Document LKRMbj9p8e6XBgLGYr9oNX8Q7

American Society of Heating and Ventilating Engineers Guide, 1929 A study of the drawing of a typical back draft diverter will make it plain that a stoppage in the chimney will cause the combustion products to come out into the boiler room, but there will be no interference with the complete combustion of the gas. Another feature in the construction of the back draft diverter is that a down draft from the chimney will have practically no effect upon the combustion of the gas, due to the baffle deflecting the draft into the boiler room. As is the case with the complete combustion of almost all fuels, the product of combustion for gas are carbon dioxide (COj) and water vapor with just a trace of sulphur trioxide (S03). Sulphur usually burns 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 con densation 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. Table 9. Minimum Round Chimney Diameters for Gas Appliances Chimney Feet 20 40 60 80 100 100 4.50 4.25 4.10 4.00 3.90 200 5.70 5.50 5.35 5.20 5.00 300 6.60 6.40 6.20 6.00 5.90 400 7.30 7.10 6.90 6.70 6.50 500 8.00 7.80 7.60 7.35 7.20 750 9.40 9.15 8.90 8.65 8.40 1000 10.50 10.25 10.00 9.75 9.40 1500 12.35 12.10 11.85 11.50 11.00 2000 13.85 13.55 13.25 12.85 12.40 A chimney for a gas-fired boiler or furnace should be constructed in accordance with the principles applicable to other boilers. Where the wall forming a smoke flue is made up of less than an 8 in. thickness of brick, concrete or stone, a burnt fire clay flue tile lining should be used. Care should be used that the lengths of flue tile meet properly with no openings at the joints. Cement mortar should be used for the entire chimney. The accompanying table gives the minimum cross-sectional areas of round chimneys (in square inches) for various amounts of heat supplied to the appliance, and for various chimney heights. This is in accordance with American Gas Association recommendations. 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, but should resist the corrosion of dilute solutions of sulphur trioxide in water. Sheet aluminum, as well as some other materials, seems to serve this purpose very well. 156 Chapter VI--Chimneys CONSTRUCTION OF CHIMNEYS For general data on the construction of chimneys reference should be made to the Standard Ordinance for Chimney Construction of the Na tional Board of Fire Underwriters. . Briefly summarized these provisions are as follows for heating boilers and furnaces: The construction, location, height and area of the chim ney to which a heating boiler or warm-air furnace is con nected affect the operation of the entire heating system. Most residence chimneys are built of brick and may be either lined or unlined, but in either case the walls must be air-tight and there should be only one smoke opening into the chimney. Cleanout, if provided, must be absolutely air-tight when closed. The walls of brick chimneys shall be not less than 3% in. thick (width of a standard size brick) and shall be lined with fire-clay flue lining. Fire-clay flue linings shall be manufactured from suitable refractory clay, either natural or compounded, and shall be adapted to withstand high temperatures and the action of flue gases. They shall be of standard commercial thickness but not less than % in. All fire-clay flue linings shall meet the standard specification of the Eastern Clay Products Association. The flue sections shall be set in special mortar, and shall have the joints struck smooth on the inside. The masonry shall be built around each section of lining as it is placed, and all spaces i' between masonry and linings shall be completely filled with mortar. No broken flue lining shall be used. Flue linings shall start at least 4 in. below the bottom of smoke-pipe intakes of flues, and shall be continued the entire heights of the flues and project at least 4 in. above chimney top to allow for a 2 in. projection of lining. The wash or splay shall be formed of a rich cement mortar. To improve the draft the wash surface should be concave wherever practical. Flue lining may be omitted in brick chimneys, provided the walls of the chimneys 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 157