Articles -
January 2021
Propylene Glycol vs. Ethylene Glycol: How to Choose
Glycols are utilized across a vast array of process cooling and heating applications—from HVAC systems and ice rinks to food and pharmaceutical processing, plastic molding, and solar thermal heat transfer. While there are many specialized formulations available, the industry relies on two primary types of glycols: Ethylene Glycol (EG) and Propylene Glycol (PG).
Toxicity and Applications
The most critical difference between the two glycols is their toxicity profile. Ethylene glycol is highly toxic to humans and animals. Conversely, propylene glycol is non-toxic and is even available in food-grade formulations (in addition to standard industrial grades).Due to these toxicity differences, Propylene Glycol is strictly required for food and pharmaceutical applications, as well as HVAC systems in schools, homes, and businesses—essentially anywhere there is a risk of human or animal exposure. Ethylene Glycol, on the other hand, is the predominant fluid used in automotive antifreeze, ice rinks, plastic molding, and in-line heaters where the system is entirely closed and human contact is not a factor.
Comparing Physical Properties
Beyond toxicity, the two glycols exhibit distinct physical properties that affect system performance. At identical concentration percentages, propylene glycol solutions generally have a higher viscosity and a higher freezing point than ethylene glycol. This makes propylene glycol slightly less thermally efficient, particularly at lower operating temperatures.
To illustrate this difference, consider the properties of a 50% volume mixture of each glycol in water:
- Freezing Point: A 50% propylene glycol mixture freezes at -31°F, whereas a 50% ethylene glycol mixture freezes lower, at -36°F.
- Viscosity: At -10°F, the viscosity of the 50% propylene glycol solution is 96 centipoise, while the ethylene glycol solution remains much thinner at just 27 centipoise.
Because of its dramatically lower viscosity at freezing temperatures, ethylene glycol can typically be used at temperatures roughly 20 degrees lower than propylene glycol while maintaining the same pumping efficiency.
Making the Right Choice
When determining which glycol is most appropriate for your specific system, you must weigh multiple factors: the type of application (open vs. closed), required toxicity and safety profiles, desired thermal efficiency, and overall cost (propylene glycol is typically more expensive than ethylene glycol). There is no “one-size-fits-all” solution in heat transfer systems, which is why your fluid selection must be made thoughtfully and carefully.
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