# 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.

**Dry Air**

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

**Wet air**

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

**Saturated air**

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

**Dry air temperature**

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

**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.

**Sling Psychrometer**

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

**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.

**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 %.

**Specific humidity**

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

w = 0.622 p_{wv}/(p_{t}–p_{wv})

**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 = p_{v}/p_{vs}

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

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

**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 represented by the symbol ‘ μ’. It is unit less. It can be as 0.56 etc.

**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, **h _{ma}**

**h _{ma} = 1 c_{p} dt + w [h_{g} +c_{pv} (t_{db} -t_{dp})]**