PRINCIPLES OF PSYCHROMETRY

The psychrometry is that branch of engineering which deals with the study of moist air i.e. dry air mixed with water vapour.

Dry air: The dry air consists of O₂ = 0.2099, N₂ = 0.7803, Ar = 0.0094, CO₂ = 0.0003 and H₂ = 0.0001 parts by volume.

Molecular weight of air = 28.966

Gas constant, R_a = 0.287 kJ/kg K

Density of air = 1.293 kg/m³ at 1.01325 bar and 0°C

Specific heat of air at constant pressure = 1.005 kJ/kg K

Molecular weight of water (vapour) = 18.02

Gas constant for water vapour, R_v = 0.461 kJ/kg K

Barometric pressure = 1.01325 bar or 760 mm Hg

Dalton’s law of partial pressures: The Dalton’s law of partial pressures states that for a mixture of ideal gases, the total pressure is equal to the sum of the partial pressures, which each constituent would exert if it occupied the whole space alone. Thus,

Barometric pressure of mixture of dry air and water vapour is given by

where p_a = partial pressure of dry air

p_v = partial pressure of water vapour

Moist air: It is a mixture of dry air and water vapour.

Saturated air: It is a mixture of dry air and water vapour when air has absorbed maximum amount of water vapour. At saturation p_v = p_s, the saturation pressure.

Degree of saturation (µ):

Specific humidity or humidity ratio w: It is the mass of water vapour present in one kg of dry air. It is expressed in terms of kg per kg of dry air (kg/kg d.a)

Absolute humidity: It is the mass of water vapour present in 1m³ of dry air. It is expressed in terms of kg per cubic metre of dry air (kg/m³ d.a.)

Relative humidity (RH or ϕ): It is the ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure.

Dry Bulb Temperature (DBT, t_d): It is the temperature of air recorded by a thermometer, when it is not affected by the moisture present in the air.

Wet Bulb Temperature (WBT, t_w): It is the temperature of air recorded by a thermometer, when its bulb is surrounded by a wet cloth exposed to the air.

The DBT and WBT are measured by a Sling Psychrometer, as shown in Fig. 20.1.

Wet Bulb Depression, WBD = DBT − WBT

Dew Point Temperature (DPT, t_dp): It is the temperature of air recorded by a thermometer, when the water vapour present in it begins to condense. Thus, DPT is the saturation temperature (t_sat) corresponding to the partial pressure of water vapour p_v

Dew Point Depression, DPD = DBT − DPT