FEATURE
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 .
Diesel-powered equipment ventilation In mechanized mines using dieselpowered mobile equipment and in the absence of continuous gas monitoring , exhaust gas dilution is used to determine the minimum ventilation air requirements where they operate . The amount of air required normally ranges between 0.03 and 0.06 m3 / s per kW of rated power at the point of operation depending on the type of the engine and whether any exhaust gas conditioning is being used .
Continuing developments in both fuel and engine technology are providing lower engine emissions while catalytic converters , wet scrubbers and ceramic filters may further reduce the leaving concentrations of carbon monoxide / aldehydes , oxides of nitrogen and diesel particulates respectively . This helps in meeting increasingly stringent contaminant limits without significantly increasing exhaust dilution rates . The minimum possible dilution limit of 0.02 m3 / s per kW is determined by the carbon dioxide emissions which are proportional to engine power and unaffected by exhaust gas conditioning . Diesel engines are about one-third efficient at converting the energy available in the fuel to useful power and most of this is then used to overcome friction resulting in a heat output which is about three times the power output .
Even when hauling rock up a decline in a truck , the useful work done is only about 10 % of energy available in the fuel .
Higher diesel engine powers are used in larger mobile equipment which require bigger excavations to operate safely . Allowing for normal vehicle clearances and a typical diesel exhaust gas dilution rate of 0.04 m3 / s per kW , the minimum air velocities where diesels operate average about 0.5 m / s .
The provision of suitable thermal conditions to minimize the dangers and adverse effects of heat stress may require mechanical cooling in addition to the ventilation necessary to control contaminants . Although the applied heat stress is a complex function of climatic variables and physiological responses to them , in practical mining terms it is the air velocity and wet bulb temperature that have the greatest influence .
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 .
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