Ventilation Part III
When a major piece of ventilation equipment such as a fan, bulkhead or other crucial component is damaged, all of the mine’s resources are mobilized to correct the situation. In the context of Stephen Covey’s prioritization matrix (from his book The Seven Habits of Highly Effective People) this type of activity would be labelled Quadrant I — “Urgent and Important”.
In the previous articles, we touched on the selection of primary and booster ventilation fans and installation guidelines to optimize fan performance. Yet, regardless of how well the fan is selected and installed for a primary ventilation circuit, the conditions upstream and downstream of the fan generally determine the fan’s effectiveness.
What we want to talk about in this article is a Quadrant II focus–on those items that are “Important but Not Urgent”. Therefore we will be exploring ways of improving airway efficiency, which is often reduced by the gradual systemic leakage that takes place in an underground mine.
Air leakage in a mine can literally drive ventilation rates through the “back”. However, in a mining environment where the concussive force of blasting is confined, reducing air leakage to zero is not a practical option. Leakage from blast concussions likely accounts for 15 to 25% of the total leakage encountered in the ventilation system. The remainder of the leakage is made up of bulkhead construction methods and the degree of maintenance of the system.
To understand how leakage affects the operating cost at your mine, think of leakage as the additional air you must provide to meet government-legislated air quality values. If an airway is leaking 20% of its air from a poorly-constructed or damaged ventilation stopping, then the ventilation system must add 20% more air to compensate. This may seem like a reasonable cost of doing business until we look at how it impacts the energy bill.
To do this we use two equations. The first is the Air Power equation (AP = Pd x Q) where AP refers to the Air Power, Pd is the pressure drop in the circuit and Q is the airflow. The second equation is known as the Atkinson Equation (Pd = RQ2). Pd and Q are the same while R is a parameter that pertains to the resistance of the airway.
The Atkinson Equation tells us that the pressure drop will not increase linearly as one might expect but by the square of the flow increase. Using the example of 20%, since the dimension of the airway remains the same, the resultant system resistance remains the same (R1/R2 = 1.0), whereas the 20% of additional airflow results in a 44% increase in static pressure. To see this, think of the 20% additional airflow as 120% of the original flow. Using units, if the original flow in this system is 1.0 m3/s, then the original pressure drop would be 1.0 Pa. Substituting these values (1.0 Pa and 1.0 m3/s) into the Air Power equation produces an energy requirement of 1.0 kW. However, with the new flow rate 20% higher, Q now equals 1.2 m3/s. When the flow parameter is squared (i.e., 1.20 x 1.20 = 1.44), a 144% increase in pressure drop results.
So because of leakage a 20% increase in flow yields a 44% increase in static pressure. But again, this is only the increase in static pressure. We haven’t yet taken into account the increased flow in the Air Power equation that allows us to calculate the new power requirement. Using the Air Power equation and inserting the new higher pressure drop value (1.44 Pa) along with the new flow (1.2 m3/s), this produces a value of 1.73 kW, or 173% of the original power.
When we realize that a 20% increase in leakage results in a 73% increase in power, 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 very difficult to measure. In fact any air flow difference measured with an anemometer is usually attributed to instrument error. Generally, air leakage can only be noted at the beginning and end of an airway system.
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, that would indicate approximately 25 m/s (~5,000 fpm) air velocity escaping through the stopping. Assuming the stopping is 3 m x 3 m (10 feet x 10 feet) with small gaps around the perimeter, leakage at this stopping could be as much as 2 m3/s (~4,200 cfm) of air.
Where background noise is too high, which is often the case, we need another approach. Maintaining a log of the amperage and voltage drawn by each fan is a valuable tool for determining whether the leakage rate is increasing. By calculating the connected load of each fan and by comparing this value with the fan motor plate’s stated horsepower–or initial commissioning values–the effectiveness of the fan system can be determined.
In the past when troubleshooting ventilation systems for clients, the authors have noted 100 kW systems only drawing one-third of the fan’s available power, an indication the fan is operating against a low system resistance. In these cases the cause of the low system resistance is often poor fan installation or inadequate ventilation bulkhead construction.
Another good tool against leakage is to standardize bulkhead construction methods and ventilation repairs. In larger operations, a crew dedicated to ventilation construction is likely a worthwhile consideration. Giving this top priority amongst construction crews is a good step.
Recognizing that leakage is expensive, yet knowing it is difficult to control with blasting concussion present, the question may arise, “What is an acceptable amount of air leakage?”
To answer this question for each unique mine site, one would need to delve into Life Cycle Cost analysis, as every mine has its own production requirements, energy costs and manpower availability. Generally speaking, however, a rule of thumb is where main and booster fans are operating at 1.0 kPa (~4.0 inches w.g.s.p.) or less, maintaining air leakage factors below 15% is considered acceptable.
In higher-pressure applications where the main and booster fans are operating at between 1.0 kPa and 2.0 kPa (~4.0 to 8.0 inches w.g.s.p.), keeping air leakage factors around 20% is reasonable. Finally for those mines where main and booster fans operate at 2.0 kPa (~8.0 inches w.g.s.p.) or greater, maintaining air leakage factors around 25% is often cost-effective.
The reason for this variation is simple. The greater the pressure, the greater the ability of the system to leak air and the more care (cost) required to control this. Bear in mind that these are rough guides as each site has different cost structures. It is anticipated that the trend will be towards further reduction of leakage rates as energy costs increase and improvements are made to sealing technology and methods.
What is often found in the field is that leakage rates are often much higher. There are many reasons why, but some are that leakage although “Important” is not deemed “Urgent”. Utilizing the Atkinson Equation will aid engineers and accountants to understand the cost ramifications of even minor air leakage.
Also working against this is the fact that the maintenance to reduce leakage comes out of the Maintenance budget, while the benefits of lower energy costs go to the Operations budget. Devising reward scenarios that recognize this maintenance effort will go a long way to lowering leakage rates.
H. Campbell (Cam) Seeber of Sault Ste. Marie, Ont., is a senior mining ventilation specialist with over 25 years of field experience in Canada and abroad. Jim Wywrot, P.Eng., is an application engineer for Canadian Buffalo Equipment in Kitchener, Ont. They maybe contacted at [email protected] and [email protected], respectively.