In the last few articles we talked about underground mining ventilation. We continue this discussion today looking at airway efficiency.
Gradual systemic leakage that takes place in an underground mine significantly increases ventilation costs if unchecked.
In a mine where the concussive force of blasting is confined, reducing air leakage to zero is not a practical option. And in fact blast concussions will likely contribute 15% to 25% of the total leakage in the ventilation system.
The balance of the airway leakage is typically in the bulkhead construction methods and the maintenance of the airways.
If an airway is leaking 25% of its air from a damaged ventilation stopping, then the ventilation system must deliver 25% more air to compensate.
Operators familiar with fan systems know that the pressure drop of the system increases not linearly, but by the square of the flow.
Using the example of 25%, since the dimension of the airway remains the same, the resultant system resistance remains the same, whereas the 25% of additional airflow results in a 56% increase in static pressure (1.25 x 1.25 = 1.56).
The actual energy requirement due to the leakage is therefore 1.25 (increase in flow) × 1.56 (increase in static pressure) which results in a 1.95 increase in power consumed.
When we realize that a 25% increase in leakage results in almost a doubling of the power consumed, then we can see the importance of good leakage control measures.
Although we can now see the significance of leakage rates, locating these sources is not easy. For instance, a 10% air leakage across individual ventilation stoppings is within the error range of an anemometer.
There are a few trouble-shooting tips that help a mine operator keep leakage rates under control. For instance, leakage can sometimes be detected by ear. If a whistling noise can be heard when walking by a ventilation stopping, it would indicate that the escaping air migrating through the bulkhead is traveling at a minimum of 5,000 fpm.
As is often the case in underground mines the background noise will often mask this whistling sound, therefore other techniques are necessary.
With the advent of energy management systems we can use transducers on the electric motors to measure and log the power drawn by each fan. This can indicate the progression of power consumption as leakage rates increase.
Another tool in a mines’ toolkit against leakage is to standardize bulkhead construction methods and ventilation repairs. In larger operations, a crew dedicated to ventilation construction has proven to pay dividends in keeping the cost of power low.
Again as we have energy management systems at our disposal, we now have a new tool to employ – ventilation on demand – which we will cover in the next article.