Coal chute design: getting it right
Design problems in coal transfer chutes may lead to spillage and blockages, resulting in lost time and lost production because of the need for manual clearing.
Here, *Tom Woods discusses typical problems, suggests ways to overcome them, and concludes that care spent in proper design and fabrication is repaid many times over.
Transfer chutes are a vital link when conveying coal and ore. Getting them right at the outset of design and fabrication can save major headaches down the track.
Over many years of producing materials handling systems for major miners we have become acutely aware of 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.
The experience of our company, T.W. Woods, and of our designers, 3D Engineering Solutions, is that this particular issue invariably leads to premature belt replacement - which is expensive on a cost basis and in downtime.
We have found also major problems resulting from coal chutes blocking due to large lumps of rock and other objects, requiring down time 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 ultimately flood the transfer chute, causing down time that no company can afford in the current economic environment. 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. Also, low headroom in both underground and above-ground chutes invariably compromises the performance of any transfer chute and demands early attention at the design stage.
Good chute design and fabrication will take account of factors such as these that may not always appear to affect the initial performance of the chutes, but which will cause substantial problems over time and which will result in the potential for early failure (generally shortly after the warranty has expired!).
Companies wanting better performance and durability from their chutes will 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 tons per hour
Flow Properties Testing
Worst Case Handling Moisture Determination
Critical Cohesive Arching Dimensions
Required Mass Flow Geometry
Wall Friction Characteristics
Internal Friction Angles
Effects of Time Consolidation and Vibration
Bulk Material Compressibility
Solids & Particle Densities
Particle Size Analysis (Sieve & Laser)
Transportable Moisture Limit (TML)
Fluidisation, De-Aeration and Air-Retention
Proper design to respond to these specifications
One solution to many of the problems stated is offered by soft loading transfer chutes incorporating adjustable hood and spoon.
These chutes - which T W Woods and 3D Engineering Solutions have been designing and building for major mines and power stations since 1999 - 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 markedly reduced by such a design approach, thereby reducing belt wear at the impact zone. Because 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 - no matter what 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 specifically 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 and 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 significantly reduced and impact idlers zones are not required. A major maintenance bonus is the significant decrease in spillage at the transfer point.
Fabrication, delivery and backup
Good specifications and design are not enough by themselves to ensure ideal performance. Selection of an experienced and well equipped fabricator is also important, and no-one who is proud of his work will hesitate to point you to customers who will comment independently on performance achieved in your industry (be it coal, ore or energy)
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. Because 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 within the narrowest possible window of opportunity so as to avoid downtime.
Not only must the supplier know every nut, bolt and plate of his own equipment - and be able to track these immediately the customer requires - but ideally he also has an established relationship with the site involved and knows their methods, preferences and operational requirements.
All these issues need to be factored in to the design and performance equation. Anything less can be asking for trouble at a time when the industry can least afford it.
*Tom Woods is a director of The T.W. Woods Group, of Tomago, which is celebrating the completion of a multi-million dollar redevelopment that will see it expand sales of its metal fabrication and coal and ore handling technology throughout Australia and internationally.Contact: Tom Woods, email - firstname.lastname@example.org