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

MULTIPLE DISTILLATION. 187 able drinking water temperature ; (2) to feed the boiler and each evaporator; and, often, (3) to force the drinking water produced to a height above or to a distance from the apparatus. This requires considerable steam power, and the steam utilised for these purposes should obviously be included in the working cost of the apparatus. If state- ments of high economy are made without counting the coal used in providing steam to drive the pump (or, indeed, to provide any source of heat used by the appar- atus), such statements are entirely delusive. 31. The method of dealing with the pump steam and exhaust will be noticed in the section on the pump, but meanwhile it may be stated that the requisite steam for driving the pump of a multiple distilling apparatus will amount to about 15 per cent, of the boiler steam used, so that if we take 10 tons of steam as the boiler evaporation by 1 ton of coal, then 25 per cent, of this 10 tons will leave 71 tons available for all evaporation purposes, and if worked out on the same lines already stated will make a considerable reduction at each stage or type of distillation. Note.—It will be seen, however, that although the steam used to drive the pump is a large item in itself, yet if the exhaust steam is properly utilised in the working of the apparatus, this expenditure of steam on the pump may be very considerably retrieved. This will be shown later on. Specified Economy. 32. It should be clearly understood that the true economy of working an apparatus is the ratio that exists between the weight of water yielded by the apparatus and the weight of coal consumed by it. Therefore, only such