1.3.1 Preamble
Due to the current electricity crisis, researching and implementing new strategies to realise energy savings by reducing energy consumption are important to ensure that mining activities can continue. However, research conducted in preceding sections concluded that underground air temperatures increase as the depth at which mining activities continue to stretch increases. Therefore, underground temperatures should be improved to ensure/
sustain productivity. This is of utmost importance as peoples’ lives are at stake.
The following section will review previous research conducted on cooling systems and/or strategies. A brief description of each study will be provided to ensure that the reader fully understands where scope for projects exists.
1.3.2 Research conducted on cooling systems and/or strategies
Stanton: Development and testing of an underground remote refrigeration plant [15]
Stanton’s paper posed developing, installing and testing a small underground mobile refrigeration plant (MRP) to mitigate the problems associated with mine cooling. Stanton researched the methods used for cooling in deep-level mining operations. In addition, he investigated the previous reasons for failures while using an MRP. The cooling requirements in the underground environment, with emphasis on the need for cooling by using an MRP, were discussed in this paper.
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Stanton used static and dynamic simulations to obtain the results presented. The strategy was implemented and tested. It improved underground conditions by 4.3 °C in close proximity to workplaces, which is a significant improvement in the mining industry. Stanton concluded that a small underground MRP can be used to improve the positional efficiency, cost per kilowatt of cooling, and cooling opportunities for remote hot areas. Significant cooling can be provided to remote areas by means of an MRP.
Even though Stanton’s strategy significantly improved underground conditions, he had to install new infrastructure. He did not improve existing cooling systems by increasing the water through them to obtain increased cooling duty without adding additional pumping costs even when no mining activities were occurring.
Van Staden: Optimal use of mobile cooling units in a deep-level gold mine [21]
Van Staden’s study used the existing mobile cooling units optimally to improve service delivery, reduce operating costs, and improve underground temperatures. It was found that these tertiary cooling systems became inefficient because of harsh working environments and a lack of maintenance. Therefore, Van Staden developed a method to accurately measure the specific operational parameters of these units and characterise their performance. Thereafter, an optimisation strategy was selected. The optimisation entailed removing 22 inefficient tertiary cooling units, which resulted in a water use reduction of 47 Mℓ and 150 Mℓ for July 2017 and August 2017, respectively. A combined cost saving of R2.38 million was achieved for the two months. Furthermore, WB temperature improvements of between 1 °C and 3 °C were achieved.
Van Staden removed inefficient tertiary cooling units as they affected underground temperatures negatively. He did not focus on increasing water supply to cooling systems and providing cooling even when no mining activities were occurring.
Mackay et al.: Refrigeration and cooling concepts for ultra-deep platinum mining [25]
This paper reviewed supplementary refrigeration technology for ultra-deep mining operations and suggested strategic recommendations. The research examined and discussed all the relevant cooling strategies and the issues they posed. By means of comparative modelling and life cycle costing, the study indicated the depths at which the efficacy of certain cooling systems runs out. The paper concluded that the introduction of cold water from surface operations for underground air-cooling should be considered. The following points were highlighted in the study:
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• Underground air cooling.
• Cold water service – priority with cold water underground.
• Secondary cooling – use of BAC spray chambers with a limited number of CWCs.
• The isolation of cold water piping will be essential.
• The introduction of ERDs into the WRS will be extremely important.
Mackay et al. suggested that ice systems should be considered for mining operations that stretch beyond 2 500 m. However, site-specific circumstances may establish an exception to this.
The research of Mackay et al. included the relevant refrigeration technologies to be used in ultra-deep mining operations with recommendations on which systems to use. The research did not focus on controlling the water flow through cooling systems to increase the cooling duties thereof. The paper only mentioned that the use of cold water from surface operations to improve cooling would need to be considered. Furthermore, cooling was not provided in times when no mining activity was present.
Greyling: Techno-economic application of modular air cooling units for deep level mining at Mponeng [26]
Greyling’s study used the VUMA simulation package to simulate a modular air-cooling unit (ACU) in a closed-loop system and evaluated it with traditional cooling methods. The paper considered five different configurations between a 500 kW conventional CWC and a 300 kW ACU. These two systems were evaluated on an economic basis for the total life cycle costing.
The emphasis was on increasing the return water temperature to as close as possible to the haulage temperature. By doing this, Greyling reduced losses in the system and increased the efficiency of the refrigeration system. As system losses were minimised, the concept of spot cooling on the development ends was motivated.
The study concluded that using CWCs in a closed-loop system with ice as a cooling medium would be the most economical option with a net present value of R206 million. However, when considering cooling on the development ends, using an ACU was considered to be the best option with a saving of approximately 49% more than using CWCs. Therefore, using ice plants located on surface and closed-loop underground chillers and BACs would provide optimal cooling for mining activities that stretch beyond 4 000 m. In addition, a CWC is considerably more sensitive to a change in water inlet temperature and water flow rate than an ACU.
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Even though Greyling’s study did consider a closed-loop u-tube system to improve underground temperatures, his study only used this system with CWCs where ice was used as a cooling medium. His study did not increase the water flow through cooling systems to improve the cooling duty, but rather just reconfigured it to work in a closed-loop system. In addition, Greyling did not provide cooling when no mining activities took place.
Belle and Biffi: Cooling pathways for deep Australian longwall coal mines of the future [53]
Belle and Biffi conducted a study to evaluate the underground temperatures profiles on deep, gassy coal mines with a propensity for spontaneous combustion. The study proposed cooling strategies to manage thermal hazards effectively. Thermodynamic modelling was performed on a longwall face. This strategy summarised the application of underground BACs, cold water sprayers and the resulting temperature profile. The implementation of cooling strategies to enhance the positional efficiency of cooling plants was discussed.
Belle and Biffi concluded that refrigeration plays an important role in gold and platinum mines in South Africa. Belle and Biffi used surface ambient temperatures, ventilation surveys and practical models to provide an adequate indication of the limits beyond which the use of refrigeration and cooling is justified.
Viljoen and Ranasinghe: Reducing chilled water requirements on an underground bulk air cooler [62]
Viljoen and Ranasinghe investigated strategies to reduce the amount of cold water required on mines by improving the efficiency of underground cooling units and reusing the outlet water of these units. Viljoen and Ranasinghe conducted on-site measurements and determined that the measured efficiency of cooling units differed significantly from design values. On closer inspection, they found that significant fouling existed on the air side of these units. The study concluded that the best method for dealing with fouling was proper cleaning and maintenance.
Furthermore, Viljoen and Ranasinghe found that old cooling systems still existed on mines, which could be used to improve the overall efficiency of the cooling system. Therefore, they used the outlet water of the BAC as inlet water to the older cooler. Viljoen and Ranasinghe arranged the two coolers as a two-stage cooling system working in a counterflow configuration. By doing this, the overall efficiency was improved. A water reduction of 30 ℓ/s was achieved while maintaining the same cooling duty. A 1.35 MW energy saving was realised.
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Viljoen and Ranasinghe reconfigured two cooling systems to provide the same amount of cooling duty while realising an energy saving. However, this study did not entirely focus on controlling water flow through cooling systems to provide additional cooling.
Du Plessis, Scott and Moorcroft: Modern cooling strategies for ultra-deep hydropower mines [64]
The study by Du Plessis et al. reviewed cooling practices to provide a cost-effective method for introducing hydropower at South African mining operations. Furthermore, this study described and discussed the equipment that has been developed to meet the requirements of both hydropower and refrigeration. The employment of these technologies proved to increase the amount of cooling effectively from 10 MW to 20 MW. This increase was obtained by utilising the hydropower water used to drive mining equipment.
The proposed strategy allowed a total heat load of 52 MW to be ventilated and cooled successfully by using both surface and underground refrigeration installations and hydropower water. The strategy entailed installing an underground refrigeration plant that provided cold water for underground BACs and spot coolers. All this cooling was done in an insulated closed- loop configuration.
Du Plessis et al. used hydropower to provide additional cooling. The study mentioned that by using hydropower, continuous cooling could be achieved. However, the study did not include the control of the hydropower water to provide the CBACs with the correct amount of water to achieve the designed cooling duty. As previous research mentioned, there is a point where the cooling duty no longer increases for very high water flow rates [62]. This means that water is wasted.
Van Rooyen: Performance and fouling prediction model for finned-tube heat exchangers [68]
Van Rooyen concluded that a need exists for a performance, fouling and ideal maintenance interval prediction for finned-tube heat exchangers (FTHXs). It was found that previous models required instrumentation to be installed and that FTHXs should be operated at design conditions. Van Rooyen’s study focused on designing a prediction model for FTHXs to predict the optimum and actual air-cooling duty as well as the outlet air temperature, performance losses experienced due to fouling, and ideal maintenance intervals at off-design conditions.
The method proposed by Van Rooyen integrated three models, namely performance, fouling and maintenance prediction. These proposed models were derived from first principles of heat
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transfer and psychometry – the approaches followed by Markov and Bayes to successfully develop maintenance policies and formulate thermal and production relations.
Van Rooyen used nine case studies to test the proposed method on underground bulk air- cooling systems. The results revealed that the actual cooling performance deviated from the optimal performance by approximately 29%, which resulted in a 2 °C increase in outlet air temperature. This was due to external and internal foulants being present on the heat transfer areas. Furthermore, the model predicted that internal fouling (18% of 29%) increased the BAC outlet air temperature by 1.3 °C, while external fouling (11% of 29%) increased the BAC outlet air temperature by 0.7 °C. Van Rooyen removed the external and internal foulants, which improved the mining revenue by R58 million and R30 million, respectively. Temperature reductions for the removal of external and internal foulants were expected to be 0.7 °C and 1.3 °C, respectively.
Although Van Rooyen improved the cooling of the CBAC by managing foulants, he did not focus on the water flow through these systems to reduce temperatures even further.