The boiler illustrated in Fig. 505 is known as the into which is welded an inner cylinder of Siemens mild-steel plate The metal is 5/16 inch thick, or. for better work, 3/8-inch. The water-space is brought down to the level of the grate-bar and the fuel is filled-in through the top hole, and falls on to the grate-bars; the products of combustion rise and fill the inner chamber, then pass out at the front opening (which is protected by cross bars) into the space between the boiler and the brickwork, travel half-way round in each direction until they strike the baffle-plates, under whieh they pass, and then proceed round the baek of the boiler to the flue. It will thus be seen that the products of combustion surround the water-ehamber on all sides. In the illustrations, two connections are shown at the top for the flow-pipes, and two inlets- at the bottom for the returns. Such a boiler, measuring 24 inches in height by 18 inches in diameter outside, is capable of heating about 350 square feet of radiating surface, while a boiler 36 inches high by 26 inches in diameter will heat about 825 square feet of radiating surface.

"Excelsior" Boiler, and consists of an external cylinder of wrought iron.

View

View.

Fig .506  View and section of the   Exceisior  Boiler.

Section.

Fig .506 -View and section of the " Exceisior" Boiler.

View

View.

Section on A B.

Section on A B.

Longitudinal Section.

Longitudinal Section.

Plan. Fig 506  Plan. View, and Section* of Simple Saddle Boiler

Plan. Fig 506 -Plan. View, and Section* of Simple Saddle Boiler.

The boiler which has found the greatest favour - owing to its very simple form - is the plain saddle boiler, shown in Fig. 506. This is made of welded wrought-iron plate, 5/16 or 3/8 inch thick, and is so set that the products of combustion play all over its inner and outer surfae The plan and sections of the setting show very clearly how this is accomplished. The flames play upon the inner part of the boiler, and the gases then pass to the back, return by the sides to the front, and pass ovrer the top of the boiler to the flue. The flues on the top of the boiler are not of much use as heating-surfaces, but the heat is radiated off the brick arch; the side flues are of much the greatest efficiency. If coal be burnt in a boiler of this class, the flues should be constantly swept out to maintain the boiler in its highest efficiency. Such a boiler, 48 inches long, by 22 inches by 19 inches over all, is expected to heat about 640 square feet of radiating surface. Saddle boilers can be obtained of smaller size, made in the independent form, that is, to be used without brick setting, as shown in Fig. 507. This cannot, of course, be so economical as the others, as the whole of the exterior of the boiler is exposed to the air, and the flames pass direct to the flue. The loss of heat from the exterior could be diminished by coating the boiler with non-conducting com-position. The saddle boiler may be made as shown in Fig. 508; the return-flue then passes back inside the boiler, and therefore more use is made of the waste gases. The cost of such a boiler is, however, higher.

View

View.

Section. Fig 507   View and section of Independent Saddle Boiler.

Section. Fig 507 - View and section of Independent Saddle Boiler.

Fig 508   Vlew of the Derosa ' Saddle Boiler.

Fig 508 - Vlew of the Derosa ' Saddle Boiler.

View.

View.

Section. Fig 509   View and section of the  Edina Boiler.

Section. Fig 509 - View and section of the "Edina-Boiler.

The boiler illustrated in Fig. 509 is a comparatively new design, in which the products of combustion are passed round a great many times before finally reaching the flue. Economy in fuel is obtained, but the first cost of the apparatus is considerably greater than that of a boiler of the plain saddle form.

Although wrought-iron boilers are more largely used in this country than cast-iron boilers, it will not do to overlook the latter. The Gurney Foundry

1 The Low Pressure Hot Water Heating System 40097Fig. 510.   View and Vertical Section of the  Oxford  Cast iron Boiler

Fig. 510. - View and Vertical Section of the "Oxford" Cast-iron Boiler.

Co., Ltd., of Toronto, Canada, make two types of boiler, which have found acceptance in Canada, the United States, and even in this country. The larger type is known as the "Oxford ". A view and section of it are given in Fig. 510, and the separate portions of which the apparatus is built in figs. 511 and 512. This "heater" is built upon the theory of directed circulation: thus, the water entering at the return heater (which is on the level of the fire-pot instead of in the ash-base) is passed to the front of the fire-pot at the bottom, and over a diaphragm which runs through the water-chamber of the fire-pot, and then back to a point above the point of entry. It is there conducted into the first section above the fire, through two openings, which run continuously from the fire-pot to the top section of the heater. These two side openings deliver the water on the respective sides of each section of the heater to a chamber which runs to the centre, to a directing diaphragm, and the two streams are thence thrown back again to a central opening, which is open to the top of the beater, and upon which the flow-heater is placed. The circulation in each of these sections is independent of the other. The theory of this circulation is, that, while there is no doubt that a directed current over a warm plate is only heated on its outer edges, the agitation and interior circulation in the liquid itself present many surfaces during the time the liquid is passing over the heated surface. In Canada and in the United States a number of heaters are dependent upon this circulation, and its success is now beyond question.

1 The Low Pressure Hot Water Heating System 40099Fig. 511   View of two Rlngs of the  Oxford Boiler, showlng circulation of water and back connection

Fig. 511 - View of two Rlngs of the "Oxford Boiler, showlng circulation of water and back connection.