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

FUELS. 65 Heating Power of Gas. 21. We have just seen that 1,000 cubic feet of gas will impart 600,000 B.T.U., and as 1,000 cubic feet weigh 33 lbs., that makes 18,181 B.T.U. per lb. of gas. We have also seen that 1 ton of coal is capable of imparting, theo- retically, 13,000 x 2,240 = 29,120,000 B.T.U. of heat, but in practice about 7,000 X 2,240 = 15,680,000. There- fore, as 600,000 : 15,680,000 :: 3s. : 79s., or say £4, to obtain the 15,680,000 B.T.U., which 1 ton of coal is capable of imparting for about 25s. 22. This difference of cost between using gas and coal is, however, counterbalanced by the great reduction in the waste of heat, for gas can be lighted and turned off as occasion may require, but a coal fire has to be started for some time before it is used, and is giving out heat to no purpose long after it is required ; besides, a very large quantity of heat goes away into space, all the time the coal fire is in use. 23. Of course, the saving would not be obtained in the case of a large-sized boiler, working continuously and steadily for several hours. A coal fire would then be much more economical than gas, but in the case of a small generator that may be required to be worked for short periods, to meet special requirements, the use of gas would be far more economical and convenient than a coal fire. (/) Recapitulation. 24. The most important point to be noted, so far as distillation is concerned, is the comparison between the heat required for water to become vaporised, and the heat that is procurable from the fuel used for that pur- 5