Periodically checking and resetting the air-fuel ratios is one of the simplest ways to get maximum efficiency out of fuel-fired process heating equipment such as ovens, heaters, boilers and furnaces.

Figure 1. Using the available heat chart, you can quickly estimate the burner flame temperature and projected fuel savings.

To prevent the formation of carbon monoxide and soot deposits on heat transfer surfaces and inside radiant tubes, most high temperature direct-fired ovens, boilers, furnaces and radiant tubes operate with approximately 10 percent to 20 percent excess combustion air at high fire. For the fuels most commonly used by U.S. industry, including natural gas, propane and fuel oils, approximately one cubic foot of air (1 ft3) 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 ppm - 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.

How to Measure Air-Gas Ratios. Air-gas ratios can be determined by flow metering or flue gas analysis. Sometimes, a combination of the two works best. Use the available heat chart (figure 1) to estimate the savings obtainable by tuning burner air-gas ratios. The excess air curves are labeled with corresponding oxygen percentages in flue gases.

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.
  Using figure 1, determine the available heat under present and desired conditions by reading up from the flue gas temperature to the curve representing the excess air or O2level; then, read left to the percentage available heat (AH). Calculate the potential fuel savings:

  % 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.

For more ways to save, visit the DOE’s Industrial Technologies Program’s web site at www.eere.energy.gov/industry/.

Sidebar
20 Ways to Save Energy Now

If you think saving energy requires costly new equipment or complicated changes to your operating practices, think again. Here are 20 steps that you can take this year for little or no cost, using in-house expertise. Simple changes can cut your energy bills. 
  1. Operate ovens and boilers at or close to design capacity.
  2. Reduce excess air used for combustion.
  3. Clean heat transfer surfaces.
  4. Reduce radiation losses from openings.
  5. Use proper oven or boiler insulation to reduce wall-heat losses.
  6. Adequately insulate air- or water-cooled surfaces exposed to the oven and steam lines leaving the boiler.
  7. Install air-preheat or other heat-recovery equipment.
  8. Improve water treatment to minimize boiler blowdown.
  9. Optimize deaerator vent rate.
  10. Repair steam leaks.
  11. Minimize vented steam.
  12. Implement a steam-trap maintenance program.
  13. Use high-pressure condensate to make low-pressure steam.
  14. Utilize a back-pressure turbine instead of pressure-reducing or release valves.
  15. Optimize condensate recovery.
  16. Minimize air leakage into the oven by sealing openings.
  17. Maintain proper and slightly positive oven pressure.
  18. Reduce weight of, or eliminate, material-handling fixtures.
  19. Modify the oven system or use a separate heating system to recover oven-exhaust gas heat.
  20. Recover part of the oven exhaust heat for use in lower-temperature processes.


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