Subject:- Refrigeration and Air Conditioning

Topic:- Psychrometry

Difficulty:- Low

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%.

Specific Humidity: Specific or absolute humidity or humidity ratio or moisture content is defined as the ratio of mass of water vapour to the mass of dry air in a given sample of air.

ω=$\frac{m_v}{m_a}$

Unit of specific humidity

ω=$\frac{(kg \ of \ water \ vapour)}{(kg \ of \ dry \ air)}$

Degree of Saturation: It is defined as the ratio of specific humidity for a given sample of air to the specific humidity of saturated air at same dry bulb temperature and same barometric pressure.

$μ=\frac{ω}{ω_s}$

For dry air, $μ=0(as ω=0)$

For saturated air, $μ=1(asω=ω_s)$

Dew Point Depression- It is the difference between the dry bulb temperature and dew point temperature of air.