The present invention relates to a radiant gas burner assembly and, in particular, to such an assembly incorporating a metal fibre radiant.

Radiant gas burner assemblies that utilize a metal fibre mat as the permeable and/or perforated medium which forms the burner radiant have been known for some time. The advantages of such mats is that they overcome the problems of slow heat up and cool down associated with other types of radiant, which also reduces the risk of fire in breakdown situations. The low thermal mass and rapid heat dissipation characteristics of these mats provide a radiant surface which is cool enough to handle within seconds of the burner being turned off. One or more such mats typically form the front face of a casing to the interior of which a gas/air mixture is supplied. The gas/air mixture permeates the mat and is combusted on the other side of the mat to the exterior of the casing. As all combustion takes place within the metal surface of the porous metal at a low temperature, emissions of carbon monoxide, nitrous oxide and unburnt fuel components are very low Mats used in such burners are typically rectangular with dimensions up to 2000 mm long and up to 200 mm wide.

One application of such burner assemblies is in conveyor ovens where they mounted above the conveyor with the metal fibre radiants in a horizontal position to radiate heat downwards on to the articles to be baked that are transported through the oven on the conveyor. However, the mounting of the burners in this position can cause problems as the air within the conveyor is also heated by the burner and this rises upwards around the casing of the burner and is lost from the interior of the oven. Apart from the ensuing loss of heat from the area around the articles to be baked, the air flow around the casing heats it and over time can cause it to become distorted in shape to the extent that it may have to be replaced to prevent leakage of the air/gas fuel mixture into the body of the oven around the sides of the radiant mat. _ O _

Similar problems arise in other applications where radiant gas burner assemblies are used.

For example, road repair work which involves the filling of potholes often requires topical heating of the area to be patched prior to and sometimes after patching to ensure fusion of repair material with an existing road surface. This involves outdoor heating of the road surface and burners are used to direct heat downwards on the road surface. However, as with the application described above, similar problems arise in that the heat generated by the burner tends to rise upward around the burner and heating can be inefficient, especially when the ambient temperature is low.

An object of the present invention is to provide a radiant gas burner assembly that overcomes or substantially mitigates the aforementioned problems and that is suitable for use in a wide range of apparatus.

According to the present invention there is provided a radiant gas burner assembly comprising a casing defining a plenum to which a gas/air mixture can be supplied, a front face of the casing comprising a metal fibre radiant, a canopy located around the casing such that an air gap is defined between the casing and the canopy, and an air blowing means for blowing air through the air gap such that it issues from the air gap to form an air curtain around the radiant.

Preferably, the assembly comprises a control means adapted to control the supply of gas/air mixture to the plenum, operation of the radiant and operation of the air blowing means.

Preferably also, the air blowing means comprises a first fan the operation of which is controlled by the control means. Preferably also, the gas/ air mixture is supplied to the plenum by a second fan the operation of which is controlled by the control means.

Preferably also, the control means controls speed of operation of the second fan whereby the temperature of the radiant is also varied.

Preferably also, a non-contact thermocouple linked to the control means is provided to detect the temperature of the area being heated and to transmit this information back to the control means which is operates to control operation of the second fan dependent on the temperature detected.

Preferably also, the control means controls operation of the first fan independently of the operation of the second fan.

Preferably also, the radiant stands proud of the edge of the canopy defining the air gap around the periphery of the radiant.

Preferably also, an ignition electrode and an ionisation probe for the burner assembly are mounted such that they are cooled by air issuing through the air gap around the periphery of the radiant.

Preferably also, operation of the ignition electrode is controlled by the control means.

Preferably also, the ionisation probe is linked to operation of a gas valve controlling the gas supply to the plenum whereby in use the gas valve is closed if the ionisation probe does not detect the presence of a flame within the radiant.

In some embodiments, the assembly preferably comprises a frame on which the casing is moveably mounted while the radiant is in operation, a drive means being provided to move the casing along the frame. In these embodiments preferably operation of the drive means is controlled by the control means. Preferably also, the frame is made up of modular sections in order that its size can be varied as required. Advantageously, the casing may also comprise a plurality of metal fibre radiants, each of which is independently operable with respect to the other or others.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. i is a side elevation of a first embodiment of a radiant gas burner assembly in accordance with the present invention such as would be suitable for incorporation in an oven;

Fig. 2 is a view from below of the assembly shown in Fig. i; and

Fig. 3 is a perspective view of a second embodiment of radiant gas burner assembly which is portable and suitable for use in road repair work.

