Patent Description:
Thermal spray apparatus and methods are used to apply coatings of metal or ceramics to different substrates. The HVOF process was first introduced as a further development of the flame spray process. It did this by increasing the combustion pressure to <NUM>-<NUM> Bar, and now most third generation HVOF torches operate in the <NUM>-<NUM> Bar range with some exceeding <NUM> Bar. In the HVOF process, the fuel and oxygen are combusted in a chamber. Combustion products are expanded in an exhaust nozzle reaching sonic and supersonic velocities.

In the first commercial HVOF system, Jet Kote™, developed by James Browning, particle velocities were increased from approximately <NUM>/s for the flame spray process to about <NUM>/s. The increased particle velocities resulted in improved coating properties in terms of density, cohesion and bond strength resulting in superior wear and corrosion properties. In the past thirty years many variations of this process have been introduced. Modern third generation HVOF guns with de Laval, convergent-divergent nozzles result in mean particle velocities on the order of <NUM>/s. High velocity air fuel (HVAF) spray processes have become more popular due to the potentially better economics using lower cost air as opposed to oxygen. HVAF torches operate at lower temperatures due to the energy required to heat the nitrogen in the air that does not participate in the combustion process in any significant way compared to HVOF torches at the same fuel flow rates.

Key high velocity torch and process design features are largely dictated by the type of fuel used. Fuels used can be gaseous such as propane, methane, propylene, MAPP-gas, natural gas and hydrogen, or liquid hydrocarbons such as kerosene, ethanol and diesel. Other considerations include: a) combustion chamber design; b) torch cooling media; c) nozzle design; d) powder injection; and e) secondary air. The choice of the combustible fuel determines the following flame parameters: a) flame temperature; b) stoichiometric oxygen requirement; and c) reaction products. These combustion characteristics along with a fixed high velocity torch internal geometry determine particle acceleration and velocity and particle temperature.

With current systems the nozzle exit of the torch must be about <NUM>,<NUM> (<NUM> inches) from the surface to be coated in order for the particles to reach sufficient velocity and temperature when they reach the target surface in order to provide a suitable coating. This makes the coating of surfaces in restricted areas, for example the inside surfaces of small pipes, difficult or impossible. There is therefore a need for a thermal spray torch in which the particle temperature and velocity is reached in a shorter distance from the nozzle to permit coating in smaller, restricted areas.

<CIT> shows a high velocity oxygen fuel (HVOF) or high velocity air fuel (HVAF) thermal spray apparatus to apply coatings to external and internal surfaces of a target, said HVOF or HVAF thermal spray apparatus comprising a combustion chamber, a divergence section downstream of said combustion chamber, an elbow housing downstream of said divergence section, a nozzle housing downstream of said elbow housing, the nozzle housing retaining a nozzle having an injection zone, a convergence section retained between said elbow housing and said nozzle housing, a feedstock injector for the injection of feedstock material for forming said coatings into an injection zone of said nozzle, and a plurality of passageways extending through said combustion chamber, said divergence section, said elbow housing, and said convergence section for passing a coolant therethrough.

The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are not meant to be limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.

The invention provides a high velocity oxygen fuel (HVOF) or high velocity air fuel (HVAF) thermal spray apparatus. The apparatus can be used to apply coatings to external and internal surfaces of a target. The apparatus comprises a combustion chamber having a primary passage for combustion of fuel received through a fuel input line with oxygen or air received through an oxidizing gas input line. A divergence section is located downstream of the combustion chamber. The divergence section has two or more channels that diverge relative to a longitudinal axis of the primary passage of the combustion chamber. An elbow housing is located downstream of the divergence section. A nozzle housing is located downstream of the elbow housing. The nozzle housing retains a nozzle having an injection zone and a nozzle throat. A convergence section is retained between the elbow housing and the nozzle housing. The convergence section has two or more channels that converge toward the injection zone of the nozzle. The apparatus also comprises a feedstock injector for the injection of feedstock material (for forming said coatings) into the injection zone of said nozzle. The apparatus also comprises a plurality of passageways extending through said combustion chamber, said divergence section, said elbow housing, and said convergence section for passing a coolant therethrough. The plurality of passageways comprise a plurality of grooves interspaced between a plurality of fins formed on a surface of the convergence section for facilitating flow of the coolant through the plurality of grooves.

