Using skirtboards and curtains to control dust

Conveyors handling dry wastes for recycling often exhibit some material loss from spillage, leakage, dust and carryback emissions, especially at transfer points. The root causes of these fugitive materials are usually obvious but rarely addressed. Yet fixing these issues by applying workable, long-term solutions is proven to increase uptime, reduce costs, improve housekeeping, and ultimately boost profitability.

Reducing visible dust emissions (usually defined as ≥ 40 μm) from conveyors is understandably a primary goal for operators, especially because it attracts the attention of workers, neighbors and inspectors. Yet it’s the dust that can’t be seen by the naked eye that’s more likely to be responsible for long-term health issues.

In enclosed operations, the use of respirators is sometimes seen as an acceptable alternative, but being right at the bottom of the hierarchy of controls should make PPE a last resort. And besides, closer evaluation shows that respirators can reduce productivity by as much as 19 %, and prolonged use can affect cognitive and sensory abilities significantly [1]. Such a reduction in productivity alone justifies taking a more sophisticated root-cause approach to containing dust.

Settling at transfer points

Once material has dropped from a chute or hopper onto on a moving conveyor belt, the skirtboard enclosure essentially creates a settling chamber. The basic concept is that dust particles will settle out of a laminar air stream based on the speed of the air flow, V_air, and the terminal velocity, V_t, of the dust particle (Fig. 2).

There are many “rules of thumb” and traditional practices for calculating skirtboard sizing and dust curtain placement in an attempt to contain the dust in the enclosure. Decades of field experience suggests most of these practices are without proof of performance other than “it’s the way we’ve always done it.” 

However, current good practice for conveyor skirtboard enclosures is to design for V_air ≤ 1.0 m/s by increasing the height of the enclosure. Two common “rules of thumb” for the enclosure length are 2 x belt width or 0.6 m for every 1.0 m/s in belt speed. It should be noted that if H is increased, the distance (L) that the average dust particle must travel also increases. 

To corroborate this practice, detailed design studies of air flow and particulate settling have been performed using flow simulation software. A “typical” conveyor was established as the baseline for the study – a 1200 mm wide belt with a 35° trough angle, traveling at 2.0 m/s. A generic material was used to produce the baseline data, with a bulk density of 1442 kg/m³ and a nominal 50 mm minus particle size distribution with a 20° surcharge. The discharge chute was sized based on a material volume equal to or less than 40 % of the chute cross section. A drop height of 3 m, an open area of 0.9 m², an average particle size of 25 mm and bulk flow of 1680 t/h were used to calculate the induced air volume. 

Several variables were explored to simplify the analysis (the combination of variables studied are given in Table 1). The complete conveyor with discharge and receiving belts was modeled and, while there were significant regions of recirculation in the upper discharge section, the air flow in the chute was reasonably consistent. So, the chute was simplified to that shown in Fig. 3, with the air volume and dust particles injected into the last 2 m of the chute. 

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 emphasizes the need for effective belt cleaning as close to the discharge as possible. A space of 1 mm between the bottom of the skirtboard and the belt was used to simulate leakage. 

Curtain designs

As part of the research, a number of experienced maintenance technicians were surveyed and their preferred curtain arrangements modeled. 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 modeled 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 600 mm a length of 3.6 m and three dust curtains placed in defined locations (Fig. 7).

Worn exit curtains were also modeled, 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. This was exacerbated when the curtain was placed close to the belt, re-entraining dust in the exiting air stream, while failing to encourage recirculation within the enclosure. When the curtain placed at the exit was worn away, the result was as if there was no curtain at all. And a curtain placed right at the exit and adjusted close to the load created another fugitive material problem, sometimes called “the popcorn effect”, where the curtain actually causes spillage by flicking material off the belt. 

Results of the study

Particle density

Solid density had little effect on the settling of nuisance dust particles from 100 to 25 μm. In every case, all of the 100 and 40 μm 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 300 mm height as shown in the standard conveyor in Fig. 4.

The tail box had little effect on dust emissions out of the exit end of the skirtboards. In most configurations, the height of the tail box was set at 300 mm. The tail box length was set at 600 mm to match the typical 600 mm idler spacing used in the load zone by most conveyor manufacturers and engineers.

Length of skirtboard

It was found that for most situations a 3600 mm long skirtboard produced the best results. Increasing the length to 4800 mm and height to 900 mm had some marginal effect, but may not be worth the extra investment.

Height of skirtboard

An enclosure height beyond 600 mm 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 2.8 m/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 50 mm wide, with slits at least 5 mm wide and the curtains extending the full width of the enclosure.

Preferred embodiments

The best value for the cost of the skirtboard enclosure and its effectiveness is judged as skirtboards 600 mm high and 3600 mm long and 3 full width slit curtains using either the retrofit or mitered discharge chute-to-skirtboard connection. 

Design recommendations

Discharge chute width across skirtboards 200 mm < width between skirtboards

Skirtboard outside width based on horizontal dimension of free belt edge for sealing and belt wander edge allowance ≥ 115 mm per side (Foundations™ method) [1]

Skirtboard height ≥ 600 mm

Inlet to skirtboards air volume flow ≤ 0.50 m³/s

Length of skirtboards for material loading turbulence ≥ 1000 mm when required

Length of skirtboards for dust settlement ≥ 3600 mm plus extra allowance for loading turbulence if necessary

Skirtboard dust curtains:

 Entrance (1^st) curtain 300 mm past end of extra allowance for material turbulence and distributing air flow

 2^nd (middle) curtain centered between entrance and exit curtains

 Exit (3^rd) curtain 300 mm from end of skirtboards

 Curtain clearance above the bulk material: 25 mm preferred, 50 mm max

 Curtain flaps: ~ 50 mm wide strips separated by slots ≥ 5 mm

Conclusion

Based on reducing spillage and clean-up labor, increased equipment life and elimination of the need for secondary dust collection, there is undoubtedly a return on investment for the control of fugitive dust with the right skirting and curtain designs for both new and retrofit installations. Additionally, in the majority of cases the improvements also reduce respirable dust emissions, not only reinforcing the financial case but also justifying root-cause engineering solutions in the interests of health, safety and the environment [2].