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5 Main Ventilation
all this reasons, natural ventilation sometimes assists the main fan and others flows
against it.
The most classical approach for the calculation of natural ventilation is to consider
the difference in density between the downcast and upcast air currents. However, in
reality, natural ventilation is due to a transformation of heat into mechanical energy
and is, therefore, a thermodynamic problem. Both approaches are taken into account
in this chapter.
Exercise 5.1 Indicate the direction of the air inside the mine for the two conditions
in the figure: (a) Outdoor temperature is lower than inside the mine, (b) Outdoor
temperature is higher than inside the mine.
a)
b)
Solution
The qualitative explanation is based on the establishing of a hydrostatic equilibrium
of pressures at the height of B and B
.
In the case of Fig. (a), when it is cold outside, the air column BC on the small shaft
weighs more than its equivalent in the deep shaft, which is hotter. For this reason,
the air circulates from the short to the long shaft.
Cold outside
Hot Outside
B
A
C
B´
A´
C´
a)
b)