The most common type of fouling is the precipitation of solid deposits in a
fluid on the heat transfer surfaces. You can observe this type of fouling even
in your house. If you check the inner surfaces of your teapot after prolonged
use, you will probably notice a layer of calcium-based deposits on the surfaces
at which boiling occurs. This is especially the case in areas where the water is
hard. The scales of such deposits come off by scratching, and the surfaces can
be cleaned of such deposits by chemical treatment. Now imagine those mineral
deposits forming on the inner surfaces of fine tubes in a heat exchanger
(Fig. 13–9) and the detrimental effect it may have on the flow passage area
and the heat transfer. To avoid this potential problem, water in power and
process plants is extensively treated and its solid contents are removed before
it is allowed to circulate through the system. The solid ash particles in the flue
gases accumulating on the surfaces of air preheaters create similar problems.