WO1995008078A1 - Radiant tube burners and methods of operating same - Google Patents

Abstract

A radiant tube burner (10) and a method of operating the same tube burner in which fuel and air are combusted as they pass through the tube (12) from one end thereof to the other, characterised in that an upstream region of the tube is heated by primary combustion in the tube of a fuel rich primary air/fuel mixture and a downstream region of the tube is heated by combustion in the tube (12) of secondary air fuel mixture by injecting secondary air into the tube at one or more downstream locations (66) intermediate the ends of the tube. The burner also comprises a system for reclamation of the heat from exhaust gases leaving the burner (10) which can be used to preheat gases entering the burner (10).

Description

Radiant Tube Burners and Methods of Operating Same

This invention relates to radiant tube burners which are burners comprising a heating tube in which a mixture of fuel and air is burned to heat the tube. The heated tube radiates heat typically into a furnace to heat same and the furnace contents. The fuel/air mixture is introduced at one end of the tube and the products of combustion (and any unburnt fuel) are discharged to waste from the other end of the tube.

Radiant tube burners have been used in furnaces for many years and owe their existence to the fact that they enable heat to be added to the furnace and furnace contents without the fuel or products of combustion coming into contact with the furnace interior or furnace contents, as it is an easy matter to locate the tube ends externally of the furnace. Thereby, the interior in the furnace can be kept contamination free, at least as concerns contamination by the products of combustion of the burning fuel/air mixture.

Radiant tube burners typically have heating tubes which are straight, or are U, M or W shaped, but the shape of the tube is not of dominating importance to this invention.

A basic problem with a simple radiant tube burner is that the products of combustion (exhaust gases) leave the tube at a high temperature in the order of 100°C greater than that in the furnace, which represents a considerable wastage of energy, and not unnaturally various methods have been proposed to recover and reuse some of the heat in the exhaust gases. Such proposals include the use of recuperators. In a true recuperator the medium to be heated, typically the air to be used for combustion in the burner and the waste gases are separated by a partition which may be made from metal or refractory material, and the unit operates continuously, the heat passing, through the partition wall which should be as thin as possible. In this manner the air to be supplied to the burner can be pre-heated to as much as 450-550°C when the exhaust gases exit the heating tube at a temperature in the order of 1000°C.

The step of pre-heating the air in this manner can result in the saving in fuel consumption of up to 22%. More sophisticated methods of pre-heating the air such as those proposed in patent specifications in the names North American Mfg Co, US 4604051, Caterpillar Tractor Co, US 4355973, and Stordy Combustion and British Gas, using regenerative units can pre-heat the air up to 900°C which can result in a saving in fuel of up to 40%.

However, by the known methods of air pre-heat as mentioned above, there is a disadvantage that in the burner the generation of oxides of nitrogen (referred to in the art generally as NOx) is increased. When the air pre-heat to 450°-500°C is used the Nox produced rises to 250 ppm (parts per million) which is barely inside new UK plant regulations on NOx emissions increasing the air pre-heat to the 900°C mentioned above it is estimated would raise the NOx level to a massive 1000 ppm, which is way outside the said regulations.

The higher NOx emissions that are produced by the introduction of air at a higher temperature results in a high temperature flame core and higher flame temperatures quite simply produce more NOx.

It is also to be noted that when there is excess air in the flame NOx again is increased, which suggests that NOx might be reduced by running the burner with a very rich flame (40% plus excess fuel) but that in itself would create all kinds of other problems such as carboning of the burner interior and the production of rich and dangerous flue gases and other negative effects.

NOx can be reduced by other methods. Thus, if the combustion air is introduced in stages and in a first or primary stage, primary air is introduced into the burner to allow the flame to run rich so that no high temperature flame core is created. The remaining or secondary air is injected into a second stage in an amount to provide the air to give complete combustion of the fuel.

Again, NOx can be reduced by what is termed flue gas recirculation in which the flue gas is injected into the pre¬ heated combustion air at the air outlet from the regenerator unit. This lowers the flame temperature by dilution of the fuel/air mixture and also lowers the amount of fuel nitrogen and oxygen making contact.

This invention is concerned with a radiant tube burner wherein fuel is burned in a burner tube to arrange for the burner to operate in as efficient a manner as possible consistent with achieving good economy of operation whilst limiting as much NOx emission as possible.

