Abstract:
An apparatus and method for forming a fusible coating or structure comprising a combustor that is operative to combust a fuel and contain the resulting flame to produce combustion products; components for cooling the combustion products to produce a hot carrier gas stream; and elements for introducing fusible material into the hot carrier gas stream; made lightweight and easier to operate through the addition of a continuous electronic ignition system and a single air manifold.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/459,631, filed Dec. 15, 2010, the disclosure of which patent application is incorporated by reference as if fully set forth herein. This application is also a continuation in part of U.S. patent application Ser. No. 12/657,211, filed Jan. 14, 2010, which claims the benefit of U.S. Provisional Patent Application No. 61/205,079, filed Jan. 14, 2009, the disclosures of which patent applications are incorporated by reference as if fully set forth herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. FA8651-04-C-0379 awarded by the United States Air Force. 
    
    
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not Applicable 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates to the formation of fusible coatings or structures (e.g., polymer or polymer composite coatings, or reinforced polymer coatings, as well as polymer, or reinforced polymer structures) via a thermal spray process. In particular, the invention relates to formation of these coatings or structures using a flameless thermal spray process. 
     The background art is characterized by U.S. Pat. Nos. 3,801,020; 3,958,758; 4,416,421; 4,694,990; 5,236,327; 5,285,967; 5,503,872; 5,932,293 and 7,216,814; by U.S. Patent Applications Nos. US2006/166153 and US2009/095823; and by International Patent Application No. PCT/US2007/009021; the disclosures of which patents and patent applications are incorporated by reference as if fully set forth herein. 
     BRIEF SUMMARY OF THE INVENTION 
     As used herein, the following terms and variations thereof have the meanings given below, unless a different meaning is clearly intended by the context in which such term is used. 
     “A,” “an” and “the” and similar referents used herein are to be construed to cover both the singular and the plural unless their usage in context indicates otherwise. 
     “About,” “approximately,” and “in the neighborhood of” mean within ten percent of a recited parameter or measurement, and preferably within five percent of such parameter or measurement. 
     “Comprise” and variations of the term, such as “comprising” and “comprises,” as well as “having” and “including” are not intended to exclude other additives, components, integers or steps. 
     “Exemplary,” “illustrative,” and “preferred” mean “another.” 
     In illustrative embodiments, the present invention is an improvement of embodiments of the invention described in the above related regular U.S. patent application. In these embodiments, the present invention comprises features to improve its usability, including an electronic ignition system, a single air supply manifold system, and an air shielded powder feed tube. These new features render the present invention easier to handle and easier to operate. 
     In illustrative embodiments of the present invention, an electronic ignition system allows the operator to light the flame with the touch of a button. In earlier embodiments, the operator ignited the burner using a butane torch or a welding spark striker. The electronic igniter makes it much easier for the operator to light the applicator. Electronic igniters are common in fixed applications such as gas hot water heaters, furnaces, and other home appliances. For these applications, low velocity gas streams are ignited using high voltage sparks ranging from 10 kilovolt (kV) to 15 kV. For the present invention, the gas velocities are too high for a conventional spark igniter. With illustrative embodiments of the present invention, a novel pilot tube electrode is used to create a region of low velocity gas to enable burner ignition at all applicator gas flow settings. 
     To generate the continuous 10 kV to 15 kV spark at the applicator required an additional novel technology. Spark generators using high turn ratio transformers and autotransformers are commonly found in home appliances and automobiles. These spark igniters are much too large to mount directly on the applicator of the present invention. The other commonly used alternative is to mount the spark igniter remotely and run a high voltage cable through the applicator umbilical. These cables are heavy and can over time leak high voltage, which could shock and startle the operator. For this application, a small high turn transformer, capacitor, and novel spark generation circuit were developed that only requires low voltage power. The power is supplied through the umbilical using a lightweight wire. 
     The applicator described in the above cross-referenced patent application and other thermal spray systems like the Xiom thermal spray system (Xiom Corp., West Babylon, New York) utilize multiple individually controlled air lines to provide the correct mix of air and fuel to get correct combustion and polymer melt conditions. These systems have numerous controls which make establishing applicator process conditions very difficult for the operator. Also, these multiple discrete air lines result in a large bundle of tubes running from a control cart to the applicator through an umbilical. These tube bundles make the umbilical very stiff and heavy. In preferred embodiments, the present invention uses a single air input line, which is split into various air streams and balanced at the applicator through a factory pre-adjusted manifold system that sets the correct air balance. 
     In an illustrative embodiment of the present invention, an air shielded powder feed tube allows the polymer to be sprayed without fouling the applicator. Earlier prototypes of the applicator, which did not have this feature, became fouled with polymer after extended use. The development of the air shielded feed tube allowed the applicants for patent to move the spray nozzle toward the outlet of the applicator shroud, which eliminated the fouling problem. In addition to this benefit, the shielded air feed tube produces a spray pattern superior to the unshielded design. The shielding air causes the polymer air stream in the feed tube to contract—just downstream of the feed tube exit—and then expand. This contraction and expansion results in an optimum spray pattern with very even powder distribution on the substrate. 
     In an illustrative embodiment, the invention comprises a thermal spray system for depositing a polymer, a polymer composite coating, or a structure onto a target substrate. In this embodiment, a stream of air is heated by a flame in a defined combustion zone, preferably using the Resodyn PTS-30 System (available from Resodyn Engineered Polymeric Systems of Butte, Mont.). In a preferred embodiment of the present invention, an igniter electrode assembly utilizes a nickel wire or another wire that is capable of withstanding a high temperature for the electrode. The wire has a nominal size of 12 American Wire Gauge (AWG). Nickel has a melting temperature of 1500° C. which is sufficient to withstand the heat at the face of the burner. In this embodiment, the electrode is installed in an electrode insulator which is made from a tube of alumina ceramic, such as Coorstek AD-998 (from Coorstek of Golden, Colo.), which has a dielectric strength of 220 Volts per millimeter. The electrode insulator preferably has a nominal outside diameter of 3/16 inch and a nominal inside diameter of 3/32 inch. The electrode insulator fits inside the electrode tube which as a nominal inside diameter of ¼ inch. The clearance between the inside of the electrode tube and the electrode insulator creates a 1/32 inch thick annular flow passage for the pilot propane gas and air mixture. 
     In this embodiment, the flow rate of the propane gas and air mixture that flows into the electrode tube is controlled by two 3/32 inch diameter electrode inlet hole(s). The hydraulic diameter of these inlet holes is significantly smaller than the hydraulic diameter of the annular flow passage between the inside diameter of the electrode tube and the electrode insulator. The electrode assembly is preferably optimized to create a pilot which has sufficient propane gas and air flow rate to ignite while keeping the pilot velocity low enough so that the spark ignites the pilot without blowing the spark out. 
     Several of the components in the illustrative igniter circuit described herein are common components which are known to those trained in the art. For example, the microcontroller used in this circuit is a CY8C24123A-24SXI manufactured by Cypress Semiconductor of San Jose, Calif., but any suitable microcontroller could be used. The STMicroelectronics FLC-200B U2 Fire Lighter Circuit thyristor type integrated circuit (from STMicroelectronics of Geneva, Switzerland) is designed to trigger using a 220 Volt alternating current (AC) signal, which is fundamental to this architecture. For this circuit, the charge capacitor must be sufficiently large to store enough energy to create a sufficiently strong spark. For this illustrative embodiment, the capacitor value used is 1 microfarad (μF). The spark transformer is a very high turn ratio transformer with an inductance ratio of 25:1. The transformer has a very high Q and a resonant frequency of 38.3 kHz. At that resonant frequency, the voltage ratio is 30:1. When the FLC-200B U2 Fire Lighter Circuit fires, a very high current pulse is induced in the spark transformer. Simply multiplying the 200V charge when by the voltage ratio of the spark transformer results in a 6 kV spike. The actual voltage is much higher because the capacitor and primary winding of spark transformer discharges very quickly. The primary winding of the spark transformer is an inductive load. The voltage across an inductive load is equal to the inductance multiplied by the rate of change of current over time. 
