Power savings through secondary fan management at Granny Smith Mine
A Broodryk1
- Superintendent, Gold Fields – Granny Smith Mine, Laverton WA 3028. Email: andre.broodryk@goldfields.com
ABSTRACT
Ventilation accounts for a large portion of energy use in the total power demand of underground mines, with secondary fans being identified as a big part of that. It has also become increasingly important to examine new technologies that enhance energy efficiencies as well as new strategies to reduce power use, not only to reduce costs, but also to reduce CO2 emissions to align with company environmental, social and governance (ESG) targets. The traditional approach to secondary ventilation involves running fans at a constant speed, often at full capacity. This method is neither energy-efficient nor adaptable to changing mine conditions. By adopting variable speed control, mines can tailor fan operations to meet specific ventilation needs, optimising energy use and reducing costs. Recently Granny Smith Mine has been exploring different new technologies and approaches to reduce secondary fan power use. These include variable speed secondary fans and controls to only run fans needed based on weekly production plans. This case study describes the outcomes and power savings achieved throughout 2023.
INTRODUCTION
The industrial utilisation of energy is extensively documented and understood as a pivotal element, not solely in business operational expenses but also in its globally significant impact on the environment. Gold Fields Ltd has integrated global sustainability standards and reporting guidelines to ensure effective management of our climate-related and other environmental risks, which include, amongst various others, emissions reduction targets through research, innovation, and technology development, as well as energy efficiency initiatives. In December 2021 Gold Fields Ltd published a comprehensive set of 2030 targets for its most material environmental, social and governance (ESG) priorities (Gold Fields Ltd, 2022). Adaptation actions that our Australian mines and projects implemented in 2022 included the investigation of ventilation on demand (VOD) and energy efficient technologies.
Ventilation and refrigeration requirements increase as mines move deeper and ambient temperature increases. Legislation also dictates minimum standards for underground ventilation in terms of diesel equipment dilution rates and subsequent air volume requirements for specific tasks. The concept of VOD aims to channel airflow exclusively to the active working zones within the mine while reducing or isolating airflow to inactive sections. This system can range from a basic set-up that activates ventilation in a zone irrespective of work activity to a relatively complex system that regulates airflow volumes in specific areas based on the ongoing activities within those zones.
The orebody layout and subsequent ventilation system design at Granny Smith Mine largely hinder the implementation of adjustments in primary air flows within specific zones. However, the ventilation system relies on the utilisation of multiple secondary fans, both in series and in parallel, to facilitate the ventilation of the necessary ore drives in its horizontal orebodies. In total, the underground mine currently uses 59 secondary fans with a combined rated power of approximately 8 MW to ventilate development and production areas. For this reason, the mine had to look at technologies and processes that would enable a more efficient use of secondary fans.
Two separate controls were recently implemented to achieve power savings through the management of secondary fans. Phase one is an administrative control that was established where working areas are identified during the weekly planning process. These areas, which do not require ventilation, have their respective fans listed on a no-start list to prevent them from energising during the start-up sequence of the supervisory control and data acquisition (SCADA) site control system. Phase 2 is an engineering control which involves the use of variable voltage, variable frequency drives (VVVF) (also called variable speed) to adjust the secondary fan speed as needed.
PRODUCTION SCHEDULE FAN MANAGEMENT
The first phase of the secondary fan management program that was implemented in June 2022 and involves fan operating requirements driven by the weekly production schedule. Switching secondary fans off when not required will always be the preferred option for power savings, since the recorded total power draw of installed twin stage fans amount to between 50 000 and 56 000 kWh per month each. This project was developed and implemented as a low-cost initiative to identify and remotely manage secondary fans in underground areas that were not required to run for production purposes during each shift. At the beginning of the mine production planning week, information about which fans are not required to run is provided to the mine control team. They then identify and tag the relevant fans on the SCADA site control system (Figure 1). Consequently, the identified fans do not start-up at the commencement of the shift following the underground blast clearing sequence.
