Few environments push heat transfer fluids harder than open bath systems. Because the fluid is continuously exposed to air, it faces the most common cause of breakdown: oxidation.

Exposure That Leads to Problems

Oxidation occurs when heated fluid interacts with oxygen, creating acidic compounds that eventually form sludge. In open bath applications, this buildup collects directly in the bath rather than in reservoirs or expansion tanks. The higher the operating temperature, the faster and more aggressive this reaction becomes. Under such conditions, most fluids last only a matter of days.

Safety Measures You Can’t Skip

Open baths also present the issue of off-gassing, often seen as smoke rising from the fluid. In reality, these are lighter molecular fractions escaping as the fluid is heated. While unavoidable, these vapors should never be inhaled. Regardless of the fluid type, every bath must have proper ventilation. Keep in mind that off-gassing escalates with temperature, making adequate airflow even more critical as operating heat increases.

Fluids That Can Withstand the Conditions

Because oxidation and off-gassing are unavoidable in open systems, the choice of fluid is crucial. Duratherm specifically recommends two products designed to cope with these harsh environments: Duratherm G and Duratherm S.

Duratherm G is a polyalkylene glycol–based fluid rated for use up to 260°C (500°F). While it is prone to oxidation, it does not typically generate sludge as a byproduct. Off-gassing is moderate at lower ranges but becomes significant near the maximum operating limit. The fluid should be tested regularly for acid buildup. Although the acids produced are generally mild and non-corrosive, prolonged neglect can allow levels to rise and potentially damage system components. To prevent this, Duratherm provides a complimentary fluid analysis program to track acidity and keep it well below harmful levels.

Duratherm S is a silicone-based (polydimethylsiloxane) fluid that delivers excellent resistance to oxidation up to 204°C (400°F). Above this threshold, however, it will begin to gel when exposed to air — a process that accelerates with increasing temperature. For instance, at 177°C (350°F), Duratherm S has an expected service life of about one year. Just above 204°C, that lifespan shortens to roughly 300 hours, and at 316°C (600°F) it may last only a single day.

Other specialized fluids are available as well. Phenylmethylsiloxane, a related silicone compound, offers greater resistance to oxidation at high temperatures and longer service life, though at a much higher cost. Fluorinated fluids, such as those from Galden or 3M, are another option for bath systems but are also significantly more expensive.

Bottom Line

Operating open baths brings unique challenges that place heavy demands on heat transfer fluids. These applications require strict safety precautions, as well as careful consideration of the balance between performance, lifespan, and cost.