Psychrometry

 

Psychrometry 

Definition

Psychrometry is the study of the properties of moist air (dry air + water vapor). It includes the study of various temperatures, humidities and psychrometric processes.

1. Dry Air

Dry air is a mixture of gases (mainly oxygen and nitrogen) which does not contain water vapor.

2. Wet air

Wet air or moist air is a mixture of dry air and water vapor.

3. Saturated air

Saturated air is one which contains maximum water vapors at a particular temperature.

4. Dry air temperature

Dry air temperature is the temperature of atmospheric air (moist air). It is measured by an ordinary thermometer.

5. Wet bulb temperature

Wet bulb temperature is that temperature at which moist air fully saturated. It is measured by a wet bulb thermometer fixed on the Sling Psychrometer.

6. Sling Psychrometer

6.Dry bulb and wet bulb temperatures can be experimentally measured with a ‘Sling Psychrometer’.

7. Dew point temperature

Dew point temperature is that temperature at which first condensation of water vapor takes place on cooling the moist air. Normally dry bulb temperature is greater than wet bulb temperature. Wet bulb temperature is greater than dew point temperature.

8. Saturated air

When the moist air is fully saturated, dry bulb, wet bulb and dew point temperatures are equal. The relative humidity is also 100 %.

9. Specific humidity 

Specific humidity (w) is the amount of water vapor per kg of dry air. Mathematically

w = 0.622 pwv/(pt–pwv)

10. Relative humidity

Relative humidity is defined at a particular temperature as the ratio of mass of water vapor per unit volume of moist air to the mass of water vapor when saturated per unit volume.It is expressed as percentage. It is represented by the symbol ‘Ф’. Mathematically

Ф = pv/pvs

Where pv is pressure of water vapors. It can be found from steam tables corresponding to the dry bulb temperature.

pvs is pressure of water vapors when saturated. This can be found from steam tables corresponding to the wet bulb temperature.

11. Degree of saturation

Degree of saturation is defined at a particular temperature as the ratio of mass of water vapor in per kg the dry air to the mass of water vapor when saturated per kg of dry air. It is representedby the symbol ‘ μ’. It is unitless. It casn be as 0.56 etc.

12. Enthalpy of moist air

Enthalpy of moist air is the sum of enthalpy of dry air and enthalpy of water vapor portion in 1 kg of dry air.

Let w be the mass of water vapor in 1 kg of moist air. Mathematically

Enthalpy of moist air/kg of dry air, hma

hma = 1 cp dt + w [hg +cpv (tdb -tdp)]