Abstract:
The invention concerns a pre-vaporizing and pre-mixing burner for liquid fuels which has a fuel feed line ( 56 ), a pump which pressurizes the fuel in the feed line, a mixing region ( 123 ) and a fuel valve ( 119 ) which opens out into the mixing region and by means of which the fuel is atomized and fed to the air for combustion ( 127 ). According to the invention, the fuel is vaporized in an optimum manner in that the fuel valve opens automatically as from a given fuel pressure, and a heating device ( 128 ) is associated with the fuel valve.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for providing a combustible mixture of a liquid fuel and combustion air, as well as a prevaporizing and premixing burner for liquid fuels, having one or several fuel heaters for heating the liquid fuel prior to combustion. 
     BACKGROUND OF THE INVENTION 
     In the field of household and small consumers (HuK) it is known to burn fuel oil EL for heating purposes or for purposes of thermal process technology in a pressure atomizer burner. The liquid fuel oil EL is converted under high pressure (500 to 2000 kPA) into a fog of droplets by means of an atomizer nozzle and is simultaneously mixed with the supplied combustion air. A method exists in addition, wherein fuel oil EL is atomized by means of compressed air. Further than that there are vaporization burner devices, wherein the liquid fuel is vaporized on the surface of a heated body which is surrounded by combustion air. 
     The following problems are connected with the present-day burners: in conventional oil burners, the liquid fuel oil EL is converted into a fog of droplets by means of an atomizer nozzle and simultaneously mixed with the supplied combustion air. The processes, such as atomizing, mixing, vaporizing and gasification of the fuel, as well as the combustion of the gasified fuel, occur unregulated side-by-side and in an interacting manner. The individual oil droplets are surrounded by a flame envelope. The high temperatures in the vicinity of the drops, together with the simultaneously occurring lack of air, trigger cracking processes, by which soot is formed. 
     Present-day blue flame burners avoid the generation of soot in that they vaporize the fuel at the flame root prior to combustion. Here hot flue gases returned from the flame zone here vaporize the oil spray emerging from a swirl nozzle. The water content of the returned flue gases prevents the formation of long-chain hydrocarbons, which can only be burned along with the generation of soot. The method of recirculating the exhaust gases lowers the nitrous oxide emissions in addition to the soot emissions. In order to convey a sufficiently large amount of hot flue gas into the flame root, a correspondingly large induction effect of the fuel/air jet within the mixture preparation is required. The induced mass flow is affected for one by the velocity of the emerging mixture flow and also by the cross section of the open jet. Both parameters can only be varied within certain limits. A high outlet velocity leads to loud flow noises, an increased blower output and large burner dimensions. An increase in the outlet cross section, together with a reduction in velocity, leads to ignition conditions already being created in the vaporization area, so that the intended fuel vaporization, which is uncoupled from the combustion reaction, does not occur. Moreover, the pulse exchange between the fuel and the combustion air is reduced, by which the mixture is also negatively affected. In addition, a high outlet velocity at the twist generator prevents a flame formation in the near range of the mixing device and thereby leads to a reduced thermal stress of these components. It follows from this that in connection with present-day mixture preparation methods for oil burners a reduction of the noxious matter emissions is always connected with an increase in the velocity of the combustion air, and therefore leads to increases in noise emissions and the required blower output. 
     In a burner system having a firing equipment output of 15 kW, a reduction of the fuel oil flow rate is not possible with conventional oil pressure atomizer nozzles. For reducing the throughput, the nozzle cross section cannot be further reduced for reasons of dependability. The pump pressure can also not be arbitrarily reduced, because the atomizing quality is clearly reduced. 
     Conventional oil burners are heterogeneous systems, i.e. the dispersed phase fuel oil EL and the dispersion medium air exist as discrete phases side-by-side and are separated by a phase boundary. The roughly dispersed fuel distribution caused by atomizing does not make it possible to mix the fuel without prior vaporization in front of the flame, because the individual fuel droplets settle under the effects of gravity and are deposited on the mixing chamber walls. For this reason a premixing surface burner device, such as can be used in the field of gas combustion, is not possible. 
