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What are the effects of inerts on the condensation of vapors?

Q: Is there a way to estimate the effect of air present in a condenser on the heat transfer coefficient? In this particular case the condenser is the OVHD condenser of a methanol distillation column in a small pilot plant. It is designed as a total condenser with sub-cooling. Reflux drum is vented to atm, therefore, condenser is also at atmospheric pressure. Condenser is horizontal BEM design. Air is present in the condenser but is stagnant. How much excess surface should be allowed?

A: There are two significant effects of inerts on the condensation of vapors.  The first is the effect on the temperature-composition profile which determines the fraction condensed as a function of temperature.  As material condenses the vapor phase composition becomes richer in air and it becomes more difficult to condense the remaining methanol.  The second effect is to add a mass transfer resistance as the vapor near the liquid film is depleted of the condensing component, resulting in a radial composition profile.  Chapter 16 of “Process Heat Transfer” (Hewitt, Shires & Bott) provides a good treatment of condensation in the presence of an inert gas.  If done correctly the amount of excess surface can be minimized.

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