What are heat transfer fluids?
In many industries, it is often said that lubrication is the life blood of our equipment. However, there is another critical industrial fluid that is also important to think about: heat transfer fluids.
Heat transfer fluids (HTF) carry thermal energy from one source to another – to either cool a system or to heat it indirectly. These HTFs can either be gases or liquids and can operate at a variety of temperatures from below -50C to over 400C (depending on the medium).
And what kinds are there?
The three most common examples of heat transfer fluids are air, water and hydrocarbons.
- Air is abundantly available, environmentally friendly, and free. On those chilly winter days when you turn on your heater, it is air that transfers the heat to you. However, air has a relatively low heat transfer rate, which makes it difficult to use in industrial applications unless it is pressurized.
- Water, like air, is abundant, low-cost, and environmentally friendly. This is why many heat exchangers in parking garages and warehouses use steam systems. The downside to water is the phase changes it goes through at 0C and 100C making it restricted in the applications you can utilize it in.
- Hydrocarbons are another popular choice for heat transfer fluids. Paraffins, like air/water are used for their non-toxic properties in industries like food, beverage, and pharmaceuticals. Aromatics are more commonly used in colder temperature applications but tend to be more toxic. We’ll discuss this type of HTF in more detail below.
Other types of heat transfer fluids include glycols, silicone, alcohols, and nanofluids.
Characteristics of Hydrocarbon-Based HTFs
Let’s look more closely at hydrocarbon-based heat transfer fluids.
Some desired characteristics include:
- Non-corrosive
- Thermal stability
- High availability
- Chemically inert
- Low freezing and high boiling point
- Non-flammable
- Low viscosity
The quantification of these properties can be broken down into two categories:
- Physiochemical – Viscosity, flash point, pour point
- Thermal properties – Viscosity, specific heat, thermal conductivity, and Prandtl number
These properties determine the chemistry and uniqueness of each fluid much like in lubrication.
Routine Monitoring
For regular condition monitoring of the health and fitness of use of your HTFs, you should test for viscosity, thermal stability, and contamination.
Viscosity is the most important property of heat transfer fluids. As the purpose of these fluid is heat transfer, they are constantly put under thermal stressors, which is why monitoring the physical breakdown is so important. A thinner fluid is desirable for its pumpability and flow properties, but thermal sheering is possible at some of the extreme temperatures HTF’s are exposed to. Alternatively, monitoring for thickening of the fluid is just as important for similar reasons.
Another tool to monitor for thermal breakdown is acid number which measures the oxidative by-products of the fluids. This can also indicate if there is contaminants or incorrect fluid in your system.
With the high temperatures that HTF’s are exposed to, flash point is a big safety parameter to monitor. Flash point is the lowest temperature that the fluid will briefly ignite at or “flash”. As the fluid breaks down and ages, the flash point can change. For safety reasons it is important to always know this value- especially with leaks.
Monitoring for contamination is important in any fluid system, whether it be lubrication or HTF’s. Similar to hydraulic systems, heat transfer systems are closed systems. The only way contamination gets in there is if we put it there, or it is being generated internally within the system itself. Using a test like ICP will help determine what contaminates are in your fluid.
A contaminate to watch out for is pentane insoluble solids. The presence pentane insoluble solids may indicate there is contamination from dirt, corrosion products, or severe oxidative or thermal stressing. These can interact with the heat transfer properties, reducing heat transfer performance, and can impact on wear generation by leaving dirt and residue on heat transfer surfaces.
HTF Testing
Be sure to refer to your suppliers’ suggested limits for HTFs as limits can vary greatly for each fluid and application. Fluid Life offers a Heat Transfer Fluid prepaid test package to monitor for viscosity, thermal stability and contamination. If you’d like more information on setting up a routine program or your industrial equipment, contact us today.
