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

94 SEA WATER DISTILLATION. Material Used for Coils—Copper and Brass. 56. The material used for the coils is either copper or brass tube, solid drawn, and the complete set of coils, fitted in place, are usually required to be sound under a test pressure of about double the maximum boiler pres- sure. Other metals are never used, but of the two (copper and brass), the former has many advantages over the latter, which may be pointed out as follows:— 1. Conductivity. 57. First, the conductivity of heat through copper is a great deal better than through brass. From the experi- ments of Péclet, with plates of various substances, of 1 foot square surface by 1 inch thick, he recorded the amount of heat (in units during one hour) passing through these plates (the two surfaces differing 1° in temperature) as follows :— A copper plate passed 515 units of heat in the hour, whilst a zinc plate similar in size and thickness passed only 225 units of heat. 58. If, therefore, we have a composite plate made in parts, the thickness being inch of copper and inch of zinc, we should have a composite plate representing the proportions of brass (70 per cent, copper + 30 per cent, zinc), and one would expect this composite plate, if experimented upon like the other two separate plates (copper and zinc) to pass heat through it as follows—viz., 515 x 0'7 = 360’5 units, and 225 x 0’3 = 67’5, and that 360'5 + 67'5 = 428 units would be the heat passing through brass, as against 515 units passing through copper, the ratio being—As 428 : 515 :: 1 : 1 2.