BURNER SHIELDS

Abstract: A burner shield (10) for an elongate burner (12) of a continuous oven is located directly beneath one run of an endless mesh conveyor band (14) on which product to be baked is deposited. The shield has an upper wall (19) of two planar portions inclined upwardly from the centre to each of the respective edges, a planar base wall (16), a planar front wall (16) and a planar rear wall (18). The two outer edges of the upper wall (19) are extended to form projecting baffles (20) for guiding combustion gases away from the conveyor (14) and the longitudinal ends of the shield are open to form outlet ports for combustion gases. The front wall (17) of the burner shield has an elongate inlet aperture (22) in which is located an elongate gas burner (12). The combustion products from the burner emerge from one of the two open ends of the shield and are deflected past the outermost edges of the conveyor (14) thus reducing burning of the product on the conveyor (14).

Description: DESCRIFΠON BURNER SHIELDS The present invention relates to burner shields and to burner assemblies incorporating burner shields. The invention is particularly, but not exclusively, applicable to burner shields for use in continuous ovens. Continuous ovens are widely used in baking foodstuffs, particularly biscuits. One such continuous oven for baking biscuits comprises an endless conveyor band and gas burners located above and below the band. Predetermined charges of biscuit mixture to be baked are deposited on the conveyor band at predetermined spacings and as the band passes through the oven the mixture is cooked by means of the burners located above and below the belt to form biscuits. However, one problem which is sometimes encountered with such known continuous ovens is the overcooking of the product around its periphery, due to the edge of the product being heated from three sides, with the convection gases of a temperature in excess of 500° C. Unless the processing conditions are accurately controlled the product can be formed with a "ring" burn mark around its outer edge. It is thought that one cause of "ringing" arises from the use of the direct gas firing bottom burners, which produce a "plume" of products of combustion which rise quickly to the top of the oven. A number of solutions to this problem have been proposed, including baffle plates over the burners, perforated plates over the burners and the use of heavier mesh conveyor bands but have not been successful. The present invention aims to overcome the aforementioned problems more reliably by controlling the heat transfer distribution within the oven. In accordance with a first aspect of the present invention a burner shield for an elongate burner comprises an elongate top wall, opposed elongate side walls extending downwardly from the top wall and an outlet for combustion gases at an end of the shield. Preferably, the shield has two outlets for combustion gases, one outlet located at each end of the shield. Preferably, the shield further comprises a bottom wall, which preferably makes the shield substantially closed in transverse cross-section. In such an arrangement, substantially all of the combustion products from the shielded burner will be forced to exit the shield via the or each outlet. A burner shield in accordance with the present invention prevents a "plume" of hot combustion products from rising directly from the burner flame. The heat is still produced by the burner but the gases which would otherwise produce the convection "plume" are guided out of the or each shield outlet, away from the area. Consequently, heat energy is transferred into the shield structure, producing a radiation effect. The heat energy from radiation is lower than that of convection and thus evens out the overall heat flux within the oven chamber. The egress of gas through the or each outlet is preferably encouraged by providing a top wall portion which is inclined upwardly towards an outlet. The top wall may comprise two inclined portions, each preferably inclined upwardly from a common, central location towards a respective outlet. The top wall of the shield is also preferably provided with a projection extending beyond the upper edge of the or each outlet, to improve the distribution of heat across the shield by increasing the radiant effect at the ends of the shield. The present invention also includes a burner assembly comprising a burner in combination with a burner shield of the present invention. The present invention also includes an oven comprising one or more such burner assemblies. In one embodiment, the shield further comprises an inlet aperture to permit the ingress of burner combustion products into the shield. The burner itself may be located in, or in the vicinity of, the inlet aperture, to maximise the amount of combustion gases entering the shield. The inlet aperture may also comprise an entrainment assembly for drawing surrounding air into the shield with the combustion products. This may take the form of a venturi member which is located, in use, in the inlet aperture. The inlet aperture may be conveniently located in one of the downwardly extending side walls. In accordance with a further aspect of the present invention, a continuous oven comprises a perforated conveyor belt for conveying food products through the oven, a burner tube situated in the oven below a run of the conveyor belt and extending transversely to the run, a burner shield including a top wall and opposite side walls extending downwardly from the top wall and so positioned in relation to the burner tube as to receive, within the confines of the shield, combustion gases emerging from the tube, the top wall and opposite side walls extending across substantially the whole width of the run of the conveyor belt and oudet ports for the combustion gases at opposite ends of the shield. Preferably, the shield further comprises a bottom wall, which preferably makes the shield