2.2.1 Preamble
The implementation of new strategies on mining systems entails having a clear understanding of the function of each component and the interaction between one another. Therefore, the first step of developing a water supply strategy to improve underground cooling is investigating the system. This involves doing a site walkthrough to question site personnel to obtain as much information on the systems as possible. It is important to obtain sufficient information on the entire system to get a better understanding of how these systems work. As mentioned in Chapter 1, each mine’s configuration will differ based on its requirements.
If the current mining operation is not satisfied with its current underground conditions, access to site data must be obtained to determine whether there is sufficient scope for increasing the cold water supply through CBACs. All measurements not logged electronically must be established by means of manual measurements. Critical outputs of the investigation step are detailed flow diagrams, all control practices on site, historical data of system parameters, and all relevant equipment specifications.
Figure 24 depicts the procedure to complete the investigation step.
High pressure water supply strategy for mine bulk air coolers 44 Investigation procedure
NO
Select mining operation
YES
NO
Identify scope
YES
Acceptable configuration?
Sufficient scope?
Collect data List all important components
Acquire WRS layouts
Identify possible systems contraints
Figure 24: Procedure to complete investigation step
2.2.2 Mining operation selection
The goal of the mining operation selection step is to identify mineshafts that have the applicable configuration to implement the proposed strategy. Firstly, identify the mineshafts where production takes place. This is important as some mineshafts are only used for ventilation, dewatering and/or water distribution purposes. Shafts where no mining takes place may not be a viable option for implementing the proposed strategy as the objective is to improve underground conditions to ensure a safe environment for workers. Next, identify which components of the WRS are present on the specific mining operation. Lastly, assign a score to each shaft based on the number of WRS components as shown in Table 4.
High pressure water supply strategy for mine bulk air coolers 45
Figure 25 shows a basic flow chart for determining which mining operations are suitable for implementing the proposed strategy. The shafts that will be considered will be shafts that does not have ERDs already installed as the study proposes to install/reconfigure pipelines to develop an ERD.
Shaft selection process
Not viable
Not viable NO
NO YES
YES
x x x
NO NO NO
x NO
YES YES YES YES
✓ ✓ ✓
Does production take place?
WRS present?
Water distribution?
Refrigeration plant system?
Secondary underground
cooling?
Dewatering?
✓
Figure 25: Shaft selection process
The selected shaft should have each one of the components as set out in Figure 25 to implement the proposed strategy. Table 4 shows an example of possible results obtained from using the flow diagram in Figure 25.
Table 4: Mine selection example
Mineshaft Water distribution
Refrigeration plants
Secondary underground
cooling
Dewatering Score
Mine A ✓ ✓ ✓ ✓ 4
Mine B x x x x 0
Mine C ✓ ✓ x ✓ 3
Mine D ✓ ✓ x x 2
High pressure water supply strategy for mine bulk air coolers 46
Use Table 4 to identify which mining shaft can be used for implementing the proposed strategy to increase the cold water supply to underground CBACs. After selecting an applicable mining operation, commence with scope identification.
2.2.3 Scope identification
Before a strategy can be developed, the scope must be identified to determine whether the specific operation has a need for increased cold water through the CBACs. The potential scope is determined by estimating the potential to increase cold water supply through CBACs to improve the underground air temperatures without adding additional energy costs. The first step is to consult with mine personnel to determine the scope for increased water.
Underground air temperatures and water flow are required to analyse potential. The scope is identified from data such as illustrated in the example of a CBAC’s water flow and outlet air temperature in Figure 26.
Figure 26: Example of the water flow and outlet air temperature of a CBAC
During periods where water flow is reduced (16:00 to 21:00), it can clearly be seen that the air temperature rises above the legal limit as set out in the Health and Safety Act 20 of 1996.
Furthermore, water reductions can be seen throughout the day, which cause the outlet air temperature of the CBAC to increase, as depicted in the example above. The water flow through the CBAC fluctuates throughout the day based on the water demand at mining sections. The water demand is reduced during non-entry periods (between 16:00 and 21:00)
0 3 6 9 12 15 18 21 24 27 30 33
0 5 10 15 20 25 30 35 40 45 50
00:30 01:30 02:30 03:30 04:30 05:30 06:30 07:30 08:30 09:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 17:30 18:30 19:30 20:30 21:30 22:30 23:30 Air temperature [°C]
Water flow [ℓ/s]
Time [hh:mm]
CBAC water flow and outlet air temperature
CBAC water flow CBAC outlet air temperature
High pressure water supply strategy for mine bulk air coolers 47
when no personnel are permitted underground. However, when personnel enter the mine for the night shift, underground conditions are not in the acceptable range.
