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

MULTIPLE DISTILLATION. 177 ment, if yielding 10,650 B.T.U. per lb. (see Chap. v.)3 which would (under the conditions specified) yield 10 tons of steam by single distillation, an economy of 10 to 1 (weight for weight). Note.—The quality of the coal—that is, its heat-giving power—is, of course, an important factor, because if the coal gave only 8,000 B.T.U. per lb., the weight of coal required would be as 8,000 : 10,650 :: 1 : 1’33 tons ; or, put in another way, the evaporation for the same weight of coal consumed would be as 10,650 : 8,000 :: 1 : 0'75 ton. The yield, therefore, will vary with the quality of the fuel. (2) Double Distillation. 16. Now, compare the above fuel cost of working single distillation with the more economical type of double dis- tillation, where one evaporator is interposed between the boiler and the condenser, and so connected up that the boiler or primary steam, instead of going directly into the condenser, where 1,065 B.T.U. were simply absorbed by the cooling water, and carried away to waste, now goes into the coils of an evaporator, and is thus able to generate a new supply of steam or vapour, which is called secondary or gained steam or vapour. The result is that the boiler steam works at the same pressure, and with the same sensible and latent heat, but the latent heat is now utilised in creating a new supply of secondary steam, amounting to about 80 per cent, of its weight, as already pointed out in the chapter on “ Steam ” ; and the pro- duction of extra or gained water is as follows :— The 1 ton of coal imparts the same quantity of heat (i.e., the same number of B.T.U.) for every lb. of the 10 tons of water converted into steam, but the 10 tons of primary steam (at 25 lbs.\ by being re-converted into 12