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MINE VENTILATION FEATURE SERIES
THE UNSEEN REVERSAL
By Eamonn Ryan
Modern mining and negative natural ventilation pressure.
At the 2025 Mine Ventilation Society of Southern Africa( MVSSA) annual conference, which coincided with its 80th anniversary, mining consultant Bruce Richards of Mikelle Consulting brought a blend of practical experience and technical understanding to his presentation on optimising natural thermodynamics in mine ventilation.
Traditional mine ventilation relies on predictable airflow principles. But Richards, drawing from global experience, has identified a new challenge: negative natural ventilation pressure( NVP) – where natural airflow can reverse, undermining conventional designs.
" To support my observations, I ran a simplified experiment using ventilation simulation software, focusing just on a mine ' s ramp – or decline. I populated the virtual ramp with a typical mix of vehicles: six haul trucks, an underground grader, a water tanker and four personnel carriers. In a conventional setup, fresh air is usually drawn down the ramp, into the workings, and then returns via the upcast shaft.
But when I conducted a thermal heat balance before introducing any forced ventilation, I discovered something critical: the heat generated by these vehicles in the decline actually induced a reverse airflow of 41m3 / s. Instead of fresh air coming down the ramp, it was being pulled down the upcast shaft, while the hot air exited up the ramp. This happened simply because heated air becomes less dense and naturally rises – essentially flipping the airflow direction.
A conventional system would require significant airflow to dilute diesel pollutants, yet Richards found heat and particulates still accumulated at the bottom, degrading air quality.
A MIND SHIFT TO EMBRACE UNCONVENTIONAL VENTILATION
Richards then introduced a single, yet profound, change to his model: he reversed the upcast fan ' s flow to become a downcast fan, effectively complementing the natural thermodynamics at play. He reviewed the results.“ At the bottom of the ramp, where air emerged from the downcast shaft, conditions were remarkably close to ambient surface temperatures. This meant the air had regained its full capacity for cooling and pollutant dilution. Crucially, the heat and pollutants accumulated in the ramp were now exhausted via the ramp access itself, rather than being drawn deeper into the mine.”
Comparing the conventional and unconventional ventilation models, Richards identified significant benefits: The revised model showed lower fan costs( R700 000), eliminated cooling needs at the ramp bottom, improved temperatures by 2 ° C, and removed DPM( diesel particulate matter) contamination – unlike the conventional model, which exceeded safety thresholds.
overlap system. Fresh air was delivered down a central shaft and returned via side shafts, reducing cooling needs and fire risk, and improving safety across all levels.
In North Africa, a gold mine with limited surface access needed cooling but lacked space for surface infrastructure. The team moved cooling underground and applied reverse ventilation, avoiding costly development and enabling
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Mining consultant Bruce Richards of Mikelle Consulting.
efficient heat rejection. This saved USD4-million in fan operating costs and delivered uncontaminated, chilled air directly to production areas.
All images by © RACA Journal
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REAL-WORLD APPLICATIONS: INDIA AND NORTH AFRICA
In India, a deep base metal mine adopted a full forced exhaust
RACA Journal I February 2026
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