TUNNEL KILN

Abstract: The invention relates to tunnel kilns. There is provided a tunnel kiln (10) comprising a heating tunnel (24) and a plurality of moving hearths (32) across the width of the tunnel, in which the moving hearths are adapted to convey the ware (44) to be treated through the heating tunnel.

Description: TUNNEL KILN Field of the Invention The present invention relates to tunnel kilns. Background to the Invention Kilns for firing ceramic wares are of two basic types namely a fixed hearth version and a moving hearth version. In the fixed hearth version, ware to be fired are placed on wheeled trollies and pushed through a kiln. The trollies are exposed to the heat. In the moving hearth version, the bottom of the kiln consisting of thermal insulation is in the form of an upper run of an endless belt as shown in United Kingdom patent application no. GB 2,080,927, the endless belt including a longitudinal succession of T-section insulating blocks which in the upper run slide in a T-section slot in a fixed base with the sides of the slot under the side walls of the kiln. The conveyor in the fixed hearth kilns and the hearth in the moving hearth kilns carry away a considerable amount of heat which is wasted. Furthermore, if the cross- sectional heating volume of the hearth is to be increased by making it wider, problems of heat expansion and heat loss are exacerbated. One aim of preferred embodiments of the present invention is to minimise the amount of waste heat. Summary of the Present Invention According to one aspect of the present invention, there is provided a tunnel kiln having a fixed hearth which is interrupted by at least two narrow conveyors which apart from their narrowness and having upper surfaces proud of the fixed hearth are similar to moving hearths. Using this configuration, the width of the kiln is not limited by the same mechanical or thermal constraints as is the case for a single moving hearth as in GB 2,080,927. Thus, more acceptable proportions of width and length of the kiln are available. Overall, the thermal mass of the moving hearths of the narrow conveyor can be considerably reduced. Thus, even in a hearth of equivalent width, the reduced volume of moving hearths in the narrow conveyors means that a lower thermal mass undergoes the cycle of heating, cooling and re-heating. Suitably, the underside of the kiln is substantially sealed, thus eliminating the ingress of cold air and improving efficiency and temperature uniformity. Sealing can be achieved more efficiently because there is a less absolute thermal expansion at any given point. Improved sealing can be achieved by the provision of a sealing plate across the underside of the kiln tunnel. The plurality of moving hearths means that the kiln overall can have a low thermal mass by including more fixed hearths across its width, enabling rapid heating and cooling. This makes production interruptions less of a problem, and less costly. The ware to be fired go through the kiln on batts straddling the proud surfaces which batts are typically thin refractory plates. Suitably, the fixed hearth comprises a pre-cast refractory material. Suitably, the narrow conveyors comprise a low thermal mass fibre. Suitably, at least one heating element is located between at least part of a fixed or moving hearth and the level of the upper surface of a narrow conveyor. Suitably, the at least one heating element is between at least one fixed hearth and the level of an upper surface of a narrow conveyor. Suitably, at least one fixed hearth includes an elongate channel in which is located a heating element. Suitably, the elongate channel is open towards the tunnel of the kiln. This configuration allows heat to be inserted into the kiln directly underneath the support batts enabling better temperature uniformity and allowing material to be fired at a faster rate with high quality. A shorter kiln can therefore be produced, reducing capital costs and floor space requirements. Optionally, there are three narrow conveyors. By having three conveyors it is possible to send batts two abreast through the kiln. More preferably, there are at least four conveyors so it is possible to have two conveyors driven at a different speed to the other or another two conveyors. So that the batts are conveyed by the conveyors, it is necessary that the narrow conveyors are proud of the fixed hearth. This can be done merely by the height of the narrow conveyors. Suitably, a narrow conveyor comprises a pier projecting from a moving hearth of the narrow conveyor. Suitably, the pier comprises a refractory