INFRARED HEATERS AND ELEMENTS THEREFOR

Abstract: A heating element for an infrared heater is made from an elongat e strip (1) of sheet metal such as nickel chrome. The strip (1) has a constant or varying cross-sectional area along its length, for example by having a non-uniform width, in order to provide an electrically resistive heating effect which varies along the length of the element. The strip (1) spans end mountings (2) and is energised by a voltage below 50 volts to provide long wave and medium wave infrared emission.

Description: TITLE: INFRA-RED HEATERS AND ELEMENTS THEREFOR This invention relates to infra-red heaters and to elements (sometimes called emitters) therefor. Such elements produce radiant heat as a result of the resistive heating effect of an electric current passing through the element. According to one aspect of the invention there is provided an electrically energisable heating element for an infra-red heater, the heating element comprising an elongated strip of sheet metal having, along its length, a constant or varying cross-sectional area chosen to impart a desired heating effect along the length of the element, wherein the strip of sheet material is planar or substantially planar and the respective ends of the strip are provided with mountings for supporting the element in the heater with the length of the strip spanning the mountings without additional support. Preferably, the resistance of the element is in the range of 0.001 to 1.0 ohms, its thickness is in the range 0.5 to 2mm and its length in the range 0.2 to 2.0 metres. The strip of sheet metal may be nickel chrome alloy strip and the mountings are preferably copper mounting brackets for mounting the element in the infra-red heater, eg by bolting or friction welding. The mounting brackets may have through passages for the passage of cooling fluid (eg water) to cool the mounted ends of the element in use. In certain preferred embodiments, the cross-sectional area of the strip varies along the length of the element, the smaller the cross-sectional area the greater the resistive heating effect and therefore the greater the local temperature of the element and the shorter the wavelength of the infra-red radiation emitted by the element. The thickness of the element may be constant, its width being varied in order to provide the desired variation in cross-sectional area. The effective width may be varied by shaping one or both longitudinal edges, by incorporating apertures in the strip or by both of these expedients. The longitudinal edges may be joggled, eg to form a sinuous shape, to increase strength and focus radiant energy in required directions. As an alternative, or in addition, to varying the width of the element, the thickness of the element may be varied. According to another aspect of the invention there is provided an infra-red heater comprising a plurality of electrically energisable heating elements each according to said one aspect. According to a yet further aspect of the invention there is provided an infra red heater comprising a plurality of electrically energisable heating elements each comprising an elongated strip of sheet metal, wherein respective ends of the strip are provided with mountings which support the element in the heater with the length of the strip spanning the mountings without additional support. The elements may be arranged in the heater in any grouping to suit the required overall heating effect, but a preferred arrangement is for the elements to be mounted in spaced parallel relationship. For example, in the application of the invention to a heater in the form of an infra-red oven (or grill), the elements are positioned in spaced parallel relationship above a conveyor which is guided for movement in a direction transverse to the direction of elongation of the elements, the conveyor being suitable for conveying beneath the array of elements items to be heated such as food items to be cooked, grilled or browned or non food items to be heat treated. The array of elements above the conveyor may be supplemented by a further array of elements positioned below the conveyor, depending on the required direction of application of radiant heat to the items to be treated. It will be appreciated that the heating elements may be positioned to one or both sides of the item to be heat treated, and that the heating elements may be shaped and positioned to treat curved or specially shaped items. The invention solves the problem of a reduced heating effect at the conveyor edges, because by reducing the cross-sectional area of certain (or all) of the heating elements at regions adjacent their ends, the radiant heating effect at these regions is increased. It will be appreciated that, in general, each element can have a particular variation of cross- sectional area along its length to impart any desired heating variation along the width or along the length of the conveyor. The elements are preferably detachably mounted in the heater, eg by fixing bolts and tensioning means such as tension springs, weights, hydraulic cylinders or gas springs, so that individual elements can be changed or replaced easily and quickly, enabling the power density distribution both across and along the conveyor to be readily modified to suit particular items to be treated. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 are plan and edge views respectively of a first embodiment of heating element, Figures 3 to 16 are similar views of second to eighth embodiments of heating element, Figure 17 is a side elevation of an oven conveyor including heating elements according to the invention, Figure 18 is a plan view of the oven conveyor of Figure 17, and Figure 19 shows various possibilities for connecting the heating elements in parallel, series or series/parallel arrangements. The heating element illustrated in Figure 1 is made of an elongate strip 1 of nickel chrome alloy of constant width (as shown in Figure 1) and constant thickness (as shown in Figure 2). At each end, a copper mounting bracket 2 is bolted to the strip. Each mounting bracket has a through bore 3 for the passage of cooling fluid. One mounting bracket, that shown at the left hand end of Figures 1 and 2, has an outer flange with spaced holes 4 for attaching this end of the element in a heater. The other bracket has an end flange with a single hole to receive a tension spring 5 for mounting this end of the element in the heater. The end flange may have a plurality of holes receiving the ends of a corresponding plurality of tension springs arranged in parallel. In Figures 3 to 16 similar parts to those in Figures 1 and 2 are given the same reference numerals. Strip 1 of the element of Figures 3 and 4 is of constant thickness but its width tapers uniformly from one end of the heating element to the other, by virtue of the longitudinal edges of the strip 1 converging in a direction progressing from left to right in Figures 1 and 2. As a result, the cross-sectional area of the strip varies linearly from one end to the other, the end having the smaller cross-sectional area producing a greater radiant heating effect and therefore producing infra-red radiation of a higher frequency. The strip 1 of the element of Figures 5 and 6 is again of uniform thickness but it is of varying width as illustrated in Figure 5. The width varies so that the strip has end regions 6 which are waisted in order to increase the heating effect at these regions near the ends of the heating element. A similar effect is achieved by the element shown in Figures 7 and 8, where adjacent each end of the strip 1 a diamond shaped aperture 7 is provided in order to decrease the effective cross-sectional area of the strip 1 in these regions. The strip 1 of the element of Figures 9 and 10 has formed therein an elongated rectangular aperture 8 which extends for the complete length of the strip, save for a small length at each end thereof. In the element of Figures 11 and 12 the strip 1 has a series of longitudinally spaced apertures 9, imparting to the strip a cross-sectional area which progressively increases and decreases in a stepwise fashion along the length of the strip. A further variation of apertures is shown in the element of Figures 13 and 14 where the strip 1 has four symmetrically arranged apertures 10, with each aperture 10 being of rectangular elongated shape and extending for rather less than half the length of the heating element. The strip of Figures 15 and 16 is of constant width but its thickness varies along its length. In particular, the thickness is reduced over two longitudinally spaced regions 11, those reduced-thickness regions 11 being shown in the enlarged scale part of Figure 16. Each heating element of Figures 1 to 16 has a total resistance between its mounting brackets in the range of 0.001 to 1.0 ohms, has a thickness in the range 0.5 to 2mm and a length in the range 0.2 to 2.0 metres. Figures 17 and 18 show a heater having an endless conveyor 12 guided for movement by rollers 13 in a direction shown by arrow 14 in Figure 18. An upper horizontal run of the conveyor path passes between an upper array 15 of heating elements and a lower array 16 of heating elements. Each element of the upper and lower array extends transversely to the direction of movement of the upper horizontal run of the conveyor. The elements of the upper array 15 occupy a common horizontal plane and the elements 16 of the lower array similarly occupy a common horizontal plane. The elements may instead be staggered vertically, or occupy a common plane inclined to the upper horizontal run of the conveyor. As shown in Figure 18, the heater has along each side a water cooled copper bus bar 17 which, by means of individual heater connections, feeds electrical power to the heating elements of the arrays 15 and 16. The bus bars 17 are powered by a transformer supply at a voltage of somewhat less than 50 volts at a current of 1000 A and at a frequency of 50 Hz. Pipes 18 along respective sides of the heater supply cooling water to the cooling passages at each end of each heating element The elements may be connected in groups in parallel, such groups then being connected in series. The elements may also be connected in series. Various connection possibilities are illustrated in Figure 19. Products to be heated, shown diagrammatically at 19 in Figure 17, are delivered to the infeed end of the conveyor 12, pass between the upper and lower heater arrays 15, 16 and are removed from the conveyor at the outfeed end. Each of the heating elements shown in Figures 17 and 18 may be of the form shown in Figures 1 to 16, or any other chosen shape for imparting a required power density to the horizontal run of the conveyor. The heating element shapes may differ along the length of the conveyor and may differ between the upper array 15 and the lower array 16, dependent on the required heating effect required. It is expected that a heating element having the shape of Figures 5 and 6 (or Figures 15 and 16) will be particularly beneficial for use in an oven conveyor because the waisted regions at each end of the element overcome the problem of undue cooling at the conveyor edges. The all-metal elements are of sufficient strength not to require intermediate support, ie each element spans its mounting brackets without additional support. The mountings accommodate thermal