A worker in a mine should be able to work under conditions which are safe and healthy for his body. At the same time the environmental conditions should be such as will not impair his working efficiency. This is possible if the mine air is nearly the same as on the surface and without toxic and inflammable gases. The humidity, temperatures and dust content of the air in the mine should also be within certain limits. This is achieved by proper ventilation of underground mine working by continuous supply of fresh surface air. As explained in the earlier chapter, the oxygen content of air is reduced for various reasons and it has to be replenished. The ventilation of a mine has therefore the following objects.
1) To restore the proper composition of mine atmosphere which should not contain less than 19% of oxygen or more than 0.5% of carbon-di-oxide.
2) To dilute other noxious and inflammable gases like CO, CH4 etc. so that they are harmless. A place is not considered to be free from firedamp if gas percentage is above 1.25.
3) (3) To provide good environmental conditions and to prevent excessive rise of temperature and humidity so that the workmen can work with maximum efficiency. The wet bulb temperature in development faces should not exceed 33.5° C
4) (4) To remove or dissipate the coal or rock dust produced in the mine.
Quantity of Air:
An idea of the approximate quantity of air required for ventilation of a mine can be had from the following empirical rules.
In gassy coal mines of Cat. II & III the major consideration to decide the quantity of air going underground is the rate of emission of gas (firedamp) which should be so diluted by the ventilating air that its percentage is not more than 0.5 in the main return airway of the mine. With this object some ventilation standards have been made by the D.G.M.S. and these are embodied in the mining regulations for coal as well as metalliferous mines. Under the Coal Mines Regulations the standard of ventilation is as follows:
(A) Quantity of air in a ventilating district should be
(i) Minimum 6 m3/min-per person employed in the district on the largest shift.
(ii) More than 2.5 m3/min. per daily tonne output
(B) Underground air should not have less than 19% O2.
(C) Underground air should not have more than 0.5% C02 or other noxious gas.
(D) Inflammable gas should be below 0.75% in the general body of return air of any ventilation district and below 1.25% at any place in the mine.
(E) Wet bulb temperature should not exceed 33.5°C; if it exceeds 30.50C at any place, air current should be faster than l m/s.
Air samples and temperature readings should be taken once in 30 days.
In gassy coal mines of Deg. II & III the quantity of air should be more than the minimum stated above and a reasonable figure should be 8 m3/min. per person employed.
The quantities of air stated above must reach the working faces. As there is leakage from intake to return at the ventilation stoppings, ventilation doors, air crossings, ventilation air locks on the surface and other places, more air should pass the downcast shaft/incline. Therefore, the quantity of air going down a mine should be as follows (per minute);
Gassiness degree (coal mines)
|
Per person in U/G mine
|
Per tonne of daily output
|
I
II & III
|
7m3
8-10 m3
|
3m3
4-5 m3
|
In Metal mines which are not deep, say upto 300 m, the quantity of air that should go down the intake shaft/incline should be 4—5 m3 per min. for every worker in the underground mine. In deep mines like those in the Kolar Gold Field where the strata temperature is high, this quantity is considered adequate to keep the mine air reasonably cool.
The instruments which are helpful for proper control of mine ventilation, to know its adequacy and assess the environmental conditions for human work are:
(1) Thermometers
(2) Barometers,
(3) Hygrometers,
(4) Kata thermometers,
(5) Air velocity meters,
(6) Water gauge for mine fans,
(7) Gas detectors.
Thermometers
These are too well-known to the readers to justify space in this book. .Temperatures are often stated in degrees Farenheit or centigrade and the conversion ratio is
C / 5 = (F— 32) / 9
where C denotes temperature in degrees centigrade and F denotes temperature in degrees Farenheit
Absolute Zero: - This is theoretically the lowest possible temperature and is the point at which all heat would be extracted from a substance if it can be cooled sufficiently. The absolute zero temperature is — 273.15°C or —459.63°F. For calculation purposes these figures are taken as —273°C and —460°F.