substantially closed in transverse cross-section. A substantially closed transverse cross-section forces substantially all of the combustion products to exit the shield via the outlet ports. The egress of gas through the outlets may be encouraged by wall portions inclined upwardly towards the outlet ports, preferably inclined upwardly from a common, central location towards a respective outlet port. The opposite ends of the top wall of the shield may also each have an extension projecting beyond the respective lateral edge of the conveyor belt, to improve the distribution of heat across the conveyor belt, by increasing the radiant effect at the edges. In one embodiment, the shield further comprises an inlet aperture to permit the ingress of bumer combustion products into the shield. The burner itself may be located in, or in the vicinity of, the inlet aperture, to maximise the amount of combustion gases entering the shield. The inlet aperture may also comprise an entrainment assembly for drawing surrounding air into the shield with the combustion products. This may take the form of a venturi member which is located in the inlet aperture. The inlet aperture may conveniently be located in one of the downwardly extending side walls. Preferably, the oven comprises a plurality of bumer tubes, each having an associated bumer shield. By way of example only, a specific embodiment of the present invention will now be described, with reference to the accompanying drawings, in which: - Fig. 1 is a side elevation of an embodiment of bumer shield in accordance with the present invention, fitted in a continuous oven (but with the bumer removed); Fig. 2 is an end elevation of the shield shown in Fig. 1 (complete with the burner); and Fig. 3 is a plan view of the shield of Fig. 1. A bumer shield 10 is used in conjunction with an elongate gas bumer 12. In use, a plurality of burner assemblies comprising a bumer 12 in combination with a burner shield 10 are located in a continuous oven. As illustrated in chain dot in the Figures the bumer shield is located directly beneath one run of an endless mesh conveyor band 14 upon which product to be baked is deposited. The conveyor band also has a return run (not shown) along the bottom of the oven. The bumer shield is formed from mild steel plate and comprises a planar base wall 16, a generally planar front wall 17 and a planar rear wall 18. The shield also comprises an upper wall 19 comprising two planar portions inclined upwardly from the centre of the upper wall to each of the respective edges. The two outer edges of the upper wall 19 are extended to form projecting baffles 20 for guiding combustion gases, as will be explained. The two longitudinal ends of the shield are open to form outlet ports, as will be explained. The front wall 17 of the bumer shield is provided with an elongate inlet aperture 22 in which is fixedly secured a burner venturi 24. An elongate gas burner 12, which is slightly shorter than the aperture 22, is mounted in the inlet portion of the venturi 24. The burner shield is secured in place by means of an adjustable bracket assembly 26 having front and rear mounting plates 28 mounted adjustably on the front and rear walls 14, 16 respectively by means of bolts 30, the plates being secured to a bracket 32 which is located in use on a bumer support bar 34 extending, in use, in the longitudinal direction of the oven. The opposite end of the bumer shield is supported by a screw-threadedly adjustable foot 36 connected t a fixed mounting bracket 38 secured to one end of the lower wall 16 of the shield. As mentioned above, in use, a plurality of bumer assemblies, each comprising a bumer shield 10 in combination with an associated bumer 12, are aligned transversely to the longitudinal direction of the continuous oven along the length of the oven, beneath the mesh conveyor band 14 on which the product is to be baked. As shown in Figs. 1 and 3, the mesh conveyor is slightly narrower than the outermost edges of the shield. Product to be baked is deposited on the mesh conveyor band in the conventional manner and passes over the bumer assemblies in the oven. In addition to the bumer assemblies illustrated, conventional unshielded burners are located above the mesh conveyor belt 14. The combustion products produced by each of the burners 12 pass directly into the associated burner shield. The venturi action produced by the bumer firing into the throat assembly 24 results in the entrainment of air within the oven chamber, causing a circulation of the oven atmosphere. The lower temperature oven atmosphere mixed with the products of combustion reduces the overall plume temperature and adds to the overall distribution of energy within the oven. As mentioned previously, the two ends of the bumer shield are open and thus the combustion products from the bumer move laterally until they emerge from one of the two open ends. The hot combustion products have a tendency to rise and the efflux of combustion gases from the bumer shield is enhanced by the inclined portions of the upper wall 18 of the shield. As the combustion products emerge from the open ends of the bumer shield they are deflected past the outermost edges of the mesh conveyor 14 by the projecting baffles 20, and thus any convection "plume" does not affect the product on the mesh conveyor. It is believed that the shield tends to cook the product by a combination of convective and radiant heat transfer rather than by convection alone, resulting in more even baking of the product. With careful selection of the materials with different emissivities used in the construction of the shield, the radiation may be increased or decreased and by careful design of the shape of the reflector 19 the radiation effect may be focused or distributed evenly across the band. The invention applies equally to burners firing vertically as well as horizontally. The invention is not restricted to the details of the foregoing embodiment.

