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

76 SEA WATER DISTILLATION. I Secondary „ — (atmospheric). Therefore, the economy of the above three cases would work out and compare as follows :— _ . . ( Primary pressure =75 lbs. per sq. in. In case (a) * „ „ a Secondary ,, =2olbs. „ As 1,108 : 1,176 :: 1 : 1 061 economy. In c se (b) Primary pressure = 25 lbs. per sq. in. As 1,093 : 1,134 :: I : 1 037 economy. f Primary pressure = 10 lbs. per sq. in. I Secondary ,, = - 6 lbs. ,, (12" of mercury). As 1,085 : 1,114 :: 1 : 1026 economy. 19. The saving or economy is, therefore, greatest in (a) and least in (c), but the difference is not even theoreti- cally very great, and a good many tons of coal would have to be burnt before a really appreciable saving would be manifested in practice. Note.—No allowance has here been made for waste of heat in actual working—i.e., for the heat lost with the discharged brine, and by radiation. The waste would be increased by irregularity of working, so that what is gained in theory would be lost in practice. In a properly designed apparatus, which utilises to best advantage the heat it works with, the pressure worked at makes but little difference in the economy obtained. Indeed, if a substantial economy were shown by working at low pressures it would conflict with the law of nature that the heat required to raise a given quantity of water from a given temperature to its boiling point (i.e., the sensible heat), and then to convert such quantity of water into steam (i.e., the latent heat), is approximately