At bulk solid handling plants, it is common for dust collection systems to have performance problems associated with the ducting system. These dust collection systems are used to keep the workplace clean and healthy for workers, and to protect equipment from fires and explosions. A high level of dust in the workplace is the first sign of underperforming dust collection systems. The main source for problems with dust collection systems is the actual design and installation. Dust collection systems that are properly designed and installed, need little maintenance and perform well. Problems with dust collection systems can be diagnosed by simple field checks. These field checks entail taking differential pressure readings and visual checks.

EFFICIENT VERSUS INEFFICIENT DUCTING SYSTEM DESIGN

Before you can use the results of your field checks to find the source of plugging problems, it helps to understand what makes a ducting system design efficient or inefficient.

EFFICIENT VERSUS INEFFICIENT DESIGN:

An efficient ducting system serves as a conduit for conveying dust-laden air at low pressures and is designed for natural airflow distribution, requiring minimal use of blast gates (adjustable dampers with a flat plate that slides across the ducting) to achieve balanced airflow. Most designers use standard computer design programs to design a ducting system with balanced airflow.

In contrast, an inefficient, poorly designed ducting system relies primarily on blast gates to force balanced airflow. With an only marginally correct collection air volume, the poorly designed system invites tampering by operators who attempt to improve airflow distribution by taking a hammer to the blast gates. Such tampering without taking any airflow measurements almost always results in plugging because a change in resistance to airflow will positively or negatively affect all the other ducting system branches and reduce air velocity in the system. As the system fan’s performance begins to climb its characteristic pressure-versus-volume curve, the airflow is restricted, causing the airflow to redistribute itself through the ducting system.

COMMON DESIGN PROBLEMS THAT LEAD TO PLUGGING:

Various ducting design flaws can lead to plugging. The flaws commonly include the use of butterfly dampers, undersized and poorly located pickup hoods, ducting without thermal insulation, and low air volume.

BUTTERFLY DAMPERS:

Using a butterfly damper in the ducting for air volume control and airflow balancing when the system handles a heavy dust loading (greater than 3 grains per cubic foot) can cause the dust to fall out of the air stream, because the damper’s valve plate and shaft partially obstruct the ducting. Using a blast gate for heavy dust loadings can prevent this problem because the blast gate’s flat plate completely pulls through the opening, preventing any obstruction. For a heavy-loading application, install the blast gate in a vertical ducting run immediately upstream of the pickup hood.

UNDERSIZED AND POORLY LOCATED PICKUP:

If an undersized pickup hood is located at a source of dust-generating turbulence — such as at a transfer chute’s end or a bucket elevator’s discharge — the resulting dust loading can be greater than your dust collection system is designed to handle. You can often detect heavy dust loading by observing the dust collector’s operation. For instance, a high but stable across the collector or a rapid pulse-cleaning cycle can indicate excessive dust loading. If your filter life is short or your system can’t meet stack emissions requirements because of the excessive dust loading, moving some pickup hoods and designing them to reduce the entrained dust loading can be cost-effective solutions. For instance, for an enclosed transfer point such as at a conveyor-to-conveyor transfer, you can design the enclosure to function as an extension of the pickup hood. This design allows maximum stilling of the airborne dust before the hood captures it, minimizing the dust load to the collector and allowing the dust collection ducting to provide a mechanical means of controlling airflow.

An efficient or inefficient ducting system design plays an important role in the success of dust collection systems. KB Duct offers design assistance and can do all of the work for you or can provide a quick quote for those who know the ducting parts they need.