Claims: CLAIMS 1. A bumer shield for an elongate bumer, comprising an elongate top wall, opposed elongate side walls extending downwardly from the top wall and an outlet for combustion gases at an end of the shield. 2. A bumer shield as claimed in claim 1, comprising two outlets for combustion gases. 3. A bumer shield as claimed in claim 2, wherein one outlet is located at each end of the shield. 4. A bumer shield as claimed in any of the preceding claims, further comprising a bottom wall, whereby the shield is substantially closed in transverse cross-section. 5. A bumer shield as claimed in any of the preceding claims, wherein a portion of the top wall is inclined upwardly towards an outlet. 6. A bumer shield as claimed in claim 5, wherein the top wall comprises two inclined portions, each inclined upwardly towards a respective outlet. 7. A burner shield as claimed in claim 6, wherein the two inclined portions are inclined upwardly from a common, central location. 8. A bumer shield as claimed in any of the preceding claims, wherein the top wall of the shield comprises a projection extending beyond the upper edge of the or each outlet. 9. A bumer shield as claimed in any of the preceding claims, further comprising an inlet aperture to permit the ingress of bumer combustion products into the shield. 10. A bumer shield as claimed in claim 9, wherein the inlet aperture comprises an entrainment assembly for drawing surrounding air into the shield with the combustion products. 11. A bumer shield as claimed in claim 10, wherein the entrainment assembly comprises a venturi member located in the inlet aperture. 12. A bumer shield as claimed in any of claims 9 to 11, wherein the inlet aperture is located in one of the downwardly extending side walls. 13. A burner assembly comprising a bumer in combination with a burner shield as claimed in any of the preceding claims. 14. An oven comprising one or more bumer assemblies as claimed in claim 13. 15. An oven as claimed in claim 14, further comprising a conveyor belt for conveying food products through the oven and wherein the top wall and opposite side walls of the bumer shield extend across substantially the whole width of the conveyor belt. 16. A continuous oven comprising a perforated conveyor belt for conveying food products through the oven, a bumer tube situated in the oven below a run of the conveyer belt and extending transversely to the run, a burner shield including a top wall and opposite side walls extending downwardly from the top wall and so positioned in relation to the bumer tube as to receive, within the confines of the shield, combustion gases emerging from the tube, the top wall and opposite side walls extending across substantially the whole width of the run of the conveyer belt and outlet ports for the combustion gases at opposite ends of the shield. 17. A continuous oven as claimed in claim 16, further comprising a bottom wall, whereby the shield is substantially closed in transverse cross-section. 18. A continuous oven as claimed in claim 16 or claim 17, wherein a portion of the top wall is inclined upwardly towards at least one outlet. 19. A continuous oven as claimed in claim 18, wherein the top wall comprises two inclined portions, each inclined upwardly towards a respective outlet. 20. A continuous oven as claimed in claim 19, wherein the two inclined portions are inclined upwardly from a common, central location. 21. A continuous oven as claimed in any of the preceding claims, wherein the top wall of the shield comprises a projection extending beyond the upper edge of the or each outlet. 22. A continuous oven as claimed in any of the preceding claims, further comprising an inlet aperture to permit the ingress of bumer combustion gases into the shield. 23. A continuous oven as claimed in claim 22, wherein the inlet aperture comprises an entrainment assembly for drawing surrounding air into the shield with the combustion products. 24. A continuous oven as claimed in claim 23, wherein the entrainment assembly comprises a venturi member located in the inlet aperture. 25. A continuous oven as claimed in any of claims 22 to 24, wherein the inlet aperture is located in one of the downwardly extending side walls. 26. A continuous oven as claimed in any of claims 22 to 25, wherein the bumer tube is positioned in the inlet.

Inventors: ATTRILL MICHAEL CEDRIC, ROLSTON ROBERT MITCHELL, ATTRILL, MICHAEL, CEDRIC, ROLSTON, ROBERT, MITCHELL
Applicants: AEROGEN CO LTD, ATTRILL MICHAEL CEDRIC, ROLSTON ROBERT MITCHELL, UNITED BISCUITS LTD, ATTRILL, MICHAEL, CEDRIC, ROLSTON, ROBERT, MITCHELL, THE AEROGEN COMPANY LIMITED, UNITED BISCUITS (UK) LIMITED
Application year: 1997
Main classification: A21B1
IPC codes: A21B1, F24C3