Martin Engineering’s president emeritus Todd Swinderman breaks down how to fix dusty conveyors.
When inspecting the conveyor transfer point, is dust escaping through gaps in the skirtboard?
Visible dust emissions (typically defined as ≥ 40 μm) from conveyors draws the attention of workers, neighbours, and inspectors. Often, the use of respirators is viewed as an acceptable workaround.
However, studies have shown that respirators can reduce productivity by as much as 19 per cent, and prolonged use can significantly impact cognitive and sensory abilities. These decreases in productivity alone can justify improvements to conveyor dust containment to lower emissions.
Dust emissions originate from air turbulence in the loading zone from the belt’s movement and tons of material impacting the belt. One of the most effective and economical solutions is to examine the skirtboard, seal the environment, and control the airflow.
Skirtboards
The skirtboard enclosure is essentially a settling chamber. The basic concept is that a dust particle will settle out of a laminar air stream based on the speed of the air flow, Vair, and the terminal velocity, Vt, of the dust particle.
distance “L” in skirtboard enclosure. Image: Martin Engineering
There are many rules of thumb, along with traditional and industry-based practices for skirtboard sizing and dust curtain placement to contain the dust in the skirtboard enclosure. Most of these practices are without proof of performance other than, “That’s the way we’ve always done it.”
Current practice for conveyor skirtboard enclosures is to design for Vair ≤ 1.0m/s by increasing the height of the enclosure. Two common rules of thumb for the enclosure length are two times belt width or 0.6m for every 1.0m/s in belt speed. It is interesting to note that if H is increased, the distance (L) that the average dust particle must travel also increases. A detailed design study of air flow and particulate settling was performed using flow simulation software.
A ‘standard conveyor’ was established as the baseline for the study. The standard conveyor is a 1200mm wide belt with a 35° trough angle, traveling at 2.0m/s. A generic material was used to produce the baseline data, with a bulk density of 1442 kg/m3 and a nominal 50mm minus particle size distribution with a 20-degree surcharge. The discharge chute was sized based on a material volume equal to or less than 40 per cent of the chute cross section. A drop height of 3m, an open area of 0.9m2, an average particle size of 25mm and bulk flow of 1680mtph were used to calculate the induced air volume.
receiving and discharge for external analysis
Image: Martin Engineering
Several variables were investigated to simplify the analysis. The complete conveyor with discharge and receiving belts was modelled and, while there were significant regions of recirculation in the upper discharge section, the air flow in the chute was reasonably consistent. The chute was simplified, with the air volume and dust particles injected into the last two metres of the chute.
The combination of variables studied are given in Table 1. Both external and internal analyses were conducted, with complete moving discharge and receiving conveyors.
The bulk material surface was set to absorb particles and the walls set to reflect particles. The effectiveness of the enclosure variations was determined by counting the number of each size particles that escaped the end of the enclosure compared to the number injected.
The results of the external analysis indicated that escaped dust particles increased in speed, as the air current is affected by traveling around the belt and the discharge pulley.
This phenomenon is known as the Magnus effect and emphasises the need for effective belt cleaning as close to the discharge as possible. A space of 1mm between the bottom of the skirtboard and the belt was used to simulate leakage.
Curtain designs
Several experienced maintenance technicians were surveyed and their preferred curtain arrangements modelled. In addition, multiple curtain designs and placement schemes were studied, including staggered, slit, curved, angled, with and without slits, with holes and no curtains. Several unconventional skirtboard enclosures were modelled to create recirculation in the enclosure and improve dust settling.
The optimum design for the standard conveyor was determined to be a conventional enclosure with a height of 600mm a length of 3.6m and three dust curtains placed in defined locations.
Worn exit curtains were also modelled, and as the spacing above the load increased, the dust settling performance deteriorated. The use of a single curtain right at the exit proved problematic in all cases, acting to speed up the exit air flow even further when close to the belt and re-entraining dust in the exiting air stream, while being ineffective in creating recirculation within the enclosure.
