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1 Psychrometry
Psychrometry is the science
of studying the thermodynamic properties of moist air and the use of these properties to
analyze conditions and processes involving moist air.
Moist air is a mixture of dry air and water
vapour. In atmospheric air, water vapour content varies from 0 to 3% by mass.
1.1 Thermodynamic Properties of Air
1.1.1 Dry Bulb Temperature
1.1.2 Wet Bulb Temperature
1.1.3 Humidity Ratio/Moisture Content
Humidity ratio w (kg/kg) of
a given moist air sample is defined as the ratio of the mass of water vapour (mw) to the mass of dry air (ma) contained in the sample.
 (1)
When the dry air and water vapour occupy
the same volume and temperature, by applying the characteristic equation of state for
perfect gas, Eqn. (1) becomes:
 (2)
where Pw = partial
pressure of water vapour in moist air
Pat =
atmospheric pressure of moist air
1.1.4 Relative Humidity
Relative humidity (Æ ) is
defined as the ratio of the mole fraction of the water vapour (xw) in a given moist air sample to the mole fraction of water vapour in an air
sample of saturated moist air (xws) at the
same temperature and pressure.
 (3)
By definition, the mole
fraction of the water vapour (xw) is the ratio of the number of mole of water vapour in a
given moist air sample to the total number of dry air and water vapour.
 (4)
When the dry air and water vapour occupy
the same volume and temperature, by applying the characteristic equation of state for
perfect gas, Eqn. (3) becomes:
 (5)
where Pw = partial
pressure of water vapour in moist air
Pws = partial
pressure of water vapour in saturated moist air
Relative humidity is usually
expressed as percentage (%).
1.1.5 Degree of
Saturation/Percentage Saturation
Degree of saturation (m ) is the
ratio of the humidity ratio of moist air (w) to the humidity ratio of saturated moist air
(ws)
at the same temperature and pressure.
 (6)
From Eqn. (2), Eqn. (6) becomes
 (7)
The difference between relative humidity Æ
and degree of saturation m is usually less than 2%.
Percentage saturation is degree of
saturation when expressed in percentage.
1.1.6 Specific Volume/Moist Volume
Specific volume v (m3/kg) is defined as the total volume V (m3) of the dry air and water vapour mixture per kg of dry
air.
 (8)
where ma = mass of dry
air, kg
1.1.7 Specific Enthalpy
The enthalpy of moist air is defined as the
sum of its internal energy and the product of its pressure and volume. Specific enthalpy h
(kJ/kg) of moist air is defined as the total enthalpy of the dry air and water vapour
mixture per kg of moist air.
 (9)
where ha = specific
enthalpy of dry air, kJ/kg
hw = specific
enthalpy of water vapour, kJ/kg
w = moisture content, kg/kg
1.1.8 Dew Point Temperature
Dew point temperature is the temperature at
which moist air becomes saturated (100% relative humidity) with water vapour when cooled
at constant pressure, i.e. temperature at which condensation of moisture begins when the
moist air is cooled.
1.2 Psychrometric Charts
A psychrometric chart graphically
represents the thermodynamic properties of moist air as shown.
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