A reverse Brayton cycle mine refrigeration system

Authors: Millar, DL

DOI https://doi.org/10.36487/ACG_rep/1710_09_Millar

Cite As:
Millar, DL 2017, 'A reverse Brayton cycle mine refrigeration system', in M Hudyma & Y Potvin (eds), UMT 2017: Proceedings of the First International Conference on Underground Mining Technology, Australian Centre for Geomechanics, Perth, pp. 115-132, https://doi.org/10.36487/ACG_rep/1710_09_Millar

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Abstract:
The discounted (i = 10%) cost of a unit of refrigeration supplied by a coefficient of performance = 4 surfacebuilt vapour compression refrigeration mine air cooling system, with a load factor of 100%, equipped with condenser water cooling towers and underground spray chambers is approximately CAD 100/MWhr. This rises to CAD 200/MWhr for the same system when the load factor of the plant falls to 25%. These high costs motivate searches for alternative mine air cooling systems. In a reverse Brayton cycle refrigeration system, where compressed air acts both as the refrigerant and the coolant, compressed air that is aftercooled and dried on-surface is delivered to an expansion device underground whereupon it is ‘let down’ to provide a very low-temperature air stream. Cooling is delivered to hot mine intake air simply by allowing it to mix with the cold exhaust of a turbo-expander or eductor. A simplified performance model for a minimum compression work hydraulic air compressor (HAC) is presented, and the formulation is extended to include the expansion device, focusing on the use of an eductor geometry. As it pressurises the mine air that passes through it, the eductor behaves as an underground booster fan, with no moving parts, that cools and dehumidifies the mine air. For the mine, HAC plus eductor systems produce compressed air very cheaply (CAD 6.11/tonne for an open loop HAC, CAD 1.52/tonne for a run-of-river HAC) but only the latter returns a discounted cost of refrigeration supplied of CAD 27/MWhr which is 33% of the CAD 83/MWhr for the incumbent vapour compression refrigeration technology.

Keywords: reverse Brayton cycle, mine refrigeration, hydraulic air compressor, eductor cooling

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