What is the purpose of adding glycol to chilled water systems?

The primary purpose of adding glycol to chilled water systems is to provide freeze protection. Glycol, typically ethylene glycol or propylene glycol, is an antifreeze that lowers the freezing point of the water mixture, preventing it from freezing and potentially damaging the pipes, coils, and other components of the chilled water system in cold weather conditions. When the ambient temperature drops below the freezing point of water (32°F or 0°C), the water in the system can freeze, expand, and cause pipes to burst or coils to crack. Adding glycol to the water lowers the freezing point to a safe level, typically below the lowest expected ambient temperature. The concentration of glycol in the water mixture is carefully controlled to achieve the desired level of freeze protection. For example, a 50% glycol solution can protect the system down to -34°F (-36.7°C). In addition to freeze protection, glycol also provides some corrosion inhibition. Propylene glycol is generally preferred over ethylene glycol in HVAC systems due to its lower toxicity. However, it's important to note that glycol solutions have a lower heat capacity and higher viscosity than water, which can slightly reduce the system's heat transfer efficiency and increase pumping power requirements. Therefore, the glycol concentration should be optimized to balance freeze protection with system performance.