PRODUCTION OF VENTILATION
Air flows from a region of high pressure to a region of low pressure. The difference of pressure may be caused by
(1) Purely natural means. It is then called natural ventilation, or
(2) By a fan; it is then called mechanical ventilation.
The ventilation in a mine using a fan is a combination of natural and mechanical means.
Natural Ventilation
How is natural ventilation produced? Consider that a level roadway joins two shafts equal in dia. and of equal depth from a level ground surface. If the air in both the shafts is at the same temperature and pressure, there are two columns of air equal in weight which balance each other and there will be no flow of air from one shaft to the other.
If, however, density of air in one shaft is more, the column of air in that shaft will have a higher weight and the difference in pressure at the bottoms of the shafts, will cause flow of air from high-pressure shaft to low-pressure shaft. The difference in density between the air of the two shafts may be produced by any of the following factors:—
i) Presence of firedamp, or steam purposely introduced in one of the shafts, which renders the air lighter.
ii) Presence of fire in one of the shafts. The fire heats up the air which has then less density.
iii) Passage of cold water down one of the shafts; the cold air is denser.
iv) Movement of cage in the shaft,
v) Unequal depth of shafts.
The last factor is important and may be explained by reference to Fig. 2.8. Consider two shafts BC and FD at different surface levels and joined at their bottoms by a level roadway DC. If we consider a horizontal imaginary plane AF passing through the point F and the point A, an extension of the shaft BC, the air at points A and F has same temperature and pressure as that of the atmosphere. It is, therefore, only the air columns below the point A and F which should create a difference of pressure if natural ventilating pressure is to cause a How of air through the shafts and through their underground connecting roadways. At C the air pressure is due to the air column AB above the pit top and the column BC; at D the air pressure is due to the air column FD.
The temperature of the rocks below ground is not the same as the temperature at the surface. The day-to-day variation and the seasonal variation in atmospheric temperature affects only the rocks near the surface but whatever be the surface temperature, at IS m below ground the temperature of rocks is constant. It is 27°C in Jharia field. Beyond that depth of 15 m the temperature of the rocks increases with depth at a definite rate. This rate of increase is known as Geothermic gradient and varies in different areas. In Jharia field this rate-increase is 1°C for every 38 m depth below the constant temperature line. In Kolar Gold field it is 1°C for 68 m; and it is 1°C for every 140 m in Gold Mines of Rand in South Africa. In Mosabani copper mines the geothermic gradient is 1°C for 52 m but indications are that it is getting steeper as the temperature recorded at 30th Level was 47.8°C against 46.2°C expected as per the rate stated above.
In Fig. 2.8 the constant temperature line is shown at 15 m below the surface and it is always parallel to the surface contour. Below that line the air in the shafts gets heated mainly due to conduction of heat from the strata. Let us consider that atmospheric temperature in Jharia Field on a summer day is 40°C. and on a winter day 20°C. The average temperature of column AC is that due to AB, and BC.
In winter column AB is colder than column FE; BC is also colder than ED which is at a greater depth from the surface. Thus AC is colder than FD and weight of air in AC is more than that of air in FD. Therefore, air at a point C is at a higher pressure than at D and current of air flows from C to D so that BC acts as a downcast shaft and FD as up cast shaft.
In summer the average temperature of air in AC is high compared to that of FD. Temperature of AB is atmospheric (40C) and higher than that of FE. I~ this particular example BC, being a shallow shaft, is a small length compared to AB and though BC is colder than ED, the mean temperature of AC is. higher than the mean temperature of FD. The air in AC is therefore less dense than in FD. Hence, air flows from D to C. There is thus a reversal of air currents in winter and in summer. In all cases, the entrance at the lower surface level is the downcast in winter.
The natural ventilation therefore causes the mine air to travel in opposite directions in summer and in winter. In deep mines, however, the air will travel in the same direction throughout the year provided the constant mean temperature of the air m the up cast shaft is higher than in the downcast shaft.
If two shafts have the same depth and are at the same surface level and if a current of air once begins to flow for any reason whatever, the cool air enters one shaft and warmer air goes up by the other, aria this results in keeping one shaft cooler and the other warmer. This causes flow of air by natural ventilation as long as difference in temperature continues and even when the surface fan is stopped. There are various ways in which the air may be induced to now, e.g. by the action of wind at one shaft mouth, and other causes mentioned earlier.
