Psychrometry
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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 ( ) 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 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.