Barometer:
It is an instrument to measure the atmospheric pressure. The standard atmospheric pressure, also called the mean or normal atmospheric pressure, is defined as that pressure which supports a column of mercury 760 mm high at the sea level when the temperature of the mercury is 0°C.
The Fortin Barometer: — This is the standard form of barometer and is of refined construction, being used in scientific work for accurate measurement of the atmospheric pressure.
It consists of a straight glass tube about 920 mm long and about 8 mm inside diameter, the upper end being sealed and the lower (open) end dipping into a small boxwood cistern of mercury having a soft chamois leather base.
An adjusting screw and plate beneath the flexible bottom enable the level of mercury in the cistern to be adjusted, (Fig. 2.1) and, when the instrument is to be moved, the plate may be raised until the lower end of the tube is sealed.
Before any reading is taken, the level .of the mercury must be adjusted so that its surface just touches the tip of an ivory pointer P. This represents the level from which the height of the mercury column must be measured. Alongside the tube near the top, is mounted a scale and vernier, which enable a reading to be made to the nearest half millimetre.
The aneroid barometer: — This instrument is much more portable and convenient than a mercury barometer for determining differences of level, and is much used by prospectors and also in certain types of ventilation surveys (Fig. 2.2). The term aneroid means that the instrument does not contain liquid. Its construction is based on a different principle of measuring atmospheric pressure. According to the Boyle's law a given mass of gas increases in volume as its pressure falls and the volume decreases as the pressure rises, temperature remaining the same. The aneroid barometer makes use of this principle.
It consists of a hollow, air-tight, gas-filled box made of thin springy corrugated metal and prevented from collapsing by means of a strong flat spring. The expansion and contraction movement of the cylinder is transmitted and magnified through a system of levers and links to a pointer or a scale which reads the atmospheric pressure. The scale is graduated to read pressure from 700 to 780 mm of mercury.
The instrument, of course, must be calibrated (i.e. tested by comparison and the graduations marked) with a mercury barometer from time to time. When subjected to sudden alterations of pressure, it is liable to give inaccurate readings, and time must always be allowed for it to adjust itself to the new conditions before a reading is taken.
The barograph or recording barometer consists of a series of thin exhausted corrugated metal shells acting together, thereby increasing the sensitivity of the barometer so that its movement can be used to operate a pen marker and give a continuous record of barometric pressure.
The barometric pressure increases with depth below the sea level and decreases with height above the sea level at a rate of nearly 1 mm Hg difference for every 12 m vertical difference.
The barometric pressure has a bearing on the safety in a mine. The noxious and inflammable gases like methane in the goaf of a coal mine expand when there is a sudden drop in the barometric pressure and overflow into the mine workings. If the influx of such gases into the workings is excessive the normal ventilation current may not be able to dilute them sufficiently, thereby resulting in high percentage of the gases in the mine air. If the barometer is steady or rising, there is no overflow and no danger of fouling of the mine ventilating air current. D.G.M.S. Circular No. 84 of 1966 directs that barometers should be provided at least at every first class mine in which depillaring is done and/or which has sealed-off underground workings.
Hygrometer and relative humidity.
The atmospheric air has the capacity to absorb moisture and air containing it is called humid air. The atmospheric air, however, can absorb moisture only upto certain limit and air which is fully saturated with the moisture content that it is capable of absorbing, is known as saturated air. A given volume of moist air is lighter than the same volume of dry air. Under normal conditions air is never completely dry and the extent, to which it is humid, is known as relative humidity.
Relative humidity = (mass of water vapour per m3 of air) / (mass of water vapour required to saturate one m3 of air)
The relative humidity of saturated air is 100%. The percentage of saturation, except in arid regions, is rarely less than 35% and is higher near the sea shore. In places like Calcutta and Bombay which are at the sea shore the relative humidity is maximum 96% during the rainy season on a hot day and the average R.H. on such day is nearly 80%.
