What Is Circulating Cooling Water? A Water-Saving Terminology Guide
Circulating cooling water is one of the largest water consumers in industry. In sectors such as petrochemicals, power generation, steel and metallurgy, circulating cooling water accounts for 50–90% of a plant’s total water use. The cooling of circulating water is achieved through contact between the water and air, and is the combined result of three processes: evaporative heat dissipation, contact heat dissipation and radiative heat dissipation.
1. How Circulating Water Is Cooled
Evaporative heat dissipation: in the cooling equipment, water forms droplets of various sizes or extremely thin water films, which enlarge the contact area with air and extend the contact time, strengthening evaporation. The water vapor carries away the latent heat of vaporization from the water, thereby cooling it.
Contact heat dissipation: when water comes into contact with air at a lower temperature, the temperature difference transfers heat from the hot water to the air, lowering the water temperature.
The heat exchange equipment used for this purpose is typically of the plate, finned-tube or spiral-plate type.
2. Water Quality Requirements of Circulating Cooling Water
Water quality standards for circulating cooling water are determined according to the chemical dosing formulation and the operating conditions of each system. The requirements for concrete materials in the system follow the current Code for Investigation of Geotechnical Engineering (GB 50021-94). For closed circulating cooling water systems, the water quality standard is set according to the production process requirements. In open circulating cooling water systems, the number of heterotrophic bacteria should preferably be less than 5×10⁵ per mL, and the slime content should preferably be less than 4 mL/m³.
3. Treatment of Circulating Cooling Water
The treatment of circulating cooling water can be summarized in the following aspects:
Corrosion inhibition: corrosion inhibitors are dosed so that a protective film forms on the metal surface, covering the metal and isolating it from the corrosive medium, thereby preventing metal corrosion.
Side-stream filtration: a side-stream filtration device is installed, with a side-stream flow generally of 1%–5% of the circulating water volume, to filter out suspended solids.
4. Why Circulating Cooling Water Matters for Water Saving
Because of the salt content of the raw (make-up) water, the circulating water must be concentrated to a certain number of cycles, at which point a portion of concentrated water has to be blown down and fresh water replenished. For a 300 MW condensing unit, the circulating cooling water flow reaches about 33,000 t/h. Assuming a raw water salinity of 1,000 mg/L and 3 cycles of concentration, the blowdown of the circulating cooling water is roughly 6′′–8′′ (about 198–264 m³/h), while the fresh water to be replenished equals the blowdown plus evaporation losses — approximately 2–2.6% of the circulating water volume, or about 660–860 m³/h. The consumption of water resources and the discharge of wastewater involved are therefore substantial.
Constrained by the cycles of concentration, circulating cooling water systems must continuously discharge a certain amount of concentrated water and replenish an equivalent amount of fresh water, keeping the salt content, pH value, organic concentration and suspended solids of the cooling water within a reasonable and permissible range. Treating and reusing this blowdown stream is of great significance: it not only raises the water reuse rate and saves water resources, but also greatly improves the overall condition of the circulating cooling water system.
