READ THE PAPER ON OUR SECONDARY MINE VENTILATION DESIGN
Secondary mine ventilation has been hamstrung by the availability of fans that satisfy many competing operational factors. The sometimes legislated requirement of adequate volume to the working face, the minimisation of excavation required to fit the selected secondary fan and the power requirements of various operational phases has, in the past, not been satisfied with just one system. The new system being introduced overcomes all these competing requirements in one simple unit with an added bonus of being very durable without loss of performance when wear occurs on the impeller.
Existing Operational Situation
Several studies1 & 2 have found that between 42% and 49% of mining energy costs are associated with mine ventilation, depending on the type of mine and the equipment used. If we then look at the general breakdown of installed power of the secondary fan to primary vent fans, the ratio is around 2:1. Thus the total energy costs of the secondary ventilation represents about 25% of the overall mining energy costs. It is of interest to note that the cost of running a typical two stage axial vs the cost of purchasing and general maintenance cost of the same fan over a 10 year period, the breakdown will be conservatively in the order of 40:1.
The currently available secondary ventilation axial fan have one duty curve at any given time. Generally this can be changed manually within the motor power parameters but requires a minor outage and removal of the fan to do the pitch adjustments. As the bag vent ducts grow longer, the supplied air reduces to a point where inadequate air is supplied to the working face. This is exacerbated by underground equipment putting additional resistance in the heading and further reducing the supplied air at the working face. This shortfall is normally corrected by the installation of an additional fan in the system to overcome losses in the bag system but comes at a cost of production as the heading needs to be shutdown to install the second fan. This also means that the heading is supplied with excessive air on the shorter runs with a wastage of fan power. Variable Speed Drives in their various forms increase the range of duties but this is limited and overspeed is usually not possible for a Standard axial impeller. There are many downsides to installing a fan with a Variable Speed Drive, as has been highlighted in a paper3 showing major fatigue issues with varying speeds of the fans. Other issues include that VSDs are generally considered unreliable in underground environments and requires cooling whilst maintaining its IP cabinet rating. The heat load of the variable speed drive has also to be accounted for. The other main downside is that altering the speed of an impeller may lead to the excitation of an impeller or blade natural frequency which will result in a very premature failure of the fan.