Powder Coating Over-spray Containment
How To Avoid Over-Spray.
Powder coating operations use a dry powder blend to produce a finish on a product. The material is applied electrostatically in dry form and then exposed to thermal energy to melt the material and create a hardened finish. The dry blend does not contain any volatile organic compounds and therefore it is not treated the same way as liquid coating that contains solvents. Exhaust systems in the spray booth capture the over-sprayed powder on a dual layer of filters and deliver the cleaned air back into the surrounding environment.
Powder coating spray operations use an enclosure called a powder booth to contain the over-sprayed powder. Airflow is used to create negative air pressure inside the booth and pull the over-sprayed powder into a collection system. The airflow is generated by a fan that is part of a recovery system that captures the over-sprayed powder for reuse or disposal.
Some percentage of the powder will attach to the part surface by virtue of the electrostatic attraction and the balance of the powder sprayed will be pulled into the reclaim system. Typically, 40% (reference Chemical Coaters Association & Society of Manufacturing Engineers on transfer efficiency) of the total powder sprayed from the powder gun will miss the part and be pulled into the recovery system.
The over-sprayed material is routed to a collector system that has a series of cartridges. The cartridges are typically 99% plus efficient at removing all of the powder from the airstream that is below 5 micron in particle size. The powder below 5 micron is typically less than 5% of the total sprayed. The air is then discharged back into the surrounding environment through a High Efficiency Particulate Arresting (HEPA) filter to ensure that no particulate is discharged back into the area. The final HEPA filter is 99.97% efficient to 0.03 micron.
Powder particle size is measured in microns using a Particle Size Analyzer. The smallest particle that the analyzer will record is 0.328 micron. The percent below 0.03 micron is so low that it cannot be measured by any known instrument. Without a leak in either filter the expected discharge through the filter is effectively zero.
The formula for a cartridge collector is as follows:
- Number of guns x output per gun per hour = total powder sprayed per hour.
- Total powder sprayed per hour x 40% = total volume of powder collected per hour.
- Total powder collected per hour x 5% = the total powder under 5 micron that reaches the cartridges.
- Total powder under 5 micron x 1% = total potential powder that passes the cartridges per hour to the final filter.
- Total potential powder that passes the cartridges per hour x 0.0001 = total particulate discharged per hour.
Example for Cartridge Collector System
- 14 guns x 15 kg/hour = 210 kg/hour (assumes 100% uptime)
- 210 x 0.40 = 84 kg/hour to collector
- 84 x 0.05 = 4.2 kg/hr. total powder that is under 5 micron that reaches the cartridges
- 4.2 x 0.01 = 0.042 kg/hour under 5 microns reaches the final filter
- 0.042 x 0.0001= 0.0000042kg/hour potentially discharged back into plant
The formula uses 14 guns. Many powder systems use only 2 or 4 manual spray guns so the potential to discharge is much lower than this automated example.
A cyclone reclaim system uses a cyclone as a separator so the amount of powder that reaches the cartridge collector is less than 10%. The total that could be potentially discharged back into the atmosphere is therefore much lower than the amount shown for the cartridge system.