Abstract:
This invention concerns a boiler fitted with a burner suitable for wall heaters or built-in kitchen heaters in which a mantle-shaped heat exchanger made of pipe elements connected in parallel and/or series divides the boiler chamber into a combustion chamber and an exhaust chamber. The heat exchanger has openings for hot flue gases distributed over its mantle. The burner head disposed in the combustion chamber is suitable for burning oil and has a flame tube with an axial flame opening and a flame baffle element disposed at a distance from the flame opening which is constructed so that the flame is diverted into the space between the flame tube and the heat exchanger. In addition, a fire chamber mantle can be disposed between the heat exchanger and the flame tube to protect the heat exchanger from direct contact with the flame.

Description:
This application is a continuation of application of U.S. application Ser. No. 09/402,133 filed Jan. 3, 2000, now U.S. Pat. No. 6,305,331 B1 which was a 371 of PCT/CH98/00112 filed May 23, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a boiler or water heater equipped with a burner, including a housing surrounding a boiler compartment, a cylindrical heat exchanger, which divides the boiler compartment into a combustion chamber, and an exhaust chamber; whereby said heat exchanger comprises passages distributed across its surface for the hot exhaust gas, and a burner head positioned in the combustion chamber. 
     2. Review of the Prior Art 
     Such a boiler or water heater is disclosed in the French Patent document No. 93 00498, and is incorporated herein by reference. It contains a number of boiler designs, which exhibit the above-mentioned design features. These boilers are designed to accommodate gas burners and comprise a cylindrical casing with the sides closed off, and a plurality of flame openings distributed across the surface area. Such a gas-fired boiler or water heater is space-saving in its design and does not require a separate furnace room. 
     It has been a long-standing desire for such a space-saving furnace to be capable of being operated with fuel oil. The disadvantage of using gas as fuel is the more complex fuel storage requirements as compared to oil. As a result, the gas furnace has to rely on either an expensive pressurized tank or on a connection to a gas distribution network. Oil, on the other hand, has been stored in sufficient quantities in tanks located at the site of the furnace in thousands of installations without any problems. Additionally, the supply and filling of the tanks with oil is substantially simpler and less dangerous as compared to gas. 
     It is therefore the purpose of the invention to provide a furnace, which can be operated by an oil burner without making it larger than a comparable gas furnace. Furthermore, the furnace should be capable to be operated with a gas or an oil burner. Additionally, it is an objective of this invention to provide a furnace characterized by low exhaust emissions, reduced heat loss, and low noise levels. 
     SUMMARY OF THE INVENTION 
     The objectives are met, in accordance to the intent of this invention, by adding a fire tube comprising an axial flame opening and a flame deflection piece positioned at a distance to the flame opening to the burner head; whereby, the fire tube is designed in such as manner that the flame between the fire tube and the heat exchanger is deflected. 
     One of the advantages of a furnace designed per this invention is that it can be operated by burners, which produce a lance-shaped flame. Such a flame usually requires a very long combustion chamber. A flame deflection piece, designed in accordance to this invention, allows the length of the combustion chamber to be significantly reduced. The deflection piece turns the flame back to its origin and, as a result, reduces the length of the combustion chamber by ½. This allows the combustion chamber to be almost fully saturated by the flame exiting the fire tube which is subsequently being deflected into the opposite direction. The deflection of the flame to its origin has the further advantage that very hot gas is present around the fire tube very soon after the flame is initiated, enhancing the cold start characteristics of the furnace. Another advantage of deflecting the flame in the above-described manner lies in the fact that the combustion chamber can be utilized much more effectively and, therefore, can be designed in a more compact fashion as compared to those systems that produce a long, thin flame. More specifically, because of the burner head being surrounded by the flame, the entire length of the combustion chamber is more thermally uniform and therefore better-suited to exchange heat energy to the heat exchange medium. 