material. With such a pier it is possible to fire heavy ware which may be able to compress ceramic fibre insulation used in the narrow conveyors. The present invention finds particular advantage in firing bricks, tiles, tableware or whiteware, but has wider application. In another aspect of the present invention, there is provided a tunnel kiln comprising a heating tunnel and a plurality of moving hearths across the width of the tunnel, in which the moving hearths are adapted to convey the ware to be treated through the heating tunnel. Suitably, the tunnel kiln further comprises any one or more of the features of the accompanying description, claims, abstract and/or drawings, in any combination. Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to the accompanying drawings; in which: Figure 1 is a side elevation, partly in section through a kiln according to the present invention. Figure 2 is a cross-sectional illustration through the kiln of Figure 1 on the line II-II. Figure 3 is a cross-sectional illustration through a conveyor configuration of a two-track kiln. Figure 4 is a schematic cross-sectional illustration similar to Figure 2 through a two-track kiln. Figure 5 is a front elevation of a moving hearth block. Figure 6 is a side elevation of the moving hearth block shown in Figure 5. Description of the Preferred Embodiments The kiln shown m Figures 1 and 2 is generally similar to the kiln described in British patent application no. GB 2,080,927 having a tunnel of thermal insulating material and means for conveying were through the kiln in the form of an endless belt. The content of GB 2,080,927 is incorporated herein by reference. Figures 1 and 2 show a tunnel kiln 10 having a frame 10 enclosing a conveyor structure 12 and an insulating structure 14. The conveyor structure 12 consists of ceramic fibre elements 16, carriers 18 therefor and transporter means 20, together constituting an endless conveyor for ware to be conveyed and heat-treated therein. A superstructure 22 (Figure 1) defines the upper region of a tunnel 24 having a preheat section 26, a firing section 28 and a cooling section 30. Cooling air can be provided by a blower 31. Referring in particular to Figure 2, there is shown a tunnel roof insulating element 100 supported by opposed side wall insulating elements 102, 104, supported on opposed base insulating elements 106, 108. A plurality of moveable 32 and fixed 34 hearth blocks extend along the base of the kiln. The insulating elements 100-108 and hearth blocks 32, 34 define the tunnel 24. A steel sealing plate 110 is provided across the underside of the kiln tunnel 24 substantially to prevent the flow of air to or from the kiln tunnel 24. The plate 110 includes U- shaped channels 112a, 112b to allow room for the endless conveyors bearing the narrow conveyors. The conveying means is in the form of two narrow conveyors comprising moving hearth blocks 32a, 32b having piers 40 which project proud of adjacent fixed hearth blocks 34a, 34b, 34c. The fixed hearth blocks 34a-34c are of a pre-cast refractory material and the moving hearth blocks 32a, 32b are of a low thermal mass fibre. The kiln illustrated is an electrically heated kiln having electrical elements 36. The fixed hearth blocks 34a-34c include elongate channels open towards the kiln tunnel 24 in which heating elements 36 are located. The ware to be fired travels on kiln furniture such as flat refractory batts 38 which are proud of the fixed hearth blocks 34a- 34c. The narrow conveyors illustrated include solid refractory piers 40 projecting through a bed of high- temperature ceramic fibre material such as SAFFIL (registered trade mark) to provide an unyielding proud part of the narrow conveyor. As shown in Figure 1, the fixed hearth blocks 34a-34c project beyond the conveyors at both ends (see generally reference numeral 42) so that the batts 38 laden with the ware can be placed on the fixed hearth 34 and slid along that hearth 34 until the batts 38 are picked up by the narrow conveyors at the input end and then will be pushed out at the other end so the batts can be removed. Figure 3 shows a conveyor configuration for a two- track kiln with four moving hearths across the width of the kiln. The full kiln arrangement is shown in Figure 4 in which similar reference numerals are used as for Figures 1 and 2 for equivalent or similar parts. The kiln of Figure 4 is generally similar to that of Figures 1 and 2 except that in this case there are four moving hearths 32a-32d and three fixed hearths 34a-34c between them. In Figure 4, two batts 38a, 38b are shown carrying