As was discussed before, heat exchanger are equipment used for heat transfer between two fluid flows with different temperatures. In design of heat exchangers such as double pipe heat exchanger, shell & tube heat exchanger, etc., it must be noted that there are some factors that affect the heat transfer rate during the equipment operation period. One of the most important factors is called fouling.
In different industries, specially chemical and petrochemical industries, fluids passing through the heat exchanger are not that clean and contain some solid material. These solids which can be metal oxides produces by corrosion, sediment, dirt, etc. and will sediment in dead zones of heat exchanger, both in tube and shell side. Bio fouling is another form of fouling. It is caused by accumulation and growth of living organisms in pipes and heat exchangers. Bio fouling mostly occurs in cooling water heat exchangers that user open waters like rivers, lakes or seas.
Accumulation of such fouling in heat exchangers will increase the thermal resistance of tube walls and as a result the efficiency of heat exchanger will reduce or even it may lead to heat exchanger malfunctioning. Since corrosion and other fouling reasons are unpreventable, it’s important to make sure that each heat exchanger fouling level is below the allowed level. To check the fouling condition of an exchanger, the outlet temperature may be observed. As fouling increases in heat exchanger, the thermal resistance between two fluids increases and the temperature difference of a fluid decreases. Another factor if the pressure drop of heat exchanger. Fouling can prevent the fluids to flow in exchanger easily and the pressure drop of each flow in exchanger will increase.
If any signs of fouling are observed, heat exchanger must be disassembled and cleaned. There are several cleaning procedures to clean different types of heat exchangers. The most common cleaning methods are washing with high pressure water jet and inserting rotating-head rods or drills or brushes.
After the cleaning step is done, the exchanger can be re installed and operated normally.
Written by: P.Jowkar
Chemical Engineer, Persian Gulf University