To properly control the combustion air, O2 and CO must be measured at the top of the radiant section where combustion is completed, regardless of the burner loading. Note that O2 and CO will coexist within the flames, where the temperature may be as high as 1200˚C.
Low NOx burners may use delayed completion of combustion through staged air to fuel mixing, or external recirculation of cooler flue gas with combustion air, reducing peak flame temperature.
In either case, to effectively control the combustion process it is essential to measure O2 and CO at the top of the radiant section, ideally 1 ft. / 0,3 m below the roof tubes where combustion reaction is expected to complete under all heater operating loading conditions.
Using a Tunable Diode Laser Spectrometer (TDLS) in concert with a dedicated controller, a cross sectional average O2 and CO density rather than a localized spot measurement can be measured to determine the right air/fuel ratio.
Using an average O2 value and CO limit value produces safer burner control and greater overall heater efficiency.