In the embodiment shown in Figs, i and 2, a radiant gas burner assembly i comprises a casing 2 defining a plenum 3 to which a gas/ air mixture is supplied via pipework 4 from a pre-mix fan assembly 5. The fan assembly 5 is linked to a gas supply 6 and comprises a gas valve 7, an air/gas mixing chamber 8 and a fan 9 which blows the air/gas mixture from the chamber 8 into the plenum 2.

The casing 2 comprises a rectangular box with a rear and side walls, 10 and 11 respectively, that are preferably made of stainless steel and a front face that comprises a metal fibre mat 12 which forms the radiant of the burner assembly 1. The mat 12 is secured to the casing 2 by mountings (not shown) which are also preferably made of stainless steel. Located around the rear and side walls 10 and 11 of the casing 2 is a canopy 13, which is attached to the casing 2 by mountings 14 such that an air gap 15 is defined therebetween. A fan 16 is provided for blowing air into the canopy 13 via pipework 17 such that this air issues from the air gap around the periphery of the radiant mat 12. Preferably, as shown in Fig. 2, the radiant mat 12 stands proud of the edge 18 of the canopy 13 defining the air gap 15 around the periphery of the radiant mat 12. As the area of the radiant mat 12 is slightly larger that the area of the aperture in the casing 2 which is covered by it, this ensures that the airflow issuing from the canopy 13 is not impeded by the mat 12.

An ignition electrode 19 and ionisation probe 20 for the burner assembly 1 are mounted on the exterior of the canopy 13 adjacent one edge of the mat 12. In this way they are cooled by air issuing from the canopy 13.

The fan assembly 5, the fan 16 and the ignition electrode 19 are controlled by a control means (not shown) which opens and closes the gas valve 7 and fires up the burner assembly 1 via the ignition electrode 19 when desired. The ionisation probe 20 is linked to the gas valve 7 controlling the gas supply to the plenum 2. A signal from the probe 20 is used to trigger closure of the gas valve 7 should no flame be detected with the radiant when the burner assembly 1 is in use.

In use, the control means opens gas valve 7 and fires up the burner assembly 1 when desired by starting the fan 9 to blows an air/gas mixture to the plenum 3 where it permeates the mat 12, is ignited by the electrode 19 and combusted on the face of the mat 12 to the exterior of the plenum 3. As indicated above, the ionisation probe 20 is used to detect that the gas is being combusted. At the same time, operation of the fan 16 is also commenced so that air is blown through the canopy 13. However, operation of the fan 16 is preferably controlled by the control means separately from that of the first fan 9 so that its operation can be continued even after the burner has been switched off in order to cool the casing 2. Typically, the air inlet for the fan 16 is to the exterior of any apparatus within which the burner assembly 1 is incorporated to prevent the recycling of hot air from within the apparatus. This hot air, which will contain combustion gases created when the gas/air mixture is combusted in the mat 12, is preferably removed from the apparatus by a suitable exhaust system (not shown) mounted above the casing 2.

It will be appreciated that in use the air passing through the air gap 15 created by the canopy 13 cools the casing 2 in order to reduce any tendency to lose its shape owing to the high temperatures created by the radiant mat 12. In addition, the air flow issuing from the canopy 13 is directed downwards in the same direction as the radiant heat from the mat 12. This forms an air curtain around the radiant and creates forced convention currents within the apparatus such as an oven within which the burner assembly 1 is incorporated. These currents assist in the transfer of heat from the burner assembly 1 to the articles to be heated and the reduce stratification of the air between the face of the mat 12 and these articles, which tends to delay heat transfer.