FIG 1 – Fans disabled from start-up.
Each week, an average of approximately 25 secondary fans of various sizes and locations are identified and then excluded from start-up and operation via PITRAM. Prior to commencing the project, the majority of secondary fan starters were not recording the operating run times of the fans. In order to determine the power savings achieved, all fans had to have their run time monitoring repaired and was only fully completed two months before the project commenced. These months were therefor the only baseline available to compare the fan power draws. The results were a 30 per cent per month energy reduction in measured secondary fan power. This level of savings has not only had a positive effect on energy consumption, but it has also provided a decrease in mine operational costs and carbon emission as well. Figure 2 provides the power cost and CO2 savings on a monthly basis throughout 2023 for Granny Smith Mine.
FIG 2 – Secondary fan power and CO2.
VARIABLE SPEED FAN
For installed fans to accommodate various demand changes, flow is controlled by different methods, such as inlet vanes and speed control. Inlet vanes were more commonly used with centrifugal fans, but this method has recently made its way into the secondary fan space by some manufacturers. Variable speed fans operate by utilising a variable frequency drive (VFD), also referred to as an ‘adjustable-frequency drive’, ‘variable-voltage, variable-frequency (VVVF) drive’, ‘AC Drive’ or ‘Inverter drive’, among other terms, to regulate AC motor speed and torque. This regulation is achieved by altering the motor’s input frequency and voltage. Over the past four decades, advancements in power electronics technology have resulted in reduced VFD costs and sizes, as well as enhanced performance due to progress in semiconductor switching, drive topologies, simulation and control techniques, and control hardware and software. Variable Speed Drives adjust the motor’s speed by modifying the voltage and frequency of the power supplied to it. Maintaining the proper power factor and preventing excessive motor heating necessitates the maintenance of the nameplate volts/hertz ratio. This constitutes the primary function of the VFD.
Traditionally, secondary fans in underground mining have been operated at a constant speed, often running at maximum capacity. This approach, while ensuring a baseline level of ventilation, is highly inefficient and leads to substantial energy waste. Constant-speed operation fails to adapt to changing underground conditions, resulting in excessive power consumption, and increased operational costs.
The adoption of variable speed control for secondary fans allows mines to precisely adjust fan speeds in response to changing ventilation requirements. During periods of lower demand, such as when there are no heavy diesel equipment operating within the ventilated drive or when only inspections occur in the drive, these fans can operate at reduced speeds. This flexibility can lead to substantial energy savings.
Actual power savings achieved follows the fan affinity laws. The fan affinity laws are fundamental principles that describe how changes in the operating conditions of a fan, such as speed, affect its performance. These laws are crucial for understanding the relationship between fan speed, airflow, pressure, and power consumption.
Fan affinity laws
The fan affinity laws describe how changes in the speed of a fan affect its performance parameters, including airflow (flow), pressure, and power consumption. There are three primary affinity laws related to speed change.
Flow rate law
The flow rate law states that the airflow or flow rate through a fan is directly proportional to the fan speed. If the speed of the fan changes, the airflow will change in the same proportion (Figure 3).
FIG 3 – Flow rate relation to fan speed.
Pressure law
The pressure law describes the relationship between fan speed and the pressure generated by the fan. It states that the pressure produced by a fan is proportional to the square of the fan speed. When the fan speed changes, the pressure changes by the square of the speed change (Figure 4).
FIG 4 – Fan pressure relation to fan speed.
Power/aw
The power law relates changes in fan speed to the power consumed by the fan. It states that the power consumed by a fan is proportional to the cube of the fan speed. When the fan speed changes, the power consumption changes by the cube of the speed change (Figure 5).
FIG 5 – Fan power relation to fan speed.
Another benefit of operating fans at reduced speed is the maintenance cost reduction: Constant speed operation can subject fan components to unnecessary wear and tear, leading to frequent maintenance and replacement expenses. Variable speed control reduces mechanical stress on these components, extending their lifespan and significantly reducing maintenance costs.
