Abstract:
A highly efficient radiant burner assembly ( 100 ) for use in a patio heater or the like in areas where people prefer or require low NOX emissions. The efficiencies are created through the use of a spherical burner element ( 116 ) that is either formed of a high temperature steel wire mesh or stamping containing apertures of a predetermined size to allow combustion to remain within the burner element and to not cause the temperature of the burner element to exceed the temperature at which NOX are developed. Coating the burner with a catalyst also aids the low emission combustion process. Additional efficiencies are provided by atomizing the fuel before it is mixed with air and by the use of a laminar flow heat exchanger ( 940 ) that utilizes a fluid media flowing in a helical coil condenser unit.

Description:
BACKGROUND 
   1. Field of the Invention 
   The invention relates to the field of radiant burner systems for providing radiant heat energy through efficient combustion of a gaseous fuel/air mixture. More specifically the invention relates to the area of burner assembly and heat exchanger configurations for use in patio heaters, room heaters and space heaters, as well as other products that require efficient sources of radiant heat energy. Additional efficiencies are provided by utilizing heat exchangers to extract otherwise wasted heat from exhaust gas. 
   2. Description of the Prior Art 
   Radiant heaters are known which use various configurations of wire mesh or perforated sheet metal burner elements to support combustion of a gaseous fuel/air mixture. The use of perforated sheet metal of the same configurations is also known to provide an even distribution of gases from the gas inlet port to the inner surface of the burner element. Some of these concepts are described in U.S. Pat. No. 5,474,443. However in the patent, the burner surface and distributors are each limited to a hemispherical shape for use in boilers, 
   SUMMARY OF THE INVENTION 
   The present invention utilizes a heavy fuel and gas burner element having a substantially spherical shape to provide a radiant heat source with improved efficiency. The outer burner element is constructed of woven high temperature rated metal wires of sufficient diameter to withstand the heat of combustion occurring at its surface and small enough to result in mesh having a predetermined porosity that allows the micro-mist or gaseous fuel/air mixture to escape there-through. 
   The invention further includes a diffuser element that is also substantially spherical in shape but smaller in diameter than the burner element so as to be concentric with and substantially equally spaced from the burner element. The diffuser element is constructed from perforated sheet metal to allow the even flow of the micro-mist or gaseous fuel/air mixture to the space between the diffuser and the burner element. A micro-mist or gas flow inlet tube delivers the fuel/air mixture to the inside of the diffuser element to propagation through to the burner element. A circular distribution disk of sheet metal or other high temperature material that is not distorted or consumed by the temperatures within the burner element, is mounted in front of the inlet tube opening and inside the diffuser element to uniformly disperse the fuel/air mixture inside the diffuser element. In another embodiment the distributor element is a perforated metal cylinder or cone mounted at the end of the inlet tube. 
   The invention also includes an embodiment with a heat exchanger located in the path of exhaust gasses in order to extract additional heat for auxiliary uses. 
   When first ignited, combustion initially occurs with a visible flame on or just external to the surface of the burner element. However, as the combustion heats the surface of the burner element, the flame disappears and combustion moves to the surface. This allows the burner element to act as a pure heat energy radiator. The spherical shape of the burner element and associated diffuser element provide a relatively large radiation surface for the overall size of the assembly. By maintaining a lean mixture, the result is a relatively cool “flameless” combustion that maintains the burner element in the range of approximately 800-1000° C. In this mode, the burner results in a substantially emission free combustion of less than 10 ppm of nitrogen oxides (NOx) w/o catalytic coating and less than 2 ppm with catalytic coating. This unique thermal process also produces very high thermal energy. The combustion is quenched, or captivated, to the surface of the burner. The actual heat is produced by radiation from the burner surface. Heat radiation is a significant factor in heat transfer, especially when the temperature is high. In this case, the burner produces radiant heat efficiently with a combustion gas temperature lower than 1300° C. w/o catalytic coating and less than 1100° C. with catalytic coating. This is in contrast to conventional burners, which produce large quantities of thermal NOx when a gas-fired combustion exceeds 1538° C. (2800° F.). 
   The radiant burner of the present invention is shown in various environments including patio heaters which provide for unique configurations as compared with conventional heaters with central posts. This invention also can be used to generate heat energy for applications such as space heaters, wall furnaces, room heaters, garage heaters, fireplace heaters, “visual flame” type heaters and floor devices for homes, offices and recreational vehicles where high efficiency and low NOx emissions are desired. 