     In an illustrative embodiment, a single air supply system employs air flow amplifiers, manifolds, and flow restrictors to achieve the correct balance of air flow rates for all air streams. In this embodiment, non-coanda air amplifiers, like those disclosed in U.S. Pat. No. 4,046,492 assigned to Vortec Corporation of Cincinnati Ohio, are used. Preferably, all of the air amplifiers used achieve an air amplification ratio of nominally 10:1. 
     In an illustrative embodiment, the system air balance is achieved by first sizing and tuning the air amplifier for the combustion gas and air stream with the applicator being designed to operate in a power range from 15 kW to 30 kW. In this embodiment, the output power is directly proportional to the volume of propane gas metered into the applicator through the propane nozzle. With complete stoichiometric combustion of propane, the combustion energy released is 43.9 kilowatts per standard cubic feet per minute (kW/scfm). So, to achieve 15 kW-30 kW requires 0.34-0.68 scfm of propane gas. For complete stoichiometric burning, the ratio of air to propane is 25:1, so the air introduced into the combustion gas and air stream is 8.5 scfm to 17 scfm. The air amplifier for the combustion and air stream is preferably tuned to generate 8.5 scfm to 17 scfm of air to assure that a stoichiometric mixture of air and gas is fed into the heater assembly for complete combustion. 
     In an illustrative embodiment, the upper air amplifier air input stream is controlled by sizing the upper air amplifier air restrictor, which for the present invention has a nominal inside diameter of 3/16 inch. With this embodiment, the upper air amplifier air input stream is routed first to the air amplifier that draws in the diverter air inlet stream. A portion of the upper air amplifier air input stream flows past the diverter air amplifier venturi and creates the air stream diverter air amplifier high velocity air stream. The remaining portion of the upper air amplifier air input stream flows through two cooling air restrictor(s) and creates the cooling air amplifier high velocity air stream. 
     In this embodiment, the total volume flow rate of the upper air amplifier air input stream is 6 scfm to 12 scfm for power levels of 15 kW to 30 kW. Sixty percent of this total upper air amplifier air input stream exits through the diverter air amplifier high velocity air stream. The remaining 40 percent exits through the cooling air amplifier high velocity air stream. 
     In an illustrative embodiment, the shielded air feed tube assembly is comprised of two concentric tubes: the inner powder feed tube and the outer shielding air tube. Shielding air is forced through the annular space between these two tubes. The shielding air stream serves two purposes. First, it creates a high velocity air steam around the powder feed outlet stream. This air steam creates a barrier that prevents the particles from drifting backwards and fouling the applicator. Second, the shielding air stream forces the powder feed output stream to converge and then expand downstream of the powder feed tube distal tip. The applicants have found that this expansion and contraction creates a well disbursed, uniform coating spray pattern. 
     In an illustrative embodiment, the invention is a thermal spray gun comprising: a lower air manifold assembly that is operative to receive a propane input stream and a main air input stream, to divide said main air input stream into a combustion air amplifier air input stream and an upper air amplifier air input stream and, to combine said propane input stream with said combustion air amplifier air input stream to produce a combustion gas and air stream; a heater assembly that is operative to receive said combustion gas and air stream, said heater assembly comprising a perforated heater plate and a diverter cone; an igniter that is operative to ignite said combustion gas and air stream to produce a flame on said perforated heater plate, said igniter comprising an electrode tip; an upper air manifold assembly that is operative to receive said upper air amplifier air input stream and to produce a diverter air amplifier high velocity air stream and a cooling air amplifier high velocity air stream, said diverter air amplifier high velocity air stream being operative to draw a diverter air inlet stream into said upper air manifold assembly to produce an inner diverter outlet steam and an outer diverter outlet stream, and said cooling air amplifier high velocity air stream acting to draw a cooling air inlet stream into said upper air manifold assembly to produce a cooling air outlet stream; and a feed tube assembly that is operative to receive a powder feed input stream and to produce a powder feed outlet stream, said feed tube assembly comprising a feed tube tip; wherein said inner diverter outlet stream is operative to cool said feed tube tip and said diverter cone and to surround said powder feed outlet stream; and wherein said outer diverter outlet stream is operative to urge said flame radially outward. In another embodiment, said lower air manifold assembly is further operative to divide said main air input stream into said combustion air amplifier air input stream, said upper air amplifier air input stream, and a shielding air inlet stream; and said feed tube assembly is further operative to receive said shielding air inlet stream and to produce a shielding air inlet stream that is operative to cause said powder feed outlet stream to contract to produce a powder contraction zone and then to expand to produce a powder expansion zone. In another embodiment, said igniter further comprises a conductive electrode tube having and electrode inlet hole and an annular void, an electrode insulator and an electrode, said electrode being separated from said electrode tube by said electrode insulator. In another embodiment, said electrode inlet hole and an annular void receive a portion of said combustion gas and air stream and produce a pilot gas air mixture stream that is ignitable by a spark traveling across a spark gap between said tip electrode and said electrode tube. 
     In another illustrative embodiment, the thermal spray gun further comprises: an electronic ignition circuit comprising: a step-up transformer, a normally-open relay, an igniter switch, a micro-controller that energizes said normally-open relay when said igniter switch is closed, a capacitor, a spark transformer, and a fire starter integrated circuit; wherein, when a first voltage is applied to said capacitor after said igniter switch is closed, said fire started integrated circuit is operative to conduct a current, said capacitor is operative to discharge said current to ground, and said spark transformer is operative to impose a second voltage across said spark gap. In another embodiment, said electrode tip is mounted substantially flush with said perforated heater plate, protruding from said perforated heater plate, or recessed from said perforated heater plate. In another embodiment, the thermal spray gun further comprises: a normally-closed dead man&#39;s switch that is operative to shut off said propane input stream when said dead man&#39;s switch is not depressed. In another embodiment, the thermal spray gun of claim  1  further comprises: a shroud that surrounds at least a portion of said heater assembly; and a heat shield that surrounds at least a part of said shroud. In another embodiment, the thermal spray gun further comprises: an umbilical comprising the following components: a propane hose, a single air hose, a powder hose, a plurality of signal wires, a spark ignition power wire, and an electrical ground; wherein said components are bundled within a protective sleeve. In another embodiment, the thermal spray gun further comprises: a valve (e.g., a needle valve) for controlling the rate of flow of said combustion air amplifier air input stream. 
     In another illustrative embodiment, the invention is a thermal spray gun comprising: means for receiving a propane input stream and a main air input stream, dividing said main air input stream into a combustion air amplifier air input stream and an upper air amplifier air input stream, and combining said propane input stream with said combustion air amplifier air input stream to produce a combustion gas and air stream; means for receiving said combustion gas and air stream, said means for receiving comprising a perforated heater plate and a diverter cone; means for igniting said combustion gas and air stream and producing a flame on said perforated heater plate, said means for igniting comprising an electrode tip; means for taking up said upper air amplifier air input stream and producing a diverter air amplifier high velocity air stream and a cooling air amplifier high velocity air stream, said diverter air amplifier high velocity air stream acting to draw a diverter air inlet stream into said means for taking up said upper air amplifier air input stream to produce an inner diverter outlet steam and an outer diverter outlet stream, and said cooling air amplifier high velocity air stream acting to draw a cooling air inlet stream into said means for taking up said upper air amplifier air input stream to produce a cooling air outlet stream; and means for accepting a powder feed input stream and producing a powder feed outlet stream, said means for accepting comprising a feed tube tip; wherein said inner diverter outlet stream is operative to cool said feed tube tip and said diverter cone and to surround said powder feed outlet stream; and wherein said outer diverter outlet stream is operative to urge said flame radially outward. In another embodiment, said means for receiving is further operative to divide said main air input stream into said combustion air amplifier air input stream, said upper air amplifier air input stream, and a shielding air inlet stream; and wherein said means for accepting is further operative to receive said shielding air inlet stream and to produce a shielding air inlet stream that is operative to cause said powder feed outlet stream to contract to produce a powder contraction zone and then to expand to produce a powder expansion zone. 