     Modem gas burner devices show that a reduction of the nitrous oxide emissions is most effectively achieved by means of a premixing burner system. 
     A gasification device for fuel oil and kerosene is known from German patent DE-C2-24 56 526, and an oil heating device from German published application DE-OS 14 01 756, wherein the fuel is heated prior to atomizing. Although heating the fuel leads to improved and finer atomizing, problems occur because of cracking product depositions, such as clogging of the lines, etc. 
     SUMMARY OF THE INVENTION 
     The above mentioned problems are solved by the present invention by the provision of a method wherein: the liquid fuel is put under pressure in a heating phase with the fuel valve closed; the fuel under pressure and in liquid form is heated; following the heating phase the fuel valve is opened and the heated liquid fuel under pressure is atomized and vaporized through a nozzle; the vaporized fuel is mixed with combustion air, where at least a part of the vaporized fuel is condensed, so that a colloid-dispersed and/or a molecular-dispersed fuel-air mixture is created; the fuel valve is closed to terminate combustion; and, with the fuel valve closed, the heated and liquid fuel is cooled; 
     The advantages which can be achieved by means of the present invention consist in particular in that, with the method on which the present invention is based, a colloid-dispersed or molecular-dispersed fuel distribution occurs, depending on the degree of air preheating. A mixed form of both distribution types is also conceivable. Because of the stability of the colloid-dispersed fuel distribution, it is possible to mix the reactants ahead of the flame without the fuel droplets being deposited on the mixing chamber walls. Here, the mixing of the reactants is possible completely spatially decoupled from the combustion reaction, and not, as with conventional emission-reducing oil burners (so-called blue flame burners), only inside a very small gasification zone ahead of the flame, which is in direct convective heat exchange with the flame via the flue gas recirculation. Because the mixture of fuel and combustion air is now no longer limited to the gasification zone ahead of the flame, the premix burner devices known from gas burner technology, which make possible a very intensive mixing of the reactants, can now also be employed for liquid fuels. The known advantages of this burner technology are therefore also now usable for liquid fuels. Among these are: 
     (a) low emissions (soot, nitrous oxide, carbon monoxide) when using a surface combustion system; 
     (b) low noise emissions; 
     (c) small blower output required; 
     (d) a combustion air blower system can possibly completely omitted (atmospheric mixture formation); 
     compact heat generator construction because of the direct coupling of the heat exchanger on the heating cycle side to the reaction zone, which can be spatially exactly determined. 
     The core of the prevaporizing, premixing combustion technique is constituted by the heating of the liquid fuel oil under pressure. Vaporization of the fuel oil only takes place at the nozzle outlet in contrast to conventional vaporization burner devices, wherein the oil impinges with almost no pressure on a hot surface, which results in the deposition of low-volatile fuel oil components. Maintaining the above mentioned pressure conditions in the operational phases, in which heated fuel oil is in the hydraulic system of the burner, prevents these deposits. Both during the heating phase and during the cooling phase, the oil lines from the pump to the heated fuel valve are pressure sealed, or the pressure in the system is maintained by means of an oil pump or of a compensation vessel (for example metal bellows). 
     A fuel valve with “atomizing characteristics” is used in the system in accordance with the present invention, which unblocks the nozzle opening starting at a defined pressure. The oil vaporization triggered by the pressure reduction at the valve outlet causes an extreme increase in volume, and therefore a considerable reduction of throughput in comparison with the operation of the fuel valve with fuel oil which was not preheated. When using a swirl nozzle, a reduction of the throughput is caused by the reduction in viscosity connected with preheating the fuel. The air core within the nozzle opening increases with increasing fuel temperature and the fuel throughput decreases. The extreme preheating makes it possible to design the nozzle opening considerably larger, in particular with small throughputs, than would be possible with conventional pressure atomizer systems. Because of employing the twist principle in a return flow nozzle with an integrated needle valve, the required output of the firing equipment can be further reduced. 
     In a further development of the present invention it is provided that the heating device is constituted by at least one electric heating rod, heating element or heating cartridge. The heating device is designed in such a way that at maximum throughput the fuel is heated to the desired temperature. It is advantageous to provide the fuel valve additionally with a temperature sensor, for example a thermal element or the like, so that its temperature can be detected for regulating the heating output of the heating device. 
