In order to provide ventilation to underground mining operations, mining ventilation engineers will put together a complex system to ensure a mine is operated both safely and economically.
Mine ventilation systems consist of one or more primary ventilation fans and many booster fans to move air underground, through the drifts (tunnels).
The primary and booster fans consume the majority of a mine’s total electrical load and as they operate continuously, mining operators and ventilation designers must take steps to provide ventilation according to the codes of the jurisdiction, while at the same time ensuring that the energy used to move the air is minimized.
Mining ventilation codes were put in place to provide minimum levels of acceptable air-quality underground. For instance, one example would be for mobile equipment. In that area, a common design parameter is to allow 100 CFM of fresh air per horsepower of operating diesel equipment.
Ventilation not only provides clean air to breathe and to be used by equipment but also is used to cool the mine.
As one travels vertically downwards the rock temperature increases, roughly 3° F per thousand feet. Keeping sufficient ventilation is important to avoid heat stress conditions in the operating personnel.
If one thinks of the main shaft as a manifold and each section as a branch, the calculations commonly used by HVAC engineers in common ducting can also derive the pressure loss in each of the sections. One calculates the pressure loss within each section by taking into account the roughness of the walls, the cross-sectional area and the length of each drift, as well as the volume flow that must pass through that section.
Pressure drop is related to the square of the velocity. Even a small increase in velocity can have a large effect.
As mentioned earlier, velocity allows cooling. However, if the velocity is too high it will not only increase the static pressure losses, it can also create other problems such as dust particles in the air becoming airborne. This can result in medical incidents, such as dust particles in the eye. For these reasons, ventilating engineers look to keep the velocity of the air under 2000 ft. / min.
Pressure losses are not only a function of the velocity but also the roughness of the walls. Anything that can be done to reduce the roughness will help in reducing the power required. Some mines will line the major air intakes with concrete to reduce the friction losses. Although expensive, this could have a reasonable payback, depending upon the airflow volumes and electricity consumed by the primary fan.
Now where do booster fans come into the equation? Well going back to the manifold and branch analogy, if one or more branches are long with smaller cross sectional areas, then a significantly higher amount of static pressure will be required to achieve proper ventilation at the end of those drifts. To avoid oversizing the primary fan to reach these areas, one would install one or more booster fans to take care of special conditions that may exist in certain drifts. Also as the drifts are lengthened, additional pressure losses result – necessitating another booster fan to look after the newly added pressure drop.
Another main consideration is density and how this affects the fan performance.
We will cover the effect of density as well as other topics such as ventilation-on-demand strategies in subsequent articles.