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The ASAC limits the secondary air surrounding the flame in the combustion chamber, resulting in a significant improvement in efficiency.
The ASAC maintains a constant fuel-to-air ratio across the full burner firing range in response to changes in the burner fuel pressure. The system
is designed to maintain the excess oxygen at 2% to 5% across the entire firing range of the burner.
Automatic Secondary Air Control (ASAC) System

U.S. Patent Number 7,905,722 B1

Firetube heaters typically use low-pressure, partial pre-mix burners in which part of the air required for combustion, the primary air, is drawn into the burner and mixed with the fuel prior to reaching the flame at the burner tip. Secondary air is drawn through the firetube by stack draft and mixed with the fuel at the flame. Any secondary air not required for combustion is excess air.
Typically, firetubes with uncontrolled secondary air operate with
stack exhaust gas temperatures of 600 to 1100 degrees and
excess air of 100 to 600 percent (11 to 18.2 % excess oxygen)
depending on the process loads and the diameter of the firetube.
This operating envelope covers efficiencies ranging from less than

10 to 60%, with large diameter firetubes tending to the low end
of the efficiency range.
In addition to operating with uncontrolled secondary air, many
firetubes operate in off-on (snap acting) mode. During the off cycle, the stack continues to draft ambient air into the firetube, thereby acting as a negative heat exchanger to the process. Large quantities of heat are lost from the process during the off cycle.