24” Tube Size for Larger Biomass Dust Collectors
Multiclone dust collectors were the main air pollution control devices for the boiler in the ‘60s, ‘70s, and ‘80s, with the collection of particles in the 5-10 micron range being most important. Today, the expectation of the dust collector is to provide a reliable, low maintenance means of protection for the ID Fan.
Currently, the greatest concern is focused on sand particles that are 20 microns and larger. Most recent wear studies show that particles less than 10 microns do not contribute much to erosion at the velocities in a typical bark boiler ID Fan. Generally speaking, the “medium size” particulate in typical wood fired boiler ash is approximately 50 microns in size.
When ProcessBarron (then Process Equipment) began installing larger tube multiclone dust collectors on paper mill bark fired boilers in the early 1990’s, it was thought the overall collection efficiency might be somewhat lower due to a drop off in the collection of the particles less than 10 microns. This was not a primary concern, as the goal was to provide equipment that would reliably collect the particles greater than 10 microns and be maintainable year after year.
After several multiclone installations were in operation, however, it was determined that the overall collection efficiencies of the larger tube multiclone biomass dust collectors were actually higher than the smaller tube units being replaced. This improved performance has since been attributed to better gas distribution throughout these large (gas volume) dust collectors that require many tubes operating in parallel.
Fighting For Air
Regardless of how evenly the airflow is distributed across the dust collector inlet, the airflow will not distribute evenly across each tube assembly in the casing. Some will be overworked and others will starve. The problem increases as the number of tubes (and tube depth) increases.
Since this condition cannot be eliminated, Process Barron has determined the best option is to reduce the cause by reducing the number of tubes fighting for airflow. Only 1/6th of the number of 24” tube assemblies are required for any application using 9” tube assemblies, and fewer tubes equal better overall performance. Process Barron’s first projects used 14” tube size assemblies.
The improvements were very good, so a transition was made to the 24" large diameter tube size assemblies, which produced even better performance. For this reason, the 24” tube size multiclone dust collector has become our “standard” offering for almost all medium and large bark/wood fired boiler applications (over 100,00 ACFM flowrate).
Operational Advantages – Large Diameter Biomass Dust Collector Tubes
In addition to improved biomass dust collection performance, there are several other operational advantages that will be realized by using 24” size tube assemblies instead of 9” size tube assemblies.
- The 24” tube dust collector requires minimum maintenance when compared to smaller tube dust collectors.
- The 24” tube dust collector can be easily inspected. In many applications, it is impossible to inspect and maintain smaller tube dust collectors because of the lack of space between the rows of tubes. Over the life of the unit, the maintenance costs of a 24” size tube dust collector will be substantially less than that of a smaller tube dust collector.
- Outage down time and installation costs will be reduced, since it takes less time and effort to replace a fewer number of 24” parts vs. a larger number of 9” parts.
- There is a less likely chance of material plugging in any part of the tube assembly (due to its large size).
- The service life of the dust collector will be longer. In typical wood fired application, a 9” size tube collector has a useful life of 1-2 years. A properly engineered 24” size tube dust collector in the same application should have a useful life of 3-5 years. This in turn means the life of the downstream ID Fan is also extended.
Process Barron still markets and sells many 9" tube size dust collectors, but assemblies are only offered for the proper biomass dust collection applications:
- When the fuel being burned does not have a tendency to plug or is not sticky
- When a large percentage of the particulate is 10 microns and smaller in size
- When the particulate loading is not excessively high
- When the air volume is 100,000 ACFM or less
- When the collector depth requirement does not exceed 8 rows on a STD design, or 15 rows on an AU design