Dust Control
In the past, three different approaches-containment suppression, and collection-have been used to control spillage and dust arising from the operation of conveyors.
Material Containment consists of those systems employed to keep material inside the transfer point with the main material body.
Dust Suppression systems increase the cohesive strength of the material body, keeping fine particles in the body before or as they become airborne.
Dust Collection is the capture and return of airborne material after it becomes airborne from the main material body.
The more successful we are at Containment, the less reliant we are on Suppression, the more successful we are in suppression the less reliant we are on Collection.
Dust Collection systems are extremely capital and maintenance intensive.
The important first step in Dust Control is efficient Material Containment. It is easy to see that the proper performance of the dust suppression and collection systems depends on the success of the containment portion of a transfer point.
Applying dust collection or dust suppression to a load zone without addressing the containment portion will greatly reduce the effectiveness of these two processes. By correctly sealing the loading zone, there is less dust in the air to be suppressed. And when dust suppression is functioning properly the need for dust collection is reduced or eliminated.
Dust Control is intrinsically linked to TMC®
Material ContainmentContainment is the mechanical control and confinement of material on the conveyor or load zone. This seems at first glance as the simplest way to prevent the escape of fugitive material. While it is in many ways the first of the three technologies, it is not so simple as it seems.
Each conveyor and each loading zone on the belt provides a different challenge due to the unique characteristics of the material and components at that given conveyor and load zone.
The sources of fugitive material include belt carryback, conveyor side spillage, tail area spillage, and exit area dust creation. Each source must be identified and monitored as part of a total fugitive material containment system.
For effective Dust Control, we must first have solid foundations. Ensure that you have the following areas addressed first.
Belt SupportProper belt support in many cases consists of the use of cradles installed in-line with the carrying idlers to absorb impact and to eliminate fluctuations at the belt edge. Insufficient belt support allows material to become entrapped at the belt sag between the idlers. This material entrapment accelerates the wear of the skirt seal, which leads to the release of fugitive material.
Skirt Spillage
Loading zone side spillage is best controlled by a three-part program incorporating proper belt support to minimise belt sag, wear liners inside the chute to protect the sealing system, and a multiple-layer, low-maintenance sealing system to capture any escaping fines and return them to the main body of material.
Chute Leakage
Often areas contributing to fugitive material are overlooked. Load chutes with holes from rust of abrasion can allow significant amounts of material to escape. Even the holes created by missing bolts can allow a visible stream of airborne dust to escape. For total dust control, these openings must be closed, either with caulk, a "scab plate", or a properly designed replacement chute.
Now that you have addressed those areas, you can now apply different products to control the Dust.
Tail Gate Sealing BoxSealing of the belt entry in the load zone is problematic. If seal at the tail end is ineffective, loading material will roll down the transition area onto the floor. But if the sealing system is against the belt tight enough to prevent leakage out the back of the loading zone, it will act as a belt cleaner, removing carryback that will accumulate at the foot of the conveyor.
A better approach is to seal this area behind the load zone with a multiple barrier sealing box. This box seals the entry area behind the load zone. A sealing strip on the inside of the sealing box forms a one-way seal to prevent material rollback. It effectively seals rollback material while preventing material build up from the belt cleaner effect.
The dust seal on the sides of the loading chute should start well behind the actual load zone and run continuously from the entry area to the end of the skirted area, eliminating the problem of sealing the high-pressure corners of the impact zone.
Dust Control Curtains
Effective sealing at the exit end of the skirted area requires dual rubber curtains hung roughly 450mm apart, forming a "dead" area to allow dust to settle. These curtains should be composed to slit rubber and extend to approximately 25mm below the conveyed product height. This forms a barrier that deflects when material is running, yet closes down the hole when there is a void in the material load.
As this exit of the transfer point represents the "last chance" for the three dust control sub-systems, it is important that the chute work and all systems be carefully engineering to work together.
Dust Socks
Some transfer points generate a great deal of positive pressure. And using the above products alone may not reduce this pressure enough. By placing a Dust Sock above the covered skirt area before the Dust Control Curtains, will eliminate this excess pressure.
This is basically a filter bag / sock that is about 600mm high that is supported by a simple frame. The material lets the air flow through while stopping the dust in its tracks. The dust then falls back onto the material flow. Depending on the application, more than one Dust Sock may be required.
Wear Liner
Wear Liner is a material dam placed inside the chute wall to prevent material side loading on the skirting dust seal. This side loading, if left uncontrolled, causes tremendous pressures on the skirting seal, leading to spillage and risking major belt damage.
Liner materials could be mild or stainless steel, abrasion-resistant plate, ceramic faced or hard-faced steel, rubber, urethane, or plastic, depending on the specifics of the application.
For all wear liners, the cross-sectional area of the chute should be checked carefully to make certain that the liner does not interfere with flow. It is critical that wear liners be installed as a smooth seamless layer that widens in the direction of belt travel, to minimise material entrapment and the risk of belt damage.