Heat Transfer Fluids

Oxidative decomposition means that oxidation splits the oil molecules. The most common reason for oxidative decomposition processes in thermal oil systems is atmospheric oxygen. However, other oxidising agents like PVC dust may have the same effect. This process is negligible below 60 °C, but the more the temperature exceeds 60 °C, the higher the decomposition rate and the more intense the oxidation process.

Oxidative decomposition can be prevented using nitrogen blanketing (for example in the expansion tank) or a “cold oil trap”, which prevents the ingress of air. The effect is two-fold: Both oxygen and moisture from the atmosphere will be kept away from the heat transfer fluid.

The degree of oxidative decomposition can be detected by an oil analysis measuring the acid or neutralisation number.

Thermal decomposition is referred to when the oil molecules are split by energy input. The decomposition rate depends on the molecular structure and the maximum supply temperature of the thermal oil (in properly designed equipment). For this reason, different operating temperatures need different chemical oil structures. General guideline: If the maximum supply temperature is below 300 °C, as a rule, mineral oils are used. If the maximum supply temperature exceeds 300 °C, as a rule, synthetic aromatic heat transfer fluids or Silicone oils are used.

The decomposition rate follows the rule of Arrhenius. This means that a temperature increase of 10 °C doubles the decomposition rate and cuts the operational lifetime by 50%. Reducing the temperature by 10 °C, however, halves the decomposition rate and doubles the operational lifetime.

This group of products is manufactured by synthesis, i.e. predefined reactants are used to produce specific products using certain production processes. The term “aromatic” means that the products contain one or more aromatic benzenetype rings.

The structure of an aromatic ring with its delocalised electron system is thermally extremely stable, which means that bulk temperatures of up to 400 °C can be realised. This gives a significant advantage in comparison to single bond hydrocarbon chains. Not all synthetic aromatic oils have pure ring structures. Many heat transfer fluids of this type do have aromatic parts with connected chain structures. The greater the share of aromatics and the smaller the share of the chain part, the more thermally stable the product.

The special structure of aromatics is also the reason for their good dissolving capacity. This is why they are used as cleaning fluids.

This product group includes synthetic aromatic fluids that can be used in the vapour phase and in the liquid phase. FRAGOLTHERM^® DPO and FRAGOLTHERM^® F-LT belong to this group.

However, nobody will get a food grade approval for this type of product.

Mineral oils represent the largest group of thermal oils. In general, the mineral oils that are used are either paraffinic or naphthenic. Paraffinic oils have a higher flash point and a higher boiling point; on the other hand, naphthenic oils have better low temperature characteristics and lower pour points. Compared to synthetic aromatic heat transfer fluids with a high aromatic content, the thermal stability is limited, as mineral oils mostly consist of molecules with single bonds in chain or ring structures without any aromatic character.

Unlike synthetic aromatic heat transfer fluids, mineral oils are distilled from crude oil in the refinery. They are natural products that consist of a large number of different molecules. Products offered on the market may have very different compositions and qualities.

Ordinary mineral oils are generally the most cost-efficient thermal oils for supply temperatures up to 290 °C. Specially refined and hydro isomerised products may be suitable up to 305 °C.

One special type of mineral oils is known as white oils. They are produced from mineral oils by further elaborate procedural steps. Due to their inertness and the purity achieved in this process, these materials can achieve food grade classifications and certificates. The classification HT1 by the American NSF or the European INS, for example, means that a thermal oil is approved for applications where unintentional contact with foods may occur. In addition, it is possible to obtain Halal and Kosher certificates.

The thermal stability is higher than that of ordinary mineral oil. FRAGOLTHERM^® FG-35 is a heat transfer fluid with food grade certificate and is used at supply temperatures of up to 305 °C.

A special type of thermal oil for the food industry is poly-alpha-olefins (PAO). They are produced by synthesis. These products may also receive approvals, but their thermal stability is significantly worse than that of white oils.

Polyglycol is a general term for polyalkylene glycols, such as polyethylene or polypropylene glycols. Due to their chemical structure, small amounts of water can be bound and this avoids significant outgassing during operation at >100 °C. In addition, they are soluble in cold water; the heat transfer fluid can easily be washed or flushed out.

Formulated polyglycols are also popular for tempering units operated in atmospheric contact, as they feature a good resistance against oxidative decomposition, which in turn leads to less fouling. The application range, however, is limited to approx. 200-250 °C, depending on the product.

Using flushing or cleaning fluids can make sense when commissioning or during an oil change. The difference between flushing and cleaning fluids is:

• Flushing fluids can be used to remove residue from fitting after commissioning, as well as to remove oil films adhering to the inside walls after an oil change. Flushing fluids do not have any dissolving capacities. This means that deposits remain in the system.
• Cleaning fluids, however, have good dissolving capacities, so they are able to dissolve oil sludge. They meet their limitations, however, when there are hard baked coke residues.

Generally, cleaning fluids are based on synthetic-aromatic structures, which is why no representatives of this group have received food grade approval. Therefore, the food industry uses almost exclusively flushing fluids.

Flushing liquids with HT1 approval are mainly used before the unit is filled with the final HT1 thermal oil. This is especially important for new systems in the food industry in order to flush any residue from the fitting process out of the unit. It is also used when switching from a conventional thermal oil to a HT1 oil (e.g. white oil), in order to safely and reliably flush out old oil residue.

The classification HT1 by the American NSF or the European INS, for example, means that a thermal oil is approved for applications, in which unintentional contact with foods is possible.

Our FRAGOLTHERM^® FG-33 flushing fluid is compatible with materials conventionally used in heat transfer technology.