**5. Heat exchanger type, geometry and process fluid influencing fouling**

Shell and tube heat exchangers are used most commonly but they are not particularly suitable for fouling conditions. Fouling can be reduced with special baffle and tube design. Several studies [32-35] have shown that finned tubes foul less than plain tubes. Non-uniform thermal expansion leads to lower deposit strength and hence less deposition. Freeman et al. [36] found that tubes with longitudinal grooves on the outside had less particulate fouling (by alumina particles) than the plain tubes.

Fluidised-bed heat exchangers are used in several applications to reduce or even eliminate fouling completely. Fluidised particles remove deposits from the heat transfer surface. They also enhance the heat transfer efficiency as they interrupt the viscous sub-layer. These heat exchangers have been used successfully to reduce fouling by hard, adhering silica deposits [37]. Graphite heat exchangers are also reported to have less fouling. Direct contact heat transfer may be another alternative to reduce fouling [38]. Properties of the process fluid such as the nature and concentration of the dissolved constituents or suspended particles, presence of any living organisms, solution pH etc. affect fouling significantly.

Excessively high over-concentration of solids in the evaporating liquid may lead to carryover in the steam and cause fouling in process heat transfer equipment. Corrosion is very important on the steam side of process equipment. Water pH, over-concentration of treatment chemicals in evaporating liquids and dissolved gases (mainly oxygen and carbon dioxide) are very important contributors to corrosion fouling [39]. The presence of living organisms causes biological fouling and makes biofilms. This can sometimes enhance other fouling mechanisms too, as microbial deposits may trap suspended particles. They may also change the chemistry of water and can cause scaling or corrosion [39].
