141. The principal systems of piping which are now in vogue for heating purposes are shown in Figs. 54 to 57. These diagrams are intended to illustrate only the general arrangement of the piping, and many details are therefore omitted. The radiators a, b, c are supposed to be located on different floors and at various distances from the vertical supply pipes or risers. It will be seen, by careful inspection of the diagrams, that the main difference between the several systems consists in the method of returning the water of condensation to the boiler.

142. The one-pipe system is shown in its simplest form in Fig. 54. Steam flows from the boiler B through the riser s, and is conveyed to the radiators through suitable branches, which are nearly horizontal. All the water of condensation flows backwards through the same pipes, moving in a contrary direction to the steam. All of the nearly horizontal pipes, such as h and e, must, therefore, be inclined back to the boiler sufficiently to secure the ready movement of the returning water.

Fig. 54.

Fig. 55.

143. The two-pipe system is illustrated in Fig. 55. Each radiator has two connections, one of which serves as an inlet for steam, and the other as an outlet for water. The steam supply passes through the pipes h and s, and the water flows back to the boiler through the return pipes r and f. The branch e which supplies steam to the radiator b, at a considerable distance from the riser, is inclined so that the water formed within it will flow towards the radiator. It is connected at k to the return pipe g by. a small relief pipe, so that the water will be drained off and prevented from entering the radiator. The steam main h is also inclined, if it is of any considerable length, so that the water formed within it will run towards the foot of the riser s. All of the water formed in the pipes h and s is drained off by the relief pipe r'. Thus the steam and the water are carefully separated at all points in the system.

144. The separate-return system is shown in Fig. 56. The steam-supply pipes are the same in every respect as in Fig. 55. The returns, however, are different, each radiator being provided with its own separate return pipe, as shown at r, r' r".

Fig. 56.

Fig. 57.

145. The drop system is shown in Fig. 57. The steam supply passes up the riser s to the top of the system, thence along the horizontal pipe h, and descends through the drop pipe d. The radiators are connected to the steam supply with single pipes, precisely as in Fig. 54. It will be seen that the water in the pipes h and d moves in the same direction as the steam, instead of in the opposite direction, as in the single-pipe system. It is not necessary that the returns should be made parallel with the steam-supply pipes, as they are shown in Figs. 55 and 56, but they may follow any convenient route back to the boiler. It is always advisable, however, to make the returns as direct as practicable, care being taken, however, to avoid straggling the pipes about the building in an unsightly fashion.

146. The circulation-that is, the supply of steam-is far more certain in the two-pipe system than in the one-pipe system, because there is nothing to oppose or interfere with it at any time. Thus, a radiator at the end of a long horizontal branch, as at b in Fig. 54, is liable to have its supply interrupted by the formation of the returning water into "slugs," which fill the bore of the pipe and cause hammering noises; but, when the pipes are arranged as in Fig. 55, the same formation may happen without causing any trouble whatever.

147. Occasionally a radiator will gradually fill up with water. This occurs, in a one-pipe system, when the steam valve remains nearly closed for a considerable time, but not shut tight. The steam is then condensed as rapidly as it enters, and the opening is so restricted that no water will escape. The same thing will happen in a two-pipe system if either of the valves is closed and the other remains open. By opening both valves wide the water will almost noiselessly pass out into the return, but in the one-pipe system, as soon as the valve is opened, a violent struggle will begin between the entering steam and the escaping water. The result will be a succession of rumbling, hammering, and snapping noises, which will continue for several minutes. If the supply pipe is long, as at e in Fig. 54, the noise is likely to be prolonged to an annoying extent.

148. It is advisable to divide all heating systems which are of any considerable extent, into several independent sections. Long or troublesome horizontal branches may be reduced to a minimum by employing independent or special risers, and carefully locating them where they will supply the largest number of radiators to the best advantage. One riser may be used to supply almost any number of radiators, provided that none of them are located so far away from it as to make it difficult to drain the supply branch. Thus the question of the number of risers to be employed will be determined mainly by considering the drainage in the horizontal pipes.

Each section of a heating system should be made independent of the others, so that it can be closed down for repairs without affecting any other part of the system. Valves should be placed, in both the supply and return riser connections, close to the mains.