Current shearer drums are of two types, the conventional pick-flushing drums and extraction drums. With the conventional wet system, the water is sprayed through sprays mounted on the top of the cutter drum vanes. The sprays can be located in front of the cutter pick, known as pick face flushing (PFF), or placed at the back and is known as pick back flushing (PBF).
Conventional suppression of dust by wet cutting is achieved by attempting to wet the dust particles at the point of crushing, i.e. where the picks impact onto the material. Thus the water is mechanically mixed with the coal as it is cut. Three possible disadvantages to the conventional system are:
a) The number of outlets is determined by the number of picks and not the size of the drum (i.e. similar numbers of picks are used on drums of all diameters),
b) One of the main dust producing areas of the drum is the clearance ring where the pick density is at its greatest with a minimal depth of cut and is the most difficult zone for mounting the water outlets,
c) Suppression of the dust made by degradation of material in the drum relies on the added moisture at the pick point. For faces with low dust make these disadvantages play a minimal part, however, if the dust make from cutting and loading is high, then the conventional suppression method will not be sufficient.
In the extraction drum system the mechanical mixing the water does not happen as the water is mixed with the cut coal at the discharged from the drum. Two versions of the extraction drums are developed. They are the rotary air curtain (RAC) extraction drum and Inverse rotary air curtain extraction drums known as CAR extraction drum. The combination of pick flushing and ED systems has been used in some situations where there is a risk of ignition temperature potential (ITP) and increased methane gas accumulation at the cut zone.
The RAC drums or Extraction drum, is a holow dust extraction system, initially designed in the UK, in 1980, for British coal mines , is aimed to reduce the amount of airborne dust generated during cutting by drawing dust laden air from the cutting zone through water powered dust capture tubes built into the shell of the shearer drum. High pressure water, up to 10MPa, is released from a spray ring manifold located on the face side of the drum which induces dust laden air into the tubes. The dusty air is cleaned as it passes through the tubes before being discharged through an annular gap between the goaf side of the drum barrel and a deflector plate (Fig. 10). The deflector plate turns the air back into the cutting zone at an angle of 45% which helps to constrain the dust inside the cutting zone. A 90 per cent reduction in dust compared with normal cutting drums is claimed. However, trials in Australia have not been successful to date due to blockage of the tubes and sprays and failure of the seals in the water distribution system. As a consequence, the ED system has been modified to include:
• A hybrid ED extraction “CAR System” with conventional pick flushing system. In this system the flow of air is reversed and forced through the hollow shafts from the goaf side of the drum to the face side. This is achived by turning water jets around towards the face side in the hollow tubes. At Oakey North Mine the application of CAR and pointed conical pick back face flushing system was introduced to control the ingnition temperature potential (ITP) methane levels at the coal cut zone. The control of ITP is attributed to PBF system. Currently this hybrid system is applied nearly to 60 % of longwall faces in Austrslia.
• The RAC extraction system with back face flushing of large radial picks. This system has recently been introduced to Baltana Mine longwall face in Hunter Coalfield, NSW. Early dust survey has indicated improvement in dust reduction at the face as well as minimising the incidence if ITP. (click for shearer drums pick flushing systems)
The depth of web cut on each pass is determined by the width of the shearer drum. An average web width is between 0.8m and 1.0 m. Coal is cut from the face by rotary drums either single or double ended drums. Each cutting drum is normally mounted at the end of a ranging arm. The shearer drum diameter can be in excess of 2.0 m, for example the shearer at Mandalong mine has a drum diameter of 2.6 metres. A shearer with twin drums mounted on ranging arms are called Double Ended Ranging Drum Shearers (DERDS). Machines with a single drum on a ranging arm are known as Single Ended Ranging Drum Shearers (SERDS).
The design of shearer drum is a complex process and is dependent on the the type of coal being cut, the thickness of seam section being cut and the power of the machine. The shearer has it’s picks arrayed around the periphery of drum helical vanes and back plates. The vanes are built around the central hub of the drum. The function of the vanes is to carry the picks as well as conveying the cut coal on to the face conveyor beneath the shearer. The number of vanes built on the drum can vary between 2 and 6 to suit the drum geometry, speed and vane angle requirements. The number of vanes on a drum are known as starts. For example a drum with three vanes is know as a three starts drum. Generally, 3 start vanes (spirals) are used in soft and easily cuttable coals. For coal which is exceptionally hard or woody and does not easily breakout, then 4 starts or higher vanes is preferable. In very thick seam mining, some drums have a pre-cutter attachment which are used to aid better coal cutting. Cutting picks are also carried on the back plates, and their function is to precut a releif for the picks mounted on the vane. The diagram opposite shows a general picks pattern drawn out on the modern three starts drum. Both vanes and back plate picks positions on the drum are shown and is known as drum pick lacing.
Picks can be mounted on the shearer drum radially or tangentially. Modern drums generally use tangentially mounted point attack picks known as forward attack picks. Radially mounted picks are of chisel type and are less favoured in Australian mines. Water is supplied to the drum to assist in reducing dust during coal cutting operation. Water sprays can be applied at the pick face (know as pick face flushing) or at the back of the pick (known as back face flushing).
Currently the are 26 longwall faces in Australia with various sheare drum types (See table on Australian Shearer Drums and Dust supression systems)