expansion and contraction of the elements. The product to be heat treated may be supplied to the conveyor in spaced lanes, with the shaping of the heating elements being chosen to suit, or the products may be fed onto the conveyor so that they crowd onto the conveyor and do not form predetermined lanes of movement. The electrical supply to the oven may be an a.c. or d.c source and at any voltage or current, but a maximum voltage of about 50 volts has the advantage of safety. This produces a watt density for each element in the range 12 to 25 watts per cm 2 , with an operating temperature in the range 650 °C to 1150°C. Further advantages of the described oven are that it is robust and safe for the food industry, without the use of glass or quartz. The inventive element offers a wide spectrum of infra-red emissions, both long wave and medium wave from a single emitter element. Heating element lengths are normally 0.2 metres to 2.0 metres but would be made to fit the production requirements, ie if products were to be heated on a conveyor 600mm wide then that would be the desired length of the elements. It is anticipated that a heater element longer than 2.0 metres would not be required, but there is scope for heater elements up to 3 metres under special circumstances. In practical terms there is an eventual limitation for heaters elements of very short length, but small domestic units requiring heater elements less than 0.2 metres are possible, provided the ratio of length of cross-sectional area is maintained. This would then mean heater element lengths of 0.1 metres would be possible. The all metal direct resistance of the elements differs from known elements which typically require a resistance of about 50 ohms for a lkW heater. By contrast, a mid range all metal element according to the invention may have a resistance of about 0.02 ohms. A typical resistance for the all-metal heater element would be within the range of 0.001 ohms to 1.0 ohms. This low resistance demands a heavy cross section for the element, and ensures the input voltage remains below 50 volts to comply with regulations for exposed electrical apparatus. Resistances quoted are for a complete heater element length rather than per unit length, because, if a change in section is incorporated, the resistance will vary along the length depending on the cross sectional area. A design feature of the heater element is to be able to work within a wet and arduous environment. This is made possible by using, as a power source for the heater, a low voltage supply via a fully isolating double wound transformer. There is no glass/ quartz, ceramics or fibres to be affected by thermal shock and the heater element will withstand complete immersion in many fluids whether energised or not, without suffering any damage. The heater element will withstand thermal shock and severe mechanical distortion without risk of failure. Under normal conditions elements with a temperature in excess of 300 deg. C. present a fire risk when used in areas that contain flammable materials. Using the all metal infra red heater element according to the invention this risk is reduced, making it possible to use the heater element in a food environment where fat is likely to splash on to the heater element. When fat drops onto the emitter or element, providing the temperature of the element is above 800 deg. C. the fat will tend to lift off the surface of the heater element and flow away on the natural repulsion created by a gas layer between the two. Heaters are quoted as emitting watts per cm 2 of heater area. The average known ceramic or metal sheathed heater emits energy at quite low power densities ranging from >0 to 10 watts/cm 2 for normal convection and radiative use, ie without forced air-cooling. If the watt density goes above 12 watts/cm 2 the heater will not be able to get rid of the heat energy and subsequently overheats and burns out. A kettle element is different in so much as it emits 25 watts/cm 2 but has the water-cooling to prevent it from burning out. The all metal heater element according to the invention will operate at values from 10 to 25 watts/cm 2 without added cooling, or risk of burn out. The main reason for this is that the strip of metal is the heating element. It does not have to radiate the energy through other media or supports, but radiates the energy as medium wave infra-red from both sides of the large and open surface area. It is anticipated that the watt density will be in the range of 12-25 watts/cm 2 giving rise to a temperature range between 650 to 1150°C. The thickness of the sheet used for the all metal heater element will be in the range of 0.5- 2mm, enabling rapid heating and cooling times from when it is first energised to being de- energised. Due to the absence of supports, ceramic backing or other materials in close proximity to the heater, there is no other thermal mass to be heated up, or cooled off with the heater. One of the main advantages of the heater element is the ability to work at very high temperatures within the medium wave infra-red wavelengths. It would be possible for the heater to operate at temperatures from > 0 up to 1150°C. However there would be little or no advantage working at lower temperatures when the design has been developed to work in the upper ranges beyond existing heater limits. Elevated temperatures beyond 650°C. long wave infra-red and convection temperatures are impossible to achieve with existing heaters of all metal self-supporting construction. All other heaters working above 650 °C need intermediate support along and within the working area. It is important with infra-red heating to ensure the items being heated