When the curtain placed at the exit was worn too much, it was as if there was no curtain at all. A curtain placed right at the exit and adjusted close to the load creates another fugitive material problem, sometimes called the popcorn effect, where the curtain causes spillage by knocking material off the belt.
Results
Particle density
Solid density had little effect on the settling of nuisance dust particles from 100 to 25μm. In every case, 100 per cent of the 100 and 40μm particles settled almost immediately. As the bulk density increased, there was a moderate reduction in respirable dust emissions.
Discharge chute and tailbox
The junction between the discharge chute and the skirtboards was found to be an important design detail for creating recirculation. Making the width of the discharge chute narrower than the width of the skirtboard helps to fold the air flow going into the first curtain, and that encourages distribution of the air flow toward the top of the enclosure, rather than along the surface of the material. The retrofit and mitered junctions were significantly more effective than a simple butt connection and 300mm height. The tail box had little effect on dust emissions out of the skirtboards’ exit end. In most configurations, the height of the tail box was set at 300mm. The tail box length was set at 600mm to match the typical 600mm idler spacing used in the load zone by most conveyor manufacturers and engineers.
Length of skirtboard
It was found that for most situations a 3600mm long skirtboard produced the best results. Increasing the length to 4800mm and height to 900mm had some marginal effect, but may not be worth the extra investment.
Height of skirtboard
An enclosure height beyond 600mm for the standard conveyor with a single exit curtain did reduce nuisance emissions but tended to increase respirable dust discharge, because the average settling path was greater with the higher enclosure.
Air flow
As would be expected, the average air velocity through the skirtboards was directly proportional to the induced air flow and cross-sectional area. Average velocities in the skirtboards due to induced air ranged from 0.8 to 2.8m/s. Belt speed has a minor effect on the average velocities. The maximum air velocities were almost always found where the air flows under the skirtboard curtains.
These high air speeds kept the respirable dust suspended, so reducing induced air into the chute is also important in improving performance.
Curtains
The best results were obtained with three or more curtains. The design of the slits in the curtains is important to allow air to pass through, allowing the airflow paths to fill the entire chamber and not just flow at high speeds under the curtains. It was found that the individual flaps should be about 50mm wide, with slits at least 5mm wide and the curtains extending the full width of the enclosure.
dust particles exiting the enclosures. Image: Martin Engineering.
Preferred embodiments
The best value for the cost of the skirtboard enclosure and its effectiveness is judged as skirtboards 600mm high and 3600mm long and three full width slit curtains using either the retrofit or mitred discharge chute-to-skirtboard connection.
Design Recommendations
- Discharge chute width across skirtboards 200mm < width between skirtboards.
- Skirtboard outside width based on horizontal dimension of free belt edge for sealing and belt wander edge allowance ≥ 115mm per side.
- Skirtboard height ≥ 600mm.
- Inlet to skirtboards air volume flow ≤ 0.50m3/s.
- Length of skirtboards for material loading turbulence ≥ 1000mm when required.
- Length of skirtboards for dust settlement ≥ 3600mm plus extra allowance for loading turbulence if necessary.
- Skirtboard dust curtains:
- Entrance (1st) curtain 300mm past end of extra allowance for material turbulence and distributing air flow.
- 2nd (middle) curtain centered between entrance and exit curtains.
- Exit (3rd) curtain 300mm from end of skirtboards.
- Curtain clearance above the bulk material: 25mm preferred, 50mm max.
- Curtain flaps: ~ 50mm wide strips separated by slots ≥ 5mm.
Conclusion
While some improvement is seen with increased skirtboard height and length, it is doubtful that it can be economically justified on the reduction of respirable dust alone. A return on investment for control of nuisance dust for new and retrofit designs can be based on reducing cleaning labor, increased equipment life and/or elimination of dust collection.
If the improvements reduce the time weighted average of respirable dust emissions to the point where engineering or administrative controls could be less stringent, then a financial case could also be made based on improvements in labour productivity.