Heat produced by secondary fans is also reduced when running at lower speeds. Implementing variable speed control to optimise fan operation, can also help minimise unnecessary heat production while maintaining adequate ventilation. The full capacity of heat reduction from the fan and addition by the VFD drive has not yet been assessed at the time of compiling this paper but would be the same as the electrical power saving achieved.
In the author’s experience, the use of variable speed fans underground has historically been isolated to primary fans due to the heat created by variable speed drives and the requirement to keep drives in clean environments. Advancements in technology have altered the capabilities of this equipment, rendering it more adaptable for a broader spectrum of applications.
In 2022 Granny Smith Mine was invited by TLT-Turbo GmbH in conjunction with SMEC Power and Technology to view new technology for secondary fans and 1000 v variable speed, liquid cooled starters being made available to the market. A viewing and inspection of the equipment in operation at their workshop was arranged with an agreement to a six month ‘fan and starter’ trial initiated soon after.
The VSD Drive topology used in the trial was 3-level switching which was developed specifically to reduce motor winding stress, decreased electrical noise, semiconductor losses, and minimise drive-induced problems associated with long motor cables and premature motor bearing failures. The reduction of motor winding stress is a result of the 3-level switching multi-step voltage waveforms which are one-half of a traditional 2-level switching VVVF drive. The reduced voltage steps, and lower line-to-line voltage decrease the non-uniform voltage distribution among motor winding turns which reduces stress on insulation material. The reduced voltage steps also significantly reduce motor shaft voltages and bearing currents which allows larger motors to be used with standard bearings.
Figure 6 shows a comparison between 3-level versus 2-level voltage wave forms, with the 3-level producing a significantly improved sinusoidal curve (wave form) and one-half voltage steps (Yaskawa Electric America, 2005).
FIG 6 – Three-level switching versus 2-level switching waveforms.
Underground installation
The underground installation position was carefully selected to assess the capabilities of the fan and starter in an area known for its elevated temperatures and considerable traffic and activity within the decline. The chosen area was a decline drive under development utilising independent firing during the shift. The rationale for selecting this location lies in the potential for achieving power savings in the mine’s most active area, thereby promising even greater benefits elsewhere. The location allowed evaluation of the reliability of the VVVF fan starter in an unfavourable location.
The installation of the secondary fan required no additional requirements to the existing site fan hanging procedure. The fan starter position was located on the intake side of the fan, off the main decline in a connection drive. The location was ventilated by only an 11 kW fan, which also provided air to a substation opposite the starter (Figure 7).
FIG 7 – Initial fan and starter position layout.
During operation of the fan starter, the internal temperature ranged between 45 and 55°C at various speed settings during the trial, while the internal alarm/trip setting is set to 75°C. This is another attribute of the 3-level switching where switching losses (losses are proportional to heat produced) are up to 44 per cent less than traditional 2-level switching drives. Three-level switching has reduced switch losses as a result of their lower switching parameter voltage values.
Operation and results
Both the fan and starter were connected to the underground long-term evolution (LTE) network, with real time monitoring through the site SCADA network. All aspects of the starter operating conditions were monitored, and remote speed adjustment was made available to PITRAM operators through Citect (Figure 8).
FIG 8 – Variable speed starter remote access.
Although the fan can operate at any required speed through the starter, it was set to operate at three different speeds, 25 per cent, 80 per cent and 100 per cent.
This allowed for three scenarios as shown in Table 1.