     In another illustrative embodiment, the invention is a method for thermal spraying comprising: receiving a propane input stream and a main air input stream in a thermal spray gun, dividing said main air input stream into a combustion air amplifier air input stream and an upper air amplifier air input stream, and combining said propane input stream with said combustion air amplifier air input stream to produce a combustion gas and air stream; receiving said combustion gas and air stream in a means for receiving, said means for receiving comprising a perforated heater plate and a diverter cone; igniting said combustion gas and air stream and producing a flame on said perforated heater plate; taking up said upper air amplifier air input stream and producing a diverter air amplifier high velocity air stream and a cooling air amplifier high velocity air stream, said diverter air amplifier high velocity air stream educting a diverter air inlet stream to produce an inner diverter outlet steam and an outer diverter outlet stream, and said cooling air amplifier high velocity air stream educting a cooling air inlet stream to produce a cooling air outlet stream; and accepting a powder feed input stream and producing a powder feed outlet stream; wherein said inner diverter outlet stream is operative to surround said powder feed outlet stream; and wherein said outer diverter outlet stream is operative to urge said flame radially outward. In another embodiment, the method further comprises: dividing said main air input stream into said combustion air amplifier air input stream, said upper air amplifier air input stream, and a shielding air inlet stream; and receiving said shielding air inlet stream to produce a shielding air inlet stream that is operative to cause said powder feed outlet stream to contract to produce a powder contraction zone and then to expand to produce a powder expansion zone. 
     In another illustrative embodiment, the invention is a thermal spray gun comprising: a manifold assembly that is operative to receive a propane input stream and a main air input stream, to divide said main air input stream into a combustion air input stream and a cooling air input stream, to combine said propane input stream with said combustion air input stream to produce a combustion gas and air stream, to divide said cooling air input stream into a diverter air amplifier high velocity air stream and a cooling air amplifier high velocity air stream, said diverter air amplifier high velocity air stream being operative to draw a diverter air inlet stream into said manifold assembly to produce an inner diverter outlet steam and an outer diverter outlet stream, and said cooling air amplifier high velocity air stream acting to draw a cooling air inlet stream into said manifold assembly to produce a cooling air outlet stream; a heater assembly that is operative to receive said combustion gas and air stream, said heater assembly comprising a heater plate; an igniter that is operative to ignite said combustion gas and air stream to produce a flame on said heater plate; and a feed tube assembly that is operative to receive a powder feed input stream and to produce a powder feed outlet stream. In another embodiment, said manifold assembly is further operative to divide said main air input stream into said combustion air amplifier air input stream, said upper air amplifier air input stream, and a shielding air inlet stream; and wherein said feed tube assembly is further operative to receive said shielding air inlet stream and to produce a shielding air inlet stream that is operative to cause said powder feed outlet stream to contract to produce a powder contraction zone and then to expand to produce a powder expansion zone. In another embodiment, said igniter further comprises a conductive electrode tube having an electrode inlet hole and an annular void, an electrode insulator and an electrode, said electrode being separated from said electrode tube by said electrode insulator. In another embodiment, said electrode inlet hole and an annular void receive a portion of said combustion gas and air stream and produce a pilot gas air mixture stream that is ignitable by a spark traveling across a spark gap between said tip electrode and said electrode tube. 
     In another embodiment, the thermal spray gun further comprises: an electronic ignition circuit comprising: a step-up transformer, a normally-open relay, an igniter switch, a micro-controller that energizes said normally-open relay when said igniter switch is closed, a capacitor, a spark transformer, and a fire starter integrated circuit; wherein, when a first voltage is applied to said capacitor after said igniter switch is closed, said fire started integrated circuit is operative to conduct a current, said capacitor is operative to discharge said current to ground, and said spark transformer is operative to impose a second voltage across said spark gap. In another embodiment, said electrode tip is mounted substantially flush with said perforated heater plate, protruding from said perforated heater plate, or recessed from said perforated heater plate. In another embodiment, the thermal spray gun further comprises: a normally-closed dead man&#39;s switch that is operative to shut off said propane input stream when said dead man&#39;s switch is not depressed. In another embodiment, the thermal spray gun further comprises: a shroud that surrounds at least a portion of said heater assembly; and a heat shield that surrounds at least a part of said shroud. In another embodiment, the thermal spray gun further comprises: an umbilical comprising the following components: a propane hose, an air hose, a powder hose, a plurality of signal wires, a spark ignition power wire, and an electrical ground. 
     In another illustrative embodiment, the invention is a thermal spray gun comprising: an umbilical comprising a gaseous fuel hose, a powder hose, and an air hose that is operative to convey a main air input stream; a manifold assembly that is operative to divide said main air input stream into a plurality of air streams, one of which is a combustion gas and air stream and another of which is a hot air stream; a heater assembly that is operative to receive and combust said combustion gas and air stream; a thermal spray gun on-off switch that is operable to turn the thermal spray gun off when released by an operator; a spark ignition electrode assembly that is operative to ignite said combustion gas and air stream within said heater assembly when activated by said operator; a shielded powder tube assembly comprising a powder feed tube that is shielded from said hot air stream by a shielding air tube powder feed tube and that extends into said hot air stream. In another embodiment, said umbilical comprises a gaseous fuel hose, a powder hose, and a single air hose that is operative to convey a main air input stream. In another embodiment, said powder feed tube extends about three inches into said hot air stream. 
     Further aspects of the invention will become apparent from consideration of the drawings and the ensuing description of exemplary embodiments of the invention. A person skilled in the art will realize that other embodiments of the invention are possible and that the details of the invention can be modified in a number of respects, all without departing from the concept. Thus, the following drawings and description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The features of the invention will be better understood by reference to the accompanying drawings which illustrate exemplary embodiments of the invention. In the drawings: 
         FIGS. 1A and 1B  are schematic perspective diagrams of illustrative embodiments of a thermal spray system in accordance with the invention, including a spray gun applicator, material and gas supply and controls. 
         FIG. 1C  is a schematic perspective diagram of an illustrative embodiment of a vibrating fluidized bed hopper providing superior powder transport capabilities. 
         FIG. 2  is a schematic block diagram of an illustrative embodiment of a heater system which produces a hot gas carrier stream. 
         FIG. 3  is a schematic flow diagram of a thermal spray process in accordance with an illustrative embodiment of the invention. 
         FIG. 4  is a schematic diagram depicting an illustrative embodiment of means for using a fluid amplifier geometry to supply excess combustion air and dilution cooling air using a small supply of compressed air relative to the total air requirement. 
         FIG. 5  is a schematic diagram depicting an illustrative embodiment of means for using a fluid amplifier geometry to supply and pre-mix oxidant and fuel to a combustion apparatus. 
         FIG. 6  is a schematic cross-sectional view of a preferred embodiment of a spray gun applicator for a thermal spray system in accordance with the invention. This view illustrates how a hot carrier gas is created in this embodiment of the invention. 
