TW Woods Constructions discuss design problems in coal transfer chutes

Transfer chutes are a vital link when conveying coal and ore. Getting them right at the outset of design and fabrication is important. There are problems caused by design and fabrication issues, such as differences in the coal industry between the velocity of the coal exiting the transfer chute and the velocity of the conveyor belt. TW Woods Constructions observe that this particular issue leads to premature belt replacement, which is expensive on a cost basis and in downtime.

TW Woods Constructions have found problems resulting from coal chutes blocking due to large lumps of rock and other objects causing downtime to clear them and the attendant loss of further output.

Some of these problems can be traced back to inadequate chute specification and design. For example, chutes often back up because of inadequate cross sectional area. While still delivering throughput, the buildup of spillage resulting from this problem can flood the transfer chute, causing downtime. Maintaining these poorly performing chutes soaks up man power required elsewhere to maintain the operational plant and equipment.

Problems can also be caused by coal leaving the head pulley and contacting the chute at a severe contact angle, causing rapid chute wear and degradation of the coal (as well as generating significant dust and noise). High speeds of the granules through the chute contribute to excessive wear of the chute. Low headroom in both underground and above-ground chutes compromises the performance of any transfer chute and demands early attention at the design stage.

Good chute design and fabrication will take into account of factors such as these that may not always appear to affect the initial performance of the chutes, but which will cause problems overtime and which will result in early failure.

Companies wanting better performance and durability from their chutes need to provide specific information on some of the flow properties (as listed below) of the material to be transferred. (Organisations such as TUNRA Bulk Solids Handling Research Associates of the University of Newcastle have most materials tested for most mines):

• Accurate tonnes per hour
• Flow properties testing
• Worst case handling moisture determination
• Critical cohesive arching dimensions
• Required mass flow geometry
• Wall friction characteristics
• Adhesion properties
• Internal friction angles
• Effects of time consolidation and vibration
• Repose angles
• Bulk material compressibility
• Solids and particle densities
• Particle size analysis (sieve and laser)
• Transportable Moisture Limit (TML)
• Fluidisation, de-aeration and air-retention

One solution to many of the problems stated is offered by soft loading transfer chutes incorporating adjustable hood and spoon. These soft loading transfer chutes which TW Woods Constructions and 3D Engineering Solutions are designing and building for mines and power stations transfer coal onto the receiving belt at the same or slightly faster horizontal component of the belt velocity.

The vertical component of the coal velocity is reduced by such a design approach, thereby reducing belt wear at the impact zone. As the spoon and hood are not necessarily attached to each other, this design controls the material flow to exit the spoon in the same direction as the receiving conveyor irrespective of the angle.

The hood is fully adjustable vertically, horizontally forward/backwards/sideways and the vertical angle (tilt) is also adjustable to cater for fine tuning the coal stream while running at full capacity. The spoon is also adjustable for forward/backwards/tilt, sideways and shim packing for vertical adjustment.

Where there are taller transfers as in above ground installations, the design incorporates intermediate segments and the transfer is designed to the geometry. The hood is still freely adjustable, but the spoon is attached at the bottom of the intermediate chute segments and different spoon/outlet segments can be bolted in place to accommodate changes to the coal flow.

For durability, the hood or spoon and intermediate segments are manufactured from chromium carbide clad plate with high wear areas in the intermediate segments being replaceable by unbolting the fabricated segment and bolting in the replacement pre fabricated section as required. The removed worn segment is renovated and held in store to be used to replace the other segment when it wears.

A particular benefit is that, if wear rates are high from impact and/or abrasion, the repaired segment can have a harder and stronger clad plate inserted in the fabrication to prolong its life span.

Time and cost savings result from the longevity of the high wear areas and lack of maintenance especially for the receiving belt, because wear is reduced and impact idlers zones are not required. A major maintenance advantage is the decrease in spillage at the transfer point.

Good specifications and design are not enough by themselves to ensure ideal performance. Selection of an experienced and well equipped fabricator is also important. On-time delivery can also be enhanced by selecting a provider that has the required range of services in-house and does not have to employ a chain of sub-contractors, (multiplying risk, lessening quality control and inviting delay by a lack of smooth continuity between the phases of the job).

Good backup is also critical. As a result of the rugged nature of the industry, all mining equipment has a finite life. When equipment does need servicing or replacement, it is important to have people who can assist maintenance staff to undertake this work to avoid downtime.

The supplier must know every nut, bolt and plate of his own equipment and be able to track these immediately to the customer requires. The supplier must also have an established relationship with the site involved and know their methods, preferences and operational requirements.

All these issues need to be factored into the design and performance equation. The T.W. Woods Group, which supply materials handling technology including transfer chutes and loading gates to mining and energy companies have undertaken a major expansion of their Tomago facility. T.W. Woods Group have begun exports to Canada and China.