In some embodiments, a fuel combusts with an oxidizer to produce a high velocity jet and further accelerating this jet with an optional accelerating gas. There are generally at least two types of accelerating gas that can be used. These include a gas such as nitrogen, carbon dioxide or argon or alternatively a combustible fuel to increase temperature and pressure. Using a high density gas such as carbon dioxide or argon increases the drag coefficient and accelerates the feedstock material faster. Increasing the pressure of the gas will also increase the density of the gas through the principles of the ideal gas law. <MAT> where ρ = density, P = pressure, R = Gas constant, T =temperature A combination of carbon dioxide and a combustion gas can also be used. It is also possible to use supercritical carbon dioxide as a supply of carbon dioxide to increase the drag coefficient.

Closer spray distance can also be obtained through a combination of the following characteristics:.

The injection of the optional accelerating gas may be upstream of the nozzle. The accelerating gas can be added to the oxidizing gas input, as is the case with HVAF where nitrogen is a dilatant of oxygen in the form of air and in effect acts as an accelerating gas. Having an accelerating gas added to the oxidant gas stream, in an amount less than the <NUM>%, which is the approximate volume fraction of nitrogen in air, can be used. For example nitrogen could be added at <NUM>% that would increase the total gas flow over a stoichiometric gas mixture, but not decrease the overall temperature as much as would be the case with air at <NUM>% nitrogen.

The high velocity torch may be water cooled or Air and/or CO<NUM> cooled. However, the use of Air and/or CO<NUM> may restrict the power level the torch can reach and therefore water cooling is preferred.

The convergence and nozzle design can result in higher injection pressures. The nozzle is characterized by the throat diameter. The smaller this throat diameter is the higher the pressure for a given gas flow. This increased pressure has the benefit of increasing heat transfer from the hot combustion gas to the feed stock material, usually a powder, and also increasing the pressure in the converging gas and feed stock region. Therefore, particles can reach the desired temperature and velocity without the use of an accelerating gas.

In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.

Examples and embodiments are illustrated in referenced figures of the drawings. It is intended that the examples, embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

With reference to <FIG>, in which the exterior powder feed line and coolant water line are removed for illustrative purposes the novel High Velocity thermal spray gun to spray wear and corrosion-resistant coatings <NUM> has a base plate <NUM> in which are located various input passages and chambers. It includes a combustion chamber <NUM>, divergence section <NUM>, elbow housing <NUM>, convergence assembly <NUM> (<FIG>) and nozzle <NUM> (<FIG>, <FIG>). Nozzle <NUM> is retained in nozzle housing <NUM>. Rigid tie rods <NUM> strengthen the torch body, by connecting base plate <NUM> at mounting holes <NUM> (<FIG>) to the elbow housing <NUM>. Water cooling, entering or leaving through water line <NUM>, <NUM> is preferred but air and/or CO<NUM> cooling may also be incorporated through the use of an accelerating fluid such as gas that goes through recuperative heating while cooling the torch. In the illustrated embodiment in <FIG> an accelerating gas enters the gas stream through passages <NUM>, <NUM> into the convergence area around the powder feed injection port <NUM> as described below. Hydrogen is the preferred fuel, however other fuel gases such as methane, ethylene, ethane, propane, propylene or liquid fuels such as kerosene, ethanol or diesel can be used. The feed stock may be powder, wire, liquid or a suspension of powder in liquid.

