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 CHAPTER IV.—PIPING AND CONSTRUCTION DETAILS.

Having selected the system of heating to be employed according to the needs of the building in hand, and having proportioned the radiator surface according to the requirements of the various rooms, it then remains to lay out the system of piping and arrange the various details of construction.

In regard to piping connections, it should be stated at the outset that the flow of steam through any system of piping depends primarily upon the difference in pressure between that at the supply end and that at the delivery or return end, and without any difference of pressure no flow of steam can exist. In exhaust heating or low-pressure gravity systems, this difference of pressure is very slight, and consequently for such systems the pipes have to be larger than for high-pressure heating or vacuum systems in which the pressure in the returns is reduced by connecting them to a vacuum pump. Again, in most systems of steam heating there is another consideration which affects the sizes of pipes; that is, the water of condensation from the radiators. If the steam circulation were uniform and continuous and the water of condensation kept separate from the steam supply, as in properly arranged two-pipe systems, the pipes might be very small; but it is necessary to allow for sudden opening of radiator valves, so as to take care of the momentary demand for steam which this causes, as well as the rush of water of condensation which accompanies it. For exhaust and low-pressure gravity systems, it may be laid down as a general rule that pipe sizes should be larger for the simple one-pipe system than for any other arrangement. They may be smaller for the one-pipe overhead, or Mills system, and still smaller for the two-pipe systems. A description of various systems of steam distribution was given in Chapter II.

Baldwin's rule for pipe sizes.—In the early days of steam heating, pipe sizes were proportioned by various empirical rules, the usual basis of which was the principle of being sure to get the pipes large enough; and such rules are, to a large extent, blindly followed to-day. Mr. William J. Baldwin, in his earlier work on