Piggyback heat
An overview: thermal fluids for process technology

Thermal fluids are used in areas of process technology where direct heating and cooling are not possible. This article will briefly introduce important thermal transfer media such as water, water/glycol solutions, mineral and synthetic fluids.
Klaus Beverungen
Liquid or vapor media for transporting heat are referred to as thermal fluids. They are used either for supplying heat (in order to heat objects) or for removing heat (in order to cool objects). The chart shows the operative temperature ranges of liquid and vapor thermal media.

The requirements on thermal media naturally depend on their use, but in general the fluids must meet the following criteria:

good heat transfer and heat transmission properties;

compatibility of the fluid with the materials installed;

no corrosive effect;

minimal sensitivity to foreign substances.

If high temperatures have to be borne, the fluids should be characterized by low flammability (high flash and ignition point), a high boiling point and low vapor pressure, and good overall thermal and oxidative stability. Uses at low temperature require a low pour point (when the medium is still just about free-flowing) and low viscosity.

In general, thermal fluids should be ecologically harmless (leaks!), as neutral as possible in terms of odor and easy to treat and recycle after use. As no thermal medium meets all requirements at once, a specialist must select the appropriate product for the respective application.

Water and water/glycol mixtures

Water is an almost ideal thermal transfer medium for use in unpressurized applications from above freezing point up to 100 °C and offers key advantages over other media, e.g. up to twice the thermal capacity and up to five times the thermal conductivity, combined with low viscosity. In addition, water is not flammable, has a low cost of procurement and disposal and poses a low ecological risk in the event of leaks.

Its disadvantage is the risk of corrosion: the corrosion of metal materials can be largely eliminated by mixing additives to the water or installing non-corrosive materials. However, anti-corrosive additives result in greater ecological and toxic risks in the event of leaks.

It must also be borne in mind that substances dissolved in water, such as iron, chloride, nitrite and calcium carbonate, are deposited at higher temperatures. This reduces the cross-section of pipes and impedes the transfer of heat between the thermal medium and the heat exchanger. This can be remedied by de-ionizing the water. If water is to be used as a thermal media at temperatures in excess of 100 °C, this is only possible under pressure at correspondingly high technical expense. Even at 200 °C, for example, a pressure of 15.5 bar is required.

Water-glycol solutions can be used for cooling and in a temperature range from –50 to 120 °C (continuous operation). In systems with forced pump circulation, such products are used in practice in the low-temperature range down to about –25 °C. Water/glycol products combine the good usage properties of water with the additional properties of a glycol additive. Water/glycol additive solutions of this kind avoid the dreaded scaling produced by water and prevent corrosion.

Mineral thermal fluids

Depending on what type of hydrocarbon determines the physicochemical properties of the compound as a whole, a distinction is made between paraffin-based, naphthene-based or aromatic versions.

The thermal and oxidative resistance and the pour point, boiling point and flame point can differ widely.

As aromatic compounds have a greater tendency to react with oxygen due to their double bonds, they are less resistant to oxidation than compounds based on naphthene or paraffin. On the other hand, cyclic compounds such as those that occur with aromatic compounds and naphthenes have greater thermal stability than the chain compounds in paraffins.

An adequate level of oxidation stability can be achieved using additives or by shutting off the oxygen in a closed system. When choosing thermal fluids based on mineral oil, it is important to know that paraffinic mineral oils have a higher flash point and boiling point, but poorer low-temperature behavior than naphthenic mineral oils. Products known as “white oil” are a particular version of thermal fluids based on paraffinic mineral oil. In the USA, products of this nature are placed in the HT1 category by the NSF (National Sanitation Foundation), i.e. they are approved for direct contact with foodstuffs.

Synthetic thermal fluids

These can be categorized as polyalkylene glycols, polybutenes and aromatic hydrocarbons. Some of the aromatic hydrocarbons can be used in both the liquid and the vapor state. Vapor thermal media have the advantage that it is possible to heat large areas or complex smaller preforms at a constant temperature at all points of the heat exchanger. The best known and longest used substance consists of a eutectic mixture, biphenyl/diphenyl oxide. Products made from benzyltoluene and diethyl benzene can also be used. The notable feature of the product based on diethyl benzene is that at –60 °C the viscosity only reaches
7.1 mm2/s, making this fluid particularly suitable for heating and cooling processes. The actual area for the use of synthetic aromatic thermal fluids in closed pressurized or unpressurized systems is in the 300 °C+ range (at these temperatures the use of mineral oils is in most cases no longer economically viable): alkyl benzenes up to 320 °C; hydrated terphenyls up to 345 °C; benzyltoluene up to 350 °C; dibenzyl toluene up to 360 °C; biphenyl/diphenyl oxide mixture up to 400 °C. If the flow temperatures are reduced by 10 °C in each case, the life can be expected to be nearly doubled. From this group, practice has shown that the substances benzyltoluene, dibenzyl toluene and the eutectic mixture of biphenyl and diphenyl oxide can be reprocessed, thereby making an ecological and financial contribution to saving resources.

Caution: Thermal fluids based on aromatic compounds are sensitive to oxidation from temperatures upwards of 80 °C. Expansion tanks that come into contact with air should therefore be superimposed with nitrogen wherever possible.

Conclusion: From cost-effective water through to the specifically “composed” synthetic thermal fluid, there is a solution for every heating and cooling task in process technology. There can be high follow-up costs if the wrong thermal medium is chosen, so it is essential that the operator seeks advice from the manufacturer.n