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NORA Oilheat Technicians Manual

Chapter 7 Combustion Figure 7-14: Relationship of excess air, flame temperature, and volume of combustion gases Figure 7-15: Efficiency vs. net stack temperature Chapter 7—Combustion 7-13 Net Stack Temperature °F Efficiency % Efficiency vs. Net Stack Temperature No.2 Fueloil, 12% CO2 % Excess Combustion Air Flame Temperature °F Volume of Hot Combustion Gas, Cubic Feet Per Gallon of Fuel Burned the more effective the heat exchanger design and heat transfer. Stack temperature is a measurement of the heat exchanger’s ability to draw the heat from the combustion gases. As the excess air goes up, so does the stack temperature, Figure 7-14. To understand why this happens, we must look at the heat exchanger. The longer the combustion gases are in the heat exchanger, the more the heat the exchanger can pull from them, and the lower the stack temperature will be. As the excess air increases so does the volume of combustion gases. The volume of gases traveling through the heat exchanger determines how fast the gases must travel. The more air we put in, the faster it goes, and the less time the exchanger has to suck the heat out of the gases. Therefore, as the excess air goes up, the stack temperature does too, even though the flame temperature is reduced. As stack temperatures go up, efficiency comes down, and our customers’ heating costs increase, see Figure 7-15. The flue gas temperature and all other tests should be measured in the flue gas hot spot. This is the point in the center of the flue where the stack temperature and the CO2 are at the highest level and the O2 is at its lowest level. The primary importance of stack temperature is to provide enough heat in the flue to prevent water formation. If the temperature is not high enough, water in the combustion gases can condense in the flue pipe or chimney. Condensing in non-condensing appliances can cause chimney


NORA Oilheat Technicians Manual
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