Ask an engineer: Excess air and boiler efficiency

How does excess air level affect boiler efficiency?

Boiler efficiency decreases as excess air increases. For instance, a boiler requiring 50% excess air at 10:1 turndown is typically 2% less efficient than a boiler operating with 15% excess air and the same turndown. Besides excess air, another factor affecting efficiency is emissions requirements. More or less excess air may be needed to maintain desired emissions levels.

You can determine the correct amount of excess air by analyzing flue-gas oxygen concentrations. Low-cost test kits measure the percentage of oxygen in flue gas. Higher-priced analysis equipment displays oxygen, temperature and boiler efficiency, but is recommended for larger boiler systems. Online oxygen analyzers should be considered when fuel composition or steam flows are highly variable. They provide immediate feedback to burner controls to reduce excess air and optimize fuel-to-air ratios.

There are two types of burner control systems:

1. Mechanical systems can be tuned to only a limited number of firing rates. Mechanical controls should be calibrated frequently, at least on a monthly basis.
2. Electronic or linkageless digital control systems can be optimized across the entire range of firing rates and allow the fuel valve and combustion air damper to operate independently of one another. Calibration is required three to four times per year.