According to the present invention there is provided a radiant tube burner particularly but not exclusively for heat treatment furnaces comprising a burner tube, a burner head arranged in relation to said tube so that fuel can be emitted from said burner head whilst primary air is passed into one end of the tube and through the tube to create a fuel/air mixture which combusts inside the tube as it passed through the tube to be discharged from the other end thereof, and including means for introducing secondary and supplementary air into the combusted fuel/air mixture at a location downstream of the burner head but upstream of said other end providing secondary combustion of said mixture inside the tube.

By this means it can be seen that in a radiant tube burner, steps are taken to operate the burner at low NOx emissions insofar as the burner operates on staged air injection. The fuel/air mixture including the primary air can be fuel rich to limit NOx production and the secondary air which is supplied to complete combustion.

Also, as combustion takes place at at least two locations namely at the burner head and where the secondary air is introduced which are spaced along the length of the tube, the tube will be heated more evenly along the length of the tube than it would be if the flame existed only at the burner head. This leads to longer tube life and eliminates hot spots.

Preferably, the burner has a branch or bridge passage connecting the ends of the tube, whereby the primary and secondary air can be supplied from the same source, the secondary air being supplied to said downstream location.

Said downstream location is defined by the inner end of a secondary air inlet pipe which extends from its outer end at said other end of the burner tube into the burner tube and the outer end of the secondary air inlet pipe connects with an end of the branch or bridge passage.

The burner tube preferably is U shaped and the secondary air inlet pipe is located in one limb of the burner tube and the burner head preferably is located in the other limb.

At the said other end of the burner tube where the fuel/air mixture is discharged from the burner tube there may be a heat retention exhaust gases regeneration cell through which the exhaust gases pass to have heat removed therefrom. The radiant tube burner is preferably adapted to be operated in reverse or in push-pull arrangement in that it has a burning mode as described above, and a reverse or retained heat transfer mode in which no combustion takes place, but the flow of air through the burner tube is adapted to be reversed, the purpose of which is to transfer heat from the exhaust gases regeneration cell to an air pre-heating cell at said one end of the burner tube.

Reversal of air can be effected quite simply by appropriate connection of an air blower or suction? fan at the appropriate end of the burner tube, which may be provided with suitable fittings, and in the reverse mode ambient air is passed through the burner by first passing it through the exhaust gas regeneration cell from which it removes heat and then it flows both through the burner tube and the bridge passage to the region of the burner head and then all of the heated air passes through the air pre-heating cell where is gives up its heat or some of it to the cell. When the burner is switched again to combustion mode, the air which is passed into the burner passes through the pre-heating cell and is pre-heated by the heat retained therein.

The said heat retention cells may comprise enclosures containing pieces of refractory material typically balls of refractory material, although any suitable heat retention cells could be used.

By providing the present invention, at least in the form preferred as set forth above, the cost of the system can be reduced. According to another aspect of the present invention there is provided a radiant tube burner comprising a burner tube, a burner head arranged in relation to said tube so that in a combustion mode, fuel can be emitted from the burner head whilst combustion air is passed through the tube to create a fuel air mixture which combusts inside the tube as it is passed through the tube to be discharged from the other end thereof, and wherein there is a regeneration unit at each end of the tube so that in said combustion mode incoming air passes through the regeneration unit at the said one end of the tube and the exhaust gases pass through the regeneration unit at the other end of the tube so as to retain heat from the said exhaust gases, and the burner can operate in reverse or heat transfer mode in which the fuel supply is cut off and air is passed through the regeneration unit at said other end of the tube to remove said retained heat, and is passed back through the regeration unit at said one end of the tube to heat same ready to pre-heat the next batch of air which flows into the tube in the next combustion mode, and the burner may have appropriate controls, operable dependent upon temperature, to switch the burner automatically in push-pull fashion between combustion and heat transfer modes.

An embodiment of the present invention will now be described, by way of example only with reference to the accompanying drawings, which are diagrammatic and wherein:-

Fig. 1 is a perspective view of a radiant tube burner according to the present invention;

Fig. la shows in enlarged and perspective view, the detail of each of the tube and fittings of the burner as ranged in Fig. 1; and

Figs. 2 and 3 are similar views which illustrate respectively the burner of Fig. 1 in its two modes of operation and wherein the representations are distorted in that the end fittings of the tube shown in Fig. 1 are turned through 90° relative to their actual disposition in the interests of clarity of illustration.

