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NORA Oilheat Technicians Manual

Chapter 7 Combustion Figure 7-22: 0% Excess air Figure 7-23: 50% Excess air 7-22 Combustion (Amount of water vapor is not considered in % of CO2 determination 1.18 lb. water 11.02 lb. nitrogen 3.16 lb. carbon dioxide 15.36 lb. total 1.00 lb. oil 14.36 lb. air 15.36 lb. total 1.18 lb. water 11.02 lb. nitrogen 3.16 lb. carbon dioxide 7.18 lb. excess air 22.54 lb. total 1.00 lb. oil 21.54 lb. air 22.54 lb. total percentage of CO2 drops and the percentage of oxygen starts to increase. (Figure 7-20). By measuring either the percentage of CO2 or the percentage of unused oxygen (O2) we can determine the quantity of excess air. They are the opposite of each other. As the percent of O2 increases the CO2% decreases. (Figure 7-21). Why we must control excess air Properly adjusting the burner air shutter is a compromise between too little air, which produces smoke, and too much air, which lowers efficiency. See Figures 7-22 and 7-23. Earlier we found that a fixed quantity of combustion air is required to burn each pound of oil. We call this the theoretical fuel-air ratio. Oilburners require a controlled amount of excess air above the theoretical value to assure complete combustion and smoke free operation. This excess combustion air serves as a safety margin to prevent incomplete combustion and smoke. The air pressure in the building is always changing, the temperature of the fuel changes, and the draft produced by the chimney is not constant. Excess air gives us the safety factor we need for all these variations. While excess air is needed for reliable clean operation, it also reduces efficiency. When increasing air for more oxygen, excess nitrogen comes along for the ride. This dramatically increases the amount of combustion gases that must be vented. Heat exchangers need time to absorb the heat from the combustion gases. The more gases forced into a heat exchanger, the faster they travel through the heat exchanger. This gives the heat exchanger less time to pull out the heat so the stack temperature goes up, and the efficiency goes down. Limiting the amount of excess air can increase efficiency and save our customers money. We measure excess air by determining


NORA Oilheat Technicians Manual
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