     A particularly simple exemplary embodiment provides that the heating device is placed into the fuel valve. In this case the individual heating cartridges or the like can be inserted into bores, for example. However, it is also conceivable that the heating device can be installed on the fuel valve, for example flanged to it, so that there is a direct contact between the heating device and the fuel valve. 
     With one exemplary embodiment, the fuel valve is embodied as a simplex nozzle with a closing piston. In this case the closing piston can be located outside or inside of the fuel valve. 
     A further development provides that the fuel valve has a return flow opening and can be combined with a return flow line. A return flow system is created in this way and the fuel valve is used as the return flow nozzle. 
     When the hydraulic system is laid out as a return flow system, direct electric heating of the valve, or respectively of the valve body, is not necessary. It is sufficient to heat the fuel by means of an electrically heated fuel heater which is remote from the fuel valve and is arranged upstream of the fuel valve, viewed in the flow direction. Transferring the fuel by pumping at a small pressure difference between the forward flow and return flow pressure prior to opening the valve causes heating of the fuel volume inside the valve. In this way the emergence of insufficiently heated fuel immediately following the opening of the fuel valve is prevented. 
     The return flow nozzle can a for example a have an integrated needle valve, which pressure-seals the nozzle opening during the heating and cooling phase. The movement of the valve tappet is made possible by means of the pressure difference between the forward and return flow pressure. Transferring the fuel oil by pumping at a small pressure difference between the forward flow and return flow pressure prior to opening the valve prevents the emergence of insufficiently heated fuel oil. For cooling the hot returned oil mass flow it is possible to additionally provide an oil cooler, which heats the combustion air, upstream of the pump inlet. Depending on the degree of air preheating, the proportion of the gaseous fuel in the fuel/air mixture increases. The pulse exchange between the combustion air and the fuel, which affects the quality of the mixture, also increases with increasing air temperature. 
     A further development provides that an adjustable flow resistor for pressure regulation, as well as an adjustable check valve, are provided in the return flow line. 
     With a return flow line which is merely used as a leakage line, no special measures for cooling the very small oil mass flow are necessary. For example, with a coaxial combination of the oil feed line and the oil return line, the cooling action of the fed-in oil mass flow is sufficient. Finally, embodiments are known, with which a return flow line is not required. 
     A burner with a fuel valve terminating into free space immediately after the valve tappet has the advantage that, depending on the degree of air preheating, a colloid-dispersed or molecular-dispersed dispersed fuel distribution occurs. Because of the stability of the colloid-disperse or respectively the molecular-dispersed distributed fuel it is possible to mix the reactants already ahead of the flame in an area of large volume without the fuel droplets being deposited on the mixing chamber walls. Therefore, mixing of the reactants is possible completely spatially decoupled from the combustion reaction, and not, as with conventional emission-reducing oil burners (so-called blue flame burners), only inside a very small gasification zone ahead of the flame, which is in direct convective heat exchange with the flame via the flue gas recirculation. The low temperature of the quasi-homogeneous mixture of the burner in accordance with the present invention permits intensive mixing in a mixing zone of large volume without the danger of spontaneous ignition. Now the mixing of fuel and combustion air is no longer limited to the gasification zone ahead of the flame. By employing a return flow nozzle in connection with extreme preheating of the fuel under pressure in particular, a small required firing equipment output can be achieved with operational dependability. Moreover, a large advantage is achieved in that deposits of cracking products are prevented, since the fuel vaporization takes place in the free atmosphere and not, as in film vaporization burners, at a hot surface in the presence of oxygen. 
     The heating zone is located in the direct vicinity of the reaction body, but is spaced apart from it. With another exemplary embodiment, the heating zone is connected directly with the reaction body. By means of this embodiment the fuel is heated during its passage past the heating zone by the reaction body which, as a rule, is red hot during operation. Therefore separate heating devices are not required during operation. In this case heating can be accomplished by means of radiated energy, by means of convection or by direct contact by means of heat conduction. 
