Fig. 67.
It will be noted that the efficiency of small pipes is much greater than that of larger ones, and in these days of high-pressure steam, much may be said in favour of the use of comparatively small wrought-iron steam-pipes instead of the larger cast metal ones. The first cost is small, the pipes are easily obtained ready screwed, and in the lengths required, and may be put together by any intelligent workman. The risk of fracture by the concussion of condensed water is very trifling, as compared to that of metal, and much lighter pipes are safe for high pressures. Steam-pipes must always be laid with an incline of say 1 in. in 10 ft. from the end where the steam is admitted, so that the condensed water may get away, and at the lowest point a steam-trap must be provided for its escape. In the writer's experience, the best form is that of Holman, made by Tangye of Birmingham, of which the principle will readily be understood from [Fig. 67]. The cup-shaped vessel a floats on the water in the outer casing, and so closes the valve b until a gets full, when it sinks and allows the water to escape until it floats up again. It is important that this trap should be set level, or the valve will not close properly. Each pound of condensed water is equivalent to about 1000 units of heat given off (see Table III.). In planning steam-pipes, it is not necessary that they should be arranged in a single line. Even if in gridiron form the steam will still reach every part, in proportion to the condensation which takes place. A series of large pipes may be supplied by small pipes from a common main, and discharge their condensed water into a common waste-pipe with branch from each. A 1/2-in. pipe from a high-pressure boiler will supply a considerable range, say 100 ft. of 4-in. pipe, though a larger size is advisable. At the farther end of a range of steam-pipes a small tap must be provided to let out the air which accumulates in them. In employing the exhaust steam of an engine for heating purposes, the pipes must be of ample size and freely open at the ends to avoid back-pressure. For this purpose the gridiron form is a very good one.
The planning of hot-water pipes is much more difficult than that of steam-pipes, but the general principle is that the pipes must rise all the way from the boiler to the farther end, where there must be an expansion-box or supply-cistern to allow the water to rise and fall and dissolved air to escape. From this the pipes must fall more or less, throughout the distance, back to the boiler, entering it at the bottom. If at any point the pipe has to fall, leaving an upward bend, a tap must be provided for the escape of air, but such upward bends are a fertile source of difficulty and failure of action. With long runs of either steam or water pipes, arrangements must be made to allow of expansion and contraction, which will amount to 1-2 in. per 100 ft., according to the temperature employed. If one end of the system can be left free, all that is needed is to support the pipes on rollers (pieces of old pipe may be used); if not, stuffing-boxes must be provided.
The air heated by boilers, and other sources of waste heat, may often be utilised for heating purposes, but generally requires to be driven by a fan, unless the drying-room can be arranged directly above the source of heat. If air has to be conveyed, the air-ways must be of ample size, and if the ascending force of heated air be relied on, passages less than 2 ft. sq. are seldom of much use. This ascending force is generally much overrated where the differences of temperature are so small as those employed in a drying-room. In a boiler chimney, where the temperature of the escaping gases is 552° F. (289° C.), the specific gravity of the air is about half that outside, and a chimney of 50 ft. in height gives a draught equal to the pressure of a column of about 1/3 in. of water, and the hot gases theoretically have a velocity of about 80 ft. per second; whereas the same chimney with a difference of temperature of 30° F. would have a draught equal to 1/300 in. of water only, and a velocity of 8 ft. per second.
The following table will enable the reader to calculate approximately loss in friction in air-passages and the pressure required to pass a given volume of air. The pressure needed increases in proportion to the length of the pipe and the square of the velocity of the current of air to be passed. Thus if we double the length of the pipe we must double the pressure to pass the same quantity; and in order to double the quantity of air through a given pipe, the pressure must be quadrupled.
Table IV.
Head, or Difference of Pressure at the two ends of a Circular Pipe 1 yd. long in inches of water required to pass 1000 cub. ft. of air per minute.
| Velocity in Ft. per Sec. | Diameter of Pipe. | Head. | ||
| in. | ||||
| 84·8 | 6 | ·186 |  | To pass 100 ft. per min. these figures must be divided by 100. To pass 10,000 ft. they must be multiplied by 100. |
| 37·7 | 9 | ·02442 | ||
| 21·2 | 12 | ·00579 | ||
| 9·4 | 18 | ·000763 | ||
| 5·3 | 24 | ·000181 | ||
| 3·4 | 30 | ·0000593 | ||
To calculate the head required for a long pipe, multiply the head given by the table by the length in yards. The air passed by square pipes of the same diameters will be 1·273 times greater with the same heads.
To be added to the pressure required to overcome friction is that needed to force the air out at the end of the pipe. This varies with the shape of the tube, &c., but for our purpose may be taken as given in the following table:—