With reference to <FIG> and <FIG>, wherein the same reference numerals are used to reference the same parts as in <FIG>, the novel High Velocity thermal spray gun to spray wear and corrosion-resistant coatings incorporating use of a high density and/or fuel accelerating gas is shown at <NUM>. It has a base plate <NUM> in which are located various input passages and chambers. It includes a combustion chamber <NUM>, divergence section <NUM> (<FIG>), elbow housing <NUM>, convergence assembly <NUM> (<FIG> not being according to the invention) and nozzle <NUM> (<FIG>, <FIG>). Nozzle <NUM> is retained in nozzle housing <NUM>. Rigid tie rods <NUM> fix the torch body, by connecting base plate <NUM> at mounting holes <NUM> (<FIG>) to the elbow housing <NUM>. Water cooling is preferred but air and/or CO<NUM> cooling may also be incorporated through the use of an accelerating fluid such as gas that goes through recuperative heating while cooling the torch. In the illustrated embodiment, the accelerating gas enters the gas stream through passages <NUM>, <NUM> into the convergence area around the powder feed injection port <NUM> as described below. Hydrogen is again the preferred fuel, however other fuel gases such as methane, ethylene, ethane, propane, propylene or liquid fuels such as kerosene, ethanol or diesel can be used.

Hydrogen gas or fuel enters central channel <NUM> (<FIG>) which communicates with central passage <NUM> of combustion chamber <NUM>. In some embodiments, the combustion stream in passage <NUM> is diverted in divergence section <NUM> into two channels <NUM>, <NUM> which pass through elbow <NUM> (<FIG>, <FIG>, <FIG>). In such embodiments, divergence section <NUM> may be integrally formed with elbow <NUM>. In other alternative embodiments, the combustion stream is diverted into two channels <NUM>, <NUM> in passage <NUM> before the diverted combustion stream passes through divergence section <NUM> (<FIG>). In such embodiments, divergence section <NUM> may be integrally formed with combustion chamber <NUM>.

Coolant water enters (or leaves) the torch body at <NUM> (<FIG>) and passes through passageways <NUM> (<FIG>) and exits (or enters) the torch body through line <NUM>. In some embodiments, passageways <NUM> are defined by or otherwise comprise longitudinal passages <NUM> extending through combustion chamber <NUM> (<FIG>), axial passages <NUM> extending through divergence section <NUM> (<FIG>), elbow cooling fluid passages <NUM> extending through elbow housing <NUM> (<FIG>), and body passages <NUM> extending through convergence section <NUM> (<FIG> not being according to the invention; <FIG> according to the invention).

As shown in <FIG>, combustion chamber <NUM> comprises a plurality of longitudinal passages <NUM> extending therethrough. Each longitudinal passage <NUM> is in direct fluid communication with water line <NUM>. Longitudinal passages <NUM> extend in a direction generally parallel to the longitudinal axis of combustion chamber <NUM>. Longitudinal passages <NUM> may be located around the circumference of central passage <NUM>. For example, combustion chamber <NUM> may comprise twelve longitudinal passages <NUM> that are spread circumferentially around central passage <NUM> as shown in <FIG>. Longitudinal passages <NUM> may be tubular shaped (<FIG>) or crescent shaped (<FIG>) although this is not necessary.

As shown in <FIG>, divergence section <NUM> comprises a plurality of axial passages <NUM> extending therethrough. Each axial passage <NUM> is located downstream of and in direct fluid communication with one or more longitudinal passages <NUM>. Axial passages <NUM> extend in a generally axial direction (i.e. in a direction generally parallel to longitudinal passage <NUM>) through divergence section <NUM>. Each axial passage <NUM> is defined by or otherwise includes a rear end 62A facing combustion chamber <NUM> (<FIG>) and a front end 62B facing elbow housing <NUM> (<FIG>). Two or more axial passages <NUM> may share a common rear end 62A as long as they do not share a common front end 62B. For example, divergence section <NUM> may comprise sixteen axial passages <NUM> located between twelve rear ends 62A and sixteen front ends 62B. In some embodiments, the number of axial passages <NUM> is greater than the number of longitudinal passages <NUM>. In other embodiments, the number of axial passages <NUM> is the same as the number of longitudinal passages <NUM> (<FIG>). Axial passages <NUM> may be tubular shaped or crescent shaped to conform to the shape of longitudinal passages <NUM> although this is not necessary.

