Most high temperature direct-fired ovens, boilers, furnaces and radiant tubes operate with about 10 percent to 20 percent excess combustion air at high fire to prevent the formation of dangerous carbon monoxide and soot deposits on heat transfer surfaces and inside radiant tubes. For the fuels most commonly used by U.S. industry, including natural gas, propane and fuel oils, approximately one cubic foot of air is required to release about 100 BTUs in complete combustion. Process heating efficiency is reduced considerably if the air supply is significantly higher or lower than the theoretically required air.
Factors Affecting Excess Air Level Requirements. Combustion systems operate with different amounts of excess air between high and low fire. Measurement of oxygen and combustibles such as carbon monoxide in flue gases can be used to monitor changes in excess air levels. For most systems, 2 percent to 3 percent oxygen with a small amount of combustibles -- only 10 to 50 parts per million -- indicate ideal operating conditions.
Processes that evaporate moisture or solvents need large amounts of excess air to dilute flammable solvents to noncombustible levels, to ensure adequate drying rates, and to carry vapors out of the oven. Lowering excess air to minimal levels can slow down the process and create an explosion hazard.
To figure potential savings, you need to know:
- The temperature of the products of combustion as they leave the oven, boiler or furnace.
- The percentage of excess air or oxygen in flue gases, at which the oven, boiler or furnace now operates.
- The percentage of excess air or oxygen in flue gases, at which the oven, boiler or furnace could operate.
% Fuel Savings = 100 X ((%AH Desired - %AH Actual) / %AH Desired)
To get the most efficient performance out of fuel-fired furnaces, ovens, and boilers: determine the best level of excess air for operating your equipment; set your combustion ratio controls for that amount of excess air; and check and adjust ratio settings regularly.
This article is adapted from Energy Tips, a publication of Industrial Technologies Program, Energy Efficiency and Renewable Energy, part of the U.S. Department of Energy, Washington, D.C. For more energy-saving tips, call the Clearinghouse at (800) 862-2086 or visit www.eere.energy.gov/industry.
7 Energy-Saving IdeasImplement Today for Immediate Results
- Operate Ovens, Furnaces and Boilers At or Close to Design Capacity. This decreases fuel consumption per unit of production. With an estimated energy savings range of 1 percent to 5 percent, this objective can be completed at a low cost.
- Reduce Excess Air Used for Combustion. Measure and control air-fuel ratio or oxygen and carbon monoxide content of flue gases from process heating equipment such as furnaces, ovens, and boilers for the entire range of operation. With an estimated energy savings range of 2 percent to 15 percent, this objective can be completed at low cost.
- Clean Heat Transfer Surfaces. Cleaning the heat transfer surfaces lowers flue gas temperature and increases efficiency by improving heat transfer. Doing so can save an estimated 1 percent to 5 percent at a moderate cost.
- Reduce Radiation Losses from Openings. Reducing radiation losses can save an estimated 1 percent to 2.5 percent at a relatively low cost.
- Use Proper Oven, Furnace or Boiler Insulation to Reduce Wall Heat Losses. Reducing wall losses produces energy savings of 1 percent to 2 percent with only a moderate investment.
- Adequately Insulate Steam Lines Leaving the Boiler as well as Air- or Water-Cooled Surfaces Exposed to the Oven or Furnace Environment. Insulating piping reduces losses from it. With an estimated energy savings range of 1 percent to 5 percent, piping insulation can be completed for a moderate investment.
- Install Air Preheat or Other Heat Recovery Equipment. This type of equipment recovers available heat from exhaust gases and boiler blowdown and transfers it back into the system by preheating. The potential energy savings range from 1 percent to 25 percent, depending on process temperatures and other variables, but the cost is high.