Document 064gOzdvkqw5kazJ69BzqVKM

340 CHAPTER 13 1951 Guide air, as they have been tested as built for good combustion and efficiency. In order to insure freedom from carbon monoxide under conditions which may obtain in installations, it is customary to design gas burning appli ances for a supply of 30 to 35 percent of excess air. In individual installa tions in which flue gas analyses are made, the excess air is sometimes re duced to approximately 20 percent. 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 in the burner head has a decided effect upon flame propa gation. It is necessary that the gas should flow out of the burner ports fast enough so that the flame cannot travei 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 permissible flow speeds from burner ports 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 low ered, and the mixture becomes more gas rich, the limiting speeds become farther apart, until with 100 percent gas, in an all-yellow flame, flash back cannot occur and a much higher velocity is needed to blow the flames off the burner ports. 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. 3272s shows that the loss of seasonal efficiency is not so great as has been be lieved, and usually is not over 6 percent 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 de crease 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 pre vent proper combustion. Soot can probably be most effectively removed by a jet of compressed air, by means of a brush, or a vacuum cleaner. 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.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 acid.9 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 appliances, unless they are made of materials that will resist these cor rosive influences. It is usually desirable to maintain a flue gas tempera ture 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 supplied to ^insure complete combustion, are shown in Table 9. Fuels' and 'Combustion 341 REFERENCES i Fjve Hundred Tests of Various Coals in Househeating Boilers {U. S. Bureau of Mines Bulletin No. 276). : i Combustion Efficiencies as Related to Performance of Domestic Heating Plants, by A. P. Kratz, S. Konzo and D. W. Thompson (University of Illinois, Engineering Experiment Station Circular No. 44). `Quality of Anthracite as Prepared at Breakers, 1935 (U. SI Bureau of Mines Report of Investigation, R. I. 3283). `Plotted from Charts 8 and 9, p. 281, Gaseous Fuels (American Gas Association, 1948). .. Hand Firing Soft Coal Under Power Plant Boilers (U. S. Bureau of Mines Tech nical Paper No. 80). > Effect of Soot on Heat Transmission in Boilers ((/. S. Bureau of Mines Report of Investigation No. 3272). 1 Effect of Soot on the Rating of an Oil-Fired Heating Boiler (National Bureau of Standards Report BMS 54). Removal of Soot from Furnaces and Flues by the Use of Salts and Compounds, by P. Nicholls and C. W. Staples (V. S. Bureau of Mines Bulletin No. 360). 8 Condensation of Moisture in Flues, by William R. Morgan (University of Illinois, Engineering Experiment Station Circular No. 22). BIBLIOGRAPHY Fuels and Their Combustion, by Haslam and Russell (McGraw-Hill Co., 1926). Principles of Combustion in the Steam Boiler Furnace, by Arthur D. Pratt (Bab cock and Wilcox Co.), Smoke-Producing Tendencies in Coals of Various Ranks, by H. J. Rose and-F. P. Lasseter (A.S.H.V.E. Transactions, Vol. 45, 1939, p. 329). Fundamentals of Combustion in Small Stokers, by C. A. Barnes (Bituminous Coal Research, Inc., Technical Report No. TV). Hand-Firing of Bituminous Coal in the Home, by A. P. Eratz, J. R. Fellows, and J. C. Miles (Illinois Engineering Experiment Station Circular No. 46). Classification and Selection of Illinois Coals, by G. H. Cady (Illinois State Geo logical Survey Bulletin No. 62). Bituminous Coal Research, Inc.: Technical Report VI, The Treatment of Coal with Oil and Other Petroleum Prod ucts, by J. M. Pilcher and R. A. Sherman. Information Bulletin No. 4, Dustless Treatment of Coals with Materials Other Than Oil, by R. A. Sherman and G. W. Land. Information Bulletin No. 6, Questions and Answers on the Use of Fuel Oil for Dust less Treatment. Technical Report VII--Application of Overfire Air Jets, by R. B. Engdahl. Bureau of Mines Publications: Bulletin No. 97, Sampling and Analyzing Flue Gases, by Henry Kreisinger and F. K. Ovitz. Report of Investigations (R. I. 2980), Coke as a Domestic Heating Fuel, by P. Nicholls and B. A. Landry. Technical Paper No. 303, Value of Coke, Anthracite, and Bituminous Coal for Generating Steam in a Low-pressure Cast-Iron Boiler, by John Blizard, James Neil, and F. C. Houghten. Bulletin 378, Effect of Preheat, and Distribution of Ash in Fuel Beds, by P. Nicholls. Handbook, Questions and Answers for the Home Fireman (Revised), by J. F. Barkley. Anthracite Industries Laboratories Publications: Report 2015, Comparison of Sizes, Egg, Stove and Chestnut Anthracite. Report 2018, Domestic Survey.