   In a further embodiment, a high efficiency laminar flow heat exchanger is located in the path of combustion gas as it is exhausted from the burner element. A liquid medium is employed in the heat exchanger to assist in the transfer of heat from the exhaust gas. The extracted heat can be provided for auxiliary storage or immediate uses. The combustion gas is condensed by the heat exchanger and the condensation is drained off. 
   In combination with a properly designed reflector, the energy is directed in a predetermined heat radiation pattern, so as to provide an even distribution pattern, preferably without hot spots. 
   In summary, the flameless surface combustion is optimized to burn below the temperature where NOx is produced, but still combusts at an optimized range that takes advantage of producing efficient radiant heat, resulting in a small, highly efficient, and emission free heavy fuel or gas-fired burner. 
   It is therefore an object of the present invention to provide a radiant burner assembly comprising a generally spherical shaped burner element having a first opening that surrounds the opening of a fuel/air delivery tube to allow for a fuel/air mixture to be delivered within said element; wherein said burner element is formed of material that has an array of apertures of a predetermined size and spacing over substantially its entire spherical surface and remains undeformed at all temperatures within the range of use. 
   It is a further object of the present invention to provide a radiant burner which is usable in a patio heater and other heat radiating devices in which low NOx emissions are desired. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional plan view of the preferred embodiment of the radiant burner assembly of the present invention configured for use in a patio heater. 
       FIG. 2A  is a plan view of a patio heater containing the radiant burner of the present invention. 
       FIG. 2B  is a plan view of another patio heater containing the radiant burner of the present invention. 
       FIG. 3A  shows a detailed area of a wire mesh embodiment of the burner element surface. 
       FIG. 3B  shows a detailed area of a perforated metal embodiment of the burner element surface. 
       FIG. 4A  is a 50/1 micrograph of the burner element surface embodiment of  FIG. 3A . 
       FIG. 4B  is a 500/1 micrograph of the burner element surface embodiment of  FIG. 3A . 
       FIGS. 5A and 5B  are cross-sectional plan views of other embodiments of the invention. 
       FIG. 6  is a perspective view of the invention used as a food cooker. 
       FIG. 7  is a perspective view of the invention used as a space heater. 
       FIG. 8  is a conceptual view of the burner element during an assembly step. 
       FIG. 9  is a plan view of the burner element after it has been assembled. 
       FIGS. 10A and 10B  are respective front and side elevation views of a room heater embodiment of the present invention. 
       FIG. 11  is a perspective view of the inner portion of another room heater embodiment of the invention. 
       FIG. 12  is a perspective view of the housing for the embodiment shown in  FIG. 11 . 
       FIG. 13  is a diagram used as a reference for calculating heat transfer efficiencies. 
       FIG. 14  is a cross-sectional view of a portion of the heat exchanger coil as may be employed in the embodiment shown in  FIGS. 11 and 12 . 
       FIG. 15  is a block diagram of a control system for use in a patio heater embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , the preferred embodiment of the radiant burner assembly  100  of the present invention is shown in cross-section and configured in a patio heater. The parts of the assembly include a gas feed tube  110 , a diffuser element  112 , a distribution disk  114 , a burner element  116  and a reflector element  120 . As can be seen from  FIG. 1 , the diffuser element  112  and the burner element  116  are substantially spherical in shape and differently sized so as to define a substantially spherical space  113  between them. The distance between the diffuser element  112  and the burner element  116  is substantially less than the diameter of the diffuser element  112 . 
   The gas feed tube  110  is connected to an extension arm  109  that serves to provide mechanical support and deliver the fuel and air mixture or a micro-mist from a pressurized source to the burner assembly. The gas feed tube  110  in turn provides a support structure for other elements of the burner assembly and a delivery path for the mixture of a gaseous fuel/air delivered to the burner. The distribution disk  114  is attached to the gas feed tube  110  and is spaced from the opening  111  to provide a uniform diversion of the fuel/air mixture as it enters the burner assembly. The distribution disk  114  is formed from sheet metal, a ceramic or another heat tolerant material and is mounted below the opening  111  of the gas feed tube  110  (in this case, approximately 1 inch) in order to evenly distribute the gaseous fuel/air mixture over the internal surface of the diffuser element  112 . Since the burner assembly is substantially spherical in shape, the distribution disk  114  is of circular configuration. However, it is contemplated that three dimensional elements may be substituted for distribution. These may have conical or other truncated shapes to provide the needed uniform distribution of the fuel/air mixture to the burner. 
