VENTILATION Main Fans

Note that the following is written using fan in the singular for the main ventilation. In fact where a large fan duty is required it is common to use 2 or 3 fans in parallel to provide the duty. The reasons for this are:

• Fan duties are becoming very large and the physical size of fans that could meet such duties singularly become impractical
  
• With multiple fans partial ventilation can be maintained in the event that one is stopped, a feature which is considered very beneficial.
  

The main fans which ventilate large mines are major power consumers, having large motors which run virtually continuously, usually only being stopped for maintenance purposes. Great care is therefore required in design and specification in order to avoid unnecessary expenditure. The fan duty required often varies greatly over the life of a mine as the workings become more extensive and it is usually best to make provision for varying the fan performance. This can be done by installing inlet vanes, which restrict the fan inlet size, or by varying the fan speed. Speed can be varied by changing motors, by use of gears or by use of variable drive systems (VVVF drives are being increasingly used).
Air quantity flowing at the main fan is large and unless care is taken with the design of airways and ducting considerable power can be wasted in overcoming the resistance and shock losses which result. Airways should be as straight and smooth as possible and sudden changes of direction should be avoided as far as practical. At the same time there is a requirement that provision be made to protect main fans from damage in the event of an explosion. This is done by mounting the fans away from the direct line of the main airway, which usually involves a 90^o deflection (or close to it), and providing a device in line with the airway that is designed to fail under pressure and release the explosion pressure (e.g.doors or a weak section of duct).
Airways should be kept as clear as possible, though it is common for main return shafts to involve winders with the associated shaft fittings.
High air flows in shafts can cause problems:

• Because of the effect of turbulence on stability of cages, counterweights or ropes
  
• If shafts are wet high velocities can result in water being held in suspension causing unstable flow (air velocities between 8 and 12 m/s should be avoided; above this range water will be carried up the shaft and will have to be dealt with at the fan site and part of the fan power cost will actually be for pumping water).
  

As fan duties become more onerous, the ability to provide ventilation using only fans on the surface becomes increasingly difficult, especially for mines with coal liable to spontaneous combustion. The use of "booster fans" sited underground and acting in series with the surface fan will become more common. While such strategies may be preferred they raise difficulties in relation to interlocking surface and underground fans, avoiding recirculation and separately ventilating underground fan motors to ensure gas-laden air does not pass over the motor(s).