2.6 Heat Inside Mines
55
This phenomenon is modelled through Fourier’s law, which allows calculation of
the heat flow for an isothermal surface, and the Laplace´s equation, which describes
the temperature field to offer an approximation of heat flow to the air (e.g. Goch and
Patterson 1940).
2.6.2 Outdoor Climate
External climate has a direct impact on mines’ internal temperatures. This affects the
ground temperature and the variation in the air temperature that is introduced from
the outside. Seasonal temperature variations generate a similar effect. In general,
below a depth of 25 m, the temperature inside mines is fairly independent of external
conditions. At this depth, which is called the neutral zone, the recorded temperature
coincides with the average annual temperature of the outside air.
2.6.3 Air Self-compression
Air self-compression is the process by which a column of descending air compresses
and its temperature increases. If one assumes that the humidity remains constant,
and there is no friction with the walls, there is no heat exchange, so the process can
be considered adiabatic. At the downcast shaft, this phenomenon is the opposite of
that which takes place at the upcast shaft. At the latter, as the air rises, a decrease in
temperature and pressure is observed.
The temperature increases at the downcast shaft, usually at a dry-bulb temperature
of about 1 ° C per 100 m of descent. Brake (2008) considers the real values of increase
to be 0.6 °C and 0.4 °C per 100 m when the wet-bulb temperatures outside are 6 °C
and 25 °C, respectively.
Starting from the equation of hydrostatic pressure in its differential form:
dP = ρgdH
Solving for dH and operating (Eq. 2.8):
dH =
dP
ρg
=
dP
γ
= V e dP
(2.8)
If the process is adiabatic, without heat exchange with the outside, then k = C p /C v
where C p and C v are the specific heats at constant volume and pressure, respectively.
The relationship is (Eq. 2.9):
PV
k
e = constant
(2.9)
55
This phenomenon is modelled through Fourier’s law, which allows calculation of
the heat flow for an isothermal surface, and the Laplace´s equation, which describes
the temperature field to offer an approximation of heat flow to the air (e.g. Goch and
Patterson 1940).
2.6.2 Outdoor Climate
External climate has a direct impact on mines’ internal temperatures. This affects the
ground temperature and the variation in the air temperature that is introduced from
the outside. Seasonal temperature variations generate a similar effect. In general,
below a depth of 25 m, the temperature inside mines is fairly independent of external
conditions. At this depth, which is called the neutral zone, the recorded temperature
coincides with the average annual temperature of the outside air.
2.6.3 Air Self-compression
Air self-compression is the process by which a column of descending air compresses
and its temperature increases. If one assumes that the humidity remains constant,
and there is no friction with the walls, there is no heat exchange, so the process can
be considered adiabatic. At the downcast shaft, this phenomenon is the opposite of
that which takes place at the upcast shaft. At the latter, as the air rises, a decrease in
temperature and pressure is observed.
The temperature increases at the downcast shaft, usually at a dry-bulb temperature
of about 1 ° C per 100 m of descent. Brake (2008) considers the real values of increase
to be 0.6 °C and 0.4 °C per 100 m when the wet-bulb temperatures outside are 6 °C
and 25 °C, respectively.
Starting from the equation of hydrostatic pressure in its differential form:
dP = ρgdH
Solving for dH and operating (Eq. 2.8):
dH =
dP
ρg
=
dP
γ
= V e dP
(2.8)
If the process is adiabatic, without heat exchange with the outside, then k = C p /C v
where C p and C v are the specific heats at constant volume and pressure, respectively.
The relationship is (Eq. 2.9):
PV
k
e = constant
(2.9)