   The diffuser element  112  is formed of perforated sheet metal to have a substantially spherical shape attached to the gas feed tube  110  and having a small opening that surrounds the end of the gas feed tube  110  and the opening  111 . The perforations in the sheet metal of the distributor element  112  are evenly spaced over the surface area of the sheet metal in order to allow an even distribution of the gaseous fuel/air mixture to the spherical zone  113  adjacent the inside surface of the burner element  116 . The burner element  116  is also substantially spherical in shape, as well as concentric with and larger than the diffuser element  112 . The burner element  116  is attached to the gas feed tube  110  for rigid support, having a relatively small opening that surrounds the opening  111  to allow entry of the gas feed tube  110  into the interior of its defined sphere. In this embodiment, the burner element  116  is formed of a high temperature steel wire mesh. As an option to provide further reduction in NOX during combustion, the mesh may be subjected to an aluminum oxide wash coat and then a catalyst coating of palladium or the like during its formation process. 
   The reflector element  120  is preferably formed of a rigid and lightweight material, such as aluminum or other metal having the desired reflective properties. Alternatively an insulated structure can be used and onto which an appropriate reflective coating can be placed on its inner surface  121 . In either event, the reflector element  120  provides a controlled pattern of heat radiation to the area below the burner assembly. In this embodiment, the reflector  120  is shown to be formed of a single unit, having a central opening  123  which is attached to the extension arm  109  for rigid support. In the shown embodiment, a cylindrical protective extension member  122  is attached to the major opening  125  of the reflector  120 . A plurality of clips  127  are used to hang the protective extension member  122  from the reflector  120  in a manner that provides limited exposure to direct radiation of heat from the burner  116 . In addition, the extension carries a light baffle  128  on a support member  129  that serves to block direct radiation and avoid a central hot spot. The protective extension member  122  may be formed of a metal having a reflective inner surface or glass with or without a partially reflective coating to allow the soft glow to be transmitted while controlling the reflective pattern of the radiated heat energy. The goal of distributing heat from the burner assembly in this patio heater embodiment is to define a circular pattern in a plane that is perpendicular to the central vertical axis “V” of the reflector. Therefore the reflector is designed so as to flood the area of the pattern closest to the axis with reflected heat while direct radiation is blocked by the baffle  128 . The intermediate area defined beyond the blocked area is flooded with both direct and reflected radiation, while the defined outer area receives only reflected radiation. In the event that it is desired to define a distribution pattern that is rectangular or non-circular, or one that provides an uneven distribution, it is certainly conceivable that one could design a reflector using know principles to accomplish such desires. 
   In  FIG. 2A , the burner assembly  100  described with respect to  FIG. 1  is shown embodied in a patio heater  130 . The extension arm  109  is shown as extending from a base unit  150  where the fuel may be stored and the control system located. The mixing of the gas from the fuel supply with air occurs in the gas feed tube  110  within the burner assembly. In the alternative, the fuel is supplied along a separate tube or capillary within the tube  100  and is emitted into the burner assembly as an atomized micro-mist of the fuel where it is mixed with air. In order to provide the actual burner assembly  100  at a location where it will supply an uninterrupted area of heat distribution, such as over a table where people may be seated or standing, the extension arm  109  extends upwards from the base  150  in an arc to a desired height and projects radially to a desired distance. Of course the base must have a great enough mass to serve as a counterweight to the extended arm  109  and the burner assembly  100 . 
   In  FIG. 2B , the burner assembly is designated as  100 ′ and is in another configuration of a patio heater  130 ′. In this embodiment a horizontal extension arm  109 ′ and counterweight  140 ′ are supported above a base  150 ′. In an alternative configuration (not shown) a suspended array of burner assemblies with reflectors are possible alternatives to the patio heaters shown in  FIGS. 2A and 2B . 