     It is advisable for the heat exchanger to comprise a blocking plate, which effectively limits the combustion system in lengthwise direction. In addition to the exhaust chamber, this creates an additional chamber into which exhaust gas flows from the exhaust chamber. The exhaust gas is cooled by the heat exchanger and is partially re-circulated back to the fire tube in order to cool the flame, and partially exhausted through the flue. A preferred blocking plate design separates the discharge chamber from the boiler compartment at its side facing away from the combustion chamber, whereby said discharge chamber is connected to the flue. Such a discharge chamber resides axially inside the boiler. This allows a uniform flow of the exhaust gas from peripheral the areas into the discharge chamber. This avoids non-uniform thermal loading issues of the heat exchanger. It is preferred for the blocking plate to separate a re-circulation chamber from the boiler compartment. Cooled exhaust gas can then re-circulate through this re-circulation chamber into the fire tube in order to cool the flame. The re-circulation chamber can also serve the function of a discharge chamber. It would be preferred, if the discharge chamber and/or re-circulation chamber (separated by the blocking plate) were surrounded by the heat exchanger. This allows additional cooling of the exhaust gas entering these chambers prior to leaving the furnace. As a result of the dual contact with the heat exchanger, the exhaust gas is cooled to approximately 80 degrees C., at continuous operation under full load. This allows for the exhaust gas to be piped directly into a flue (made of a plastic compound) upon exiting the boiler. 
     Further, it would be preferred that the blocking plate between the combustion chamber and the exhaust discharge chamber is shaped in a curved manner in such a way as to allow an increase in the length of the combustion chamber while minimizing the space requirements of the exhaust discharge chamber. Such a design feature results in a relatively large ratio of the heat exchanger surface surrounding the exhaust discharge chamber with respect to its volume. 
     In order to reduce the number of required parts, the flame deflection piece should form the blocking plate. In doing so, the position of the deflection piece in relation to the housing wall has certain acoustic advantages. The domed surface should point towards the exhaust discharge chamber. The purpose of this domed surface is to deflect the flame without the participation of any heat exchange elements, permitting the use of the entire heat exchange area since the deflection piece does not obstruct any passages for the hot exhaust gas. A preferred flame deflector design comprises a flame separator, positioned along the centerline of the flame and a ring-shaped deflector dish, which surrounds the flame separator. The flame separator divides the flame, and the deflector dish reflects the flame in order to change the flame direction by 180 degrees. The deflector dish should be made circumferentially uniform in order for the flame to retain a uniform shape after its deflection. 
     The casing of the heat exchanger consists of pipes positioned adjacent to one another with clearance between the pipes; whereby, said pipes are positioned to surround the combustion chamber and are connected to a supply and a return line. The heat exchange pipes should be wound in a screw-like manner. Such a heat exchanger unit is easy to manufacture; it comprises a large surface area and provides for passages between the pipes. Additionally, pipes can be made to thinner wall thicknesses as compared to castings, and therefore offer a more dynamic heat transfer characteristic which is reflected by higher performance at reduced space requirements. It would also be beneficial if this heat exchanger were assembled from a plurality of heat exchanger units. The individual heat exchanger units have the advantage of using shorter pipe lengths, as compared to a single unit having one long pipe system, resulting in higher through-flow velocities. 
     The heat exchanger units should be connected to the supply and return in parallel. Heat exchanger units using individual elements disclosed in the French Patent No. 93 00498 are applied successfully. The units described in said documents are characterized by a flat cross-sectional area of the pipes, resulting in an increase in the heat exchange area compared to typical pipes with round cross-sectional areas. Furthermore, an essential advantage of these heat exchanger units is due to the fact that these units are in series production for gas furnaces, and are therefore readily available in the marketplace at excellent quality levels. 
     The burner should be equipped with exhaust gas re-circulation in order to exceed the current regulated emission values, especially during cold starts. 
     Even if gas burners are applied to the boiler described by this invention, it is still preferred to use oil burners, simply because oil can be stored in inexpensive tanks, which can be readily refilled. The dependence on a supply distribution network can thus be avoided. Furthermore, the handling of oil is much less hazardous as compared to the handling of gas, which must be stored under pressure in appropriate tanks, in cases where it is not supplied through a gas distribution network. 
     It would be of benefit, however, to have the capability of the burner to operate on both fuels, oil as well as gas. If the burner head is designed to accommodate oil as well as gas, both fuels can be used in the same furnace alternatively with only minimal effort. This has the advantage that price changes and supply shortages can be dealt with in a more proactive fashion, or in a case of having to wait for a hook-up to be in place to provide gas to the furnace, oil can be used as a temporary means to power the furnace until the gas supply is secured. 