As shown in <FIG>, elbow <NUM> comprises a plurality of elbow cooling fluid passages <NUM> extending therethrough. Each elbow cooling fluid passage <NUM> is located downstream of and in direct fluid communication with one or more axial passages <NUM>. Elbow fluid cooling passages <NUM> may be curved relative to the longitudinal axis of combustion chamber <NUM>. In some embodiments, elbow fluid cooling passages <NUM> are curved relative to the longitudinal axis of combustion chamber <NUM> by an angle greater than <NUM> degrees. In some embodiments, elbow cooling passages <NUM> are curved to conform to the curvature of channels <NUM>, <NUM> (<FIG>). This can provide improved cooling to the areas around channels <NUM>, <NUM>.

Convergence section <NUM> comprises a plurality of body passages <NUM> that form part of passageways <NUM>. Each body passage <NUM> is located downstream of and in direct fluid communication with one or more elbow passages <NUM> and water line <NUM>. In some embodiments, body passages <NUM> are tubular shaped and extend in a generally axial direction (i.e. in a direction generally parallel to the longitudinal axis of powder feed tube <NUM>) through the body of convergence section <NUM> (<FIG> not being according to the invention). According to the invention (e.g. see <FIG>), body passages <NUM> include slots <NUM> formed around the circumferential edge of convergence section <NUM>, grooves <NUM>, and/or transverse passages <NUM> (i.e. passages that facilitate fluid flow in directions generally orthogonal to the longitudinal axis of powder feed tube <NUM>). As described in more detail below, coolant flow paths through body passages <NUM> can in some cases be controlled through the physical coupling between convergence section <NUM> and elbow section <NUM>.

<FIG> illustrates the convergence section <NUM> of torch <NUM> according to an embodiment. Convergence section <NUM> may be adapted for use with both HVOF torches and HVAF torches. For the purposes of facilitating the description, it is assumed that coolant (e.g. water) enters torch <NUM> from line <NUM> of combustion chamber <NUM> and exits torch <NUM> through line <NUM> of nozzle housing <NUM>. Convergence section <NUM> is retained between elbow <NUM> and nozzle housing <NUM>. Convergence section <NUM> comprises powder feed injection port <NUM> and hot gas channels <NUM>, <NUM> for the diverted hot gas combustion stream to converge and flow therethrough, as described in more detail elsewhere herein. Convergence section <NUM> comprises a plurality of grooves <NUM> interspaced between a plurality of <NUM>(<FIG>). The interspaced grooves <NUM> and fins <NUM> are formed on the top side (i.e. the side facing toward elbow section <NUM>) of convergence section <NUM>. That is, grooves <NUM> and fins <NUM> face toward elbow section <NUM>. Grooves <NUM> and fins <NUM> extend in a direction generally perpendicular to the longitudinal axis of powder feed tube <NUM>. Advantageously, fins <NUM> provide increased surface area to facilitate good heat transfer between hot gas flowing through channels <NUM>, <NUM> and coolant flowing through grooves <NUM>.

Some or all of grooves <NUM> may be in fluid communication with a respective slot <NUM>. Slots <NUM> are formed around the circumference of convergence section <NUM> Slots <NUM> extend in a generally axial direction (i.e. in a direction generally parallel to the longitudinal axis of powder feed tube <NUM>). Slots <NUM> are shaped to allow coolant to flow from elbow section <NUM> through convergence section <NUM> and into nozzle housing <NUM>. Slots <NUM> may be arranged to encourage optimized (e.g. balanced) coolant flow through convergence section <NUM>. Since convergence section <NUM> is subject to extreme heat due to hot gas flowing through channels <NUM> and <NUM>, encouraging optimized coolant flow through convergence section <NUM> can be desirable.

