The main objective of mine ventilation is the provision of sufficient quantities of air to all the working places and travel ways in an underground mine to dilute to an acceptable level those contaminants which cannot be controlled by any other means. Where depth and rock temperatures are such that air temperatures are excessive, mechanical refrigeration systems may be used to supplement the beneficial effects of ventilation.

     The contaminants to be controlled by dilution ventilation are primarily gases and dust, although ionizing radiations associated with naturally occurring radon may present problems, especially in uranium mines and where the background uranium concentrations of the host or adjacent rocks are elevated. The amount of air required for dilution control will depend on both the strength of the contaminant source and the effectiveness of other control measures such as water for dust suppression or methane drainage systems in coal mines. The minimum dilution air flow rate is determined by the contaminant requiring the greatest dilution quantity with due cognizance of the possible additive effects of mixtures and synergism where one contaminant can increase the effect of another. Overriding this value could be a minimum air velocity requirement which is typically 0.25 m/s and increasing as air temperatures also increase.

     Both axial and centrifugal fans are used to provide air circulation in mine ventilation systems, with fan efficiencies of over 80% being achievable. The selection between axial flow or centrifugal for main mine fans depends on cost, size, pressure, robustness, efficiency and any performance variation. In mines where a fan failure may result in dangerous methane accumulations, additional fan capacity is installed to ensure continuity of ventilation. Where this is not so critical and with a twin fan installation, about two-thirds of the mine airflow will continue if one fan stops. Vertical axial flow fans installed over the airways have low costs but are limited to about 300 m³/s. For larger air quantities, multiple fans are required and they are connected to the exhaust with ducting and a bend.

     To obtain the highest efficiencies at reasonable cost, axial flow fans are used for low pressure (less than 1.0 kPa) applications and centrifugal fans for high pressure (greater than 3.0 kPa) systems. Either selection is suitable for the intermediate pressures. Where robustness is required, such as with exhausts with air velocities above the critical range, and water droplets are carried up and out of the system, a centrifugal fan will provide a more reliable selection. The critical air velocity range is between 7.5 m/s and 12.5 m/s where the water droplets may stay in suspension depending on their size. Within this range, the amount of suspended water can build up and increase the system pressure until the fan stalls. This is the region where some of the air recirculates around the blades and fan operation becomes unstable. Although not desirable for any type of fan, the possibility of a centrifugal fan blade failure is significantly less than an axial blade failure in this region of flow fluctuation.

     It is rare that a main fan is required to operate at the same duty point over the life of the mine, and effective methods of varying fan performance are desirable. Although variable speed results in the most efficient operation for both axial and centrifugal fans, the costs, particularly for large fans, is high. The performance of an axial flow fan can be varied by adjusting the blade angle and this can be carried out either when the fan is stopped or, at a significantly higher cost, when it is rotating. By imparting a swirl to the air entering a fan using variable inlet vanes, the performance of a centrifugal fan can be varied while it is running.
     The efficiency of the centrifugal fan away from its design point falls off more rapidly than that of an axial flow fan and, if a high performance is required over a wide range of operating points and the pressures are suitable, the axial flow fan is selected.

     The first mine refrigeration system was installed at Morro Velho, Brazil, in 1919. Since that date, the growth in worldwide capacity has been linear at about 3 megawatts of refrigeration (MWR) per year until 1965, when the total capacity reached about 100 MWR. Since 1965 the growth in capacity has been exponential, with a doubling every six or seven years. The development of mine refrigeration has been influenced both by the air conditioning industry and the difficulties of dealing with a dynamic mining system in which the fouling of heat exchanger surfaces may have profound effects on the amount of cooling provided.

     Initially, the refrigeration plants were installed on surface and the mine intake air was cooled. As the distance underground from the surface plant increased, the cooling effect was reduced and the refrigeration plants were moved underground closer to the workings. Limitations in underground heat rejection capacity and the simplicity of surface plants has resulted in a move back to the surface location. However, in addition to the intake air being cooled, chilled water is now also supplied underground. This may be used in air-cooling devices adjacent to the working areas or as the service water used in drills and for dust suppression.