Patent Publication Number: US-6220188-B1

Title: Windbox burner

Description:
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 tightness. The force moving the ring damper towards its one extreme position in the swing direction may be obtained, for example, by weight-loading one end of the ring damper, by displacing the pivot pins in relation to the middle of the ring damper, or by biasing the ring damper by an elastic force. 
     In a further embodiment of the invention, the ring damper is arranged inside the register casing. This has been found to entail considerable advantages, especially during dispatch and installation of windbox burners in the windbox of the boiler or furnace. Letting the ring damper be enclosed by the register casing leaves the movable parts, such as the trolley or the carriage and the ring damper itself, protected against any damage that may occur during dispatch or at the installation. If the ring damper is arranged to enclose the outside of the register casing, the ring damper with trolley or carriage must be secured so as not to move during transportation, and likewise the risk of damage to the ring damper is great, for example if the whole windbox burner is lifted in straps placed around the ring damper. 
     The ports of the register casing may be formed so that the flow area in the first part of the opening movement of the ring damper increases progressively, while the flow area in the subsequent part of the movement of the ring damper increases proportionally with the axial displacement. This renders it possible to achieve a more even adjustment of the air flow over the full opening stretch of the ring damper. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cross-sectional, schematic side view of part of a windbox burner according to the invention with a ring damper shown in a closed position, 
     FIG. 2 shows a cross-sectional end view of the ring damper of the windbox burner with a secondary air pipe, 
     FIG. 3 is a view like that shown in FIG. 1 with the ring damper shown in a partially open position, 
     FIG. 4 is a view as that seen in FIG. 3 with the ring damper shown in a more open position, 
     FIG. 5 is a partially cross-sectional, schematic side view of a windbox burner in which the ring damper encloses the register casing, 
     FIG. 6 is a schematic end view of the ring damper of the windbox burner of FIG. 5, 
     FIG. 7 is a part view of a leaf spring, and 
     FIG. 8 is a plane view of a port in the register casing of the windbox burner for supply of secondary and tertiary air. 
    
    
     As shown in FIG. 1, the windbox burner is fitted into a windbox  1  and has a tubular register casing  2  with ports  3  arranged along the periphery for an inward flow of combustion air from the windbox. As shown in FIG. 8, the width of the port  3  measured along the periphery of the register casing is relatively small at the end furthest away from the burner tip, while the width increases towards the burner tip. At one end the register casing is closed by means of a bolted on front plate  5 . Inside the register casing and coaxially therewith is a secondary air pipe  35  having openings  36  for secondary air. The secondary air pipe  35  is closed at one end by means of a bolted on plate. 
     A tubular ring damper  20  enclosing the secondary air pipe  35  is supported on a trolley  7  with wheels  8  running on two mutually parallel rails  6  welded on to the secondary air pipe. By means of a pull rod  9  connected with, for instance, a compressed-air cylinder, the trolley  7  can be reciprocated along the secondary air pipe. The ring damper  20  is suspended in the trolley  7  by means of a semicircular bracket  10  which is welded at the middle to the trolley by means of lugs  12  and journalled swingably about pivot pins  11  welded to the inner surface of the cylindrical wall of the ring damper. As indicated in FIG. 1, the ring damper can swing through an angle α to either side of a starting position in which the longitudinal axis of the ring damper coincides with the centre axis of the windbox burner. The swings of the ring damper are limited by two stops  13  arranged on the trolley  7 . 
     At one end the ring damper has a welded-on ring  21  of stainless round bar steel which, as shown in FIG. 1, abuts an annular leaf spring  23  which is retained by means of a retaining ring  25  against a flange  24  welded into the register casing  2 . At the other end of the ring damper is a similar annular leaf spring  26  made of stainless spring band steel which by means of a holding ring  28  is clamped on to a flange  27  welded on to the ring damper. In the closing position, the leaf spring  26  abuts a ring  22  of stainless round bar steel welded on to a stop  4 , which is against fixedly connected with the front plate  5  on the register casing. As it appears from FIG. 1 it is possible by removal of the front plate  5  to take out the ring damper  20  with trolley  7  for overhauling or replacement, and there is also access for a possible removal of the leaf spring  23 . As shown in FIG. 7, the leaf spring has radial slits  30  along its outer periphery. 
     About the secondary air pipe  35  is arranged a tubular damper  37 , which is longitudinally adjustable and has a collar  38  of an outer diameter so that the collar substantially blocks the space between the outer surface of the secondary air pipe  35  and the inner surface of the ring damper  20 . For the sake of clarity, the tubular damper  37  and the collar  38  are shown only in the lower half of FIGS. 1,  3  and  4 . It is not shown in detail how the tubular damper is adjusted axially, but this may be done, for instance, by means of a manually operated screw rod mounted in the removable front plate  5 . 
     The ring damper in the windbox burner according to the invention operates in the following manner: In the closed position of the ring damper, as illustrated in FIG. 1, the ring  21  at one end of the ring damper abuts the leaf spring  23 , and at the same time the leaf spring  26  at the other end of the ring damper abuts the ring  22  on the stop  4 . Because of the flexibility of the leaf springs good sealing is ensured between the respective sealing means, and any inaccuracies are compensated for by the ring damper having the possibility to tilt about its axis through the pivot pins  11 . The combustion air supplied to the windbox  1  in a manner not shown, is thus prevented from flowing through the openings  3  in the register casing and on to the burner tip. When the opening of the ring damper is commenced, as illustrated in FIG. 3, the air can flow through the openings  3  in the register casing  2  as shown by the arrows indicated and on through the air openings  36  in the secondary air pipe  35 . On the other hand any air passage through the ring damper is blocked by the collar  38 , and the passage around the ring damper is blocked by the leaf spring  26 . At the same time the tubular damper  37  by its overlapping of the ports  36  prevents secondary air from flowing from the secondary air pipe  35  into the ring damper to the space  39  for tertiary air. 
     When the ring damper opening is increased, as illustrated in FIG. 4, it can be seen that the combustion air in the windbox  1  can still flow through the ports  3  in the register casing  2  and on into the secondary air pipe  35  through the ports  36  therein. However, since the ring damper now no longer overlaps the collar  38 , the combustion air which has flowed in through the ports  3  can now also continue through the ring damper into the space  39  for tertiary air. Since the tubular damper  37  with the collar  38  is axially adjustable, it is thus possible to adjust the time of opening of the passage for tertiary air through the ring damper. 
     As shown in FIGS. 5 and 6, the windbox burner can also be designed so that the ring damper  20  encloses the register casing  2 . As shown in the upper part of the Figure, the trolley can run on rails  6  mounted on the upper surface of a secondary air pipe  35 . Also in the upper part of the Figure, the ring damper is shown in a closed position in which flow of combustion air from the windbox through the ports  3  in the register casing  2  is blocked. In the lower part of the Figure, the ring damper  20  is shown in an open position in which it does not overlap the outward collar  38  on the tubular damper  37  so that as indicated by arrows both secondary air and tertiary air flows into the burner. For the sake of clarity, the tubular damper  37  with the collar  38  is only shown in the lower part of the Figure.