Existing mine cooling concept
Chilled Water Cars (CWC) are widely used in underground mine cooling. These units mainly utilize chilled water supplied from large-scale surface chillers as the heat sink to cool local ventilation air via conventional finned‑tube cooling coils.
The effective cooling capacity attained per CWC is highly dependent on the supply temperature of the chilled water. Unfortunately, the water temperature is adversely affected by the depth underground as well as heat gains over long distances. This implies that the system performance degrades rapidly with depth and distance from the main water supply shaft. In addition to this, a relatively small temperature rise must be maintained through the cooling coil. This requires large quantities of water to be pumped back to the surface once it has absorbed the heat extracted from the ventilation air.Due to this, the required number of cooling cars and the quantity of chilled water that must be supplied can increase exponentially with depth and distance. This implies high capital expenditure and excessive energy costs associated with the underground pumping power.
What is the ACU concept?
The ACU is a modular underground Air Cooling Unit that can utilize water at temperatures of up to 40°C as the heat sink, as opposed to chilled water only. It is a robust, low maintenance vapour compression heat pump cycle with efficiency comparable to that of surface chillers.
Heat is extracted locally from the ventilation air via a finned‑tube evaporator coil. The heat is transferred to the water in a special condenser unit that can operate efficiently with a wide range of inlet water temperatures and with a large water temperature rise through it.The fact that relatively warm water can now be supplied to the ACU means that the heat gains from the surroundings can be reduced. Furthermore, the fact that the water temperature rise through the ACU can be much larger than for the CWC, means that the required water quantities are significantly reduced.Since the performance of the ACU does not degrade much with higher inlet water temperatures, the number of units and therefore the capital expenditure, is not really affected by depth and distance. Furthermore, the decreased water quantities significantly reduces the underground pumping power requirements, and therefore also the energy consumption.
ACU versus CWC
The ACU is also mobile and comes standard fitted on rolling stock to suite any mine infrastructure. It is available in two models namely an 80 kW unit utilising R134a as refrigerant and a 100 kW unit utilising R407c. Multiple units can be connected in parallel to obtain larger cooling capacities.
The positional efficiency of the ACU is extremely high in the sense that primary cooling can be provided exactly where and when required.
For a typical mine layout where cooling is required at a depth of 3500 m and at an equal distance away from the main shaft, the capital cost of the ACU system per MW cooling is comparable to that of a CWC system. The energy consumption of an ACU system is typically 20% to 50% less than an equivalent CWC system, mainly due to the reduced pumping power requirement.
Deep mine ACU application
The ACU offers an especially cost effective and energy efficient solution for application in deep mine cooling below 4000 m. The required capital expenditure of an ACU system is 25% to 35% less than that of an equivalent underground refrigeration plant with CWC network. Energy consumption of the ACU system is typically 25% to 45% less than that of the underground refrigeration plant system, due to reduced pumping power as well as cooling better efficiency.
An advantage of the high return water temperature is that the bulk of the surface cooling can be done directly via cooling towers, instead of through expansion of costly large‑scale chiller plants.
Air Cooling Units for Mine Cooling