Referring to the drawings, and firstly to Fig. 1, a radiant tube burner is illustrated by reference numeral 10, and it will be seen to comprise a tube 12 of a material suitable for use in connection with radiant heating. The tube 10 is of U shape as shown, ( although it could be of other configuration such as straight or M or W shaped) , so that it has two limbs 14 and 16.

When the radiant tube burner is connected in a furnace the tube 12 will mainly be inside the furnace, but the ends of limbs 14 and 16 will project from the furnace and these ends mount two fittings 18 and 20 which as shown are of inverted T shape. The bases 22 and 24 of the fittings which are of box shape are in alignment with and are connected to the ends of the tube 12. The upright legs 26 and 28 are of square cross section, but at the top are formed into pyramidal shape at shoulders 30 and 32 and lead to inlet sleeves 34 and 36 which are vertical.

As shown in enlarged detail in Fig. la, each sleeve 34 and 36 forms a casing of an eductor or jet driven pump 37,39 of which the jet is indicated by 38. The jet 38 is connected to the end of a pipe 40 for the supply of air under pressure which forms the drive medium for the pump and this supply is controlled by a manually or automatically operable control valve 42. The jet driven pumps provide respectively for the sucking of air through the burner in opposite directions as will be described. Connecting the bases 22 and 24 of the fittings 18 and 20 is a bridge pipe 44, whose function will be explained hereinafter, and connected into the base 22 of fitting 18 is a gas supply pipe 46 which forms the sole burner unit in the head of the apparatus.

In the use of the burner shown in Fig. 1, a combustion flame is established inside the tube 12 in order to heat same, the fuel being supplied by the fuel pipe 46, and the air to support the combustion during the combustion mode is pulled through the burner by the jet pump 37 which also discharges the exhaust gases in a known manner. The apparatus can operate in a reverse or heat transfer mode and in that case the supply of fuel through fuel pipe 46 is terminated, and the air is induced to flow through the apparatus by operation of the jet pump 39. The air exits via sleeve 34. The jet pumps 37,39 operate alternatively and provide a suitable suction for generating the air and exhaust gas flows mentioned above. It will be understood that any conventional type of suction and/or pressure pump may be adopted instead of or in addition to the jet pumps 37,39. A single unit may be adopted or twin units may be adopted depending upon the pipe work connected to the sleeves 34 and 36 in accordance with the requirements of the designer.

Figures 2 and 3 show the tube 12 and the fittings 18 and 20 in cross section, the fittings being turned on their side relative to the Fig. 1 position for the purposes of illustration only.

Referring to Fig. 2 which shows the apparatus in the combustion mode referred to above, suction is applied to the burner by jet pump 39 to draw gaseous products therefrom as indicated by arrows 48. This suction as described creates the flow through the entire apparatus as illustrated by the arrows in Fig. 2 and air is drawn into the apparatus through sleeve 34 and the air passes through the leg 26 of fitting 18 such leg being hollow and containing heat retention material suitably in the form of refractory pieces or balls 50. The air after passing through the refractory material emerges at location 52 into the interior of the base 22 of fitting 18 and that base is hollow so that the air flow in fact splits into first or primary air flow through the right hand side of the base, and second or secondary air flow through the left hand side of the base.

The primary air passes over a burner head 54 to which pipe 46 is connected. In the combustion mode fuel is supplied to pipe 46. The fuel issues from apertures in the burner head 54 and a burner flame is created in the region of head 54 as indicated by reference 56 by virtue of the mixing of the primary air and the fuel. At this stage it should be mentioned that the flame 56 runs rich i.e. it has an excessive amount of fuel as compared to a stoichiometric mixture and therefore NOx generation is reduced or eliminated. The flame 56 in fact extends through the limb 14 of the burner tube and partially around the tube bend thereby heating at least the first section of the tube.

The secondary air which deflects to the left hand side of the base 22 of fitting 18 is directed into the bridge pipe 44 which connects the interiors of the respective bases of the fittings 18 and 20.