         FIG. 7  is a cross-sectional view of a preferred embodiment of a spray gun applicator for a thermal spray system in accordance with the invention. This view illustrates how fusible powder is entrained into a hot carrier gas in the embodiment of the invention presented in  FIG. 6 . 
         FIG. 8A  is an exploded isometric view of a preferred embodiment of means for deflecting or diverting excess air into a flame and across a powder injection nozzle. 
         FIG. 8B  is a perspective view of a preferred embodiment of a subassembly comprising a powder nozzle, a powder tube, a deflector and a burner plate. 
         FIG. 8C  is a cross sectional view of the subassembly shown in  FIG. 8B . 
         FIG. 9A  is a diagram showing flame length without an air deflector 
         FIG. 9B  is a diagram illustrating the flame shorting effect of the air deflector. 
         FIG. 10  is a front elevation view of a preferred embodiment of a spray gun applicator with an annular burner plate disposed coaxially to a powder injection nozzle and excess air deflector. 
         FIGS. 11A and 11B  are diagrams that illustrate burner plates geometries that mitigate combustion noise. 
         FIGS. 12A-12D  are illustrations of combustion chambers that were experimented with. 
         FIG. 13  is a perspective view of another illustrative embodiment of the single air source applicator of the present invention. 
         FIG. 14  is an exploded view of an illustrative embodiment of the single air source applicator of the present invention. 
         FIG. 15  is a cross-sectional view of an illustrative embodiment of the umbilical of the present invention. 
         FIG. 16  is a cross-sectional view of an illustrative embodiment of the single air source applicator of the present invention. 
         FIG. 17  is a perspective view of the heater assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 18A  is an elevation (front) view of the heater assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 18B  is a cross-sectional view of the heater assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 19  is a perspective view of the electrode assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 20  is a cross-sectional view of the electrode assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 21  is a block diagram of the electronic ignition system in accordance with an illustrative embodiment of the present invention. 
         FIG. 22  is an exploded view of the lower air manifold assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 23  is a cross-sectional view of the lower air manifold assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 24  is an exploded view of the upper air manifold assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 25A  is a front view of the upper air manifold assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 25B  is a cross-sectional view of the upper air manifold assembly in accordance with an illustrative embodiment of the present invention. 
         FIG. 26  is a cross-sectional view of the single air supply applicator showing the air shielded feed tube in accordance with an illustrative embodiment of the present invention. 
         FIG. 27  is an isometric view of the shielded air feed tube assembly. 
         FIG. 28  is a cross-sectional view of the shielded air feed tube assembly. 
         FIG. 29  is a cross-sectional view of the shielding air tube inlet section in accordance with an illustrative embodiment of the present invention. 
         FIG. 30  is a cross-sectional view of the feed tube exit in accordance with an illustrative embodiment of the present invention. 
     
    
    
     The following reference numerals are used to indicate the parts and environment of an illustrative embodiment of invention on the drawings:
           1  thermal spray system     2  cart     3  umbilical cable, umbilical     4  spray gun applicator     5  air supply     6  fluidized bed hopper, hopper     7  propane/fuel and air/gas controls     8  propane/fuel, combustible fuel gas, combustible fuel, fuel gas     9  primary air, primary oxidant gas, motive air     10  mixing chamber     11  combustion chamber     12  excess air, excess oxidant gas     13  cooling or dilution air, cooling or dilution gas     14  hot carrier gas     15  burner nozzle, burner plate     16  vibrator     17  propane tank     28  powder injection nozzles/nozzle     29  fusible powder entrained in hot gas     30  fluid amplifier, second fluid amplifier     31  compressed air     32  annular manifold     33  annular nozzle     34  Coanda profile     35  low pressure area     36  pre-mix fluid amplifier, pre-mixer     52  flame/combustion gas     100  mixing and combustion step     102  flame anchoring step     104  combustion containment step     106  temperature reduction step     108  create and project carrier gas stream step     208  propane fuel gas nozzle     209  educted primary air, additional air, additional oxidant     210  combustible gas mixture     211  deflector, gas diverter     212  educted excess air     228  powder transport tube     229  powdered coating material, fusible powder, powder     231  round hole mesh     233  square mesh     301  single air source applicator     302  heater assembly     303  electrode assembly, igniter     304  upper air manifold assembly, first air manifold     305  feed tube assembly     306   a  left hand grip     306   b  right hand grip     307  igniter circuit board, electronic ignition circuit     308  lower air manifold assembly, second air manifold     309  shroud     310  heat shield     311  support handle     312  main air input stream     313  propane input stream     314  main air tube     315  propane input tube     316  propane valve     317  propane valve lever     318  feed switch button, robust feed switch     319  combustion gas and air stream     320  powder feed input stream     321  diverter air inlet stream     322  cooling air inlet stream     323  powder feed outlet stream     324   a  inner diverter outlet stream     324   b  outer diverter outlet stream     325  cooling air outlet stream     326  diverter cone     327  perforated heater plate     328  electrode tip     329  electrode inlet hole     330  electrode tube     331  electrode insulator     332  electrode     333  electrode seal plug     334  electrode lead wire     335  pilot gas air mixture stream     336  spark gap     337  lower air manifold assembly, lower air manifold     338  propane nozzle     339  combustion air amplifier base     340  combustion air amplifier tube     341  combustion air amplifier shim     342  combustion air amplifier air input stream     343  upper air amplifier air input stream     344  upper air amplifier manifold     345  cooling air restrictor     346  upper air amplifier input tube     347  divert air amplifier input tube     348  upper air amplifier air restrictor     349  cooling air amplifier shim     350  diverter air amplifier shim     351  cooling air amplifier venturi     352  diverter air amplifier venturi     353  diverter air amplifier high velocity air stream     354  cooling air amplifier high velocity air stream     355  shielded air feed tube assembly, shielded powder tube assembly     356  shielded air supply tube     357  powder feed tube     358  shielding air tube     359  shielding air manifold     360  shielding air inlet     361  powder feed hose barb     362  shielding air inlet stream     363  shielding air stream     364  powder contraction zone     365  powder expansion zone     367  hose barb fitting     370  dead man&#39;s switch     371  shielding air hose     373  propane hose     375  air hose     377  cable     383  protective sleeve, sleeve     384  igniter button       

     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1A and 1B , illustrative embodiments of thermal spray system  1  are presented. In this embodiment, thermal spray system  1  includes cart  2 , spray gun applicator  4  and umbilical  3  connecting spray gun applicator  4  to cart  2 . Fluidized bed hopper  6  and propane tank  17  are mounted on cart  2 . Spray gun applicator  4  is preferably portable and has a handle grip. In this embodiment, spray gun applicator  4  has conduits for passing powdered coating materials, combustible fuel gas, oxidant gas, excess and cooling gas and compressed air through spray gun applicator along a path or a plurality of paths. Spray gun applicator  4  also includes an assembly mounted on a distal end portion of the gun body including a nozzle for directing and controlling the hot gas flow and a channel or plurality of channels for ejecting powdered materials into the hot gas flow and a means for supplying coating material to spray gun applicator  4 . 
     In this embodiment, material is supplied to spray gun applicator  4  by means of a fluidized bed hopper  6 . The rate of supply is controlled by two venturis (not shown). The first venturi transports a stream of the powder material particles in compressed gas from fluidized bed hopper  6  to umbilical  3 . The second venturi adds additional transport air to the umbilical  3  and ejects the stream of powder material particles into spray gun  4 . Each of the first venturi and second venturi is independently controlled by a different individual stream of compressed gas. Fluidized bed hopper  6  is commercially available in several hopper sizes from a number of manufacturers, such as Powder Parts Inc., Elgin, Ill. 60123. 
