Abstract:
A naturally aspirated, fully aerated radiant burner and optional heat exchanger arrangement where the radiant burner has a generally enclosed cavity defined, at least in part, by fuel gas impermeable surroundings and a lower surface of fuel gas permeable burner element, wherein cavity preferably has two opening exposed to an oxidizer source. Sealingly coupled to openings are mix tubes, each having respective first ends and second ends, wherein first ends occupy openings and second ends extend into and are exposed to cavity. Fuel gas injectors, which during use are in fluid communication with fuel gas, are positioned to introduce fuel gas into mix tubes and entrain only slightly more air than needed for stoichiometric combustion. Pre-combustion gasses migrate to upper surface where stable stoichiometric combustion occurs, resulting in low CO and NOx emissions, increased wind resistance and elevated combustion gas temperatures Connecting the heat exchanger directly to the burner further increases its wind resistance and prevents dilution of the combustion gases by wind or free convection.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to controlled, low emissions, combustion and more particularly to pressurized hydrocarbon gas burners and most particularly to a liquid pressurized gas (LPG) burner/heat exchanger system that Includes a high efficiency heat exchanger working in conjunction with a fully aerated radiant burner. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Conventional gas external combustion apparatus traditionally use partially aerated fuel-air mixtures and require introduction of relatively large quantities of secondary air for complete combustion to occur. This dilution of the post combustion gases reduces heat transfer efficiencies into a heat transfer surface, such as a fluid container in a cooking or water heating system, e.g., a pot, or a commercial or residential hot water tank. Additionally, the volume of introduced secondary air is dependent on the apparatus&#39; natural convection and diffusion properties, which limit the driving pressure of the pre-combustion gases and excess air to pressures that can be attained only by the buoyancy effect of the hot rising gases—this mode of heat transfer is know as free convection. Thus, while partially aerated, free convection combustion apparatus are generally simple in construction and operation, they suffer from a loss of heat transfer potential through thermal dilution by secondary air and lack the ability, particularly in smaller scale implementations, to create significant heating through convection. 
         [0003]    In addition to the foregoing, apparatus relying on free convection heat transfer are further limited by the need to allow adequate space between the apparatus burner and target surface (container surface and/or heat exchanger) so that sufficient secondary air is available for complete combustion and so that the flame does not impinge upon, and is quenched by, the cooler target surface. Therefore, while minimizing the distance between the burner flame and the target surface increases heat transfer efficiencies, both inadequate air and flame quenching lead to elevated CO production, which is particularly undesirable in small and substantially enclosed environments. 
         [0004]    One approach used in the prior art to increase heat transfer efficiencies involves the use of a heat exchanger. A number of companies (for example, Cascade Designs Inc. and JetBoil, Inc.) offer portable gas combustion apparatus with heat exchangers that boost efficiency from conventional portable stove and pot combinations (35%-55%) to (45%-65%). However, because these apparatus are limited by free convection heat transfer coefficients and dilution of the combustion gases with secondary air, higher efficiency opportunities for apparatus of these designs are limited. Another approach used in the prior art to increase heat transfer efficiencies is to exploit the advantages of forced convection. Significantly higher heat transfer rates can be obtained if the combustion gases can be driven at elevated pressures (forced convection). U.S. Pat. Nos. 4,773,390 and 5,749,356 describe hot water heaters, for example, that combine forced convection and a heat exchanger to boost heat transfer efficiencies. In order to achieve this, however, the invention described in U.S. Pat. No. 4,773,390 requires a blower to introduce adequate air for combustion and the invention described in U.S. Pat. No. 5,749,356 demands that fuel and air be mixed via a gas-air feed circuit. Thus, both systems show increased complexity and/or infrastructure requirements over a naturally aspirated burner. 
         [0005]    While manufacturers of combustion-based heat transfer apparatus continually strive for increased combustion and heat transfer efficiencies, they must also address environmental concerns relating to combustion by-products. One such class of combustion by-products, nitrous oxides (NOx), is of particular concern with respect to domestic gas water heaters. Initial combustion of gases in most partially aerated burners occurs at high temperatures, which are conducive to nitrous oxide formation. U.S. Pat. Nos. 5,645,413; 6,446,581 and 6,508,207 claim naturally aspirated burners with reduced NOx emissions. However, these designs require the introduction of excess secondary air (between 20-100% excess air) and are thus subjected to the heat transfer penalties imposed by its diluting effects, as described above. Finally, staged-air combustion chambers, as described in U.S. Pat. No. 5,645,413, present a trade-off between NOx and CO emissions—“the lower the NOx . . . the more difficult it becomes to burn out CO.” (column 7). 