The location of grooves <NUM> and fins <NUM>, the spacing of grooves <NUM> and fins <NUM>, and/or the size of slots <NUM> can be configured to guide the water coolant to flow across the fin and groove pattern (i.e. from front side <NUM> to back side <NUM> as shown in <FIG>) to enhance cooling of convergence section <NUM>. In some embodiments, grooves <NUM> may be arranged to provide improved cooling around O-rings and/or seals. Controlling water flow path through convergence section <NUM> can improve cooling, thereby allowing torch <NUM> to be operated at higher powers.

In some embodiments, convergence section <NUM> comprises transverse passages <NUM> that facilitate fluid flow across convergence section <NUM>. For the purposes of facilitating the description, the term "across" (as used in this context) refers to a direction that is generally orthogonal to the direction of extension of slots <NUM>(e.g. direction <NUM> as shown in <FIG>). Transverse passages <NUM> may be in fluid communication with one or more grooves <NUM> (<FIG>). In the example embodiment shown in <FIG>, some of grooves <NUM> are in fluid communication with transverse passage <NUM> while others are not. This can increase water velocity in transverse passage <NUM> and/or improve cooling to selected areas (i.e. improve cooling to areas where grooves <NUM> are in fluid communication with transverse passage <NUM>). Transverse passages <NUM> are located between elbow <NUM> and the top of convergence section <NUM>. Transverse passages <NUM> may be enclosed or otherwise bounded between elbow <NUM> and the top of convergence section <NUM> in some cases. This allows water flowing through transverse passages <NUM> to cool both convergence section <NUM> and elbow <NUM>. Transverse passages <NUM> are arranged or otherwise configured to direct water flowing down from elbow passages <NUM> toward the back side <NUM> of elbow <NUM> and convergence section <NUM> (e.g. to direct water to flow along direction <NUM>). For example, transverse passages <NUM> may be arranged to connect grooves <NUM> located at front side <NUM> to grooves <NUM> located at the back side <NUM>. Since water line <NUM> is located at the front side <NUM> of convergence section <NUM>, water flowing down elbow passages <NUM> will tend to flow through body passages <NUM> (e.g. slots <NUM>) located at the front side <NUM> of convergence section <NUM> to follow the passage of least resistance. Advantageously, body passages <NUM> can be configured to constrict water flow through slots <NUM> located at the front side <NUM> of convergence section <NUM> and/or to direct water toward back side <NUM> to encourage more water to flow through slots <NUM> located at the back side <NUM> of convergence section 20A. For example, the number and/or size of slots <NUM> located at the front side <NUM> (and in fluid communication with transverse passages <NUM>) can be reduced relative to the number and/or size of slots <NUM> located at the back side <NUM> to enhance water movement from front side <NUM> to back side <NUM>.

In some embodiments, powder feed injection port <NUM> and channels <NUM>, <NUM> extend through a protrusion <NUM> of convergence section <NUM> (<FIG>). In such embodiments, transverse passages <NUM> may be arranged to extend or otherwise curve around protrusion <NUM>. For example, transverse passages <NUM> may be arranged to form an obround shape around protrusion <NUM> as shown in <FIG>. Water flow across transverse passages <NUM> can be increased by increasing the pressure differential between the back side <NUM> of convergence section <NUM> and the front side <NUM> of convergence section <NUM>. In some embodiments, the top of some or all of the grooves <NUM> located at the back side <NUM> of convergence section 20A is covered by a cover <NUM>, or the like (<FIG>). Cover <NUM> can be a part of elbow <NUM> that extends to contact the top of fins <NUM> (<FIG>), thereby enclosing grooves <NUM> in effect. Enclosing grooves <NUM> located at the back side <NUM> of convergence section <NUM> in this manner (e.g. enclosing grooves <NUM> with a bounding solid) can reduce the pressure at the back side <NUM> of convergence section <NUM> partly due to the Venturi effect, thereby encouraging water to flow from front side <NUM> of convergence section <NUM> through transverse passages <NUM> to back side <NUM> of convergence section <NUM>.

