Axial Fans for Underground Mining: Auxiliary and Booster Ventilation

Underground mines cannot work without ventilation, and axial fans are the workhorses of it. They are compact enough to sit in an airway, they can be arranged in series and parallel, and their duty can be tuned by blade angle. But real mines also show how easily ventilation goes wrong: one study of underground auxiliary fans in deep South African gold mines found that the fans and the systems around them operated far below their potential. This page explains where axial fans fit in a mine, the evidence on what goes wrong, and where our range starts and stops.

The Four Fan Roles in a Mine

Mine ventilation engineers describe fans by their role in the network.

Role	What it does
Primary (main) fans	Move the whole mine’s air through the network from the surface
Booster fans	Smaller fans in series with primary fans, helping overcome airway resistance
Development-end (auxiliary) fans	Ventilate a dead-end heading with no air flowing through it, usually through a duct
District or circuit fans	Direct air into a specific area, such as a mining district, bulk air cooler, or shaft

Both axial and centrifugal fans are used as main fans, and the axial type is favoured for underground locations. Our range covers auxiliary and booster duties. Primary main fans for large mines are far bigger: some manufacturers rate mine main fans up to about a million cubic feet per minute, roughly 470 m³/s, at pressures of several thousand pascals, which is well beyond our maximum of about 51 m³/s per fan. We do not build primary main fans.

What a Study of Real Mines Found

An evaluation of underground auxiliary fan systems in four South African deep-level gold mines found an average fan efficiency of 38% across 33 fans, and a total auxiliary fan system efficiency of only 5% across six systems. The fans deviated significantly from their design operating points, and the authors concluded there were significant shortcomings in current underground fan practice. The same body of research reports that fan assemblage components can consume 40 to 80% of the energy that primary ventilation fans use.

These are figures from one study and should not be read as a universal rule. But the pattern is common: the fan is often blamed when the cause is the system around it. Long, leaky, restrictive ducting, fans operating off their design point, and poor matching between fans in series or parallel waste far more energy than the fan’s own efficiency.

Duct Sizes and Fan Diameters

Auxiliary fans are sized to match the ventilation ducts used underground, and standard duct diameters run from about 700 mm up to 1,600 mm. Our maximum impeller diameter of 1,600 mm sits at the top of that range, which suits auxiliary and booster applications.

Ducted Ventilation: Leakage and Resistance

Flexible ducting is cheap and quick to install, but it leaks and its resistance grows with length. If 20% of the air leaks from a long duct run, only 24 m³/s of a 30 m³/s fan reaches the face. Resistance also rises with length and with every bend, kink, or sag, so the fan must be selected against the duct system’s real resistance, including leakage.

Good practice: – Select the fan for the flow required at the face plus leakage, not the face flow alone. – Keep ducts straight, tight, and properly supported. – Use larger ducts where possible, since resistance falls steeply with diameter. – Check the operating point at the longest duct length the heading will reach.

Stall, Series, and Parallel Operation

Axial fans have a pronounced stall region at high resistance. In a mine, resistance changes as ducts lengthen, doors open, and workings advance, so an operating point that was safe at the start can drift toward stall. Some designs allow blade angles to be adjusted so a single-speed fan can cover a wide range of duties, and anti-stall arrangements exist for high-pressure transients. See how to read an axial fan curve.

Fans can be arranged in series to add pressure, for example to push air through a long duct, and in parallel to add flow. Both need matched fans and a check of the combined curve, since mismatched fans can hunt or operate one fan in stall.

Diesel Particulates and Airflow Demand

Airflow demand in many mines is driven by diesel equipment. Limits on diesel particulate matter can require mines to greatly increase ventilation, and the low-resistance, large-opening mines that result need fans that move large volumes at low pressure. In such stone mines, propeller fans, designed for low pressure and large flow, can suit the duty better than axial vane fans, which do not operate efficiently at low static pressures.

Gassy Mines: Group I Equipment

Mines with firedamp or combustible coal dust require Group I equipment under ATEX, and fans for mining are outside the scope of EN 14986, which covers Group II. Group I equipment follows other standards. This page covers ventilation in non-gassy mines and general duties. We do not manufacture Group I equipment, so if your mine is gassy, the fans and motors in the affected workings must come from a specialist supplier. Read ATEX and spark-resistant axial fans.

Energy and Control

Because power varies with the cube of speed, fans that run at full duty when only part is needed waste a great deal of energy. Speed control, blade-pitch control, and staging all help. See adjustable pitch versus VFD control.

Maintenance Underground

Underground conditions are hard: dust, moisture, blasting vibration, and limited access. Fans should be robust, corrosion-resistant where the air is wet or acidic, and easy to inspect. Vibration monitoring and regular cleaning help. See axial fan vibration and maintenance.

Where Our Range Fits

NextAir Systems manufactures axial fans with impeller diameters up to 1,600 mm, static pressure up to 2,400 Pa, and air volume up to 185,000 m³/h. That covers many auxiliary, booster, and district fan duties. It does not cover primary main fans for large or deep mines, or gassy-mine Group I fans, which we do not manufacture. We will say so early instead of forcing a selection.

What to Specify

1. Fan role: auxiliary, booster, or district.
2. Face airflow required, plus duct leakage.
3. Duct diameter, length, and resistance at the longest extent.
4. Air density at the mine’s depth, temperature, and altitude.
5. Whether the mine is gassy, and any Group I requirement.
6. Control approach: speed, blade pitch, or staging.
7. Corrosion, dust, and access conditions.

Tell Us Your Duct Length and Duty

Send your fan role, duct size, length, and site conditions, and we’ll size the fan and check the operating point. Contact us for a free quote.

Frequently Asked Questions