In case a pressure in excess of the atmosphere is required, the vent pipe is closed and a safety-valve attached which will open when the pressure reaches the desired point. By increasing the pressure on the system the boiling temperature of the water will be much increased, and hence it will be possible to maintain a higher temperature throughout the system. As showing the increase in temperature of the boiling point with excess of pressure, the following table is inserted:

Pressure systems of hot-water heating were used at one time to a considerable extent in England, under what was known as the Perkins* system, in which small pipes and exceedingly high pressures and temperatures were used. It has also been used to some extent in this country in the Baker system of car-heating.

The advantages of the pressure system are those which are due simply to the use of higher temperatures and smaller radiating surfaces; the disadvantages are the danger of an explosion

* Hood's "Heating and Ventilating of Buildings."

which would be likely to happen were the safety-valve inoperative, or did any part of the apparatus give way. The sudden liberation of a considerable body of water having a temperature above the boiling point would result in the instantaneous pro. duction of a large amount of steam, which might produce disastrous results.

With the open expansion-tank it seems hardly possible that any serious accidents could result even from the most careless management, since the escape of steam from the top of the expansion-tank would prevent the accumulation of pressure. To prevent accident the expansion-tank should be connected to the heater by a pipe protected from frost and without stop or valve, so as to render it impossible to increase the pressure on the system by stoppage of the connection.

It is desirable to provide the expansion-tank with a glass water-gauge showing the depth of water, and a connection to the supply-pipe for adding water to the system. In case the expansion-tank occupies a cold location where it might freeze in extreme weather, a small pipe connected with the circulating system, in addition to those described, should be run to the tank and connected at a higher level than the expansion-pipe, so as to insure circulation of warm water.

94. Form of Chimneys.-The form and size of the chimney is of great importance in connection with the satisfactory operation of a heating plant, and it should in every case receive the closest inspection before guarantees of capacity are made.

It will be found that for a specified area a round chimney will have the most capacity, but in ordinary building construction such a chimney is difficult to construct and is not ordinarily built. A square chimney of the same area has somewhat more friction, and one with a rectangular narrow flue very much more, so that an increase in area proportional to excess of perimeter should be made for such cases. The chimney should be as smooth as possible on the inside in order to prevent loss of velocity by friction, and, if of brick, the flue should in every case be plastered. In the construction of chimneys it is better that the inside be made with a thin wall not connected in any way with the outside, both in order to permit

free expansion of the inner layer of the chimney with the heat and also to secure the advantage of the non-conducting power of an air space between the inside and outside walls. Such a construction is common for chimneys for power purposes, but is not ordinarily applied to those used in buildings.

95. Sizes of Chimneys.-The area of cross-section required for a given chimney will depend upon its height and also upon the amount of coal to be burned. The conditions which affect chimney draft are so numerous, and so difficult to consider in any theoretical discussion, that empirical or practical formulæ derived from the study of actually existing plants are probably more satisfactory than those obtained from purely theoretical computations. Of the various formula which have been given for the capacity of chimneys the writer prefers that of William Kent, from which the accompanying table is computed.

Kent's formula is computed on the assumption that the chimney shall have a diameter two inches greater than that required for passage of the air, in order to compensate for friction. The following is his formula:

=

in which A actual area in square feet of the chimney, E effective area, h = height in feet, S = side of the square in inches, H = horse-power of plant. If we let R number of square feet of radiating surface to be supplied, then, Article 73, page 173,

The table gives the diameter of round or side of square chimneys in inches for various heights computed from the above formulæ, with the diameter increased by 2, to allow for friction. A square chimney is considered the equivalent of the inscribed round one.

DIAMETER OR SIDE OF CHIMNEY IN INCHES REQUIRED FOR VARYING AMOUNTS OF DIRECT STEAM-RADIATING SURFACE.

For other kinds of heating multiply the radiating surface by the following factors: Hot-water heating 1.5, indirect steam 0.7, hot-blast heating 0.2.

96. Chimney-tops.-The draft of a chimney is influenced to a great extent by the conditions of the surrounding space. If other buildings exist in the vicinity of such a form as to deflect the currents of air down the chimney, the draft will be impaired and may be entirely destroyed. The objects which tend to produce downward air-currents may sometimes be situated a considerable distance from the chimney and thus render the specific cause of poor draft very difficult to determine. The remedy for a smoky chimney is sometimes difficult to apply, but usually the draft will be improved, first, by increasing the height of the chimney; second, by adopting some form of chimney-top which utilizes the force of horizontal currents to aid by induction in increasing the draft.

The writer found that curved trumpet-shaped tubes located with the small ends projecting into the chimney in an upward direction increased the draft materially when the wind was blowing into the openings, and there is little reason to doubt but that a chimney-top may be constructed in such a manner as to materially increase the draft.

97. Grates. For supporting the fuel during its combustion in such a manner as to allow a free passage of air, a perforated metallic construction of some sort is required. For burning very fine coal the perforation must be small and close together; for burning larger sized coal the perforations may be larger and further apart. The area of the air-spaces compared with the total area of the grate should be about 50 per cent in order to secure best results, but they will more generally be found to be 30 to 40 per cent. The grates are usually constructed of cast iron and in a very great variety of forms, as shown in Figs. 157 and 158. In some instances a series of parallel bars is used; in others the grates are made in one solid

casting. This latter practice is never one to be recommended. The solid grate is likely to break from expansion strains due to heating unless made in such form that the various parts are free to expand independently.

Nearly all heating-boilers, hot-water heaters, and furnaces are supplied with some form of shaking- and dumping-grate. Many of these grates are known from experience of the writer to give most excellent satisfaction, and doubtless all present points of merit. The various shaking-grates operate in nearly every way, and it is hard to conceive either a form of grate-bar or a method of shaking which is not exemplified in some of these grates. Some of the bars are flat or rectangular in shape, and are operated by shaking backward and forward; others are triangular and are continually rotated so as to present successively new surfaces to the fire each time they are shaken. The shaking-grate will, in general, be found much superior to the fixed one, and a furnace fitted with such grates