     With a particularly preferred exemplary embodiment the heating zone is designed as a ring conduit. In this way a comparatively large surface for the inflowing fuel is created, so that it can be rapidly heated, for example by radiation. With a sleeve-like reaction body enclosing the ring conduit in particular, a very large surface for heating is available. 
     With another embodiment it is provided that the heating zone is designed as a spiral tube. The fuel to be heated is conducted through this spiral tube, wherein the reaction body directly radiates against the spiral tube. 
     In the preheating phase prior to the start of the burner, the fuel is heated in that an electric heating cartridge is provided, which is connected with the heating zone. In particular, the heating zone rests directly against the heating cartridge, so that the heat from the heating cartridge is transferred by heat conduction to the heating zone and from there to the fuel. In this case the heating cartridge can be designed as a heating rod or a heating spiral. 
     In a preferred embodiment, sections of the heating cartridge are in connection with the area through which the fuel-air mixture is conducted, wherein a flashback arrester is provided in the direction toward the mixture preparation. In this exemplary embodiment the heating cartridge is additionally used as an ignition device, wherein the fuel-air mixture is ignited on the casing of the heating element, which as a rule is red hot. Separate ignition devices are therefore superfluous. 
     Further advantages, characteristics and details ensue from the claims as well as from the following description, in which several preferred exemplary embodiments and variations are described in detail, making reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 : is a schematic representation of a prevaporizing, premixing burner for liquid fuel according to the present invention; 
     FIG.  2 : a shows an arrangement for the regulation of fuel feeding; 
     FIG.  3 : is a first exemplary embodiment of a fuel valve with a closing piston according to the present invention; 
     FIG.  4 : a fuel valve with a closing piston designed in the way of a simplex nozzle; 
     FIGS. 5 to  7 : are longitudinal sectional views of embodiments of the burner in accordance with the present invention; and 
     FIG. 6 a : is a cross section of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fuel lines  113 , the air-conducting components  114 , the components  115  conducting the fuel/air mixture, the components  116  conducting the flue gas and the water lines  117  of the heating circuit  144  are schematically represented in FIG.  1 . 
     The burner in accordance with FIG. 1 comprises the functional units  118  for air preparation, fuel preparation,  121  for air regulation,  122  for fuel regulation, mixing zone  123  and reaction zone  124 . 
     The air preparation unit  118  consists of a heat exchanger  125  for preheating air, which removes heat from the return flow of fuel  126  and transmits it to the supplied combustion air  127 . 
     The fuel preparation unit  119  consists of an electrically heated fuel heater  128 , the heat exchanger  125  in the return flow line  126 , which is connected with the air preparation unit  118 , and a heat exchanger  129 , which transfers a portion of the heat being released during the combustion reaction to the fuel preparation unit  119 , and a return flow nozzle  130  with an integrated needle valve. 
     The air regulation unit  121  consists of a blower  131  and an air throttle  132 , which can be actuated electro-mechanically or mechanically, by means of which an automatic adaptation of the conveyed air mass flow to the actual air requirements of the firing equipment is possible. 
     When the burner is started, the burner control switches the burner motor on, which is coupled with the oil pump  62  (FIG. 2) and the blower  131 . The check valves  53 ,  54  and  55  are initially closed. Thereafter, the electro-mechanically actuable check valve  53  in the forward flow line  56  and the electro-mechanically actuable check valve  55  in the side branch  58  of the return flow line  57  are opened. Simultaneously the burner control switches on the electrically operated heating element  133  in the fuel heater  128 . In this operational phase, the oil pump  62  conveys the fuel through the fuel preparation unit  119  and the heat exchanger  125  coupled to the air preparation unit  118 . Because of the low pressure difference between the measuring points  33  for the forward flow pressure and  134  for the return flow pressure  134 , the needle valve in the return flow nozzle  130  remains closed. This pressure difference can be variably set by the mechanically operated pressure regulating valves  60  and  59 . Here, the minimum pressure in this system corresponds to the pressure value which can be detected at the measuring point  134  for the return flow pressure. Thus, an overpressure in comparison with atmospheric pressure prevails in all components through which the heated fuel flows. By means of this it is assured that no low-boiling fuel components are vaporized during the heating of the fuel and the remaining high-boiling fuel components do not form deposits in the hydraulic system. Moreover, the transfer of fuel by pumping prevents the premature emergence of insufficiently preheated fuel from the return flow nozzle  130 . 
