What is Psychrometry?
The term air conditioning means treating of air or conditioning the air to change its temperature or the moisture as per the requirements of various applications. Air conditioners are used throughout the world for a number of applications and one just can’t imagine the life without them. They are the devices or machine that condition or alter the state of the air by changing its temperature and the humidity level. Though you may have read about the various types of air conditioning systems, you may have hardly bothered about reading the various constituents of the air and what changes occur in them that lead to reduction in the temperature of the air and changes in its humidity.
Psychrometry is the science of study of various properties of air, method of controlling its temperature and moisture content or humidity and its effect on various materials and human beings. Studying Psychrometry helps understanding different constituents of air and how they affect each other, which in turn unravels various mysteries of the atmosphere and the nature. Some of the psychrometric properties of air that we are going to study are: dry bulb temperature, wet bulb temperature, dew point temperature, relative humidity etc.
Composition of Air
Air comprises of mixture of various gases and water vapor or moisture. The air without any water vapor is called as dry air, thus the ordinary air is the mixture of dry air and water vapor. As such the air always contains some amount of water vapor so the pure dry air doesn’t really exists, however its concept is very important in understanding the properties of the air and how various changes occur in the air conditioning process. The dry air and water vapor mixture is merely physical one as there is no chemical reaction between the two.
The dry air is composed of various gases, chiefly nitrogen (78%), and oxygen (21%). The remaining 1% of the gases includes carbon dioxide, and very small quantities of inert gases like hydrogen, helium, neon, and argon. The water vapor is also small part of the air included among remaining 1% of the gases.
The amount of moisture in air by its mass keeps on varying from place to place and depending on the atmospheric conditions at a particular place. The places located close to the sea areas contain more moisture while the desert areas contain less moisture. Similarly, during the raining seasons the moisture content of the air is high while during summers and winters its low. The air contains usually 1% to 3% of moisture by mass.
At the normal atmospheric temperature conditions oxygen gas exists in superheated conditions as gas since its boiling point is -182.7 degree C or -297 degree F. By nature oxygen is highly active agent causing rusting and corrosion of metals. Nitrogen too exists in superheated condition as gas in the atmosphere since its boiling point is -195 degree C or -319 degree F. However, nitrogen is an inert gas and does not cause any chemical reactions in the atmosphere. Since the chief constituents of the air are oxygen and nitrogen and they both exist in superheated condition, the air also exists in the superheated conditions as the gas.
It is important to note here that small changes in the temperature of the dry air during the air conditioning process cause very small changes in its volume and density. It is also important to note that all the heat that is added or removed from the air during air conditioning process is the sensible heat and no latent heat is involved since the boiling point temperatures of oxygen and nitrogen are very low.
Another important point to note is that the water vapor exists in the superheated condition, but when it is cooled or heated there is change in its phases, hence it absorbs or liberates sensible heat as well as the latent heat due to changes in its phases. This is what makes the whole process of air conditioning highly complicated. Cooling of water vapor results in its condensation, whiles its heating leads to superheating.
1) Book: Principles of Refrigeration by Roy J. Dossat
2) Book: Basic Refrigeration and Air Conditioning by P N Ananthanarayan