Document QJBKEgw8e7Rp71Bm4orLQ91x7

HEATINC VENTILATING AIR CONDITIONING GUIDE 1940 As is the case with the complete combustion of almost all fuels, the products of combustion for gas are carbon dioxide (Cft) 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 gasTired 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. In localities where the price of gas requires Table 4. Minimum Round Chimney Diameters for Gas Appliances (Inches) Height of Chuinbt Feet 100 Gas Consumption in Thousands of Btu per Hour 200 300 400 500 750 1000 1500 2000 ` 20 4.SO 5.70 6.60 7.30 8.00 9.40 10.50 12.35 13.85 40 4.2S 5.50 6.40 7.10 7.80 9.15 10.25 12.10 13.55 60 4.10 5.35 6.20 6.90 7.60 8.90 10.00 11.85 13.25 80 4.00 5.20 6.00 6.70 7.35 8.65 9.75 11.50 12.85 100 3.90 5.00 5.90 6.50 7.20 8.40 9.40 11.00 12.40 highly efficient appliances, the material used for the flue connection should not only be resistant to the corrosion of water but should resist the corrosion 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. This may be done by raising the flue temperature slightly or increasing the size of the back-draft diverter. 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 asphaltchromate provides an excellent protection. A chimney for a gas-fired boiler or furnace should be constructed similarly to 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 at the joints. Cement mortar should be used for the entire chimney. Table 4 gives the minimum cross-sectional diameters of round chim neys (in inches) for various amounts of heat supplied to the appliance, and for various chimney heights. This is in accordance with Ametican Gas Association recommendations. 198 Chapter 11 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 (Fig. 2) or sprinkler type in which coal is distributed either mechanically or by air over the entire grate surface. This type of stoker has a wide application on small sized fuels and on certain special fuels such as lignites, high-ash coals, and coke breeze. 199