To comply with the Mines Act a current of air sufficient to dilute and render harmless all dangerous gases must be circulated through the workings of the mine. The air enters one of the two shafts, which must be provided, and leaves by the other. The shaft by which the air enters is called the downcast shaft. After passing round the workings, it leaves by the upcast shaft, as it is termed. In large collieries where several districts, or perhaps different seams, have to be ventilated, the main current of air is split up at convenient points, and carried round the different districts in separate smaller currents. These currents usually join at or near the bottom of the upcast shaft. The production, regulation, measurement, and direction of these currents form the work of ventilation.

Production of Air Currents. — In most mines, owing to the natural heat of the strata, a small current of air will pass, but it is only in rare cases that the natural current is sufficient to ventilate the mine. It therefore becomes necessary to resort to some method of producing the air current required for this purpose. The air required to ventilate the workings is set in motion by means of a fan which may reduce the pressure at the top of the upcast shaft below atmospheric pressure, or it may be done by a fan which forces air into the downcast shaft at a higher pressure than atmospheric pressure. The former type of fan is called an exhausting fan, while the latter is called a forcing fan.

It is desirable, especially in deep mines, to use an exhausting fan, as a sudden stoppage of the fan causes a rise of pressure in the mine air, and, consequently, the gases are driven back for a time, whereas if a forcing fan is used a stoppage of the fan causes an immediate fall of pressure in the mine air, and gases in the wastes expand and pass out into the airways.

Whatever type of fan be adopted it is necessary to make such arrangements as will allow of the air current being reversed should an emergency render such a course desirable.

Fans may be of large diameter, and running at a low number of revolutions per minute, or they may be of smaller diameter with a higher speed of rotation. In general the greater the diameter of the fan the better it is suited for circulating large quantities of air against a low resistance, whereas fans of small diameter are better adapted for dealing with small quantities of air against a high resistance.

Guibal Fan. — This fan is made from 15 to 45 feet in diameter according to the volume of air required. The ear or opening and the width of the fan are usually about one-third of its diameter. It consists of a large wheel with arms projecting from a central boss and carrying flat blades or vanes at their outerend. The wheel is enclosed in a casing which entirely covers the fan, but opens to the mine at one or both sides, and to the atmosphere immediately in front of the wheel. The blades of the fan are set backwards and slightly curved at the tip. This type of fan (Fig. 61) is suitable for moving large volumes of air, but it cannot produce high water-gauges.

Schiele Fan. — This is a fan of small diameter from 4 to 12 feet, or thereby, and it revolves at speeds varying from 100 to 300 revolutions per minute. The fan is formed by a central disc to which the blades are riveted on each side. These blades diminish in width from the centre to the tip, and are curved backwards. The fan runs in a spiral-shaped casing of cast-iron, and is so placed that the discharged air enters a gradually widening passage before it reaches the expanding chimney through which it is discharged into the atmosphere. Owing to its rigid construction and small clearance spaces, this type of fan (see Fig. 62), although only able to produce moderate quantities of air, produces high water-gauges.

Sirocco Fan. — This class of fan is of recent design and aims at obtaining large volumes of air by means of a rotating wheel of comparatively small dimensions. It is made both as a single and a double inlet fan. In the double inlet type the central or driving disc is of heavy steel plate, supported from a cast-iron hub made in two pieces. The blades, which are numerous, are of pressed steel, and are fixed between the central discs and a front plate, which is stayed by tie-rods from the hub. This arrangement provides a wide inlet which is practically unobstructed. This type of fan (see Fig. 63) deals with large volumes of air and produces fairly high water-gauges. These features give it a wider range of effective action than the other fans just described.

The whole question of fans is too complicated to be dealt with here, but it may be mentioned that in addition to the types described we have the 'Waddle,' 'Cappell,' 'Walker,' 'Rateau,' and others.

