Where heat transfer is taking place at the saturation temperature of a fluid, evap­oration or condensation (mass transfer) will occur at the interface, depending on the direction of heat flow. In such cases, the convective heat transfer of the fluid is accompanied by conduction at the surface to or from a thin layer in the liquid state. Since the latent heat and density of fluids are much greater than the sen­sible heat and density of the vapour, the rates of heat transfer are considerably higher. The process can be improved by shaping the heat exchanger face (where this is a solid) to improve the drainage of condensate or the escape of bubbles of vapour. The total heat transfer will be the sum of the two components.

Rates of two-phase heat transfer depend on properties of the volatile fluid, dimensions of the interface, velocities of flow and the extent to which the trans­fer interface is blanketed by fluid. The driving force for evaporation or con­densation is the difference of vapour pressures at the saturation and interface temperatures. Equations for specific fluids are based on the interpretation of experimental data, as with convective heat transfer.

Mass transfer may take place from a mixture of gases, such as the conden­sation of water from moist air. In this instance, the water vapour has to diffuse through the air, and the rate of mass transfer will depend also on the concen­tration of vapour in the air. In the air-water vapour mixture, the rate of mass transfer is roughly proportional to the rate of heat transfer at the interface and this simplifies predictions of the performance of air-conditioning coils.

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