In some embodiments, transverse passages <NUM> include a hole or passage <NUM> that extend in a transverse direction (e.g. in direction <NUM>) and through protrusion <NUM>. Such hole or passage may be located to place a groove <NUM> located at front side <NUM> in fluid communication with a corresponding groove <NUM> located at back side <NUM> (<FIG>).

While the disclosed embodiment uses water cooling, and air cooling is not incorporated, air cooling and /or CO<NUM> cooling could be used as coolants and air cooling could be added when combined with CO<NUM> as the coolant.

Referring back to <FIG>, powder feed line <NUM> supplies the spray powder or other feedstock such as wire, liquid or a suspension. Oxygen or air enters the combustion chamber through passages <NUM> and <NUM> and combusts with the fuel in passage <NUM> in combustion chamber <NUM> to form the torch flame. The accelerating gas can also be added through passages <NUM> and <NUM>. When the accelerating gas is added in this location, it is added after initial combustion in an amount not great enough to extinguish the flame. While the illustrated embodiment shows the use of o-ring seals which seal axially throughout, including the combustion chamber <NUM> in <FIG>, it will be apparent that radial o-ring seals may also be used throughout, as illustrated in the alternate embodiment of the combustion chamber <NUM> in <FIG>, wherein o-rings are seated in co-axial sealing grooves <NUM>.

Air can be used as a replacement for oxygen. In this case the torch becomes a High Velocity Air Fuel (HVAF) torch. The amount of oxygen in air is approximately <NUM>% so the volumetric air flow will be approximately <NUM> times higher to reach the same stoichiometric conditions used for pure oxygen.

In some embodiments, the combustion stream in passage <NUM> is diverted in divergence section <NUM> into two channels <NUM>, <NUM> which pass through elbow <NUM> (<FIG>, <FIG>, <FIG>). In other alternative embodiments, the combustion stream is diverted into two channels <NUM>, <NUM> in passage <NUM> before the diverted combustion stream passes through divergence section <NUM> (<FIG>). Powder feed tube <NUM> is a stainless steel or tungsten carbide tube attached to the convergence assembly <NUM>. It is supplied by powder feed line <NUM> which is a synthetic polymer hose, preferably a Teflontm hose which fits over the end of powder feed tube <NUM>. In some cases a metal powder feed tube is preferred. The metal tube can be made from materials such as stainless steel, copper or brass. Powder feed tube <NUM> passes through powder channel <NUM> in elbow <NUM> (<FIG>) and communicates through powder feed injection port <NUM> in convergence assembly <NUM> (<FIG>) into the center of nozzle entrance <NUM>. Channels <NUM>, <NUM> open into a crescent shape in cross-section within the convergence assembly <NUM> as shown in <FIG> and <FIG> and converge around the entry point of powder feed injection port <NUM> at the nozzle entrance <NUM>.

<FIG> shows an embodiment of a convergence nozzle configuration that creates a higher pressure in the converging nozzle region than would otherwise be the case for a straight nozzle with exit internal diameter. With reference to <FIG>, the convergence assembly <NUM> and nozzle <NUM> are shown in cross-section. Nozzle <NUM> has throat <NUM>, injection zone <NUM>, entrance <NUM>, exit <NUM>, entrance diameter A, exit diameter B, total length L, throat diameter D, converging length M and diverging length N. Powder feed tube communicates through powder feed injection port <NUM> in convergence assembly <NUM> into the center of nozzle entrance <NUM>. Channels <NUM>, <NUM> converge around the entry point of powder feed injection port <NUM> at the nozzle entrance <NUM>.