The quantity of moisture present in air is chiefly dependent on temperature and pressure, other things being equal.
When a man works, his body temperature goes up and the body perspires. Normal human body temperature is 98° F (36.6°C) and a person feels discomfort if the body temperature goes up or below this figure by 1°C. During work if the perspiration given out by human body covers the skin, the latter will not be cooled in stagnant air and temperature of the body will rise, thereby causing discomfort to the worker. If the atmosphere in which man works is already full with moisture, it will have no capacity to absorb moisture of perspiration. The drier the air, the more comfortable it is for the worker, as evaporation of perspiration from his body is brisk and the body temperature does not rise to the limit of discomfort. The cooling effect of the air depends not only on the temperature and humidity, but also on the velocity of air.
The sources which contribute to the moisture content of mine air are:
(i) Original moisture content of air.
(ii) Moisture given off from the strata in wet downcast shafts
(iii) Wet roadways, working places and drains.
(iv) Perspiration of men.
(v) Water vapour given off during burning of lamps.
(vi) Water introduced in the mine for wet cutting, water infusion, spraying on coal dust, etc.
Hygrometer: —
An instrument to determine relative humidity of air, i.e., the extent to which it is saturated with moisture, is known ashygrometer, a typical example of which is the dry and wet bulb hygrometer. Essentially, it consists of two thermometers mounted side by side on a suitable frame. One thermometer has a dry bulb and it indicates the actual temperature of the surrounding air. The other thermometer has its bulb covered with a moist cloth which dips into a small bottle filled with water. Constant evaporation of moisture takes place from the wet bulb, thereby cooling it and bringing down its temperature. When the air is relatively dry, i.e., it has a low relative humidity, there is a large difference between readings, of dry bulb and wet bulb. When the air is nearly saturated, the two readings have hardly any difference.
A hygrometer convenient to carry underground is Whirling Hygrometer (Fig. 2.3). Two thermometers are placed on a frame and bulb of one is covered with wet cloth. When the frame along with the thermometers is whirled at 200 r.p.m. for about a minute the readings of dry and wet bulbs enable the operator to calculate the relative humidity of air from tables Relative humidity of mine air in the temperature range obtained in Indian mines can be roughly calculated as follows:
Deduct from 100, 7%per°C temperature difference between dry and wet bulb readings above 25°C of dry bulb reading, 8% per °C difference for dry bulb temperature from 20 to 25°C, and 9% per °C difference for dry bulb temperature of upto 20°C.
If the relative humidity of air is high and the air velocity is brisk the combined effect is drying of the coal dust and creation of dusty condition in a coal mine. It is not a desirable feature when viewed in the context of coal dust suppression measures. Moreover, high velocity of air causes discomfort to the workers. For these reasons the maximum air velocity has been laid down by D.G.M.S. by Cir. No-42 of 1974 (See Chapter on coal dust). The velocity of air at the working face at 0.5 m to 2.0 m/s is reasonable for comfortable working conditions.
In addition to these considerations the capacity of a worker to work in mines with humid conditions depends on the extent of his acclimatisation. An Indian worker may continue to work reasonably well in an underground atmosphere where the wet bulb temperature is 30°C or near about.
The temperature and relative humidity of air increase in the vicinity of a place of spontaneous heating of coal and if the hygrometer readings are taken regularly they provide an indication whether or not heating is taking place.
Kata Thermometer—
To judge whether a working place is suitable for a man to work efficiently and without discomfort, it is necessary to know the temperature of air at the working place, the relative humidity and air velocity. The joint effect of all these factors can be known with the help of kata thermometer which measures the cooling power by a combination of the above mentioned three factors at the instrument temperature of 36.5°C, the normal temperature of human body. Kata consists of an alcohol thermometer with two marks graduated on the stem at 35°C and 38°C. The instrument is used as follows:
The bulb is immersed in hot water carried in a thermos flask until the alcohol rises above the upper graduation. The bulb is wiped dry and the time required for the alcohol column to drop from the upper mark to the lower mark is noted. The procedure is repeated with a wet muslin cloth wrapped on the bulb.
