UNIT NO.
: 03
NATURAL VENTILATION
1.1. NATURAL VENTILATION –
When a mine consists of two shafts connected at some depth below the surface, some air will flow
through the mine even if no fan is provided. This flow is known as the “Natural Ventilation”.
NATURAL VENTILATION PRESSURE –
The pressure producing Natural ventilation in mine atmosphere is known as the “Natural
Ventilation Pressure”.
GEOTHERMIC GRADIENT :
The rate of increase of temperature with depth is called the GEOTHERMIC GRADIENT.
The Geothermic or Geothermal Gradient varies from place to place and is dependent on the
physical properties of the rock such as thermal conductivity, specific heat and density which
govern the rate of heat transfer in the rock.
The geothermic gradient is steeper, or the rise in temperature is with depth faster for rocks of
lower thermal conductivity such as coal – measure rocks than for rocks of higher thermal
conductivity.
Other minor factors which affect the geothermic gradient are the age of the rock and the
presence of dykes, sulphide ore bodies etc. nearby.
Geothermic gradient is determined by observing the virgin – rock temperature at various depths
in a mine.
For obtaining accurate virgin – rock temperatures, readings should be taken at the bottom of
boreholes where the temperature of the rock is not affected by ventilating air – currents.
The depth of the borehole required for accuracy depends on the time for which the rock face has
been exposed.
FACTORS CAUSING NATURAL VENTILATION PRESSURE (N.V.P.) :
The factors which results in causing natural ventilation are as follows –
(1) Temperature –
Natural ventilation can be visualized to be caused by the difference in densities of air in the
upcast and downcast shafts.
The heavier air sinks down and the lighter air moves up thus setting up an air – current.
The difference in air densities in the upcast and downcast shafts is mainly caused by the heating
and rarefaction of air in the mine workings due to the addition of heat from rocks, men,
machinery, lights, spontaneous heating, etc.
(2) Moisture content of the air –
Addition of moisture in the downcast shaft decreases the density of air as moisture is lighter than
air, but this also causes evaporative cooling of the downcast air and consequent increase in its
density so much so that, in effect, evaporation of moisture in the downcast shaft usually aids
natural ventilation.
(3) Barometric pressure –
It is well known that air density is a function of barometric pressure.
If the mean barometric pressure of the downcast air column is higher than that of the upcast air
column, it helps natural ventilation and vice versa.
However, since barometric pressure rarely varies to any appreciable extent from place to place
within the limits of a mining property, the effect of such variation n natural ventilation is
negligible.
(4) Addition of gases-
Methane emitted from the workings of coal mines reduces the density of return air thus aiding
natural ventilation.
This effect may, however, be slightly offset by the cooling of mine air produced by the desorption
of methane.
Large addition of carbon dioxide on the other hand has the opposite effect.
Addition of compressed – air from the exhaust of the compressed – air machinery used
underground, however, has little effect on the density of air.
(5) Leakage –
In multilevel mines, leakage of denser downcast air to the upcast shaft causes an increase in the
density of upcast air, thus reducing natural ventilation.
In view of the low efficiency of natural ventilation, of the order of 1.4 to 1.6% it is important to
minimize leakage of air from the downcast to the upcast shaft in order to get the maximum
benefit of natural ventilation.
(6) Circulation of refrigerated air –
Circulation of refrigerated air through the downcast shaft increases the density of downcast air
thus aiding natural ventilation to a large extent.
(7) Other factors –
Other factors such as spraying of water in the downcast shaft for preserving shaft timber from dry
rot or minimizing fire hazard etc. or having steam pipes through the upcast shaft where steam is
used underground (this is, however, a rare practice now) help natural ventilation by increasing
the downcast air density or decreasing the upcast air density.
EFFECT OF SEASONAL CHANGES ON DIRECTION OF NATURAL VENTILATION :
This depends mainly on the depth of the mine as the depth generally controls the amount of heat
added to the mine air.
In shallow mines, where the underground temperature is not very high, the surface temperature
is the major contributing factor to the direction of natural ventilation.
Let us try to understand the effect of seasonal changes on direction of natural ventilation with the
following example:
Let’s consider two shafts with their collars at different elevations as illustrated in the figure below
–
Let the ground temperature be 298 K at an average depth of 20m from the surface and the
geothermic gradient, 1 K for every 40m depth.
(A) IN WINTER :
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 upcast shaft.
(B) IN SUMMER :
In summer, the average temperature of air in AC is high compared to that of FD.
Temperature of AB is atmospheric (40 ℃ ) and higher than that of FE.
Here, BC being a shallow shaft, is of 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.
Thus, in shallow mines, there is 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.
In deep mines, however, the air will travel in the same direction throughout the year provided
the constant mean temperature of the air in the upcast shaft is higher than in the downcast
shaft.
In the intervening periods of spring and autumn being periods of stalemate, there is little or no
natural ventilation.