     Referring to  FIG. 1C , a schematic diagram of an illustrative embodiment of the invention is presented. In this embodiment, fluidized bed hopper  6  is mounted to a suspended plate to which a vibrator  16  is attached in order to vibrate the fluidized bed hopper assembly. The vibrator is added to fluidized bed hopper  6  to assist in de-agglomerating powdered materials within hopper  6  and to assist in fluidizing the powdered material. Vibrators are commonly added to powder transport systems to shake boxed powdered materials and such box shakers may be purchased from several manufacturers, such as Powder Parts Inc., Elgin, Ill. 60123. Vibrators are not added to background art fluidized bed hopper systems because the types of powder used with typical commercial powder spray equipment only requires one fluidization technique, that is, use of a box shaker to vibrate a box of powder or a fluidized bed hopper, but not both fluidization techniques. 
     In a preferred embodiment, a combination of vibrator  16  and fluidized bed hopper  6  provides superior powder transport capabilities. The combination is effective at de-agglomerating and fluidizing powders for transport between fluidized bed hopper  6  and spray gun applicator  4  through a powder hose within umbilical  3 , with the types of thermoplastic powders used to create thermoplastic fusible coatings. 
     The thermal spray system described herein may be used for depositing a variety of coating materials, including zinc, aluminum, zinc-aluminum alloy, ferrous metal alloys, copper, copper alloys, ceramics, carbon, graphite and combinations thereof. They may also be used for depositing other materials, such as colorants, electrically conductive materials, fluorescent materials, phosphorescent materials, anti-fouling agents, reflective materials, radar absorbent materials, anti-microbials, microballoons, foaming agents, leveling agents, lubricants, ultraviolet (UV) protectors and combinations thereof. Still other materials suitable for deposition using thermal spray system  4  include thermoplastic or thermoset polymeric materials, such as epoxy resins, polyurethanes, polyethers, nylons, polyesters, polycarbonates, polyethylene, polypropylene, acrylic polymers, polyvinylchloride (PVC) resins, fluorocarbon polymers, ethylenevinylacetate (EVA), ethyleneacrylicacid (EAA), acrylonitrilebutadienestyrene (ABS), polyetheretherketone (PEEK), Polyvinylidenfloride (PVDF), silicones and chemical or physical combinations thereof. Coating materials may be combined with other materials. Particle sizes for the coating materials may range from about 5 microns to about 5,000 microns. 
     Referring to  FIG. 2 , a schematic diagram of an illustrative embodiment of the invention is presented. In this embodiment, combustible fuel  8 , typically a gas, for example, propane, and oxidant  9 , typically air, are mixed prior to combustion chamber  11  (e.g., in mixing chamber  10 ) or within combustion chamber  11 , at near (approximately) stoichiometric ratios. As used herein, the stoichiometric ratio is the exact ratio of fuel molecules that will combine with oxidant molecules to yield a complete combustion reaction. Combustible fuel  8  and oxidant  9  may also be mixed at sub-stoichiometric ratios (rich in combustion fuel  8 ) with additional oxidant  9  brought in later in order to complete the combustion reaction. Combustion occurs within combustion chamber  11  and produces combustion products. Excess air or other gas  12  is next introduced to the combustion process in order to complete combustion and begin the cooling of the combustion products. Cooling or dilution gas  13 , typically air, is finally introduced near the forward end of combustion chamber  11  to reduce the gas temperature to the final desired process temperature and to produce hot carrier gas stream  14 . Here, “near” means located closely in space to the object it precedes. In addition to propane, other gaseous fuels, such as acetylene, butane, isobutane, hydrogen, or natural gas may be used as the combustible fuel, as well as atomized, or vaporized liquid fuels such as kerosene, white gasoline or diesel fuel. 
     Referring to  FIG. 3 , a process flow diagram of an illustrative embodiment of the invention is presented. In this embodiment, there are five steps involved in creating a flameless heat suitable for processing polymer powders using a combustion process. First, in mixing and combustion step  100 , fuel  8  and oxidant  9  are mixed within an appropriate range of ratios (fuel/oxidant) and exposed to a critical ignition temperature which causes combustion to occur. Second, in flame anchoring step  102 , the flame from combustion is “anchored” in order to provide a stable ignition temperature for the combusting mixture. Third, in combustion containment step  104 , combustion products are contained within an enclosed or partially enclosed volume. Fourth, in temperature reduction step  106 , the temperature of the combustion products is reduced to the desired process temperature. Fifth, in create and project carrier gas stream step  108 , a carrier gas stream having the appropriate process temperature is created and projected from the outlet of the heater unit, preferably toward a target. 
     In the embodiment of  FIG. 3 , in order to achieve appropriate process temperature conditions, the flame is anchored within the combustion chamber. Otherwise, the flame would exit the nozzle and either extinguish due to overly lean conditions, or burn outside of the nozzle, causing the fusible particles to degrade as explained above. In order to anchor a flame, the velocity of fuel/oxidant gas mixture is reduced to a level at which the combustion reaction can occur and a proper residence time is provided for the combustion reaction to complete. Velocity reduction is achieved in certain embodiments disclosed herein by influencing back flowing eddies in the gas stream through the use of a burner nozzle. The burner nozzle may be of the form of a blast nozzle or that of a perforated flame anchoring plate within an enclosed or partially enclosed volume. 
     A person having ordinary skill in the art would know that a variety of other flame anchoring means are used in flame systems, such as stoves and fueled jets. These flame anchoring means may also be incorporated into embodiments of the invention. Thus, the foregoing examples provide a basic insight into the process of flame anchoring and should not be construed as limitations on the invention. 
     The heat of combustion at stoichiometric conditions for burning propane in air is 1,980° C. This temperature is too high to be contained by most common refractory materials. For example, high temperature steel alloys have a service temperature of 537° C. Nickel-chromium-iron alloys are used up to 677° C. Even ceramic coated jet engine parts only operate at a maximum temperature of 1,371° C. Therefore, background art flame generating devices are configured so that the flame burns outside the device architecture in free air. For these reasons, in certain embodiments of the invention, in order to contain combustion, film cooling on the flame containment surfaces and heat transfer management are employed. 
     The desired process temperature for a thermoplastic sprayer device is a hot gas temperature that exits the device in the neighborhood of 700° C., but could range from 100° C. to around 1,000° C. Here, “around” means “approximately” as it is defined above. Most fusible materials are processed in this temperature range. Because combustion temperatures are much higher than preferred fusible material processing temperatures, and to provide a stream of heated carrier gas, in illustrative embodiments of this invention, excess air  12  and cooling gas  13  are introduced to the process during combustion and after combustion is completed. 
     Referring to  FIG. 4 , a schematic diagram of burner nozzle  15  contained within combustion chamber  11  shows how excess air  12  and cooling air  13  may be supplied by fluid amplifier  30 . In this embodiment, fuel gas  8  and oxidant  9  enter burner  15  from the left. Compressed air  31  enters via annular manifold  32 . Compressed air  31  is throttled through an annular nozzle  33  at a high velocity to create a primary airstream. This primary air stream adheres to a Coanda profile  34 , which is an annular convex curve in this case. A low pressure area is created at the center  35  which induces (draws) a high volume of surrounding excess air  12  and cooling air  13 , into the air stream, thus amplifying the primary air flow rate typically by an order of magnitude. The compressed air along with the induced air supplies the total excess air  12  and cooling air  13  required to produce flameless hot carrier gas  14  without requiring a high volume blower, i.e., a relatively small amount of compressed air becomes adequate for supplying much larger amounts of excess combustion and cooling air. 
