The basis for rationalisation

Perhaps the most important thing for the student of psychrometry to appreciate from the outset is that the working fluid under study is a mixture of two different gaseous substances. One of these, dry air, is itself a mixture of gases, and the other, water vapour, is steam in the saturated or superheated condition. An understanding of this fact is important because in a simple analysis one applies the Ideal Gas Laws to each of these two substances separately, just as though one were not mixed with the other. The purpose of doing this is to establish equations which will express the physical properties of air and water vapour mixtures in a simple way. That is to say, the equations could be solved and the solutions used to compile tables of psychrometric data or to construct a psychrometric chart.

The justification for considering the air and the water vapour separately in this simplified treatment is provided by Dalton’s laws of partial pressure and the starting point in the case of each physical property considered is its definition.

It must be acknowledged that the ideal gas laws are not strictly accurate, particularly at higher pressures. Although their use yields answers which have been adequately accurate in the past, they do not give a true picture of gas behaviour, since they ignore intermolecular forces. The most up-to-date psychrometric tables (CIBSE 1986) are based on a fuller treatment, discussed in section 2.19. However, the Ideal Gas Laws may still be used for establishing psychrometric data at non-standard barometric pressures, with sufficient accuracy for most practical purposes.

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