Praxair TAFA 5220 HP/HVOF Gun
The TAFA Model 5220 gun, also referred to as an HVOF thermal spray torch, has key benefits afforded by the 5220’s high particle velocities including extremely high coating density, high hardnesses, low oxide content, and better overall coating integrity.
Density is achieved by particles packing tightly and uniformly to reduce porosity. Low oxide content results from reduced particle temperature and shorter dwell time in the flame. These two features are especially beneficial when applications require high hardness to combat wear and dense, low oxide coatings to resist corrosion.
The TAFA Model 5220 gun’s combustion chamber exit is sized and shaped to create a supersonic over-expanded jet and to maintain a low-pressure region where the powder is introduced. This combustion design produces exceptionally high chamber pressure of up to 120 PSIG (8.2 bar) (vs. 60-80 PSIG (4-5.5 bar) in gaseous-fueled HVOFs) and superior gas velocities (7,200 fps (2,190 m/sec) for the 5220 vs. 4,800-5,900 fps (1,460-1,800 m/sec) in others). Another central design feature is its radial powder injection. The powder is injected downstream of the combustion nozzle. Injection into this lower pressure area promotes better powder mixing, more even heating, less oxidizing, and more uniform, higher particle velocities. The particles impact with kinetic energy and have a lower, but more consistent temperature compared to other HVOF thermal spray torches.
Because of the extremely high gas and particle velocities achieved, coatings can be sprayed very thick and exhibit favorable compressive stresses that enhance bond strength and performance. The proven gun design is rugged, yet elegantly simple, featuring durable, long-lasting components that have withstood the harshest industrial spray environments repeatedly and reliably for many years.
Advantages of the 5220 Gun
- Rugged, simple design
- Converging / diverging combustion chamber throat allows high gas and particle velocities and low-pressure powder injection zone
- Dual, radial powder injection ensures uniform powder mixing into flame
- Variable barrel lengths to optimize powder heating
- Electronic, internal ignition
- Monitored combustion pressure for repeatable coating quality
- Safe, non-volatile liquid fuel
- Low operating cost
- Lower flame temperature
- Less carbide solutioning
- Higher coating hardness
- Exceptional & Reproducible coating quality
- Compressive coating stress
- Thick coating capability
- High spray rates when needed
- Long, insensitive stand-off
The Model 5220 HP/HVOF gun offers outstanding coating characteristics at low to high spray rates without loss of coating characteristics or deposit efficiency
Efficient water-cooling allows operation for extended periods, increasing uptime. Unlike gaseous-fueled HVOF thermal spray torch designs, which require closely controlled water temperatures to reproduce coating quality, the 5220 is not sensitive to water temperature. This unique design permits the use of inlet water temperatures from 40-80°F without effect on gun performance or coating quality.
The TAFA Model 5220 gun is interchangeable with the Model 8200.
A key benefit to the high particle velocities generated by the TAFA Model 5220 gun is the extremely high coating density and low oxide content. Low oxide content is simply due to the particles having less time in the atmosphere to oxidize. The high velocity also packs the particles more tightly, reducing porosity and creating a denser coating. This is especially beneficial in corrosion resistance applications where a denser barrier is needed and when grinding, where high-density coatings produce a finer finish.
Another key factor in the Model 5220 gun’s coating quality is the heating consistency of the powder as it exits the gun. The radial powder injection design with variable length barrels of the Model 5220 gun heats the powder uniformly. Independent laboratory tests have shown the WC-Co (tungsten carbide-cobalt) and CrC – NiCr (chromium carbide-nickel chromium) coatings have superior wear resistance – in some cases one-third of the wear of coatings from gaseous-fueled HVOF guns. This is due to uniform powder heating and the lower flame temperature, which allows a much higher percentage of carbides to remain in their original, less brittle state. The powder heating consistency also improves bond strength and overall coating quality.
Photomicrographs of typical coatings (Figures 4 Carbide and 5 Metallic) show exceptionally clean, low porosity, low oxide content coatings, and in the case of carbide coatings, with uniformly dispersed carbides.
|Model 5220 Gun Specifications|
|Weight (gun only)||9.5 lbs (4.3 kg) net|
|Height||6.25" (159 mm)|
|Weight||3.25" (83 mm)|
|Length||16.5" (419 mm)|
|5220 Gun Requirements|
|Cooling Water Flow||7-12 gpm
(26.5 - 45.4 I/min)
|Minimum Pressure||100 psi (6.8 bar)|
|Maximum Pressure||150 psi (10.2 bar)|
|Oxygen Flow (maximum)||2300 SCFH (47 I/min)|
typically K1 Kerosene)
|10.0 gph (38 I/hr)|
|50 SCFH (24 I/min)|
Optional 3-Way Powder Splitting
Praxair Surface Technologies has introduced a unique 3-port powder feed system for the TAFA Model 5220 and 8200 HP/HVOF® guns. The new 3-way splitter and 3-port interconnectors allow a more balanced powder distribution to the powder injection ports. This allows for more consistent coating quality and reduced barrel wear. With the improved powder introduction into the flame, the overall target “spot” size on the coating substrate is reduced thus improving target efficiency.
3-Port Powder Injection System
This 3-port powder injection system reduces the powder buildup and wears in the barrel. This powder injection design allows for a more balanced and consistent introduction of powder into the supersonic gas stream with less powder near the edges of the flame. This leads to better target efficiency by having a more concentrated spray “spot” with slightly higher deposit efficiency. These benefits allow for longer run times, reduced barrel replacement, and lowers maintenance shutdown requirements.
The Praxair Surface Technologies TAFA Model 5220 and 8200 guns are the heart of the JP-5000® and JP-8000TM HP/HVOF® systems. These elegantly simple and effective designs produce coatings of the highest qualities.
Of critical importance to both guns is powder injection into the flame at the Interconnector located at the low-pressure region after the converging/diverging combustion chamber nozzle. Proper powder distribution to the powder injection ports is important for minimizing barrel wear as well as producing consistent coating quality. In order for the Interconnector to perform properly, the powder must be supplied to the powder injector ports in as equal distribution as possible.
For a gun equipped with a 2-port Interconnector, the optimal way to achieve a 50/50 distribution of powder is to use two powder feeders supplying equal amounts of powder and carrier gas to each side of the gun. However, this is more expensive upfront because of the cost of the second powder feeder.
System Spray Setup
The typical system spray setup is a simple, and less expensive, use of a 2-port Interconnector on the gun, a single powder feeder with a “T” powder splitter. Using this setup, powder distribution is theoretically supposed to be 50% to each powder injection port. However, the “T” splitter never gives a 50/50 powder split. It is usually a 60/40 or 70/30 split which, over time, is erratic.
Though this “T” splitting of the powder has worked extremely well throughout the years, this new option may work better for your application needs. With the 3-way powder splitting arrangement, powder distribution from a single feeder is intended to be divided 33/33/33. Even if this becomes unbalanced the amount of powder going to each injection port is more evenly distributed than what is achieved using a two-way “T” splitter.
Additionally, having 3 separate powder streams flowing almost equal mass that intersect in the center of the flame not only reduces barrel wear but can in some instances, eliminate or minimize barrel loading from fine particle-sized or low melting powders.
Switching from a 2-way to 3-way powder split is easy, simply replace the Interconnector, powder splitter, and tubing from the splitter to the gun. Then, while maintaining the same powder feed rate, adjust carrier gas flow higher to approximately 11.8 – 13.2 l/min (25 - 28 SCFH).