What are the water quality treatment requirements for the air-conditioning water system? How can pipe corrosion and scaling be prevented?
I am an HVAC expert. Water quality treatment of air-conditioning water systems is an easily overlooked yet crucial aspect in HVAC system design, construction, and operation and maintenance, as it directly affects equipment lifespan, heat exchange efficiency, and operating energy consumption. Based on my years of engineering experience, water quality issues are often the "hidden killer" that causes premature failure of chiller evaporators, condensers, and piping. Below I will provide a detailed analysis combining standards and practice.
First, regarding water quality treatment requirements, the main references are the Code for Design of Heating Ventilation and Air Conditioning for Civil Buildings GB 50736-2012 and the Code for Design of Industrial Circulating Cooling Water Treatment GB 50050-2017. Article 8.6.2 of GB 50736 explicitly stipulates: "Air-conditioning cold and hot water systems shall adopt water quality stabilization measures combining chemical treatment or physical treatment," and points out that key indicators of circulating water systems such as pH value, hardness, alkalinity, and chloride ion content must meet equipment requirements. Specifically, for closed systems (such as air-conditioning cold and hot water), the pH value should be controlled between 8.0 and 9.5 to inhibit corrosion; for open cooling tower systems, attention should focus on the concentration ratio (usually controlled at 3 to 5 times) to prevent scaling. GB 50050 has more detailed limits for turbidity, total hardness, iron ions, etc.; for example, the turbidity of cooling circulating water should not exceed 20 mg/L.
In preventing pipeline corrosion and scaling, the core is a dual approach of "physical + chemical." Starting from the design stage, I usually recommend:
1. **Install water treatment devices**: Install fully automatic water softeners or electronic descaling instruments on the system return water main to reduce water hardness (e.g., reduce calcium and magnesium ion concentrations to below 50 mg/L), reducing scaling risk from the source. At the same time, configure side-stream water processors to control microbial growth through filtration and sterilization, because mucus produced by microbial metabolism will aggravate under-deposit corrosion.
2. **Chemical dosing treatment**: According to the water quality analysis report, regularly add corrosion inhibitors (such as molybdate, zinc salts) and scale inhibitors (such as polyphosphates). For example, in closed systems, I often use molybdate formulations, which can form a protective film on metal surfaces and slow oxygen corrosion. In cooling tower systems, biocides and algicides (such as isothiazolinone) also need to be added to prevent the proliferation of microorganisms such as Legionella.
3. **Control flow velocity and temperature**: The design flow velocity of pipelines must comply with Article 6.3.6 of GB 50736; the flow velocity of cold and hot water pipelines should preferably be controlled at 1.0 to 2.5 m/s to avoid low-speed deposition or high-speed erosion. At the same time, control the supply water temperature not to exceed 60℃, because high temperatures accelerate the precipitation of scale.
Actual engineering case: I participated in the renovation of the air-conditioning system of a five-star hotel. After the original system had operated for 5 years, the chiller evaporator frequently alarmed, and heat exchange efficiency dropped by 30%. Upon inspection, it was found that the cooling tower system was not equipped with an automatic blowdown valve, and the concentration ratio was as high as 8, resulting in severe scaling of the condenser copper tubes. We took the following measures: first, chemical cleaning of the pipelines (using sulfamic acid descaler); second, adding an online water quality monitor and automatic dosing device on the cooling tower return water pipe to control the pH value at about 8.5 and add corrosion and scale inhibitors; finally, setting the concentration ratio to 4. After the renovation, no scaling problems occurred within 2 years of system operation, and energy consumption was reduced by about 12%.
Summary: Strictly carrying out water quality treatment in accordance with the requirements of GB 50736 and GB 50050, combined with physical filtration, chemical dosing, and operational control, is the key to preventing pipeline corrosion and scaling. The above are reference opinions. The actual project situation still needs further comprehensive evaluation and implementation in combination with specific operating conditions, site conditions, and other factors.