Steam Heating

Overhead System of Steam Heating. The overhead system, sometimes designated as the Mills System, is so called from the fact that all radiating surface on the job is supplied from overhead, or from a steam supply which has been taken directly from the boiler to the attic or an upper floor of the building, the main there dividing or branching to feed a number of "drops" or pipes extending through and con necting with the radiators on the various floors below.

Although styled the "Mills System," pre sumably so from the originator's name, Mr. Mills was not the father of the overhead idea in steam and hot-water piping. A Mr. Perkins, of London, England (1830), made use of it, as did also a Mr. Charles Hood at a somewhat later day, but prior to the advent of Mr. Mills.

Fig. 27 shows the general design of the sys tem. The main steam supply is carried from the boiler to a point in the basement where it is convenient to rise with it to the top of the build ing. From this extreme end of the main riser, which is the high point of the system, the dis tributing mains are taken, the number of the branches being as many as are necessary to sup ply the drops to the floors below. The attic mains and all branches pitch down from the high point named, in order that the entire system of piping may have a thorough drainage. The branches, as a rule, are taken from the side, as shown in Fig. 27. The drop pipes supplying radiators are connected in the basement into wet returns.

In so far as the mains and returns are con cerned, the system is of the two-pipe style; that part relating to radiator connections is arranged on the one-pipe plan. The one-pipe systems de scribed and illustrated in the preceding pages are largely used for small work. The overhead system is particularly adapted for heating a hotel, apartment, or other building of consider able height, where, by reason of the space on each floor being similarly divided, the radiators are placed in the same relative positions on each floor. A line of pipe or a drop can feed a num ber of such radiators so located without friction and with freedom from any of the water-hammer so prevalent in large installations erected in accordance with ordinary methods.

Although Fig. 27 shows a direct connection at the base of each drop, it is good practice to use an expansion joint at this point, or to allow for the expansion by making what is known as a double-swing joint, illustrated in Fig. 24: By providing for expansion in this manner, a short, stiff connection may be used in connecting each radiator.

The attic mains should be hung on expansion hangers or supported by roller hangers in such a manner as to afford the pipes freedom to ex pand and contract. In order to imderstand the importance of this feature, Table TTT is given. showing the increase in length (or the expan sion) of a line of pipe 100 feet long when heated to various temperatures.

From the information afforded by this table, it will also be readily understood bow easily leaks may be caused in the system or fittings cracked from expansion when no provision for it has been made.

With this system, automatic air-valves arc attached on each radiator in the usual manner. As the distributing supply is separate from the returns, and the steam and water of condensa tion both flow downward through the drops, fric tion is reduced to a minimum; and with this system, therefore, pipe sizes may be somewhat reduced from those required by the ordinary one-pipe method.

Two-Pipe System of Steam Heating. This is the oldest method extant, and while it is to day practiced chiefly on high-pressure work, we find many such systems in use for low-pressure heating.

The piping for a system of this character may properly be, and usually is, somewhat smaller in size than that necessary for a one pipe system, for the reason that the flow of steam and of the water of condensation are separated, each having its own service pipes. Fig. 28 is an elevation of a two-pipe system, and, from its ex amination, it will be noted that all return pipes are connected below the water-line of the boiler. This is an important feature to be observed in order to prevent the short-circuiting of the steam into the returns, which action would result in cutting off or retarding the flow of the conden sation—a condition which might not only block the circulation but would probably also cause water-hammer.