What speeds are suggested in the CEMA "Belt Conveyors for Bulk Materials" publication? ■

There are coal conveyors operating at > 10 m/s. If you are refering to the necessary speed to create airborne particles, you can make such a rough evaluation based on the Reynolds Number that incorporates, particle size, air velocity, and time.

Micron sized particles can become suspended at rather low velocities (< 6m/s). Roughness of the coal stream on the conveyor's surface, also carries influence in creating air turbulence beyond the general air flow velocity.

You must determine what particle size you are willing to liberate and the portion of such particles that might be exposed to the air stream. Further, moisture additives and conveyor belt surface covering can restrain most critical surface dust for some time and must be weighed in the cost assessment to control futigive dust.

So, in the final analysis, you must engineer your solution. ■

You talk of particle liberation at 3 m/s. What size are you refering to? What is the percent passing your reference size in the flow stream? ■

If the coal is dry you might release airborne dust under aerodynamic influences. If it is very wet you might bounce off some slurry as the material rocks to & fro on its journey.

As stated before the CEMA/MHEA/DIN people have considered these phenomena in their recommended practices.

If you are honestly concerned about fugitive coal at 3m/s put a lid on it & get your punter to fork out for conveyor covers. ■

Where are the >10m/s speed you mention. Are you talking about boom belts or about long overland? ■

Dear John,

I am not well informed of all installations throughout the world.

In Germany, the lignite open-pit operations have cross pit conveyors operating at 10 m/s.

In South Africa, Eskom operates a 1.1 km Power Strip ash handling conveyor that has attained speeds to 12.5 m/s. The system was installed in 2001, from memory. The conveyor is horizontally curved . They have been experimenting with the behavior of the Power Strip to apply the principle to +20 km overlands. If you wish to see the concept, see our website noted below.

I digress to add:

The Power Strip is a concept I invented in 1992 to overcome the problem of open-pit and long distance transport where transfer stations are not allowed and high rate of curvature are desired.

The invention was first conceived to replace 35 haul trucks in a deep open-pit copper mine in South Africa. The Power Strip could hung the existing truck haul road. It could curve in a 250m horizontal radius to lift ore 300m along the 1500m roadway. The economics were strong and a BOOT system was proposed. Right-hand-left-hand internals then came to light that the Client had already funded another plan that essentially squashed further interest.

In a 1995 Eskom feasibility study, CDI was contracted to provide a Power Strip concept for a 143 km overland conveyor system to transport coal from Witbank coal fields to the Majuba power station. The economics favored the conveyor over trains by over 3:1 on a 40 year plan. Politics intervened. It had a 19% capital benefit over conventional conveyors.

Today, Eskom are still looking to apply it in a conventional program of +20 km overlands. ■

Lawrence,

Thank you. Very interesting. Is the system at ESKOM still working?

Any feedback from operators?

I think 12m/s is quite amazing, as any minor trouble at that speed, must lead to a nightmare...

I'll have a look to your system. Sounds interesting as always.

Best Regards

JP ■