Influence of non-condensable gases

If a vapour condenses in the presence of a non-condensable gas it has to diffuse through this gas to the phase interface. This means that a drop in the partial pressure to the phase interface is required. As can be seen in Fig. 4.8, the partial pressure pl of the vapour drops from a constant value p1G away from the phase interface to a lower value pu at the phase interface. Correspondingly, the associated saturation temperature ds(p ) also falls to the value dj at the phase interface. The pressure p0 of the inert gas rises towards the phase interface, so the sum px + p0 always yields a constant total pressure p. 

The saturation temperature at the phase interface can, depending on the gas content, lie considerably below the saturation temperature t s(p) associated with the pressure p, which would occur if no inert gas was present. The temperature difference between the phase interface and the wall is lowered because of the inert gas and with that the heat transfer is also reduced. In order to avoid or prevent this, it should be possible to remove the inert gas through valves. Large condensers are fitted with steam-jet apparatus which suck the inert gas away. In other cases, for example the condensation of water out of a mixture of steam and air or in the condensation of ammonia from a mixture with air, it is inevitable that inert gases are always present. Therefore their influence on heat transfer has to be taken into account. 

The influence of the inert gases on heat transfer can be determined with the 

The heat transfer coefficient aG signifies the heat flow transferred from the vapour-gas mixture to the phase interface. The superscript point indicates that the heat is not only transferred by conduction but also by a material flow normal to the wall. In a flowing gas, the size of aG is determined from the material properties 

The mass flow rate M of the vapour transported at the condensate surface increases with the difference in the partial pressure p1G = y1Gp in the vapour space and pn at the phase interface. 

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