Choosing a heat transfer fluid
Weighing up heat transfer fluid properties and cost
When it comes to heat transfer fluids there is no one size fits all. Their suitability depends on a variety of factors, such as the application, operating temperature ranges, thermal stability, thermal conductivity, heat capacity and the chemical composition of the thermal fluid.
To meet varying customer needs, many high performance heat transfer fluids are available on the market and it can be difficult to select the right one. Here, we share some advice on the different types of heat transfer fluid and the main considerations when making a selection.
Heat transfer fluid systems allow manufacturers to indirectly heat products across a wide range of industry sectors including pharmaceuticals, food and chemical processing. The thermal fluid moving through the system transfers heat to base, intermediate and final products during manufacturing or processing. It’s important that manufacturers choose the right thermal fluid for their system and application to get the best results.
Manufacturers can select a heat transfer fluid based on its properties, such as thermal stability and heat transfer efficiency, but must also consider the requirements of the heat transfer system and cost.
One factor on which to select a heat transfer fluid is its chemical composition, which can be an organic or a synthetic compound.
A process based on one of these heat transfer chemicals has several benefits over a traditional steam and water approach. This is because they do not require high pressures, are less reactive and less corrosive to system components.
Heat transfer fluids are typically an organic or a synthetic compound. Chemical based, synthetic, heat transfer fluids can also be based on several chemicals, including silicone, terphenyl and alkylated aromatic compounds. Synthetic heat transfer fluids, such as a propylene glycol or an ethylene glycol based fluid, are commonly chosen because they offer better heat transfer efficiency and excellent thermal stability. As a result, these fluids often have a longer lifespan than mineral fluids.
Synthetic fluids are also more resistant to fouling, which means they tend to form less coke on the internal pipework and heater and produce less light ends. Light ends, which are lower-boiling, volatile thermal oil components generated at high operating temperatures, often form as fluid degrades.
Temperature and fluid type are also important considerations. When selecting a thermal fluid, look at both the highest and lowest operating temperatures, as well as the boiling point and bulk temperature. Choosing a heat transfer fluid with a broader operating temperature range can provide better temperature control for your application.
Manufacturers should look at both the highest and lowest temperatures to ensure the fluid operates effectively during a system start up or if any of the system, such as pipework, is located outside of the facility.
Because of their chemical compositions, heat transfer fluids have differing properties. Manufacturers should be aware that the chemical composition of the fluid has an impact on how suitable a fluid is for a particular application. Though all thermal fluids will degrade over time, matching the fluid to the system and the operating temperature can help to preserve fluid life.
Synthetic heat transfer fluids, such as a silicone, have a lower propensity to form carbon than mineral based oils, offering better heat transfer efficiency, a high boiling point and thermal stability. They are also more resistant to fouling, which means they tend to form less coke on the internal pipework and heater.
For example, a eutectic based fluid containing biphenyl and diphenyl oxide, such as Globaltherm Omnitech, is generally considered to have better stability at high temperatures compared with a mineral based fluid. A manufacturer looking to operate a system at a high temperature should consider thermal stability to ensure fluid breakdown isn’t accelerated at high temperatures.
Fluid viscosity can also influence a manufacturer’s choice, as it impacts how easy it is to pump around the system and therefore how much energy is used. Synthetic fluids tend to have a lower viscosity than mineral-based fluids, so a manufacturer looking to reduce energy costs may prefer this option.
Match the application
In certain industries, including food and beverage, fluid choice may be dictated by legislation surrounding which oils are suitable for a manufacturer’s process. In this example, the manufacturer should opt for a food grade oil because it is certified non-toxic, as opposed to another fluid type. Pharmaceutical manufacturers may also want a specific heat transfer fluid to ensure product for human ingestion is safe.
Ultimately, a manufacturer must weigh up heat transfer performance with the cost of filling the system. Filling a new system, or re-filling an existing system, can be an expensive task. However, poor fluid choice can lead to inefficient system operation and accelerated thermal fluid breakdown, which can increase maintenance requirements and impact system availability and production.
By carefully considering the requirements of the system and the application, manufacturers can make an informed heat transfer fluid choice.