Psychrometry
Introduction: The psychrometry is a branch of engineering science, which deals with the study of moist air i.e. dry air mixed with water vapour or humidity. It also includes the study of behavior of dry air and water vapour mixture under various sets of conditions.
Though the earth’s atmosphere is a mixture of gases including nitrogen (N2), oxygen (02), argon (Ar) and carbon dioxide (C02), yet for the purpose of psychrometry, it is considered to be a mixture of dry air and water vapour only.
Psychrometric Terms: Though there are many Psychrometric terms, yet the following are important from the subject point of view:
1. Dry Air: The pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon dioxide, hydrogen, argon, neon, helium etc. But the nitrogen and oxygen have the major portion of the combination.
2. Moist air: It is a mixture of dry air and water vapour. The amount of water vapour, 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 vapour, when the air has diffused the maximum amount of water vapour into it. The water vapours, usually, occur in the form of superheated steam as an invisible gas. However, when the saturated air is cooled, the water vapour 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 actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature.
5. Humidity: It is the mass of water vapour present in 1 kg of dry air, and is generally expressed in terms of gram per kg of dry air (g / kg of dry air). It is also called specific humidity or humidity ratio.
6. Absolute humidity: It is the mass of water vapour present in 1 m3 of dry air, and is generally expressed in terms of gram per cubic-meter of dry air (g/m3 of dry air). It is also expressed in terms of grains per cubic meter of dry air.
7. Relative humidity: 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. It 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 (briefly written as DBT) is generally denoted by td or tdb.
9. Wet bulb temperature: It is the temperature of air recorded by a thermometer, when its bulb is surrounded by a wet cloth exposed to the air. Such a thermometer is called wet bulb thermometer. The wet bulb temperature (briefly written as WBT) is generally denoted by tw or twb.
10. Wet bulb depression: It is the difference between 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 a thermometer, when the moisture (water vapour) present in it begins to condense. In other words, the dew point temperature is the saturation temperature (tsat) corresponding to the partial pressure of water vapour (P). It is, usually, denoted by tdp.
12. Dew point depression: It is the difference between the dry bulb temperature and dew point temperature of air.