The base 24 of the fitting 20 is also hollow, but is subdivided by a partition plate 58 into a left hand chamber 60 and a right hand chamber 62. The secondary air passing through pipe 44 enters the chamber 60, and then is directed along an injection pipe 64 which extends from chamber 60 through the base of fitting 20 and into and along the leg 16 of tube 12 so that it has an inner end 66 open at a location inside the tube 12 which is downstream of the flame 56, but is upstream of the end of the length 16 of tube 12. At said location 66 the secondary air issues and is deflected around the end of the tube as shown by the arrows so as to mix with the fuel rich partly combusted mixture arriving via limb 14 and the bend 12 with the result that there is secondary combustion at the location 66 and along the annular space 68 between the outside of pipe 64 and the inside of the limb 16 of tube 12. The products of combustion of this secondary combustion in relation to which the amount of the air added is calculated to give a stoichiometric or leaner mixture, are passed into the interior of base 24 of fitting 20 and the products of combustion then pass up through leg 28 of fitting 20 which contains heat retention matter of refractory material 70 which is similar to the material 50 to extract the heat from the exhaust gases which are then finally ejected through sleeve 36.

The above describes the apparatus in the combustion mode but when the combustion mode has been operational for a time such as to result in the temperature of the exhaust gases in sleeve 36 reaching a predetermined temperature, the apparatus is then operated in reverse or transfer mode as indicated in Fig. 3. In this mode, heat is transferred from the hot refractory material 70 in leg 28 of fitting 20 by operating the jet pump 37 ( the pump 39 being non-operational) and in this mode, the supply of fuel through pipe 46 is terminated. The arrows in Fig. 3 show the flow arrangements through the apparatus it being understood that the suction applied by jet pump 37 causes the flow indicated in all sections of the apparatus. With a suction applied at sleeve 34 air is drawn in through sleeve 36 and through the heated refractory material 70 so that the air is heated thereby, and the heated air passes through the annular space 68 until it reaches location 66 where it splits and some of the heated air passes through the tube bend and limb 14 until it reaches the interior of the base 22 of fitting 18, and the remainder of the air is drawn along the inside of the pipe 64 to the chamber 60 and then through the bridge pipe 44 and into base 22 of fitting 18. All of the heated air is then drawn through the refractory material 50 transferring the heat to same and the now relatively cool air is discharged from sleeve 34. When the air discharged from sleeve 34 indicates a predetermined temperature, the apparatus can again be switched back to the combustion mode.

The apparatus therefore in fact operates in a push-pull or switching manner operating first in one mode and then the other, and thereby heat is continuously being transferred from one regeneration unit (refractory material 70) to the other (refractory material 50). The control of the switching may be manual or automatic.

The burner is assymetri in so far as it has only one burner unit but two regenerative units and two jet pumps 37,39. The latter operate alternatively and the burner is on only when jet pump 39 is operational.

The apparatus described has the advantages that having regard to the combustion conditions in limb 14 during the combustion mode, NOx should not be produced in quantities associated other than those which are produced when stoichiometric combustion takes place, and because the distance allowed between the initial combustion flame 56 and the secondary combustion at location 66 which effects final burning of carbon monoxide and hydrogen remaining in the products of combustion arriving at location 66, is sufficiently great for the gases to pass their heat to the tube in limbs 14 and 16 before the second stage air is applied, the temperature of the flame reduces to a figure below that at which high NOx formation levels takes place, and this is so even if the flame temperature is again raised by the final combustion.

The secondary air supplied at location 66 should be sufficient to complete combustion of the waste gases in limb 16 and to maintain the temperature of the waste gases high enough give the tube a substantially uniform temperature throughout its whole length.

It is anticipated that pre-heating of the air for combustion should enable the air temperature to be raised to 0.8 of the waste gas temperature which should be higher than that associated with normal non-stage combustion and specifically could be 0.8 of 1150°C which is almost equal to that of a double regenerator burner system.

All of this is achieved by the use of a single burner unit which improves favourably with a situation in which two burner units in symmetric array are used.

Claims

1) A radiant tube burner (10), particularly but not exclusively for heat treatment furnaces, comprising a burner tube (12), a burner head (54) arranged in relation to said tube so that fuel can be emitted from said burner head whilst primary air is passed into one end of the tube (12) and through the tube (12) to create a fuel/air mixture which combusts inside the tube (12) as it passes through the tube (12) to be discharged from the other end thereof characterised in that the burner (10) also comprises means for introducing secondary and supplementary air into the combusted fuel/air mixture at one or more locations (66) downstream of the burner head (54) but upstream of said other end providing secondary combustion of said mixture inside the tube (12) .

