Air Prep / Dust Collection
Air PreparationIt is important that the gas used is clean and dry to maximize the life of the high-power heater core and ensure the smooth operation of the powder feeding system. If clumping of the feedstock powder due to moisture and contaminants is avoided, the Cold Spray process provides higher deposition efficiency and ensures that the applied coating or the substrate will not become compromised during the spraying process.
SST recommends that a 2-micron filter is used and that the carrier air has a -40 °C dew point. This is a little drier than the standard -33 °C rating common to many air preparation equipment specifications.
- Cartridge filters are comparatively low in cost but they require maintenance. Usually, a pre-filter of between 20 microns to 5 microns is used to remove the bulk of moisture and contamination. This technique improves the service life of the cartridge filter. As the filter cartridge becomes burdened with contamination, the output flow rate will decrease. Routine maintenance is necessary to ensure that the process efficiency will not be impacted by the flow rate decline.
- Membrane filters require less maintenance than cartridge filters because the membrane does not trap but rather isolates the contamination from the downstream system. Typically, these devices are self-cleaning by either gravity or an active process. It is common for membrane filters to consume about 5 m3/h (3 SCFM) of air.
- A refrigerated dryer can also be used to eliminate moisture from the compressed air supplied to the cold spray system. If the refrigerated dryer is part of an already existing air compressor system, ensure that the dryer is properly set and capable of getting the air dry enough.
- To eliminate the inconvenience of setting all the above filters together, SST offers an Air Preparation Package that consists of one 20-micron and one 2-micron cartridge filters, plus two of the 22 SCFM membrane filters. The four filters, altogether with a 20-gallon surge tank are then mounted on a wheeled cart, offering great mobility and flexibility. If your application has special requirements, SST can tailor equipment to your particular process.
Dust CollectionConsiderable care must be taken to manage finely divided metal particles and dust involved in the cold spray process. The appropriate NFPA or other applicable standards should be followed when designing, installing, operating, and maintaining dust collection systems that work in conjunction with the SST equipment.
While the following list is not all-inclusive, it may help to know some of the common design requirements. Some key requirements for safe operation include:
- The air velocity in conveying pipes must be greater than 4,500 fpm.
- Downdraft or backdraft tables should have a minimum of 150 fpm air velocity at the surface.
- An air curtain should have a flow of at least 400 fpm.
- The mass of accumulated aluminum powder should never exceed 1 pound.
- Only custom explosion-rated vacuums can be used with SST equipment and can be either integrated into custom SST systems or fitted with casters for portability. Typically, these vacuums have a 1-1/2 HP motor operating at 220 V, 60 Hz, and include a HEPA filter for the discharged air.
- SST offers 2,000 cfm and 5,000 cfm sized wet filtration systems. The advantage of these systems is that they trap the waste powder in water so that combustion does not occur. A supply of water and a drain must be provided with these systems. The escaping moist air is commonly vented to an area where it will not adversely affect the Cold Spray process, such as outside the plant or office.
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