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Right-Sizing Mechanical Dust Collector Inlet Tubes

During the 60-plus years they have been in use, the inlet tube diameter used in mechanical dust collectors has undergone some dramatic shifts. These changes — from the original 6-inch to the 9-inch and 24-inch sizes used today — have been motivated by the changing role dust collectors have come to play in air handling systems. 

Up until the early 1980s, when EPA industrial emission regulations became much more rigorous, mechanical dust collectors served as the primary air pollution control (APC) device in most biomass-fired boiler systems. Their designs were entirely geared toward removing pollutants from emissions. Very small diameter (6 inch) tubes were favored since they created greater centrifugal force, which caused more particulate to be separated from the airstream.

In response to the EPA regulations, air handling systems began to incorporate more sophisticated APC equipment such as fabric filter baghouses, wet and dry scrubbers, and electro static precipitators (ESPs). Dust collectors took on a supporting role, removing larger, more destructive debris to protect systems’ air handling equipment — induced draft (ID) fans, in particular. Thus the now-familiar role of mechanical dust collectors as “precleaners” was established.

To remove larger particles, inlet tubes that were 9 and 10 inches in diameter were typically engineered for biomass-fired boiler applications. Many manufacturers and users felt that this was the ideal size, striking a perfect balance between air volume handling and collection efficiency. A 9- or 10-inch inlet tube is still a good selection when the air volume requirements are small and grain loading is light. However, medium- to large-size boilers that produce large flue gas volumes (100,000 acfm and greater) exhibit poor gas distribution and underperform when outfitted with 9- and 10-inch tubes.

Larger systems function much better with 24-inch inlet tubes, each of which can handle the airflow of seven and a half 9-inch inlet tubes while maintaining the same pressure drop across the system. Furthermore, controlled laboratory testing comparing 9-inch and 24-inch tube assemblies has revealed that the smaller size is a little more efficient at collecting particles less than 10 microns, but that the difference is insignificant when it comes to particles that are greater than 15 microns. Since other studies have established that particles less than 10 microns in size do not contribute to the wear of centrifugal machinery — e.g., the fans, duct, economizers, and air heaters that the mechanical dust collector, in its role as “precleaner,” is supposed to protect — there is no reason to retain 9- or 10-inch tubes in larger systems when one considers the performance gains possible with a dust collector outfitted with 24-inch tubes.

Moreover, today’s 24-inch design is fundamentally more reliable because larger tubes become clogged less often than smaller ones. Over time, some guide vane plugging will occur in all systems, but the possibility of total bridging is greatly reduced in systems with 24-inch inlet tubes.

To learn more about the history and benefits of the various sizes of mechanical dust collectors out there, visit our Learning Center and download Philip Gore’s comprehensive article, “Evolution of the Mechanical Dust Collector in Biomass Applications.” You can also call 1-888-663-2028 and speak to Philip directly.