   The burner element  116  shown in  FIG. 1  is preferably formed from a woven mesh  170  and is shown in  FIG. 3A . The mesh is formed from the highly compact weaving of high temperature steel wire having a thickness “A” of approximately 1 mm so as to provide a spacing gap “B” of no more than 0.8 mm. The spacing gap “B” is critical to keeping the combustion of the gaseous fuel/air mixture within the fairly low temperature range of between 900-1100° C. and thereby keeping the burner element below 1600° C. and maintaining the low NOX characteristics of the invention. 
   Backfiring in the burner has been found to be prevented when the gaps are held to less than 0.8 mm. 
   In  FIG. 3B  an alternative material is shown that can be used for the burner element  116 . A sheet metal stamping  180  is shown that contains a continuous array of spaced apart apertures having openings “D” sized to no more than 0.8 mm and being evenly spaced apart “C” by 0.4 mm. 
   Either of the materials shown in  FIGS. 3A and 3B  may be formed into a substantially spherical shape to form the burner element  116 . However, in order to maintain an efficient combustion over the surface of the burner element  116  and resultant low NOX emissions, it is necessary that the 0.8 mm openings not become distorted and altered as the burner element is heated to its operating temperature. 
   It is expected that other materials may be substituted for those suggested here for the various elements. While the inventors have found that those described here are adequate and perform well, other materials such as porous ceramics or high temperature tolerant materials may perform equally as well. 
   The flexible wire mesh  170  shown in  FIG. 3A  is preferred to avoid heat stresses in the reaction zone of the burner element  116 . 
   If a catalyst is used, the diameter of the wires has to be as large as possible to be able to provide a wash-coat (aluminum-oxide: AL 2 O 3 ) on the wires. With reference to the micrograph in  FIG. 4A , a rough whiskered surface  171  is created by heat treating the wires  170  (from the Al content in the steel). The wash-coat adheres to this surface. The wash-coat carries the catalyst which is preferably Palladium to support lean methane combustion. The purpose of providing the rough whiskered surface  171  on the wires  170  is to increase the surface area onto which the catalyst is applied and exposed to the combustion/oxidation of the gas. In  FIG. 4B , the palladium catalyst is shown as globules  172  attached to the Al 2 O 3 . 
   In  FIG. 5A , a burner assembly  200  is shown as an alternative embodiment of the invention. In this embodiment, the distributor element  214  is formed as a right cylinder having a closed end that is attached to and extends from the opening of the gas feed tube  210 . The distributor element  214  is formed of perforated sheet metal with an even distribution of apertures on its curved cylindrical surface with its end closed with a disk piece  215 . The distributor element  214  provides an acceptable uniform distribution of the gaseous fuel/air mixture. The spherical burner element  216  is substantially the same as that shown in  FIG. 1 , as is the diffuser element  212 . In this embodiment, a reflector  220  is shown in which an outer lip  222  is formed at its periphery to define a gutter  224  that extends just below the maximum extension of the inner surface  221 . The gutter  224  acts to collect any condensation that may initially form on the inner surface  221  during start up of the burner and prevent the condensation from dripping from the burner assembly  200 . After initial ignition when the burner has stabilized its combustion, condensation is no longer a concern and the condensation collected in the gutter  224  evaporates. 
     FIG. 5A  further shows the location of a capillary tube  190  within the gas feed tube  210  to provide atomized fuel as a micro-mist. In a conventional manner, liquid fuel within the capillary is heated to near its boiling point. In this embodiment, the heating may be performed by either an auxiliary electrical source applied to the capillary, an adjacent heating element and/or by the excess heat generated by the burner assembly after start up. Under pressure, the fuel flows through the capillary and exits the open end of the capillary  191  as an atomized micro-mist. The micro-mist of fuel then mixes with the air flowing through the gas feed tube  210  and enters the burner assembly via the distributor element  214 . Upon entry into the burner assembly, the fuel/air mixture migrates to the burner element and combustion takes place. With this alternative fuel delivery technique, the burner can be made to provide complete combustion of heavy fuels as well as gas fuels. It is expected that the use of the micro-mist injection technique can be used with any of the distributor configurations shown or described herein as well as others yet to be created. 
   In still another embodiment shown in  FIG. 5B , a burner assembly  300  corresponds to the burner assembly shown in  FIG. 1 . However, in this embodiment, a baffle element  319  is added. The substantially spherical baffle element  319  is formed of perforated sheet metal similar to the distributor element  312  and, like the distributor element  312 , has evenly distributed perforations to allow the gaseous fuel/air mixture to pass towards the burner element  316 . The baffle element  319  is also attached to the gas feed tube  310  for rigid support. 
