Psychrometric Properties Of Air

INTRODUCTION

Psychrometry is the study and measurement of properties of air-vapour mixture.

The study is used in air conditioning, cooling tower, etc.

The atmospheric air generally consists of water vapour. The amount of water vapour plays important role in psychrometry.  It is very important to keep the moisture content in the air within the specified limit in the case of processing industries and air-conditioned buildings.

PSYCHROMETRIC PROPERTIES

The properties of air-vapour mixture are known as psychrometric properties.

The different psychrometry properties are 

• Dry air
• Moisture
• Moist air
• Saturated air
• Saturation temperature
• Dry bulb temperature (DBT)
• Wet bulb temperature (WBT)
• Wet bulb depression (WBD)
• Dew point temperature (DPT)
• Dew point depression (DPD)
• Humidity
• Specific humidity or humidity ratio or moisture content
• Absolute humidity
• Relative humidity
• Degree of saturation 
• Enthalpy of moist air (Total heat of air)
• Adiabatic saturation of air by evaporation of water

1.DRY AIR:

The dry air is considered as a mixture of nitrogen and oxygen, neglecting the small percentages of other gases. The composition of air by weight and volume is as follows:

a.       Dry air = Nitrogen 77% + oxygen 23% (by weight)

b.      Dry air = Nitrogen 79% + oxygen 21% (by volume)

2.MOISTURE:

The water vapour present in the air is known as moisture.

3.MOIST AIR:

It is a mixture of dry air and water vapour. The amount of water vapour present in the air depends upon the pressure and the temperature of air.

4.SATURATED AIR:

When the moist air at a given temperature contains the maximum amount of water vapour (moisture) that it can hold is called saturated air.

5.SATURATION TEMPERATURE:

The temperature at which the moist air is saturated is called saturation temperature. If the temperature is reduced below the saturation temperature, the water vapour present in the air will begin to condense.

6.DRY BULB TEMPERATURE (DBT):

The temperature of air measured with ordinary thermometer is known as dry bulb temperature.  It is unaffected by the moisture content of the air-vapour mixture.

7.WET BULB TEMPERATURE (WBT):

The temperature measured with the thermometer with its bulb covered with wet cloth and exposed to a current of moving air is known as wet bulb temperature.

8.WET BULB DEPRESSION (WBD):

The difference between the dry bulb temperature and wet bulb temperature is called as wet bulb depression. It becomes zero when the air is fully saturated.

        WBD = DBT – WBT

9.DEW POINT TEMPERATURE (DPT):

When the temperature of the air is reduced by continuous cooling, the water vapour in the air will start condensing at a particular temperature. The temperature at which the condensation of water vapour in air starts is known as dew point temperature.

10.DEW POINT DEPRESSION (DPD):

It is the difference between the dry bulb temperature and dew point temperature.

      DPD = DBT – DPT

11.HUMIDITY:

The quantity of water vapour present in the moist air is known as humidity. It depends on the temperature of air and independent of the pressure of air.

12.SPECIFIC HUMIDITY OR HUMIDITY RATIO OR MOISTURE CONTENT:

It is the mass of water vapour presents in one kg of dry air. It is denoted by “w”

     

13.ABSOLUTE HUMIDITY:

The mass of water vapour present in unit volume of air is known as absolute humidity.

   

14.RELATIVE HUMIDITY:

It is defined as 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 air when it is fully saturated at the same temperature. It is denoted by  φ

   

15.DEGREE OF SATURATION:

It is defined as the ratio of the actual specific humidity to the specific humidity of saturated air at same temperature. It is denoted by μ

      

16.ENTHALPY OF MOIST AIR (TOTAL HEAT OF AIR):

Enthalpy (total heat) of moist air is the sum of the enthalpy (sensible heat) of dry air and enthalpy (latent heat) of water vapour associated with dry air.

     Enthalpy of moist air (h_t) = enthalpy of dry air/kg + enthalpy of water vapour/kg of dry air

       H_t = h_S + h_L

             = (C_p. Td) + w(h_{L dpt} + 2.3 Tdp) in KJ/Kg of dry air

            = 1.022 Td + w(h_{L dpt} + 2.3 Tdp) in KJ/Kg of dry air

Where

h_t – Total enthalpy (total heat) of moist air in KJ/Kg.

h_s – Enthalpy (sensible heat) of dry air part in KJ/Kg.

h_L – Enthalpy (latent heat) of water vapour part in KJ/Kg.

C_p – Specific heat capacity of dry air at constant pressure.

T_d – Dry bulb temperature of the air.

w – Humidity ratio in Kg/Kg of dry air.

h_Ldpt – Latent heat of vaporization of water at dew point temperature.

Note: It is observed that

1.   The sensible heat of the air (h_s) is a function of the dry bulb temperature.
2.   The latent of air (h_L) is a function of the dew point temperature.
3.   The total heat of air (h_t) is a function of the wet bulb temperature.

17.ADIABATIC SATURATION OF AIR BY EVAPORATION OF WATER:

      When unsaturated air flows over a long sheet of water in an insulated chamber as shown in fig, the water evaporates, and the specific humidity of air increases. Both the air and water are cooled as evaporation takes place.

The process continues until the energy transferred from the air to the water is equal to the energy required to vaporize the water. When this point is reached, thermal equilibrium exists with respect to the water, air and water vapour and consequently, the air is saturated. The equilibrium temperature is called the adiabatic saturation temperature or the thermodynamics wet bulb temperature. The make up water is introduced at this temperature to make the water level constant.

The adiabatic cooling process on T-s diagram is shown in fig for the water vapour in the air-vapour mixture. The vapour is initially at the unsaturated state (t_dbl) and is cooled adiabatically to the adiabatic saturation temperature (t_db2 = t_wb2). The adiabatic saturation temperature and the wet bulb temperature are taken to be equal for all practical purposes.