A: Eric Maynard, Jenike & Johanson, says:
Dust collection system plugging manifests itself in many forms, such as in the duct, hopper, or filter. The root causes of plugging in a system can be plentiful and include:
- Poor duct layout
- Overfeeding the dust collection line
- Hopper design
Poor duct layout. There are many instances where the conveying velocity was correctly selected and the fan and motor operated as required, but the dust didn’t convey effectively through the duct. This is generally the result of too many elbows being placed in close proximity to one another in the system and poor branch-to-trunk layouts, as shown in Figures 1a and 1b.
As demonstrated in American Conference of Governmental Industrial Hygienists (ACGIH), the branch entries to the trunk line, as well as throughout the duct layout, are important, and 90-degree entries should be avoided. In Figure 2, various branch entry layouts are shown, labeled with how acceptable each is from a sound design perspective. Furthermore, the duct diameter should be expanding along its run to the dust collector to maintain the minimum conveying velocity, a factor that’s changing with the cumulative airflow gain caused by an increase in pipe diameter.
Overfeeding the line. Overfeeding of a dust collection system can present problems because the additional material in the conveying airstream increases system resistance, which then reduces airflow and velocity. Some designers will use a dust collection system as a part of a material recycling loop, for which the system usually wasn’t originally designed. As a result, heavy solids loading in the line, often erratically introduced to the system, degrades system performance. If a feeder, such as a rotary airlock valve or screw feeder, can be used to modulate the solids flowrate into the duct, then this can be a simple improvement to regulate the feedrate and prevent plugging.
Leaks. Leaks in a dust collection system can “rob the system” of conveying energy and cause material settlement and buildup. Just like a leak in your household wet-dry vacuum hose, the system’s dust collection performance will degrade. Leaks can occur at duct or pipeline couplings, diverters, elbows where holes have formed, blast gates, and in dust collector housings (especially at the maintenance doors). Leaks can be tested via use of talcum powder, helium tracing, or noncombustible smoke around suspected leak points.
Buildup. Figure 3 shows a significant buildup problem with resin pellets in a conveying line. The air, alone, moving through the system with the buildup exceeded the system resistance design condition, thereby rendering dust collection impossible as solids were introduced to the system. The buildup problem was a result of temperature-induced softening of the resin, allowing the material to fuse to the pipeline’s interior. This created increased friction and reduced the pipeline’s diameter, which both significantly increased the system’s resistance to air and solids throughput.
Buildup in the ductwork can be addressed by various methods, including: periodic manual cleaning; “pigging,” whereby a semi-rigid projectile with flexible ribs and brushes is sent through the ductwork; or dry ice chips are used, where the scouring action cleans the duct and the dry ice evaporates, leaving no trace. At a minimum, the ducts should be inspected quarterly to ensure buildup isn’t affecting the system performance or leaving residual combustible dust or other solids that can allow dust explosion propagation to occur.
Hopper design. An often-overlooked issue with dust collection system operation is the plugging of the collected material in the hopper attached to the filter-receiver. The following are possible problems that can arise when material builds up:
- Bridging: A no-flow condition in which material forms a stable, arch-shaped obstruction over the outlet of a hopper.
- Ratholing: A no-flow–erratic-flow condition in which material forms a stable open channel within the hopper.
If the dust or collected bulk solid is allowed to accumulate in the hopper, and if the material is cohesive (tends to stick to itself, especially when packed), then these flow problems are likely to occur. These flow problems are the result of a hopper discharging material in an undesirable flow pattern. The type of flow pattern you choose for your dust collection hopper can directly influence the type of material flow performance you’ll experience.
Unfortunately, the strong majority of standard-design collector hoppers can yield undesirable flow patterns with hard-to-handle materials since these hoppers discharge bulk materials in a funnel-flow pattern. With funnel flow, some material moves while the rest remains stationary. Most dust collector conical hoppers are sloped at 60-degree angles or have a shallow pyramidal geometry that encourages stagnation in the corners (called valley angles). Though both of these hopper types are easy to build and have a relatively low cost, they generally don’t allow reliable flow with cohesive and adhesive (sticks to walls) bulk solids.
Flow problems can be prevented with hoppers specifically designed to discharge materials in a mass-flow pattern. With mass flow, all material moves whenever any is discharged. Flow is uniform and reliable due to the first-in first-out flow sequence; ratholing is prevented; and there are no stagnant regions, so dusts and powders won’t cake. Information about how to generate mass-flow designs for hoppers can be found in Andrew Jenike’s published bulletin on the subject.
Bridging and ratholing problems can often be avoided in a dust collection hopper if the hopper is frequently emptied and the material isn’t allowed to accumulate. In some cases, controlled vibration applied to the hopper in a short burst (and not continuously) or injecting pulsed air into the material or inert gas sweeps may be needed to dislodge the dust adhered to the hopper walls. In those cases, it’s not ratholing, but rather buildup of several inches of dust on the hopper’s wall surface that’s preventing proper mass flow.
Eric Maynard holds a master’s degree in granular mechanics and is the vice president of Jenike & Johanson.