Heat transfer fluids play an invaluable role in circulation heaters and industrial heating processes. While all sorts of processes use these fluids, there is no single fluid type that is ideal for all operations. Instead, there are a number of factors that determine the right heat transfer fluid for each process.
There are four main factors that influence what type of transfer fluid fits a specific project. The first is temperature. The minimum and maximum operating temperatures correlate directly to fluid types. Water, for instance, has a lower maximum temperature than other transfer fluids, and salt melts can operate at significantly higher temperatures.
Pumpability is another important consideration. The fluid must be able to be pumped and circulated at the desired flow rate and efficiency to maintain heating and effective operations.
Similarly, pressure requirements also determine the suitability of a heat transfer fluid. If the fluid is better suited to pressures above or below the project requirements it can lead to inefficiency..
Finally, thermal stability must be considered. If the fluid will break down under the heat of regular operation, it will change the fluid’s physical properties. While this will of course affect operational efficiency, it can also be hazardous and present a serious damage risk to equipment.
Other factors in choosing the right heat transfer fluid include:
Of all of these factors, temperature is often the first consideration. So it is through the lens of temperature that we here discuss the types of heat transfer fluid.
Water is the most inexpensive heat transfer fluid. As such, it’s almost always used where applicable. However, it comes with a set of limitations that make it ineffective for many operations. Its operable temperature range, for instance, is fairly low.
Freezing occurs at 0℃ and boiling at 100℃. As such, high-temperature operations cannot use this fluid, and low temperatures require additives or other heating solutions.
Oils can provide a greater utility range than water as a heat transfer fluid. However, natural oils still experience fairly limited temperature ranges. For applications exceeding 175℃ (350℉), it’s best to switch to a synthetic organic oil.
Synthetic organic oils provide improved thermal stability, as well as having greater operating temperature ranges. A variety of different fluid types in this category provide minimum temperatures as low as -80℃ (-110℉) and maximum temperatures of -400℃ (-750℉).
Although the minimum temperatures are lower than water, synthetic organic heat transfer is more often used in the 175℃ to 400℃ range than below 0℃. For lower temps, we more often look to inhibited glycol-based fluids.
Solutions made of water and inhibiting glycols, these heat transfer fluids are most commonly used for operations under 175℃ (350℉). They can perform, however, at temperatures as low as -50℃ (-60℉).
These can be either ethylene glycol or propylene glycol fluids depending on your application. For instance, if there is potential contact with drinking water, ethylene glycol-based fluids are ideal due to their low oral toxicity.
Using molten salts as a heat transfer fluid is ideal for high-temperature operations. They are ideal for temperatures ranging from 285℃ to 565℃ (545℉ to 1049℉). For temperatures above 565℃ chloride and fluoride salts can be used as they are more stable. However, these are more corrosive than nitrate salts.
Even at temperatures as low as 285℃, where synthetic oils are usable, salt melts may be preferable. This is because molten salts are at less risk of degradation at these temperatures and have a lower vapor pressure.
The advantages of molten salts in heating systems include their operating temperatures, low vapor pressure, and efficiency.
Picking the right heat transfer fluid for your operation is just as important as the equipment itself. To choose the right heaters and heating systems, read How to Pick the Right Thermal Fluid Heating Equipment.