AFCs | Equipment Selection | Longwall | underground COAL

EQUIPMENT SELECTION

a) Armoured Face Conveyors (AFCs)

Although frequently considered after supports and cutting machines, AFCs can be the ultimate limitation on face width. Given that geological, ventilation, financial and such issues would not limit a longwall width, it will almost certainly be the AFC which governs the final size. In most cases, adding extra supports to widen a face would have minimal effect other than the additional unit cost. There is not even a large effect on pump capacity as the same number of supports will be operated at any time as for a shorter face (though there will be extra pressure losses in longer hoses and increased leakage). The cutting machine would be the same size regardless of face length and a longer face would only entail additional incremental costs for longer cables, hoses, etc.

With the AFC however, any additional length requires additional power in the drives, additional strength in the transmission equipment and additional strength (which usually involves additional size) in the AFC chain.

The additional power will normally entail an increase in physical size of drive units. In thinner seams, simply fitting the larger equipment in the seam height may be difficult, will minimise any clearance left to allow for strata movement (roof or floor) and will increase the resistance to ventilation, all factors which merit close consideration at the planning stage. In thicker seams, the effect of these factors will of course be less, though increased ventilation resistance may still be important, but in all seams the increased power will give rise to increased heat production, heat which is bound to be released into the ventilating air at some point. This will be a major consideration in mines where heat is a problem.

Additional power will also dictate an increased chain size (48mm chain is quite common on large longwalls and 50mm chain is now in use) together with increased sprocket size which in turn impacts on the size of face end equipment. Large chain brings with it material handling difficulties when sections have to be moved on or off the face.

Calculation of power requirements is by no means a simple procedure and large factors have to be included to allow for worst case situations. The starting point is simply the power required to drag the empty chain around the pan line and overcome the friction involved. Consideration of the "payload" to be carried must allow for the possibility of a completely full load, at least to the top of the AFC back plates, and the possibility that a proportion of this may be stone (cut intentionally or because of a loss of roof control). There is then the probability (certainty in some respects) that the AFC will seldom be completely straight horizontally and vertically, and the face grade can be in favour of or against the load, frequently some of each. Added to all this is the possibility that all this resistance may have to be overcome from standstill. While there are drive arrangements that can ensure a "soft start" and avoid the very high initial demand of a direct start, the energy still has to be transmitted through the system in some way to get things moving.

There are computer programs available to calculate power requirements for AFC's, given that the properties of the material to be transported and seam conditions are known, but a decision based on experience (especially if there are other mines in the vicinity) or on a risk assessment has to be made to decide the economic optimum level of "overdesign" to cater for the worst cases. Inadequate power or strength of components may require the excess load to be shovelled off the AFC by hand in order to restart after a stoppage, a lengthy and therefore expensive exercise. Faces with 2 or even 3 motors rated at 1MW each (including main and tail gate drives) are in use successfully on some faces.

Large AFC capacity entails more robust and wear resistant line pans, unless a shorter life is acceptable, which is seldom the case. Improved design and construction methods and use of more wear resistant materials have all led to better AFC performance. Separation of structural and wear components enables the use of abrasion resistant steel on the wear sections. Thicker, and in some cases replaceable deck plates help to extend the working life. The improved design and construction methods have led to closer tolerances being possible which in turn assists in reducing wear and increasing life and in allowing greater flexibility. Modern pan lines are capable of around 7° of vertical flexure and around 1° horizontal.

All this extra power and load capacity leads to a requirement for stronger attachments between supports and pans, particularly at face ends where all the larger equipment has to be advanced by the few main gate and tail gate supports, usually the same number of supports being involved regardless of face width.

While dog bones have had improvements in design and manufacture, some being rated at up to 450t, these are still intentionally designed as the weak link to fail before more expensive damage occurs (i.e. equivalent to shear pins).

Shearer drive racks have also increased in size along with larger and/or faster shearers. While the racks are carried on the AFC pans, they are essentially an add-on item and can be changed out if an increased size is required, possibly with an upgraded mounting arrangement. However, extra weight would still be involved.

Increased AFC capacity entails increased BSL, crusher and boot end capacity all of which results in increased power requirements and increased component strength in their turn.

All the above power and strength increases may be desirable from the point of view of longwall operators, but all come at an additional financial cost and with additional weight. The latter is not usually a problem during operation, unless a soft floor is involved which fails under the extra load, but may become an issue during face relocation when the heavier items have to be transported. As usual, the optimum size will probably be a compromise between several factors.