Therefore, from the example shown, scope exists to increase cold water through the CBAC to lower the outlet air temperature. This can be used to a mine’s advantage as temperatures underground will be improved to sustain production and provide safe working environments.
However, the potential scope is only possible when the water flow is below the critical flow of the CBAC. The critical flow is determined by using Equation 3 discussed in Chapter 1. As mentioned, optimum heat transfer takes place when turbulent flow is present. Therefore, a Reynolds number above 2 300 results in turbulent flow, whereas laminar flow occurs at a Reynolds number less than 2 300 for internal flow. Furthermore, as research has shown, a too high water flow decreases efficiency. Therefore, the CBAC water flow should be controlled between the design water flow and the critical water flow. The designed water flow is obtained from the CBAC specification sheets.
The scope of each mining operation must be determined by calculating the percentages of the day during which the water flow through the CBAC was lower than the critical water flow. Use Equation 4 to determine the scope percentage.
Equation 4: Potential scope to increase the CBAC’s water flow
𝑆𝑐𝑜𝑝𝑒 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 = (𝑡𝐶𝐵𝐴𝐶 𝑤𝑎𝑡𝑒𝑟 𝑓𝑙𝑜𝑤<𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙
𝑡𝑑𝑎𝑦 ) × 100
Where:
𝑆𝑐𝑜𝑝𝑒 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒
= 𝑡ℎ𝑒 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑡𝑖𝑚𝑒 𝑤ℎ𝑒𝑟𝑒 𝑡ℎ𝑒 𝐶𝐵𝐴𝐶 𝑤𝑎𝑡𝑒𝑟 𝑓𝑙𝑜𝑤 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙 [%]
𝑡𝐶𝐵𝐴𝐶 𝑤𝑎𝑡𝑒𝑟 𝑓𝑙𝑜𝑤<𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙
= 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 ℎ𝑜𝑢𝑟𝑠 𝑤ℎ𝑒𝑟𝑒 𝐶𝐵𝐴𝐶 𝑤𝑎𝑡𝑒𝑟 𝑓𝑙𝑜𝑤 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙 [ℎ𝑜𝑢𝑟𝑠]
𝑡𝑑𝑎𝑦= 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 ℎ𝑜𝑢𝑟𝑠 𝑖𝑛 𝑎 𝑑𝑎𝑦 [ℎ𝑜𝑢𝑟𝑠]
The potential scope will differ for each day as the mining demand is not the same every day.
The scope percentage will give a good indication whether sufficient scope exists to increase the CBAC’s water flow. A month’s data should be used. When potential scope exists to increase the water flow through CBACs, the next step is to collect all the relevant data needed as well as the listing of all the important components of the system.
High pressure water supply strategy for mine bulk air coolers 48
2.2.4 Data collection and listing of important components
The main objective of the data collection phase is to gather all the necessary information needed. Most gold mines use a supervisory control and data acquisition system (SCADA) to log data received from sensors installed on certain machinery. These data logs are better known as tags, which are stored in the mine’s historian. These tags are usually logged by the energy management software (EMS) used by ESCos, which are third party companies.
One of the known EMS platforms is the Real-Time Energy Management System (REMS), which is used for logging data. REMS logs the required data in two-minute intervals and saves it in a comma separated value (.CSV) data file. The historian’s data consists of daily totals and averages. The REMS platform stores data at a higher resolution than the historian. However, any monitoring and/or data capturing method can be used if it reads data in sufficient resolutions as discussed in this methodology. Mine personnel and the SCADA system are used to characterise the system and verify all components and their installed capacities. Table 5 lists all the major components in a typical mine’s WRS.