An instrument factor is marked on every instrument. The kata factor of an instrument is the number of milli-calories of heat which it loses per sq. cm. of surface area of the bulb on cooling from 38° to 35°C. It is nearly 480.
Cooling power = (Kata factor) / Time in sec. for alcohol to fall from upper mark to lower mark
The cooling power calculated is called dry if no vet cloth is used on the bulb, and wet if wet cloth is used. The cooling power is given in milli-calories per. sq. cm. per second, and it is the relative value that is important. Minimum limits of the Kata thermometer cooling power for comfortable working are as follows:
| Dry Kata | Wet Kata | |
| For sedentary workers | 6 | 18 |
| For light manual workers | 8 | 25 |
| For hard manual workers | 10 | 30 |
If the dry time is used, the result is the cooling power by conduction and convection. Cooling power by conduction, convection and radiation is given by using the wet bulb time.
A temperature and humidity monitor marketed by Uptron Ltd. is as follows:
The Sieger model TH1 monitor is an intrinsically safe, low cost device for continuously measuring the temperature and humidity in underground mines. The instrument consists of three units, a control module, a remotely sited transducer complete with sensors, and connecting cable, which can be up to 400 metres long. A number of alternative power inputs can be used, including 12V DC, 18V DC, or 15V AC. intrinsically safe supplies, ail of which wilt also charge the in-built nickel cadmium battery. This battery can be used as an automatic stand-by supply if the input power is interrupted and also allows the instrument to be operated for many hours in a new development or areas where there is no power available. When used with an intrinsically safe power input a Special Junction Box is used which prevents incorrect connection of the various leads.
The transducer unit contains separate temperature and humidity sensor together with voltage regulation and signal conditioning circuits, power for which is obtained from the control module. This is contained in a metal case and in addition to a long-life, back-up battery supply, also contains the main amplifier and display driver circuits associated with the LCD indicators on the instrument front panel. These give continuously readings of both temperature (°C) and humidity (in percentage RH), in addition to warning of low battery or fault conditions.
Air velocity meters: -
The meters used to determine the velocity of air in a mine are the commonly used instruments like anemometer, velometer and pilot tube. They have been described elsewhere in this book.
Measurement of air pressure
The pressure of atmospheric air is measured by Fortin's barometer or aneroid barometer. A barometer recordsabsolute pressure in mm of mercury column. The value of atmospheric pressure varies from place to place and depends further on climatic conditions. It is maximum at the sea level and gradually decreases with altitude above seal level. The standard atmospheric pressure, also called the mean or normal atmospheric pressure, is defined as that pressure which supports a column of mercury 760 mm high at sea level when the temperature of mercury is 0°C, This is stated as absolute pressure of 760 mm of mercury. In the metric units the value of standard atmospheric pressure is very nearly 1 kgf/cm2. The internationally accepted value of standard atmospheric pressure in SI units is 101, 325 N/m2, i.e. approximately 100,000 N/m2. The unit bar (b) used by the meteorologists to express atmospheric pressure is equivalent to 105 N/m2, and therefore very nearly equal to the atmospheric pressure. A 760 mm column of mercury is equivalent to (760 X 13.6) / 1000 metres of water column i.e. 10.34 m of water column sp, gr. of mercury is 13.6). If the barometric pressure at a place is h mm of mercury, its conversion into SI units is simple.
h mm height of Hg = (101, 325/760) X h N/m2
= 133.3 h N/m2.
