Heat Transfer Fluids Comparison of Ethylene Glycol vs. Propylene Glycol

As Canadians, we know all too well that sub-zero temperatures can wreak havoc with our engineering infrastructure—in particular, water freezing when it shouldn’t. Thus, we need to choose the right solution (pardon the pun). It’s this need that brings us to this tip of the week: heat transfer fluids—ethylene glycol vs. propylene glycol.

It’s widely known that solutions of both ethylene glycol and propylene glycol will effectively lower the freezing point of water, and for most applications they can be used interchangeably or even mixed. That said, while there are many minor variations in the properties between the two solutions, two key differences usually dictate which of the glycols to choose for a particular application: toxicity and viscosity.

As to toxicity, ethylene glycol is by far the more toxic of the two liquids—two to four ounces of the substance can be fatal if ingested by an adult. That is why many choose the alternative. Furthermore, the law regulates use of ethylene glycol more closely than propylene glycol.

Propylene glycol is completely different—not only is it safer, but it is actually used as a food additive. Therefore, propylene glycol solutions are favoured for many applications, particularly when human exposure can occur.

As to viscosity, while differences between ethylene glycol and propylene glycol at elevated temperatures are minimal, the viscosity differences become significant at temperatures below 0°F. If heat transfer at these low temperatures is required, it is much better with ethylene glycol solutions. In addition, ethylene glycol solutions require less pumping horsepower under cold conditions. If these fluid characteristics are important, ethylene glycol is usually chosen for the application.

Performance differences between ethylene glycol and propylene glycol

Ethylene glycol solutions
higher heat transfer coefficient
better freeze point depression
less swell-ability to plastics/elastomers
higher surface tension, fewer leaks

Propylene glycol solutions
better cavitation erosion protection
concentrate does not freeze
not harmful to humans if ingested in moderate amounts