[0001] The present invention is related to an infrared heating device, primarily for outdoor heating, though not limited thereto. The invention is primarily related to large area IR-devices, also referred as Mull emitters' , preferably in the form of emitting panel surfaces, which emit essentially non-visible IR-radiation (as opposed to IR-lamps that emit visible IR-light) .

State of the art

[0002] Infrared heaters exist in many variations.

Generally, an infrared heater has two main functional components: a heat source (generator) and an emitter material. In most known infrared heaters, the source is electricity that flows through an ohmic resistor an thus generates heat. There are also infrared heaters that operate with burnt fuel (e.g. gas or petroleum) . The emitter material can be any suitable material thermally connected to the heat source. Sometimes the heat generator and the emitter material are the same, for example in heaters where a resistor wire inside a lamp is at the same time the main infrared source.

[0003] In IR heating panels, the emitter material is a rather smooth or large flat sheet or flat surface that is heated by a heat source that is thermally coupled to the emitter sheet.

[0004] Depending on the emitter material, the physical form and the application, infrared heaters will operate at different operating temperatures. Woven cloth and infrared lamp wire often operates at temperatures in the range of 500°C to more than 2000°C (a box heater) . Thermal stress in these cases is very high.

[0005] Current day Mull emitter' IR heating panels operate at temperatures in the range of 50°C to 200°C. Temperatures are limited by technical regulations and deformation of the flat surface in case of overheating. For this reason, Mull emitters' have so far not been applicable as outdoor heaters.

[0006] Today there are no specific efficient panel- based infrared systems that work effectively in the range 200°C - 500°C. The main reason for this is that sheet expansion and heat source expansion are unequal which causes local interruption of the thermal interconnect. This local interruption will immediately lead to additional local heating because of the emerged air gap thus causing even higher local deformation of the sheet. The sheet will deform increasingly in waves causing increasing dysfunctions and finally failure. Apart from the dysfunction, the loss of smoothness (waving deformation) of the surface is not acceptable from an aesthetic point of view . Aims of the invention

[0007] The present invention aims to provide a solution to the above described drawbacks of existing IR heating panels. Summary of the invention

[0008] The invention is related to an IR heater as described in the appended claims. The invention is thus related to an infrared heater comprising a panel having an emitting surface configured to be heated by a heat source, characterized in that said panel is mounted in a housing, in such a way that - at least in operation - the panel is in a stressed condition. According to a preferred embodiment, the panel is curved in a convex or concave shape .

[0009] Said panel may be fitted into a housing having two lateral flanges, the panel being clamped between said lateral flanges. Said flanges may be configured to be removable, so as to allow the removal and replacement of the panel. Said flanges may be pivotable along a pivot line parallel to the longitudinal direction of the flanges.

[0010] According to an embodiment, the panel is provided with a T-shaped profile along each of the lateral sides, configured to fit into a rectangular shaped profile (11) in the housing, wherein at least one side of said rectangular shaped profile is part of the flanges. The space between said T-shaped profile and said rectangular profile may be filled with a heat-insulating material.

[0011] The panel may be formed from a plurality of panel modules, assembled together via connection means along their lateral sides.

[0012] According to a preferred embodiment, said panel or said modules is (are) provided on the backside with a plurality of heat conducting profiles, the cross section of said profiles taken perpendicularly with respect to the panel, consisting of a cradle portion and two or more diverging leg portions, with a clamping profile releasably attached on top of each heat conducting profile, and said clamping profile being configured to push a heating element into said cradle portion. Said clamping profile may comprise two leg portions and a central clamp portion configured to fit around a heating element, to thereby clasp the heating element between the clamp portion and the cradle portion. The cradle portion as seen in cross section perpendicular to the panel, may have a rounded shape on one side and a planar shape on the opposite side.

[0013] According to a preferred embodiment, the panel or modules are produced from aluminium provided with an enamel layer on top of said aluminium.

Brief description of the figures

[0014] Figure 1 is a 3D view of a heater panel according to the invention, shown from the front (fig. la) and back (fig. lb) .

[0015] Figure 2 is a cross-sectional view of a heater panel according to a preferred embodiment.

[0016] Figure 3 shows a detail of the heater panel of figure 2.

[0017] Figure 4a and 4b illustrate embodiments wherein the heating elements are electrical resistance elements or fluid tubes respectively.

