48
2 Environmental Conditions in the Mine
ν =
V
M dry
(2.6)
where
• ν: Specific volume (m
3 kg
−1 ),
• V: Total volume of the mixture (m
3 ), and
• M dry : Dry-air mass (kg).
Question 2.4 Work out whether dry air has a higher or lower density than humid
air. Base your assumptions on the molecular weight of the substances that compose
air.
Answer
Air is composed mainly of N 2 and O 2 . The molecular weight of N 2 is 28 g mol
−1 ,
and that of O 2 is 32 g mol
−1 . Water vapour has a molecular weight of 18 g mol
−1 ,
thus the more the water vapour displaces other gases, the lighter the gas mixture will
be. Therefore, dry air is heavier than wet air.
Dew Temperature (DT)
Dew temperature is the temperature to which air must be cooled for it to become
saturated with water vapour. When air cools below the saturation temperature, water
begins to condense and become dew. In the SI, it is expressed in K.
Enthalpy (H)
Variation in enthalpy expresses the amount of energy absorbed or transferred by the
air. In the SI, it is expressed in J kg
−1 .
Sensible Heat and Latent Heat
The total amount of energy in the air is composed of sensible heat and latent heat.
Sensible heat is the heat obtained or transferred during changes in air temperature.
Latent heat is the heat exchanged during liquid–vapour phase changes. In the SI, it
is expressed in J kg
−1 .
Psychometric Chart
The psychometric chart represents the curves of the state equations for the air–vapour
mixture. The atmospheric pressure is fixed and the dry temperature (abscissas) and
the humidity ratio (ordinates) are left as variables. For any point within the chart, the
parameters of the air can be determined according to the arrows in Fig. 2.7.
2 Environmental Conditions in the Mine
ν =
V
M dry
(2.6)
where
• ν: Specific volume (m
3 kg
−1 ),
• V: Total volume of the mixture (m
3 ), and
• M dry : Dry-air mass (kg).
Question 2.4 Work out whether dry air has a higher or lower density than humid
air. Base your assumptions on the molecular weight of the substances that compose
air.
Answer
Air is composed mainly of N 2 and O 2 . The molecular weight of N 2 is 28 g mol
−1 ,
and that of O 2 is 32 g mol
−1 . Water vapour has a molecular weight of 18 g mol
−1 ,
thus the more the water vapour displaces other gases, the lighter the gas mixture will
be. Therefore, dry air is heavier than wet air.
Dew Temperature (DT)
Dew temperature is the temperature to which air must be cooled for it to become
saturated with water vapour. When air cools below the saturation temperature, water
begins to condense and become dew. In the SI, it is expressed in K.
Enthalpy (H)
Variation in enthalpy expresses the amount of energy absorbed or transferred by the
air. In the SI, it is expressed in J kg
−1 .
Sensible Heat and Latent Heat
The total amount of energy in the air is composed of sensible heat and latent heat.
Sensible heat is the heat obtained or transferred during changes in air temperature.
Latent heat is the heat exchanged during liquid–vapour phase changes. In the SI, it
is expressed in J kg
−1 .
Psychometric Chart
The psychometric chart represents the curves of the state equations for the air–vapour
mixture. The atmospheric pressure is fixed and the dry temperature (abscissas) and
the humidity ratio (ordinates) are left as variables. For any point within the chart, the
parameters of the air can be determined according to the arrows in Fig. 2.7.