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pipe w leading to the chimney is cylindrical. The purpose of the air space s is to prevent the conduction of heat to the outer walls and thus keep them cool. Its utility is somewhat doubtful and many of the best boilermakers do not recommend it.
APPROXIMATE NUMBER OF BRICKS REQUIRED FOR SETTINGS OF RETURN-TUBULAR BOILERS.
Number of Bricks Required Number of Bricks Required
Above Floor Level.
The chimney serves the double purpose of creating a draft and carrying away obnoxious gases.
The draft pressures ordinarily produced by chimneys vary from 0 to 2 in. of water. A water-gauge pressure of 1 in. is equivalent to .03617 lb. per sq. in. To successfully burn anthracite, slack, or culm, a draft of 14 in. is necessary.
EXAMPLE.-A round chimney 100 ft. high is to be used for a battery of boilers of 550 H. P. What should be the internal diameter?
SOLUTION.-Looking under column 100 in "Height of Chimney Feet," the nearest horsepower is 565, and the diameter corresponding is 60 in., which should be the internal diameter of the chimney. Ans.
The flue through which the gases pass from the furnace to the chimney should have an area equal to, or a little larger than, the area of the chimney. Abrupt turns in the flue and contractions in its area should be avoided, as they greatly increase the resistance to the flow of the gases.
HOT-AIR FURNACE HEATING.
The method of warming buildings by hot-air furnaces may be briefly described as one in which air passed over heated surfaces within a furnace and warmed thereby is distributed
to the various rooms throughout the building by means of metal pipes or ducts, as indicated in the accompanying illustration. Ordinarily, the only motive force producing the
flow of air is that due to the difference in density, or weight, between that of the heated air and that of the outer atmosphere. In other words, on being heated the air expands and thereupon becomes lighter, being forced upwards from the furnace into the rooms to be warmed by the pressure due to the greater density of the outer atmosphere. This method is known as the gravity system of furnace heating.
When, instead of utilizing the force of gravity, a fan or blower is used to force the air over the furnace and through the ducts, the method is known as the fan-furnace system of heating. The internal circulation system of furnace heating is one wherein a supplemental air-supply duct is arranged between the furnace and the first floor of the dwelling, so that at night, or on extremely cold days, the supply of air to the furnace is taken from within the building.
The hot-air furnace, a common form of which is shown in the illustration, is essentially composed of a firebox a, combustion chamber b, an air space e surrounding them, an ashpit r, and a jacket e enveloping the whole. The lower end of the air space c communicates with the outer atmosphere by a cold-air duct v, through which fresh air enters the air space of the furnace by way of the cold-air pit g. The slide m in the cold-air duct v is provided for shutting off the supply of fresh air. The air is heated by contact with the heated plates or castings that envelop the firebox and combustion chamber. Hot-air pipes s, s, etc. are attached to the top of the jacket to convey heated air from the air space to the rooms to be warmed, the air being discharged through floor registers i, i, or by wall registers n, n. The chief object to be considered in putting together the parts that form the firebox a, combustion chamber b, and ash-pit r, is to make the joints permanently gas-tight, so that the gases of combustion will not have access to the air space c through open joints. The products of combustion flow from b through the smoke pipe o to the chimney u, and escape to the atmosphere.
Hot-air furnaces are usually supplied with a water pan, placed inside the jacket, for moistening the heated air.