Where there is a large diurnal variation of temperature, ventilation may be reversed during the
course of the day.
LIMITATION OF NATURAL VENTILATION :
The limitations or disadvantages of Natural ventilation are –
Natural ventilation is usually poor.
Fluctuates to a large extent.
Is subject to reversal of direction.
In some cases, such as underground mine fires, natural ventilation cannot be controlled.
It is for these reasons that natural ventilation is prohibited in underground mines.
1.2. MOTIVE COLUMN :
Motive Column is used to express Natural Ventilation Pressure (N.V.P.).
Motive Column is the height of the air column in the downcast shaft which causes the natural
ventilating pressure.
In other words, motive column is the N.V.P. expressed in terms of the height of a column of air
with a density equal to that of air in the downcast shaft.
Mathematically, motive column is given as follows –
h=
N .V .P.
where,
ρd×g
h = height of motive column (in m)
ρd= density of air in the downcast shaft (in kg/m³)
g = acceleration due to gravity (in m/s²)
N.V.P. = natural ventilating pressure (in Pa)
CALCULATION OF NATURAL VENTILATING PRESSURE :
Natural Ventilating Pressure (N.V.P.) can be calculated from the following considerations –
(a) Calculation of N.V.P. from Air Densities –
Let the mean density of downcast and upcast air be ρdand ρu kg/m³ respectively.
Let the shafts be of equal depths, D m (or, D should be taken as equal to the difference in
height between the higher shaft collar and the deepest ventilated point in the mine).
The N.V.P. = D (ρd - ρu)g Pa
Now, ρd = kg/m³ .........................equation (1)
( B d −0.378 e d ) ×10³
287.1
ρu = kg/m³ .........................equation (2)
( B u−0.378 e u ) ×10³
287.1
inserting the values of equations (1) and (2) in the formula of N.V.P.
For mine shafts in general, Bd= Bu = B say. Further, let us neglect the effect of moisture.
Then,
N.V.P. = Dg ( - ) × 10³
B B
287.1Td 287.1Tu
= × 10³ Pa
gDB Tu−Td
Where,
287.1 Tu × Td
ρd = density of air in the downcast shaft (in kg/m³)
ρu = density of air in the upcast shaft (in kg/m³)
D = depth of shafts (in m)
g = acceleration due to gravity (in m/s²)
B = mean barometric pressure (in kPa)
Bd = mean barometric pressure in downcast shaft (in kPa)
Bu = mean barometric pressure in upcast shaft (in kPa)
Td = temperature in the downcast shaft (in K)
Tu = temperature in upcast shaft (in K)
(b) Calculation of N.V.P. from Motive Column –
In terms of Motive Colum, N.V.P. can be calculated in the following manner –
h= m
N .V .P.
ρd × g
N.V.P. = h × ρd × g Pa
where,
h = height of motive column (in m)
ρd = density of air in the downcast shaft (in kg/m³)
g = acceleration due to gravity (in kg/m²)
N.V.P. = Natural ventilating pressure (in Pa)
(c) From pressure and quantity measurements with fan running and fan stopped :
With the fan running, the mine resistance,
R= ................equation (1)
P f +P n
With the fan stopped, the mine resitance,
Qr²
R= ......................equation (2)
Pn
Qn ²
From equations (1) and (2), we get
=
P f +P n Pn
af ter solving the above equality, we get,
Qr² Qn ²
Pn= Pf
Qn²
Where,
Qr ²−Q n ²
Pn= N.V.P.
Pf = fan pressure (fan – drift pressure)
Qn = quantity flowing through the mine when fan has stopped
Qr = quantity flowing through the mine when fan is running
R = resistance of mine
(d) From pit – bottom with fan running and fan stopped :
Pn = Pn – Rs Qn² = Pn (1 - ) ...............equation (1)
Rs
Similarly,
R
Pf = (Pn+ Pf) (1 - ) .................equation (2)
Rs
Combining equations (1) and (2)
R
=
Pf P n+ P f
Pn Pn
Or, Pn =
P f Pn
Where,
P f −P n
Pn = pit – bottom pressure with fan stopped = N.V.P.
Pf = pit – bottom pressure with fan running
R = mine resistance
Rs = shaft resistance
(e) From Pressure across a stopping in fan drift :
N.V.P. can also be found by erecting a stopping in the fan – drift (in fact in any part of the main
circuit of ventilation) and measuring the pressure across the stopping with the fan idle, but this
will vary from the N.V.P. measured with air flowing through the workings, since in the latter case,
the difference in air densities causing natural ventilation will change as a result of the addition of
a variable quantity of heat from outside depending on surface conditions.
(f) From fan pressures and quantities at two different speeds :
R= =
P n+ P f 1 P n+ P f 2
Pn can be found from the above equation
Q1² Q2²
Where,
Pn= N.V.P.
Q1 & Q2 = quantities
Pf1 & Pf2 = fan pressures
N1 & N2 = fan speeds
R = resistance of the mine