Table 5: Major components in a typical mine’s WRS
Components Data required
Water dams (hot and cold)
• Volumes [m3]
• Operating limits [%]
• Water temperatures [°C]
• Configuration [–]
Water valves • Positions [–]
• Automated/manually control [–]
CBACs
• Type of CBAC [–]
• Utilisation [%]
• Installed cooling duty [kW]
• Current cooling duty [kW]
• Water flow [ℓ/s]
• Air flow [kg/s]
• Water and air inlet and outlet temperatures [°C]
Refrigeration plants
• Compressor installed capacity [kW]
• Configuration [–]
• Evaporator inlet and outlet temperature [°C]
• Condenser inlet and outlet temperature [°C]
• Evaporator water flow [ℓ/s]
Piping • Length [m]
High pressure water supply strategy for mine bulk air coolers 49
• Diameter [m]
• Material [–]
Dewatering pumps
• Pump installed capacity [kW]
• Water flow [ℓ/s]
• Pumping head/elevation [m]
However, not all data may be available on the mine’s SCADA system. Outdated and/or faulty equipment and instrumentation together with communication failures of operating equipment may result in data loss, preventing the SCADA from logging data, or let the SCADA log incorrect data. In instances where data loss occurs or incorrect data is logged, manual readings should be taken to verify the data. This is done by using portable instrumentation such as water flow meters, mobile temperature sensors, and/or pressure loggers. As this equipment can be expensive, it should be used as a last alternative.
All information stated in this section is required before potential strategies can be simulated and verified. After all the necessary information has been collected, the next step is to identify and understand the operation of each component.
2.2.5 WRS layout
Figure 27 shows a typical layout of a mine WRS. The main purpose of this simplified layout is to identify all major components and fully understand their operation and interaction with one another. The potential scope for increased cold water through CBACs must be verified once all components and their operations have been identified. The potential scope is verified by evaluating the current operation of the specific system.
High pressure water supply strategy for mine bulk air coolers 50 Pre-cool water
dam
Surface hot water dam Surface cold
water dam
Underground cold water dam
Settlers
Surface BAC
Underground refrigeration plants
Underground hot water dam
Underground hot water dam EAST
WEST EAST WEST
Surface refrigeration plants
Precooling towers
Dewatering pumps Dewatering pumps Secondary and tertiary cooling Dewatering
Figure 27: Simplified layout of a mine WRS
The purple boundary seen in Figure 27 displays the dewatering system, whereas the green boundary displays the secondary and tertiary cooling systems as well as the mining activities for a typical deep-level mine. These two systems must be investigated to develop a proposed water supply strategy.
It is of great importance that an accurate layout of the operation and its components is compiled. Doing so simplifies the process of determining whether the specific operation can be adapted to incorporate the increased cold water supply through CBACs. Before developing a strategy, the next step is to evaluate the system to identify all constraints that may prohibit the implementation of the proposed water supply strategy.
2.2.6 System constraints
Increasing the cold water supply through the CBAC will have an impact on certain systems in the current operation. Therefore, it is of critical importance that the constraints that may prohibit the implementation of the proposed strategy are identified and understood. Some familiar constraints for increasing water supply to CBACs are summarised below.
High pressure water supply strategy for mine bulk air coolers 51
Cold water demand
Increasing the cold water supply through CBAC increases the underground cold water demand. It is important that the system can meet this increased demand without affecting the current operations negatively. It should be ensured that the increase water does not compromise the current refrigeration systems’ operational integrity by adjusting control to allow for more water.
Cold water storage capacity
As the water demand is increased, it is important to ensure that cold water storage dams have sufficient storage capacity to account for the increased demand. If the current cold water storage dams cannot account for the increased water demand, then the project may not be viable.
Cooling system performance
Previous studies conducted by authors such as Van Rooyen [68] concluded that fouling occurs at a CBAC during operation due to impurities in the hot and cold fluid streams and, as a result, the CBAC’s performance deteriorates. Therefore, increasing the water supply will not necessarily lower the outlet air temperature if fouling occurs. Therefore, maintenance on these machines is important to avoid any fouling that will make the proposed strategy unsustainable.
Valve issues
Several water valves are used within a CBAC setup. The performance of the valves may deteriorate over time due to impurities, which makes it difficult to control the amount of water.
Therefore, it is important to ensure that these valves are maintained.
Personnel cooperation
As mentioned in Chapter 1, the CBACs are installed in a so-called ‘BAC loop’ with doors located at the BAC outlet and inlet in the main haulage, which prevents the cooled air from recirculating. However, the main haulage is a travelway for ventilation air, workers and locomotives carrying ore. As personnel and/or locomotives move through these doors, they sometimes forget to close them, causing cold air to recirculate and working areas to reach temperatures above the legal limits. Therefore, it is of critical importance that mining personnel keep these doors closed at all times. Otherwise, the effects of the increased cold water will be non-existent at working areas and faces.
High pressure water supply strategy for mine bulk air coolers 52