Mass transfer in buildings
The phrase ‘mass transfer’ describes the net movement of mass from one place to another in liquids and gases.
In the simplest interpretation, mass transfer refers to the movement of mass by diffusion at a molecular level. Mass will diffuse from areas of high concentration to areas of low concentration, as described by Fick's law. In this interpretation, the movement of mass by fluid motion (such as convection) is more correctly a part of fluid dynamics rather than mass transfer. That is, mass transfer is dependent on a concentration gradient whereas fluid motion is dependent on a temperature or pressure gradient.
The evaporation of water is an example of simple mass transfer, as the humidity of the air close to the surface of water is higher than that in the surrounding air and so moisture vapour diffuses away from the surface of the water, allowing more water to evaporate.
However, mass transfer is also sometimes taken to refer to fluid flows, driven by temperature and pressure gradients and to phase change.
Mass transfer in buildings is particularly important in cooling processes, such as in cooling towers, chiller units, heat pumps and so on.
Mass transfer is sometimes considered alongside heat transfer, as they are very similar and related processes.
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