     Coanda or attached flow fluid amplifiers are known in the art of fluidics. It is the coupling of a fluid amplifier to a burner or flame tube located within combustion chamber  11  that provides at least two functions. First, excess air  12  serves to complete combustion and begin cooling the flame. Second, the cooling or dilution air  13  serves to further reduce the temperature of the combustion products to achieve the desired flameless hot carrier gas for processing of polymer powders or other materials. Both described functions are accomplished using relatively low quantities of compressed air by means of a Coanda fluid amplifier. 
     Referring to  FIG. 5 , a schematic diagram of an illustrative embodiment of the invention is presented showing how a Coanda pre-mix fluid amplifier  36  may serve to pre-mix fuel gas  8  and oxidant  9 . Fuel gas  8 , such as propane, is metered through propane fuel gas nozzle  208  to pre-mix fluid amplifier  36  acting as a pre-mixer. Motive air  9  is introduced to pre-mix fluid amplifier  36  and as previously described, the geometry of the pre-mix fluid amplifier  36  draws in additional fluid, in this case additional oxidant  209 , e.g., air. Pre-mixed fuel/oxidant  8 ,  9  is then delivered via a first fluid path to a flame source, e.g., burner  15 , located inside a combustion chamber  11 , said combustion chamber being located within an exterior surface. A second fluid amplifier  30 , previously discussed, may then be used to reduce the temperature of the combustion products (e.g., a combustion gas) in order to produce hot carrier gas  14 . 
     Background art venturi style eductors generally do not provide enough primary air to create a stoichiometric mixture and therefore tend to burn rich and require additional oxidant air at the burner. This problem is solved by the applicants by de-coupling the propane gas flow  8 , which is typically the motive flow in a pre-mix venturi eductor, from the air venturi and instead using an independent Coanda pre-mix fluid flow amplifier  36 , run by primary air  9  and educting additional air  209 , in combination with propane fuel gas nozzle  208 , e.g., a propane jet orifice, that discharges into the entrance of pre-mix fluid amplifier  36 . 
     Referring to  FIG. 6 , an illustrative embodiment is presented that incorporates many of the features discussed previously into hand held spray gun applicator  4 . In this embodiment, propane  8  is throttled through a propane fuel gas nozzle  208  into pre-mix fluid amplifier  36 . Primary air  9  is introduced to the pre-mix fluid amplifier  36  and, through fluid amplification, additional primary air  209  is educted into pre-mix fluid amplifier  36  where the gases are mixed to create a stoichiometric combustible gas mixture  210 . Combustible gas mixture  210  is introduced to mixing chamber  10  which functions as a plenum to uniformly distribute combustible gas mixture  210  across burner plate  15  via a first fluid path. A flame, flame front, or series of smaller flames  52  is created and is anchored by the burner plate  15 , burner plate  15  thereby acting as a flame source. Motive excess air  12  is used with second fluid amplifier  30  to educt additional excess air  212  into and through the center of spray gun applicator  4  via a second fluid path. Excess air  12 ,  212  is drawn around powder transport tube  228  and flows to deflector  211 . Here, “around” means on all or various sides. Deflector  211  diverts excess air  12 ,  212  into flame  52 . Excess air  12 ,  212  is mixed with flame  52  which insures complete combustion and begins to cool the combustion gas. Deflector  211  also diverts excess air  12 ,  212  across powder injection nozzle  28  and keeps the nozzle  28  cool so that powdered coating materials do not stick to and foul the nozzle  28 . Cooling or dilution air  13  is emitted through an annular orifice via a third fluid path, which serves to keep the walls of combustion chamber  11  and the exterior surface of thermal spray gun  4  from overheating and to further cool the combustion products/combustion gas and create hot carrier gas  14 . 
     Referring to  FIG. 7 , a diagram is presented that illustrates how a gas-particle mixture, e.g., fusible powder  229 , is entrained in hot carrier gas  14  in the embodiment of the spray gas applicator presented in  FIG. 6 . Fusible powder  229  is transported though powder transport tube  228  to powder injection nozzle  28 . Fusible powder  229  then mixes with hot carrier gas  14  and becomes fusible powder entrained in hot gas  29 . 
     Referring to  FIG. 8A , an exploded isometric view of a preferred embodiment of air deflector  211 , powder nozzle  28  and powder transport tube  228  is presented. Air deflector  211  serves to mix excess air  12 ,  212  with the flame in order to rapidly complete combustion and allow the flame to remain within combustion chamber  11 . 
     Referring to  FIG. 8B  a perspective view of a preferred embodiment of a subassembly comprising powder nozzle  28 , powder tube  228 , deflector  211  and burner plate  15  is presented. 
       FIG. 8C  presents a cross sectional view of the subassembly shown in  FIG. 8B . In this embodiment, powder nozzle  28  is disposed concentric to and attached to powder tube  228 . Deflector  211  is disposed concentric to powder nozzle  28 . There is an annular space between deflector  211  and powder nozzle  28  to allow for gas flow. Burner plate  15  is disposed concentric to deflector  211 . There is also an annular space between burner plate  15  and deflector  211  to allow for gas flow. There is also a standoff space between burner plate  15  and deflector  211  to allow for gas flow. 
     Referring to  FIGS. 9A and 9B , schematic diagrams are presented that show that without air deflector  211 , the flame is 8 inches to 10 inches long when operating at 120,000 BTU per hour. With the air deflector  211  the flame is reduced to approximately 1 inch long when operating at 120,000 BTU per hour. 
     Referring to  FIG. 10 , a front elevation view of an embodiment of spray gun applicator comprising burner plate  15 , deflector  211  and powder injection nozzle  28  is presented.  FIGS. 11A and 11B  show burner plate designs that mitigate burner noise. Burner noise is problematic with many burner designs and becomes evident as loud screech noises. The applicants discovered that burner plate geometries that served to “break-up” the flat face of burner plate  15  were effective at mitigating noise.  FIG. 11B  illustrates a preferred embodiment. In this view, burner plate  15  is a combination of perforated round hole mesh  231  in combination with an annular ring of square hole mesh  233  around the perimeter.  FIG. 11A  shows a geometric configuration that works to some extent but not as well as the preferred embodiment shown in  FIG. 11B . 
     Referring to  FIGS. 12A-12D , the applicants discovered that the shape of the semi-enclosed combustion chamber  11  was important in keeping the flame from exiting the chamber and in preventing the powder injection nozzle  28  from heating up. A preferred embodiment of combustion chamber  11  has the shape of a diverging frustum of a cone as illustrated in  FIG. 12D . This shape was determined through experimentation with converging, straight, and diverging shapes of different lengths. The shape of the diverging cone enables the hot gases from combustion chamber  11  to expand. Hence, the flame is not propelled out of combustion chamber  11  but stays anchored to burner plate  15 . The applicants also discovered that the diverging shape also discouraged the heating up of powder nozzle  28 . In contrast, straight walled and converging shapes for combustion chamber  11  caused powder nozzle  28  to heat up and foul with fusible powder. 
     In a preferred embodiment, thermal spray system  1  comprises single air supply applicator  301 . Referring to  FIG. 13 , a perspective view of single air supply applicator  301  of an illustrative embodiment of the thermal spray system  1  is shown. Referring to  FIG. 14 , a number of subassemblies and components of single air supply applicator  301  are shown in exploded view. These include heater assembly  302 , electrode assembly  303 , upper air manifold assembly  304 , feed tube assembly  305 , left hand grip  306   a , right hand grip  306   b , igniter circuit board  307 , lower air manifold assembly  308 , shroud  309  and heat shield  310 . 