   The function of the baffle element  319  is to allow uniformly constant migration of the fuel/air mixture from the diffuser element  312  to the burner element  316  and to reduce noise that is generated by the combustion of the gaseous fuel/air mixture occurring at the surface of the burner element  316 . The baffle element  319  is formed to be larger than the diffuser element  312  and smaller than the concentric burner element  316 . In this manner, substantially spherical zones  313  of predetermined thicknesses are defined between the diffuser element  312  and baffle element  319 , and between the baffle element  319  and the burner element  316 . 
   While the present invention is ideally suited for use in a patio heater configuration, it is also seen as being uniquely suited in other configurations in which highly efficient heat is required along with very low emissions. For instance, the invention could be used as a food cooker, as shown in  FIG. 6 , or as a space heater, as shown in  FIG. 7 . Other uses such as wall furnaces and garage heaters are also envisioned, but not shown in the drawings. 
   In  FIG. 6 , a cooker unit  600  is shown in its basic form to include an insulated housing  601  having a floor  603 , heat vents  605 , a cooking platform  609  and rotating plate holder  611 . The burner assembly includes the gaseous fuel/air mixture deliver tube  610  that supports the burner  616  and reflector  620 . The reflector in this embodiment is curved to redirect the radiation “R” towards the food  619  placed on the plate holder  611 , in an even and efficient distribution. An optional splatter shield  607  is shown as being located between the burner  616  and the cooking area in order to prevent grease and food particles from reaching and touching the burner element which may cause back-flashing which is a form of uncontrolled burning. The splatter shield is preferably made of material that is rigid, essentially transparent to heat radiation and easily cleaned. Various glasses are suitable for this purpose. Alternatively, the shield  607  could be a ceramic glass, that re-radiates the heat energy received from the burner  616 . 
   In  FIG. 7 , a space heater  700  is shown in which a reflector element  720  is oriented to direct heat from the burner  717  to a space selected by the user. In this case, a safety shield  719  is attached to the front of the reflector so as to prevent any foreign contact with the burner element  717 . Alternatively, the shield  719  could include an intermediate ceramic glass, which re-radiates the heat energy received from the burner  717 . A gaseous fuel/air mixture delivery tube  710  is shown as providing the fuel from the base supply  750  and providing support for the reflector and burner assembly. 
   The fuel used in the present invention is preferably natural gas or propane. However, it is contemplated that other fuels can also be used, provided they meet the criteria for delivery to the burner in a gaseous state at low pressure on the order of 1-2 atmospheres. 
   The preferred method of forming the spherical shape of the burner element  116 , shown in  FIG. 1 , is to prepare two hemispheres  802  and  803  having the equal diameters, as shown in  FIG. 8 . In preparing the hemispheres, the mesh is placed over a die and compressed into shape. The result is that a flange  810  and  813  of material is formed around the respective open diameter of each hemisphere. The flanges  810  and  813  can then be clamped together and the flange is welded. The resulting sphere  116  is shown in  FIG. 9  with a weld  820  around its equator, following trimming of the excess flange material. 
   A room heater embodiment  10  is shown in  FIGS. 10A and 10B . In this embodiment, a housing  11  is formed of a rigid material such as sheet metal and may be coated with a fire resistant insulation layer, not shown. Three radiant burners  16  of the present invention are shown disposed in a linear array. Depending on the room size and heating capacity of the unit, the burners may be scaled in size and more or fewer radiant burners may be employed. 
   Due to the extremely low emissions produced by the radiant burners of the present invention, it is understood that the heater  10  of this embodiment could be used as a “ventless” heater without utilizing outside combustion air. However, in this instance the use of outside combustion air and outside exhaust is shown in a conventional way. 
   The radiant burners  16  are mounted on a reflector support element  19  and are connected to a combustion air inlet duct  34 . A horizontal manifold duct  32  is also shown to provide distribution of combustion air to the burners  16 . The radiant burners  16  extend into a volume defined by a reflector  26  and a ceramic glass  18 . An opening  22  in the reflector  26  and the reflector support  19  allow the combustion gas to be exhausted through vent duct  24 . 
   The housing  11  defines a plenum space  20  that becomes heated by the combustion gases and the heat that migrates from the reflector area. A room inlet vent  12  is provided at the bottom front of the housing  11  and a corresponding room outlet vent  14  is provided at the top front of the housing  11 . In this manner convection heat is produced by the heater  10  and dispersed to the room. A fan (not shown) also may be incorporated within the plenum space  20  to increase the air flow and distribution of the convection heat. 
