Sometimes the bigger saving is to be had with a bit of strategy and foresight, investing a little more capital in order to realise larger gains on hidden costs.
In the case of mine ventilation, that hidden cost can be found in energy use, which can be greatly lessened by giving attention to quality and design, particularly of secondary ventilation systems.
The secondary or auxiliary system is the infrastructure responsible for reticulating air around underground, while the primary system draws the air from the surface.
Where a ventilation system is not delivering enough air to the work face some operators might make costly assumptions.
“One of the habits in the industry has been that if you hang the duct up, turn the fan on and you don’t get enough air, they’ll say ‘go and get a more powerful fan’,” AC Industries sales director Tony Wigg told Australia’s Mining Monthly.
“What needs to be considered, is with a bit more planning and design , including using a quality, low-leakage and low-resistance duct system they wouldn’t necessarily need to get a bigger fan and they could actually save themselves a lot of money.”
Leakage and resistance problems in auxiliary system ducting can generate enormous power bills for miners, with that cost often slipping through the cracks, unattributed to its cause.
For Oz Vent Consulting director Andrew Derrington, secondary ventilation systems are often viewed as a basic off-the-shelf commodity.
“There are a couple of standard duct and fan sizes to choose from, but the choice seems to be dictated mainly by the cost of purchase,” he said.
“This is puzzling when you consider that fan purchase costs are completely eclipsed by the power costs incurred in running these systems.
“Perhaps the reason for this is that we focus on what is tangible – and a large power bill paid each month by the mine accountant is fairly intangible to most.
“Assuming 33¢ per kilowatt hour for site-generated power, the annual cost for continuous operation of a single 220kW secondary fan is about $636,000.
“For smaller mines, this cost impost for the single fan amounts to several dollars per tonne. Put another way, a diesel-powered gen-set would consume roughly 60 litres per hour of fuel in order to supply power to this single fan.”
It is a common sentiment among the broader mining services industry at present that the need for cost reductions is leading a lot of producers to opt for the cheaper products.
The obvious cautionary message here is ‘you get what you pay for’, and for those in the mine ventilation business, this sentiment seems to hold true, though the price difference between various ducting options is closing.
“Looking at the good ducting and the cheap ducting, the price difference is not that great now,” Gillies Wu Mining Technology senior mine ventilation engineer and consultant Dr Hsin Wei Wu said.
“We’re talking about a difference of less than a few thousand dollars for a whole system and with the benefit of reduced energy consumption you can probably pay that back in three to six months.
“And then you also have more reliable working conditions for the worker.”
Wu has extensively tested airflow performance in duct systems at the University of Queensland, recently presenting a research paper alongside colleague Stewart Gillies to the International Mine Ventilation Congress.
The paper, A Comparison of Predicted Performance of Auxiliary Ventilation Ducting System, examined the efficiency of different types of duct construction and measured their leakage coefficients and resistance.
“Leakage of air from ducting is affected by ducting material and construction methodology [such as welded or sewn], quality of installation, number of joints, total length, pressure differences between the inside and outside and diameter,” the report said.
“… the best ducting system with welded construction can have up to two times less leakage as compared with some ducting systems with sewn construction.
“… welded type ducting requires less fan energy to deliver more air hence delivering increased air cooling power or dilution of contaminants.
“Using a ventilation ducting system with a lower leakage coefficient results in saving in capital and operating cost as in most situations a smaller fan can be used.”
Limiting the amount of leakage and air flow resistance in a mine’s ducting not only leads to power savings but also has the flow on effect of less downtime to fix ducting sections and increased safety for workers in an increasingly strict regulatory environment.
Diesel particulates have become a primary concern in underground mine safety in recent years, with the World Health Organisation raising the status of diesel engine exhaust to a Grade 1 Carcinogen in 2012.
Add to this the more recent re-emergence of black lung in the Queensland underground coal industry, and it is plain to see the importance of mitigation measures such as mine ventilation.
For Ozvent’s Derrington, there are a range of actions that can be taken to better manage air quality and flow underground while still striving to reduce cost.
Some of these include reducing primary ventilation rates where there is less diesel equipment being operated and bypassing areas of the mine which are no longer active through the use of ventilation on demand systems.
“Other options include using single-stage fans for shorter duct lengths and potentially also fitting variable speed drive units so the fan speed can be optimised to produce the required airflow rate,” Derrington told AMM.
The senior mine ventilation engineer also said bigger ducts could be used to supply more air flow.
“Given the space restrictions underground, this can be done using oval duct profiles,” he said.
Finally, Derrington also recommended giving the ventilation system a helping hand by reducing pollutants at their source.
“Less ventilation is then needed to dilute these pollutants,” he said.
“For example, specify equipment that uses the latest low-emissions diesel engines. For older equipment, consider fitting diesel particulate filters. Also ensure that dust-producing locations such as roadways are adequately watered.”
