Latest round of 'Inline versus Bifurcated' debate
CDC Technical Paper

The debate on the relative merits, in terms of safety, of the inline and bifurcated fan designs used on wet fan scrubber boxes mounted on continuous miners was opened with a hard-hitting article in the June 1997 issue of CDC News.

In that article we set out to show that the inline system is superior to the bifurcated system from a safety point of view. The inline fan design is used in all CDC wet fan scrubber boxes.

The article evoked a swift response from Alex McGaw, managing director of Locked Torque, with a length counter-argument in a paper entitled "Is the risk worth it?" The company of course uses a bifurcated fan design in its Engart scrubber boxes.

In the present article we take the debate further and, in doing so, hope to keep the record straight.

Since there has been no extensive testing of the two systems to substantiate – or disprove – the claims made by either party, it is inevitable that opinions will be formed on the basis of good engineering practice, scientific data established from research and development in associated parameters, and, in the final analysis, the actual practical performance of the systems in operation in the workplace.

The safety of mine personnel is paramount. The secret of good engineering design is to reduce the inherent risks of the mining environment to an acceptable level.

How the two systems differ

Before examining the pros and cons of each system, it is perhaps advisable to review the differences between them.

The bifurcated design, where the motor is situated out of the air stream of the fan, uses a standard flame-proof air-cooled motor.

CDC, in developing its current wet fan scrubber box, also initially went this route, but found that that the air-cooled motor was unacceptably unreliable and therefore investigated the use of a specially-designed motor for the application. It selected a water-cooled motor similar in design to the cutter head motors used on the CM. These motors are situated close to the cutter head in areas of high methane concentrations and are renowned for their ruggedness, reliability and safety in operation.

It was therefore a natural step to abandon the bifurcated design in favour of the much more compact inline design, taking advantage of all the design features of the water-cooled motor.

A couple of legal niggles

In reference to current legislation, McGaw raises the question: Is a wet fan scrubber box a return airway? Regarding auxiliary ventilation fans, Regulation 10.14.1 states: "Where it is intended to install a fan in a return airway from a ventilation district the installation should be such that the motor is situated in intake air and there shall be no possibility of return air passing over the motor."

Talk to any mining engineer about a return airway and he will describe a ducted column fitted with one or more axial flow fans with a system length of at least 10 m, but more often often hundreds or even thousands of metres. A scrubber box is 1 m long – so where’s the comparison?

In fact neither the bifurcated fan nor the inline fan on a scrubber box system are capable of meeting these requirements.

Fresh air? What fresh air?

In support of his claim that the bifurcated design is safer than the inline design, McGaw argues that methane-laden air is drawn over the inline fan motor, whereas only fresh air is drawn over the bifurcated fan motor.

We beg to differ over the latter claim. In fact the methane-laden air is drawn over both the inline and bifurcated fan motors. As is explained in McGaw’s paper, South African mining law demands that if the methane content in the general air body exceeds 1,4 per cent, then all electricity is cut off. Since methanometers are mounted in strategic points to ensure that this regulation is adhered to, it is difficult to imagine how methan fare in excess of 1,4 per cent concentration can be drawn in by the inline fan.

On the other hand he describes a situation with a bifurcated fan motor where a methanometer mounted close to the intake of the dust extractor and a second one mounted at the dust extractor discharge show a methane content, at both points, in excess of 2 per cent and sometimes as high as 4 per cent.

Since the motor of the bifurcated fan is situated midway between these two points, how can only fresh air be drawn over the motor by the cooling fan? In short, the fresh air argument is a myth!

Methane concentrations

McGaw goes on to claim in his paper that a high concentration of methane is drawn into an inline fan on start up, after interruption of the cutting cycle.

The answer of course is that, in these conditions as in any other, the air drawn over the inline fan is the same as the air drawn over the bifurcated fan, as already stated.

However, addressing this point specifically, it is true to say that methane builds up at roof level during a stoppage. But if the methanometer cuts-outs are working as they should, then the CM will be unable to start until the methane concentration has been reduced by the action of the jet fan and the general ventilation system.

However, in the event that no cut-out occurs under these conditions despite a methan build-up, it must still be taken into consideration that the scrubber fans are operating at 10-15 cu m/sec and whilst methane may well be drawn from just below the roof area, a lot of air is also drawn from the rest of the cutting to dilute the methane concentration.

What routine inspections do and don’t reveal

In response to McGaw’s contention that the inline motor is less accessible for inspection than the bifurcated unit, it has to be stated that of all the motors returned to us for repair, including a few damaged mechanically, we have yet to ascertain the cause of failure from an external inspection. In every case the motor has had to be stripped to find out what actually happened.

Therefore routine inspection of a motor is extremely unlikely to reveal any potential cause of failure.

While we’re on the subject of motor inspections it is instructive to note incidentally that we’ve found most of the bifurcated fan motors we’ve come across during inspections covered by a pile of coal dust. And in the case of bifurcated fans mounted with the opening top-to-bottom rather than side-to-side, the whole motor cavity is filled with coal. None of this bodes well for the performance and cooling of the motor.

Would you like to read that comparison again?

McGaw says that we have been misleading our clients by arguing that the gathering arm motors at floor level are in the same atmosphere as an inline motor, which is situated at the top of the coal cutting machine. CDC has never made this comparison – only a comparison between the inline fan motor and the cutter head motors which are situated at the top of the machine. These motors are similar in construction and, if anything, the cutter head motors are located even closer to high methane concentrations than the inline fan motor.

To date there is no record of the cutter head motors – or the inline fan motors, for that matter – having been the source of an ignition. If McGaw’s argument gained enough credence to result in the inline fan motor being banned, then the cutter motors would have to be banned as well!

The proof’s in the practice

The Inline versus Bifurcated debate will no doubt rage on for some time to come. However ultimately what counts are the practicalities and the performances of the units themselves.

It’s a fact that more and more mines are switching to the inline fan design.

The reasons are simple and straightforward: the units are more reliable, more accessible, easier to maintain and have proved to be extremely safe in operation.

The simple fact of the growing acceptability and increasing success of the inline units could explain why the supporters of the bifurcated design have become increasingly more vociferous in their condemnation of the inline design.

Trying to create a smoke screen to hide the deficiencies of the bifurcated design, perhaps? Sorry, I’ll re-phrase that – a dust screen.