![]() The diluted flue gas is less effective at reducing NOx. When these older boilers are retrofitted with low NOx burners using flue gas re-circulation, the high draft condition needs to be controlled because the recirculated flue gas is diluted by fresh air (called tramp air) that leaks through the boiler casing. ![]() The combustion controller can’t precisely control the air flow through the burner if the boiler draft is constantly changing.īoilers that operate with excessively negative pressure will draft too much air through the furnace, resulting in poor burner turndown and poor efficiency because excess air cannot be controlled-especially at lower firing rates. Burners utilizing high flue gas re-circulation rates, and ultra low NOx burners have narrow limits of flammability and require precise fuel air ratio control. If the stack draft is not repeatable, the fan will induce varying amounts of flue gas re-circulation that will make the fuel air ratio control unstable. If the stack draft is too negative, the forced draft fan will not be able to induce enough flue gas to meet the required NOx emissions. Flue gas re-circulation is often induced by the combustion air fan. ![]() However, even air-tight forced draft boilers built today often need draft controls to help stabilize burners using flue gas re-circulation for NOx control. There are still many balanced draft boilers in operation that require draft controls for the same reasons they did in 1964. Then as now, proper draft control was also important for flame stability and maintaining the correct fuel air ratio in the boiler. To control the negative pressure generated by a tall stack or an induced draft fan, stack outlet dampers were installed and controlled to maintain a set point typically about 0.1” negative pressure measured at the back of the furnace. Allowing these furnaces to “go positive” for even a short amount of time could result in damage to the boiler casing or injury to boiler operators.īoilers made in this era typically had tall stacks to induce a negative pressure (or draft) in the boiler, or induced draft fans. Because the furnaces were not air tight, the furnace walls were kept cool by a constant stream of cool air drawn in by the slightly negative pressure of the furnace. In 1964, many more boilers were of brick-set construction, required to be run at negative draft or balanced draft pressure.
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