How is the makeup water volume for cooling towers calculated? What are the water quality treatment standards? How can Legionella growth be prevented?

How is the makeup water volume for cooling towers calculated? What are the water quality treatment standards? How can Legionella growth be prevented?

I am an HVAC expert. The calculation of cooling tower makeup water, water quality treatment standards, and Legionella prevention and control are key aspects of HVAC system design and O&M, directly affecting system energy efficiency, equipment lifespan, and public health safety. In accordance with 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), and drawing on years of engineering experience, I provide the following detailed analysis:

1. Cooling Tower Makeup Water Calculation

• Evaporation loss: 1.0%–1.5% of the total circulating water volume (estimated based on a supply-return water temperature difference of 5–10℃), calculable by the formula \( W_e = 0.00153 \times \Delta t \times Q \), where \(\Delta t\) is the supply-return water temperature difference (℃) and \(Q\) is the circulating water volume (m³/h). For example, at a temperature difference of 5℃, the evaporation loss is approximately 0.765% × Q.
• Wind drift loss: For mechanical draft cooling towers, approximately 0.1%–0.2% of the circulating water volume; for natural draft towers, it may be neglected (estimated at 0.1%).
• Blowdown loss: Depends on the cycles of concentration of the circulating water (typically controlled at 3–5), calculated by the formula \( W_b = W_e / (N-1) \), where \(N\) is the cycles of concentration. For example, at 4 cycles of concentration, the blowdown loss is approximately 1/3 of the evaporation loss.
• Leakage loss: Generally estimated at 0.1%–0.3% of the circulating water volume, but must be adjusted according to the sealing conditions of the piping and tower body.

2. Water Quality Treatment Standards

• pH: 7.0–9.0 (to be adjusted according to chemicals to prevent corrosion or scaling).
• Total hardness (as CaCO₃): ≤450 mg/L (to prevent scaling).
• Total alkalinity (as CaCO₃): ≤300 mg/L (must be balanced with hardness).
• Chloride ion: ≤1000 mg/L (lower for stainless steel materials, e.g., ≤300 mg/L).
• Suspended solids: ≤20 mg/L (to prevent fouling of fill).
• Conductivity: ≤2000 μS/cm (to control cycles of concentration).
• Total bacteria count: ≤1000 CFU/mL (heterotrophic bacteria).

3. Legionella Growth Prevention and Control

• Water temperature control: Ensure the cooling water temperature is below 20℃ or above 50℃; however, this is difficult to achieve in actual operation, so periodic disinfection is required. It is recommended to alternately dose oxidizing biocides (e.g., sodium hypochlorite, with a free chlorine residual of 0.5–1.0 mg/L) and non-oxidizing biocides (e.g., isothiazolinone) once a week.
• System design optimization: Avoid stagnant water zones (e.g., install air release valves at pipe ends), provide a cooling tower basin slope of ≥1% for drainage, and clean the fill regularly (once per quarter).
• Water quality testing: Conduct Legionella culture testing of the cooling water quarterly (standard limit ≤100 CFU/L); if exceeded, perform shock chlorination or use ultraviolet disinfection.
• Operation and maintenance: If shut down for more than 48 hours, drain and flush the system, and perform high-temperature disinfection (circulation at above 60℃ for 30 minutes) before startup.

Summary

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.