     By opening the electro-mechanically actuable check valve  54 , the return flow pressure drops in the swirl chamber of the return flow valve  130  when the required oil temperature has been reached, and the needle valve unblocks the nozzle bore. Because of this, the fuel in the mixing zone  123  is atomized and forms a combustible mixture with the fed-in combustion air  127 , which burns in the reaction zone  124 . Either a conventional high voltage ignition system or an electrically heated ignition element is provided for igniting the mixture. Another possibility lies in using the high surface temperature of the electrically heated fuel heater  128  for igniting the mixture. 
     The return flow nozzle  130  is designed as a swirl nozzle, the same as in a conventional pressure atomizer burner. The throughput is reduced with increasing fuel temperatures. Moreover, the employment of a return flow nozzle  130  has the advantage that, at constant forward flow pressure, the ratio of fuel conveyed back and the amount of atomized fuel can be changed over a large range 1:10 by throttling the return flow pressure. 
     Preheating the fuel and the use of a return flow nozzle make it possible to select the nozzle cross section considerably larger at the same oil throughput than is possible with conventional pressure atomizer burners. A reduced clogging tendency of the nozzle opening and increased operational dependability of the system are the result. 
     With this method, the heated fuel, which is under pressure, is atomized in the dispersion agent air. A portion of the vaporized molecules is condensed into a colloid-dispersed system, the remaining portion is maintained as a stable gas and forms a homogeneous mixing system, the same as in a gas burner. The proportion of colloid-dispersed oil droplets and homogeneously mixed molecules is a function of the fuel composition, which is influenced by temperature- and pressure-dependent chemical reactions (for example cracking reactions in connection with the fuel oil EL as fuel), and the degree of air preheating. 
     The colloid-dispersed distributed fuel in this system is aggregated into droplets of such length that they are separated by a phase border from the dispersion agent, the air. On the other hand, however, the particles are so small that in their behavior they correspond to a large degree to dissolved molecules. 
     From this, the following advantages over a roughly dispersed fuel oil distribution of conventional pressure atomizer systems result: (a) the colloid-dispersed oil droplets are distributed in the combustion air through Brownian movements until their concentration has reached the same value at all locations of the system; (b) excellent constancy of the colloid-dispersed fuel; (c) the gaseous portion of the fuel forms a combustible mixture with the combustion air immediately following mixing by molecular transport. The laws of spray combustion apply to the colloid-dispersed portion of the fuel, the same as with conventional oil pressure atomizer burners. The accelerated heating and vaporization of the drops because of the small drop diameters, and the chronologically advanced combustion and the temperature increase connected therewith of the portion of the fuel already in the gas phase make it possible to speak of a “quasi-homogeneous combustion systems , in spite of the portion of the fuel which is in the liquid phase. 
     The electrical heating element  133  is switched off during the operating phase of the burner. The energy required for heating the fuel is taken out of the reaction zone  124 . The heat exchanger  129  in the reaction zone and the fuel heater  128  are designed as a structural unit. 
     For turning off the burner, the burner control initially closes the check valve  54  in the return flow line  57  and the check valve  55  in the side branch  58  of the return flow line  57 . Because of this the pressure difference between the forward and return flows at the measuring points  33  and  134  is reduced and the needle valve in the return flow nozzle  130  is closed. The combustion reaction is interrupted by this. The high pressure in the fuel preparation unit  119  prevents the vaporization of the still hot fuel after the burner has been shut off. At the end, the burner control closes the electro-magnetic check valve  53  in the forward flow line  56  and turns off the burner motor. 
     A first exemplary embodiment of a burner valve, identified as a whole by  201 , is represented in FIG.  3 . This burner valve  201  has a housing  202  with a valve nozzle bore  203 , in which a feed line (not represented) terminates through an opening  204 . Optimally the fuel valve  201  can be provided with an additional opening  205 , which also terminates in the valve nozzle bore  203 . A return flow line (not represented) can be connected to this additional opening  205 , so that the fuel valve  201  can be used in a purely forward flow system as well as in a return flow system. When used with a forward flow system, the opening  205  is closed by a plug. 