[0018] Figure 5 illustrates an embodiment wherein the heating element is a conductive woven carbon cloth.

[0019] Figures 6 and 7 illustrate some details of embodiments involving springs supporting the heating layer.

[0020] Figure 8 shows a heater with two emitting panels . Detailed description of the invention

[0021] The invention is related to an infrared heater equipped with a panel, configured to emit heat from the panel's surface by IR-radiation, by thermally coupling the panel to a heat source, e.g. a set of electrical heating elements. Figure 1 shows a heater according to the invention. Figure la shows the panel 100 and its housing 101, figure lb shows a possible arrangement at the back of the panel, with supports 102 for attaching the panel to a wall or other support structure. The characteristic aspects of the panel heater are described hereafter. In a heater of the invention, the panel 100 is mounted in a housing 101 so that when the panel is in operation, i.e. heated to an operational temperature at which it emits heat in the form of IR-radiation, the panel is in a stressed condition. In other words, the panel is subjected to an elastic deformation as a consequence of the heating, due to an expansion or shrinking of the panel material when heated and due to the boundaries of the housing which constrain the panel. In the most common case of a panel which expands when heated, this can be done by mounting the panel into the housing in such a manner that in operation, the emitting surface has a concave or convex shape with a more pronounced curvature than the curvature of the panel's surface when it is not heated. In other words, the concave or convex shape is obtained or enhanced by stressing (heating) the panel: ^x obtained' in the case of a flat panel forced - when heated - into a curved shape by fitting into the boundary, and ^enhanced' in the case of a pre-curved panel fitted into the boundary. The panel can be mounted in the housing so that it is ^x pre-stressed' , in which case it is already under elastic deformation when it is not in operation. In that case, the elastic deformation is enhanced during operation. Alternatively, a flat or pre- curved panel can be installed under pre-stress by applying a pulling force on the panel. In the latter case, a shrinking of the panel material increases the elastic deformation .

[0022] Preferably, the panel is replaceable, so that it can be removed from the housing and replaced by another panel. According to a further embodiment, the panel is built from a number of modules, assembled together to form a larger (pre) stressed emitting surface. Due to the stress

(elastic deformation) of the panels, wave-like deformation of the panel is suppressed, which allows heating to higher temperatures than is currently possible before the onset of deformation .

[0023] Figure 1 shows an infra-red heater according to a preferred embodiment of the invention. The heater comprises two panel modules 1 and 1', assembled together to form a rectangular emitting surface. Each module is a flat or pre-curved plate, preferably a metal plate, provided with interconnect means 2 and 3 at the vertical lateral sides, configured to interlock two or more modules together so as to form a large emitting surface. The modules are clamped inside a housing. In the embodiment shown, this housing consists of several profiles 4, 5 and 6 fitted together to form the back wall of the IR heater, and by removable side flanges 7. The modules are clamped inside this housing so that the emitting surface is curved in concave shape and possibly pre-stressed between the side flanges 7. In the embodiment shown, the modules are provided at their lateral edges with extended profiles 10 having a T-shaped cross-section, and which are arranged to fit into rectangular shaped recesses 11 in the housing. An isolating material is provided between the T-shaped profiles 10 and the rectangular recesses 11, in order to isolate the modules thermally from the housing. The isolating material must be an essentially non-compressible, low compliance material, in order to ensure a sufficiently strong mechanical support to the modules.

[0024] In the embodiment shown, the lateral flanges

7 have a cross-section in the shape of a semi-circle or similar rounded shape, possibly with a recess 16 for allowing an easy grip of the flanges. Each flange is connected to the housing via a pivot connection 17 along a pivot line which is parallel to the longitudinal direction of the flange, so that the flange may be opened and pivoted away from the housing. Each flange is further provided with a securing means for securing the flange to the housing. The securing means can consist of a resiliently arranged strip 18 configured to removably click behind a rim 19 on the housing, but other forms of securing means may be applied. The securing means is such that it can be manually opened, so that the flanges 7 may be pivoted and the modules become accessible. In the embodiment shown, the rectangular shaped recesses 11 have one side wall 20 which is integral with the flanges, so that when the flange is opened, the recess 11 becomes accessible. The space 50 between the back wall and the modules 1,1' is preferably filled with a heat insulating material, preferably a hydrophobic insulating material.