Position of Fan. — The fan, whether it is used as an exhausting fan or a forcing fan, must be placed on the surface, and should be sufficiently far back from the shaft to avoid damage in the event of an explosion. The fan must also be arranged so that the direction of the air current can be reversed. This is usually done by means of doors in the fan drift, so placed that by opening one set and closing another the fan is made to draw its air from the atmosphere and discharge it into the fan drift, instead of drawing its air from the drift and discharging into the atmosphere as it usually does.

Water-Gauge. — This instrument is used to determine the ventilating pressure, or otherwise the pressure which, acting on the mine air, causes it to flow round the various airways in the mine. It is simply a glass tube shaped like the letter U and open at both ends. Water is placed in the bent portion of the tube, and stands at the same height in both arms so long as each end of the tube is subjected to the same pressure. If, however, one end of the tube is put in communication with the interior of the fan drift, and the other exposed to the atmosphere, the water will rise up one of the arms and fall down in the other, the difference in height of the two surfaces representing the weight of water column that is balanced by the difference of pressure between the atmosphere and the air in the fan drift. If the fan is exhausting the water will rise in that arm of the gauge which is connected to the inside of the drift, as the pressure there is lower than the pressure of the atmosphere. Should the fan be a forcing one, the water will rise in that arm of the gauge which is open to the atmosphere, since the pressure in the other leg connected to the fan drift is higher than that of the atmosphere. The reading of the watergauge is taken in inches, and the ventilating pressure is expressed as so many inches of water-gauge. One cubic inch of water weighs 0.036 lbs., so that a column of water 1 square foot in area and 1 inch high weighs 0.036 x 144= 5.2 lbs.

This gives the rule that: Inches of water-gauge x 5.2 = pressure in pounds per square foot producing ventilation.

The question of how the end of the water-gauge in the drift should be placed in relation to the air current so as to measure static pressure is one that is still undecided for reasons that cannot be dealt with here. A water-gauge such as is commonly used for mining purposes is shown in Fig. 64.

Measurement of the Ventilating Current. — If Q be taken as the quantity of air passing a given point in cubic feet per minute, V the velocity of the air in feet per minute, and A the area of cross section of the airway in square feet, then

Q=VxA.

From this it is obvious that before the quantity of air passing can be ascertained, two measurements are necessary: first, the dimensions of the airway which supplies the data from which the area is calculated; and second, the velocity or rate of motion of the air. Once the dimensions of the airway are obtained by measurement with an ordinary footrule, or tape, the area is found by the usual rules of mensuration. The velocity is measured by (a) allowing some light body to be carried in the air current, and noting the time it takes to travel a measured distance, (b) by the use of the anemometer, an instrument which is constructed to indicate the rate at which an air current is moving.

When the former method is used the area of the airway must be uniform over a length such as will admit of the velocity being measured with a fair degree of accuracy. If such a place is unobtainable, then the mean section over the distance chosen must be determined. Various light substances may be used, but smoke generated by the ignition of a pinch of gunpowder gives good results. The flash of the gunpowder indicates that the smoke has been set free, and its arrival at the end of the measured distance enables the time the air carrying it has taken to travel that distance to be obtained. This method is seldom used in practice, but it is one of the tests laid down for the Firemen's Examination by the authorities at the Home Office. Where the velocity is obtained bya smoke test, the quantity of air passing will be obtained as follows

but since Q = V x A, we may obtain by substitution

The foregoing method is only available as described in non-fiery mines, where naked lights are allowed. If, however, the fumes formed by bringing Ammonia (NH₃) and Hydrochloric Acid (HCl) together be used, the method can be applied to safety lamp mines. If this be done a signal must be given to the observer when the Ammonium Chloride fumes are liberated into the air current.

These smoke tests give fairly accurate results provided the proper part of the airway is chosen, the errors that may arise being chiefly errors of observation. In practice, however, they are rarely used, unless, perhaps, in cases where the velocity of the air current is too sluggish to move the vanes of the anemometer, or where a rough check on the reading of that instrument is desired.