     A valve nozzle  206 , into which a valve tappet  207  has been inserted, is screwed into the valve nozzle bore  203 . This valve tappet  207  is maintained in the closed position by means of a closing spring  208 . If the pressure in the valve nozzle bore  203  is increased past a defined value, the valve tappet  201  is pushed and the valve nozzle  206  automatically opens. 
     It can moreover be seen in FIG. 3 that heating cartridges  209  have been inserted into appropriate bores or other recesses of the housing  202 . If the housing  202  is heated by means of these heating cartridges  209 , which are electrically operated, the fuel present in the valve nozzle bore  203  is also heated. In this way the valve nozzle bore  203  is used as a preheating chamber  210 . The fuel emerging from the valve nozzle  206  is preheated, from which the above mentioned advantages ensue. 
     FIG. 4 shows a second exemplary embodiment of a fuel valve, identified as a whole by  211 , which has a slightly altered construction. The preheating chamber  210  terminates in a simplex nozzle  212 , which is closed by a valve tappet  213 . Here, too, the valve disk  214  is lifted off the opening of the simplex nozzle  212  when the fuel in the preheating chamber  210  has reached a defined pressure. Since the characteristics of a simplex nozzle are known, i.e. the reversely proportionate connection between throughput and temperature of the fuel, this will not be further discussed here. It should be noted that the seat  215  of the valve tappet  213  has been represented in FIG. 4 merely by way of example. Other constructions are conceivable and should also be covered by the present invention. 
     However, it is important that the fuel valves  201  and  211  are provided with a heating device  216  constituted by heating cartridges  209 , wherein the heating cartridges  209  have been inserted into appropriate openings in the exemplary embodiments. It is, however, also conceivable that the heating device  216  is interlockingly mounted on the fuel valves  201  and  211 . The housing  202  of the fuel valve  201 , or respectively  211 , is heated by means of the heating cartridges  209 , and the fuel in the preheating chamber  210  is heated via this housing  202 . When a defined temperature has been reached, or when the pressure of the fuel in the preheating chamber  210  has reached a defined value, the valve tappet  207 , or respectively  213 , is lifted and fuel can emerge from the fuel valve  201 , or respectively  211 . The fuel, which is under pressure and heated, is nebulized in the course of expansion and can be optimally mixed with the possibly preheated combustion air. 
     A burner, identified as a whole by  301 , is represented in FIG. 5, which has the construction described in what follows. A heat exchanger element  303 , in which the fuel is preheated, is located inside a dynamically balanced reaction body  302 . It is fed through a supply line  304  to the heat exchanger element  303  and reaches a ring conduit  305  constituted by two concentric sleeves  306  and  307 . The fuel is introduced into the ring conduit  305  through a connecting line  308 , or respectively it is removed from the ring conduit  305  through a line  309 . The line  309  terminates in a return flow nozzle  310 , which is opened, starting at a defined pressure prevailing in the line  309  and atomizes the fuel into an inner mixing chamber  311 . Air conduits  312  furthermore terminate in this mixing chamber, through which combustion air is supplied. This combustion air flows through the heat exchanger element  303  via a line  313  as well as a ring conduit  314 . 
     With the return flow nozzle  310  closed or opened, the fuel supplied through the line  309  is returned into the tank through a return flow line  315 . This return flow line  315  is located near the line  313 , so that the fuel in the return flow line  315  is cooled by means of the air flowing through the line  313 , or respectively the air is heated by this fuel. With large amounts of fuel conveyed back, a separate oil cooler is provided, through which either the supplied combustion air or the oil mass flow, or both, flow. 
     The heat exchanger element  303  has a circumferential groove  316 , into which a heating element  317  in the form of a heating spiral  318  has been inserted. In the starting phase, this heating spiral  318  preheats the inner sleeve  306 , and by the latter the fuel in the ring conduit  305 . The fuel in the ring conduit  305  is under pressure here. The inner sleeve  306  is pressed on the heating spiral  318  and welded on its front faces, so that the heating spiral  318  is fixed in place and protected. The heating spiral  318  can be additionally provided with a thermal element (not shown). 
