84. This can be had with a bibb if desired, also with various forms of handles or keys, and with nickel or brass finish.

Automatic air-valves are made of a great variety of patterns. Those for steam-radiators are all closed by the expansion of some material. Fig. 85 shows an expansion air-valve, in which the valve is closed by the expansion of a curved metallic strip. The valve will remain open until this curved

strip becomes nearly equal in its temperature to that of the steam; the heat then increases its length and it bends out sufficiently to close the valve. A drip-pipe is provided for removing any water of condensation escaping from the air-valve.

Another form, which has in the past been extensively used, is shown in Fig. 86. In this case the interior tube A is heated more than the frame bb; this serves to press the valve c against the end of the tube when it is heated, thus closing the orifice. This is best adapted for use in a vertical position.

A form of air-valve now in extensive use is shown in Fig. 87. In this a composite material which expands rapidly when heated is used instead of metal. It is claimed for some of these valves that with suitable adjustment of the top screw the temperature of the radiator will be automatically maintained at any desired point-a mixture in any required proportion of air and steam being maintained in the radiator by this action.

To prevent escape of water and injury to furniture a radiator-valve with a float attachment is often used, as shown in Fig. 88. The valve is closed when heated, as in Fig. 87, by the expansion of a composite substance; it is connected to a float, so that if water passes into the air-valve the float will rise and close the orifice regardless of the temperature.

An automatic air-valve for hot-water radiators is shown in the sketch, Fig. 89. The air escapes at A, the orifice being closed by the float Facting on the lever L. So long as only air surrounds the float it sinks and keeps the orifice open, but

as soon as water surrounds it it rises and closes the orifice. 62. Expansion-joints. In the erection of any system of piping means must be provided so that the elongation of the

pipe due to expansion will not cause a leak.* For all ordinary purposes of heating the expansion can be provided for by the use of elbows and right-angled offsets, of such length that the expansion will simply cause one pipe to slightly unscrew in one or more joints. This requires the use of two or three elbows, and so causes a slight increase of resistance to flow due to friction; but it is a very satisfactory arrangement, and will stand for years without developing leaks, even with high-pressure steam, if properly erected.

It is sometimes necessary to provide for expansion in a long line of straight pipe, in which case expansion-joints of some kind must be used. The ordinary expansion-joint, Fig. 90, consists of a sleeve sliding into an exterior pipe, provided with a stuffing-box. This joint, when heavy and provided with a catch to prevent it pulling apart, is a very durable and satisfactory construction. The packing will have to be renewed occasionally, and one part needs to be solidly anchored to prevent motion.

Expansion-joints are often used constructed of copper pipe

in form of a U-shaped bend; also of one or more diaphragms connected to each other at the edges and to the pipes near the centre (Fig.91). The copper bend is always satisfactory. The last-named device works very well if means can be adopted to thoroughly drain off any water lodging against the diaphragm. If used in a horizontal position, and on large pipes it is likely to gather sufficient moisture to form a water-hammer that may produce rupture when steam is turned on.

ELASTIC COUPLING.

*The expansion of iron is one part in 148,000 of length per degree. This is equivalent to about 1.45 inches per 100 feet in changing from temperature of freezing to boiling.

CHAPTER VI.

RADIATORS AND HEATING SURFACES.

63. Introduction. The amount of heat which will pass through various kinds of radiating surface is determined largely by experiment, and has been fully discussed in Chapter IV. In this chapter we will consider briefly the methods of construction.

When steam and hot water-heating were first employed the radiating surface consisted almost entirely of cast-iron pipe arranged in horizontal lines, as shown in Fig. 35, page 88. With the invention and use of wrought-iron pipe, cast-iron pipe was superseded by coils of this pipe, and at a somewhat later day largely by the radiator with vertical surfaces made either of cast or wrought iron. The change from pipe surfaces to radiators was, no doubt, largely due to the attempt to economize space in the room, as well as to improve the appearance.

64. Radiating Surface of Pipe.-Very efficient radiating surfaces can be made of coils of piping arranged as shown in Figs. 92 and 93. The return-bend coil shown in Fig. 92 is made by connecting return-bends, Fig. 61, page 96, with lines of straight pipe. The pipe mostly used is one inch in diameter, although, when the bends are numerous, 1 or 2-inch pipe should be used to reduce the friction. In use the flow is continuous, the fluid entering at the top and thence with a gradual descent flowing to the right and left alternately, finally discharging at the bottom. There is a great deal of friction in coils of this class, and air is likely to gather in the bends and stop circulation. The writer would, therefore, recommend that they be employed only when other forms will not answer.

The branch-tee or manifold coil is constructed by connecting branch-tees with parallel lines of pipe. In each pipe-line one or more elbows must be placed to counteract the effect of unequal expansion.

The coil may be arranged on a flat wall-surface so as to form a mitre branch-tee coil as in Fig. 93, lower part, or with both branch-tees at one end and elbows and nipples at the opposite end; the fittings at ends being connected by pipes having the proper pitch. Such a construction is called a return branch-tee coil, see upper part Fig. 93. The coil may be arranged on two sides of a room with the elbows placed in the intervening corner, in which case it is called a corner coil.

The various types of branch-tee or manifold coils as described present small frictional resistance to the flow of steam or water and give satisfactory service for either steam or hotwater heating.

FIG. 93.-BRANCH-TEE MITRE COIL AND RETURN-COIL.

If two connections are used the steam should be supplied at the highest point of the coil, and the return taken off at the lowest; if one connection, steam is to be supplied at the lowest point. The horizontal portion should be given a