It is well known that there are a number of safety issues in regards to underground mining in Australia and around the world. One of the hottest topics associated with this form of mining is the high risk of heat illness. Exposure to heat illness can lead to a loss of productivity, health issues and high worker turnover rates.
Auto compression, strata heat, water, people, explosives, oxidation, compressed air equipment, electric and diesel machinery are all heat load sources in underground mining that can result in operating temperature conditions well above safe working limits. As an underground mine progresses further, heat load becomes an increasingly larger issue, with standard induced air flow ventilation methods beginning to lose effectiveness, due to the nature of auto compression heat load. Auto compression heat load is the conversion of the potential energy of the surface air to heat energy as it makes its way deeper through the mine. This means that once you reach a certain mining depth no matter how much unrefrigerated air volume is circulated via ventilation it will be physically impossible to lower the operational temperature to that below safe working limits. Auto compression heat load and localised hot spots in deep underground mines or relatively shallow mines with high surface temperatures, can therefore result in temperatures above safe working limits which will inevitably affect or stop production.
Often excessive heat load problems can be controlled through various ventilation techniques, however if the heat loads are too high and safe working limits are exceeded then the only means of maintaining acceptable working temperatures is through correct application of refrigeration. Most mines in Australia, where refrigeration is required, make use of Bulk Air Cooling (BAC) prior to entry of the mine’s air intakes. This method is ideal where most of the air cooled will be utilised and has the advantage of all maintenance activities on the refrigeration system being surface based. If the mine is very deep, or only a small proportion of the air chilled on the surface is of benefit, then other forms of cooling may be considered. Chilling of water or production of ice at the surface for use in underground bulk air coolers can be an option, or so too can the installation of substantial refrigeration equipment underground as is the case in many South African mines.
There are two main refrigeration systems that can provide the refrigeration required by the bulk air coolers; vapour compression systems and absorption systems.
Vapour compression refrigeration systems are your standard refrigeration setups in which a refrigerant vapour is compressed, condensed, expanded, and then evaporated in a continuous cycle. Refrigerant is generally compressed with a screw or centrifugal compressor, condensed via air or evaporative cooling, then evaporated to chill water which is then passed through a bulk air cooler to cool the air. Vapour compression is generally the most common refrigeration system in place for the mining sector offering high COP’s (coefficient of performance), flexibility, and versatility as they can operate with closed circuit evaporators and air cooled condensers in locations where water is scarce. With appropriate maintenance, vapour compression systems can see a lifetime of over 20 years.
Absorption refrigeration systems are similar to vapour compression systems in many ways except instead of a refrigerant passing through a compressor, it now passes through a generator, an absorber, and an absorbent pump. During this process the refrigerant is absorbed into and evaporated from an absorbent solution. The main advantage that absorption systems have over vapour compression is they can be driven by waste heat or low cost forms of heat which supply the energy for the internal chemical reactions to take place instead of electrical power. The waste heat required can be obtained from a variety of sources such as exhaust heat generated from gas and diesel power station exhausts and jacketing, and via the burning of coal seam methane. Utilising this waste or lost energy source heat provides substantial financial and environmental benefits in terms of power use and CO2 production compared to vapour compression systems, especially for those mines operating from local diesel or gas generator power stations, or a ready supply of low cost gas.
Through the installation of bulk air cooling equipment utilising vapour compression and absorption refrigeration systems it is possible to provide the required cooling to maintain safe working temperatures for employees, maintain or increase production, and take your mine to new depths.