Australia’s mining industry adds about $60 billion to the country’s GDP. It is an important industry, both in terms of export revenue as well as OH&S issues. More than most industrial activities, mining is constantly under close scrutiny from all angles over occupational health and safety matters. Although Australia is a leader in workplace safety for the mining industry, the sheer scale of the industry and its activities has its inherent dangers.
Mine ventilation is one such area, which calls for a number of work practices and technologies to be well developed so as to assure the safety of all mining employees. Eximo , which supplies ducting technologies that often form an integral part of an overall ventilation and/or filtration system for mine ventilation, have worked closely with their technical partners to make sure such installations and identified key objectives are implemented.
According to Melissa Phelps, General Manager, Eximo, when designing a mine auxiliary ventilation system, the basic objectives are the sufficient provision of clean air for individuals and equipment, and adequate ventilation for the removal, or dilution of hazardous gases or dust. The quantity of air required, physical restraints present, and other factors should be considered, and taken into account.
The air quantity should be considered in terms of both velocity and volume, the air velocity in the entry or tunnel and the volume of air required for workers, the operation of diesel equipment, the dilution or removal of gases and dust, cooling, and other needs. There are also physical constraints, which demand consideration, which include:
- Head room over haulage or material transport equipment
- Length of duct
- Associated handling and hanging problems
- Potential for damage from blasting and other activities
- Correct sizing of duct and fans to ensure that the fans are not working in a stall
- A larger duct allows for an efficient and lower horsepower system
- Legal and contractual requirements for fans and duct
- Preferred directional airflow
- Suction or blowing
- Acceptable levels of leakage for the determination of duct length
- With leakage occurring at the joints, longer length has less leakage
When evaluating the different types of duct available, there are four broad categories to consider, namely steel, fibreglass, flexible suction and lay-flat.
For suction ducting, steel requires the low initial investment, it is non-flammable, holds high positive and negative pressure, allows medium leakage, is low friction, and is suitable for moderate length ventilation runs (less than 1000m), such as drill and shoot operations, and single use applications.
Fibreglass, the most durable of suction ducting material, is flame resistant, holds up well to abuse, is light-weight, gives high positive and negative pressure, has a low friction factor, and is suitable for short-length ventilation runs (less than 500m), multiple reuse operations, drill and shoot operations and mine development.
While flexible suction has the distinct advantage of being both expandable and contractible in length so that it can be stored in a small area, its weight is dependent on the negative pressure rating. It is also flame-resistant and suitable for starter tunnels, short ventilation runs and flexible connections with other types of duct.
Lay-flat, also of a low initial investment, has a pressure rating dependent on material and construction, has a medium friction factor and low leakage in long length, but requires an additional fan for reversing airflow on OSHA jobs. This technology requires special expertise in applications for booster fans. It is flame resistant, available in many grades, and is suitable for short ventilation runs using low cost duct, long ventilation runs using high-quality long-length ducts and TBM tunnels.
When calculating ducting air pressure losses, losses can be categorised as either dynamic or friction induced. With dynamic losses, the pressure loss in each fitting, inlet and outlet should be calculated separately and is based on the velocity pressure of air at that point in the system, while with friction losses, friction loss curves and formula are used to give the pressure loss due to friction in the duct.