Subject:- Refrigeration and Air Conditioning

Topic:- Psychrometry

Difficulty:- Low

Dry bulb temperature (DBT or $t_d$):

The temperature of moist air as measured by ordinary thermometer when placed in air is called dry bulb temperature.

Wet bulb temperature (WBT or $t_w$):

It is the temperature recorded by a thermometer whose bulb is covered with a wick or cloth saturated with water and is exposed to a current of moving air. When air is passed over the wet wick, moisture contained in the wick evaporates and cooling effect is produced at the bulb. Hence WBT is always less than DBT for unsaturated air and is equal to DBT for saturated air. WBT value is affected by moisture content of air.

The difference between DBT and WBT is known as Wet Bulb Depression (WBD) and is measure of degree of saturation of air. WBD=DBT-WBT

Relative Humidity: It is defined as the ratio of actual mass of water vapour $(m_v)$ in a certain sample of moist air at temperature $‘t_d’$ to the mass of water vapour in the same volume of saturated air $(m_vs)$ at same temperature ‘$t_d’$ and same barometric pressure.

$R.H.=φ=\frac{m_v}{m_vs} =\frac{p_v}{p_vs}$

Relative Humidity can also be defined as the ratio of actual partial pressure of water vapour in unsaturated air to maximum partial pressure of water vapour at the same temperature when air becomes saturated. It is expressed in %. For saturated air R.H.= 100%.