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

THE EVAPORATOR. 95 The proportion or ratio of effectiveness between the two metals, copper and brass, shows that copper is about 20 per cent, better than brass, so that from the foregoing figures one would expect that it would require about 20 per cent, more heating surface if made of brass than of copper. A brass plate made of an alloy of 70 per cent, of copper and 30 per cent, of zinc does not appear in the list of metals experimented on, but in actual practice with distilling machinery it has been found that a copper sheaf lias about 15 to 20 per cent, more effec- tiveness than a precisely similar sheaf of brass tubes. Therefore, it may be said that a copper coil has practi- cally about 20 per cent, more effectiveness than if the coil were made of brass, and, consequently, if copper is used instead of brass, about 20 per cent, less surface will be found sufficient for the same work. The metals copper and brass, are usually left to the choice of the evaporator-maker in Admiralty specifications, copper is usually selected for the heating surfaces of the evaporator. Note.—It should be noted that the above comparison in the passage of heat through the two metals is not at all in the same ratio as what is usually termed the con- ductivity of heat by metals. The following Table L gives the ordinary conductivity of heat by various metals, and if copper and brass be compared in that list it will be seen that whilst copper is placed at 73’6 per cent., brass is placed at 24 per cent. This would lead one to suppose that copper is about three times as effective as brass, which is not at all borne out as regards the passage of heat through these metals as shown in practice.