Fouling
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The general term "fouling" refers to the accumulation and deposition of living organisms (biofouling) and certain non-living material on hard surfaces in an aquatic environment. This can be the fouling of ships, pilings, and natural surfaces in the marine environment (marine fouling), fouling of heat-transferring system components through ingredients contained in the cooling water, and even the development of plaque on teeth, among other examples. This article is devoted to the fouling of heat exchanger systems, although many of the points made are applicable to other varieties of fouling.
In the cooling technology and other technical fields, a first rough distinction is made between macro fouling and micro fouling.
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[edit] Macro fouling
Macro fouling is caused by coarse matter of biological origin or by industrially produced refuse. Such matter enters into the cooling water circuit through the cooling water pumps from sources like the open sea, rivers or lakes. Even in closed circuits, like cooling towers, the ingress of macro fouling into the cooling tower basin is possible through open canals or by the wind. Sometimes, parts of the cooling tower internals detach themselves and are carried into the cooling water circuit. All such substances foul the heat-transferring surfaces of the coolers and heat exchangers and deteriorate the relevant heat transfer coefficient.
- Examples
- Manmade refuse
- Internal parts of cooling towers
- Algae
- Mussels
- Leaves, parts of plants up to entire trunks
[edit] Micro fouling
As to micro fouling, distinctions are made between:
- Biological fouling, like settlements of bacteria and algae
- sedimentation of sand and mud
- scaling as crystallization of salts:
Through evaporating and degasifying, the concentration of salts can rise above the saturation point, leading to a precipitation of salt crystals. For the equilibrium between the readily soluble calcium bicarbonate - always prevailing in water - and the unsoluble calcium carbonate, the formula reads:
The calcium carbonate that has formed through this reaction precipitates in the tubes. Due to the temperature dependence of the reaction, the scaling is higher at the outlet than at the inlet.
[edit] Chemical reactions
Chemical reactions may happen on the grounds of the contact with the warmer surface of the condenser tube. In such cases the metallic surface sometimes acts as a catalyst. These reactions are mainly corrosion and polymerization occurring in cooling water for the chemical industry which has a minor content of hydrocarbons. Higher tube wall temperatures may lead to carbonizing.
[edit] The economic importance of fouling
Fouling creates considerable operational costs. These costs result initially from corrosion damages and impaired heat transfer. However, also the ecological costs have to be considered that arise from the use of biocides for the avoidance of bio fouling, or from the increased fuel input to compensate for the reduced output caused by fouling. For example, normal fouling at a conventionally fired 500 MW power station unit accounts for output losses of the steam turbine of 5 MW and more, and in a 1300 MW nuclear power station the losses may reach even 20 MW. In seawater desalination plants, fouling may reduce the Gained Output Ratio by two-digit percentages (the Gained Output Ratio is an equivalent that puts the mass of generated distillate in relation to the steam used in the process). The extra eletrical consumption in compressor-operated coolers is also easily in the two-digit area.
In addition to the operational cost, also the capital cost increases because the heat exchangers have to be designed in larger sizes to make good for the heat transfer losses through fouling.
[edit] Fouling control
The most reasonable way of controlling fouling is to keep the cause of fouling out of the cooling water circuit. In steam power stations and other major industrial installations of water technology, macro fouling is avoided by way of pre-filtration and cooling water debris filters. In the case of micro fouling, filtration is possible only with extensive methods of water treatment or membrane technology. Chemical or mechanical cleaning processes for the elimination of scales are recommendable in this regard. These processes comprise pickling with acids or cleaning with recirculating sponge rubber balls by the so called "Taprogge System". Whereas pickling causes environmental problems through the handling, application and storage of acids, mechanical cleaning by means of circulating cleaning balls is a renowned method.
The applied biocides may be classified as follows: inorganic chlorine and bromide compounds, chlorine and bromide cleavers, ozone and oxygen cleavers, unoxidizable biocides. One of the most important unoxidizable biocides is a mixture of chloromethyl-isothiazolinone and methyl-isothiazolinone. Also applied are dibrom nitrilopropionamide and quaternary ammonium compounds.