The following equations characterize particle velocity and temperature that are important to the thermal spray process.

The present invention uses relatively short nozzles at nominal length of approximately <NUM> and <NUM>. The nozzle length is set at less than or equal to about <NUM> times the nozzle throat (bore) diameter D. With the nozzle length being less than or equal to about <NUM> times the throat or bore diameter. Total nozzle length L to Throat Bore ratio for different nozzle bore diameters used herein is provided in the following Table <NUM>.

The injection zone <NUM> is the area within the torch where the hot gas and feedstock injection come together upstream of the nozzle throat. In the case where the nozzle throat diameter D is the smallest area that hot gas will pass through, the injection zone pressure will be representative of the combustion pressure subject to pressure losses through the elbow <NUM> and convergence section <NUM>.

For the described embodiment, the high injection pressure increases the gas density and thermal conductivity which results in an increase in heat transfer from the hot gas to the particle. Heat transfer to a particle in thermal spray applications is commonly calculated through the Ranz and Marshall correlation. As can be seen, heat transfer increases with increasing thermal conductivity k, increasing density ρ to the power <NUM>. In the pressure ranges <NUM>-<NUM> bar, the viscosity will change very little and can be considered a constant for analysis purposes.

The accelerating gas used in the embodiment of <FIG> may be introduced at inlet port <NUM> (<FIG>) from an accelerating gas source through high pressure tubing of stainless steel or copper (not shown). The accelerating gas travels from inlet port <NUM> to gas chamber <NUM> and then through accelerating gas connecting hole <NUM> into accelerating gas reservoir <NUM> which is sealed and surrounds powder feed tube <NUM>. The hole to form accelerating gas connecting hole <NUM> is drilled from the exterior of the torch and plugged from the exterior of the torch <NUM> by plug <NUM>. Accelerating gas ports <NUM> in convergence assembly <NUM> carry the accelerating gas from accelerating gas reservoir <NUM> to powder feed injection port <NUM>. Accelerating gas ports <NUM> can vary in number and diameter. These ports <NUM> are preferably equally spaced around the central powder feed injection port <NUM> in convergence assembly <NUM>. A preferred number of accelerating gas ports <NUM> is three (<FIG>).

The accelerating gas from ports <NUM> thereby is injected into the powder feed stream in powder feed injection port <NUM> in convergence assembly <NUM> which is joined in the nozzle entrance <NUM> by the converging combustion streams in <NUM> and <NUM>. The accelerating gas joining the combustion flow increases the mass and force of the combustion stream as it accelerates through the convergent/divergent nozzle <NUM>, allowing the flame to reach its necessary force and temperature in a shorter distance from the nozzle outlet <NUM> than would otherwise be possible. Hence the closer spray distance is obtained through the use of accelerating gas combined with a small physical size of the torch, increased injection pressure and increased power relative to torch size through increased power via increased fuel through the primary fuel supply and/or accelerating gas ports exiting inside the nozzle. This is partially facilitated by optimizing heat transfer resulting in improved torch cooling.

If supercritical CO<NUM> is to be used as accelerating gas, accelerating gas orifices must be such that for a given flow rate, the upstream pressure must be above the critical point of <NUM> MPa (<NUM> atm, <NUM>,<NUM> psi) and the accelerant temperature must be above <NUM> degrees C. For example, for a flow of <NUM> liter per minute CO<NUM> with a density of <NUM>/m<NUM>, a total orifice area of <NUM><NUM> would necessitate a back pressure of <NUM> atm which would meet the supercritical pressure requirement. For <NUM> ports <NUM> this would equate to a hole diameter of <NUM> microns and for <NUM> ports <NUM> this would equate to <NUM> microns.

Particle acceleration in a gas flow is given by the equation: <MAT>.

