Algae formation in wet cooling systems: Technical, energy, and hygiene implications for operation and system stability
Evaporative cooling systems are among the most efficient and economical systems for industrial process cooling. They use the physical principle of evaporative cooling to dissipate large amounts of heat cost-effectively. However, this efficiency comes with a systemic disadvantage. The open design with intensive air-water contact creates ideal living conditions for microbiological growth. Algae in particular find optimal conditions for spreading in these systems.
This analysis focuses on the systematic presentation of the technical, economic, and hygienic effects of algae growth in evaporative cooling systems.
Biological basis of algae formation in open cooling systems
Algae are photosynthetically active microorganisms that use light as their primary energy source and are bound to humid to water-bearing environments. Open wet cooling systems offer ideal conditions for this due to constant humidity, mild temperatures, and an open structure that allows daylight to penetrate unhindered.
The circulating water is continuously sprinkled over packing material, brought into contact with air over a large area, and typically moves within a temperature range that provides favorable growth conditions for many types of algae. At water temperatures between 25 and 35 °C, numerous species of green and blue-green algae can multiply rapidly, provided that sufficient nutrients are available. The latter enter the system via the ambient air that is sucked in, among other things. Nutrient-rich raw water with elevated nitrate, ammonium, or phosphate levels can further "fertilize" the system and thus accelerate algae formation.
Mechanisms of impairment in wet cooling systems
3.1. Influence on heat transfer
In trickle fill areas, uniform wetting is crucial for heat and mass transfer between water and air. If algae accumulate on these surfaces, they change the physical properties of the boundary layer. This results in the formation of an additional, biologically influenced layer with its own thermal conductivity. This acts as an insulating film and increases the thermal resistance between water and ambient air.
As the layer thickness increases, heat transfer deteriorates. Even thin organic coatings can reduce the effective heat transfer coefficient because they impair the direct wetting of the structured packing surfaces. At the same time, the surface roughness can change, which in turn influences the flow behavior of the water. In practice, this manifests itself in an increasing approach temperature or reduced cooling capacity with the same fan and pump capacity.
3.2. Hydraulic effects
Packing elements are designed for defined flow cross-sections and droplet formation. If their geometry is altered by biological deposits, partial constrictions may occur. The water film becomes uneven, with some areas drying out while others are subjected to greater stress.
If the packing structure is partially blocked or the water distribution is disrupted, the effective wetting area of the packing is reduced. Experimental studies show that even partial blockages or nozzle defects can lead to significant losses in tower characteristics (KaV/L). Thermal efficiency decreases significantly as a result of uneven water distribution and reduced effective exchange area.
Deposits can also accumulate in the basin area, binding sediments and forming sludge structures. This not only increases the cleaning effort, but can also impair the function of pumps, screens, or sensors.
3.3. Effects on energy efficiency
Thermal and hydraulic effects have a direct impact on the energy requirements of the system. If the cooling capacity decreases as a result of deposits or biofilm formation, the system often has to compensate by using higher volumes of air or water. Fans run longer or at higher speeds, and pumps work against increased pressure losses. A large-scale study showed that the coefficient of performance (COP) decreases by up to 18% on average when system efficiency is reduced, and by more than 40% when the cooling load is high. This led to an additional energy requirement of 5 to 13% per cooling season.
3.4. Corrosion and material damage
As layer formation increases, the microchemical conditions on the wetted surfaces also change. Local environments develop under algae and mixed coatings, which can differ significantly from the free circulating water in terms of oxygen concentration, pH value, and material turnover. Such gradients promote electrochemical processes that accelerate corrosion reactions.
Even though algae themselves are not primarily considered to be classic causes of corrosion, in open systems they can contribute to the biofilm matrix and thus create a stable structure in which corrosion-causing microorganisms can settle. Within these structures, small-scale oxygen differences arise, leading to local element formation and potentially endangering metallic components such as basins, fixtures, heat exchangers, or pipes. In addition, deposits and particles can accumulate under dense coatings. Such deposits promote crevice and pitting corrosion, as they undermine protective passive layers or hinder mass transfer.
3.5. Hygiene and safety risks
The continuous formation of aerosols in the cooling tower causes fine water droplets to be released into the ambient air. If the circulating water contains microbiological contaminants, these can in principle be transported with the aerosol. Algae themselves are not generally considered to be primary pathogens. Their significance lies rather in their role as structure-forming organisms within complex biofilms. Such biofilms create protected habitats in which other microorganisms can also settle. The matrix of organic material provides nutrients and protection against hydraulic wash-off or chemical treatment.
Final consideration
Algae formation in open wet cooling systems affects key technical functions, reduces energy efficiency, and acts as a catalyst for microbiological processes that can jeopardize both operational safety and material integrity in the long term. The gradual development of biological deposits, their interaction with water chemistry, and their role in biofilm formation make it clear that algae should not be viewed in isolation or merely as a visual problem.
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