By Hannes Potgieter, Senior Ventilation Consultant at BBE Consulting
Energy makes up a considerable portion of operational mining costs with operators increasingly looking for ways to reduce consumption as the price of energy continues to escalate worldwide. Mine ventilation and cooling systems are major energy consumers, specifically in South African narrow reef operations. Secondary ventilation systems are generally energy inefficient and comprise hundreds of fans with motor sizes ranging between 7 kW and 45 kW. These fans operate in conjunction with transient ventilation ducting systems that transfer air to advancing production headings up to more than 400m beyond through ventilation.
A recent study conducted at a mine with close to 3 MW underground auxiliary fan power indicated electrical inefficiencies of 25% whilst at the same time delivering 40% less ventilation, or air, to the work faces. Operating duty variations are significant, and mines often end up trying to make-do with inefficient, low-performing fans by installing additional fans in series. This only results in additional energy wastage, and an increase in the mine heat load as all electrical energy from fans appear as heat in the mine’s ventilation system.
Since auxiliary fans are often installed close to the working areas, which are challenging environments, failures cannot be avoided and subsequent replacement is common. Premature failing and fans not performing to expected operating duties leads to “lost-blasts”, unsafe environmental conditions, and increased electrical power demand. Auxiliary fans generally operate until failure, at which point they are returned to surface and sent for repairs to external service providers. In many cases, mines use local engineering facilities to perform repairs without issuing repair specifications, i.e. specifying the use of OEM parts and providing performance envelope requirements. This often leads to fans returning to the mine that are unable to operate at their intended duty points.
The three factors that generally influence energy efficiency (and minimise the risk of production losses due to premature failures) are purchasing policies, repair specifications, and correct fan selection. Buying the cheapest fan on the market, and specifying motor sizes instead of required fan duties and efficiencies, normally proves to be an expensive decision over the average 7-year operational life of an auxiliary fan. A fourth factor is ventilation ducting resistance to airflow, which provides an opportunity to improve power consumption and system efficiency.
In South Africa, legislative changes in 1996 meant that the role of the ventilation department has changed, with them taking on greater operational responsibilities. New legislation has shifted focus to occupational exposure measurements and technical ventilation practice such as fan maintenance, and procurement responsibilities have merged into the functions of other departments. Understandably, with less direct involvement from the ventilation departments, emphasis may shift from maintaining optimum fan performance to the preservation of budgets.
The change in responsibility has in many instances led to the ventilation department, the operational end user of the auxiliary fans, being left out of important decisions and quality checks. This is unfortunate, as ventilation departments are ultimately responsible to ensure the performance is acceptable over a wide operating range under demanding conditions. Substandard fans, whether procured or repaired, leads to an increase in power costs and production losses. Across the industry, significant increases in energy costs mean that emphasis must shift from reducing the hard cost of repair to looking at power saving efficiencies.
In this time of global energy consciousness, it may be time for mining companies to re-evaluate the roles and functions of the “Occupational Hygienist” versus the role of a “Ventilation Specialist”. Mines in South Africa are ever deepening and moving further away on strike that require strategy changes to manage both working environments and energy consumption.
Having the right management structures in place will assist mines to lower their energy consumption by building efficiency into ventilation systems. Efficient systems are good for owners, as they cost less to operate, are more reliable and will ultimately have a positive impact on the underground workplace environment, while assisting to achieve production targets.