Particle acceleration can therefore be increased by increasing the gas density. The density of the gas can be determined using PV=nRT. Substituting n = m/ Mw <MAT> Therefore, density can be increased by increasing the gas molecular weight and pressure.

Carbon dioxide may be used as a coolant and accelerating gas. Carbon dioxide has a density that is <NUM> times greater than steam (H<NUM>O) generated from hydrogen fueled torches. At temperature and pressures above <NUM>, <NUM> atm respectively carbon dioxide is supercritical. Supercritical CO<NUM> has a density <NUM>/m<NUM> at its critical point. This compares to a density of <NUM>/m<NUM> at standard temperature and pressure. Using liquid carbon dioxide that is widely available, and is denser than other alternative accelerant gases at the operating temperatures is therefore preferred. Once the accelerant gas is injected, the super critical fluid pressure will decrease and the fluid will transform into a gas and rapidly expand, thereby adding to the acceleration.

The use of carbon dioxide also has the added benefit of reducing the tendency of tungsten carbide (WC) to oxidize to W<NUM>C through the following equation. <MAT> By increasing the partial pressure of CO<NUM> in the system, this reaction is suppressed.

Typical initial conditions for an operating torch are as follows:.

If fuel is used as an accelerating gas, the amount of fuel accelerating gas can be greater, less than or equal to the primary fuel gas flow and does not need to be the same as the primary gas type. The oxidizer will be adjusted accordingly.

Typical operating parameters at <NUM> kW are as follows:.

A gaseous fuel such as: hydrogen, methane, ethylene, ethane, propane, propylene, or liquid fuel such as kerosene or diesel can be added through the accelerating gas inlet ports <NUM>, <NUM> into the convergence to increase gas temperature and velocity. Increased temperature and pressure with transfer to the particles increase these particles temperature and velocity. With fuel accelerant being used, excess oxygen in the primary flow is used to combust the fuel in the nozzle region. The amount of accelerant fuel can be used to control the temperature and velocity of the flame and particle velocity.

Claim 1:
A high velocity oxygen fuel (HVOF) or high velocity air fuel (HVAF) thermal spray apparatus (<NUM>) to apply coatings to external and internal surfaces of a target, said HVOF or HVAF thermal spray apparatus (<NUM>) comprising:
a combustion chamber (<NUM>) having a primary passage (<NUM>) for combustion of fuel received through a fuel input line with oxygen or air received through an oxidizing gas input line;
a divergence section (<NUM>) downstream of said combustion chamber (<NUM>), the divergence section (<NUM>) having two or more channels (<NUM>, <NUM>) diverging relative a longitudinal axis of the primary passage (<NUM>) of the combustion chamber (<NUM>);
an elbow housing (<NUM>) downstream of said divergence section (<NUM>);
a nozzle housing (<NUM>) downstream of said elbow housing (<NUM>), the nozzle housing (<NUM>) retaining a nozzle (<NUM>) having an injection zone (<NUM>) and a nozzle throat (<NUM>);
a convergence section (<NUM>) retained between said elbow housing (<NUM>) and said nozzle housing (<NUM>), the convergence section (<NUM>) having two or more channels (<NUM>, <NUM>) converging toward the injection zone (<NUM>) of said nozzle (<NUM>);
a feedstock injector for the injection of feedstock material for forming said coatings into the injection zone (<NUM>) of said nozzle (<NUM>); and
a plurality of passageways (<NUM>) extending through said combustion chamber (<NUM>), said divergence section (<NUM>), said elbow housing (<NUM>), and said convergence section (<NUM>) for passing a coolant therethrough,
characterized in that the plurality of passageways (<NUM>) comprise a plurality of grooves (<NUM>) interspaced between a plurality of fins (<NUM>) formed on a surface of the convergence section (<NUM>) for facilitating flow of the coolant through the plurality of grooves (<NUM>).