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NORA Oilheat Technicians Manual

Chapter 7—Combustion 7-5 Heating oil flames Combustion is a series of exact chemical reactions that create exact quantities of combustion gases. It takes 14.4 pounds of air to burn each pound of heating oil and we produce 15.4 pounds of combustion gases, Figure 7-3. If we could achieve stoichiometric, or perfect combustion, each gallon of oil consumed would need 1,360 cubic feet of combustion air. The actual amount of air required will vary by the heat value of the fuel and the design of the burner. Heating oil contains about 19,500 Btus per pound. Non-flame retention burners need at least 1,700 cubic feet of air to burn clean. Current burners need about 1,500 cubic feet. Buildings today are so well insulated and weather-stripped that getting adequate combustion air to the burner is becoming a problem. When troubleshooting combustion problems, get into the habit of asking yourself, “Where is my combustion air coming from?” and “Do I have enough to burn all of my fuel?” Oilburner flames produce various combustion gases in fixed quantities. With perfect combustion, every pound of oil burned will produce 3.2 pounds of carbon dioxide, 1.1 pound of water vapor, and 11.1 pounds of hot nitrogen. This constant ratio of combustion gases allows us to test the quality of a flame against this perfect standard to determine optimum adjustment. Heating oil atomization and vaporization Heating oil will not burn as a liquid. It must be converted to a vapor before the rapid reaction between the fuel and the air can produce a flame. The oilburner’s job is to convert the liquid fuel into a vapor so it can be burned. The oil is pumped to the nozzle at high pressure (100 psi or more) where it is broken up into a mist of small droplets (atomized). The droplets evaporate quickly when exposed to the heat of the spark or flame, producing vapors that burn easily with the air supplied by the burner fan. Combustion air supply and air-oil mixing The better the air and the oil vapor are mixed, the better our combustion. Burner air parts, (including turbulators, spinners, end cones, and flame retention heads), are designed to give good mixing of the atomized fuel droplets and the combustion air. Good fuel and air mixing assures that all the fuel vapors contact enough oxygen for complete burning. In high pressure atomizing burners, several factors control the quality of air-oil mixing. The spray pattern of the oil droplets must be similar to the air pattern created by the burner. Flame retention burners have much better air/oil mixing capabilities than older burners. They use high speed burner Figure 7-3: Fueloil combustion in air Chapter 7 Combustion


NORA Oilheat Technicians Manual
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