Document QkQY69kDK1JQ0eLzRQnzzx2Z7

HEATING VENTILATINC AIR CONDITIONING GUIDE 1944 COMBUSTION OF GAS The majority of gas burners utilized in central domestic heating plants are of the Bunsen type and operate with a non-luminous flame. In this Table 9. Representative Properties of Gaseous Fuels, Based on Gas at 60 F and 30 in. Hg. Gas Natural gas-- California, Natural gas-- Mid-Conti nental Natural gas-- Ohio Natural gas-- Pennsylvania Retort coal gas Coke oven gas Carbureted water gas Blue water gas Anthracite pro ducer gas Bituminous producer gas Oil gas Btu feb Cu Ft (GHrioss*s) Low (Net) Specific Grafitt Am 1.00 Am Required fob Combus tion, (Cu Ft) Products of Combustion 3ubic Feet Total COj E& with N Ulti mate COj Dry Baas Theoretical Flame Tem perature, (deo F) 1200 1087 0.67 11.26 1.24 2.24 12.4 12.2 3610 967 873 0.57 9.17 1130 1025 0.65 10.70 1232 575 588 1120 510 521 0.71 11.70 0.42 5.00 0.42 . . .5.19. 536 496 0.65 308 281 0.53 4.37 2.26 134 124 0.85 1.05 150 140 0.86 575 510 0.35 1.24 4.91 0.97 1.92 10.2 11.7 3580 1.17 2.16 11.8 12.1 3600 1.30 2.29 0.50 1.21 0.51 .1.25 12.9 5.7 5.9 12.3 11.2 11.0 3620 3665 3660 0.74 0.75 0.46 0.51 5.0 17.2 2.8 22.3 3815 3800 0.33 0.19 1.9 19.0 3000 0.35 0.19 2.0 19.0 0.47 1.21 5.6 10.7 3160 3725 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 die 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 9 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 182 CHAPTER 8. COMBUSTION AND FUELS 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. 32726 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 547 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. CONDENSATION AND CORROSION Sulphur dioxide or trioxide formed by the combustion of fuels is the corroding element in flue gases and becomes active whenever moisture is available for the formation of sulphurous or sulphuric acid. It is there fore necessary to maintain a flue gas temperature above 175 F in all parts of appliances, and consequently it is not practicable to recover the latent heat in flue gases. Since some unpreventable condensation occurs during -the warming-up period, it is important to design appliances so that- all surfaces reach quickly a temperature above the dew-point of the flue gases or have corrosion resistant properties enabling them to with stand the corrosive effect. REFERENCES Fuels and Their Combustion, Haslam and Russell (McGraw-Hill Co., 1926). Principles of Combustion in the Steam Boiler Furnace, Arthur D. Pratt (Babcock and Wilcox Co.). Effect of Soot on Heat Transmission in Boilers (U. S. Bureau of Mines Report of Investigation No. 3272). TEffect of Soot on the Rating of an Oil-Fired Heating Boiler {National Bureau of Standards Report BMS 54). 183