Parameters that influence fouling resistances

Many operational and design variables have been identified as having well-defined effect on fouling. Some of those parameters include the following:

Fluid Temperature

A good practical rule to follow is to expect more fouling as the temperature rises. This is due to a “baking on” effect, scaling tendencies, increased corrosion rate, faster reactions, crystal formation and polymerization, and loss in activity by some antifoulants.

Lower temperatures produce slower fouling buildup, and usually deposits that are easily removable. However, for some process fluids, low surface temperature promotes crystallization and solidification fouling. For those applications, it is better to use an optimum surface temperature to overcome these problems.

For cooling water with a potential to scaling, the desired maximum surface temperature is about 140°F (60°C). Biological fouling is a strong function of temperature. At higher temperatures, chemical and enzyme reactions proceed at a higher rate with a consequent increase in cell growth rate.

Velocity and Hydrodynamic Effects

Hydrodynamic effects, such as flow velocity and shear stress at the surface, influence fouling. Within the pressure drop considerations, the higher the velocity, higher will be the thermal performance of the exchanger and less will be the fouling. Uniform and constant flow of process fluids past the heat exchanger favors less fouling. Foulants suspended in the process fluids will deposit in low-velocity regions. Higher shear stress promotes dislodging of deposits from surfaces. Maintain relatively uniform velocities across the heat exchanger to reduce the incidence of sedimentation and accumulation of deposits.

Material

The selection of tube material is significant to deal with corrosion fouling. Carbon steel is corrosive but least expensive.
Noncorrosive materials such as titanium and nickel will prevent corrosion, but they are quite expensive.
Although the construction material is more important to resist fouling, surface treatment by plastics, vitreous enamel, glass, and some polymers will minimize the accumulation of deposits.

Impurities

Fluids are rarely pure. Intrusion of minute amounts of impurities can initiate or substantially increase fouling. They can either deposit as a fouling layer or acts as catalysts to the fouling processes.

In crystallization fouling, the presence of small particles of impurities may initiate the deposition process by seeding. Sometimes impurities such as sand or other suspended particles in cooling water may have a scouring action, which will reduce or remove deposits.

Surface Roughness

Better surface finish has been shown to influence the delay of fouling and ease cleaning. Similarly, non-wetting surfaces delay fouling. Rough surfaces encourage particulate deposition.

After the initiation of fouling, the persistence of the roughness effects will be more a function of the deposit itself. Even smooth surfaces may become rough due to scale formation, formation of corrosion products, or erosion.

Suspended Solids

Suspended solids promote particulate fouling by sedimentation or settling under gravitation onto the heat-transfer surfaces. Since particulate fouling is velocity dependent, prevention is achieved if stagnant areas are avoided. Often it is economical to install an upstream filtration.

Plate Heat Exchangers

High turbulence, absence of stagnant areas, uniform fluid flow, and the smooth plate surface reduce fouling and the need for frequent cleaning. Hence the fouling factors required in plate heat exchangers are normally 10-25% of those used in shell and tube heat exchangers.

Seasonal Temperature Changes

When cooling tower water is used as coolant, considerations are to be given for winter conditions where the ambient temperature may be near zero or below zero on the Celsius scale. The increased temperature driving force during the cold season contributes to more substantial overdesign and hence over performance problems, unless a control mechanism has been instituted to vary the water/air flow rate as per the ambient temperature.