1. introduction to air conditioning

Manoj PJ Associate prof(Mech) 1

Air conditioning

maintaining the temperature, humidity, air flow and purity of a

particular place


Psychrometry

• the study of properties of mixtures of air and water vapour.


Dry air.

• Air without moisture .


Moist air.

• it is a mixture of dry air and water vapour• depends upon- the temperature of the air.• Maximum quantity - saturation capacity .


• The maximum amount of vapour that can be mixed with any given volume of dry air depends only on the temperature of the air. Since the amount of water vapour in the air determines the partial pressure exerted by the water vapour. It is evident that the air will contain the maximum amount of water vapour when the water vapour in the air exerts the maximum possible pressure.


• Since the maximum pressure that can be exerted by any vapour is the saturation pressure corresponding to its temperature, air will contain the maximum amount of water vapour when the pressure exerted by the water vapour is equal to the saturation pressure corresponding to the temperature of the air


Dalton’s Law of partial pressure:

• total pressure of a mixture of gases is equal to the partial pressures exerted by each gas when it occupies mixture volume at mixture temperature.


Dry bulb temperature.

• Temperature measured by ordinary thermometer


Wet bulb temperature

• Temperature measured by a thermometer when it’s bulb is covered by wet cloth and exposed to a moving air.


Dew point temperature

• The temperature of the air is reduced by continuous cooling and then the water molecules in the air starts condensing.

• The temperature at which the water molecules in the air start condensing is known as due point temperature.


• Since partial pressure of water vapour very small , it exists in superheated state in the atmospheric temperature.


Absolute humidity.

• The weight of water vapour present in unit volume of air mixture is known as absolute humidity.


Specific humidity or humidity ratio (ω).

• It is the mass of water vapour present per kg of dry air. ( kg of water per kg of dry air).


Relationships between mass and specific volume


Degree of saturation(µ).

• Ratio of mass of water vapour associated with unit mass of dry air to mass of water vapour associated with unit mass of dry air saturated at the same temperature


Degree of saturation(µ).

• At the same temperature


OR


Relative humidity (ф)


Enthalpy of Moist air.

• Enthalpy of moist air = enthalpy of one kg of dry air + enthalpy of water vapour associated with one kg of dry air


• considering the change in enthalpy of a perfect gas as a function of temperature only

• Enthalpy above a datum of 0 degree C can be expressed as


• Note:• For convenience in air conditioning

calculations, the enthalpy and specific volume are based upon unit mass of dry air.


Empirical relationship to find Pv

1. introduction to air conditioning

Engineering