Document MJv96Vgen4Nyzg5r6oepqNoBa

358 CHAPTER 14 1955G The operating efficiency of a gas appliance can then be computed with sufficient precision by application of Equation 18. nti Percent Combustion Efficiency = ' -',Ti (Gross Btu of fuel \ _ /total flue losses per\ gas per cubic foot/ \cubic foot fuel gas)/ Gross Btu of fuel gas per cubic foot - (18) Reference to Table 9 will show that ultimate. C02 percentage values of fuel gases vary. While personal errors involved in C02, temperature, and chart determinations, would doubtless more than offset any inaccuracies due to universal use of the alignment charts shown, precise laboratory work may require a more exact method. For more complete information the reader is referred to Combustion, 3rd Edition, and Gaseous Fuels, (published by American Gas Association) and particularly to tables covering various properties of different commercial gases included in these publications. CONDENSATION AND CORROSION Sulfur dioxide or sulfur trioxide, formed by the combustion of sulfur in fuels, are the principal corroding substances in flue gases. They become active whenever sufficient moisture is present for the formation of sulfurous or sulfuric acid,22 and they lower the dew points of flue gases appreciably. Therefore, unless heating equipment is designed for operation at flue gas temperatures below the dewpoint, which is seldom the case, it is always ad visable to maintain temperatures above this value in all parts of the ap pliance. Excessive spot temperatures in the combustion chamber or else where, on the other hand, are also destructive in that they may result in rapid oxidation of ordinary heating surfaces. American Standard Require ments for gas furnaces, floor furnaces, and recessed heaters, for example, specify that minimum spot heating surface temperatures during normal operation must neither fall below 178 F (50 F above average dewpoint) nor exceed 875 F on any portion of the heating surface. In any event* it is usually desirable to maintain flue temperatures within the limits in dicated not only throughout the appliance, but in its connecting vent, flue, or chimney as well. Otherwise, excessive condensation and corrosion problems, with resultant customer dissatisfaction, will in all probability be the result. Average dewpoint temperatures of flue gases resulting from the combustion of various fuels, when burned with the amount of excess air normally supplied to insure complete combustion, are shown in Table 10- 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 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 5424 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 Pr' vent proper combustion. Soot can probably be most effectively removed by a jet of compressed air, by means of a brush, or a vacuum cleanerHowever, it has been found that copper chloride, lead chloride, tin chloride; zinc chloride, common salt and some other salts are partially effective 111 Fuels and Combustion 359 Table 10. Average Flue Gas Dew-Point fob Various Fuels* Type of Fuel *-- Average Dew-Point Temperature, F Butane-Air Gas Mixture (535 Btu/cu ft.................................. 68 84 93 111 127 137 119 124 121 removing soot from furnaces and boilers when properly used.25 A discus sion of instruments and methods of evaluating smoke will be found in Chapter 52. REFERENCES 1 Five Hundred Tests 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. Kratz, S. Konzo and D. W. Thompson ( University of Illinois, Engineering Experiment Station Circular No. 44). "Quality of Anthracite as Prepared at Breakers, 1935 ((/. S. Bureau of Mines Re port of Investigation, R. I. 3283). 1 Hand Firing Soft Coal Under Power Plant Boilers (U. S. Bureau of Mines Tech nical Paper No. 80). jCombustion Flames and Explosions of Gases. Lewis and von Elbe (Cambridge University Press, 1938). * Oil Burning, by H. A. Romp (Martinus Nijhoff, 1937). I ASTM Test Designation D287-39. `ASTM Test Designation D611-43T. 'Tomorrow's Fuel Oil, by W. A. Sullivan and E. B. Glendenning (Fueloil and Oil Heat, Vol. 4, No. 1. May 1945 p. 36). 14 The Correlation of Cetane Number with Other Physical Properties of Diesel Fuels (Journal of the Institute of Petroleum, Vol. 30, 1944, p. 193-197). "Characterization of Petroleum Fractions, by K. M. Watson, E. F. Nelson and L. B. Murphy (Industrial and Engineering Chemistry, Vol. 27, Dec. 1935, p. 1464). 12 Burning Index for Distillates, by J. C. Reid and A. B. Hersberger. Fueloil and Oil Beal, Vol. 5, No. 9, Jan. 1947, p. 90). II Carbon Hydrogen Ratio of Catalytically Cracked Distillate Fuel Oils, by S. P. Oauley and E. B. Delgass (The Oil and Gas Journal, Vol. 45, No. 12 July 27, 1946, P-166). 11 Rating of Fuel Oils by a Test Unit, by D. W. Locklin and G. V. 1 armelee (ASHVE Transactions, Vol. 57,1951, p. 139). Is Gaseous Fuels (American Gas Association, 1948, p. 32). 16 Gas Analysis and Testing of Gaseous Materials by V. J. Altieri (American Gas Association, First Ed. 1945). " Tentative Methods of Test for Specific Gravity of Gaseous Fuels (American society for Testing Materials, ASTM Designation: 1070-49). 14 Standard Method of Test for Calorific Value of Gaseous Fuels by the WaterHow Calorimeter (American Society for Testing Materials, ASTM Designation: 900-48). 14 American Standard for Installation of Gas Piping and Gas Appliances in Build,0gs. ASA Z 21.30 1950 (American Gas Association). NBFU Standard for the Installation of Oil Burning Equipments--NBFU pam phlet no. 31 (National Board of Fire Underwriters). - **l IDI omestic Gas Range Research (American Gas Association Laboratories Bulletin L P- 64).