Document vVKKYYOJ7LKX37y5pgLQEGM2Z

318 CHAPTER 16 1948 Guide type of burner part of the air required for combustion is mixed with the gas as primary air, the air and gas mixture being fed to the burner ports; Additional secondary air is introduced around the flame by draft inspi ration. In the luminous flame burner, which is sometimes used, all of the air for combustion is brought in contact with the flame as secondary air. This secondary air should be brought into intimate contact with the gas. Some makes of burners use radiants or refractories to convert some of the energy in' the gas to radiant heat. The radiants also serve as baffles in directing the flow of the products of combustion. The quantity of air given in Table 6 is that required for theoretical combustion, but with a properly designed and installed burner the excess air can be kept low. In order to insure freedom from carbon monoxide under conditions which may obtain in installations, it is customary to design gas burning appliances for a supply of 30 to 35 per cent of excess air. In individual installations in which flue gas analyses are made, the excess air is sometimes reduced to approximately 20 per cent. The division of the air into primary and secondary is a matter of burner design, the pressure of gas available, and the type of flame desired. The air gas ratio has a decided effect upon flame. propagation. It is necessary that the gas will flow out of the burner ports fast enough so that the flame cannot travel back into the burner head, i.e. flash back, but the velocity must not be so high that it blows the flame away from the port. The maximum and minimum flow speeds from burner ports which may be permitted are known to be very close together when air-gas mixtures in theoretical proportions are being supplied to the burner. As the air-gas ratio is lowered, and the mixture becomes more gas rich, the limiting speeds become farther apart, until with 100 per cent gas, in an all-yellow flame, flash back cannot occur and a much higher velocity is needed to blow off the flames. SOOT .The deposit of soot on the flue surfaces of a boiler or heater acts as an insulating layer over the surface and reduces the heat transmission to the water or air. The Bureau of Mines Report of Investigations No. 3272*5 1 * * 4 shows that the loss of seasonal efficiency is not so great as has been believed and usually is not over 6 per cent because the greater part of the heat is transmitted through the combustion chamber surfaces. The Bureau of. Standards Report BMS 546 points out that, although the decrease in efficiency of an oil fired boiler due to soot deposits is relatively small, the attendant increase in stack temperature may be considerable. The soot accumulation clogs the flues; reduces the draft, and may prevent proper combustion. Soot can probably be most effectively removed by a jet of compressed air or by means of a brush. However, it has been found that copper chloride, lead chloride, tin chloride, zinc chloride, common salt and some other salts are partially effective in removing soot from furnaces and boilers when properly used.7 8 CONDENSATION AND CORROSION Sulfur dioxide or sulfur trioxide formed by the combustion of sulfur in fuels is the principal corroding element in flue gases, and becomes active whenever moisture is present for the formation of sulfurous or sulfuric add. It is necessary, therefore, to maintain a flue gas, temperature in excess of the dew-point temperature of the flue gases in all parts of Table 10. Average Flue Gas Dew-Point for Various Fuels' Type op Fuel . Anthracite,________________ Semi-Bituminous Coal......... Bituminous Coal--------------OiL________________ _____ a Natural Gas_________ ____ _ Manufactured Gas.............. Average Dew-Point Temperature. F 68 84 93 111 127 137 appliances unless they are made of materials that will resist these cor rosive influences. It-is usually desirable to maintain a flue gets tem perature above the dew-point temperature throughout the heating appliance and the chimney or smokestack because of these same corrosive effects. The average dew-point temperatures of the flue gases from the several fuels, when burned with the amount of excess air usually sup plied to insure complete combustion, are shown in Table 10. LETTER SYMBOLS USED IN CHAPTER 16 hi = heat loss in the dry chimney gases, Btu per pound of fuel. hi = heat loss in water vapor from combustion of hydrogen, Btu per pound of fuel. hi = heat loss in water vapor in combustion air, Btu pier pound of fuel. ht -- heat loss from incomplete combustion of carbon, Btu per pound of fuel. hi = heat loss from unburned carbon in the ash, Btu per pound of fuel. wg. = weight of dry flue gas per pound of fuel (from Equation 6), pounds. cp = mean specific heat of flue gases at constant pressure. tg = temperature of flue gases at exit of heating device, Fahrenheit degrees. . /a .* temperature of combustion air, Fahrenheit degrees. Hi percentage of hydrogen in the fuel by weight from ultimate analysis of fuel as fired. M = humidity ratio of combustion air, pounds of water vapor per pound of dry air. ua = weight of combustion air per pound of fuel used, pounds. C = weight of carbon burned per pound of fuel corrected for carbon in ash, pounds. Cu = percentage of carbon in the fuel by weight from the ultimate analysis. CO, COi = percentages of CO, COi in the flue gases by volume. Wa = weight of ash and refuse, pounds. Ca = per cent of combustibles in ash and refuse by weight. W = weight of fuel used, pounds. REFERENCES 1--Five Hundred Testa of Various Coals in Househeating Boilers (U. S. Bureau of Mines Bulletin No. 276). *--Combustion Efficiencies as Related to Performance of Domestic Heating Plants, by A. P. Kiatz, S. Konzo and D. W. Thompson (Illinois Engineering Experiment Station Circular No. 44). ^"Quality of Anthracite as Prepared at Breakers. 1935 (U. S. Bureau of Mines Report of Investigation, R. I. 3283). 4--Hand Firing Soft Coal Under Power Plant Boilers (U. S. Bureau of Mines Technical Paper No. 80). 5--Effect of Soot on Heat Transmission in Boilers (U. S. Bureau of Mines Report of Investigation No. 3272). 4--Effect of Soot on the Rating of an Oil-Fired Heating Boiler (National Bureau of Standards Report BMS 54). 7--Removal of Soot from Furnaces and Flues by the Use of Salts and Compounds, by P. Nicholls and C, W. Staples (U. S. Bureau of Mines Bulletin 360). 8--Condensation of Moisture in Flues, by William R. Morgan (Illinois Engineering Experiment Station Circular No. 22).