An important, but often overlooked, component of these systems is the dust collection equipment at the end of the system that filters dust out of the air used to convey the materials.
Generally speaking, an undersized or poorly configured dust collector can lead to several issues including dust emissions, high differential pressure (DP) across the media, excessive media cleaning functions, and reduced media life – regardless of the application. However, the nature of pneumatic conveying requires additional scrutiny to not only avoid these issues but also to prevent additional issues such as over-pressurization of the terminal equipment, which can lead to dusting out of equipment openings and may impact system performance.
Typical Dust Collection Equipment for Pneumatic Conveying Systems
For positive pressure conveying systems, the typical filtration equipment configurations include bin vent dust collectors (Figure 1) that mount on silos, hoppers or process equipment, and filter receivers (Figure 2) that include a hopper with a rotary valve, butterfly valve, or slide gate at its discharge. With bin vent applications, the silo or connected equipment is the terminal point for the conveying system while the hopper of a filter receiver serves as the terminal point for the system to which it is connected.
For negative pressure (vacuum) conveying systems, a vacuum filter receiver is the typical terminal point of the system; it is similar to a filter receiver used in positive pressure systems but is designed for the vacuum pressure of the application.
Dust Collector Configuration
Dust collectors are configured for a quantified airflow using target values for two parameters: air-to-cloth ratio (acfm/sq. ft. of media) and interstitial velocity (fpm).
Air-to-cloth ratio is the rate of “dirty” air passing through the effective media cloth area; target values mainly depend on media configurations and dust characteristics. Essentially, air-to-cloth ratio is the speed at which the dust-laden air approaches the media. As such, filtration velocity (fpm) is used interchangeably with air-to-cloth ratio. Figure 3 illustrates air-to-cloth ratio.
High filtration velocity can lead to dust being retained on a media’s surface with high force, driven into the media where it is trapped, or passing through the media as emissions. A high DP level due to surface-retained or embedded dust requires increased cleaning energy to recondition the media, which can shorten media life, especially when trapped dust leads to a rapidly increasing residual DP.
Interstitial velocity is the speed of air and dust upwards through the media cross-section; target values depend on the dust particles’ terminal velocity.
Excessive interstitial velocity inhibits proper dust discharge off the media and down into the connected vessel. As such, it often builds up underneath the tube sheet or between the media elements which reduce effective cloth air, thereby increasing the filtration velocity.
Additionally, built-up dust can rapidly drop, flooding connected vessels or damaging the filter media, including dismounting the elements (bottom-load collector).
In some applications, the “can” velocity of a dust collector (the velocity in the filter housing below the filter media) should be reviewed to ensure excessive values do not cause issues.
Typically, the “can velocity” area is greater than the interstitial velocity area and is not a concern with sizing a collector. However, if there is a converging transition to a smaller opening on the connected vessel, the velocity in the smaller opening may be sufficiently high to prevent dust from dropping out.
Figure 4 illustrates filtration, interstitial and “can” velocities.
Determining Applicable System Airflow
For vacuum conveying systems, the flow at the inlet of the air mover (ICFM) is used to configure the collector.
For positive pressure conveying systems, the determination of applicable airflow is dependent on the conveying regime of the process. In dilute-phase conveying applications, typically fed via a rotary valve, the applicable airflow is the SCFM of the blower multiplied by a small surge factor to account for potential surges in pressure due to material surges.
For semi-dense and dense phase conveying applications, the surge factor is much higher due to the fact that the typical pressure vessels discharging materials into a conveying line serve as potential energy storage devices. When a vessel empties of material, the instantaneous drop in pressure leads to a very high momentary surge of airflow that can overwhelm a collector if it is not configured properly.
Conclusion
Properly sizing a dust collector for a pneumatic conveying application requires an understanding of the operating characteristics of the system, the characteristics of the dust, and the available media configurations.
Sized correctly, a dust collector in a pneumatic conveying application will provide consistent performance and achieve long media life.
Cyclonaire is a recognized leader in the engineering and manufacturing of pneumatic conveying equipment. We take great pride in providing value-based solutions for bulk material transfer applications, railcar unloading, locomotive track sanding systems, dust collection and aeration systems. We also produce a full line of parts and accessories that integrate ...
