This experimental and analytical study focuses on the formation of banded condensate films on a horizontal tube, when the ammonia mass fraction of the vapor is less than 0.0126. The experimental results show that there is a step change in the heat transfer coefficient when a banded film is formed. This change develops when the vapor mass fraction is approximately 0.004 and the heat transfer coefficient at the end of the change is 15% greater than the value expected for steam. The banded films are initiated at the base of the tube; midway between the regions where droplets detach. Photographs taken during the tests show that the development starts with the formation of a single band; when this band is complete, secondary bands are formed on either side and then further bands are added until the complete tube surface is covered with a banded film. The reason for the delay in the formation of the bands is examined using a simple two-dimensional force balance. The success of this examination provides an insight to the mechanisms employed in the formation of banded films.

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