     In this embodiment, heat shield  310  is a secondary cone that surrounds shroud  309 . Shroud  309  may reach temperatures in excess of 500° C., but the addition of heat shield  310  reduces the temperature of the exterior of heat shield  310  to less than 250° C. In this embodiment, shroud  309  is passively cooled by air educted between shroud  309  and heat shield  310  and heat shield  310  blocks heat radiating from shroud  309 . 
     In a preferred embodiment, dead man&#39;s switch  370  and robust feed switch  318  are provided in the handle of single air supply applicator  301 . Referring to  FIG. 15 , a cross sectional view of an illustrative embodiment of umbilical  3  is presented. In this embodiment, umbilical  3  comprises propane hose  373 , air hose  375 , powder feed hose  392 , and cable  377 , preferably bundled within sleeve  383  which is made from a protective fabric cover. In a preferred embodiment, umbilical  3  comprises a single air hose  375 . In a preferred embodiment, the six wires in cable  377  are: +24V DC—digital ground; 220V AC—chassis ground, feed, igniter; +24V DC—power supply to the PCBA on the applicator; digital ground—ground for igniter circuit board  307 ; 220V AC—step up transformer output voltage for igniter circuit board  307 ; chassis ground—chassis ground for igniter circuit board  307  and return for the 220V AC; feed −24V—output to drive the feed solenoid; and igniter −24V output to drive the igniter relay. All these wires are preferably in a single multi conductor cable  377 . 
     In this embodiment, three air streams and a propane stream come together in single air supply applicator  301  to create an ideal nameless spray environment. These flow streams are illustrated in  FIG. 16 . Referring to  FIG. 16 , main air input stream  312  and propane input stream  313  are fed into a fitting at the base of single air supply applicator  301 . 
     In this embodiment, propane input stream  313  flows through propane input tube  315  and is normally shut off with propane valve  316 . Propane valve  316  may be opened by depressing propane valve lever  317  and allowing the propane to flow into lower air manifold assembly  308 . The propane valve  316  and propane valve lever  317  comprise dead man&#39;s switch  370 . 
     Main air input stream  312  flows through main air tube  314  and into lower air manifold assembly  308 . In lower air manifold assembly  308 , main air stream  312  is split to travel into heater assembly  302  and into upper air manifold assembly  304 . The portion of main air supply input stream  312  that is directed into heater assembly  302  by lower air manifold assembly  308  is mixed with propane from propane input stream  313  to create combustion gas and air stream  319 . Combustion gas and air steam  319  is preferably a stoichiometric mixture of propane gas and air, which can burn cleanly and completely. 
     In this embodiment, three other air streams are input to the back end of single air supply applicator  301 . The first air stream, powder feed input stream  320 , is a mixture of pressurized air and powder supplied to single air supply applicator  301  by a powder feed pump (not shown). Powder feed input stream  320  carries the fusible powder that is melted in single air source applicator  301 . The second air stream is diverter air inlet stream  321 , which is drawn into the back of single air supply applicator  301  by upper air manifold assembly  304 . The third air stream is cooling air inlet stream  322 , which is also drawn into the back of single air supply applicator  301  by upper air manifold assembly  304 . 
     As the air flows from right to left in the embodiment shown in  FIG. 16 , four separate output air streams may be described: The first air stream is powder feed outlet stream  323 , which flows directly through single air supply applicator  301  from powder feed input stream  320 . The second air stream is inner diverter outlet steam  324   a , which originates as part of diverter air inlet steam  321 . The third air stream is outer diverter outlet stream  324   b , which originates as part of diverter air inlet stream  321 . The fourth air stream is cooling air outlet stream  325 , which supplies cool air to blanket the inside surface of shroud  309  to reduce the temperature of shroud  309 . 
     In this embodiment, inner diverter outlet steam  324   a  and outer diverter outlet stream  324   b  divide diverter air inlet stream  321  as the air flows over diverter cone  326 . Inner diverter outlet stream  324   a  keeps the tip of feed tube assembly  305  and diverter cone  326  cool so that powder  229  will not stick to these surfaces. Also, inner diverter outlet stream  324   a  surrounds powder feed outlet stream  323  to prevent flames from heater assembly  302  from directly contacting powder  229  in the powder feed outlet stream  323 . Outer diverter outlet stream  324   b  forces the flame on the outlet side of heater assembly  302  radially outward toward shroud  309 . Forcing the flame radially outward prevents the flame from directly contacting powder  229  in powder feed outlet stream  323  and keeps the flame anchored within shroud  309 . Referring to  FIG. 17 , heater assembly  302  is shown in perspective view. 
     Referring to  FIGS. 18A and 18B , the location of perforated heater plate  327  and electrode tip  328  are shown. In this embodiment, heater electrode tip  328  is nominally disposed flush with the front of perforated heater plate  327 ; however, electrode tip  328  may be slightly protruding or slightly recessed from the front of perforated heater plate  327 . The entire heater assembly  302  is electrically grounded, which provides an electrical return path for the spark originating at electrode tip  328 . 
     Referring to  FIG. 19 , an illustrative embodiment of electrode assembly  303  is shown in perspective view. One electrode inlet hole  329  is shown in this view. Preferably, there are two electrode inlet hole(s)  329 . A metered portion of the combustion gas and air stream flows into electrode inlet hole(s)  329  and passes into electrode assembly  303 . 
     Referring to  FIG. 20 , a cross-sectional view of an illustrative embodiment of electrode assembly or igniter  303  is presented. In this embodiment, electrode tube  330  is comprised of a conductive metal (e.g., stainless steel), which is connected to electrical ground through heater assembly  302 . Inside electrode tube  330  is electrode seal plug  333 , which is made from an electrically insulating material (e.g., aluminum oxide ceramic). Inside electrode seal plug  333  electrode insulator  331  (preferably made of an aluminum oxide ceramic tube) is bonded to electrode seal plug  333 , preferably using a ceramic adhesive. In this embodiment, there is an annular void between electrode insulator  331  and electrode tube  330 . The combustion gas and air mixture enters electrode inlet hole(s)  329 , flows through this void, and exits as a controlled velocity stream of combustible gas, which is shown as pilot gas air mixture stream  335 . 
     During operation, a high voltage electrical potential is repeatedly applied to electrode lead wire  334  and passes through electrode  332 . The high voltage potential is sufficient to create a spark, which travels from the electrode tip  328  across the pilot gas air mixture stream  335  to electrode tube  330 . By forcing the spark through pilot gas air mixture stream  335 , the flame can reliably be ignited in heater assembly  302 . 
     Referring to  FIG. 21 , an illustrative embodiment of electronic ignition circuit  307  is presented. In this embodiment, 110 VAC power for igniter  303  is supplied by step up transformer T 1 , which generates a 220 Volt AC current. The current output of the transformer is normally off when relay S 2  is not energized, i.e., relay S 2  is normally open. Microcontroller U 1  energizes relay S 2  when igniter switch S 1  is depressed. When the relay S 2  is energized, 220V is applied across capacitor C 1  and the primary windings of spark transformer T 2 . In this embodiment, STMicroelectronics FLC-200B U2 fire starter integrated circuit (IC) behaves like an open circuit until a voltage of 200V is reached. When the voltage reaches 200V across capacitor C 1  and the primary coil of spark transformer T 2 , FLC-200B U2 starts conducting current and becomes a direct connection to ground. When the FLC-200B U2 conducts current, the charge in capacitor C 1  discharges to ground though LFC-200B U2. The primary coil of spark transformer T 2  resists this sudden change in current and creates a very large back EMF voltage, which couples with the secondary coil of spark transformer T 2 . Transformer T 2  is a very high ratio transformer, so the back EMF from the primary coil of spark transformer T 2  creates a voltage in the secondary coil of spark transformer T 2  on the order of 10 kV to 15 kV. This voltage is applied to the center conductor of electrode assembly  303  and creates a spark in spark gap  336  between electrode tip  328  and the distal end of electrode tube  330 . 
