent heat for evapor- ating the sea water, and thus generating the secondary steam.