are within line of site of the heater element. Any other materials within the working area will influence the heat transfer and distort or filter the broad spectrum of infra-red energy. The all metal heater is best suited to temperatures in the range of 650-1150°C. This equates to wavelengths between 2-4μm. The all metal heater will potentially radiate a much broader spectrum of infra-red from 1.9-5μm. Existing heaters using quartz supporting tubes, filter out large sections of infra-red within the longer wavelengths of 3-4μm leaving less heat available to heat the product, thus reducing their efficiency. A flat strip can radiate considerably more heat from its larger surface area than a round bar, because it has a large surface area per unit length in relation to cross-sectional area. This large surface area enables the all metal flat heater element according to the invention to work at much higher watt densities and temperatures compared to conventional heaters, thus allowing a much greater wattage of electricity to be converted into heat energy than has been possible before. A conventional strip or rod heater is limited in power output or power density because of its very small surface area. If more power is applied, then the heater gets hotter, however, if it does not have the surface area to radiate the heat energy, it just gets hotter and burns out. This is staged as watt density. To obtain medium wave infra-red, the radiating surface must have a temperature above 650°C. and this requires a high power input. The new all metal flat heater according to the invention is able to convert this increased electrical input into heat energy without giving rise to existing limitations. Furthermore the all-metal flat strip heater is able to profile this elevated power density along its length when it is formed to the various shapes as indicated in the accompanying drawings.

Claims: CLAIMS 1. An electrically energisable heating element for an infra-red heater, the heating element comprising an elongated strip of sheet metal having, along its length, a constant or varying cross-sectional area chosen to impart a desired heating effect along the length of the element, wherein the strip of sheet material is planar or substantially planar and the respective ends of the strip are provided with mountings for supporting the element in the heater with the length of the strip spanning the mountings without additional support. 2. A heating element according to claim 1, wherein the resistance of the element is in the range 0.001 to 1.0 ohms. 3. A heating element according to claim 1 or 2, wherein the element has a thickness in the range 0.5 to 2mm and a length in the range 0.2 to 2.0 metres. 4. A heating element according to any of the preceding claims, wherein the strip of sheet metal is nickel chrome alloy strip. 5. A heating element according to any of the preceding claims, wherein the mountings are mounting brackets having through passages for the passage of cooling fluid to cool the mounted ends of the element in use. 6. A heating element according to any of the preceding claims, wherein the thickness of the strip is constant, its width being varied along its length in order to provide the desired variation in cross-sectional area. 7. A heating element according to claim 6, wherein the effective width of the strip is varied by shaping one or both longitudinal edges of the strip. 8. A heating element according to claim 6, wherein the effective width of the strip is varied by incorporating apertures in the strip. 9. A heating element according to any of claims 1 to 5, wherein the thickness of the strip is varied to vary the effective cross-sectional area. 10. An infra-red heater comprising a plurality of electrically energisable heating elements each according to any of the preceding claims. 11. An infra red heater comprising a plurality of electrically energisable heating elements each comprising an elongated strip of sheet metal, wherein respective ends of the strip are provided with mountings which support the element in the heater with the length of the strip spanning the mountings without additional support. 12. An infra-red heater according to claim 10 or 11 and including energising means for electrically energising each element with a voltage not exceeding 50 volts. 13. An infra-red heater according to claim 12, wherein the watt density of each element is in the range 12 to 25 watts per cm 2 , with an operating temperature in the range 650 °C to 1150°C. 14. An infra-red heater according to claim 12 or 13, wherein the elements are mounted in spaced parallel relationship. 15. An infra-red heater according to claim 14, wherein the elements are positioned in spaced parallel relationship above a conveyor which is guided for movement in a direction transverse to the direction of elongation of the elements, the conveyor being suitable for conveying beneath the array of elements items to be heated such as food items to be cooked, grilled or browned or non food items to be heat treated. 16. An infra-red heater according to claim 15, wherein the array of elements above the conveyor is supplemented by a further array of elements positioned below the conveyor. 17. An infra-red heater according to claim 15 or 16, wherein the elements are detachably mounted in the heater, so that individual elements can be changed or replaced, enabling the power density distribution both across and along the conveyor to be readily modified to suit particular items to be treated.

Inventors: LEECH JONATHAN PATRICK, LEECH, JONATHAN, PATRICK
Applicants: LEECH JONATHAN PATRICK, LEECH, JONATHAN, PATRICK
Application year: 1998
Main classification: H05B3
IPC codes: H05B3