Recirculation can be defined as any portion of mine airflow that travels through the same area more than once.
For the purposes of this discussion, it is NOT the reuse of air in other parts of the mine, although this can also have undesirable consequences.
While some mines allow a limited amount of controlled recirculation, for most mines it is an undesirable and sometimes dangerous result of poor ventilation design, which is often exacerbated by poor maintenance of ventilation controls. In fact, in many countries, the use of recirculated air is strictly forbidden by mining legislation.
Recirculated air has a number of undesirable and possibly dangerous consequences.
- Buildup of heat and humidity – As air recirculates, it repeatedly picks up increasing heat from machines, rock strata and ventilation fans. Moisture and humidity is increased by ground water and mining activities.
- Build of fumes and dust – Mining activities need fresh air to clear out noxious gases and dust. Recirculated air prevents this and allows fumes and dust to accumulate.
- Recirculated blasting fumes and dust may often prevent re-entry into blasted areas for extended periods of time, delaying production and other activities.
- Gas buildup, particular in coal mines can create a dangerous environment, prone to explosion or poisoning of personnel.
Chasm Consulting has been fortunate to have had the opportunity to review many mine ventilation systems around the world. Unfortunately, in a remarkably high portion of mines, recirculation is very prevalent. In some cases, mine personnel are not even aware of it.
In response to the poor ventilation conditions caused by recirculation, a common response is to install more fans or larger fans, which can often make conditions even worse.
Mine Design Issues
The vast majority of recirculation in mines is caused by two aspects of mine ventilation design.
- Underground booster fans. Booster fans are normally designed to ‘push’ air through a mine, usually to assist surface mounted fans to circulate air into further reaches of the mine. This configuration creates a high pressure zone through the region in front of the fans. ANY connection back to the mine behind the fans (drives, declines, shafts, mined out voids and stopes) provide an opportunity for air to leak back into the mine and recirculate. Even closed doors and other ventilation control may allow for unacceptable recirculation, particularly if high pressures are present.
- Solution – Use booster fans only when necessary and limit or eliminate pathways for boosted air to re-enter the mine. If possible, place booster fans near intake or exhaust shafts to limit re-entry points. Ensure any doors or controls to prevent recirculation are of high quality and regularly maintained.
- Underground development fans are another source of recirculated air regularly encountered in mines. In many cases, the auxiliary fan consumes more air than it receives, resulting in used air being redrawn back through to feed the fan intake.
- Solution – Any free standing (or hanging) auxiliary fan MUST have more air feeding past the inlet than it consumes. As a rule of thumb, try to ensure at least 0.25m/s or more of excess air moves past the fan when running to prevent air being drawn back from the working face. Test installed fans using smoke tubes or similar to ensure air is not drawn back through the fan.
- Finally, avoid using multiple in line auxiliary fans to extend duct length, unless the duct is solid, low leakage duct and cannot draw air through gaps under negative pressure. Do not use open flexible duct feeding into downstream fans and duct, as recirculation will be significant and inevitable.
Using Ventsim™ Recirculation Features
Ventsim™ Advanced has an automatic recirculation detector (the green toolbar button). This will highlight any area of a mine which has recirculation levels above a limit defined in the settings (which can be changed in the Tools > Settings > Air Simulation section).
To determine the exact portion of recirculation, place a contamination source (smoke) in any portion of the recirculated part of the mine. Set the concentration strength to ‘100’ if not already set, and perform a ‘steady state’ contaminant simulation. If recirculation is present, some contaminant will return to the same point. For example, if a contaminant value of ’44’ is seen to return to the point of original contamination, then this indicates 44% combined recirculation of the original source.
‘Acceptable’ recirculation limits need to be determined by the individual mine and subject to local mine regulations and acceptable workplace atmospheric standards.