71. Heating surfaces which have no projections of any kind are classified as plain surfaces, while those having ribs, knobs, pins, or other projecting parts are called extended surfaces.

The object sought in the construction of extended surfaces is to make the area of the emitting surface greater than that of the absorbing surface. By this means, heat may be transferred from a fluid which gives it off readily to one which takes it up slowly, with but little decrease in temperature of the heat-transmitting surfaces.

72. The effectiveness of a radiator will depend, to a considerable extent, upon the direction in which the air is moved over the heating surfaces. Fig. 22 shows a vertical tube standing in still air.

The tube is heated by steam, and its surface has a temperature which is practically uniform throughout. The air, which is warmed at the lower end of the tube, flows upwards and envelops the upper part in a current of hot air. The emission of heat will be slower from the upper part of the tube than from the lower part, because the difference in temperature between the air and metal is less.

A similar loss of efficiency occurs in a common coil of horizontal pipes laid vertically over one another, as shown in Fig. 23. The upper pipes are enveloped in the warm air which has been heated by the lower pipes. The maximum efficiency can be attained by placing the coil or radiator in a horizontal position, as indicated in Fig. 24. Each tube will then operate upon air of equally low temperature, and consequently the rate of emission will be greater than in the cases shown in Figs. 22 and 23.

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Fig. 22.

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Fig. 23.

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Fig. 34.

73. If horizontal radiator tubes are grouped together in large numbers, the efficiency of the tubes in the interior of the group will be much less than that of the outside tubes, because the access of cold air to them is practically cut off, and they can act only upon air which has been already warmed by the outer tubes.

Their efficiency is still further reduced by the fact that nearly all of the heat which they emit by radiation is intercepted and cut off by the outer tubes.

If the inner tubes of a group can be fully supplied with cold air, in some manner, they will be as useful as the outer tubes. When forced circulation is employed, there is little difficulty in driving the cold air over all the tubes; but, with natural draft only, it is necessary to modify the shape and arrangement of the tubes to secure a satisfactory result.

Figs. 25 and 26 show varieties of radiator tubes which are so shaped that, when they are assembled in a group, they enclose vertical air flues, as shown at a. The bases of the tubes are set high enough above the floor to permit an abundant flow of air into the flues at the bottom. Radiators constructed in this manner are called flue radiators.

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Fig. 25.

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Fig. 26.