TABLE 1
Fan speed settings used.
| Fan Speed | Fan Flow | Fan Pressure | Fan Power Draw | Scenario |
| 100% | 100% | 100% | 100% | Loading of trucks, Re-entries |
| 80% | 80% | 36% | 51% | Jumbo drilling, IT’s etc |
| 25% | 25% | 6% | 2% | LV’s, inspections, vent bag extension |
The initial plan ideally aimed to have the fan operate autonomously, relying on vehicle movement tracked through the site’s vehicle tracking system. However, this integration was unfortunately unavailable at the time, necessitating manual adjustment of the fan speed by PITRAM operators. This adjustment was based on the work being conducted in the drive at that time and relied on operators notifying PITRAM of their movements in and out of the drive. This resulted in some instances where the fan was running at full speed when not required. For the first two and a half months the fan was operating in the decline development. This was during mid-summer in January 2023. During the start of the trial the vent bag was short, leakage was low and the air discharge volume reduction at 80 per cent fan speed was barely noticeable. As the vent bag length increased in the following months it became evident that a 20 per cent reduction in flow, and 36 per cent reduction in fan pressure became more noticeable at the face during the 80 per cent fan speed setting. This resulted in the fan operating at 100 per cent speed for most of February. Power reduction in January and February of 2023 were 21 per cent and 6 per cent respectively. The power saving was calculated based on the duration of time the fan was operating at the set reduced speeds and power draw calculated as per Table 1 (The fan speed setting is recorded at one-min intervals).
In mid-March 2023, the fan was relocated further up the decline to provide ventilation for an Incline development. This particular drive was shorter and not included in the independent firing zone. The shorter vent bag, combined with the decreased production rate, facilitated significantly greater power savings in the last three months. The power reductions achieved from April to June 2023 were 49 per cent, 53 per cent, and 53 per cent, respectively. Throughout the 196 days of data collection, the fan operated at the speeds described in Table 2.
TABLE 2
Fan speed setting and saving per month.
| Fan Speed | January | February | March | April | May | June |
| 100% | 74% | 90% | 65% | 40% | 39% | 37% |
| 80% | 24% | 9% | 14% | 21% | 14% | 17% |
| 25% | 2% | 1% | 21% | 39% | 47% | 46% |
| Total power saving | 13% | 6% | 27% | 49% | 53% | 53% |
There were several days when the fan did not operate at all, and these instances were excluded from the calculations. The cumulative reduction in electrical power draw over the data collection period amounted to 35 per cent. The actual power draw of the fan is dependent upon the resistance it encounters; however, compared to a full-time power draw of, say 190 kW for a twin stage 110 kW fan, the resulting power saving can amount to 48 MW per fan per month, equating to 292 t of CO2 per annum.
Figure 9 shows the total power saving achieved compared to when a fixed speed fan was to be installed in the same position.
FIG 9 – Power saving of variable speed fan compared to a fixed speed fan of the same size during trial period.
The initial assessment comparing maintenance costs to those of normal direct online (DOL) fan starters has indicated that only fortnightly dust filter checks and cleaning are required, in addition to the existing periodic maintenance checks.
CONCLUSION
The implementation of secondary fan management controls at Granny Smith Mine has been successful in reducing the power use on-site, with the next phase of fan speed automation having the ability to reduce power even further. The magnitude of power savings through variable speed secondary fans depends on each fan’s installation location, but if carefully selected, can amount to large power savings. Through the use of variable speed fans, as well as the management of operating fans based on the production schedule, it has thus far shown at Granny Smith Mine that power savings of 30 to 35 per cent are very achievable.
ACKNOWLEDGEMENTS
- Martin Law, Warren Ellis, SMEC Power and Technologies
- Kenny Kramara, Divergent Engineering
- Paul Michetti, TLT Turbo
- Data was collected and reproduced with permission from Gold Fields’ Granny Smith
REFERENCES
Gold Fields Ltd, 2022. Climate Change Report 2022. Available from: <https://www.goldfields.com/pdf/investors/integrated
-annual-reports/2022/ccr-2022-report.pdf> [Accessed: December 2023].
Yaskawa Electric America, 2005. Motor Bearing Current Phenomenon and 3-Level Inverter Technology. Available from:
Leave a Reply