![draft fan with draft control for owb draft fan with draft control for owb](https://airprofan.com/wp-content/uploads/2019/11/induced-draft.jpg)
Changes in wind velocity blowing across the stack.Changes in stack temperature as the boiler warms up or changes firing rates.Today draft controls are required to accurately maintain draft conditions in the furnace and compensate for changes in outside conditions including: The boiler and breeching are designed to withstand this positive pressure without the need for cooling air. The burners always include forced draft fans sized to pressurize the burner windbox, the furnace, and sometimes even part of the stack.
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Boiler combustion chambers now are entirely steel encased and air tight. To meet typical NOx guarantees, effective draft controls need to be installed, and the boiler casings often need to be repaired to reduce air leakage.īoiler construction today is almost entirely different, but draft controls are still required in many applications for mostly different reasons. The diluted flue gas is less effective at reducing NOx. When these older boilers are retrofitted with low NOx burners using flue gas recirculation, 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 recirculation 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 recirculation 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.
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Flue gas recirculation is often induced by the combustion air fan. However, even airtight forced draft boilers built today often need draft controls to help stabilize burners using flue gas recirculation 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 setpoint typically about 0.1” negative pressure measured at the back of the furnace. Boilers made in this era typically had tall stacks to induce a negative pressure (or draft) in the boiler, or induced draft fans. 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. 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. Namely, boiler construction. Since then, many more boilers were of brick-set construction, required to be run at negative draft or balanced draft pressure. Today, draft controls are still common on all types of boilers, but for very different reasons. (Heating oil was 25¢ a gallon in 1964!) Additional benefits included more reliable burner performance, reduced burner emissions, and increased safety by tripping a boiler off line if the draft turned positive.Ĭheck out the first page of that article below. Danbury, CT – In 1964, Preferred Instruments published an article in the Fuel Oil & Oil Heat magazine. During that time, draft controls were used primarily to control excess draft from tall chimneys and lower excess air to conserve fuel.