Condensation with non-condensable gas — What is the effect of a non-condensing gas mixed with a condensing vapor on the condensation process?

Introduction:
In many condensers, the vapor stream contains a non-condensable gas (e.g., air with steam). This component significantly hinders heat transfer and mass transfer, reducing condenser performance. Understanding its effect is critical to design venting strategies and sizing surface area adequately.

Given Data / Assumptions:

• Binary mixture: condensable vapor + non-condensable gas.
• Film condensation on a cooled surface.
• No chemical reaction between components.

Concept / Approach:

The non-condensable gas accumulates near the cold surface, creating a gaseous boundary layer with reduced partial pressure of the condensable component at the interface. This lowers the driving force for condensation (difference between bulk vapor partial pressure and saturation pressure at the interface) and adds a mass-transfer resistance. Simultaneously, the stagnant gas layer adds thermal resistance. Net effect: lower heat flux and reduced condensation rate; hence it is undesirable.

Step-by-Step Solution:

Verification / Alternative check:

Condenser design manuals show dramatic fall in heat flux with even small percentages of non-condensables; venting points are standard practice.

Why Other Options Are Wrong:

A/B/C contradict the established detrimental effect; E is incorrect because the impact is substantial.

Common Pitfalls:

Ignoring non-condensables in energy balances or assuming their effect is minor; in reality, they often control performance.

Final Answer:

None of these.