THE CONDENSATION OF COAL GAS
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some extent, not only witli tlie difference between the temperatures involved, but with the condition of the surface, i.e. whether clearr or coated. Haug, in careful experiments, found that when the difference in temperature between the two surfaces (i.e. the heat potential) was 30° Fahr., the heat transmission amounted to approxi-mately 6 B.Th.U. per square foot of surface per minute under average working conditions. It must be remembered, however, that as temperature differences decrease the unit given will slowly decrease, and vice versa.
THE DESIGN OF CONDENSERS
Fra. 272.— Typical Water-tube Condenseb.
Several types of water-cooled condensers are in use to-day, but in many cases the difference is only a matter of detail. Older patterns having cast-iron shells are still in evidence, but in general the construction will conform to that shown in Fig. 272. This condenser may be used singly or in batteries, and some means should always be pro vided where-with. the flow of gas and water may be reversed. So far as general dimensions are concerned, the height of the apparatus should be from two to five times the diameter. For smaller condensers 2-inch water tubes may be employéd, but the 3-inch is preferable for all larger types. A point to remember is tha.t the length of the tube should not be such as to make it structurally weak; it is as well to confine the ratio of length to diameter within the limit of 80 to 1. The larger tubes are certainly more easily kept free from stoppage and are more readily cleaned, but there is some limit to their size in that as the diameter increases the cooling surface per unit area of grotmd surface is curtailed. As regards length, it is as well to confine this within the limits of manufacturers’ stock sizes, otherwise additional expense will be incurred. The 20-foot tube is, perhaps, most suitable for all the