     In this embodiment, only igniter switch S 1 , electrode  303 , microcontroller U 1 , fire starter integrated circuit FLC-200B U2, capacitor C 1 , and spark transformer T 2  are attached to single gas supply applicator  301 . The other components are preferably installed on a separate cart  2 . The circuit connections between the components attached to the single gas supply applicator  301  are connected to the components installed on cart  2  by an electrical umbilical cable  3 . 
     Referring to  FIG. 22 , an exploded view of an illustrative embodiment of lower air manifold assembly  308  is shown. In this embodiment, propane nozzle  338  threads into the bottom of lower air manifold  337 . Combustion air amplifier base  339  is housed inside lower air manifold  337 . Combustion air amplifier shim  341  is placed on top of combustion air amplifier base  339 . Combustion air amplifier tube  340  is then threaded into the top of lower air amplifier manifold  337 . Hose barb fitting  367  allows access to the interior of lower air amplifier manifold  337 . 
     Referring to  FIG. 23 , an illustrative embodiment of lower air manifold assembly  308  is shown in a cross-sectional view. Main air input stream  312  enters the bottom of lower air manifold assembly  308  and is divided into two air streams: combustion air amplifier air input stream  342  and upper air amplifier air input stream  343 . The air from combustion air amplifier air input stream  342  combines with propane gas, which flows out of propane nozzle  338  to create combustion gas air stream  319 , which flows into heater assembly  302 . Upper air amplifier air input stream  343  flows out of lower air manifold assembly  308  into upper air manifold assembly  304 . 
     Referring to  FIG. 24 , an illustrative embodiment of upper air manifold assembly  304  is shown in an exploded view. Upper air amplifier air input stream  343  enters upper air manifold assembly  304  through upper air amplifier air restrictor  348 , diverter air amplifier input tube  347 , and upper air amplifier input tube  346 . Upper air amplifier air input stream  343  enters upper air amplifier manifold  344  and is divided into an air stream which flows past diverter air amplifier venturi  352  and an air stream that flows through (preferably) two cooling air restrictors  345 . The air stream that flows through two cooling air restrictors  345  flows past cooling air amplifier venturi  351 . In this embodiment, cooling air amplifier shim  349  and diverter air amplifier shim  350  are selected to achieve desirable air flows for cooling air amplifier venturi  351  and diverter air amplifier venturi  352 , respectively. 
     Referring to  FIGS. 25A and 25B , an illustrative embodiment of upper air manifold assembly  304  is shown from a rear (inlet side) view and a cross-sectional view, respectively. Diverter air inlet stream  321  and cooling air inlet stream  322 ,  FIG. 16 , are educted into single air supply applicator  301  by diverter air amplifier high velocity air stream  353  and cooling air amplifier high velocity air stream  354 , respectively. 
     Referring to  FIG. 26 , an illustrative embodiment of shielded air feed tube assembly  355  with powder feed tube  357  at its center is shown in a cross sectional view. Powder feed input stream  320  enters shielded air feed tube assembly  355 . The shielding air is forced into the shielded air feed tube assembly  355  through shielding air tube  358  which is attached to lower manifold assembly  308  by a shielding air hose  371  held in place by barbed fittings. 
     Referring to  FIG. 27 , an illustrative embodiment of shielded air tube assembly  355  is shown in an isometric view. The shielding air is contained within shielding air tube  358  and shielding air manifold  359 . Referring to  FIG. 28 , shielded air tube assembly  355  is shown in a cross sectional view. Powder feed input stream  320  is introduced into powder feed tube  357  through powder feed hose  392  which is attached to powder feed hose barb  361 . The distal tip of powder feed tube  357  preferably extends beyond the distal end of shielding air tube by about ⅜ inch and by about three inches into the hot air stream that includes inner diverter outlet stream  324   a  and outer diverter outlet stream  324   b . The shielding air is introduced through shielding air inlet  360 . 
     Referring to  FIG. 29 , a cross-sectional view of an illustrative embodiment of the inlet section of shielded air feed tube assembly  355  is presented. In this embodiment, powder feed input stream  320  is introduced into powder feed tube  357 . Shielding air input stream  362  enters shielding air manifold  359  through shielding air inlet  360 . The shielding air is forced into the annular gap between powder feed tube  357  and shielding air tube  358 , thereby creating shielding air stream  363 . 
     In a preferred embodiment, powder feed tube  357  is made from 5/16 inch outside diameter stainless steel tubing with a 0.028 inch wall thickness. Shielding air tube  358  is preferably made from ½ inch outside diameter stainless steel tubing with a 0.028 inch wall thickness. In this embodiment, the volumetric flow rate of shielding air input stream  362  ranges from 1.0-5.0 standard cubic feet per minute (SCFM), and is approximately 1.5 SCFM for a 30 kW power setting. 
     Referring to  FIG. 30 , a schematic illustration of powder feed outlet stream  323  is shown exiting powder feed tube  357 . As shielding air steam  363  exits shielding air tube  358 , it expands and forces powder feed outlet steam  323  to contract. Powder contraction zone  364  is shown downstream of the distal tip of powder feed tube  357 . Further downstream, powder feed outlet stream  323  expands to create a conical powder expansion zone  365 . As the powder passes through powder contraction zone  364 , it undergoes turbulent mixing so that the powder in powder expansion zone  365  is evenly disbursed. This even dispersion enables single air supply applicator  301  to deposit a very uniform coating. 
     Powder contraction zone  364  and powder expansion zone  365  may be altered by adjusting the shielding air volume flow rate. In addition, alternate feed tube and shielding tube diameters and shapes are envisioned, which can create a variety of spray patterns. 
     In preferred embodiments, the invention possesses a number of advantageous features. These embodiments comprise air manifold assemblies  308  and  337  which divide air provided to single air source applicator  301  in air hose  375  into a plurality of air flow streams. The single air supply embodiment is lighter and less cumbersome than embodiments that require multiple air supplies. A propane dead-man&#39;s switch  370  is provided on handgrips  306   a  and  306   b  as a safety feature. If applicator  301  is dropped, propane flow stops and combustion stops. A spark ignition electrode assembly  303  is provided to light the combustor for the convenience of the operator. When the operator picks up the unit, he squeezes dead-man&#39;s switch  370 , presses igniter button  384 , and applicator  301  begins generating large quantities of heat. Providing shielded powder tube assembly  355  results in an improved powder deposition pattern and eliminates fouling. The presence of shielding air tube  358  allows powder feed tube  357  to be extended (e.g., approximately three inches) into a hot air stream that includes inner diverter outlet stream  324   a  and outer diverter outlet stream  324   b . Igniter  303  provides a more reliable means for igniting the propane/air mixture. 
     Many variations of the invention will occur to those skilled in the art. All such variations are intended to be within the scope and spirit of the invention. Some variations include trip plates, trip lips and/or bluff bodies. Other variations call for flame tubes, holes or perforated walls, serpentine paths and/or fluid amplifiers with annular nozzles and/or air knives. All such variations are intended to be within the scope and spirit of the invention. 
     Although some embodiments are shown to include certain features or steps, the applicants specifically contemplate that any feature or step disclosed herein may be used together or in combination with any other feature or step on any embodiment of the invention. It is also contemplated that any feature or step may be specifically excluded from any embodiment of the invention.