Ideal gas theory is very important for analysis of processes because in most of the situations moisture content is extracted in the form of water vapour, which behaves as an ideal gas. An ideal gas can be described in terms of three parameters: the volume that it occupies, the pressure that it exerts, and its temperature. According to the definition, ideal gas represents a special state of mater which can be delimited by a system boundary. It is assumed that: All particles have rest mass( m > 0 the particles are not photons). The number of particles with respect to the system volume is small. Total volume of particles is negligible with respect to system volume. Collisions of particles with each other is much less probable than the collisions with system boundary.

Real gases are non-ideal gases which occupy space and have mass. These gases do not obey the ideal gas laws. When they are cooled to their boiling point, they liquefy. Real gases have velocity and volume. Examples of real gases are : O 2 , N 2 , C O 2 etc.