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=Factors Governing Biological Fouling in Seawater Systems =

Surface Temperature
The temperature at the interface of the seawater and the solid has a significant effect on the fouling deposition rates. In biological fouling, the rate initially increases with temperatures up to 30-40°C, and then rapidly starts decreasing since many of the biofouling organisms cannot survive at higher temperatures. Figure 1 shows the effect of temperature on the fouling factor of biological fouling compared to other types of fouling mechanisms in seawater heat exchanger systems.



Bulk Temperature
The bulk temperature has the same effect on biological organisms as the surface temperature. Initially as temperatures rise above ambient temperature, biological growth increases. After reaching a peak quite soon, at higher temperatures, these organisms start dying. Thus, with increasing temperatures, the rate of biological fouling decreases. Ideally higher temperatures should be used to decrease biofouling, but it is not always possible due to thermal expansion constraints.

Water Velocity
Velocity is a significant factor for biological fouling. Biological fouling rate increase with velocity at low flow velocities, as organisms are benefited with faster transport of their nutrients to them. A low flow rate is required to sustain the mass transfer of nutrients to the metal surface. At much higher velocities, water shears off much of the fouling deposition and the rate decreases. Figure 2 shows the effect of velocity on the fouling factor of biological fouling compared to other types of fouling mechanisms in seawater heat exchanger systems.



Oxygen Concentration
Oxygen is necessary for most biological organisms to survive. Some anaerobic slimes are still present with low oxygen levels. The fouling rate is dependent on the survival of these organisms, and therefore oxygen in a major factor for biofouling.

Heat Exchanger Material
The material used for heat exchanger play a role in the biofouling rate. Some materials are more susceptible than others to attachment of biological organisms. For example, titanium is an excellent material for seawater heat exchangers since it can be used for temperature up to 300 degrees Celsius. However, it is more prone to biological fouling. The effect of the material on the fouling rate may be offset by adjusting the temperature or velocity rate. Biofouling on titanium heat exchangers can be avoided since titanium can withstand high velocities.

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