Understanding Heat Transfer Fluid Degradation
Heat transfer fluid (HTF) systems use HTFs as a heat carrier that circulates through a closed system that starts with a heater and ends with an object being heated. The process of heat transfer refers to the transfer of heat from a higher temperature object to a relatively lower temperature object. This process is maintained until the object being heated reaches thermal stability, and is vital in the process of generating homogenous end products, from processed foods to thermoplastic polymer resin used to make fibres for clothing, and containers for liquids and foods.
As you might expect, there are many applications for high temperature HTFs and a wide variety of fluid chemistries are needed to suit the processes in which they are used. The composition of high-temperature fluids can be organic (for example Globaltherm M); highly-refined organic fluids (for example Globaltherm FG); or synthetic (for example, Globaltherm Syntec). The choice of fluid is affected by many factors including the cost, the type of process application, safety, the particular system requirements of the fluid (such as thermal stability and heat transfer efficiency), and particular system design features.
In our experience, few companies understand that servicing their HTFs and HTF systems can sustain their longevity.
For example, in extreme cases, a fluid with a high carbon level may cause the carbon to weld to the internal surfaces of plant heaters and pumps and in such cases will mean that they will need to be replaced. The cost is multi-faceted, as the manufacturer would have to shut down the plant, stop production, replace plant parts, flush the new system once repaired to remove the old fluid and, lastly, fill the system with a virgin HTF.
The objective of fluid sampling is to chemically analyse the HTF to gain insights into its status and, where necessary, take proactive corrective measures. These include a wide array of techniques such as complete or partial (dilution) replacement of the fluid. In the case of complete replacement it is important to consider using a synthetic fluid rather than a mineral fluid as these tend to have a higher purity and better thermal stability. Hence, a thermal degradation can be slowed using a well-engineered synthetic HTF. Contamination of the HTF can be managed by installing fine pore filters which remove circulating particles that may have originated from environmental contamination during construction or from wear metals circulating through the system. Water and thermal degradation byproducts can also be removed through the installation of temporary or permanent distillation columns. Sustained use of a HTF at high temperature (often defined by the upper operating temperature of a fluid, and Globaltherm, for example, has fluids operating to up to 320–600oC) leads to thermal degradation and formation of by-products. These are commonly referred to as light and heavy ends, and are in fact short- and long-chain hydrocarbons. Long-chain hydrocarbons can be detected in a fluid through changes in carbon levels. Heavy ends are detected as a high carbon in a HTF and the system problems they cause relate to welding on to internal surfaces. During operation this can lead to reduced heating efficiency and increased energy consumption and cost. Heavy ends can also lead to reduced thermal conduction due to the insulating effect of carbon and can also cause localised hot spots on heating surfaces, which can lead to heater failure.
Meanwhile, the presence of volatile, flammable light ends reduces the flash temperature of the HTF fluid. Light ends are problematic to any system as they boil and ignite at lower temperatures than the operating temperature of the HTF. Light ends have the potential to cause cavitation in thermal fluid system pumps and cause operational problems. Light ends are detected from the reduction in the ignition point (flash point) of the HTF and a potential fire hazard. Manufacturers have a duty to manage light ends in line with DSEAR/ATEX Health and Safety Regulations.
A further problem is the degradation of HTF in the presence of oxygen. Indeed, this is the most common factor that threatens every hot fluid when operating above 60oC (from 60oC up to the maximum operating temperature) and in contact with the atmosphere (air). This reaction forms acids in the fluid and these form sludge deposits and will restrict or even block HTF flow, foul a HTF system and, as mentioned, reduce fluid life. This is a problem for all systems, and installing a nitrogen blanket is advised to prevent this reaction. Indeed, for each 10oC rise in a fluid’s temperature, the rate of oxidation doubles. This is, surprisingly, quite a common complication and is detected through acidification of the fluid and also soft carbon formations where the fluid is in direct contact with air. This degradation can be observed in a sampled fluid or seen (when a system is shut down and drained) inside the system or on the external surfaces of the system.
In order to properly assess the HTF in your plant, it is imperative to:
- gain a true representative sample of a HTF whilst it is hot and circulating; and
- routinely assess light ends, heavy ends and oxidative status to get a representation of the HTF’s status over time.