radiator at the end opposite the steam inlet. As steam flows through the main and the risers, part of it will be condensed

and the water of condensation will fall by gravity to the bottom of the steam main, flow to its lower end f, and enter the bottom of the boiler through the return pipe g. The water

of condensation from the radiators first accumulates in the base of the radiators until a sufficient hydrostatic head is formed to cause it to flow out of the radiators against the inflow of the steam. It then falls down the risers, through the riser connections, and into the steam main, also againstthe flow of the steam. If the riser connections to the steam main or radiator connections to the riser have too little pitch, or if the pipes are too small, the flow of the water of condensation through them will be resisted to such an extent by the flow of steam that the water will not flow off as quickly as it is formed, the result of which will be that the water will accumulate in the pipe until it entirely closes it, when water hammer will take place. The steam main should be made sufficiently large to prevent such a difference between the pressure in the boiler and that at the point ƒ as will cause the water to back up in the main and retard the flow of steam to any riser connection.

The two-pipe system is illustrated by the risers and radiator connections at the right of Fig. 2. Each radiator has two connections, one of which serves as an inlet for steam and the other as an outlet for water of condensation.

One of the many modifications of the two-pipe system is that known as the separate-return system shown at the left of Fig. 2. From the boiler a the return main b pitches downwards to the point at which connection with the return main c is made by the relief pipe d.

It will be observed that in the method of piping radiators e, e, e, the radiator return connections discharge into the same return riser ƒ above the water-line. Consequently, steam is liable to flow from one radiator into another by way of the return riser. This is objectionable, because the inflow of steam to any of the radiators against the outflow of the water of condensation tends to cause the water to back into the radiator and thus flood it.

When a radiator is double-piped there should be a continuous inflow of steam through the inlet only and an outflow of water of condensation through the outlet only, the water and steam always flowing in an unchanging

direction. This, however, is not always accomplished when the radiators are connected up by the two-pipe method shown at B.

In order to obtain a positive circulation through the radiators and throughout the system, the return riser from each radiator may be continued down separately and connected

together below the level of the boiler water-line, as shown at i, i, i2.

The drop system, sometimes called the Mills system, is shown in Fig. 3. The steam supply passes up the riser s to the top of the system, thence along the horizontal branch main h, and descends through the drop riser d. The radiators have

single-pipe connections to the steam supply. The water of condensation in the pipes h, d moves in the same direction as the steam, instead of in the opposite direction, as in the single-pipe system.

PIPING DETAILS.

Standard Pipe Threads.-What is known as the Briggs standard pipe threads have an angle of 60°, and are rounded off slightly at the top and bottom, so that the depth of the thread is only four-fifths as great as it would be if the threads were sharp. The outside surface of the pipe is tapered to a PROPORTIONS OF STANDARD PIPE THREADS.*

*For actual external and internal diameters, thickness of metal, etc., see table of standard dimensions of wrought-iron

pipe.