Heat Transfer Fluid: What it is and Why Your Industry Needs It

Many industrial processes require equipment, products, or operating environments to stay within specific temperature ranges. To achieve that, systems can use engineered products known as heat transfer fluids (HTFs).

 

Heat transfer fluids come in many different forms, including glycol-based HTFs. Glycol heating systems prevent freezing, transfer heat at high efficiency, keep temperatures stable, and protect against corrosion.

 

Old World Industries (OWI) senior account manager Vache Soghomonian is an expert in glycol-based heat transfer fluids. We sat down with Vache to learn more about glycol HTFs and their use cases across diverse industries. Vache's insights cover:

 

• Types of heat transfer fluids and their properties
• Glycol HTFs and their main applications
• Ethylene or propylene glycol: Which should you choose?
• Heat transfer fluid FAQs

 

Let's dive in:

 

Types of heat transfer fluids and their properties

Technically speaking, HTFs include any fluid with the predictable and controllable ability to transfer heat. This broad definition includes compounds like water, steam, mineral oils, molten salts, liquefied metals, and engineered synthetic solutions. 

 

However, glycol heat transfer fluid systems often make the best choice — and not just because they tend to cost less. Glycol HTFs have low freezing points, enhanced thermal stability, and excellent corrosion resistance. These characteristics create a versatile profile that transfers well to different equipment and conditions.

 

"Glycol HTFs have operating temperatures in the range of minus-60 to 250 degrees Fahrenheit," Vache says. "Typically, systems operating in the heating-cooling space are running within those temperatures and are not really going anywhere past 180 degrees."

 

Vache notes that other HTFs, including synthetic ones and mineral oil or silicone-based ones, function in unique ways. "These HTFs have varying operating temperatures and different applications," he says.

 

This table compares the major characteristics and performance features of glycol-based HTFs against synthetic, mineral oil, and silicone heat transfer fluids:

 

HTF Type	Key Properties	Operating Temperature Range	Thermal Stability	Viscosity	Heat Transfer Properties	Cost
Glycol	Freeze Protection Thermal Stability Corrosion Resistance	-60°F to 250°F	Good	Low	Very Good	Low
Synthetic	Oxidization Stability Strong high-temperature performance	-75°F to 750°F	Excellent	Wide Range	Excellent	High
Mineral Oil	Cost-effective Most common in food and textile industries	0°F to 575°F	Moderate	Moderate	Good	Varies
Silicone	Thermal Stability Non-toxic Most common in medical and pharmaceutical industries	-150°F to 750°F	Excellent	Wide Range	Excellent	High

 

 

 

Glycol Heat Transfer Fluids and their main applications

With respect to HTFs, glycol heating systems include two main types: ethylene glycol and propylene glycol.

 

What is ethylene glycol?

Ethylene glycol is a viscous, colorless, and odorless liquid made up of carbon, hydrogen, and oxygen. It offers:

 

• Excellent heat transfer properties
• Lower viscosity and lower temperatures
• High thermal stability for extended system performance

 

Ethylene glycol is toxic and potentially harmful to humans, animals, plants, and ecosystems. It therefore requires more stringent handling practices.

 

What is propylene glycol?

Propylene glycol is also composed of carbon, hydrogen, and oxygen, but it has an extra carbon atom and a slightly different molecular structure. Its performance profile is broadly similar to ethylene glycol, but with a few important differences. These include:

 

• Higher viscosity
• Slightly lower thermal efficiency
• Wider safety margin

 

Propylene glycol's low toxicity makes it better-suited to applications in the medical, pharmaceutical, and food and beverage industries.

 

 

Glycol heat transfer fluid applications

According to Vache, ethylene and propylene glycol HTFs are widely used in building HVAC systems, hospitals, universities, data centers, geothermal and renewable energy systems, and the oil and gas industry. "We also learned during the pandemic that our infrastructure and food distribution systems were not that strong, and we now see cold storage popping up as an application throughout the food and beverage space," Vache adds.

 

This table summarizes the major industrial applications for glycol heat transfer fluids:

HVAC and Building Services	Renewable Energy Systems	Oil and Gas	Food and Beverage
Closed-loop HVAC systems Field subsurface heating Cold room dehumidification systems Ice rinks Refrigeration warehouses Data centers	Solar power systems Geothermal heat pump systems	Thermal energy storage Line heaters	Meat and poultry seasoning Dairy plants Cold storage

  

 Ethylene or propylene glycol: Which should you choose?