A second embodiment of radiant gas burner assembly 20 shown in Fig. 3 is a portable assembly adapted for localized heating of an area such as part of a road surface. Such a portable assembly 20 could incorporate a burner arrangement similar to that described above. However, in this embodiment a housing 21 is split into a plurality of separate burner arrangements 22a, 22b, 22c each of which is similar to the casing 2 described above and comprises a plenum to which a gas/air mixture can be independently supplied via a pre-mix fan assembly 23 and its own metal fibre mat forming a radiant. The fan assembly 23, although not shown in detail, is again similar to that described above but each arrangement 22a, 22b and 21c has its own gas valve so that a fan of the assembly 23 can blow the air/gas mixture from an air/gas mixing chamber into the relevant plenum dependent on which radiants are to be switched on. Each of the burner arrangements 22a, 22b, 22c may also comprise its own ignition electrode and ionisation probe (not shown) that are again mounted so that they are cooled in the same way as in the first embodiment. It will be appreciated that any one, any two or all three of the burner arrangements 22a, 22b, 22c can be used as desired. However, in a modification, one of the burner arrangements 22a, 22b, 22c may be used to ignite the other two so that only one ignition electrode and ionisation probe will be required. In this modification either said one or all three of the burner arrangements 22a, 22b, 22c are ignited at any one time.

The fan assembly 23 is linked to a gas supply 24, which in this embodiment comprises a bottled gas supply such as bottled liquefied petroleum gas (LPG) or an equivalent rather than a mains gas supply.

The housing 21 forms a single canopy defining an air gap around all of the burner arrangements 22a, 22b, 22c and a fan 25 mounted on top of the housing 21 is provided for blowing air into this gap. As in the first embodiment, operation of the fan assembly 23, the fan 25 and the ignition electrodes is controlled by a control means 26 which in this embodiment is powered by a generator 27. However, as in the first embodiment the control means 26 could be adapted to be powered by a mains electrical supply.

Mounted on the exterior of the housing 21 is a non-contact thermocouple 28 that is also linked to the control means 26. The thermocouple is orientated to detect the temperature of the area being heated and adapted to transmit this information back to the control means 26. The control means 26 is adapted to automatically control operation of the fan assembly 23 dependent on the temperature detected so that the area reaches the desired temperature quickly but is not over-heated. Such a thermocouple could be included in the first embodiment shown in Figs. 1 and 2 if desired.

It will be appreciated that the burner arrangements 22a, 22b and 22c each operate in the same manner as that described above with reference to Figs. 1 and 2. The fan assembly 23 and ignition electrodes are controlled by the control means 26 which opens the appropriate gas valve or valves and fires up the appropriate arrangement 22a, 22b, 22c when desired, the ionisation probe or probes being used to detect the presence of a flame. However, in this embodiment, the housing 21 and burner arrangements 22a, 22b and 22c are moveably mounted on a frame 29, for example on rails (not shown) in order that they can be reciprocated to and fro along the frame 29 while the burner arrangements 22a, 22b and 22c are in operation. A drive means 30 comprising a motor powered by the generator 27, a gearbox and a pinion drive system is provided to move the housing 21 along the frame 29. The frame 29 is preferably modular so that additional frame sections 31 can be added to increase its length as desired dependent on the total surface area it is desired to heat.

In use of this embodiment, for example to effect localized heating of part of a road surface to be repaired, the assembly 20 is transported to the appropriate location and erected over the relevant part of the road surface with the modular frame 29 made large enough to cover it. The control means 26 is set up to heat the relevant area, an operator programming it so that all or some of the relevant burner arrangements 22a, 22b, 22c will be deployed to heat only that portion of the road surface which requires repair. On operation of the relevant burner arrangements, the housing 21 is moved to and fro over the road surface by the drive means 30 so that the area to be repaired is evenly heated. Patching of the road surface can then be accomplished in a conventional manner and, if necessary, the patching material can also heated and cured using the assembly 20. It will be appreciated that the control means 26 controls operation of the fan assembly 23 dependent on the data received from the thermocouple 28 to give controlled heating of the area of the road surface being repaired.

As in the first embodiment, the air passing through the air gap defined by the housing 21 cools the burner arrangements 22a, 22b, 22c. In addition, the air flow issuing from the housing 21 is directed downwards in the same direction as the radiant heat from the arrangements 22a, 22b, 22c. This forms an air curtain around the area being heated to contain the heat within the confines of the curtain. This significantly reduces loss of heat to the environment and makes the heating process more efficient.