Condensation occurs in two distinct modes, and the heat transfer difference between them is a matter of factors:
Filmwise condensation. If the surface is wettable the liquid forms a continuous film. That film is a thermal resistance layer between the vapour and the wall, and the resistance grows as it thickens.
Dropwise condensation. If the surface is hydrophobic the liquid collects as droplets; as drops grow and run off, the clean surface beneath is exposed again. The heat transfer coefficient is an order of magnitude above the filmwise value.
Why designs are based on film
The dropwise regime is attractive but not durable. Surface coatings become wettable over time through fouling, oxidation and erosion, and the system reverts to film. Heat exchanger design is therefore based on the filmwise assumption, which is safe over the life of the equipment; dropwise behaviour is treated as a bonus.
Non-condensable gases: small fraction, large effect
A small amount of non-condensable gas (air, nitrogen) in the vapour reduces heat transfer far more than expected. As the vapour condenses at the surface, the gas accumulates there; the gas layer that forms at the interface blocks vapour from reaching the surface.
A few percent of air can markedly degrade condenser performance. In systems operating under vacuum, sealing is therefore a performance matter and not just a maintenance one.
Modelling
Resolving the film with the mesh is rarely necessary in industrial problems; the film is represented as a surface thermal resistance and a mass sink. If the film itself is the question (dripping, entrainment, erosion), then a free-surface approach is needed.
Where non-condensable gas is present, species transport must be solved; otherwise all the model sees is vapour and the result comes out optimistic.
References
- Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605
- Ranz, W. E. & Marshall, W. R. (1952). Evaporation from drops. Chemical Engineering Progress, 48, 141–146 (Part I) and 173–180 (Part II).