Pressures of fluids are measured by pressure gauges or manometers. A manometer measures small pressure differences above or below the atmospheric pressure. It consist of a vertical U-tube containing a liquid and one limb of the U-tube is connected to the vessel containing the fluid under pressure while the other limb is open to the atmospheric air. The liquid in the U-tube is generally mercury but if the pressure difference is small, water or other lighter liquid is used. The difference between the levels of the liquid in the two vertical limbs of the U-tube is a measure of the pressure of the fluid above the atmospheric pressure (Fig. 2.4, b) or below the atmospheric pressure (fig. 2.4c). In fig. 2.4 if the prevailing atmospheric pressure is P mm of mercury and if the liquid used in the manometer is mercury, the absolute pressure of fluid in the cylinder at b is P + h mm of mercury and at c, it is P -h mm of mercury. The amount by which a pressure is below the prevailing atmospheric pressure is referred to as vacuum and it is quoted in mm of mercury or water column. Fig. 2.4c, represents a vacuum of h mm of mercury or water column. Fig. 2.4c, represents a vacuum of h mm of mercury. Most pressure measuring devices indicate pressures above or below atmospheric pressure. Such devices indicate zero when exposed to the atmosphere; consequently pressures obtained from such instruments are reported as gauge pressures.
Absolute pressure = prevailing atmospheric pressure ± gauge pressure.
The minus sign is to be used when the pressure gauge records vacuum. Such pressure gauge recording vacuum is called a vacuum gauge.
If a manometer employed for finding out the pressure of a fluid uses water in place of mercury and the difference of water levels in the two limbs of U-tube is h mm, then absolute pressure in mm of Hg
= (barometric height of water column in mm ± h) / 13.6
The pressure developed by a mine fan is small and it is measured by a manometer using water. If very small pressure differences are to be measured, say, upto 50 mm of water, inclined manometers are used in which the limbs of the U-tube are inclined to the horizontal.
Water gauge
The atmospheric pressure in an underground mine can be measured by the aneroid barometer. The difference of pressure between nearby points is however known by a water gauge which is essentially an ordinary glass tube of U-shape containing water and having one open end of U-tube connected to one point of low pressure and the other open end connected to another point having high pressure. The difference between the water levels of the two legs of the U-tube records the pressure difference between the two points. Water rises in the leg which is connected to the point of low pressure, and falls in the other leg. A difference of one mm (stated as 1 mm water gauge) represents a pressure difference of 1 kgf/m2. A water gauge is placed in the fan draft; one end connected to the atmosphere and the other in the fan drift very near to the fan (towards the mine).
The term pressure is used in mine ventilation with the following refinements.
(1) Static pressure: This is the pressure exerted by a moving fluid on a surface parallel with the direction of movement.
(2) Velocity pressure: This is the pressure exerted by fluid by virtue of its motion and it varies with the square of its velocity.
Fig. 2.5. U tube and various methods of measuring air pressure
A, B and C give static pressure.
D - Not recommended
E - gives total pressure or fan pressure
(3) Total pressure: This is the algebric sum of (1) and (2). In the case of a fan the total pressure or fan pressure (or ventilating pressure) is the difference between the mean total pressure of the fan and the mean total pressure of the air entering the fan. For the fan located on the surface it’s is the difference between the atmospheric pressure and the total pressure in the fan drift. The total pressure (i.e. fan pressure or ventilating pressure) is measured by a facing tube i.e. with the open end of the tube facing upstream side of air current (e.g. at E in Fig. 2.5).
Measurement of ventilating pressure
The static pressure ps may be measured in several ways by a U-tube gauge having one leg exposed to the atmosphere and the other in the fan-drift, as shown at A, B and C in fig. 2.5 At A, the tube projects into the drift at right angles to thedirection of air-flow and is provided with a thin, sharp-edged flat disc, called a d'Arcy tip. At B, a recess is cut in the side of the drift and is closed by a smooth metal plate fitted flush with the drift-side and perforated with a small hole. At C is shown a hooked tube, sealed at the point and provided with four small holes without burr. This has the advantage that it furnishes (he maximum reading when correctly aligned in the air-current. All three methods give reasonably accurate results. On the other hand, a plain-ended tube, like D, invariably gives incorrect results (too low a reading with a compressing fan and too high with an exhausting fan.)