     For oil operation, an injector sprays oil into the exhaust gas being re-circulated into the fire tube; the inlet openings into the fire tube for fresh air as well as exhaust are designed so that fresh air and exhaust are mixed together inside the hollow cylinder or the hollow truncated cone making up the turbulent zone. As the oil mixes with the exhaust, it fully evaporates prior to its mixing with air. This assures very low exhaust emission values and an excellent starting behavior of the burner. 
     For gas operation, an air supply passage is used for introducing the gaseous fuel. The inlet openings into the fire tube for the fuel/air mixture and the re-circulated exhaust gas, respectively, are designed so that fuel/fresh air mixture and the exhaust are mixed together inside the hollow cylinder or the hollow truncated cone making up the turbulent zone. Because of these similar methods of introducing the fuels, the same fire tube can be applied to both fuels, gas and oil. It is feasible, to leave the oil injector in place during gas operation and to leave the gas supply system in place during oil operation, in order to provide a dual-fuel furnace with a single burner. A system, such as the one described above, is capable of achieving exhaust emission values of below 60 mg Nox per KW when operating on oil and under 20 mg Nox per KW when operating on gas. The CO values also lie under 20 mg Nox per KW at a very low level. In addition to these very low emission levels, the furnace is capable of performing very well under cold start conditions. 
     In the combustion chamber, in the area between the fire tube or deflected flame and the heat exchanger, resides a cylindrical combustion chamber shroud, comprising openings for the hot combustion gas. This combustion chamber shroud provides a uniform distribution of the hot combustion gas to the heat exchanger and also serves as an ash collector. It protects the heat exchanger from direct contact with the flame. This allows the distance between the flame and the heat exchanger to be very small. Additionally, this combustion chamber shroud has a positive influence in terms of noise suppression. The openings are arranged so that the combustion gas exits the combustion chamber shroud tangentially in order to facilitate the flow of the gas through the heat exchanger casing to also occur in a tangential direction. This improves the heat transfer as compared to a system that utilizes a radial through-flow direction. 
     The housing of this unit should be proportioned to be similar in size of a wall heating unit or a “plug-in” kitchen unit. The housing, including the air supply line and exhaust line, should have a length of approximately 50 cm. A short design should be able to operate with a boiler length of 30 cm. This means that a separate room for this furnace is not required. It can be stored in a cabinet. The air supply line should surround the exhaust gas line in a counter-flow arrangement in order to pre-heat the incoming air by the warm exhaust gas. The blower should be placed next to the housing with the air supply line leading from the blower to the end face of the housing and the burner head, in order to minimize the length and depth of the unit. 
     It is practical to line the end faces of the combustion chamber with fireproof tiles with a labyrinth-like inner structure. These protect the underlying metal parts, isolate the housing from the heat of the flame, and dampen the sound emissions of the burner. It is furthermore appropriate to provide one end face of the housing with a removable cover. It would be advantageous to mount the burner on this cover because this allows easy access to the boiler compartment and the burner. 
     It is further advisable to use austenitic stainless steel, which resists the aggressive exhaust gas and condensates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of reference to the drawings, where: 
     FIGS.  1 . 1 - 1 . 4  shows schematically four arrangements of boilers; 
     FIG. 2 shows a longitudinal cross-sectional view, of the boiler according to the preferred embodiment of the invention; 
     FIG. 3 shows a longitudinal cross-sectional view, of the boiler with a combustion chamber shroud according to the preferred embodiment of the invention; 
     FIG. 4 shows a cross-sectional view of the embodiment according to FIG. 3; 
     FIG. 5 shows a longitudinal cross-sectional view of an oil burner head according to the preferred embodiment of the invention; 
     FIG. 6 shows a schematic view of the combustion process using liquid fuel; 
     FIG. 7 shows a top view of a panel insert with guide plates cut out but not twisted; 
     FIG. 8 shows a cross-section through a panel insert according to FIG. 7; whereby, the guide plates are twisted for swirl generation; and 
     FIG. 9 shows a longitudinal cross-section of the gas burner head and schematic of combustion process using gaseous fuel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1.1 depicts schematically a simplified illustration of a boiler  11 ′ in accordance to this invention. Housing  13  is divided by heat exchanger  15  into a combustion chamber  17  and an exhaust chamber  19 . Fire tube  23  is positioned at one of the end faces of the combustion chamber  17 ; flame  25  exits the fire tube axially. Fresh air flows through the mixing pipe  21  into the fire tube  23 , leading to combustion in flame  25  and subsequently flows as hot combustion gas through the passages in heat exchanger  15  into the exhaust chamber  19  (arrows). From there, the combustion gas exits the exhaust chamber  19  through an opening (not shown in FIG. 1) in housing  13 . FIG. 1.2 illustrates a design variation, whereby, a blocking plate  27  limits the length of the combustion chamber  17  in boiler  11 ″. 