         [0006]    In view of the foregoing, a need exists for an external combustion, naturally aspirated heating apparatus that provides both inherent and exploitable heating abilities while mitigating NOx and CO emissions. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a naturally aspirated, fully aerated burner, an optional heat exchanger arrangement optimized for fluid containers, and systems incorporating the combination thereof. Burner embodiments of the invention use premixed fuel-air in conjunction with relatively low burner surface temperatures and incandescing surface combustion to efficiently create heat with minimal CO and NOx combustion by-products. High heat transfer values to containers exposed to the burner are achieved through forced convection of hotter, undiluted, combustion gases to optimized heat exchangers associated with the containers, which increase overall efficiency of system embodiments (70% to 85% in the embodiment described in detail herein), without adding excessive heat exchanger surface area or materially impeding the convective gas flow. An additional benefit realized by system embodiments of the invention is markedly increased resistance to the deleterious effects of wind, particularly in exposed conditions. 
         [0008]    Unlike free convention prior art burners that rely upon the introduction of secondary air to provide sufficient oxidizer for proper combustion, burner embodiments of the invention exploit forced convection principles. Thus, as heat output is increased, the driving pressure for forced convection is also increased, and heat transfer efficiency is generally constant over a wide range of heat level outputs. Because complete combustion is achieved at the burner element outer surface without the addition of secondary air, the optional heat exchanger can mate directly with the burner, eliminating the cooling effects of convecting air and making the burner essentially impervious to wind. An optional thermally activated fuel flow interrupt increases the safety of the burner by stopping fuel flow in the case of an overheat scenario. The result of this arrangement provides for a radiant burner that is has increased resistance to the deleterious effects of wind on the burner, that greatly increases the safety of operation of the radiant burner, and that significantly reduces the output of NOx and CO. 
         [0009]    Burner embodiments of the invention comprise a generally enclosed cavity defined, at least in part, by a fuel gas impermeable surrounding and an inner surface of a fuel gas permeable burner element, wherein the cavity has at least one opening exposed to an oxidizer source, preferably oxygen present in the ambient environment. Sealingly coupled to the at least one opening is fuel-air mixing element or mixing means having a first end and a second end, wherein the first end is exposed to and/or is fluidly coupled with the at least one opening of the cavity and the second end extends into and is exposed to and/or is fluidly coupled with the cavity. In a preferred embodiment, the mixing element or mixing means is a mix tube, which maximizes both the air entertainment and resulting momentum transfer as well as thorough mixing of the air with the fuel gas. As those persons skilled in the art will appreciate, any structure capable of mixing a gaseous fuel with a gaseous oxidizer, preferably air from the ambient environment, can be used as, or in place of, a mix tube, and therefore such structures are considered equivalent thereto. 
         [0010]    A fuel gas injector, which during use of the burner embodiments is in fluid communication with a source of fuel gas, is positioned to introduce fuel gas into each mix tube, preferably at or proximate to the first end, thereby encouraging momentum transfer from the fuel gas to the air when the air is also introduced at or proximate to this location. Variables such as fuel gas pressure, location of gas introduction, mixing element volume, orifice size and related parameters are assessed and established to ensure proper fuel-air ratios in support of proper combustion. For example, in a preferred embodiment, the fuel gas injectors, mix tubes, burner permeability and port area are designed such that the injectors entrain approximately 1-6% more oxidizer than necessary for stoichiometric combustion. In addition, the port area is sized in a way to optimize port loading and consequent combustion surface temperatures such that NOx and CO production is minimized. As a consequence, the resulting pre-combustion gas (fuel-air mixture) requires no additional oxidizer in order to achieve proper combustion, which would normally be introduced at the situs of combustion, thereby decreasing the temperature of the resulting convective flow. 
         [0011]    Because of the porosity of the burner element and the momentum transfer of the fuel gas to the fuel-air mixture, a pressure gradient exists between the cavity and an outer surface of the burner element. Consequently, pre-combustion gasses diffuse from an inner surface of the burner element, which is exposed to the cavity, to the burner element outer surface. Fully aerated, pre-combustion gasses at the outer surface of the burner element may then be ignited, such as by an igniter that is associated with the burner, whereupon combustion takes place. 
