Document 6vK8wD7eBXyM7k92xgzgEJk6

HEATING VENTILATINC AIR CONDITIONING GUIDE 1942 products 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 condensation of a large amount of the water vapor. A good chimney draft draws air 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 should be of a non-corrosive material. The material used for the flue connection should not only be resistant to the corrosion of water but should resist the cor rosion of dilute solutions of sulphur trioxide. Local practice should be followed in the selection of the most appropriate flue materials. When condensation in a chimney proves troublesome, it may be necessary to provide a drain to a dry well or sewer. The cause of the excessive condensation should be investigated and remedied if possible. The protection of unlined chimneys has been investigated and the results indicate that after the loose material has been removed, the spraying with a water emulsion of asphalt-chromate provides an excellent protection. A chimney for a gas-fired boiler or furnace should be constructed similarly to the principles applicable to other boilers. Table 2 gives the minimum cross-sectional diameters of round chimneys (in inches) for various amounts of heat supplied to the appliance, and for various chimney heights. This is in accordance with American Gas Association recommendations. CONSTRUCTION DETAILS For general data on the construction of chimneys reference should be made to the Standard Ordinance for Chimney Construction of the National Board of Fire Underwriters. Briefly summarized, these provisions are as follows for heating boilers and furnaces: The walls of brick chimneys shall be not less than in. thick (width of a standard size brick) and shall be lined with fire-clay flue lining meeting 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 between masonry and linings shall be completely filled with mortar. No broken flue lining shall be used. Flue lining 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 the chimney top to allow for a 2 in. projection of lining. 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, but no such cap or coping shall decrease the flue area. 196 Chapter 10 AUTOMATIC FUEL BURNING EQUIPMENT Classification of Stokers, Combustion Process and Adjust ments, Furnace Design, Classification of Oil Burners, Combus tion Chamber Design, Classification of Gas-Fired Appliances AUTOMATIC mechanical equipment for the combustion of solid, liquid and gaseous fuels is considered in this chapter. MECHANICAL STOKERS A mechanical stoker is a device that feeds a solid fuel into a combustion chamber, provides a supply of air for burning the fuel under automatic control and, in some cases, incorporates a means of removing the ash and refuse of combustion automatically. Coal can be burned more efficiently by a mechanical stoker than by hand firing because the stoker provides a uniform rate of fuel feed, better distribution in the fuel bed and positive control of the air supplied for combustion. Stokers may be divided into four types according to their construction, namely, (1) overfeed flat grate, (2) overfeed inclined grate, (3) underfeed side cleaning type, and (4) underfeed rear cleaning type. Overfeed Flat Grate Stokers This type is represented by the various chain- or traveling-grate stokers. These stokers receive fuel at the front of the grate in a layer of uniform thickness and move it back horizontally to the rear of the furnace. Air is supplied under the moving grate to carry on combustion at a sufficient rate to complete the burning of the coal near the rear of the furnace. The ash is carried over the back end of the stoker into an ashpit beneath. This type of stoker is suitable for small sizes of anthracite or coke breeze and also for bituminous coals, the characteristics of which make it desirable to burn the fuel without disturbing it. This type of stoker requires an arch over the front of the stoker to maintain ignition of the incoming fuel. Frequently, a rear combustion arch is required to main tain ignition until the fuel is fully consumed. A typical traveling-grate stoker is illustrated in Fig. 1. Another and distinct type of overfeed flat-grate stoker is the spreader (Figs. 2 and 3) or sprinkler type in which coal is distributed either by 197