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Explain the terms DBT, WBT and RH

Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

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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

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.