         [0012]    A feature of selected burner embodiments of the invention is the incorporation of a thermal fuse disposed between the fuel gas source and the gas injector(s). This fuse may be constructed from any material that will be predictably responsive to heat such that when exposed to heat higher than a certain temperature for an established period of time, the material changes form, which operates to interrupt fuel flow to the gas injector(s). In one series of embodiments, the fuse comprises a eutectic metal, such as a cadmium-lead-tin alloy, which is formed into a washer that operatively keeps a check valve between the gas source and the burner in the open position. Thus, in the event of a light-back or thermally derived malfunction, the increased temperature will cause the washer to undergo a phase change, such as from solid to glass or liquid, and thereby permit the check valve to close and terminate fuel gas delivery from the fuel gas source. 
         [0013]    In order to increase the efficiency of burner embodiments of the invention, containment vessels, such as pots, can be specially adapted to exploit the quantity and quality of heat output of such burners, as previously intimated. A primary mode of efficiency enhancement comprises the use of integrated or removable heat exchanging structure at or near the bottom of containment vessels. Such structure preferably comprises a plurality of fins, either as fin elements integral with the vessel or as fin bodies attachable to the vessel, arranged to maximize radiant and convective heat transfer of combustion gasses from the burner. Alternatively, efficiency enhancement comprises the use of heat exchanging structure at or near the outer surface of the burner element, which may or may not be removable. Efficiency can be further increased by maximizing the thermal absorptivity of the vessel surface to optimize radiative heat transfer. Each relevant containment vessel will have a bottom surface and a lower side surface that is linked to the bottom surface by a shoulder. 
         [0014]    The burners described and illustrated below provide a user with exceptional efficiency and significantly decreased undesirable combustion byproducts. For example, CO emissions are about 8 times less than a comparably sized conventional portable stove. Similarly, nitrogen oxides are significantly reduced (approximately 80-93%) when compared to commercially available competing portable stoves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an elevation view of an assembled burner and heat exchanger equipped pot system; 
           [0016]      FIG. 2  is a cross section elevation view of a burner; 
           [0017]      FIG. 2A  is a detailed cross section of a thermal fuse/trip that can be used in the embodiment shown in  FIG. 2 ; 
           [0018]      FIG. 3  is a cross section plan view of the burner of  FIG. 2 ; 
           [0019]      FIG. 4  is a cross section elevation view of a first heat exchanger equipped pot; 
           [0020]      FIG. 5A  is a perspective view of the first heat exchanger equipped pot wherein post pot manufacture fin elements are attached to the bottom of the pot and external covers and rings are removed for clarity; 
           [0021]      FIG. 5B  is a perspective view of the first heat exchanger equipped pot wherein fin bodies are integrated into the bottom of the pot during manufacture of the pot and external covers and rings are removed for clarity; 
           [0022]      FIG. 6  is a cross section elevation view of second heat exchanger equipped pot wherein a peripheral heat exchanger ring is employed to increase the surface area available for heat transfer; and 
           [0023]      FIG. 7  is a perspective view of a peripheral heat exchanger ring segment for use with the embodiment of  FIG. 6 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention as defined by the appended claims. Thus, the present invention is not intended to be limited to the embodiments show, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0025]    Unless otherwise noted herein, most parts of burner  10  and heat exchanger  90  are constructed from metal. Depending upon the part&#39;s application, the metal may be aluminum, steel, copper, brass or similar conventional metal. The selection of metal is primarily driven by thermal transfer considerations, although resistances to corrosion and high temperatures, as well as weight considerations are also valid criteria for material selection. In a preferred embodiment, burner element  60  comprises a porous metal foam material sold under the trademark METPORE by Porvair Advanced Materials, Inc. of Hendersonville, N.C. However, those persons skilled in the art will appreciate that other gas porous, heat resistant materials can be used, such as ceramics and metal-ceramic composites. 