   The majority of the heat energy produced by the heater  10  is in the form of radiant heat that is projected by the burners  16  and the associated reflector  26  directly into the room. The ceramic glass  18  functions to allow a high percentage of the radiant heat to be transmitted into the room and to separate the radiant burners  16  from coming into contact with foreign objects. Alternatively, radiant heat emanated from the burner(s)  16  and the associated reflector  26  will transfer to the ceramic glass  18  (designed for this purpose). The glass  18  will then radiate the heat to the room. 
   Although not shown in  FIGS. 10A and 10B , the heater embodiment includes the appropriate sensors and systems for ignition control, thermostatic control and high temperature safety cutoff control. 
   A further alternative to the spherical burner assembly is foreseen as a right cylinder which has its central axis aligned with the vertical. In this manner, the gravitational effects on the cylindrically configured burner assembly will be minimized, while maintaining many of the efficiencies of the other embodiments. 
   Another embodiment of the invention is shown in  FIGS. 11 and 12  as a heater  900  with a high efficiency heat exchanger  940 . In this embodiment, heat exchanger  940  is employed to draw additional heat from the exhaust gas resulting from the controlled combustion in the burner  916 . The embodiment includes a base  951  that serves to support the other components of the assembly. A component housing  913  is shown which contains the electronic control unit, valves for controlling the air fuel mixture from the gas supply inlet  932  and the air inlet  934 , as well as a fan (not shown) if the mixture requires pressure to the burner  916 . Electrical wiring to the main supply is provided at  915 . It should be noted that the external air supply  934  may be eliminated if the unit is intended to burn ambient air or if the vaporized fuel is premixed with air prior to being furnished to the unit. 
   A feed tube  910  extends from the component box  913  to the radiant burner  916 , as disclosed above with respect to other embodiments. The feed tube  910  also mechanically supports the reflector  920  and the heat exchanger  940 . 
   In this embodiment the reflector is used to radiate heat from the burner  916  in a predetermined pattern away from the assembly. Combustion gases pass through apertures  924  formed in the top portion of the reflector  920  into the heat exchanger  940 . The combustion gas rises through the components of the heat exchanger  940  and is exhausted through exhaust pipe  924 . 
   The heat exchanger  940  is comprised of a helical tubing  942  that is structured to allow laminar flow of the exhaust gas between the individual coils segments where heat is transferred from the gas to the tubing  942 . Water or other similar heat transfer media enters through tube extension  946 , is passed through the tubing  942  and exits through tube extension  944 . 
   The heat exchanger coil  942  has gaps created by the helical shape of the tubing  942  that are very narrow “h” (about 0.8 mm) and comparably long “l”(shown if  FIG. 13 ). The gaps provide a laminar flow path for the exhaust gas. This results in a very efficient heat exchange process. The exhaust gas enters the heat exchanger  940  at about 900° C. and exits at less than 100° C. Water, or other heat absorbing liquid medium, flows through the tubing  942  and extracts the heat energy from the heated tubing  942 . The heated water can be used for various purposes, such as the bathroom or kitchen, or simply as an addition to the home water heater, or for heating other rooms. Because the laminar flow heat exchanger  940  acts as a condenser, condensate water is produced on the outside of the tubing  942  as a by-product of the heat exchange process and is collected at the base of the heat exchanger. The condensate water is drained at tube extension  948 . This water has no impurities and may be consumed (assuming it does not pick up impurities from the collection vessel). 
   The outer housing  950  for the embodiment described with respect to  FIG. 11  is shown in  FIG. 12 . The housing  950  includes a cylindrical tube that mates with the base  951  and has a top panel  953  to form an enclosed space in which the components described with respect to  FIG. 11  are contained. 
   A ceramic glass element  918  is attached to a corresponding aperture in the housing  950  in registration with the reflector  920  in order to allow heat radiation to be directed outward from the unit. The diameter of the cylindrical tube exceeds the diameter of the heat exchanger  940  so as to define a heating space that allows heat which radiates from the back side of the reflector  920  to rise in the housing. Grill like apertures  952  and  954  are formed in the respective lower and upper portions of the housing  950  to allow convection heat to flow out of the unit into the room in which the unit is located. Of course, a fan may be employed within the housing in order to increase the air flow and decrease the housing temperature, if desired. 
   The heat transfer from exhaust gas to water can be significantly intensified by using the laminar flow of the exhaust gas. The theory is shown below: 
   