     The return flow nozzle  310  is located in a union nut  319 , so that it can be rapidly removed when needed, for example for repair or maintenance purposes. The valve tappet  320 , which is prestressed by a compression spring  321 , can be seen at the rear of the nozzle  310 . The fuel valve can also contain the spring as a structural unit. 
     The inner mixing chamber housing  322 , around which the outer mixing chamber housing  323  is wrapped, has been placed on the union nut  319 . Thus, there is a further mixing chamber  324  between the inner and outer mixing chamber housing, which is used for further homogenizing the fuel-air mixture. The mixture is supplied from this outer mixing chamber  324  to the reaction body  302  and flows through the latter radially toward the outside. After ignition, the mixture burns outside of the reaction body  302 , so that the reaction body  302  is red hot during operation. The heat radiated by the reaction body  302  is radially transmitted toward the interior as well as toward the fuel-air mixture located between the reaction body  302  and the heat exchanger element  303  and to the outer sleeve  307 , so that the mixture and the fuel in the ring conduit  305  are heated. The heating element  317 , which is supplied with energy via the electric conductors  325 , is switched off during operation, or respectively is operated in cycles, for example by means of a regulator, for maintaining a defined temperature. 
     Flame monitoring at the exterior of the reaction body  302  takes place by means of a flicker detector  326  facing the inside of the fire chamber or the premixing area and looking from below through the reaction body  302 . Also possible is flame monitoring by means of an ionization electrode arranged above the reaction body or projecting inside it. 
     The embodiment of a burner represented in FIG. 5 has the considerable advantage that the fuel is heated within a very short time, in particular in the starting phase, because of the short distance of the oil film in the ring conduit  305  from the radiation source constituted by the reaction body  302 . In this case the heat flows radially from the inside to the oil film. During burner operation the output of radiated heat from the reaction body heats up the outer sleeve of the ring conduit. The latter transfers the heat to the oil film. During the startup of the burner, the oil film is correspondingly heated radially from the inside, during burner operation heating is provided by the heat output (radiation, conduction) of the reaction body. 
     The ring conduit  305  moreover offers a large heat exchanging surface. The dynamically balanced reaction body  302  can alternatively also be embodied as a flat bottom wherein, in place of the ring conduit  305 , a heat exchanger for heating the fuel must also be provided directly underneath this flat body. 
     In the exemplary embodiment of FIG. 6, the heat exchanger element  303  directly contacts the reaction body  302 , so that the fuel in the connecting line  308  is heated by heat conduction. The heating element  317  for heating the fuel in the starting phase is designed as a heating rod  327 , which is inserted into a corresponding bore  328  (see FIG. 6 a ) of the heat exchanger element  303 . This bore  328  is broken open segment-like over a part of its length, so that the heating rod  327  is openly accessible in this area  329 . This area  329  is in connection via an opening  330  and a connecting line  331  with a chamber  332 , which itself is connected via the ring conduit  333  with the outer mixing chamber  324 . In this way the fuel-air mixture, which can enter the opening  330  via the connecting line  331 , can be ignited by the red hot heating rod  327  at the end of the starting phase, so that the flame can penetrate through the reaction body  302 , by means of which the burner  301  is started. A flashback of the flame through the opening  330  into the chamber  332  is achieved by means of the comparatively small cross section of the connecting line  331  and its length, so that a dependable flashback arresting device is created. The great velocity of the fuel-air mixture and the small distance between the surfaces and the relatively great length of the surfaces (extinguishing distance) of the connecting line  331  prevent the ignition of the mixture in the chamber  332 . 
     In the exemplary embodiment in FIG. 7, the heat exchanger element  303  is wrapped in a spiral tube  333 , in which the fuel is conducted. This spiral tube  333  is connected both to the connecting line  308  and the line  309 , wherein the flow through the spiral tube  333  is also a counterflow. This spiral tube  333  is irradiated by the reaction body  322 , so that the fuel flowing in it is heated.