Glycol is an organic compound that is a viscous, sweet-tasting liquid commonly used in thermal applications. While often associated with cooling systems, glycol itself is not a refrigerant in the traditional sense. It is instead known as a heat transfer fluid (HTF), which serves a distinct role in thermal regulation. Glycol’s primary purpose is to enable cooling and heating systems to operate effectively across a broad temperature range where water alone would fail.
Glycol’s Function as a Heat Transfer Fluid
The fundamental difference between a refrigerant and a heat transfer fluid lies in the process by which they move heat. A primary refrigerant, such as ammonia or Freon, absorbs large amounts of heat by undergoing a phase change, transitioning from a liquid to a gas in a cycle of evaporation and condensation. This change of state is the core mechanism of refrigeration.
Glycol does not change its state during the cooling process; it remains a liquid throughout the cycle. It functions as a secondary coolant, transporting thermal energy from a chiller or heat exchanger to the point of use. This makes it a heat transfer fluid, a medium that moves heat from one place to another without a phase change.
Glycol is almost always mixed with water, which significantly alters the solution’s physical properties. The main value of this mixture is its ability to lower the freezing point of water dramatically, acting as an antifreeze agent. Depending on the concentration, this solution can resist freezing down to temperatures as low as -60 degrees Fahrenheit, preventing damage to pipes and equipment. The solution also often contains corrosion inhibitors, which protect the metal components within the system.
Ethylene Glycol Versus Propylene Glycol
The two most common types of glycol utilized in cooling systems are ethylene glycol (EG) and propylene glycol (PG). Ethylene glycol provides superior thermal performance, offering better heat transfer efficiency and lower viscosity compared to PG. This lower viscosity means it can be pumped with less energy and offers a better rate of heat removal, especially at colder temperatures.
Ethylene glycol is classified as moderately toxic, making it unsuitable for applications where incidental contact with humans, animals, or food is possible. Due to its toxicity, EG is reserved for closed-loop industrial systems or automotive applications where efficiency is prioritized and exposure risk is minimal.
Propylene glycol, in contrast, has a very low level of toxicity and is often available in food-grade formulations. This safety profile makes PG the preferred choice for processes where the fluid could potentially leak and contaminate consumables or potable water. While PG has a slightly higher viscosity and lower heat transfer capability than EG, its safety advantage makes it the industry standard for uses like food and beverage processing.
Systems That Utilize Glycol
Glycol solutions are widely implemented across various sectors where temperature regulation is necessary across long distances or in freezing environments. Large-scale commercial HVAC systems frequently use glycol in their chilled water loops. This is especially true for systems with outdoor piping, where the glycol prevents the water from freezing and bursting the lines during winter.
Industrial process cooling relies heavily on glycol to maintain precise temperatures for manufacturing equipment or product streams. For example, breweries circulate a chilled propylene glycol solution to quickly drop the temperature of beer after fermentation. Geothermal heat pump systems also utilize glycol to transfer thermal energy between the building and the underground heat exchange loop.