Types of fouling(Precipitation fouling or crystallization fouling,Particulate fouling,Corrosion fouling,Chemical reaction fouling,Biological fouling)
Six types of fouling
Precipitation fouling or crystallization fouling
Also called crystallization fouling. A fluid or gas used in a heat exchanger can contain dissolved inorganic salts. Given certain conditions, there’s a maximum amount of salt that can be dissolved in this fluid or gas. When the process conditions inside the heat exchanger differ from the conditions at the entrance, supersaturation may occur. This means that part of the dissolved salt will crystallize on the heat transfer surface. Figure 1 gives a clear example.
Figure. 1 Percipitation fouling
Particulate fouling
This is when the gas or fluid inside the heat exchanger contains small particles which will attach to the heat transfer surface. Examples are dust or sand. The deposition occurs mostly as a result of gravity.
Chemical reaction fouling
This type of fouling considers the deposits that are formed as a result of chemical reactions within the fluid. The heat transfer surface itself is not consumed in the reaction, although it could operate as a catalyst. This type is a common problem in for example petroleum refining or polymer production.
Corrosion fouling
This fouling is also caused by some chemical reaction, but this time the surface is a reactant and will be consumed. The surface reacts with the fluid or gas to form corrosion products on itself. The rusting of steel parts is a well-known example, as can be seen in figure 2.
Figure 2 Corrosion fouling
Solidification fouling
When the heat transfer surface is low enough, a fluid flowing through a heat exchanger can actually freeze at the surfaces. In case of a multicomponent fluid, it’s the high melting point constituent that will solidify. This is easy to imagine for fluids, like water cooling, but in practice this phenomenon can also occur when the medium is a gas.
Biological fouling
It’s also possible for biological micro- and macro-organisms to stick to the heat transfer surface. In this case not only the attaching of the material is a problem, but also it’s growth. In many cases this will result in a slime layer. This can be seen in figure 3.
Figure. 3 Biological fouling
To understand more about the influence of fouling on the performance of a heat exchanger, one must consider the heat transferred q:
Here LMTD stands for the Logarithmic Mean Temperature Difference, AO is the outer surface area and U stands for the overall heat transfer coefficient. The influence of fouling can be seen in the coefficient U. In the past various equations for U have been developed to capture fouling factors, but the most widely used is this:
The term inside the first brackets stands for the ordinary heat coefficient, when there is no fouling (or for an unused heat exchanger); h stands for the convective heat transfer coefficient, RW and AW are thermal resistance resp area of the wall. The second term is the extra term because of fouling. Here RF is the fouling resistance. The indices I and O stand for inner and outer surfaces.
It’s clear to see that for increasing fouling factors, the thermal coefficient U will drop, causing the transferred heat q to drop too. One way to compensate this effect is to over dimensionize the heat exchanger, which is increase the heat transfer area. One disadvantage is of course that this will result in a more expensive device.
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