         [0026]    Turning then to  FIGS. 2 and 3 , a burner embodiment of the invention is shown in cross section elevation and plan views, respectively. Burner  10  comprises metallic base  12 , which provides fuel delivery infrastructure  30  (discussed below) and which partially defines cavity  24 . Cavity  24  is further defined by metal surround  14  and burner element  60 . As will be described in more detail below, cavity  24  is generally sealed from the environment with two major exceptions. First, mix tubes  50   a  and  50   b  are sealingly attached to surround  14  and are exposed to the environment via proximal ends  52   a  and  52   b  (see  FIG. 3 ). Second, burner element  60  is porous to gasses (see  FIG. 2 ). As a result of this arrangement, gasses introduced at proximal ends  52   a  and  52   b  of mix tubes  50   a  and  50   b  may travel the length of the mix tubes and into cavity  24  through distal ends  54   a  and  54   b . Because of the momentum transfer of the fuel gas to the fuel-air mixture, a gas pressure gradient exists between cavity  24  and the environment at outer surface  64  such that gasses present in cavity  24  will diffuse through burner element  60  towards outer surface  64 . 
         [0027]    Fuel gas, such as Liquid Pressurized Gas (LPG), is delivered to burner element  60  in the following manner. An LPG bottle (not shown) is rotationally coupled to fuel delivery infrastructure  30 , as is best shown in  FIGS. 2 and 2A . To permit such coupling, fuel delivery infrastructure  30  includes inlet housing  27  having threaded portion  32 , preferably conforming to the B-188 standards, as described in EN  521 —Specifications for Dedicated Liquefied Petroleum Gas Appliances-Portable Vapour Pressure Liquefied Petroleum Gas Appliances, to ensure wide compatibility with gas bottle suppliers. Once securely coupled and referring to  FIG. 2A , probe  36  opens a valve in the LPG bottle and pressurized gas travels through probe  36  and into chamber  26 . Chamber  26  is generally defined by inlet housing  27  and seat  28 . Within chamber  26  are sealing plug  29  and compression spring  25 . Compression spring  25  provides an outward bias to sealing plug  29 , which is prevented from translational movement by seat  28  reacting against outlet housing  31  via thermal fuse body  38 . LPG occupies both chamber  26  and area  26 ′, which is in fluid communication with outlet conduit  40  via port  39  and prevented from escape to the environment by O-ring  34 . Outlet conduit  40  then permits LPG to discharge into pressure regulator  42  (see  FIG. 2 ). 
         [0028]    A feature of the disclosed arrangement is directed towards a thermal LPG interrupt that functions to autonomously stop the flow of gas from the container to the burner. As briefly described above and as best shown in  FIG. 2A , seat  28  functions to prevent sealing plug  29  from extending into contact with sealing surface  41 . In turn, seat  28 , which is in a compression mode through the bias imparted by spring  25  to sealing plug  29 , reacts against outlet housing  31  via thermal fuse body  38 . But for the presence of fuse body  38 , seat  28  would be urged to translate away from compression spring  25 , thereby permitting sealing plug  29  to come in sealing contact with sealing surface  41 , and thereby occlude further gas passage into outlet conduit  40 . Therefore, fuse body  38  is intentionally constructed to loose structural cohesion at or above a general temperature to prevent potentially dangerous conditions such as might be encountered during a “light back” or reverse ignition propagation event. While the ultimate determination of the appropriate temperature is a matter of design consideration, the disclosed embodiment contemplates thermal conditions of between about 145° C. to 200° C. as being candidate temperatures for a thermal trip. 
         [0029]    While those persons skilled in the art will appreciate the broad selection of candidate materials, particularly satisfying results have been objected when eutectic alloys are chosen. A benefit of using eutectic alloys concerns both the precise nature of their phase conversion and the very sharp transition provided by them. This second characteristic is of importance to the operational life of the burner; because the thermal fuse is in an axial compression mode, mechanical creep can occur, particularly at higher temperatures, thereby potentially decreasing the performance of the system during normal conditions. Creep is further limited by keeping thermal fuse body  38  well contained. By doing this, as the material creeps, it is forced to “flow” through small gaps between seat  28  and outlet  31 . This would require large shear stresses when fuse body  38  is solid but very low stresses once the fuse has melted. One alloy that has yielded favorable results comprises cadmium −18.2% wt.; lead −30.6% wt.; tin −51.2% wt. This alloy has a melting point of about 145° C.±1.5° C. 
         [0030]    Upon passing into outlet conduit  40 , the compressed gas is directed towards regulator  42  and valve assembly  44  for pressure and flow regulation. Regulated gas is then directed to both gas jets  48   a  and  48   b  via distribution manifold  46 , which in turn direct fuel gas into mix tubes  50   a  and  50   b . Entrainment of an oxidizer, in this case oxygen bearing air, occurs at the injector and throughout the length of the mix tube by drawing air into the mix tube at openings  16   a  and  16   b  to create pre-combustion gasses. Those persons skilled in the art will appreciate that other forms of oxidizer introduction could take place via the same or different structure. However, the present embodiment represents an efficient and cost-effective approach to the production of a combustible gas. Because the described method and related structure rely upon momentum transfer (a venturi effect is established at opening  16   a  and  16   b , which creates a localized area of low pressure, thereby drawing in ambient air to aid in combustion), mixing of the fuel gas with an oxidizer is accomplished efficiently and inexpensively. Moreover, because there are no moving parts, reliability and longevity are also increased. 
