Novel approach for improved control processes on ageing mine refrigeration systems

Abstract

Worker safety remains one of the top priorities in South African deep-level gold mines. Due to their operating depths, these mines are heavily dependent on artificial cooling. Large refrigeration systems are commonplace in the South African mining environment, ensuring cold water and air are supplied to underground working areas. The advanced age of the industry in South Africa unfortunately leads to one of its biggest challenges: old equipment using outdated control methods and processes. There is also a second challenge brought about by the industry’s mature age: that of depleted ore grades as areas are mined out over time. Gold mines in South Africa have seen increased underground expansion in search of new gold-bearing ore bodies to keep operations profitable. The nature of narrow reef mining also means that these expanding and complex underground areas are frequently changing, resulting in an extremely dynamic environment. Infrastructure past its prime, together with increased distances to active mining areas, presents the gold industry in South Africa with a unique set of cooling challenges. Decades-old refrigeration equipment is operated on original design parameters while mines are deeper and more complex than ever. It is therefore vital that deep-level mine refrigeration systems operate dynamically to account for changes within the environment they service. Current control optimisation methods found in literature mainly focus on operation for cost benefit, and rarely employ strategies for improved service delivery. Economic stresses on the industry, however, requires a novel approach to improve service delivery of deep-level mine refrigeration systems while still prioritising operational cost. Analysis of available literature focusing on refrigeration control highlighted a clear need to develop an optimisation technique to integrate cost savings and service delivery improvements using existing infrastructure. Using existing equipment would prove crucial to the study as capital expenditure on gold mines has become extremely limited in recent years. A method was developed to implement improved control processes on ageing refrigeration systems in gold mines for improved performance. The knowledge from the commercial and the mining refrigeration control sector was critically analysed and integrated into a novel solution. This solution took proven methods (e.g. temperature resets) from the commercial field and assimilated them into a mining refrigerating environment. Detailed investigations focusing on the control characterisation of a refrigeration system proved that evaporator temperature resets can be employed to improve cooling performance while simultaneously reducing energy consumption. The improved control processes were implemented on a case study, referred to as Mine A, at a South African deep-level gold mine. The refrigeration system at Mine A was determined to be over 25 years old and operating ineffectively, as original design parameters were used in the control strategy. An evaporator temperature reset and updated flow control were implemented as part of the improved control processes. After implementation of the improved control, chilled water temperatures sent underground was reduced by 16% during summer months. This was achieved even with the total refrigeration system using 18% less energy. The improved control meant service water temperatures were reduced by more than 1°C and bulk air cooler air temperatures were reduced by over 2°C during summer months. The control processes also improved the system’s temperature control accuracy by 12% by using 20% more dynamic flow capacity. The dynamic nature of improved control was thus validated, as the refrigeration system could better adapt to external changes in demand and the environment. All these improvements were present even as Mine A increased production output by 5%. Combined, the improved control processes implemented on Mine A also resulted in daily energy savings of 38 MWh. This energy saving equates to an annual reduction in electricity cost of R 9 million. The results on Mine A validated the effectiveness of improved control processes on ageing mine refrigeration systems. The main objective of the research presented in this thesis was thus achieved: that is, improving ageing mine refrigeration performance at reduced operating cost.