     The boiler compartment is thereby divided into 3 zones: The combustion chamber  17 , the exhaust chamber  19  and an exhaust discharge chamber  29 . The exhaust gas now flows from the exhaust chamber first through the heat exchanger  15  into the exhaust discharge chamber  29 , and from there, through an opening  31  into the flue. FIG. 1.3 depicts a simplified version of the FIG. 1.2; here, the heat exchanger  15  does not separate the exhaust chamber  19  from the exhaust discharge chamber  29 , but simply surrounds the combustion chamber  17 . Arrows in FIGS. 1.2 and  1 . 3  indicate how the exhaust gas is being re-circulated into the fire tube  23 . FIG. 1.4 displays a boiler  11 ″″ in whose boiler compartment an additional blocking plate  27 ′ is added to blocking plate  27 . This additional blocking plate creates a recirculation chamber  33 . The re-circulating exhaust gas from combustion chamber  17  reaches the re-circulation chamber  33  after having traversed through heat exchanger  15 , the exhaust chamber  19 , and, again, through the heat exchanger  15 . Once there, it is being drawn into the fire tube  23  through its re-circulation openings. 
     FIG. 2 displays via a cross-sectional view through an embodiment of boiler  11  including the heat exchanger  15 , the combustion chamber  17 , and the exhaust chamber  19 . Combustion chamber  17  contains fire tube  23 , which comprises re-circulation openings  35  and one flame opening  37 . The heat exchanger  15  consists of pipes  40  with a flat cross-sectional area, which are wound in a screw-like manner. The pipes  40  are positioned at a distance from one another, so that exhaust gas can flow through the gaps  41  between the pipes  40  of the heat exchanger  15 . The heat exchanger  15  consists of individual elements  43 , which are connected in series and/or parallel to a supply and a return line, respectively. Opposing the flame opening is a deflector piece  39 . This deflector piece  39  forms a blocking plate  27  or is connected to a blocking plate  27 . The blocking plate  27  is positioned between two pipes  40  or between two elements  43 ,  50  that the hot exhaust gas is forced to flow from the combustion chamber  17  through the gaps  41  into the exhaust chamber  19  and from there, once again through the pipes  40  into the exhaust discharge chamber  29 . The exhaust gas can then finally exit the exhaust discharge chamber  29  via a passage  31  into the flue or into an exhaust pipe. 
     The deflection piece  39  comprises an elevated area  47  on axis  45  of the fire tube  23  or boiler  11 , which is facing the flame and symmetrically separates the flame. The flame is subsequently re-directed into a direction, opposite its original direction and impacts in the area between the fire tube  23  and the heat exchanger tubes  40  against the root of the flame. This effect creates an approximately cylindrical flame body equal to approximately twice the fire tube diameter. The hot exhaust gas exits through the gaps  41  between the pipes  40  throughout the length of the combustion chamber  17 , causing an energy exchange with the exchange medium flowing through the pipes  40 . 
     The deflector piece  39  exhibits a dish-like shape and is positioned with its floor near the end face of housing  13 , which is positioned opposite to the fire tube. The inner diameter  51  connects with the outer diameter  53  of the dish-shaped deflector  49 ; whereby, the surface created by  51  and  53  seats flush against the heat exchange pipes  40 . Surface  55  of the deflector piece runs from the outer area  51  away from the heat exchange pipes in a slanted fashion, so that none of the pipes  40  are obstructed by the depth occupied by the deflector piece  39 , thereby allowing passage of the exhaust gas. The space occupied by the dish-shaped deflector piece  39  goes on the expense of the exhaust discharge chamber  29 , which, as a result, is reduced to the minimum required. Because of the shape of blocking plate  27 , the combustion chamber  17  increases in length towards the exhaust discharge chamber  29 . This allows the length of the boiler to be reduced to a minimum. 
