Condenser and auxiliary heat exchanger tube and cooling tower film fill fouling create difficulties that can cost a plant hundreds to thousands of dollars per day in lost efficiency. In extreme cases, solids deposition within cooling tower film fill has caused structural failures of the cooling tower, requiring unit shutdown. Repairs might take weeks or longer, which can result in excessive monetary hardship due to lost production and equipment repair costs.
Cooling water fouling mechanisms
Fouling and scaling are the two primary mechanisms of deposition in cooling systems. Fouling in particular can be very troublesome and dangerous. Suspended solids that enter cooling systems might settle in the cooling tower basin, at low-flow spots within the system, on condenser tubes and in the tower fill. Solids are introduced with the makeup, particularly if the supply is surface water, but cooling towers also are very efficient air scrubbers. Suspended solids will naturally cycle up in the system, and without a control mechanism can reach overwhelming concentrations.
Particularly
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Bacteria are separated into the following three categories:
- Aerobic: Utilize oxygen in the metabolic process
- Anaerobic: Live in oxygen-free environments and use other sources, i.e., sulfates, nitrates or other donors for their energy supply
- Facultative: Can live in aerobic or anaerobic environments
A problem with microbes, particularly many bacteria, is that once they settle on a surface, the organisms secrete a polysaccharide layer for protection. This film will collect silt from the water and grow even thicker, further reducing heat transfer. But, this is just part of the problem. Even though the bacteria at the surface might be aerobic, the secretion layer allows anaerobic bacteria underneath to flourish. These organisms generate acids and other harmful compounds that directly attack the metal. Microbial deposits also establish concentration cells, where the lack of oxygen underneath the deposit causes the locations to become anodic to other areas of exposed metal.
Metal loss occurs at anodes, with pitting as the result.
Fouling also will cause significant – and at times devastating – buildups in cooling tower film fill.
Film fill is common in modern cooling towers, as the large surface area provided by the packing enhances contact between air flowing up through the fill with the cooling water traveling down.
Fouling disrupts the water-air flow patterns, and in severe cases completely plugs passageways. In some severe cases, the buildup of materials in the fill has even caused structural failure of internal tower components or complete tower sections.
Fungi will attack cooling tower wood in an irreversible manner, which can eventually lead to structural failure. Algae will foul cooling tower spray decks, leading to reduced performance and unsafe working locations.
Monetary costs of fouling
In previous work, I examined the costs of degraded condenser performance due to fouling, scaling or excess air in-leakage, all of which can be equally troublesome. Poor condenser heat transfer during the summer months at a large plant might result in increased fuel costs of 6 figures or more. [2] Consider a top-flight expert’s evaluation, in which he examines fouling of condenser tubes that increase the condenser backpressure from 1? to 2? Hg. This loss of efficiency can “increase the heat rate by as much as 200 Btu/kWh. This can increase fuel costs significantly. For example, in a 500 MW unit, this increase in heat rate will increase fuel costs by $4,800 per day if the [base] fuel cost is $2 per MMBtu.” [3] Costs will be even higher if fouling forces a unit derating.
