Document bq20oE50vbdQKddGgm739jGo

American Society of Heating and Ventilating Engineers Guide, 1930 amounts to about 10 per cent of the total heat value. It is practically impossible to utilize the entire higher heat value of the gas in any house heating appliance, because to do so it would be necessary to cool the products of combustion down below their dew-point, which is ordinarily in the neighborhood of 130 deg. A stack temperature of 130 deg. is not sufficient to produce a good draft under actual installation conditions. The heat balance of a gas-burning heating boiler or furnace, therefore, includes the following items: Heat in the dry flue gas. Heat in the water vapor. Heat loss due to incomplete combustion. Heat loss through radiation. Heat absorbed by the water or air in the boiler or furnace. Table 1 shows the maximum theoretical efficiencies when burning a typical manufactured gas with various stack temperatures. These, are based on the higher heat value of the gas, and do not include any cor rection for radiation from the boiler covering. Radiation will reduce these efficiencies as much as 5 per cent, depending on the insulating properties of the covering. The gas is assumed to be burned with 35 per cent excess air and has the following composition: CO, 8.6 per cent H, 52.5 " " CH4 31.6 " " C,H. 1.1 " " C,H, 1.1 " " O, 0.1 " " CO, 1.5 " " N, 3.5 " " Heat value per cubic foot at 60 deg. fahr. and 30 in. hg., 535 B.t.u. Specific gravity 0.58 (air = 1) Air Temperature 60 deg. fahr. Atmospheric moisture neglected. 100.0 per cent Table 1. Products of Combustion and Theoretical Efficiencies with a Typical Manufactured Gas Stack Temperature (deg. fahr.) 250 Heat in Dry Flue Gas above 60 deg. (%) _____ 3.57 Heat in Water Vapor above 60 deg. (%).._____ 9.13 Heat absorbed by boiler, or efficiency (%) 87.30 .......... 0 275 4.05 9.22 86.73 0 300 4.52 9.30 86.18 0 350 5.48 9.47 85.05 0 Flue Gas Analysis 100.00 100.00 100.00 5.63% Na 83.19% 100.00 It will be seen from Table 1 that the maximum attainable efficiency under practical operating conditions, which include the necessity of having a stack temperature sufficiently high to insure a good draft, and with a normal radiation loss; is approximately 80 per cent. AIR FOR COMBUSTION Most gas appliances consume the gas with a blue or Bunsen flame, although some room heaters use a luminous or yellow flame. Bunsen flames and luminous flames differ in the.way in which the air necessary 218 ' ,i | .^ It .r - i> i Chapter 11--Heating by Gas = for combustion is supplied. Bunsen type burners are provided with an external mixer, in which a portion of the air is mixed with the gas previous to ignition. This is called primary air. The aspirating effect of the jet of the mixture issuing from the burner ports, and the chimney effect of the gas passages above, draw currents of air past the flame and into it, in sufficient quantity to cause complete combustion. This is known as secondary air. In order for combustion to be complete and for the products to be free of carbon monoxide, it is necessary that heating surfaces be sufficiently high above the burner to prevent the pale blue inner cone, which is a characteristic part of a Bunsen flame, from striking any cold heating surfaces. A yellow tip on the flame indicates insufficient primary air and is corrected by opening the adjustable air shutters. Some room heaters which consume gas in very small amounts use . the luminous flame. A luminous flame is limited in its application to appliances burning small amounts of gas. No part of a luminous flame can be allowed to touch any cold surface, as this will result in incomplete combustion with the formation of soot and carbon monoxide. Table 2 shows the heat values of a cubic foot of each of three common varieties of gas, together with the theoretical air requirements and the ' air requirements with 50 per cent excess. In practice excess air must be admitted to the fire in order to insure complete combustion. Table 2. Volume of Air Required for Combustion of Different Gases Gas Mixed Coke Oven and Water Gas...... B.t.u. pea Cubic Foot 1131 560 537 Theoretical Am pee Foot op Gas Cu. Ft. 10.70 4.78 5.03 Actual Am with 50 Pea Cent Excess Cu. Ft. 16.05 7.17 7.55 In designing and installing gas-burning equipment, particularly where large amounts of gas are to be consumed, it is important that provision be made for the easy access of sufficient air to support combustion and to provide boiler room ventilation. Very little motive force exists to draw air into the boiler room. Therefore, the opening through which it enters must be large enough to keep the velocity to a very low value. FUEL COSTS WITH GAS In comparing the cost of gas with that of other fuels, a common basis .should be used. This is usually taken as the cost per 1,000,000 B.t.u. and for gas involves consideration of the calorific value of the fuel, the overall efficiency and the average gas rate, which should include the de.mand rate as well as the commodity rate. The chart (Fig. 1) can be used for estimating the cost per 1,000,000 B.t.u. for both manufactured and natural gas for various efficiencies and average gas rates. This chart is based on a heat content of 535 B.tTu. per cubic foot for manufactured gas and 1,000 B.t.u. for natural gas. By reference to the Chapters on Heating by Goal, Heating by Oil and Heating by Electricity, a comparison can be made as to heating costs with these other heating media. 219