2) A radiant tube burner (10) in accordance with claim 1, characterised in that the burner (10) operates by staged air injection wherein the fuel/air mixture including the primary air is fuel rich to limit NOx production and the secondary air is supplied to complete combustion.

3) A radiant tube burner (10) in accordance with claim 1 or 2, characterised in that combustion takes place at the burner head (54) and where the secondary air is introduced at one or more spaced locations along the length of the tube, so that the tube is heated evenly along the length of the tube.

4) A radiant tube burner (10) in accordance with any preceding claim, characterised in that the burner (10) has a branch or bridge passage (44) connecting the ends of the tube (12), whereby the primary and secondary air can be supplied from the same source. 5) A radiant tube burner (10) in accordance with claim 4, characterised in that the secondary air is supplied to the or each downstream location.

6) A radiant tube burner (10) in accordance with claim 4 or 5 characterised in that the or each downstream location is defined by the inner end of a secondary air inlet pipe (64) which extends from its outer end at said other end of the burner tube (12) into the burner tube (12) and the outer end of the secondary air inlet pipe connects with an end of the branch or bridge passage.

7) A radiant tube burner (10) in accordance with any preceding claim, characterised in that at the said other end of the burner tube (12) where the fuel/air mixture is discharged from the burner tube (12) there is provided a heat retention, exhaust gases regeneration cell (20,28) through which the exhaust gases pass to have heat removed therefrom.

8) A radiant tube burner (10) in accordance with claim 7 characterised in that the radiant tube burner (10) is adapted to be operated in reverse or in push-pull arrangement in that it has a burning mode and a reverse or retained heat transfer mode in which latter mode no combustion takes place and the flow of air through the burner tube (12) is adapted to be reversed so that heat from the exhaust gases regeneration cell is transferred to an air pre-heating cell (18) at said one end of the burner tube (12) .

9) A radiant tube burner (10) in accordance with claim 8, characterised in that the reversal of air is effected by appropriate connection of an air blower or suction fan, provided with suitable fittings, at the appropriate end of the burner tube (12) . 10) A radiant tube burner (10) in accordance with claim 9 when dependent on claim 4, characterised in that during the reverse mode ambient air is passed through the burner by first passing through the exhaust gas regeneration cell from which it removes heat and then flowing both through the burner tube (12) and the bridge passage to the region of the burner head (54) and then all of the heated air passes through the air pre-heating cell where it gives up its heat or some of it to the cell.

11) A radiant tube burner (10) in accordance with claim 10, characterised in that as soon as the burner (10) is switched again to combustion mode, air which is passed into the burner (10) passes through the pre-heating cell and is pre-heated by the heat retained therein.

12) A radiant tube burner (10) in accordance with claim 11 characterised in that said heat retention cell comprises enclosures containing pieces of refractory material.

13) A radiant tube burner (10) in accordance with claim 11 or 12, characterised in that a suction is applied to the other end of the tube to draw the the air and exhaust gas through the apparatus.

14) A radiant tube burner (10) in accordance with claim 12 or 13, characterised in that the pieces of refractory material comprise balls or any other suitable heat retention means.

15) A radiant tube burner (10) in accordance with claims 11,12 13 or 14 characterised in that the preheated air enters the tube at a temperature of about 80% of the temperature of the exhaust gas. 16) A radiant tube burner (10) comprising a burner tube (12), a burner head (54) arranged in relation to said tube so that in a combustion mode, fuel can be emitted from the burner head (54) whilst combustion air is passed through the tube (12) to create a fuel air mixture which combusts inside the tube (12) as it is passed through the tube to be discharged from the other end thereof, and wherein there is a regeneration unit (18,20) at each end of the tube so that in said combustion mode incoming air passes through the regeneration unit (18) at the said one end of the tube and the exhaust gases pass through the regeneration unit (20) at the other end of the tube so as to retain heat from the said exhaust gases, and the burner (10) can operate in reverse or heat transfer mode in which the fuel supply is cut off and air is passed through the regeneration unit (20) at said other end of the tube to remove said retained heat, and is passed back through the regeration unit at said one end of the tube to heat same ready to pre-heat the next batch of air which flows into the tube in next combustion mode the burner (10) may have appropriate controls, operable dependent upon temperature, to switch the burner (10) in push-pull fashion between combustion and heat transfer modes.

