As we move into the summer months in Australia, we are looking forward to the warmer weather where we can spend more time outdoors, enjoying blue skies and sun. Air-conditioning systems become synonymous with summer to stay cool. In the context of underground mining operations where heat becomes an issue at depth, mine cooling systems should be considered.
Mine cooling systems generally comprise three major components, namely, bulk air coolers (BACs), refrigeration machines or chiller units and heat rejection for the condensers. Open circuit BACs tend to be quite popular in mine cooling installations and tend to be more efficient compared to closed-circuit BACs. However, closed-circuit BACs also operate efficiently but at a particular set of operating conditions e.g. ambient air temperatures and evaporator water temperatures. The selection of open- or closed-circuit BACs needs to be evaluated based on the merits and appropriateness for the mine site. Chilled water supplied to the BAC is produced at the evaporator in the refrigeration machine or chiller unit.
The mechanical vapour compression refrigeration machine in its most basic form consists of four components: evaporator and condenser heat exchangers, compressor and electrical motor and expansion valve. The supply of electrical power to a compressor allows for the production of cooling at the evaporator and ensures conformance to the Second Law of Thermodynamics (a reverse heat engine).
A simple part of the Second Law of Thermodynamics states that heat flows from a high temperature to a low temperature. In a refrigeration cycle, the supply of mechanical work (compressor and electrical motor) to a refrigerant allows for heat to be transferred from a low temperature to a high temperature. Heat from the air is supplied to the evaporator via the evaporator water circuit to vaporise the refrigerant at constant temperature and pressure (path AB). The compressor increases the pressure of the refrigerant vapour (path BC). Heat is removed from refrigerant in the condenser via the condenser water circuit (path CD) at a higher pressure and temperature compared to the evaporator. Liquid refrigerant downstream of the condenser passes through a valve to reduce the pressure (path DA) and return to the start of the cycle (point A).
Absorption chillers have also been installed at mine sites in Australia but similar to the selection of BACs, the merits of the absorption chiller and vapour compression chiller will be evaluated against the mine’s requirements. Condenser heat rejection generally takes place in condenser cooling towers that circulate water between the condenser and condenser cooling towers (CCTs). The CCTs are induced-draft units with fans installed at the top of the tower and air flows through inlet louvres on the sides of the tower. Heat exchange takes place directly with water droplets produced in the tower and air flow through evaporation.
The presence of evaporation results in water losses through water vapour and results in a build-up of salts in the condenser water circuit. This necessitates the requirement of good quality makeup water to the condenser water circuit. Also, chemical dosing with be necessary to ensure maintenance of the pH, minimise build-up solids precipitation and prevent bacterial growth (e.g. legionella). Alternative heat rejection systems include the evaporative condensers where the refrigerant is piped from the refrigeration machine to cooling towers for indirect heat exchange with water sprays and air-cooled condensers where condenser fans form part of chiller unit. Selection of the condenser heat rejection system should be appropriate for the mine site and prevailing ambient temperatures.
Mine cooling refrigeration systems are initially installed on surface (BACs, refrigeration machines, CCTs and associated equipment). When cooling of the intake air has been maximised by the surface refrigeration system, underground mine cooling systems will subsequently be considered. Underground cooling systems can have various combinations and will introduce several complexities that need to be considered in this case.
BBE is well positioned to assist with all mine refrigeration and cooling needs. For more information, view our capabilities here