Ventilation part 1
This is the first in a series of articles about the design considerations of main and booster fan installations. Articles in the next two issues will cover general mine ventilation practices including equipment selection and installation, operation and maintenance.
An underground mine’s primary and booster fans are large pieces of equipment that are similar to the power that comes into your house–nary a thought is given to the work they do until they stop.
These fans consume the majority of a mine’s total electrical load as they operate 24 hours per day, seven days a week. They form the base load of a mine’s electrical system that cannot be shed during peak power situations. Because the cost of running these fans is high, it is important that the power consumed is used effectively in achieving the goal of delivering the correct ventilation amounts to the various work areas.
In terms of the fan selection, the flow component can be readily calculated. In Canada, provincial legislation mandates the proper dilution ratio. For example, Ontario specifies that 0.06 m3/s must be supplied for each operating kW of diesel-powered equipment (~100 cfm/BHP) operating underground. Additional cooling is required for the really deep mines, where the rock temperature increases 2*C for every 250 metres (3*F per 1000 feet) vertical on average.
Cooling is usually based upon air velocities within the underground drifts or mine openings, although air-cooling plants can also be utilized. With regards to mine air cooling utilizing air velocities, the American Conference of Governmental Industrial Hygienists (ACGIH) have a number of heat/stress charts, based on the work activity, and Wet Bulb Globe temperature which will aid the ventilation engineers and designers in determining what additional air velocity and thereby air volume, is required.
The fan static pressure (FSP) component of the rating is determined from the flow requirements. The pressure loss is a function of the velocity through that section and the roughness of the wall. Knowing each section’s air volume flow and using the cross-sectional area for that section, one can readily calculate the velocity of the air travelling through each mine section. Totaling up the losses of each section, one will arrive at the total pressure loss and the amount of FSP that the fan must deliver to move this volume of air.
A good rule of thumb for air velocity calculations is to keep it under 10 m/s (2000 fpm) for each section. The rationale is based not only on energy losses, but on safety issues as well. Where air-flow velocities exceed 10 m/s, an increase in medical incidents is usually reported, the most common being dust particles in the eye.
As pressure losses are a function of air velocity and wall roughness, anything that can reduce roughness should also be considered. Some mines have lined major air intakes (raises) with concrete–a costly but effective way to reduce pressure losses.
As a mine has many branches, often ‘booster’ fans are required to overcome shortcomings in the main ventilation fans. Generally they are used where additional fan static pressure is needed to overcome pressure drops in long or narrow sections of airway. Again, the same rules apply in selecting a booster fan as they do for the primary system
Determining the flow and pressure are two main considerations to select a fan, but there are of course many more. Density of the air is also an important parameter. Air density can either be increased or decreased by the air temperature, the air/gas molecular weight, its humidity, the elevation in which the fan is operating and amount of suction the fan will experience. The topic of density is a topic in itself and we will return to that in another issue. The main thing to know is that density has a real effect on the static pressure requirements and the horsepower consumed.
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., an application engineer for Canadian Buffalo Equipment in Kitchener, Ont. They may be contacted at [email protected] or [email protected], respectively.