17) A radiant tube burner (10) in accordance with any preceding claim, characterised in that the tube (12) is in a U-shaped, W-shaped or linear configuration.

18) A radiant tube burner (10) in accordance with any preceding claim characterised in that the burner tube (12) is U shaped and the secondary air inlet pipe (44) is located in one limb of the burner tube (12) and the burner head (54) is located in the other limb.

19) A radiant burner tube (10) in accordance with any preceding claim characterised in that the burner head (54) is the only burner unit of the burner (10).

20) A method of operating a radiant tube burner in which fuel and air are combusted as they pass through the tube (12) from one end thereof to the other, characterised in that an upstream region of the tube is heated by primary combustion in the tube of a fuel rich primary air/fuel mixture and a downstream region of the tube is heated by combustion in the tube of primary air fuel mixture by injecting secondary air into the tube at one or more downstream locations (66) intermediate the ends of the tube (12) .

21) A method in accordance with claim 19, characterised in that the primary and secondary air can be supplied from the same source.

22) A method in accordance with claim 20, characterised in that the secondary air is supplied to the or each downstream location (66) .

23) A method in accordance with claim 19-22, characterised in that the secondary air passes through a branch or bridge passage (44) connecting the ends of the tube (12), whereby the primary and secondary air can be supplied from the same source.

24) A method in accordance with claim 19, 20, 21 or 22 characterised in that the exhaust gases pass through a heat retention, exhaust gases regeneration cell (20) at the said other end of the burner tube (12) where the fuel/air mixture is discharged from the burner tube (12) so that heat is removed therefrom.

25) A method in accordance with claim 23 characterised in that the radiant tube burner (10) operates in reverse or in push-pull arrangement in that it has a burning mode and a reverse or retained heat transfer mode in which latter mode no combustion takes place and the flow of air through the burner tube (12) is adapted to be reversed so that heat from the exhaust gases regeneration cell (20) is transferred to an air pre-heating cell (18) at said one end of the burner tube (12).

26) A method in accordance with claim 24, characterised in that the reversal of air is effected by appropriate connection of an air blower or suction fan, provided with suitable fittings, at the appropriate end of the burner tube (12).

27) A method in accordance with claim 25 when dependent on claim 22, characterised in that during the reverse mode ambient air is passed through the burner (10) by first passing through the exhaust gas regeneration cell (20) from which it removes heat and then flowing both through the burner tube (12) and the bridge passage (44) to the region of the burner head (54) and then all of the heated air passes through the air pre-heating cell (18) where it gives up its heat or some of it to the cell (18).

28) A method in accordance with claim 26, characterised in that as soon as the burner (10) is switched again to combustion mode, air which is passed into the burner (10) passes through the pre-heating cell (18) and is pre-heated by the heat retained therein.

29) A method in accordance with claim 27, characterised in that a suction is applied to the other end of the tube (12) to draw the the air and exhaust gas through the apparatus. 30) A method in accordance with claims 26 or 27 or 14 characterised in that the preheated air enters the tube at a temperature of about 80% of the temperature of the exhaust gas.

31) A radiant tube burner (10) characterised in that it has two regenerators (18,20) with a single burner head (54) with its operation being unsymmetrically divided between a firing phase and a heat redistribution phase.

32) A radiant tube burner (10) comprising a burner tube (12), a burner head (54) arranged in relation to said tube so that in a combustion mode, fuel can be emitted from the burner head (54) whilst combustion air is passed through the tube (12) to create a fuel air mixture which combusts inside the tube (12) as it is passed through the tube to be discharged from the other end thereof, and wherein there is a regeneration unit (18,20) at each end of the tube so that in said combustion mode incoming air passes through the regeneration unit (18) at the said one end of the tube and the exhaust gases pass through the regeneration unit (20) at the other end of the tube so as to retain heat from the said exhaust gases, and the burner (10) can operate in reverse or heat transfer mode in which the fuel supply is cut off and air is passed through the regeneration unit (20) at said other end of the tube to remove said retained heat, and is passed back through the regeration unit at said one end of the tube to heat same ready to pre-heat the next batch of air which flows into the tube in next combustion mode the burner (10) may have appropriate controls, operable dependent upon temperature, to switch the burner (10) in push-pull fashion between combustion and heat transfer modes characterisedin that the burner comprises a single burner head (54) which is used in conjunction with the two regeneration units(18,20) as aforesaid.