The total pressure P (fan pressure) is measured correctly by a facing tube, as shown al E, i.e. with the open end of the tube facing to windward. It is this pressure that must always be used when calculating the "H.P. in the air" or when determining the efficiency of a fan. It is sometimes, however, convenient and more accurate (owing to the turbulent and gusty nature of the airflow in a drift or airway) to measure the average static pressure and make allowance for the velocity pressure by calculation from the observed mean air-velocity.
The water gauge produced by fans in our mines is small, varying between 25 mm and ISO mm, though it is higher for the deeper mines. Very small water gauge as in the case of underground auxiliary fans or booster fans is measured by U-tubes with inclined limbs and necessary corrections applied to convert the observed inclined water gauge reading into vertical water gauge reading.
For the measurement of water gauge smaller than 50 mm an inclined water gauge is generally used (Fig. 2.6). The reading is convened into equivalent vertical w.g. as follows:
Vertical w.g. = L X Sin α, where α is the angle of inclination with the horizontal and L the inclined reading of manometer. The manometer can be made more sensitive by using a lighter liquid instead of water e.g. alcohol. The vertical w.g. is then.
Vertical w.g. = L X sin α X w where w is the sp. gr. of the liquid.
The inclination of the water gauge is conveniently adjusted to 5° 44' as the sine of that angle is 0.1.
Inclined water gauges can be read upto 0.2 mm and in some cases even upto 0.02 mm depending upon scale graduation.
The pilot tube
A pilot tube is a device which can be used to measure the static pressure, the velocity pressure or the total pressure. The principle involved is shown at F in Fig. 2.5 which shows one leg utilised to obtain the static pressure and the other to obtain the total pressure, the actual reading recording the velocity pressure which is equal to the difference between the limbs of the U-tube. If the velocity pressure of air at any point has to be measured the two limbs shown at F should be near each other. In a pitot tube, as shown in Fig. 2.7 the inner tube is connected via nipple N1 to one leg of the w.g. and is subjected to total pressure. The outer (static) tube is sealed at the end (as indicated by the shaded portion) but is pierced as indicated with seven holes. It is connected via nipple N2 to the other leg of the w.g. and the instrument is placed in the airway so as to face the air-current. The relation between the observed w.g. and the velocity of the air is given by the following equation:
V = 4.43√ (w.g)/w or,
at standard air density
V = 4.015 √ w.g.; V
4√w.g. (Approximately)
where
V = Velocity of air in m/sec.
w. g. = Pressure in mm of w.g. or in kgf/m2
w = Density of air in kgf/m2.
A pilot tube should be used in conjunction with a sensitive inclined manometer.
Airflow meter of Nanda Manufacturing Co. comprises
(a) portable inclmed manometer, and
(b) pilot static tube.
Portable inclined manometer
Pilot readings are very small for the common range of air velocities. Ordinary U-tube manometers are not useful for measurement of gas velocities below 12 m/s. For this use, inclined tube manometer is suggested. These manometers are well type with single limb indication. “NMC” inclined manometer can be swivelled in 4 positions— 1 in 20, 1 in 10, 1 in 5 and vertical- Maximum limit of pressure measurement is 250 mm Wg. Flexible PVC connecting tubing in contrasting colours, each of 9 m length, is provided. A bottle of manometer fluid with the density labelled on it and a funnel for topping up the reservoir fluid is included among the scope of supply. A table showing the conversion of w.g. into Pascals is supplied.
Levelling is accurately and conveniently done by means of screw threaded feet in conjunction with spirit levels.
Pressure range: 0-125/250/500/2500 Pascals (N/m3)
0-12.5/25/50/250 mm wg.
Velocity range: 0-14.2, 0-28 m/sec.
Gas Detectors
These have been described in the earlier chapter on mine gases.
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