     On the side of the boiler  11  that houses the burner head resides cover  57 , which is bolted against housing  13 . Cover  57  comprises an opening  59  on whose inside resides a panel or baffle plate  61 , onto which the fire tube  23  is mounted. A ring-shaped disk  63  is positioned at a distance around the fire tube  23 . This disk is made of fireproof, porous, or felt-like material and because of its insulating properties, tends to reduce heat loss and noise emissions. The deflector piece  39  has the same structure and, therefore, the same effect. 
     The fire tube  23  comprises re-circulation openings  35  located near the baffle plate  61 , through which exhaust gas is being circulated into the fire tube from the area  65  between the heat exchanger  15  and fire tube  23 . The exhaust envelops an air stream, which is being centrally admitted into fire tube  23 . This causes the fire tube to be surrounded by hot exhaust gas upon ignition of a flame and therefore heats up rapidly. An oil injector  67  is provided for liquid fuel operation, which sprays the fuel through the centered airflow into the shroud-like exhaust flow enveloping the airflow. The fuel evaporates within the exhaust flow. The evaporated fuel mixes with the air and exhaust in a turbulent fashion. The flame burns in blue color since all the fuel is converted into gaseous fuel prior to the generation of a flame. 
     The same burner head can be used for operation with gas. However, the gaseous fuel should be introduced into the low-pressure side of the air blower. A shroud of exhaust, having been re-circulated through the recirculation openings  35  into fire tube  23 , envelops the incoming air/fuel stream, mixes with said air/fuel stream in the swirl zone between the shroud and core flow and, as a result, the flame burns very similar to the flame generated by the evaporated liquid fuel. The fire tube  23  becomes hot with both fuels and transfers a certain amount of energy onto the heat exchanger  15  through radiation. This effect is desired, especially since blue flames have relatively little radiation energy. Exhaust emission values are very low with both fuels. Nox emissions are under 60 mg Nox per KW when operating on oil and below 20 mg Nox per KW when operating on gas. The CO values also lie under 16 mg Nox per KW at a very low level. 
     Burners that are built and functioning per the above-described methodology are described in detail in the submitted European notifications “process and devices for combusting liquid fuel” and “process and devices for combusting gaseous fuel”, both of which are based the Swiss priority notifications No. 1997 0718/97 and 0719/97, respectively. 
     FIGS. 3 and 4 illustrate an additional embodiment of a boiler in accordance to this invention. FIG. 3 depicts a longitudinal cross-section of a boiler; FIG. 4 depicts a cross-section in the transverse direction of the same boiler. In this boiler  11 , the blocking plate is designed as a simple deflector plate without a specific form. A further substantive difference relative to the boiler  11  shown in FIG. 2 is the presence of a combustion chamber shroud  69  in combustion chamber  17  on the burner side of heat exchanger  15 . The combustion chamber shroud  69  comprises slots  71  on its cylindrical outer surface, as well as guide plates  73 , which serve to release the hot exhaust gas from the inner area of the combustion chamber  17  and to guide the gas by means of a rotating flow field around axis  45  through the gaps  41  located between the pipes  40  of the heat exchanger  15  (ref. Arrows in FIG.  4 ). The flame is reflected back to the end face of housing  13  near the fire tube via the area between fire tube  23  and the combustion chamber shroud  69 . The combustion chamber shroud directs the exhaust gas into a spiraling motion. 
     The bottom area  75  of the combustion chamber shroud contains a zone without any slots  71 . This allows any ash present on the combustion chamber shroud to collect at the bottom  75  for easy removal. The combustion chamber shroud  69  also serves as protection for the heat exchanger  15  as it keeps it from direct contact with the flames. The combustion chamber shroud is therefore closed at the front of the shroud, near the blocking pate  27  or the deflector piece  39  and has no slots  71 , through which a partially reflected flame could potentially reach the pipes  40  of the heat exchanger  15 . 
