Auxiliary Mine Fan

Overview

An auxiliary mine fan ventilates the dead-end workings that the main mine ventilation circuit cannot reach. A mine's primary fans move enormous volumes of air through the through-going airways, but a development heading being driven into solid rock is a blind tube: air will not flow into it on its own. The auxiliary fan, working with a length of ducting, forces fresh air to the face or draws contaminated air away from it. This is what makes it safe for people to work at the end of a heading, clearing diesel exhaust, blasting fumes, dust, and any gas seeping from the rock, and supplying the oxygen the crew and equipment need.

The fan itself is an axial machine: a bladed Impeller Assembly spinning inside a cylindrical Fan Casing, driven by a flameproof Drive Motor. Air enters along the axis, is accelerated by the blades, and leaves along the same axis into the Ducting that carries it down the heading. Because mines are noise-sensitive and explosion-risk environments, the design wraps this simple aerodynamic core in Silencer units, a robust Skid Frame, and a protective Control Panel panel.

Forcing, exhausting, and overlap

Auxiliary ventilation is run in one of three modes, and the fan is selected to suit. In forcing ventilation the fan pushes fresh air through the duct to the face, where it scours the working area and the stale air returns along the open heading. This delivers clean air right to the crew but lets contaminated return air travel back past them. In exhausting ventilation the fan pulls air out of the heading through the duct, drawing fresh air in along the open roadway; this clears blasting fumes well but needs rigid or spiral-reinforced duct that will not collapse under suction. An overlap system uses both, a forcing duct and an exhausting duct working together, for long or gassy headings. The same fan platform serves these modes with changes to ducting and fan position.

Impeller and motor

The Impeller Assembly is the working element. Aerofoil Aerofoil Blade units, of cast aluminum or glass-reinforced polymer, are set into a central Impeller Hub. Most auxiliary fans have adjustable blade pitch: each blade root turns on a Blade Pitch Bearing so the angle of attack can be set to match the duct length and the airflow needed. A steeper pitch moves more air against more resistance but draws more power, so pitch is tuned to the heading as it advances and the duct gets longer. The impeller is balanced with trim Balance Weight pieces because it spins fast and any imbalance shortens bearing life. It is driven directly by a Drive Motor, a flameproof induction motor whose Motor Housing is built to contain any internal spark, an essential feature where flammable gas may be present.

Casing, aerodynamics, and noise

The Fan Casing is a steel tube that houses the impeller and forms the airway. An Inlet Bellmouth bellmouth smooths air into the blades, internal straightening vanes on the Motor Mount and Vanes remove the swirl the impeller imparts, and an Outlet Diffuser cone at the outlet recovers some of the air's velocity back into useful pressure. Axial fans are efficient but loud, and a heading concentrates the sound, so Silencer units fitted at the inlet and outlet pass the air through absorptive Acoustic Splitter splitters lined with sound-absorbing material. Bringing the noise down protects the hearing of crews who work near the fan for a full shift.

Frame and ducting

The fan and its silencers sit on a Skid Frame, a welded skid with Vibration Isolator mounts that keep vibration out of the structure and Lifting Lug points for moving the unit as the heading advances. The air is delivered by the Ducting system. Forcing duct is usually Flexible Duct, a coated fabric tube that packs flat for storage and inflates under pressure; exhausting duct must be rigid or spiral-reinforced Rigid Duct so it does not collapse under suction. Lengths are joined by Duct Coupling clamps and hung from the roof on Duct Hanger supports. Duct leakage is the main loss in the system: a long run of poorly sealed duct can lose most of the fan's output before it reaches the face, so duct quality and jointing matter as much as the fan rating.

Control and protection

The Control Panel panel starts and protects the fan. A Motor Starter, commonly a soft starter or variable frequency drive, brings the heavy impeller up to speed without a large inrush current and lets the airflow be matched to demand. An Overload Relay relay and motor Winding Thermistor guard the windings, and an Airflow Monitor confirms that air is actually flowing, because a fan that runs but moves no air, through a torn duct or a stalled blade pitch, gives a false sense of safety. The whole panel is housed in a sealed Panel Enclosure.

Selection and variants

Selecting a fan means matching its pressure-flow curve to the resistance of the duct run, which rises with length and falls with diameter. As a heading advances, operators either change blade pitch, add a second fan in series for more pressure, or run twin fans in parallel for more flow. Variants range from small single-stage fans for short development headings to twin-stage, high-pressure machines that push air through two kilometers of duct in long tunnels. Larger units may be diesel-driven where electrical reticulation has not yet reached the working area. Across the range the job is the same: keep a controlled stream of fresh air reaching the one part of the mine that the main circuit cannot serve by itself.