Document Nzy6BLvQvr74eobxY1ewNbdQ

American Society of Heating and Ventilating Engineers Guide, 1936 are not less than 8 in. thick, and that the inner course shall be a refractory clay brick. All brickwork shall be laid in spread mortar, with all joints push-filled. Exposed joints both inside and outside shall be struck smooth. No plaster lining shall be permitted. Chimneys shall extend at least 3 ft above flat roofs and 2 ft above the ridges of peak roofs when such flat roofs or peaks are within 30 ft of the chimney. The chimney shall be high enough so that the wind from any direction shall not strike the top of the chimney from an angle above the horizontal. The chimney shall be properly capped with stone, terra cotta, concrete, cast-iron, or other approved material; but no such cap or coping shall decrease the flue area. There shall be but one connection to the flue to which the boiler or furnace smokepipe is attached. The boiler or furnace smoke-pipe shall be thoroughly grouted into the chimney and shall not project beyond the inner surface of the flue lining. The size or area of flue lining or of brick flue for warm-air furnaces depends on height of chimney and capacity of heating system. For chimneys not less than 35 ft in height above grate line, the net internal dimensions of lining should be at least 7 x 11H in. for a total leader pipe area up to 790 sq in. Above 790 and up to 1,000 sq in. of leader pipe area the lining should be at least ll}i x 11J4 in. inside. In case of brick flues not less than 35 ft in height with no linings, the internal dimensions should be at least 8 x 12 in. up to 790 sq in. of leader area, and at least 12 x 12 in. for leader capacities up to 1,000 sq in. Chimneys under 35 ft in height are unsatisfactory in operation and hence should be avoided. CHIMNEYS FOR GAS HEATING The burning of gas differs from the burning of coal in that the force which supplies the air for combustion of the gas comes largely from the pressure of the gas in the supply pipe, whereas air is supplied to a bed of burning coal by the force of the chimney draft. If, with a coal-burning boiler, the draft is poor, or if the chimney is stopped, the fire is smothered and the combustion rate reduced. In a gas boiler or furnace such a condition would interfere with the combustion of the gas, but the gas would continue to pass to the burners and the resulting incomplete com bustion would produce a dangerous condition. In order to prevent incom plete combustion from insufficient draft, all gas-fired boilers and furnaces should have a back-draft diverter in the flue connection to'the chimney. A study of a typical back-draft diverter shows that partial or complete chimney stoppage will merely cause some of therproducts of combustion to be vented out into the boiler room, but will not interfere with com bustion. In fact, gas-designed appliances must perform safely under such a condition to be approved by the American Gas Association Laboratory. Otlier functions of the back-draft diverter, are to protect the burner and pilot from the effects of down-drafts, and to neutralize the effects of variable chimney drafts, thus maintaining the appliance efficiency at a substantially constant value. Converted boilers or furnaces, as well as gas-designed appliances, should be provided with back-draft diverters. Since back-draft diverters have a special function to perform in pro tecting gas burning appliances, it is necessary that they should be built to the proper size as shown in Table 2. Work is now in progress on the development of a horizontal diverter for use where there is not enough room to install a vertical type of diverter. Information on the approved proportion of such equipment may be secured from the American Gas Association Testing Laboratory. As is the case with the complete combustion of almost all fuels, the 466 Chapter 26--Chimneys and Draft Calculations Table 2. Suggested General Dimensions for Vertical Back-Draft Diverter cAPipe Size B D E F- G H I J K L M 3 3 3 5.5 7.0 3.8 0.7 4.4 3.0 1.5 2.5 0.7 1.5 2.3 4, 4 -4 7.2 9.5 5.0 1.0 6.0 4.0 2.0 3.5 1.0 2.0 3.0 5 5 5 9.4 10.8 5.3 1.5 8.0 5.0 2.3 4.0 0.9 2.4 3.5 6 6 6 11.5 12.0 5.6 1.9 9.8 6.0 2.5 4.5 0.8 2.7 4.0 7 7 7 13.5 13.9 6.4 2.3 11.6 7.0 2.9 5.3 0.9 3.1 4.6 8 8 8 15.5 15.8 7.1 2.7 13.4 8.0 3.2 6.0 1.0 3.5 5.3 9 9 9 17.5 17.5 7.7 3.1 15.2 9.0 3.5 6.7 1.0 4.0 5.8 10 10 10 19.7 18.8 7.9 3.6 17.2 10.0 3.8 7.3 1.0 4.3 6.2 11 11 11 22.2 20.7 8.4 4.3 19.6 11.0 4.1 8.0 1.5 4.6 6.6 12 12 12 24.7 22.2 8.7 5.0 22.0 12.0 4.4 8.5 1.7 5.0 7.0 products of combustion for gas are carbon dioxide (CO,) and,water vapor with just a trace of sulphur trioxide (S03). Sulphur usually bums to the trioxide in the presence of an iron oxide catalyst. The volume of water vapor in the flue products is about twice the volume of the carbon dioxide when coke oven or natural gas is burned. Because of the large quantity of water vapor which is formed by the burning of gas, it is quite important that all gas-fired central heating plants be connected to a chimney having a good draft. Lack of chimney draft causes stagnation of the products of combustion in the chimney and results in the condensation of a large amount of the water vapor. A good chimney draft draws air into the chimney through the openings in the back-draft diverter, lowers the dew point of the mixture, and reduces the tendency of the water vapor to condense. The flue connections from a gas-fired boiler or furnace to the chimney should be of a non-corrosive material. In localities where the price of gas requires the use of highly efficient appliances, the material, used for the flue connection not only should be resistant to the corrosion of water, 467