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Introduction:-
The psychrometry is that branch of engineering science, which deals with the study of moist air
i.e. dry air mixed with water vapour or humidity. It is 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 (O2), argon (Ar) and carbon dioxide (CO2), yet for
the purpose of psychrometry, it is considered to be a mixture of dry air and water vapour only.
1.2- Psychrometric Terms:-
Though there are many Psychrometric terms, yet the following are important from the subject
point view:
1. Dry air: The pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon
dioxide, hydrogen, argon, helium etc. but the nitrogen and oxygen have the major portion of the combination.
Moist air: it is a mixture of dry air and water vapour. The amount of water vapour 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 vapour, usually, occurs 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 the water vapour in 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 generally is expressed in
terms of gram per kg of dry air (g/kg 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. Mathematically, one kg of water vapour is equal to
15430 grains.
7.Relative humidity: It is the ratio of actual mass of the 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 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 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. Dew point temperature: It is the temperature of air recorded by 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 (pv). it is, usually, denoted by tdp. Since pv is very small, therefore saturation temperature
by water vapour (pv) is also low (less than the atmospheric or dry bulb temperature). The water
vapour in air exists in the superheated state and the moist air containing moisture in such a form
(i.e. superheated state) is said to be unsaturated air. When the partial pressure of water vapour
(pv) is equal to the saturation pressure (ps), the water vapour is in dry condition and the air will be
saturated air.
Notes: a) The dew point temperature is the temperature at which the water vapour begins to
condense.
b) For saturated air, the dry bulb temperature, wet bulb temperature and dew point
temperature is same.
11. Dew point depression: It is the difference between the dry bulb temperature and dew point
temperature of air.
12. Wet bulb depression: It is the difference between the dry bulb temperature and wet bulb
temperature at any point. The wet bulb temperature indicates relative humidity of air.
Psychrometer: There are many types of psychrometers, but the sling psychrometer as shown in
Fig.(1.1) is widely used. It consists of a dry bulb thermometer and a wet bulb thermometer
mounted side by side in a protective case that is attached to handle by a swivel connection so that
the case can be easily rotated. The dry bulb thermometer is directly exposed to air and measures
the actual temperature of the air. The bulb of the wet bulb thermometer is covered by a wick
thoroughly wetted by distilled water. The temperature measured by this wick covered bulb of a
thermometer is the temperature of liquid water in the wick and is called wet bulb temperature.
The sling psychrometer is rotated in the air for approximately one minute after which the
readings from both the thermometers are taken. This process is repeated several times to assure that
the lowest possible wet bulb temperature is recorded.