Thermal-fluid modelling of the transient behaviour of a capacity-controlled underground refrigeration unit

Abstract

South African mining operations are continuously expanding deeper underground to reach the available ore bodies, resulting in a higher demand for cooling. The traditional cooling approach by using a vast chilled water network with a surface fridge plant, becomes less effective for deep mining operations, especially in remote underground locations. Spot cooling in remote areas has become a necessity to comply with mining regulations. The modular and mobile Air Cooling Unit (ACU) (patent 2007/04679) was specifically developed to achieve more effective spot cooling. The ACU extracts energy from the air and reject it into service water via a vapour compression cycle. The ACU is dependent on a constant water flowrate to stay operational. Unfortunately the water supply fluctuates and results in the ACU tripping on over pressure as a safety mechanism. To improve operations and reduce down time, infinite capacity control of the compressor has been implemented to cope with the fluctuating water flow. The effect of the infinite capacity control strategy on the thermal fluid dynamics of the refrigeration cycle under varying environmental conditions is not well known. This study investigated the refrigeration cycle's reaction when subjected to a fluctuating feedwater or air supply. A detailed thermal fluid model of the ACU was developed using Flownex®. Each of the main cycle components were first individually modelled, and their accuracy verified using third-party software. These components were then integrated in a cycle model, the accuracy of which was validated against both surface and underground data. The model was then used to simulate transient scenarios of feedwater and air availability. The effect on the refrigeration cycle and its response was observed. The study identified the operational limits for feedwater and air supply, accurately captured the response of compressor infinite capacity control to fluctuating feedwater supply and determined the obtainable cooling duty for limited feedwater availability. The operational limits that led to ACU downtime were restricted feedwater supply, less than 1 kg/s at 21°C, and total fan failure. In response to fluctuating feedwater supply, the compressor actively decreased capacity for reduced feedwater supply and increased capacity when the feedwater supply was restored. Obtainable cooling duty did not dramatically improve when feedwater was oversupplied. Undersupplying feedwater below 4 kg/s at 21°C resulted in a dramatic decrease in obtainable cooling due to compressor capacity decrease. The study concluded that the compressor infinite capacity control strategy is advantageous to maintain continuous ACU operation in the mining environment.