ceramic ware 44. The kiln arrangement shown in Figures 3 and 4 is of double width so batts can go through the kiln side-by- side. There would of course have to be at least three conveyors with the batts bridging between the central conveyor and one or other outer conveyors. However, if four conveyors are used, as here, the batts 38a, 38b can bridge between two moving hearths 32a, 32b on one side of the kiln and between two other moving hearths 32c, 32d on the other side of the kiln. The first mentioned and second mentioned pairs of moving hearths have independent drives 46a, 46b independently operable at different speeds so that two different types of ware can be fired with different timings. Figures 5 and 6 show a narrow conveyor in more detail comprising a moving hearth block 32 and a pier 40 therethrough located in a blind bore 48. The pier 40 includes an upper surface 50 proud of the moving hearth block 32 (and of the fixed hearth blocks 34) , which upper surface supports a batt 38 in use. A moving hearth support structure 52 is also shown in more detail . The narrow conveyors must be proud of the fixed hearth and can simply be the ceramic hearth. It is possible to use all solid piers or a tube to provide a proud surface. The embodiment described herein is suitable for firing almost any ceramic material up to a temperature of about 1450°C and probably about 1600°C. Although it is advantageous to use moving hearths in an inverted "T" configuration (as shown in Figure 2) so that the minimum thermal mass undergoes heating, cooling and re-heating, an upright "T" configuration (in which the fixed hearth blocks 34a-34c are shown in Figure 2) can be used. Instead of electrical heating elements, other heating such as gas or oil burners can be used. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) , may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims: Claims 1. A tunnel kiln having a fixed hearth which is interrupted by at least two narrow conveyors which apart from their narrowness and having upper surfaces proud of the fixed hearth are similar to moving hearths. 2. A tunnel kiln according to claim 1, in which the underside of the kiln is substantially sealed. 3. A tunnel kiln according to claim 2, in which a sealing plate is provided across the underside of the kiln tunnel . 4. A tunnel kiln according to any preceding claim, in which the fixed hearth comprises a pre-cast refractory material . 5. A tunnel kiln according to any preceding claim, in which the narrow conveyors comprise a low thermal mass fibre. 6. A tunnel kiln according to any one of claims 1 to 5, in which at least one heating element is located between at least part of a fixed or moving hearth and the level of the upper surface of a narrow conveyor. 7. A tunnel kiln according to claim 6, in which the at least one heating element is between at least one fixed hearth and the level of the upper surface of a narrow conveyor. 8. A tunnel kiln according to claim 6 or claim 7, in which at least one fixed hearth includes an elongate channel in which is located a heating element. 9. A tunnel kiln according to claim 8, in which the elongate channel is open towards the tunnel of the kiln. 10. A tunnel kiln according to any one of claims 1 to 9, in which there are three narrow conveyors. 11. A tunnel kiln according to any one of claims 1 to 9, in which there are at least four conveyors. 12. A tunnel kiln according to claim 11, in which separate conveyor driving means are provided for each or two pair of conveyors, whereby one pair of conveyors can be driven at a different speed to the other or another pair of conveyors. 13. A tunnel kiln according to any preceding claim, in which a narrow conveyor comprises a moving hearth and a pier projecting therefrom, which pier is proud of the fixed hearth. 14. A tunnel kiln according to claim 13, in which the pier comprises refractory material . 15. A tunnel kiln comprising a heating tunnel and a plurality of moving hearths across the width of the tunnel, in which the moving hearths are adapted to convey the ware to be treated through the heating tunnel . 16. A tunnel kiln according to any one of claims 1 to 17, in which the tunnel kiln further comprises any one or more of the features of the accompanying description, claims, abstract and/or drawings, in any combination.

Inventors: BOYLETT WILLIAM FREDERICK, BOYLETT, WILLIAM, FREDERICK
Applicants: BOYLETT WILLIAM FREDERICK, CONSULTANT GAS ENG LTD, BOYLETT, WILLIAM, FREDERICK, CONSULTANT GAS ENGINEERS LIMITED
Application year: 1997
Main classification: F27B9
IPC codes: F27B9, F27D3