WATER TREATMENT

However, chlorine used in biocide treatment for cooling towers also has some serious disadvantages. To begin with, the chlorine in bleach breaks down relatively quickly. Its shelf-life is therefore far from ideal. As a gas, chlorine becomes very dangerous to handle. Lethal chlorine gas could escape if something goes wrong. But the biggest disadvantage of all is that chlorine forms trihalomethanes (or AOX) in the cooling water when used as a biocide chemical for cooling tower. They are believed to be carcinogenic and to cause nerve damage. Because of this, many countries impose trihalomethane emission thresholds on cooling towers.

A popular cooling tower biocide water treatment product: chlorine dioxide
Despite the similarity in the name, chlorine dioxide is NOT the same as chlorine. However, like chlorine it disinfects. Chlorine dioxide can be produced on site directly from sodium chlorite, making it an easy sell. However, as chlorine dioxide is rather dangerous – mixtures with air containing more than 10% chlorine dioxide by volume are potentially explosive – elaborate measuring and regulating equipment becomes necessary. Otherwise, safety cannot be ensured in compliance with the law. These instruments are quite expensive, which means chlorine dioxide is only really a viable choice for large cooling circuits.

The applications for chlorine dioxide are based on its oxidising effect. It is often used instead of chlorine because it forms fewer toxic and harmful chlorinated organic compounds when reacting with organic substances.

However, its aggressive action on materials is a serious disadvantage of chlorine dioxide. It even attacks plastic pipes, seals, stainless steel and glass.

The advantages of a hydrogen peroxide / silver combo as a cooling water biocide
Less prevalent in cooling tower biocide water treatment is a combination of hydrogen peroxide and silver, such as Sanosil C, for use as a biocide used in cooling water. Although this approach seems more expensive than chlorine at first glance, this seeming disadvantage is easily offset by the greater effectiveness and longer-lasting effect of Sanosil C.

Unlike chlorine and chlorine dioxide, hydrogen peroxide and silver do not merely oxidise. Silver, even though only traces of it are present, has an incredibly strong catalytic effect on peroxide while also possessing excellent potential to inhibit new microbial growth.

Sanosil C shows its true strength as a cooling tower biocide when combating biofilms in cooling circuits; an area in which it is outstandingly effective. Whereas chlorine has a hard time in the face of old, thick biofilms, Sansosil breaks up biofilm structures much more easily. Because Sanosil C biocide is able to penetrate biofilm before it degrades, bubbles of air form within the slime. The increase in volume literally blows the slime structures right off the surfaces.

As a biocide in cooling water, another positive aspect of Sanosil C is that it does not generate trihalomethanes during disinfection. This means the biocide cooling tower dosage can be adjusted without restrictions or concern for the environment. This is especially advantageous during ‘shock dosing’ to combat biofilms.

Calcification, precipitation, stabilisation: Descaling chemicals for cooling tower
Water either corrodes or forms deposits. Depending on its composition, water contains a certain amount of dissolved solids such as calcium and magnesium which are suspended in carbon dioxide. When water is heated, carbon dioxide is decomposed and calcium (actually, all dissolved salts) begin to precipitate and form deposits in pipes or heat exchangers.
This isn’t necessarily a bad thing – the deposits protect metal against corrosion. HOWEVER, they also reduce the heat transfer coefficient (k value) in the heat exchanger. In turn, this reduces the plant’s performance and effectiveness considerably. Of course it’s possible to desalinate the cooling water to prevent calcification, but then you can practically watch the metal corrode in real-time and risk overusing corrosion inhibitors for cooling water systems etc.

Modern water stabilisers for cooling tower scale removal like phosphates can’t prevent calcium from precipitating, but they do prevent it from forming scale and crystals. After using a cooling tower descaler, calcium is kept in suspension, allowing it to be flushed out of the circuit with the blowdown. The calcium suspended in the water is actually good for the system, because it prevents corrosion in cooling water systems.

Corrosion inhibitors for cooling water systems and protection
Wherever soft water is used in cooling towers – no matter whether it is surface water or desalinated water – corrosion inevitably follows. All metals are affected to a greater or lesser degree. Even stainless steel will succumb in time. The advantage is that the heat exchanger always achieves optimal thermal conductivity, as there are no deposits to prevent heat transfer.

But in that case it is mandatory to protect the system against aggressive water of this kind. This is accomplished with a cooling water corrosion inhibitor or an anti-corrosive which cover all metal surfaces with a thin protective layer – without the insulating effect of calcification. Ideally, a phosphonate is chosen as it protects against corrosion and acts as a hardness stabiliser. Corrosion inhibitors for cooling water systems do this by keeping calcium suspended in the water, preventing it from crystallising and thus protecting metal surfaces. Excess calcium is then flushed out of the circuit during blowdown.

Dispersant for cooling tower, preventing sedimentation
The moment water comes into contact with air, it begins to accumulate dust, soot, insects, pollen, leaves and so forth. This is because a cooling tower acts like a gigantic air scrubber. Floating in the water and aided by vortices, these dissolved solids precipitate at critical junctures throughout the system.

At some point, these sediments prevent water throughput and displace cooling water from the compensating reservoir.

Naturally, it is possible to remove the sediments mechanically on a regular basis – for example, by using shovels, sludge extractors and water jets during annual maintenance. However, a simpler solution is to prevent the formation of these sediments in the first place. Sediment and corrosion inhibitors for cooling water systems / dispersant for cooling tower keep deposit-forming solids suspended in the water and prevent them from settling on surfaces. Like calcium, they get flushed out of the system during blowdown before they can contribute to blockages or throughput issues.

Cooling tower corrosion inhibitor Sanosil Corfit A and Corfit B each combine stabilisers, cooling water corrosion protection and dispersion in one useful, efficient recipe.

Biofilms, bacterial slime and algae
Bacteria are number four of the bothersome quartet and thrive in warm cooling water. They form slimy layers which prevent heat transfer and can drift, causing blockages in pipes and pumps. As if that weren’t bad enough, warm water in a cooling tower can become a habitat in which legionella and other harmful bacteria can flourish.

Adding a biocide or disinfectant keeps these bacteria in check and prevents slime and algae from forming. Additionally, it protects the vicinity from dangerous aerosols containing microorganisms. This enables a technically and legally sound operation of cooling towers.

Sanosil C is an excellent biocide for cooling circuits. It is fast-acting, reliable and long-lasting against biofilms and microbes in water.
Plus, it is an excellent companion for the corrosion inhibitors for cooling water systems Corfit Protect B and Corfit Protect A for stabilising, corrosion in cooling water systems protection and dispersion.

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.

Cooling systems 2: Closed cooling circuit with air cooling / dry recooler
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.

Semi-open evaporative cooler / hybrid system
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.

Problems in the operation of a cooling circuit and their solution
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.