Freezing Protection for Closed-Circuit Cooling Towers

As severe cold approaches, many people in northern China have probably experienced water pipe bursts caused by low temperatures. Most factory facility managers are familiar with open-circuit cooling towers but have little knowledge of closed-circuit ones. Closed-circuit cooling towers are commonly used for equipment requiring high-quality cooling water. This article mainly explains the structure of closed-circuit cooling towers and their winter freezing protection measures!

 Introduction to Closed-Circuit Cooling Towers

 Working Principle

A closed-circuit cooling tower features two water circulation loops: one for the internal spray water (tap water) and the other for the supply and return of cooling water (RO water). The main water inlet pipe of the cooling tower enters the tower and splits into multiple small heat exchange copper tubes, which are closely arranged with the internal packing. The copper tubes run from bottom to top and then converge to the main water outlet pipe. The outlet water enters external heat exchange equipment for circulation before re-entering the cooling tower, forming a fully closed cooling water cycle. The internal spray water system is open, supplemented with tap water. In the sump at the bottom of the tower, a spray pump delivers water to the distributor at the top of the packing, which sprays water downward onto the packing and cooling water copper tubes. The spray water undergoes forced heat exchange with the cooling water to lower the temperature of the cooling water inside the copper tubes. Meanwhile, under the action of the top fan, the temperature of the spray water is reduced through water evaporation.

 Key Components of Closed-Circuit Cooling Towers

Spray Pump: Provides power for the internal spray water, enabling it to flow downward for forced heat exchange with the cooling water, thereby reducing the cooling water temperature.

Fan: Equipped with a variable-frequency motor, the fan controls the evaporation rate of the spray water by adjusting its frequency, thus regulating the spray water temperature.

Heat Exchange Copper Tubes: While ensuring the quality of the cooling water, these tubes effectively enhance the heat exchange efficiency between the cooling water and spray water (due to the use of high thermal conductivity materials). However, their pressure loss is relatively high due to structural requirements.

Packing: Increases the contact time between the spray water and copper tubes, ensuring more sufficient heat exchange.

Winter Freezing Protection Measures for Closed-Circuit Cooling Towers

 Anti-Freezing Using the End Heat Load of the Project Itself

This is the most energy-efficient method. The end load raises the temperature of the returned cooling water. When the returned water is sent to the closed-circuit cooling tower, the fan facilitates heat exchange between the copper tubes and cold air, lowering the water temperature while preventing freezing of the cooling tower and outdoor pipelines.

 Anti-Freezing with Antifreeze Fluid

Antifreeze fluid is the optimal choice to prevent pipe freezing accidents. Widely used in automotive, electronic, and other industries, it offers excellent antifreeze performance. Physical antifreeze fluids are convenient, safe, and reliable for operation and maintenance.

The electrical control system has potential uncontrollable factors, making operation and maintenance less convenient with relatively higher costs. During winter operation, it is necessary to consider actual weather conditions. In case of extreme or sudden weather changes, unused cooling towers must have the water in the coils forcibly drained in advance or in a timely manner, or operate with heat load to maintain sufficient circulating water flow and heat load circulation, preventing water in the coils from freezing. A common misunderstanding among operators is that a small temperature difference causes freezing, so they lower the pump frequency to reduce flow and increase the temperature difference for antifreezing. However, closed-circuit cooling towers consist of multiple coil groups. When the system flow decreases, pressure imbalances among the coils lead to slow or no water flow in some coils, which is highly likely to cause freezing and pipe bursting. Therefore, when using closed-circuit towers in winter, the circulating water flow should be appropriately increased to ensure water flow in each coil group.

Forced Drainage for Anti-Freezing

When there is no demand for cooling towers in winter, before the ambient temperature drops below 0℃, open the coil drain valve and force compressed air into the coils to drain all water. Note that the pressure of the compressed air should be neither too high nor too low: too low may fail to completely dry the water in the coils, while too high may exceed the pressure-bearing capacity of the coils. The optimal pressure range is 0.3-1MPa.

Thermal Insulation for Spray Pumps and Piping

If heat exchange between cold air and the cooling water copper tubes (with the fan running at full frequency) cannot meet the required water temperature, it is necessary to supplement spray water and start the spray pump. However, due to load fluctuations, the spray water and pump may freeze. In this case, thermal insulation and heat tracing should be installed for the spray piping and pump. Start the spray pump to keep the spray water circulating, preventing freezing of the piping and pump. Meanwhile, prevent ice formation on the packing surface, which may block air intake and affect heat dissipation. Alternatively, adopt an intelligent control system where the fan runs forward for a set period, stops, and then runs in reverse for a few minutes after a delay. The reverse rotation of the fan blows hot air inside the tower outward to melt the ice.