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Cooling SystemsAs soon as a cooling water circuit (especially open evaporative coolers) is operated, one very quickly encounters a number of points without whose treatment the cooling water not only contributes to heat dissipation, but also to the destruction/decrease in efficiency of the system. The correct application of chemicals for cooling water treatment such as corrosion protection agents, hardness stabilisers and biocides is an art in itself.
In practically every industrial production, energy generation, building services or IT computer centre, large amounts of heat are generated that must be dissipated to prevent the systems from overheating. Wet cooling towers are often used for this purpose, in which the excess heat is released into the environment. The cooling water circulating between the heat source and the cooling tower, together with pumps, heat exchangers and pipes, is called the cooling circuit for short.
Since cooling water, depending on its chemical composition, tends to cause either deposits or corrosion, and germs such as bacteria, fungi and algae multiply explosively in warm cooling water without treatment, it is imperative to counteract this with suitable chemicals for cooling water treatment. Treating cooling water without chemicals leads to malfunctions within a very short time and can even be dangerous for the environment (legionella). But first let’s take a look at the most common cooling systems:
In this system, the heated cooling water in the cooling tower is sprayed onto a system of fins that create a large surface area. A powerful draught of air is ignited in the cooling tower by a pressure or suction fan or chimney effect. By evaporating the water, heat is extracted from it and the water temperature of the cooling water is lowered. The cooling water can be fed directly into the cooling process. The design of open cooling towers can vary and be round, square, but also V-shaped.
Since the cooling water in open systems comes into intensive contact with dust and dirt, germs and biofilms quickly form in the cooling water without biocide treatment or disinfectants. Evaporation also increases the salt/lime concentration in the cooling water (thickening). Therefore, in addition to hardness stabilisers and corrosion protection, dispersants are also needed to prevent dirt and lime deposits.
The cooling water (water, corrosion protection and antifreeze) flows through the pipes of a heat exchanger. Fans force air through the cooling fins, releasing heat into the air. The cooling water does not evaporate – a classic closed cooling circuit.
The system is widely used (e.g. in smaller form in cars). It requires little or no maintenance of the softened / desalinated cooling water after initial conditioning during refilling (which, however, requires a particularly high-quality cooling water treatment with chemicals such as molybdates as corrosion inhibitors). However, the energy consumption for the fans is generally higher at warm temperatures compared to wet cooling due to the poorer efficiency of pure air cooling.
In this model, part of the cooling water is circulated in a closed circuit (similar to a dry recooler) via a heat exchanger and therefore does not come into direct contact with the ambient air. The tube bundles of the heat exchanger, which are usually smooth because they are easier to maintain, are sprayed with water in a second, open cooling circuit, whereby the cooling water cools down more quickly in the closed part of the cooling circuit system. The cooling water evaporates only in the open part.
At very low air temperatures (winter) and low cooling loads, it may even be possible to dispense with water cooling (dry operation) and maintain cooling solely through the air draught of the fans. In this case, the cooling water is either drained off or placed in a frost-proof storage tank. In this type of cooling system, the open and closed parts are treated separately with chemicals. The open part requires regular treatment of the cooling water with hardness stabilisers, corrosion protection agents and biocides. In the closed part, on the other hand, a high dose of anti-corrosion agent and a little biocide is only needed when filling.
Corrosion:
Cooling water is either sedimenting or corrosive. The softer the cooling water in the circuit, i.e. the less dissolved substances it contains, the more aggressively it dissolves materials such as iron and other metals. It thus causes pitting, leaks and other corrosion-related problems.
Corrosion inhibitors or corrosion protection agents (e.g. phosphonates for open cooling circuits and molybdates for closed cooling circuits protect the surfaces of the cooling system in the process.
Deposits:
Hard cooling water contains a high dose of dissolved substances such as salts and lime. If the cooling water is heated during operation, CO2 escapes, which keeps the lime in suspension. The lime precipitates and begins to produce deposits. On the one hand, these protect against corrosion, but on the other hand they hinder heat transfer in the heat exchanger. This greatly reduces the efficiency of the cooling circuit system.
Hardness stabilisers such as phosphonates prevent the lime from crystallising and thus forming a crust. This can thus simply be flushed out via the desalination system.
Biofilms:
Warm water is an excellent breast site for microorganisms of all kinds. These very quickly form slimy deposits, which can quickly reach a considerable thickness. Like limescale deposits, they hinder the heat transfer in the heat exchanger and reduce the efficiency. In addition, these germs pose a health risk that should not be underestimated, if cooling water aerosols containing e.g. legionella spread in the vicinity of the cooling towers and are inhaled. With the help of biocides such as Sanosil C in the cooling water, these biofilms and pathogenic germs are eliminated and prevented from forming.
Automated chemical dosing for cooling water controls and regulates the content of all necessary chemicals and is strongly indicated for smooth cooling water treatment.
