Frequently Asked Questions
About Glycol-Based Heat Transfer Fluids
This depends on a number of factors, including the temperature range, the materials of construction, whether the system is open or closed, and if it is pressurized. These answers to these questions will dictate if you should use a thermal oil (hydrocarbon), a brine (salt solution), a glycol, or another chemistry.
Once the basic fluid category has been selected, it’s time to consider other properties: viscosity, thermal conductivity, toxicity, flammability, durability, cost. The best way to pick a fluid for your specific system is to speak with an expert.
The number one difference between the two glycols is that ethylene glycol is toxic and propylene glycol is not, allowing propylene glycol to be purchased as food grade in addition to the technical or industrial grade.
Propylene glycol has higher viscosity than ethylene glycol, especially at lower temperatures, making it the less thermally efficient of the two. Propylene glycol is also more expensive than ethylene glycol. Because of this, propylene glycol is most often used in applications where human or animal contact is expected and toxicity is a primary concern.
Glycol is almost never used at full strength. The goal should be to minimize the amount of glycol in the fluid mixture, while maintaining the necessary freeze protection and corrosion inhibitor content.
For example, if the freeze protection needed is -20°F, then the minimum amount of propylene glycol needed is 46% by volume (47% by weight). You can consult the technical data sheets provided by the manufacturer to determine the percentage of glycol that will give the desired freeze protection.
Dynalene sells glycol at full strength, which is more cost-effective but requires the user to do their own dilution on-site, or as a pre-mixed solution which can be added to a system with no additional water required.
Corrosion inhibitors have multiple functions in a fluid. The first and primary function is to stop the metallic parts in a system from corroding. The second is to extend the life of the coolant itself. Glycol-based coolants will degrade over time to form acids. Corrosion inhibitors provide buffering to help mitigate the effects of these acids, and some may actually slow the degradation.
Even if you have a stainless steel or non-metallic system, you should consider using an inhibitor to protect the fluid. The only time Dynalene recommends not using an inhibitor is when there is galvanized (zinc-coated) piping or other parts in the coolant loop.
No. When glycols are manufactured, they are clear and mostly colorless. The color in a glycol is added to improve leak detection. Individual manufacturers may associate certain colors with certain products, but this practice is not universal and color choices may vary across manufacturers, so color should not be used to determine the type of glycol in a system.
This is a difficult question to answer because of the range of different variables that determine coolant longevity. Temperature range, materials of construction, and fluid maintenance are all factors affecting coolant lifetime. Some Dynalene customers have systems where the original glycol has lasted more than 15 years. On the other end of the spectrum, for example in systems that run at very high temperatures (> 300°F), the glycol may last only a few years.
One easy test is to check the pH of the fluid. Normally the pH should be above 8.0, but if it drops below that you should have it tested professionally. Next, you can use a hydrometer or refractometer to determine the glycol percentage, to see if your fluid has significantly changed in concentration.
Finally, if you observe a large number of particulates in the interior of the cooling loop (a possible indicator of severe corrosion) then a thorough testing of the fluid is necessary. Based on the test results, the glycol manufacturer can recommend any adjustment to or change-out of the fluid.
It is always good to have a filtration system (in-line or slip-stream) in the heat transfer fluid circulating loop. Fluids over time accumulate debris, corrosion products, and precipitates due to contamination, system corrosion or fluid degradation. If the fluid is not filtered then it may lead to system clogging, a reduction in heat transfer capability, and even accelerated corrosion.
Aluminum is a great material to use in a heat transfer fluid system due to its light weight and excellent heat transfer properties. However, typical corrosion inhibitor packages may not work to protect the system. The pH of common glycol-based heat transfer fluids can be too high for aluminum, which is least likely to corrode at a pH between 4.5 and 8.5. Options for addressing this include adjusting the pH of your fluid or adding an aluminum-specific corrosion inhibitor.
It is not always straightforward to call a system open or closed. A truly open system is a bath that is completely open to atmosphere. A truly closed system is air-tight and completely purged of air. There are a number of different types in between, such as covered baths, which limit the amount of air that contacts the fluid but do not eliminate it, or systems using reservoirs with an air-filled headspace.
The recommended temperature range for an open system versus a closed system is based on the fluid vapor pressure (which affects boiling or evaporation), moisture condensation from humid environments, and the potential for oxidation and/or degradation due to oxygen exposure.
You can use properly inhibited glycols at high temperatures up to 350°F.
Typical inhibited glycols are generally rated up to 250°F. At temperatures higher than 250°F, glycols begin to degrade severely, forming acid by-products. However, with proper buffering to maintain the pH and corrosion inhibitors to protect the metal surfaces, a glycol can be used at high temperatures.
A fluid’s freezing point is the temperature where ice crystals can begin to form. Slush may form in the lines, but the fluid volume will remain stable. The burst point is the temperature at which the fluid will freeze all the way through, and the resulting ice can expand to break pipes or damage other parts of the equipment. Increasing the percentage of glycol in a heat transfer fluid will lower the freezing and burst points.
When using glycol/water mixtures it is important to look at how cold the fluid will get and how it will be used at that temperature. Will the fluid circulate, or will the system be shut down? How long and how frequent will the low temperature periods be? All of these things should be considered when choosing a glycol percentage for your system.