[183] E. & F. N. Spon, Ltd., London.
Loss of Heat through Walls.
| Thickness of Wall in Inches. | K.-calories per Sq. Foot per Hour. | — |
|---|---|---|
| 4·5 | 1·76 | Stone walls must be about one-half thicker, to afford equal warmth with brick ones. |
| 9 | 1·44 | |
| 14 | 1·20 | The loss from glass windows amounts to 3 or 4 k.-calories per square foot per hour. |
| 18 | 1·06 |
If the building is of several stories, the loss to the roof in the intermediate ones need hardly be taken into account, but if the ceiling is not tight, and open to the roof, the loss may be great, but difficult to estimate. If we consider the drying room already described, the total area of the walls and ceiling is about 4000 feet, and to maintain its temperature 30° F. above the atmosphere at 1·2 cal. per sq. foot would require 4800 cal. per hour or 80 cal. per minute, a very small amount compared to that consumed in drying.
The following table calculated from data given by Box will give some idea of the amount of steam or hot-water piping required for heating. The sizes given are for the internal diameter of the pipe, allowance being made for the increased heating surface of pipes of ordinary thickness. Small pipes are considerably more effective in proportion to their surface than large ones, and for high-pressure heating 11⁄2 or 2-inch wrought-iron pipes are to be recommended as in many ways preferable to cast iron. The gilled or ribbed pipes now often used are also advantageous as giving a greatly increased heating surface.
Heat given by Steam-pipes.
| Steam Pressure, lb. per sq. in. | Temperature of Pipe. | K.-calories per hour per foot run of Pipe. | ||||
|---|---|---|---|---|---|---|
| °F. | 2 in. | 3 in. | 4 in. | |||
| 52 | 300 | 102 | 137 | 169 | ||
| 35 | 280 | 92 | 121 | 148 | ||
| 21 | 260 | 81 | 106 | 130 | ||
| 10 | 240 | 68 | 92 | 113 | ||
| 2·5 | 220 | 59 | 81 | 97 | ||
| 210 | 54 | 72 | 89 | |||
| 200 | 49 | 66 | 81 | |||
| 190 | 45 | 60 | 74 | |||
| 180 | 40 | 54 | 67 | |||
| 170 | 36 | 49 | 60 | |||
The temperature of the air to be heated is understood to be 60° F.; at lower temperatures the quantity of heat given off by the pipes would be greater, and at higher temperatures less; the amount being approximately proportional to the difference of temperature between the air and the hot pipes. It is also important to note that the table refers to steam-pipes in still air, and that if placed in a powerful draught, (as immediately before or behind the fan), their heating effect may be at least doubled. This has not been considered in the following calculations.
Applying these figures to the estimate of 1110 calories per minute required for drying in our building, and assuming 80 calories per minute for the loss of heat through the walls, we have a total of about 71,400 calories per hour, and to obtain this would require 736 feet of 4-inch pipe at 220° F. (heated by exhaust steam) or 700 feet of 2-inch pipe heated to 300° F. by steam at 52 lb. pressure.
If we adopt the estimate of 1620 calories of No. 5 and 6, we shall require 1050 and 1000 feet of the two pipes respectively, and this covers approximately the worst conditions. We must, however, remember that these estimates are made for continuous drying during the twenty-four hours, and that if the fan and steam are only applied during a portion of this time, the supply both of air and steam must be proportionately increased, or the time of drying correspondingly lengthened.