     The screw-like windings  77  of the heat exchanger  15  are connected with a straight connecting piece  79  (FIG. 4) to a supply line  81  and return line  83 . The individual heat exchanger units  43  consist of four windings of pipe  40  having a flat cross-sectional flow area and are connected in parallel to the supply line  81  and the return line  83 . Local enlargements of the pipe walls (not shown) maintain a distance between the pipes  40  of the windings  77 . 
     FIG. 5 displays a burner head  111  for liquid fuels, including baffle plate  113 , which can be mounted on a wall surface (not shown) of combustion chamber  112 . Fire tube  115  is mounted on the baffle plate in direction of the flow (indicated by arrow  114 ); whereby, said fire tube has a diameter to length ratio of approximately 1:2. In addition, a fuel injector  119  is positioned along the centerline  117  of the fire tube. The devices used to mount the fuel injector and the baffle plate  113  form together a panel insert, as described in document No. EPA 0 650 014. The fuel injector head  123  seats centrally in panel insert  125 . The spray opening  121  of fuel injector  119  lies in the plane of the baffle plate  113  and the panel insert, respectively. Panel insert  125  is mounted on the baffle plate and covers the opening  127  of the baffle plate  113  but leaves a ring-shaped air vent  129  around the fuel injector head  123  open. This ring-shaped air vent  129  occupies an area of approximately 8% of the cross-sectional area of the fire tube  115 . 
     Air vent  129  is furthermore equipped with swirl generating guide plates  131 . These guide plates  131  are positioned radially and are slanted relative to the centerline  117  of fire tube and the direction of flow  114 , so that air flowing through air vent  129  is energized into a rotary motion around centerline  117 . The guide plates  131  are manufactured from one piece together with the panel insert  125  (FIG.  7  and FIG.  8 ). The guide plates are cut or stamped from the panel insert  134  and subsequently twisted by 60 to 80 degrees relative to the mounting surface. In doing so, round cut-outs are added in the area of the metal that receive most of the deformation as a result of the twisting action, in order to prevent the initiation of cracks (ref.  136  in FIG.  7 ). 
     Fire tube  115  is mounted on the baffle plate  113  by means of connecting tabs  133 . These connecting tabs  133  are formed as a single unit with the wall surface  139  of fire tube  115 , protrude above the end of fire tube  115  facing the baffle plate and are inserted through slots in the baffle plate  113 . On the upstream side of the baffle plate  113 , the connecting tabs re twisted upon insertion so that a tight connection between the baffle plate  113  and fire tube  115  is established. 
     The connecting tabs  133  are shaped in step-like manner, becoming smaller towards the end. The steps  137  in the tab contact the baffle plate  113  on the side of the fire tube and define the opening width of the recirculation slots. Exhaust gas is being drawn through this re-circulating slot  135 , along the baffle plate  113  and the panel insert  125  into the fire tube  115  in order to avoid soot in this area. A favorable opening width is approximately 1 mm. 
     The fire tube  115  comprises recirculation openings  139  near the baffle plate, through which exhaust gas is being drawn, primarily due to the sub-atmospheric pressure which is generated downstream of the baffle plate  113  as a result of the present air flow. In the case being described, there are 18 circular recirculation openings  139 , each with a diameter of 6 mm. The openings  139  can be lower or higher in number and/or can also be shaped differently. 
     The fire tube  115  comprises an inner tube diameter of approximately 80 mm and a length of 160 mm. The end of the fire tube  15  facing the combustion chamber  112  is narrowed. This contraction  141  reduces the flame exit area  143  relative to the fire tube cross-sectional area. The outer area of  145  of the fire tube  115  is rounded off towards the inside in order to create the contraction  141 . 
     The spark electrodes  147  are inserted through the baffle plate  13  near the periphery of the fire tube  115  with insulating elements  149  and protrude with their ends  151  into the fire tube  115 . The point  153  at which the spark is generated is placed at a distance from the baffle plate  113  equal to approximately ⅖ of the length of fire tube  115 . 
     FIG. 6 displays the various zones during combustion schematically. Because of air being forced through the air opening  129 , a sub-atmospheric pressure is being generated in area  161 , downstream of the baffle plate  113 . Through this sub-atmospheric pressure, exhaust gas is being drawn into the combustion zone as indicated by the arrows  163  and  165 . The exhaust gas forms a shroud  167  around the core of flow  169 . The exhaust gas entering a long arrow  165  moves along the outer surface of the baffle plate and thus protects it from carbon deposits. Regions of turbulence  171  are being generated in the area between the core flow  169  and the exhaust shroud  167 , which facilitate mixing of the two mediums, air and exhaust. 