     
       
             
           
             
             
           
             
             
             
           
             
             
           
             
             
             
           
         
             
                 
             
             
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                           · 
                           
                             η 
                             ρ 
                           
                           · 
                           
                             l 
                             
                               
                                 h 
                                 3 
                               
                               · 
                               b 
                             
                           
                         
                         ⁢ 
                         dm 
                       
                     
                   
                 
               
             
             
                 
                 
             
           
        
         
             
                 
               η: Dynamic Viscosity 
                 
             
             
                 
               ρ: Density 
               b = width 
             
             
                 
               dm: Mass Flow 
               h = height 
             
             
                 
               w: Velocity 
             
             
                 
                 
             
           
        
         
             
                 
               
                 
                   
                     
                       
                         1. 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Q 
                       
                       = 
                       
                         
                           α 
                           · 
                           F 
                         
                         ∝ 
                         
                           
                             b 
                             · 
                             l 
                           
                           h 
                         
                       
                     
                   
                 
               
             
             
                 
                 
             
             
                 
               
                 
                   
                     
                       
                         2. 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         p 
                       
                       ∝ 
                       
                         l 
                         
                           
                             h 
                             3 
                           
                           · 
                           b 
                         
                       
                     
                   
                 
               
             
             
                 
                 
             
             
                 
               
                 
                   
                     
                       
                         2 
                         → 
                         
                           
                             1 
                             ⁢ 
                             
                               : 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Q 
                           
                           ∝ 
                           
                             
                               
                                 b 
                                 h 
                               
                               · 
                               Δ 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               p 
                               · 
                               
                                 h 
                                 3 
                               
                               · 
                               b 
                             
                           
                         
