Furnaces – Draft regulators – Damper
Reexamination Certificate
1999-03-09
2001-04-24
Ferensic, Denise L. (Department: 3749)
Furnaces
Draft regulators
Damper
C110S147000, C110S297000, C110S314000, C110S261000
Reexamination Certificate
active
06220188
ABSTRACT:
The present invention relates to a windbox burner comprising a register casing with ports for inflow of combustion air, a secondary air pipe coaxial with the register casing and enclosed by it and having inlet ports for secondary air, a ring damper substantially coaxial with the register casing and axially displaceable and having at each end an annular sealing means which in the closed position of the ring damper sealingly abuts a corresponding annular sealing means at each axial end of the ports, thus preventing the combustion air from flowing through the ports to the burner tip, in which each pair of mutually abutting sealing means is made of a stainless and heat-resistant material, and in which one sealing means comprises at least one plane leaf spring the plane of which is substantially perpendicular to the longitudinal axis of the ring damper.
In large furnace plants, such as refuse incineration plants or steam boilers, it is common that one or more windboxes for supply of combustion air are provided on the front wall of the boiler. Each windbox comprises several windbox burners, each capable of containing, for example, a combined coal/oil burner. Coaxially with and enclosing the coal/oil burner one or more pipes are provided for supplying air to the burner tip, which air supply pipes are enclosed by a substantially tubular register casing having openings that permit combustion air from the windbox to flow into one or more of the air supply pipes. The air flow through the openings in the register casing is controlled by means of an axially displaceable ring damper.
In many known windbox burners the sealing means at the ends of the air damper are usually only the roughly machined ends of the tubular ring damper abutting corresponding flanges in the register casing or a stop mounted thereon. Such a known ring damper may have a diameter of about 2 m, and owing to the tolerances used, a large leakage loss occurs both in the closed position of the ring damper and in its partially open position, resulting in an inadequate control of the air supply, which again may give rise to poor combustion with a consequent high NOX figure and adversely affecting the environment.
In GB-A-1 527 072 and, in particular, FR-A-1 561 182, improved tightness has been achieved by the use of sealing means of a stainless and heat-resistant material, of which one sealing means comprises at least one plane leaf spring.
As controlling the air flow in such burners is essential in order to create combustion conditions under which the formation of nitrogen oxides is as low as possible, several other attempts have been made to reduce the formation of NOX.
In GB patent application No. 2 146 112 the quantity of combustion air through two secondary air passages is controlled by axial displacement of a ring damper in the form of a sleeve, the swirl in each passageway being regulated by swirl vanes.
U.S. Pat. No. 3,904,349 discloses a burning apparatus in which the combustion air supplied from the windbox is distributed among three separate passageways. The quantity admitted to each passageway is controlled by separate flow regulating means.
The object of the invention is to improve a windbox burner of the type mentioned in the introduction, in which the combustion conditions in the burner are optimized with respect to maximum reduction of the formation of NOX.
This is achieved according to the invention in that the secondary air pipe supports an axially adjustable tubular damper adapted to overlapping at least a portion of each of the inlet ports and having an external collar, that in one open position the ring damper overlaps the external collar, whereby the flow of air through the space between the secondary air pipe and the register casing is substantially blocked by the external collar, and that in a second, more open position the ring damper is free of the external collar, whereby air is permitted to flow through said space as tertiary air.
By this design it is possible to divide the flow of combustion air flowing through the openings in the register casing into a secondary air flow being passed to the secondary air pipe already when the ring damper begins opening, and into a tertiary air flow formed at a later time by the opening of the ring damper. The tertiary air flow is passed to the burner tip through the space or annular channel defined between the secondary air pipe and the register casing. The axially adjustable tubular damper can, for example, be adjusted by means of a manually actuated screw rod or a pneumatic working cylinder receiving control impulses from the control system of the boiler or the furnace.
It is thus possible to maintain favourable combustion conditions even at partial or low-load operation as the quantity of tertiary air may be reduced and eventually eliminated when reducing the total quantity of air to the burner by movement of the ring damper.
In an embodiment of the windbox burner according to the invention, the leaf spring of one sealing means has radial slits arranged equidistantly along the periphery to increase the flexibility of the leaf spring. The increased flexibility of the leaf spring thus obtained means that the seal can compensate for larger deviations in shape and inaccuracies in the manufacture of the ring damper and the corresponding parts of the register casing.
In another embodiment one sealing means may comprise a second leaf spring abutting the first leaf spring and being substantially formed in the same way as the first leaf spring, and the slits in the two leaf springs may be mutually displaced in the tangential direction to prevent air from flowing through the slits.
Use of two leaf springs with mutually displaced slits prevents any leakage loss that might occur through the slits of the individual leaf spring.
In further embodiments the leaf spring may be made of stainless spring band steel, and the corresponding sealing means may be a ring made of stainless round bar steel.
To ensure accurate guidance of the ring damper in relation to the register casing and thus an accurate mutual location of the sealing means at closure of the ring damper, and to ensure that the often very heavy ring damper is easily mobile in the axial direction, the ring damper in embodiments of the windbox according to the invention may be mounted on a carriage adapted to slide on rails mounted in parallel with the centre axis of the windbox burner, or the ring damper may be mounted on a trolley with wheels adapted to run on said rails.
To further ensure that the ring damper can compensate for any inaccuracies in the alignment of the individual parts of the register casing, the ring damper may be pivotable about an axis being substantially perpendicular to the centre axis of the windbox burner and intersecting it. It is also possible in one embodiment to let the ring damper be resiliently supported on the carriage or the trolley, which again causes the ring damper in the closed position to be able to compensate for any inaccuracies.
In one embodiment, the carriage or the trolley may support a fitting formed as a half-ring having at each end a pivot pin on which the ring damper is pivotably journalled, and there may be stops to limit the swinging movement of the ring damper to a swing of about ±2.5° in relation to the centre axis of the windbox burner.
It has been found that letting the ring damper be biased to assuming one of its extreme positions in the swing direction when in the open position provides an especially good seal when the ring damper is moved to its closed position. It is not quite clear why this effect is obtained, but it is presumed that since the sealing means of the ring damper are moved against each other not only in the axial direction of the ring damper, but also in a direction perpendicular thereto, this may cause better abutment and thus improved tightness. It is also possible that the movement transversely to the longitudinal direction may contribute to removing any collections of impurities on the sealing means and thus contribute to improved tight
Burmeister & Wain Energi A/S
Ferensic Denise L.
Rinehart K. B.
Shannon John P.
Venable
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