In some cases, you can use your choice of ethylene or propylene glycol without impacting performance. In others, one formulation or the other clearly makes a better option.
 

"With ethylene glycol, you're going to have greater thermal efficiency," Vache explains. "That means the heat is going to be able to move a little better through the system." If optimizing thermal efficiency is a priority and all other considerations are equal, ethylene glycol is probably your best option.

 

However, there's an important caveat. "Ethylene glycol is deemed toxic and hazardous," says Vache. "Propylene glycol has a lower toxicity and it's generally recognized as less toxic compared to EG. That's why you're more likely to find it anywhere there might be incidental contact with people or food."

 

For a quick comparative summary of the major functional differences between ethylene and propylene glycol, refer to this chart:

	Ethylene Glycol (EG)	Propylene Glycol (PG)
Thermal Efficiency	High	High, but slightly lower than EG
Toxicity	Toxic to humans, animals, and plants	Lower toxicity, with non-toxic formulations available Suitable for food, beverage, and pharmaceutical uses
Additional Benefits	Costs less than PG	Generally recognized  as safe (GRAS)
Application Considerations	Use if environmental safety is not a major consideration	Use if any contact with people, animals, ecoystems, food or beverages could occur

 

 

Heat transfer fluid FAQs

Some common questions about glycol HTFs include:

 

What color should glycol heat transfer fluid be?

Color distinctions can help you identify the type of glycol HTF you have. They also give indications about the product's safety and toxicity, and can aid in leak detection and preventing contamination.

 

"Typical industry standards have non-food-grade versions of propylene glycol as a fluorescent yellow," Vache specifies. "For food-grade versions, you'll find either a clear or an orange color."

 

My glycol smells foul. What does this mean?

"That's not so good," according to Vache.  "If you're having some foul odors in your glycol, that means there could be some bacteria growth." Should this occur, you may need to flush out your system and replace the contaminated glycol with fresh fluid.

 

"There are ways to correct the situation before things get that far," he says, "but it would take a lab analysis to understand it better."

 

What is the recommended course of action if ethylene glycol may have become cross-contaminated with propylene glycol?

"Take a look at whether you're around people or food," Vache says. Ethylene glycol is toxic, and you should err on the side of caution if there's any possibility of accidental cross-contamination.

 

If ethylene and propylene glycol HTFs are ever mixed, intentionally or otherwise, know that the two fluids are compatible from a functional perspective. However, you must treat the entire system as toxic, since it now contains ethylene glycol.

 

How should I change from ethylene glycol to propylene glycol due to environmental liability concerns?

"First, check with your local environmental protection authority (EPA) to see what is and is not acceptable," Vache advises. "From there, you can flush the system and hire a third party to haul your ethylene glycol away if you're dealing with a volume that goes beyond what's acceptable in your local EPA's handling standards."

 

Once you complete the system flush, you can then introduce the propylene glycol.

 

How will storage in direct sunlight affect glycol heat transfer fluid?

In a recent research project, Vache investigated how glycol HTF held up after six months of exposure to the Arizona sun. "We discovered that the glycol wasn't the same color and it had some unusual properties," he said. "I would strongly recommend using a tarp if you're going to store it outside."

 

Are there regulations against draining ethylene glycol into a sewer?

As a general rule of thumb, you should never dispose of industrial fluids or chemicals into sewer systems or sewage drains.

 

"Consult your local EPA for specific rules on how to dispose of ethylene glycol," Vache says. "In general, you usually need to hire a third party to handle disposal of more than 500 pounds or about 55 gallons of HTF."

 

Learn more about HTFs and browse OWI's industry-leading product lineup

OWI's Thermal Charge^Ⓡ product line includes an extensive catalog of ethylene glycol and propylene glycol heat transfer fluids with advanced performance capabilities. Every Thermal ChargeⓇ product delivers:

 

• Superior corrosion protection
• Extended fluid life
• Specially formulated inhibitors for enhanced operational efficiency

 

You can explore all the technical details in this Thermal Charge^Ⓡ spec sheet, which explains product properties and their specific use cases in full depth. 

 

Vache recently joined HPAC Engineering for an informative webinar presentation that explains everything you'll ever need to know about thermal transfer fluids. You can access the free webinar on the HPAC Engineering website.