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Cooling Tower Mainteance Cooling water treatment is very important: the moment a cooling water circuit (particularly an evaporative cooler) is put into operation, several issues quickly arise which, unless taken care of, will result in the cooling water not only performing heat exchange but also degrading the system’s efficiency. Using cooling tower water treatment chemicals such as corrosion inhibitors, hardness stabilisers and biocides for cooling tower water treatment is an art unto itself.
Nearly all industrial manufacturing processes, energy generation, building services and data centres generate large quantities of heat, which needs to be removed to prevent the systems from overheating. Wet cooling towers are a preferred method for releasing surplus heat into the environment. The pumps, heat exchangers, pipes as well as the cooling water which circulates between the heat source and the cooling tower are collectively referred to as the cooling circuit.
As cooling water tends to cause sediment or corrosion, depending on its chemical makeup, and pathogens such as bacteria, fungi and algae multiply explosively in untreated warm water, it is mandatory counteract these effects by treating the cooling water with suitable chemicals. Cooling water without chemicals quickly leads to problems, and can even become hazardous for the environment (legionella). Let’s take a look at the most prevalent cooling systems:
In this system, the heated cooling water is sprayed onto a system of fins in the cooling tower, which provide a greater surface area. A push or pull fan, or the stack effect, generates a powerful draft. As the water evaporates, heat is removed from it, reducing the water temperature. The cooling water can be fed directly into the cooling process.
Open evaporative cooling towers can vary in design and be round, square or even V-shaped.
Because the cooling water in open systems has intensive contact with dust and dirt, pathogens and biofilms can quickly form unless it is treated with biocides. Also, evaporation increases the concentration of salt and chalk in the water (thickening). Therefore, when it comes to water treatment for cooling towers of this kind, in addition to hardness stabilisers and corrosion inhibitors, it is also necessary to add dispersants to prevent dirt and limescale.
The cooling water (water, corrosion inhibitor and antifreeze) flows through the pipes of a heat exchanger. Fans push the air through the cooling fins, transferring heat energy to the ambient air. The cooling water does not evaporate. This makes it a conventional closed-circuit system.
The system is widespread (cars make use of a small version, for example). After the initial conditioning when filling (which, however, does require a particularly high-quality cooling water treatment with cooling water treatment chemicals such as molybdates to inhibit corrosion), the softened/desalinated water needs little maintenance.
Admittedly, the inferior efficiency of air cooling means the power usage for the fans tends to be higher than for wet cooling with higher temperatures.
In these cooling tower water treatment systems, a closed system (similar to a dry cooler) is run across a heat exchanger and does not come into direct contact with the ambient air. The heat exchanger’s tube bundles are usually smooth for easier maintenance and are sprayed with water in a second, open cooling circuit, which results in the cooling water in the closed part of the circuit cooling more rapidly. The cooling water only evaporates in the open section.
When temperatures are very low (winter) and if the cooling load is low, it may even be possible to dispense with water cooling (dry operation) and maintain cooling with the draft from the fans alone. In this case the cooling water is either drained or stored in a frost-proof storage tank.
Cooling systems of this type require separate chemical cooling water treatment for the open and closed sections. The open section periodically requires hardness stabilisers, corrosion inhibitors and biocides, whereas the closed section only needs a high dose of corrosion inhibitor and a little biocide when filled.
Cooling water either scales or corrodes. The softer the cooling water in the circuit, i.e. the fewer dissolved solids it contains, the more aggressively it will dissolve materials such as iron and other metals. It will cause pitting, leaks and other problems resulting from corrosion.
Corrosion inhibitors or anti-corrosives (e.g. phosphates for open and molybdates for closed cooling circuits) protect the cooling system’s surfaces.
Hard cooling water is rich in dissolved solids such as salts and calcium. When the cooling water heats up during operation, CO2 escapes, which keeps the calcium in suspension. The calcium precipitates and begins to form limescale. This protects against corrosion, but also prevents the transfer of heat in the heat exchanger. As a result, it substantially reduces the effectiveness of the cooling circuit.
Hardness stabilisers such as phosphates prevent the calcium from crystallising, and therefore from forming crusts. Instead, it can be flushed out via the desalination system.
Warm water is a perfect breeding ground for all kinds of microorganisms. They rapidly form slimy deposits which can become quite dense in a very short time. Like calcium deposits, they prevent heat transfer in the heat exchanger and reduce its efficiency. What’s more, these germs can be a health hazard that should not be underestimated, if for instance cooling water aerosols containing legionellae are spread near the cooling towers and inhaled.
Treating the cooling water with biocides like Sanosil C eliminates these biofilms and sources of pathogenic bacteria and prevents them from growing.
An automated dosing of cooling water treatment chemicals monitors and regulates the content of all necessary chemicals, and is imperative for unproblematic cooling water treatment.