[0025] An alternative to this embodiment is an embodiment wherein the flanges 7 are not removable, but wherein the modules can slide out of the rectangular profiles 11, in the direction perpendicular to the plane of the drawing (possibly after removing lateral flanges on opposite sides of the panel, said flanges being perpendicular to the flanges 7) .

[0026] In the embodiment of figure 2, the heat source may be formed by electrical resistance heating elements, which are to be placed in the locations indicated by reference numeral 30. In any embodiment where electrical power is used for heating the emitting panel, electrical heating elements are in physical contact with the emitting panel. In the preferred embodiment of figure 1, this contact is optimized by the formation of heat-conductive profiles 31 and clamping profiles 40 at the back of the panel modules.

[0027] This arrangement is shown in detail in figure

3. The cross-section of each profile 31 consists of two leg portions 32, connected by a cradle portion 33, the latter being configured to receive for example a resistance heating element. The leg portions 32 extend from the cradle portion 33 towards the back of the emitting surface 1 in a diverging way. Clamping profiles 40 are then provided on top of the heat-conductive profiles 31. The cross-section of each clamping profile consists of two leg portions 41 and a central clamp portion 42 designed to fit around the heating elements, to thereby clasp the heating elements between the clamp portion 42 and the cradle portion 33. Preferably the cradle 33 and the clamp 42 are rounded in shape, to fit around cylindrical heating elements. However, as shown the cradle portion 33 preferably has a rounded shape on one side and a planar shape on the opposite side, to make it easier to accommodate looped heating elements without further machining operations. The clamping profile 40 is fitted onto the conductive profiles 31 of the modules by interlock mechanisms, for example a set of hook profiles 43 which can be clicked fast, said interlock mechanisms being configured to fit the profiles together with a certain pre-stress, so that the clamping profiles 40 exert a pressure onto the heating elements, to thereby improve the thermal contact between the heating elements and the cradle portions 33. The outwardly diverging shape of the leg portions 32 ensures an optimal distribution of heat from the heating elements to the front of the emitting surfaces .

[0028] Figure 4a shows the embodiment with electrical resistance heating elements 35. Alternatives to the above described embodiment are possible without deviating from the scope of the invention. For example, the heating elements in locations 30 can be replaced by any other suitable heat source layer, such as tubes 36 carrying heated fluid, as illustrated in figure 4b. The heat source layer can also be a conductive woven carbon cloth 50, as shown in the schematic view of figure 5. The latter view also shows a functional layer 51 between the carbon cloth and the emitting panel 52, the functional layer being for example a layer for improving the thermal interconnect between the heat source and the emitting panel (e.g. a silicone thermally conductive path) . An insulating layer 53 is further present between the heat source layer 50 and the housing 60. Figure 6 shows an embodiment wherein the conductive layer 50 has a smaller surface than the emitting panel, in order to compensate for dilatation deformations. Lateral springs 61 may be present to maintain the layer 50 in place.

[0029] According to another embodiment, illustrated in figure 7, springs 65 are present between the housing 60 and the heat source layer 50, in order to increase the contact pressure between the heat source layer 50 and the emitting panel 52. In any of the embodiments described above, the housing 60 (or 4,5,6,7) may be installed inside an outer housing 70, via connections 80.

[0030] Figure 8 shows an embodiment, wherein several panels are mounted in a common inner housing 60, which is itself mounted in a common outer housing 70. One panel is (pre) stre s s e d in concave form, while the other is (pre) stressed in convex form. The connections 80 between the inner and outer housing may be rotatable connections, allowing the inner housing 60 and thus the emitting panels to be rotated around a horizontal axis, so as to change the direction of the radiated heat. The outer housing 70 is preferably dimensioned such that the space between the inner and outer housing may form a layer of hot air, which will improve the ratio radiation vs. conduction of the heater .

[0031] The invention is also related to an IR heater with a non (pre) stressed emitting surface, provided with the heat-conductive profiles 31 and clamping profiles 40 as described above.

[0032] According to a preferred embodiment, the material of the emitting surface in a heater of the invention is aluminium provided with an enamel coating. Said coating is suitable also for receiving printed images thereon. An enamel based coating that can be used in the invention is an enamel coating with an organic binder.

[0033] The invention is equally related to an IR heater provided with an aluminium emitting panel as described above, however not necessarily (pre) stressed, but provided with the above-described enamel coating.