Psychrometric- Dry, Moist, Saturated Air, Degree of saturation, Absolute, Relative Humidity, Dry and Wet Bulb Depression & Temperature, PDF
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The Psychrometric is the branch of engineering science which deals with the study of moist air i.e dry air mixed with water vapor or humidity.
It also includes the study of the behavior of dry air and water vapor mixture under the various set of condition. Tough the earth's atmosphere is a mixture of gases including Nitrogen (N2), Oxygen (O2), Argon (Ar), and Carbon Dioxide (CO2), yet for the purpose of Psychrometric, It is considered to be a mixture of Dry air and water vapor only.
The Psychrometric terms are:
1. Dry air:
This is defined as the pure dry air is a mixture of a number of gases such as Nitrogen, Oxygen, Carbon Dioxide, hydrogen, Argan, neon, helium, etc.
But the nitrogen and oxygen have the major portion of the combination.
The dry air is considered to have the composition as given in the following table:
Carbon Dioxide (CO2)
The molecular mass of dry air is taken as 28.966 and the gas constant of air (Ra) is equal to (0.287 kJ/kg K) or (287 J/kg K).
The molecular mass of water vapor is taken as 18.016 and the gas constant of air (Rv) is equal to (0.461 kJ/kg K) or (461 J/kg K).
An important point in dry air are:
1. The pure dry air does not ordinarily exist in nature because it always contains some water vapor.
2. That term air wherever used in this text means dry air containing moisture in the vapor form.
3. Both dry air and water vapor can be considered as perfect gases because both exist in the atmosphere at low pressure.
Thus all the perfect gas terms can be applied to them individually.
The density of dry air is taken as 1.293 kg/m3 at pressure 1.0135 bar or 101.5 KN/m2 and at the temperature (0 degree Celsius that is 273 Kelvin).
2. Moist air:
It is a mixture of dry air and water vapor.
The amount of water vapor present in the air depends upon the absolute pressure and temperature of the mixture.
3. Saturated air:
It is a mixture of dry air and water vapor in the air has diffused the maximum amount of water vapor into it.
The water vapor usually occurs in the form of superheated steam as an invisible gas.
However, When the saturated air is cooled the water vapor in the air starts condensing and the same may be visible in the form of moist, fog or condensation on cold surfaces.
4. Degree of Saturation:
It is the ratio of an actual mass of water vapor in a unit mass of dry air to a mass of water vapor in the same mass of dry air when it is saturated at the same temperature.
It is the mass of water present in 1 kg of dry air and is generally expressed in terms of a gram per kg of dry air ( g/kg of dry air).
It is also called as specific humidity or humidity ratio.
6. Absolute Humidity:
It is the mass of water vapor present in 1 m3 of dry air and is generally expressed in terms of a gram per cubic meter of dry air (g / m3 of dry air).
It also expressed in terms of grains per cubic meter of dry air.
1 kg of water vapor is equal to 15430 grains.
7. Relative Humidity:
It is the ratio of an actual mass of water vapor in a given volume of moist air to the mass of water vapor in the same volume of saturated air at the same temperature and pressure.
This is briefly written as RH.
8. Dry Bulb temperature:
It is the temperature of air recorded by a thermometer when it is not affected by the moisture present in the air.
The dry bulb temperature is briefly written as DBT.
This is denoted by td or tdb
9. Wet Bulb Temperature:
It is the temperature of air recorded by a thermometer when it's bulb is surrounded by wet clothes exposed to the air. Such a thermometer is called Wet bulb thermometer.
The wet bulb temperature is briefly written as WBT.
This is denoted by tw or twb
10. Wet Bulb Depression:
This is the difference between the dry bulb temperature and wet bulb temperature at any point. The wet bulb depression indicates relative humidity of the air.
11. Dew Point Temperature:
It is the temperature of air recorded by the thermometer when the moisture (Water vapor) present in it begins to condense.
In other words,
Dew point temperature is the saturation temperature (tsat) corresponding to the partial pressure of water vapor pv
This is usually denoted by tdp
For saturated air, the dry, wet and Dew point temperature is the same.
12. Dew point Depression:
This is the difference between the Dy bulb temperature and dew point temperature of the air.
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