                       
                       = 
                       
                         
                           
                             b 
                             2 
                           
                           · 
                           
                             h 
                             2 
                           
                           · 
                           Δ 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         p 
                       
                     
                   
                 
               
             
             
                 
                 
             
           
        
         
             
                 
               Q and Δp are given: 
               b · h = Constant 
             
             
                 
                 
             
           
        
       
     
   
   As can be seen from the above theory and with reference to  FIG. 13 , for a given heat Q to be transferred at a given pressure loss Δp:
     1. The necessary area F=b*l=Constant/b of the plates (gives size and mass of the heat exchanger) is indirectly proportional to the width of the gap b. As wider the width dimension b, as smaller is the area F of the heat transfer surface and as lighter the heat exchanger.   2. Since the necessary length l is indirectly proportional to b 2 , the length l can be kept very short when the dimension of width b is increased.   That means for the design of heat exchanger  942 , as shown in enlarged cross-section in  FIG. 14 :   1. The dimension of height h should be as small as possible and should be sufficient to allow laminar flow of the exhaust gas. The height h should be &lt;1 mm (for practical reasons, it cannot be much smaller).   2. The dimension of width b should be as great as possible.   

   These geometric goals can be achieved with a helical tube  942  by providing rectangular a cross-section indicated by stacked sections  944   i - 944   n  having the length l, separated by a gap of height h and an extremely long width dimension b running the length of the helical tube. 
   The laminar flow heat exchanger works very effectively as a condenser. The exhaust gas enters the narrow gap at a temperature of &gt;900° C. and is cooled to less than 100° C. With methane as fuel, a theoretical additional 11% heat can be generated by condensing the water content in the exhaust gas. The condensate forming on the outside of tube  942  from the natural gas combustion is very clean, if the condenser is fabricated from metal that does not contain heavy metals. 
   Alternatively, an exhaust fan can be provided down stream from the heat exchanger to make sure that the cooled combustion is exhausted from the unit. Heat control from the unit can be provided by several means. A first method of control is to regulate the fuel flow to the burner with an adjustable thermostat feedback. A second method is by including several choices of ceramic glass windows having varying transmission characteristics for manual placement on the front of the unit. 
     FIG. 15  shows a block diagram of a control system  1000  as may be employed for the radiant burner of the various embodiments described herein. The radiant burner is represented as block  1010  which receives regulated fuel vapor input from gas line  1048  and controlled air from air tube  1056 . The fuel is derived from source  1040 , which may be natural gas, methane, propane, butane, diesel or other bio or petro products which can be provided in a vapor state. The fuel passes through a tube  1041  to a pressure regulator  1042  of a conventional type and a manually adjustable valve  1044 . An electrically controlled gas shut-off valve  1046  may also be used for added safety. Following the gas shut-off valve  1046 , the fuel is piped to the radiant burner  1010  where it is mixed with air for controlled combustion. Electrical power for system is supplied from a source  1030  through a manually activated switch  1032 . 
   An electronic controller  1020  receives power from source  1030  and switch  1032  on line  1033 . After sensing on line  1045  that the gas valve  1044  is turned on, electronic controller  1020  provides initial ignition to the burner  1010  through line  1015  to spark igniter/flame sensor  1017 . The controller then monitors the existence of a flame via the flame sensor  1017  on line  1019 . And regulates the air flow into the burner by controlling the speed of the blower  1050  on line  1049 . The air flow control is performed in response to the manual setting of gas valve  1044  to maintain the fuel/air mixture at the desired level that provides substantially complete combustion on the surface of the burner. Other safety devices in the controller  1000  include an air flow sensor  1052  and a tip-over sensor  1054 . When either of these sensors are tripped, for the lack of air flow in the case of sensor  1052  or tip over of the unit in the case of sensor  1054 , the electrical controller deactivates the gas shut-off valve  1046  to cause the burner to be turned off. 
   It should be understood that the foregoing description of the embodiments is merely illustrative of many possible implementations of the present invention and is not intended to be exhaustive.