Natural ventilation in a shallow mine assists the flow of air caused by the fan for part of the year and opposes the pressure difference produced by the fan in the remaining part of the year. In those mines where there is much difference between day temp erature and night temperature, natural ventilation flow during night may be opposed to that during day. When the temperature difference of the two air columns is maximum the N. V. P. is maximum and it ceases altogether when there is no temperature difference.
The natural ventilation pressure produced in the above exa mple may be measured by a water gauge placed at the bottom of the two air columns AC and FD. The water gauge should be placed on a partition erected across the connecting road CD; one leg of the water gauge should be exposed to the air column AC and the other to the air column FD.
It should be noted that in all questions concerning N. V.P. (1). The two air columns should be considered between two ima ginary horizontal planes, one at the shaft collar of the higher-level shaft and the other at the lower point of the two shafts. When one of the outlets is not a shaft but an inclined drift or an adit, the same principle applies. (2) Total N. V, P. can be calculated considering the two air columns stationary. To cause flow of air through the shafts, some of the water gauge of the N. V. P. is expended in overcoming resistance of the shafts and the connec ting roadways. If air flows through the shafts and underground roadways, a water gauge connected at shaft bottom between UC and DC shaft across a partition door will record the total N.V.P. minus the pressure spent on causing air flow through the shafts and it is the balance N. V. P. recorded on the water gauge which will be available for circulating air to the in bye workings. In contrast to this when a mine is ventilated by a surface fan, the maximum water gauge developed by it and available for ventila tion of the mine including the shafts is that which is recorded on a U-tube placed at the fan drift (one leg exposed to the atmo sphere and the other exposed to the air in the fan drift).
Motive column: In the case of N. V, P. it is the excess weight of air in D. C. air column which gives rise to the N. V. P. The height of this excess weight of air column of DC shaft, 1 m2 in cross section which gives rise to N. V. P. is called motive column. In other words, the motive column, when referring to N. V. P., is the preponderant or unbalanced part of the whole D, C. column, 1 m2 in cross section, i. e. that part of the D.C. column which is not balanced by the UC air column. The following example will make it clear.
Example: Mean air temp, in a D. C. shaft 400 m deep is 28°C and in the U. C. shaft is 38°C. Calculate
(i) the motive column, and
(ii) the N. V. P. assuming average barometric pressure in D. C. shaft to be 750 mm of Hg.
Answer: The height of the motive column is given by the formula,
Where h = height of motive column (m)
Tu = Average temp, in up cast shaft (°C)
Td = Average temp, in down cast shaft (°C)
D = Depth of column between top of the higher-level shaft and bottom of the deeper shaft, ( in m )
N. V. P. = Motive column x density of air in D. C. shaft.
Motive column, h = {(Tu-Td) / (273+Tu)} x D
= {(38-28) / (273 + 38)}x 400
= (10 x 400) / 311
= 12.8 m.
Density of air in the D.C. shaft = (0.4645 B) / (273 + Td)
Where B is the barometric pressure in mm of Hg.
Density, w = (0.4645 B) / (273 + Td)
= (0.4645 x 750) / (273 + 28)
= 1.157 kg/m3
N. V. P. = motive column x density of air in D.C. shaft
= 12.8 x 1.157
= 14.81 kgf/m2 = 148.1 Pa
Density of air (mass per unit volume) varies with its temperature, pressure and moisture content. The formula for density of moist air in SI units is as follows:
Density w= (B ^°ff j 103 Kg per m3
where B = barometric pressure in kPa
T = temperature of air in K
e = water vapour pressure in air in kPa
The various causes which tend to heat the air as it travels through a mine are:—
(1) Conduction of heat from the strata.
(2) Compression of the air due to depth in the D. C. shaft and dip workings.
(3) Burning of lamps.
(4) Oxidation of carbonaceous material.
(5) Heat given out by men.
(6) Heat given out by machinery and subsiding strata.
Of these, the first three are the most important, the others being of relatively minor consequence except in confined places.
The various causes which tend to cool the air are:
(1) Evaporation of moisture from wet shafts and roadways, or from the coal itself.
(2) Expansion of the air as it rises up the U. C. shaft, or up the rising roadways, to a higher level.
(3) Local cooling effect due to expansion of compressed air at the exhaust of compressed air motors.
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