Combustion – Combustion bursts or flare-ups in pulses or serial pattern
Reexamination Certificate
2000-03-09
2001-11-06
Clarke, Sara (Department: 3743)
Combustion
Combustion bursts or flare-ups in pulses or serial pattern
C431S012000, C431S060000, C431S285000, C239S415000
Reexamination Certificate
active
06312250
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a burner apparatus having a reaction zone in which a premix of fuel and oxidant is ignited and undergoes combustion to heat a process chamber communicating with the reaction zone.
BACKGROUND OF THE INVENTION
A premix burner is part of an industrial furnace having a process chamber in which a drying or heating process is performed. The burner has a reaction zone communicating with the process chamber. A mixture of fuel and oxidant, which is known as a premix, is ignited and burned in the reaction zone to provide thermal energy for heating the process chamber. The premix is formed upon intermixing of the fuel and oxidant along flow paths that convey the fuel and oxidant to the reaction zone.
The combustion conditions in the reaction zone can be controlled by controlling the firing rate at which the premix is ignited upon entering the reaction zone. The firing rate is generally controlled by modulating the velocity at which the premix enters the reaction zone. The velocity is modulated uniformly throughout all of the premix flow paths leading to the reaction zone.
Modulating the premix flow velocity has certain limitations as a way to control the firing rate of the burner. First, the practical velocity turn-down range is limited by flashback. Flashback occurs when premix flow velocity decreases sufficiently to allow flame to propagate upstream along the flow paths leading to the reaction zone. Second, ultra low NOx emissions, and to some extent very low CO emissions, depend on excellent mixing of the fuel and oxidant forming the premix. Unfortunately, mixing quality can deteriorate as the flow path velocity and pressure drop decrease when the burner is turned down in a conventional manner.
Additionally, premix burners can amplify or cancel noise, depending in part on the velocity at which the premix flows toward and into the reaction chamber. The burner can be tuned for noise accordingly, but conventional turn-down changes the premix velocity and thus changes the noise tuning of the burner. This limits the velocity turn-down range for some noise-prone applications. The minimum velocity may thus be limited by flashback, emissions levels, and noise tuning limits.
Increasing the maximum velocity in a premix burner is one way to increase the turn-down range. Increasing the maximum velocity and reducing the size of the burner increases the turn-down range by increasing the amount of turn-up. However, increasing the turn-down range with a higher maximum velocity can significantly increase pressure requirements and, therefore, power costs. Accordingly, increasing the maximum premix flow velocity can be an expensive way to increase the turn-down range.
Conventional control of the burner firing rate can also be rather slow. The transition from a low to a high firing rate may take from thirty seconds to several minutes, depending on the speed of the fuel and oxidant control devices, and also on the ability of the ratio control system to maintain the fuel to oxidant ratio. Many low NOx burners require precise ratio control that can be maintained adequately only when the firing rate is changed slowly. This might not be suitable for applications that require a rapid firing rate response for optimum performance.
SUMMARY OF THE INVENTION
In accordance with the invention, a premix burner apparatus includes a burner structure and a firing rate control system. The burner structure defines a premix reaction zone configured to communicate with a process chamber. The burner structure further defines a plurality of separate entrances to the reaction zone, and a corresponding plurality of separate premix flow paths, each of which is configured to direct both oxidant and fuel to a respective one of the entrances to the reaction zone. The control system is operative to control flows of oxidant and fuel along at least one of the flow paths separately from flows of oxidant and fuel along at least one other flow path.
In a preferred embodiment of the invention, the control system is operative to interrupt flows of oxidant and fuel along at least one of the flow paths while oxidant and fuel continue to flow along at least one other flow path. The control system includes a plurality of separately shiftable valve assemblies. Each valve assembly has a closed condition blocking a combined flow of oxidant and fuel along a single corresponding one of the flow paths, and has an open condition not blocking that flow.
The invention enables the firing rate to be controlled precisely and rapidly at the entrances to the reaction zone because the flow paths conveying premix to the entrances are controlled separately from each other. Since the firing rate can be affected by affecting the premix flow at one or more of the entrances to the reaction zone, the premix velocity and the fuel to oxidant ratio can remain substantially constant at the other entrances to the reaction zone.
In accordance with another principal feature of the invention, the firing rate control system further includes a controller which is operative to cause a shift of one or more of the valve assemblies between their open and closed conditions, and thereby to switch the corresponding premix flows ON and OFF. This type of on-off control can be practiced in a number of different modes, including modes in which premix flows are cycled ON and OFF for sustained control of the burner firing rate, and modes in which premix flows are turned ON and OFF for turn-up and turn-down of the burner firing rate.
Another principal feature of the invention relates to a fuel supply system that conveys fuel from a fuel source to the burner structure. The fuel supply system includes a flow measuring device, a pressure regulating device and a valve. The pressure regulating device provides and maintains a selected value of fuel pressure between the flow measuring device and the burner structure. The valve provides and maintains a selected value of resistance to fuel flow between the flow measuring device and the burner structure. This configuration of fuel supply components enables the ratio of fuel to oxidant to remain substantially constant throughout on-off control of the premix flow paths. In a preferred embodiment of the invention, the values of fuel pressure and fuel flow resistance that are provided and maintained in the system are selected by a controller with reference to the fuel flow rate indicated by the flow measuring device.
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International Search Report of Application No. PCT/US00/10293 dated Nov. 8, 2000.
Neville Thomas B.
Saker Naim H.
Schmotzer Brian J.
Clarke Sara
Jones Day Reavis & Pogue
North American Manufacturing Company
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