The sole purpose of any HTF is to transmit heat from one location to another and in manufacturing, such as the chemical and pharmaceutical industries, this is from a heat exchanger to a source requiring heat during the production phase. Hot heat transfer fluids can generally be divided into two groups - mineral or synthetic and it is very common to compare the relative characteristics of these fluids, especially when choosing a new fluid as these are very expensive investments. Synthetic fluids are generally considered to be superior to mineral fluids, however, there may be scenarios where the use of a mineral fluid may be appropriate.
Decision 1 – Choosing steam or an engineered heat transfer fluid
Mineral and synthetic HTFs are viable alternatives to water/steam and the preferred choice at temperatures above 200 degrees C (392 degrees F). At higher temperatures they can be operated at much lower pressures than steam – i.e. at a temperature of 343 degrees C (650 degrees F) steam has a vapour pressure 100-to-200-hundred times that of a synthetic terphenyl-based fluid HTF such as Globaltherm Syntec. Mineral and synthetic fluids are also preferred at sub-zero temperatures as their freezing points tend to be lower than water. For example, Globaltherm Syntec has a freezing point around -28 degrees C [-18.4 degrees F]. HTFs also tend to have a higher purity and less reactive/corrosive to system pipes and components.
Decision 2 – Ensuring the fluid is appropriate for use in the respective sector
In the food sector, there are food-grade HTFs and approved for incidental contact with foods during manufacture and have physical properties that mean they are colourless, non-toxic, non-irritating and non-fouling. In the chemical sector a fluid needs to be matched to the system and the requirements of the process and this means the operation may be run using either a mineral or synthetic HTF. Appropriateness may also include restrictions imposed in the sector or by company policy and insurance.
For example, the ability to store and handle the fluid on-site. As a general rule, mineral HTFs are less restrictive in their handling requirements and less hazardous to humans and the environment than more highly refined synthetic chemicals, so this needs to be part of the decision making process. Equipment manufacturers may also recommend the use of particular fluids. This is something that needs to be considered during the buying process.
Decision 3 – The upper operating range for the fluid
The operating temperature for an HTF depends on its base chemistry and purity. Mineral HTFs tend to have a lower maximum operating temperature than their synthetic equivalents. So, the fluid needs to be matched to the upper operating temperature of the system. This is also important to the potential effect of prolonged operations at high temperatures on the inevitable ageing of the fluid. Synthetic fluids can also operate at higher temperature and better resist thermal degradation – for example Globaltherm Omnitech can operate up to 400 degrees C [752 degrees F]).
Decision 4 – The key product features of the fluid
Key criteria for an HTF include high temperature thermal stability, product purity and heat transfer efficiency.The thermal stabilities of mineral and synthetic HTFs differ and so do mineral HTFs. For instance, lower quality mineral HTFs (e.g. group I base oils) are considered to be less stable (i.e. a higher risk of fouling) and to have a lower maximum operating temperature (260 degrees C [500 degrees F]) than group III base oils (~316 degrees C [600 degrees F]). So, the choice of an HTF is a trade-off between cost and high thermal stability and performance. Mineral HTFs are a good trade-off in this situation as they are cheaper than synthetic HTFs.
Decision 5 - The ancillary features of the HTF
Ancillary features need to be considered. These include viscosity, expansion rate, flash points and resistance to oxidation.
- Viscosity: The objective is to get a fluid that has a low viscosity at low temperature as this affects the lowest start-up temperature for the system. In certain situations, mineral-based HTFs may have an advantage over synthetic fluids, operating down to lower/sub-zero temperatures than synthetic fluids.
- Expansion rate: The fluid manufacturer should be consulted for the given rates of a fluid’s expansion as this is relevant to the rate of expansion of the fluid and whether the system is sufficiently designed to accommodate the fluid as it expands during operations.
- Flash and fire points: A decline in closed flash point temperature indicates a rise in the formation of short-chain hydrocarbons (also known as ‘light-ends’) and their formation can signify thermal degradation. This rate tends to be faster for mineral HTFs. However, this can be slowed by operating at below the maximum operating temperature and the use of routine condition monitoring of the fluid to monitor and plan interventions to slow the overall process.
- Resistance to oxidation: Oxidation of saturated hydrocarbons leads to the formation of polymerisation products (e.g. sludge) which can accelerate fluid breakdown and system fouling. Again, HTF condition monitoring can be used to detect these effects and be used as a basis to plan interventions to slow the process and limit its overall impact on the fluid and system.
It is important that end-users choose an HTF based on ‘value’, which is ultimately a trade-off between fluid cost and fluid performance. Once a decision has been made to use a high temperature HTF, the end user/buyer needs to ensure the fluid is appropriate for use in their facility and sector.
Author Chris Wright is a research scientist for Global Group of Companies. He can be reached at chrisw@globalgroup.org.
