Fouling (cooling water)

Heat exchanger in a steam power plant soiled by macrofouling

Condenser tube with residues of biofouling (cut open)

Condenser tube with a layer of calcium carbonate (cut open)

Brass pipe with traces of corrosion (cut open)

Cost relationships between the individual forms of fouling

Under fouling (engl. Dirt, mold) is defined as the contamination of heat-transferring system components by ingredients of the used cooling water . In cooling technology and other technical areas, a distinction is initially made roughly between micro and macro fouling.

Macro fouling

Macrofouling is caused by coarse substances of biological origin or industrially produced waste . These substances are brought into the cooling water cycle by the cooling water pumps from cooling water sources such as the open sea , rivers , lakes and groundwater . Even with closed systems in cooling towers , entry through open channels or through the wind into the cooling tower bowls is possible. In the case of cooling towers, parts of the internals sometimes come loose and get into the cooling circuit. All of these substances contaminate the heat transferring surfaces of the coolers and heat exchangers and worsen their associated heat transfer coefficient .

• Waste produced by man
• Installation of cooling towers
• Seaweed
• Shellfish
• Polyps
• Leaves , parts of plants up to the tree trunk

Microfouling

In microfouling, a distinction is made between the following processes:

• Biological fouling such as bacterial colonization and algae colonization
• Sedimentation of sand and mud
• Scaling as a crystallization of poorly soluble salts :
  • Calcium hydrogen carbonate
  • Calcium sulfate
  • Silicates
  • Magnesium hydroxide

Due to evaporation and degassing, the concentration of salts can move above the saturation point and thus lead to flocculation of salt crystals. The formula for the equilibrium between the readily soluble calcium hydrogen carbonate always present in water and the insoluble calcium carbonate is:

${\displaystyle \mathrm {Ca^{2+}+2\ HCO_{3}^{-}\leftrightharpoons \ CaCO_{3}\downarrow +CO_{2}\uparrow +H_{2}O} }$

The calcium carbonate produced in this reaction is deposited in the pipes . Due to the temperature dependence of the reaction, the deposits are higher at the outlet than at the inlet.

Chemical reaction

These can occur due to contact with the warmer surface of the condenser tube. In these cases the metallic surface sometimes acts as a catalyst . Most of these reactions are corrosion and polymerizations . This occurs with cooling water for the chemical industry that is contaminated with a low content of hydrocarbons . Higher pipe wall temperatures can lead to coking .

Economical meaning

Fouling is costly. These costs arise initially from corrosion damage and poor heat transfer . Furthermore, the ecological costs due to the necessary use of biocides to prevent biofouling or the increased use of fuel to compensate for the reduced performance caused by fouling are also to be seen.

Even normal fouling in a conventionally fired 500 MW power plant block causes  power losses of the steam turbine of 5 MW and more, in a 1300 MW nuclear power plant the losses can reach 20 MW. In seawater desalination , fouling reduces the Gained Output Ratio by high double-digit percentages (The Gained Output Ratio is a ratio that relates the mass of the distillate produced to the mass of the steam used in the process). Additional electrical power consumption in compressor-operated refrigeration machines is also often in the double-digit percentage range.

In addition to the operating costs, the investment costs also increase , since the heat exchangers have to be designed larger in order to compensate for the poorer heat transfer caused by the fouling.

Combat

The most sensible way to combat fouling is not to let the substances causing the fouling get into the cooling water circuit. In steam power plants and other large industrial facilities for water use, macrofouling is prevented by pre-filtration and cooling water filters. With microfouling, filtration is only possible with very complex water treatment processes or membrane technology . For this reason, chemical or mechanical cleaning methods are recommended to remove deposits. These methods are the pickling with acid or the recirculating ball cleaning with sponge rubber balls . While pickling creates environmental problems through the supply, use and storage of the acid , mechanical recirculating ball cleaning is now a recognized process. Another option is treatment with all-metal catalysts. They attract free germs in the water by means of electrostatic charges and decompose them. The decomposition process produces environmentally friendly biosurfactants that detach and thus cause biofilms to die. The biocides that have often been used up to now can be divided into four groups: inorganic chlorine and bromine compounds, chlorine and bromine releasers, ozone and oxygen releasers and non-oxidative biocides. One of the most important non-oxidative biocides consists of a mixture of chloromethylisothiazolinone and methylisothiazolinone . Dibromonitrilopropionamide and quaternary ammonium compounds are also frequently used .

See also

• Fouling (shipbuilding)

Individual evidence

1. ↑ Sustainable disinfection using catalyst technology ( Memento of the original from October 3, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. 

Web links

• Biofouling in heating, drinking water and cooling systems
• Antifoulings and cooling water systems (PDF file; 329 kB)
• Use of environmentally friendly chemicals in cooling water conditioning (PDF file; 1.32 MB)