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Mine Ventilation

In between 4000 and 1200 BC, European miners dug tunnels into chalk deposits searching for flint. Archaeological investigations at Grimes Graves in the south of England have shown that these early flint miners built brushwood fires at the working faces presumably to weaken the rock. However, those Neolithic miners could hardly have failed to observe the currents of air induced by the fire. Indeed, the ability of fire to promote airflow was rediscovered by the Greeks, the Romans, in medieval Europe and during the Industrial Revolution in Britain.

The Laurium silver mines of Greece, operating in 600 BC, have layouts which reveal that the Greek miners were conscious of the need for a connected ventilating circuit. At least two airways served each major section of the mine and there is evidence that divided shafts were used to provide separate air intake and return connections to the surface. Underground mines of the Roman Empire often had twin shafts, and Pliny (AD 23-79) describes how slaves used palm fronds to waft air along tunnels.

Ventilation in a mine serves three main purposes: (a) to provide fresh air for respiration by the miners, (b) to dilute any noxious gases that may be formed underground and (c) to lower natural heat of the rock. The underground temperature rises with increasing depth-on an average about 1oC for every 30m (100ft.)- so that the deeper the mine, the hotter is generally is.

Mine Ventilation

In simple horizontal-tunnel mining it is usually sufficient to rely on natural ventilation by utilizing the difference in air pressure associated with the difference in level between two openings – the mine entrance and the top of a ventilation shaft (Chimney effect, Fig.1). Depending on the external temperatures prevailing at different times of the year, the direction of flow of the draft is subject to change.

Diffusion (as distinct from draft) also plays some part in changing the air in a tunnel; thus, a large-diameter tunnel can be driven to a distance of several hundred feet without requiring artificial ventilation. The exhaust air from pneumatic tools is also helpful in promoting air circulation.

In deep mining it is necessary to use fans, that is also of very large size. In large coal mines fresh air may have to be drawn in at a rate of 20,000 m3 (7,00,000 ft3) per minute. These fans are installed at the air-extraction shafts at the edge of the mined area the main winding shaft or shafts, through which the fresh air is drawn in, being located in the central part of this area. With every method of ventilation the above mentioned chimney effect is utilized as fully as possible: the fresh air descends by gravity to the lowest levels of the mine and is heated by the natural heat of the rock, so that it becomes specifically lighter and tends to rise.

Mine Ventilation

The rising air makes its way by various paths to suction zone of the main extraction way or shaft, in which suction pressure up to 400mm (17 in) water gauge are maintained. Distribution of the fresh air over the various levels, main roadways, crossnuts, rooms and workings is assisted by ventilation doors (designed as air locks), stoppings, air crossings and other devices (Fig.2 and 3).

Mine Ventilation

Planning of a mine-ventilation system includes the preparation of so called air-flow sheets-diagrams comprising data on airflow conditions. These diagrams are prepared for each section and for the mine as a whole, the data being checked against measurements of the actual flow underground.

For reasons of safety the main flow has to be split up into the largest possible number of circulating currents, and it is essential to prevent “short circuits” – circumstances causing the air to take a shortcut and thus bypass certain parts of the mine. Parts that are not accessible to the natural ventilation have to be provided with auxiliary ventilation. For this purpose air is piped to those parts through large-diameter ducts through which it is impelled by powerful fans.

Mine Ventilation

This auxiliary ventilation possesses a separate system whose proper functioning has to be supervised and controlled with considerable care. It may operate by suction or by pressure (blowing) or a combination of both (Fig 4). Particularly in deep and hot mines e.g., in South African gold mining – air conditioning (as distinct from mere ventilation) may be used to maintain the atmosphere of the workings at suitable temperature and humidity for men to work in. Because of the high cost involved, it is seldom used, however.