For safety - bifurcated or inline?
CDC Technical Paper

A bifurcated fan is inherently safer than an inline fan, which is always running in a methane-laden atmosphere – true or false?

The regulation applicable to axial flow fan motors says they may not be situated in a return air stream. This regulation was formulated with ducting systems in mind. Long ducting systems driven by a number of axial flow fans at strategic points draw air from the rock face and transport it to the surface. Clean air is drawn down the shaft along the tunnel to replace the methane-laden air that is extracted.

If the long duct run is contaminated with methane at the correct concentration then the system is potentially a large bomb. So it makes sense to move the motor out of the air stream and into the clean air being drawn into the mine.

However the government mining engineer rightly concluded the same situation doesn’t apply with the motor on the fan on a scrubber box on a continuous miner. The motor on this fan is surrounded by methane-laden air, regardless of the type of fan used.

It is also argued that the fan of an onboard scrubber of the continuous miner "concentrates" the methane in the atmosphere and discharges it from the rear of the box, the inference being that the rest of the area is methane-free. To answer this, we must first look at the overall ventilation system.

Air is circulated past the driver aided by jet fan, round past the cutter head aided by the spray fan system, collecting any methane that is released during cutting which together with the coal dust is swept towards the scrubber box. Dust and methane-laden air will be drawn into the scrubber box, the balance going past it, continuing on and out of the cutting.

Up to 50 per cent of the air is re-circulated, which means that methane-laden air is circulated back round again into the cutting.

Where does the air that surrounds the scrubber box come from? There is little doubt that the methane in the air passing through the scrubber box and the fan is at the same concentration as the air surrounding the scrubber box.

Let’s examine the relative positions of the fan motors in the bifurcated and inline units.

In the bifurcated design (motor positioned outside the air stream) the air around the motor – and in the case of air-cooled motors, drawn over the motor by the cooling fan – will have the same concentration of methane in it as that going through the scrubber box. The air around the motor will be relatively dry.

On the other hand, the air being drawn over the motor in the inline design (motor positioned in the air stream) is saturated in a cloud of fine water droplets at 30 to 40 litres of water per minute.

When a spark is generated by a failure in the motor, which of the two units is inherently safer – the motor in a relatively dry atmosphere or the one in a water-saturated atmosphere? In both cases the atmosphere will carry the same amount of methane, if it’s present.

It doesn’t take a rocket scientist to conclude that the inline design with its water-laden atmosphere will quickly quench any spark or potential ignition source resulting from a motor failure.

Finally, the motor in the CDC inline fan is exactly the same design as the cutter head motors, which are located right next to the potential methane source. So if you were to ban the inline fan motors you would be obliged to ban the cutter motors as well!