How should scale buildup in closed cooling towers be removed?

It's quite common for cooling towers to develop scale buildup—similar to how limescale forms when you boil water in a kettle at home. Tap water is typically hard water, containing a higher concentration of dissolved minerals and impurities. When exposed to high temperatures, these substances rapidly break down, and the resulting ions combine to form calcium carbonate, which settles at the bottom. Over time, this accumulation leads to the formation of scale. In closed-loop cooling towers, cooling coils circulate the coolant fluid, while the outer surface is continuously sprayed with copious amounts of recirculating water. Next, we'll use the example of a closed-loop cooling tower to explain how scale develops—and discuss its impact on the cooling tower system.

The cooling coils in a closed cooling tower reach very high temperatures. A large volume of spray water is circulated through the sprinkler system, evenly distributing the coolant over the surface of the cooling coils. As a result, the water on the surface quickly absorbs the heat conducted from the coils and evaporates. However, the spray water contains significant amounts of calcium bicarbonate and magnesium bicarbonate, which rapidly decompose under the intense heat. These compounds then combine with carbon dioxide in the air, forming calcium carbonate. Over time, this calcium carbonate adheres to the outer layer of the cooling coils, gradually accumulating and eventually creating thick white scale—essentially pure calcium carbonate—that continues to build up on the coil surfaces.

Scale is primarily weakly alkaline in nature, so dilute hydrochloric acid is the main cleaning agent used to remove it. Dilute the hydrochloric acid with water, then evenly spray the diluted solution onto the surface of the scale. The hydrochloric acid will neutralize the scale, causing bubbles to form. If the reaction is slow, you can slightly increase the concentration of the acid. During the neutralization process, never touch the area directly with your hands—always wear protective gloves. Once the surface-scale has been dissolved and neutralized, you can rinse it off using a water hose or gently scrape it away with a hardwood tool. If the neutralization isn’t thorough enough, simply reapply the diluted hydrochloric acid to complete the dissolving and neutralizing process.

Scale buildup in closed cooling towers can significantly impact the cooling efficiency. When scale forms on the surface, spray water cannot directly reach the cooling coils—instead, it absorbs only the heat transferred from the coils to the scale. This greatly reduces both the water's heat-absorbing capacity and the overall heat-transfer efficiency. Using distilled water when operating a cooling tower is an excellent solution to this issue. Distilled water is pure and free of any impurities. However, during operation, air, dirt, and other contaminants often get introduced into the system, further complicating the task of removing scale buildup.

How should scale be removed from closed cooling towers?

1. Acid-alkali neutralization method. When acid and alkali are mixed, a chemical reaction occurs—specifically, an acid-alkali neutralization reaction—which produces water and sodium chloride, commonly known as salt. Since scale buildup is often extensive, manual removal can be costly. Therefore, we typically use a spray of mildly acidic water to neutralize the deposits. Once most of the scale has been removed, we switch to pure water circulating continuously for thorough rinsing, leaving surfaces remarkably clean in most cases.

2. Cooling tower descaling agents contain a certain amount of mild acid, but also include penetrants, corrosion inhibitors, and other additives. These agents can quickly penetrate even thick layers of scale, dissolving it so that the scale naturally falls off under the force of water hammer. There’s a key difference in how open and closed cooling towers should be treated with descaling agents: In open cooling towers, scale forms as water flows through, allowing it to remain immersed for extended periods. In contrast, scale in closed cooling towers tends to accumulate primarily on the surface of the condenser, which is made of metal. Since strong acids could potentially corrode metal components, it’s crucial to carefully control the dosage of the descaling agent when treating closed systems.

3. Electromagnetic descaling is a relatively new method that uses magnetic fields to alter the structure of scale, preventing it from clumping together after oxidation. Currently, electromagnetic descaling is considered a safer approach, as it poses no harm to metal materials. Additionally, this method helps save significant costs. Electromagnetic descalers are primarily installed within circulating pipelines for optimal performance.

Closed-circuit cooling towers require regular descaling, but this issue doesn't occur when stainless steel pipes are used. Stainless steel pipe condensers undergo surface oxidation treatment, making it difficult for scale to adhere to the surface. Compared to copper tubes, stainless steel is far more effective at preventing scale buildup.