ing (Fig. 173), where the distributing-pipes are taken off, or it may run in a horizontal or vertical direction from the heater, and branch pipes taken off as required. The pipes in which the flow takes place from the radiating surface toward the boiler are called return-pipes. The pipes which extend in a vertical direction are termed risers; when the flow in these pipes is downward they are called return-risers.

A relief or drip is a small pipe run from a steam-main, so as to convey any water of condensation to the return; it must be employed at all points where water is likely to gather. For illustration of use see Fig. 176.

Pitch is the inclination given to any pipe when running in nearly a horizontal direction. In general the inclination or pitch of a supply-pipe should, in steam-heating, be downward from the boiler, and arranged so that the water of condensation will move in the same direction as the current of steam. In hot-water heating the pitch should be upward from the boiler. In all return-pipes the inclination should be downward, toward the heater or boiler.

A relay is a term sometimes used to describe a sudden change of alignment, or "jumping up," of a horizontal pipe. This is often necessary in a long line of piping to keep the pipe near the ceiling and preserve the necessary pitch. At such points a drip or relief must permit water of condensation to flow into the return.

Water-line is a term used to denote the height at which the water will stand in the return-pipes. It is usually very nearly the same as the level of the water in the boiler, being higher only in case there is considerable reduction in pressure due to friction. In heating with high-pressure steam it is desirable to have all the relief-pipes discharge into a return filled with. water, so that circulation of steam shall be continuously in one direction; this is of less importance with low-pressure steam, provided the water which gathers in returns can move freely and quickly to the boiler.

The term siphon is applied to a bend below the horizontal; it is sometimes used in the main return to hold water at a different level from that in the boiler. This is done by admitting steam to the top part of the bend on the boiler side by a relief

from the main steam-pipe. It is similar to the siphon-trap, Fig. 159, Article 98. If the relief were not connected to the top of the bend the water would pass over by suction into the boiler.

Steam-traps are vessels designed with valves which open automatically so as to preserve the water-level in the returns at any desired point. Various kinds are described in Chap. VIII, Article 98.

Water-hammer is a term applied to a very severe concussion which often occurs in steam-heating pipes. It is caused by water accumulating to such an extent as to condense some of the steam in the pipe, thus forming a vacuum which is filled by a very violent rush of steam and water. The water strikes the side of the radiators or pipes with great force, and often so as to produce considerable damage. In general a water-hammer may be prevented by arranging the piping in such a manner that the water of condensation will immediately drain out of the radiator or pipes.

A bend in the return of a steam- or water-heating system, when convex upward, will frequently accumulate air to such an extent as to prevent circulation in the system. This is designated as an air-trap. When bends of this character must be used a small pipe for the escape of the air should be connected with the highest portion of the bend and led to some pipe which will freely discharge the entrapped air.

An air-valve is not ordinarily to be recommended for such situations.

107. Systems of Piping.-The systems of piping ordinarily employed provide for either a complete or a partial circulating system, each consisting of main and distributing pipes and returns. Several systems of piping are in common use, of which we may mention :

First, the complete-circuit system, often called the one-pipe system, in which the main pipe is led directly to the highest part of the building; from thence distributing-pipes are run to the various return-risers, which in turn connect with the radiating surface and discharge in the main return. The supply for the radiating surface is all taken from the return-risers, and in

some cases the entire downward circulation passes through the radiating system.

This system was employed by Perkins in his method of highpressure hot-water heating, and is mentioned by Péclet as

r

09

FIG. 173.-DIRECT-CIRCUIT SYSTEM OF PIPING-RADIATORS TO

HOT-WATER HEATING.

THE RIGHT ARRANGED AS FOR

3

b

in use in France in 1830. In this country it seems to have been introduced into use by J.

the Mills system of piping.

H. Mills, and is often spoken of as

The system is equally well adapted for either steam or hot-water heating, and on the score of positiveness of circulation and ease of construction is no doubt to

be commended as superior to all others. It is principally objectionable because the horizontal distribution-pipes have to be run in the top story of the building instead of the basement, which may or may not be of serious importance.

Water feed..

Draw off

FIG. 174.-ELEVATION OF PIPE SYSTEM USUALLY EMPLOYED IN STEAM- HEATING.

Second, a partial-circuit system, in which the main flow-pipe rises to the highest part of the basement by one or more branches, from whence the distributing-pipes run at a slight incline, often nearly around the basement, and finally connect with the boiler below the water-line. The radiators are con

nected by risers which carry both flow and return from and to the distributing pipes, as shown in elevation in Fig. 174 and in plan in Fig. 175. This method of piping is employed extensively for steam-heating, and is perhaps less open to objection than any other.

FIG. 175.-PLAN OF DISTRIBUTING-PIPE TO BE PLACED IN BASEMENT.

Third, a system of circulation in which each radiator is provided with separate flow- and return-pipes (Fig. 176). In this case the riser and distributing pipes are run as before, but are connected to the return by a drip-pipe; the return is located