         [0031]    To optimize the introduction of air as an oxidizer and minimize the effects of the environment (primarily wind for portable burner operations), surrounding  14  is coaxially surrounded by perforated housing  18 . Consequently, a generally annular space is created between surrounding  14  and housing  18 , from which air is drawn into openings  16   a  and  16   b . In this manner, any wind impacting perforated housing  18  is diffused prior to entering opening  16   a  and  16   b.    
         [0032]    The fuel gas and air combination (pre-combustion gases) exit from ends  54   a  and  54   b  of mixing tubes  50   a  and  50   b , and enters cavity  24 , where upon it impinges heat transfer posts  56 . Because posts  56  are thermally coupled to base  12 , heat generated by burner  10  and transferred to base  12  by radiation, conduction and/or convention is partially removed by incoming cool pre-combustion gases contacting posts  56 . Beneficially, this drawing of heat from base  12  not only decreases the handling temperature of base  12 , but also increases the heat content of pre-combustion gas, which promotes more efficient combustion thereof. 
         [0033]    As noted earlier, during operation of burner  10 , a pressure gradient exists between upper surface  64  of burner element  60 , which is exposed to ambient conditions, and lower surface  62  of burner element  60 , which is exposed to slightly pressurized pre-combustion gases. After transport of pre-combustion gases from cavity  24  to upper surface  64 , a piezoelectric igniter (not shown) may be operated to initiate combustion of pre-combustion gasses, in a manner well known in the art. Upon ignition, combustion migrates to just below upper surface  64  of burner element  60 , and is prevented from further propagation by the low bulk thermal conductivity and small pore size of burner element  60 . At this point, burner  10  becomes a radiant burner with virtually no perceptible freely convective frame. The incandescing filaments at the burner surface help to sustain and catalyze combustion, even in the presence of wind. 
         [0034]    Screen  20  is provided as a protective feature to prevent unintentional physical contact with burner element  60  and to serve as an interface with cookware employing a heat exchanger as described in detail below. Both screen  20  and perforated housing  18  are secured to burner  10  by way of screen retainer ring  22 . 
         [0035]    As mentioned earlier, the fuel gas injectors, mix tubes, burner permeability and port area are designed in this embodiment such that the injectors entrain approximately 1-6% more oxidizer than necessary for stoichiometric combustion. This limits the energy diluting effects of excess air while ensuring sufficient oxidant for complete combustion. In addition, the port area is sized in a way to optimize port loading and consequent combustion surface temperatures such that NOx and CO production is minimized. When run at maximum output (approximately 9,000 BTU/hr in this embodiment), NOx concentrations average 11 ppm corrected to 3% oxygen, well below the industry accepted value of 55 ppm corrected to 3% oxygen and below Southern California&#39;s SCAQMD Rule  1121 , which limits NOx emissions of residential gas-fired water heaters to 15 ppm at 3% oxygen as of Jan. 1, 2008. When run in conjunction with the optional heat exchanger at 5,750 BTU/hr, air free CO emissions average 78 ppm. Testing shows that CO emissions drop as the stove power increases beyond this output. European standard EN  521  (Specifications for Dedicated Liquefied Petroleum Gas Appliances-Portable Vapour Pressure Liquefied Petroleum Gas Appliances) and CSA 11.2-2000 (American National Standard/CSA Standard for Portable Type Gas Camp Stoves) allow air free CO values of 2000 ppm and 765 ppm, respectively. 
         [0036]    While radiant burner  10  represents a significant advance in heating technology with respect to efficiency, emissions, wind resistance, safety and reliability, further advances have been achieved when this technology is used in conjunction with a heat exchanger purposefully adapted to extract the maximum amount of heat from burner  10 . As best shown in FIGS.  1  and  4 - 7 , heat exchanger  90  can be integrated into a fluid vessel, and more particularly vessel or pot  70 . The purpose of heat exchanger  90  is to efficiently extract heat generated by burner  10  by taking advantage of its combustion mode. In this respect, the mass flow and temperature attributes of heat generated by burner  10  are considered in the design of heat exchanger  90 . 
         [0037]    As shown in the several drawings, the constitution of heat exchanger  90  can take many forms. The ultimate selection of one form over another may be driven by design considerations such as the volume of vessel  70 , the nature of the liquid to be heated, the fluid dynamic properties of the post-combustion gasses, and similar factors. Thus, the presently illustrated embodiments are intended to show several variations, but are by no means representative of an exhaustive inventory of available heat exchangers adapted to exploit the combustion mode of the burner. However, the presently illustrated embodiments all attempt to maximize the surface area exposed to the radiant heat and combustion gasses from burner  10  without significantly increasing the pressure drop though the system, and consequently reducing air entrainment below stoichiometric levels. Thus, the illustrated embodiments employ a plurality of channels having relatively unobstructed exit paths where the channels maximize the distance the combustion gasses must travel from burner element  60  to the ambient environment. Additionally, constructing heat exchanger  90  out of high absorptivity material, or using high absorptivity coatings (e.g., hard anodizing) increases radiative heat absorption to maximize heat transfer efficiency. 
         [0038]    Turning first to  FIG. 5A , a weld-on heat exchanger arrangement is shown. Here, a plurality of fin elements  80  are formed separately from pot  70 , and subsequently attached to pot  70  such as by spot welding, brazing, laser welding or similar heating techniques to create a plurality of channels  86  through which combustion gasses may travel. Fin elements  80  are preferably constructed from aluminum by stamping or similar high volume creation means. Fin elements  80  are preferably formed for placement on bottom surface  78  of pot  70  in a spiral or involute pattern to maximize exposure time of the combustion gasses with the elements. The curved fin shape also acts to minimize thermal boundary layers of the flowing combustion gases, further increasing heat transfer.  FIG. 5B  shows a similar pattern of fin bodies  82  formed on bottom surface  78  of pot  70 , however, fin bodies  82  are integral with bottom surface  78 . In this embodiment, fin bodies  82  may be formed by machining the desired pattern in bottom surface  78  or during casting of bottom surface. While the thermal transfer rates from fin bodies  82  to pot  70  and overall durability are greater than the thermal transfer rates from fin elements  80  to pot  70  due to the more robust association of the former with the pot, manufacturing costs are higher. 
         [0039]    In addition to machining or casting methods for creating suitable fin bodies, a preferred means of manufacturing integral fin bodies is by impact extrusion processes. These processes provide the benefits of exceptional thermal conductivity (superior to that of casting), desirable surface finish for the cooking surface (superior to that of casting or machining), low weight (superior to that of casting and machining, which also generates avoidable waste) and low cost (superior to that of machining and welding). While there are size limitations using these processes, they are not material to the form factors commonly used in portable cookware. 
         [0040]    The embodiment of  FIG. 6  illustrates a perimeter heat exchanger arrangement that can be used in conjunction with the heat exchangers of  FIGS. 5A and 5B , or with other arrangements. By linking a plurality of perimeter elements  84  as shown in  FIG. 7 , for example, and surrounding the perimeter of pot  70  with such elements, waste heat exiting from channels  86 , for example, impinges upon perimeter elements  84  and is redirected along reduced diameter portion  74  of pot  70 . In this manner, additional surface area for heat exchange is created at both perimeter elements  84 , which are thermally linked to heat exchanger  90 , as well as directly to pot  70 . To prevent the unintentional migration of fluid in pot  70  from entering heat exchanger  90 , drip ring  76  is provided above reduced diameter portion  74 . 
         [0041]    Heat transfer and wind resistance can be further improved by mating the heat exchanger  90  to retainer ring  22 , eliminating the cooling effects of convecting air and making the burner essentially impervious to wind. As mentioned above, this is possible because complete combustion is able to take place without the addition of secondary air. In such embodiments, bottom surface  78  is not planar or flat. Again depending upon design parameters, bottom surface  78  can be conical or frusto-conical like, with the apex at the center of the vessel. Such a geometry will not only beneficially modify the residency of any combustion gasses during operation of a burner, but when used in conjunction with a burner such as burner  10  having screen  20 , will restrict the selection of containment vessels to those that properly mate with the burner. Alternatively, a plurality of surface features, such as convex or concave features, can be established in or on bottom surface  78  to alter the egress of post-combustion gasses to the environment.