EFFECTS DUE TO PRESENCE OF AIR IN STEAM SYSTEM:

Air acts as an insulating layer and lowers the temperature when mixed with steam.

Dalton’s law of partial pressure states that “In a mixture of gases and/or vapors total pressure of the mixture is made up of the partial pressures exerted by each gas or vapors. The partial pressure exerted by each is the fraction of the total pressure equal to the fraction of the total volume of each”.

Consider an example where the total pressure of the mixture is 1.4 bar absolute (20.30 psi). The mixture is made up of 3 parts of steam and 1 part of the air.    

The partial pressure of steam is ¾ X 1.4 = 1.05 bar absolute (15.22psi) The partial pressure of air is ¼ X 1.4 = 0.35 bar absolute (5.07psi)
The temperature of steam at 1.4 bar (20.30 psi) is 109 Deg C (228.2DegF). When air is mixed with steam the partial pressure and temperature are reduced to 1.05 bar (15.22psi) and 101 DegC (213.8 0F).

The presence of air also causes a wrong impression in the reading of the pressure gauge. The fall in temperature can be noted but the subsequent fall in pressure does not appear on the gauge. In these cases, even though steam is supplied at 1.4 barg the temperature of steam corresponds to 1.05 barg. Lowering a steam temperature results in a drop in LMTD and hence the rate of heat transfer.