Combustion – Mixer and flame holder
Reexamination Certificate
2000-05-05
2001-07-31
Price, Carl D. (Department: 3743)
Combustion
Mixer and flame holder
C431S010000
Reexamination Certificate
active
06267586
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a burner and, in particular, to a bumer designed to have low NOx emissions.
BACKGROUND OF THE INVENTION
Burners such as those employed in domestic water heaters are typically formed of two plates having a chamber between them when the plates are fastened together. The burner includes a central axis and the upper and lower plates include outer peripheries that are radially spaced from the central axis. The lower plate includes an opening for air and fuel to enter the chamber. A bracket is fastened to the lower plate for securing the burner in place. A plurality of ports extend radially outwardly from the chamber.
The products of complete combustion are carbon dioxide, water and, in the presence of excess air, nitric oxides, which include nitric oxide (NO) and nitrogen dioxide (NO
2
) (collectively referred to as NOx). The primary source of nitrogen is the air, both primary and secondary, required to combust gaseous fuels. The mechanism for formation of oxides of nitrogen is dissociation of nitrogen during the combustion process at flame temperatures in excess of 2600° F. and reaction with oxygen present. Nitrogen dioxide in particular is undesirable in that it is a toxic gas and a major constituent of smog. Some states have laws that require water heater burners to have low levels of NOx not greater than 40 Ng/j. Therefore, burners are designed to reduce NOx emissions and achieve suitable combustion efficiency.
One way that gas burners have been designed in an attempt to reduce NOx emissions is to position a portion of the burner in a flame hot zone. This is intended to lower the temperature of the combustion reaction to a point below that which is conducive to NOx formation. Bumer portions placed in the path of the flame in an attempt to reduce flame temperature include rods and a descending peripheral edge of the upper plate, as disclosed in U.S. Pat. No. 5,913,675. However, such burners are typically effective in only specific water heaters, and do not necessarily produce the same emissions when applied to heaters made by various manufacturers. The location of the flame hot zone is believed to move as a result of different conditions including input rate, distance the buner is located from the tank wall, shape of the combustion chamber and the method and amount of secondary air introduced into the combustion chamber. When the location of the hot zone changes due to a change in flame geometry such that the burner portion is not positioned in the flame hot zone, NOx emissions rise above legislated levels. This renders such burners unacceptable for use in a variety of water heaters.
There is a need in the industry for a gas burner which achieves acceptably low NOx emissions when used in a variety of sizes and in water heaters made by various manufacturers, while achieving suitable combustion efficiency.
SUMMARY OF THE INVENTION
In general, the present invention is directed to a burner that can receive air and fuel and discharge a fuel/air mixture for combustion at a periphery of the burner while limiting NOx emissions. The burner has a central axis and includes first and second plates constructed and arranged to form a central chamber between them and a plurality of burner ports extending outwardly of the chamber. The first and second plates each have an outer periphery spaced outwardly from the central axis. An opening is disposed in the second (e.g., lower) plate for receiving the air and fuel into the chamber. The outer periphery of the second plate extends outwardly from the central axis by a greater distance than the outer periphery of the first plate. In preferred form, the second plate may have an outer peripheral portion in which a plurality of spaced (secondary) openings are formed. The inventive burner design advantageously utilizes a mechanism that reduces the temperature of the flame thereby reducing NOx emissions without requiring that portions of the burner be consistently located in a hot zone of the flame.
Particular features of the inventive burner will now be described. The first plate may include an outer peripheral portion that extends toward the outer peripheral portion of the second plate to enable deflection of flame resulting from combustion of the fuel/air mixture without reducing a temperature of the flame effective to significantly reduce NOx emissions as a result of the deflection. The outer peripheral portion of the first plate extends outwardly from the central axis by a greater distance than the secondary air openings. The outer peripheral portion of the second plate may comprise an inner annular portion that extends from the ports at an angle away from the first plate and an annular lip portion that extends outwardly from the inner annular portion. The fuel/air mixture travels in each of the ports along a flow path, and each secondary air opening may be located in one of the flow paths. The first and second plates may have generally circular shapes.
Many advantages are achieved by the inventive burner. The invention does not suffer from the problem of not being able to consistently locate a portion of the burner in a flame hot zone in water heaters by different manufacturers and in different sizes of burners. The present burner employs a different mechanism for reducing NOx emissions. In general, the outer peripheral portion of the upper plate deflects the flame downwardly so that it rides on the long outer peripheral portion (e.g., lip) of the lower plate. The lower plate, being cooled by secondary air flow, acts as a heat sink which reduces the temperature of the flame. A contributing factor to the mechanism may also be that a portion of secondary air flow is delayed from reaching the flame until after the flame passes the outer edge of the lower plate. The relatively small sized secondary openings act to meter the secondary air in order to facilitate the combustion process, although the inventive design may facilitate the combustion reaction and low NOx emissions even without secondary air openings. According to the foregoing, the inventive burner may achieve low NOx emissions not more than 40 Ng/j and even significantly lower, throughout a variety of water heater designs and burner sizes.
In general, a method of operating the burner to limit NOx emissions according to the present invention indudes directing air and fuel into the central chamber thereby forming a fuel/air mixture and directing the fuel/air mixture through the plurality of ports. Flame formed as a result of combustion of the fuel/air mixture travels on the outer peripheral portion of the second plate, effective to reduce its temperature.
More specific aspects of the present method will now be described. Unlike the burner disclosed in the 5,913,675 patent, the flame may be deflected against the outer peripheral portion of the first plate without reducing the temperature of the flame effective to significantly reduce NOx emissions as a result of the deflection. The lip portion is cooled by the secondary air flowing along the underside of the lower plate and is believed to act as a heat sink which reduces the temperature of the flame as it travels on the lip. Also, since only a limited portion of secondary air is introduced through the small secondary openings in th second plate, the balance or major portion of secondary air that reaches the flame around the lower peripheral edge, is not introduced until flame is below the reaction temperature for dissociation of nitrogen and formation of oxides of nitrogen. In the case where no secondary openings are used, the secondary air cools the lip to achieve the heat sink effect and is limited from contacting the flame in that it is introduced around the elongated outer peripheral portion of the second plate. Thus, the present invention strikes a balance between the need for sufficient secondary air to facilitate combustion and to avoid CO production, while maintaining low flame temperature and thereby reducing NOx emissions by limiting the secondary air from reaching the flame.
Beckett Gas, Inc.
Price Carl D.
Watts Hoffmann Fisher & Heinke
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