     The fuel is introduced into the airflow along the shortest way possible, as depicted by the dashed lines  172 . The conical shroud of the spray plume comprises an angle of between 60 and 90 degrees. The fuel injector should preferably be designed to provide a spray plume angle of 80 degrees. The fuel evaporates in the area  173  of exhaust shroud  167  and mixes inside the exhaust shroud with the exhaust gas by means of the turbulence  175  generated in this area. Since there is no fuel present upstream of the evaporation zone  173  that could ignite, and because of the short distance that the fuel has to travel through the airflow  169 , practically all of the fuel is evaporated in the exhaust shroud  167  by the time it comes in contact with fresh air to initiate combustion. 
     Evaporated fuel is mixed with the exhaust gas and air inside the turbulence  171  and combustion initiates only in the area of said turbulence  171  with only minimal emissions. 
     The flame begins to develop in its root area  177 , approximately at the end of the first third of fire tube  115 . The flame root is embedded in the shape of a ring in the area between the exhaust shroud  167  and air stream  169 . The center air stream  171  terminates near the last third of the fire tube and serves to cool the tube. The thickness of the exhaust shroud  167  is decreasing as it traverses downstream, while the exhaust/fuel mixture mixes with air along the same distance. The fuel vapor is being supplied to the flame over a distance of approximately two thirds of the length of the fire tube. Thus, the flame has a ring-shaped, long-drawn root region and is being fed by shroud  167 . 
     Because of the contraction  141  of the fire tube, shroud zone  167  is limited in the downstream direction. The gas in the shroud region  167  is impeded while exiting the fire tube, which favors a mixing of the two media. Inside the fire tube, the exiting flame remains stable. 
     FIG. 9 illustrates schematically burner head  111 ′ for operation with gas and the different zones present during combustion of gaseous fuel. Burner head  111 ′ corresponds, in essence, to burner head  111  for liquid fuels. However, a perforated plate  157  is positioned downstream in front of the baffle plate  113  and a distance to said baffle plate  113 . The perforated plate  157  comprises an opening  158  through which the oil injector  119  penetrates. Several holes are places around this area which serve to create a pressure loss designed to avoid a blow-back of the flames into the supply passage  155 . A fuel supply line and a blower (not shown) are attached to the supply passage. 
     Since the air/fuel mixture is forced through passage  129 , a sub-atmospheric pressure is created in area  161 , downstream of the baffle plate  113 . Because of this sub-atmospheric pressure, exhaust gas is being drawn into this area, as indicated by arrows  163  and  165 . The exhaust gas forms a shroud  167  around the core flow  169 . The exhaust gas entering along arrow  165  flows along the surface of the baffle plate, which serves to protect it from excessive carbon or soot deposits. Turbulence is generated between the core flow  169  and shroud  167  within which the two media air/fuel and exhaust are being mixed. Gaseous fuel is mixed with air and exhaust gas inside the turbulence  171 , and combustion initiates only in the region of said turbulence  171  with only minimal emissions. 
     The flame begins to develop in its root area  177 , approximately at the end of the first third of fire tube  115 . The flame root is embedded in the shape of a ring in the area between the exhaust shroud  167  and air stream  169 . The center air stream  171  terminates near the last third of the fire tube and helps cool the tube. The thickness of the exhaust shroud  167  is decreasing as it traverses downstream, while the exhaust/fuel mixture mixes with air along the same distance. The flame burns steadily with only minimal emissions. 
     The gas burner, designed in accordance to the intent of this invention, functions practically independently of the form or shape of the combustion chamber. It is especially suited for compact furnace designs with relatively short combustion chambers. The burner is not only suitable for burning gas. By replacing item  119  by a fuel injector that is appropriate for liquid fuels with the capability of generating a conical shroud spray pattern, the burner is then suitable for the combustion of heating oil extra light, “Eco-oil” or kerosene. With liquid fuels, the burner achieves exhaust emission values for Nox of less than 60 mg/KW.