mining Ventilation

1. Natural Ventilation
By A K NAVEEN
1GV21MI001

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CONTENTS
Introduction
Causes
Direction
Motive Column
& NVP
Artificial aids

3. Introduction
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 Natural mine ventilation is the process of supplying fresh air without using any
mechanical systems. The external air moves into an enclosed space because of the pressure
differences arising from natural forces. Natural mine ventilation distributes oxygen from
fresh air, which enhances the pulmonary and heart functions.
 Air flows from a region of high pressure to a region of low pressure.
 The difference of pressure may be caused by
 Purely natural means: called natural ventilation, or
 By a fan: called mechanical ventilation.
 Small and shallow mines are sometimes ventilated by natural means.
 Ventilation only through natural means is usually
 Poor
 Fluctuates to a large extent and
 Subject to reversal of direction.
Fig. Natural Ventilation

4. CAUSESOF NATURALVENTILATION
The causes of natural ventilation are
 Temperature
 Moisture content of the air
 Barometric pressure
 Addition of gases
 Leakage of air
 Circulation of refrigerated air
 Other factors

5. Temperature
 Natural ventilation is 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 is mainly caused by the heating of air in the mine
workings due to the addition of heat from rocks, men, machinery, lights
spontaneous heating etc.
 Auto-compression in the downcast shaft changes the air density according to
the theoretical relation
ρ α P1/2

6. Moisture content of the air
As moisture is lighter than air, addition of moisture in the downcast shaft decreases the
density of air.
Presence of moisture also causes evaporative cooling of the downcast air and consequent
increase in its density which aids natural ventilation.

7. 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 on natural
ventilation is negligible.
Addition of gases
 Methane emitted from the workings of coal mines reduces the density of return air
thus aiding natural ventilation.
 Addition of compressed air from the exhaust of the compressed-air machinery used
underground, has little effect on the density of air.

8. Leakage of air
 In multilevel mine, 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 downcast to upcast shaft in order to get the
maximum benefit of natural ventilation.

9. Circulationof refrigeratedair
 Circulation of refrigerated air through the downcast shaft increases the density of
downcast air thus aiding natural ventilation.
 In some of the deep and hot mines such as Robinson Deep of the Rand, SA, where
refrigerated air is sent down the intake shaft, the N.V.P. is estimated to be of the
order of 1200 Pa.

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Otherfactors
 Presence of fire in one of the shafts. The fire heats up the air which has then less density.
 Passage or spraying of cold water in the downcast shaft for preserving shaft timber from
dry rot or minimizing fire hazard etc.
 Having steam pipes or stem purposely introduced in upcast shaft help natural ventilation
by decreasing the upcast air density.
 Movement of cage in the shaft.
 Unequal depth of the shafts.

11. DIRECTIONOF NATURALVENTILATION
 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 temp. is the major contributing
factor to the direction of natural ventilation.
 The temperature in the Shallower shaft is more affected by variations in surface temperature than in the deeper one.
Hence, the deeper the shaft, the less is the effect of changes in surface temperature on natural ventilation.
 The reversal of air current in shallow mines usually takes place seasonally, from summer to winter, and vice versa.
The intervening periods of spring and autumn being periods of stalemate with very little or no natural ventilation.
 Although natural ventilating pressure in the mine is mainly generated in shafts, it is also generated in the workings. If
the air traveling through them travels an inclined path.
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12.  With normal ventilation of dip workings in an inclined seam, the cool intake air travels down an inclined airway to
the face and gets heated up there so that the return air traveling up gets hot and rarified.
 As a result, a positive NVP, which adds the NVP produced in the shaft, is generated in the workings.
 In case of rise workings, however, a negative NVP is produced, which opposes the pit bottom NVP.
 For similar reasons, A higher NVP is produced in the workings with Ascensional Ventilation, where the downcast air
is taken directly to the bottom of the workings and allowed to travel up through them into the return.
 Whereas with decensional ventilation, the NVP produced in the workings is considerably less.
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13. Motivecolumn and N.V.P.
 In case of natural Ventilation it is the excess weight of air in D.C. air column which
gives to the Natural Ventilating Pressure (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, it is the unbalanced part of the whole DC column, 1 m2 in cross
section, i. e. that part of the DC column which is not balanced by the UC air
column.
N. V. P. = Motive column × density of air in DC shaft, pa

14. Artificial Aids To NaturalVentilation
Stacks or chimneys:
 Stacks built on the top of a cast shafts aid natural ventilation, but their application is limited to
shallow mines, where the length of the stack can be fairly large in comparison to the depth of the
shaft.
Furnaces:
 NVP can be considerably increased by putting furnaces at the bottom of the upcast shafts. In coal
mines where cheap fuel is available. These were quite common before the introduction of fans, but
were later discarded in favor of the latter on account of various dangers involved in having furnaces
in the underground.
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15. Use of a water or steam jets and falling water:
 Jets pointing in the direction of Airflow or waterfalling along the direction of air current helps
natural ventilation to some extent, which, though not substantial, aids in maintaining the
direction of flow when there is a chance of reversal of air current.
Surface winds:
 These, if suitably directed into the mine, aid natural ventilation by adding velocity pressure to the
natural ventilating pressure sometimes, if the mine exhaust is guided in the direction of surface
wind, the latter produces suction effect, aiding natural ventilation.
 Surface wind velocities are, however, usually small and vary considerably in direction and
amount and deflection of surface wind into a mine for aiding natural ventilation should be
considered only when a steady flow of wind with little variation of direction occurs.

16. Conclusion
Natural ventilation, the use of natural forces like temperature and pressure differences to drive air
circulation, has been a mainstay in underground mines for centuries. While its simplicity and lack of
reliance on energy-intensive machinery are appealing, its limitations become increasingly evident as
mines grow deeper and more complex.
Overall, natural ventilation remains a viable option for small, shallow mines with favorable
conditions. However, for larger, deeper, or more complex operations, reliance on mechanical
ventilation systems becomes essential to ensure adequate airflow, worker safety, and regulatory
compliance.

17. Thank You
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