Scaling is the most common type of fouling and is commonly associated with
inverse solubility salts such as calcium carbonate (CaCO3) found in water. Reverse solubility salts become
less solute as the temperature increases and thus deposit on the heat exchanger surface. Scale is difficult to remove
mechanically and chemical cleaning may be required.
Sedimentation occurs when particles (e.g. dirt, sand or rust) in the solution
settle and deposit on the heat transfer surface. Like scale, these deposits may be difficult to remove mechanically
depending on their nature.
Results from a chemical reaction which involves the heat exchanger surface material. Many metals such as copper
and aluminum form adherent oxide coatings which serve to passivate the surface and prevent further corrosion. Metal
oxides which are corrosion products exhibit quite a low thermal conductivity and even relatively thin coatings of oxides may
significantly affect heat exchanger performance.
Fouling from chemical reactions in the fluid stream which result in the deposition of material on the heat exchanger
surface. This type of fouling is common for chemically sensitive materials when the fluid is heated to temperatures
near its decomposition (degradation) temperature. Coking of hydrocarbon material on the heat transfer surface is also
a common chemical fouling problem.
Occurs when a portion of the hot stream is cooled to near the freezing point of one of its components. An example
in refineries is when paraffin solidifies from a cooled petroleum product. Another example is freezing of polymer products
on the heat exchanger surface.
Occurs when biological organisms grow on heat transfer surfaces. It is a common fouling mechanism where untreated
water is used as the coolant. Problems range from algae to other microbes such as barnacles and zebra mussels.
During seasons when these microbes are said to bloom, colonies several millimeters deep may grow across the surface within
hours, impeding circulation near the surface wall and impacting heat transfer.
It is important to consider fouling in the design of a heat exchanger. There are different methods to provide
the added heat transfer area needed to account for the expected fouling and maximize runtime between cleaning. For shell
and tube heat exchanger, the common method is to use fouling factors. For other types of heat exchangers, excess heat transfer area is often used. However, fouling is a self-fulfilling
prophecy and the selection of fouling factors or excess area must be done carefully.
Fouling tendencies depends on the type of heat exchanger and the fluids. During the design stage certain considerations
may help minimize fouling experienced in the field: