The availability of ventilation capacity is critical to mining operations, and the costs of inadequate fan performance will inevitably have some effect on mine viability and safety. The equipment used and the material being mined produces various toxic gases, including Carbon Monoxide, Methane, and Sulphur Dioxide and Nitrogen Oxides. The air may also contain high dust, humidity and heat loads.

Reliability is thus an indispensable requirement of ventilation system design. While some airflow and efficiency reduction or excess noise may be tolerated, production losses will occur, and the costs of correcting these problems once they become unacceptable can be very large. The initial price of the ventilation is thus far from the only consideration.

Achieving maximal ventilation performance cost-effectively and with the necessary flexibility demands a systems approach taking all the components necessary for ventilation into account. This should lead to reduced downtimes, lower maintenance and energy expenditures and maximised production.

CFW is an established leader in the ventilation industry, creating ventilation systems on all scales and designing and producing fans for a wide range of applications. We also provide cyclone and venturi scrubbers, ducting and parts and accessories.

The equipment we offer includes:

• Dust control systems
• Surface-located radial blade fans for accurate air volume control
• Robust underground booster fans
• Axial main or auxiliary fans (fixed or variable pitch)

Main Fans

As with any ventilation system, mining ventilation systems consist of air movers (fans) and an air distribution system.

Main fans are usually installed on the surface to make installation, access, maintenance and testing easier. It also helps to safeguard the fan in a dangerous situation. Main fans are sometimes placed underground to prevent noise on the surface or to make shafts accessible for hoisting.

If gases and dusts found in the mine are explosive, as is often the case in coal mines, main fans must be on the surface and may be subject to other safety precautions such as blow-out panels or placement out of line of the shaft or drift it is connected to. Main fans must handle all the air passing through the ventilation system.

Mine Airways

Intake airways (“intakes”) such as downcast shafts introduce air to areas with potentially poor air quality, and contaminated air moves through return airways (“returns”) and exits at the surface through upcast shafts or drifts. Ventilation safety limits are generally imposed for all areas of the mine, including returns.

A critical decision regarding the ventilation system is whether to implement an exhausting system, where main fans are connected to the upcast shafts, or a forcing (blowing) system, where they are joined to downcast shafts. Exhausting systems are more conventional and common, and remain preferable in most cases. Exhausting fans help to avoid high strata gas concentrations during periods of low surface pressure, but may also suffer from corrosion, need more maintenance and use slightly more power.

During the development of a mine, some ventilation or access openings may lose their significance and be cut off using stoppings so that they do not short-circuit the ventilation airflow. These often require special sealing and protection from fire, especially in mines where chemically active strata exist. Two or more stoppings are used when explosion-proof isolation of disused areas is required.

System Efficiency

The factors that contribute to system performance are connected in a complex network. Some important variables include leaking stoppings, high fan pressure (which contributes to more leakage), and the seals and mined-out or gob areas that need ventilation. All mines are somewhat affected by ventilation inefficiencies arising from some of these factors, which require care to minimise.

As much as a third of the underground electricity expenditure of the mine is accounted for by the ventilation fans, so that fan efficiency is a key factor in keeping costs down. Energy-efficient fan motors and proper fan sizing and type selection can achieve this goal as well as reducing maintenance costs and ensuring a longer fan life. Variable fan speeds are an option that can provide more control over air quality while reducing total energy consumption and carbon emissions where appropriate to the application. To accomplish this, fans are often fitted with variable frequency drives (VFD’s).

Fan Performance

Suboptimal fan performance can result in low air flows, increased noise levels, poor efficiency, high maintenance costs, shorter fan life and the risk of a catastrophic failure. In the last case, large fan components break up and damage fan components and housing, and may even cause severe injuries or fatalities if the impeller breaks through the housing.

In order to achieve optimal fan performance, allowances need to be made for

• Fan manufacturing tolerances
• Installation effects
• Pressure loss from ducts

It is essential to take all three these issues into account to arrive at a useful analysis of the system’s stability.

Fan Installation Considerations

Changing the fan geometry and clearances for competitive reasons without carefully assessing the effects results in failure to keep within manufacturing tolerances. This can reduce efficiency and increase noise levels and maintenance requirements. For axial fans, common design alterations detrimental to performance include leaving out an inner fairing downstream from the propeller, adding a self-closing damper too near the diffuser inlet, modifying blade profiles and tip clearances and failure to make the impeller and casing concentric. In the case of centrifugal fans, incorrect impeller geometry or positioning in the casing, incorrect inlet cone clearances, geometry or location or incorrect blade setting angles.

Inlet and discharge flow conditions are another frequent problem that may go unnoticed. The fan inlet might be placed too close to walls, vehicles, other fans or fan accessories. This will result in flow pressure problems and efficiency losses. While both centrifugal and axial fans can become unstable or stall, the problem has especially severe effects on axial fans, since the angle at which air approaches the impeller (the angle of incidence) has large effects and pressure drops can be dramatic. Instability and stall both damage fan components, and the rotating assembly will eventually fail catastrophically. Ideal inlet locations are not always practically possible, and losses from inlet flow effects are difficult to quantify, so that professional judgement is often the best guide.

System Pressure Drops

Overestimating system losses does not have serious consequences on most kinds of fans, although blade pitch may have to be adjusted for some axial fans. Underestimating pressure drop, however, can have severe effects and even cause fan components to fail.

Air system velocity changes cause pressures to fluctuate. After velocity changes, static pressure gradually returns to normal after a drop. Velocity redistribution, however, depends on lengths of straight ducting that are not present in mining ventilation, which means that pressure losses may be much higher than expected. Applying existing published data would lead to flawed estimates, so that once again professional experience is crucial in making the necessary allowances.