Heat Transfer Fluids

Traditionally, manufacturers use steam for indirect heat transfer applications. Steam based heat transfer systems may seem beneficial because water is cheap, easy to source and has a very low environmental impact.

However, to operate effectively, steam based systems require additional equipment such as surge tanks, water softener and blowdown heat recovery technology.

Instead, thermal fluid systems are simpler and cheaper because, unlike steam, they do not have to work at high pressure to maintain a constant temperature. Heat transfer oils offer excellent thermal stability at the intended operating temperature, can operate for extended periods of time and manufacturers can control the temperature based on the application.

The chemical composition of a thermal fluid is integral to its performance. Engineers often choose between two main types of fluid — mineral based or synthetic thermal oils. Chemical based heat transfer fluids can also be based on several chemicals, including silicone, terphenyl and alkylated aromatic compounds.

Be aware that chemical composition impacts how suitable a fluid is for an application. So correctly matching the fluid to the thermal oil system and application can help preserve fluid life in addition to a robust approach to condition monitoring and maintenance.

You should consider purchasing synthetic heat transfer fluids because they have a lower propensity to form carbon than mineral based oils, offering better heat transfer efficiency and thermal stability. They are also more resistant to fouling, which means they tend to form less coke on the internal pipework and heater.

Thermal fluids must operate at the correct temperature for prolonged amounts of time, so you should pick a HTF that is stable at the temperature required for your application.

You should look at both the highest and lowest operating temperatures, as well as the boiling point, when selecting a mineral or synthetic thermal fluid. The fluid must be able to work efficiently at the highest temperature but should also work at the lowest temperatures required during a system start up or if any of the system is outside of the facility.

Heat transfer fluids can operate in liquid phase or vapour phase, depending on the application. Manufacturers should consider that changing the state of the fluid also changes its stability and operating temperatures. When purchasing a fluid, manufacturers should find out its minimum and maximum operating temperatures in both phases to know if it is the best choice.

Fluid viscosity can 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 thermal fluids tend to have a higher thermal capacity and lower viscosity than mineral-based fluids, so a manufacturer looking to reduce energy costs may prefer this option.

In certain industries, including food and beverage, fluid choice may be influenced by HTF legislation. In food manufacturing, for example, you should opt for a food grade fluid because it is certified non-toxic.

Our Globaltherm FG fluid, for example, has outstanding thermal oxidation stability to meet the demands of food, beverage or pharmaceutical manufacturing.

You should also consider the fluid’s base to ensure consumers are safe from toxicity. For example, in applications where there is a chance of ingesting the product, you should consider purchasing a propylene glycol-based fluid, as it has a lower toxicity than other HTFs.

There are two ways in which heat transfer fluids can degrade.

1. Oxidation.
   A thermal fluid oxidises when it reacts with oxygen in the air by a free radical mechanism. The rate of oxidation increases with temperature and the reaction causes carbon to form.
2. Thermal degradation, also known as thermal cracking.
   When a thermal fluid is heated above the maximum film temperature specified by the manufacturer, it will start to degrade rapidly. This leads to vaporisation and the formation of carbon.

Vaporisation results in increased viscosity of the fluid, which means more energy is required to pump it around the system and this leads to increased costs for businesses.

The formation of carbon in heat transfer fluid also increases the viscosity. When its concentration reaches a certain level, it starts to deposit in a sludge on the insides of the pipework, in a process known as fouling. The sludge accumulates, particularly in low flow areas such as reservoirs and expansion tanks, and reduces the efficiency of heat exchange. This also increases costs for businesses.

A heat transfer fluid will degrade over time because it has to operate at high temperatures for long periods of time. However, you may not be able to see degradation until it impacts production.

You can slow degradation by implementing a continuous preventative maintenance programme and frequently monitor both the thermal fluid and the system. You should take a hot, closed sample, to make sure it is representative. You should then analyse the sample to understand what is happening inside the system.

We can help you create the right programme for your plant. We provide a condition based thermal fluid lifecycle maintenance programme, Thermocare, where our experts will work with you to improve accuracy when monitoring fluid, organise maintenance schedules and train staff at the facility.

Once you have chosen your fluid, you should consider how you will introduce it to the system. You cannot simply remove one thermal fluid and replace it with the next. Instead, you must drain, flush and refill the entire heat transfer system. This will increase the lifespan of the new fluid and keep the system running efficiently for longer.

Globaltherm® C1 is specifically designed to remove harsh by-products of any heat transfer fluid, such as carbon deposits, volatile light ends or loose debris to ensure there are no contaminants that may compromise the new HTF.

Price will always be important when purchasing a new heat transfer fluid for an application. The price of a fluid will depend on its properties, so the more specialised the fluid, the higher the price.

You should choose a thermal fluid that is cost efficient for the facility without compromising on the most important properties required for the application.

Selecting the correct heat transfer fluid is important. Taking the time to carefully consider what fluid will work best for your application will help improve efficiency and increase the lifespan of the HTF and system.