tubes which serve as passages for the smoke or heated gases. The fire is built underneath, and the smoke passes horizontally either twice or thrice the length of the boiler. The general form of this boiler is shown in Fig. 114. This boiler is also

FIG. 114.-HORIZONTAL TUBULAR BOILER.

used sometimes in a vertical position with the fire beneath one head, in which case it is called a vertical tubular. The watertube boilers have the water in small tubes, and the heated gases pass out between the tubes. In this class of boilers the steam is contained in drums or horizontal cylinders, which are located above the heating surface. The tubular boilers are made in small sizes, 10 horse-power and larger, while the water-tube boiler for power is seldom less than 60 horsepower capacity.

Heating-boilers.-The boilers which are used for steam-heating are designed in a multiplicity of forms, and present examples of nearly every possible method of producing extended surfaces, both of the water-tube and fire-tube types. They are generally built for low-pressure steam, and are expected to be used mainly in buildings where the condensed water is returned by gravity to the boiler without pumps or traps. They are usually built in small sizes having a capacity of 250 to 2000 ft. of radiating surface (2 to 20 H.P.), and are fitted with safetyvalves, water and steam gauges and damper regulators.

The limits of this book prevent a detailed description of any make of heating-boiler, but the leading general types are described. Several types of the power-boiler are described quite in detail, and much that is said with respect to them will apply in a general way to heating-boilers.

The following classification of steam-heating boilers was suggested by one presented by Mr. A. C. Walworth in a paper before the New York Convention of Master Steam and Hotwater Fitters, June, 1894:

78. The Horizontal Tubular Boiler. This boiler is manufactured in many places, so that in many respects it is a standard article of commerce, and it can be purchased in nearly every market for a slight advance over the cost of materials and labor used in its construction. In the construction of this boiler the shell is now almost invariably made of soft steel of a thickness depending upon the pressure which the boiler is expected to sustain. The heads of the boiler are made of flange steel, and are generally inch thicker than the material in the shell. Lap-welded iron tubes are almost invariably used, the standard sizes being as given in Table XVII. The tubes are expanded into the heads of the boiler and may or may not be beaded, and are generally arranged in parallel vertical rows in the lower twothirds part of the boiler. In some instances the middle row of tubes is omitted with good results. It is not a good plan to stagger the tubes, since in that case they are difficult to clean,

and also act to impede the circulation of the water. The boiler should be provided with manholes, with strongly reinforced edges, so that a person can enter for cleaning. The heads of the boiler above the tubes should be thoroughly braced in order to sustain safely any pressure from the inside of the boiler.

Domes are often placed above the horizontal part of the boiler, and serve to increase the capacity for the storage of steam and also provide ready means of drawing off dry steam. The dome is always an element of weakness, and if used it should be stayed and reinforced in the strongest possible manner. The dome is frequently omitted, and steam taken directly from the top of the shell or drawn through a long pipe with numerous perforations, termed a petticoat pipe.

In construction this boiler must be strongly braced wherever any flat surfaces are exposed to pressure, and the girth and longitudinal seams must be riveted in such a manner as to secure the maximum strength.

The following table gives principal dimensions for a series of horizontal tubular boilers designed for a working pressure of 80 to 100 pounds per square inch :

Fifteen square feet of surface to each horse power.

79. Locomotive and Marine Boilers. Boilers of the horizontal tubular type with a fire-box entirely enclosed and surrounded by heating surface are usually termed locomotive boilers from the fact that such construction is common on locomotives. Boilers of this style are sometimes used for sta

tionary power purposes, and possess the advantage over the plain tubular boiler of requiring no brick setting. They are not, however, as strong in form as the plain tubular, since large flat surfaces have to be used over the fire-box.

Marine Boilers.-A cylindrical boiler with an internal cylindrical fire-box is principally used on large boats. The fire-box

FIG. 115.--LOCOMOTIVE BOILER.

is often corrugated. This form of boiler is very strong and efficient, but because of cost of construction has been little used for stationary purposes.

79. Vertical Boilers.-Vertical boilers of large size are made in every respect like the horizontal tubular boiler, but are set so that the flame plays directly on one head and the heated gases pass up through tubes. These boilers are generally provided with a water-leg which extends below the lower crown sheet and is intended to receive deposits of mud, etc., from the boiler. They are usually made so that the heat passes directly out of the top of the flue, but in some cases the heat is made to pass down a portion of the length of the exFIG. 116.-UPRIGHT TUBU ternal shell before being discharged. They are economical in the use of fuel

LAR BOILER.

and occupy very small amount of floor-space; they require,

however, a great deal of head-room, are very easily choked up with deposits and sediment, very difficult to clean, and very likely to leak around the tubes in the lower crown-sheet, and consequently have a short life.

Vertical boilers with horizontal radial tubes projecting outward with ends closed, known as porcupine boilers, are also on the market, and quite recently a vertical boiler of the water-tube type has been constructed.

80. Water-tube Boilers.-The water-tube boilers, which are used for power purposes, are designed to withstand great pressures, and can be purchased in sizes ranging from 60 to 500 horse-power per boiler. The general construction of these boilers is such as to have the water on the inside of the tubes and the fire without. There are two general forms: first, those with straight tubes, and second, those with curved tubes.

In all cases they have large steam-drums at the top, which are connected to the heating-surface by headers filled with water. In the Babcock & Wilcox, Heine, and Root the tubes are inclined and parallel, and are connected at the end with headers, the fire being applied in each case under the elevated portion of the inclined tube, so as to insure circulation uniformly in one direction.

FIG. 117.-BABCOCK & WILCOX BOILER.

In the Babcock & Wilcox boiler, cast-iron zigzag headers are used; in the Root boiler, the tubes are connected together by external U-shaped bends; in the Heine boiler (Fig. 120), the tubes are connected to large, flat-stayed surfaces. In the Babcock & Wilcox and Heine boilers, feed-water is supplied at the lower part of the top drums; while, in the Root boiler, it is supplied to a special drum in the down-circulation tubes at the back end of the boiler. The Stirling boiler has three horizontal drums at the top connected by curved tubes to a single lower drum at the back end of the boiler; the Hogan has one drum at top and two at bottom, which are parallel and