Which type of liquid or gas must not be used as a heat transfer fluid?

The use of flammable liquids or gases as heat transfer fluids poses significant safety risks, as they can ignite under certain conditions and create hazardous situations. The primary function of a heat transfer fluid is to facilitate efficient thermal exchange without compromising safety. Flammable substances can lead to explosive environments, especially if there is a leak or if the fluid is subjected to high temperatures.

In contrast, non-toxic, non-flammable, and even viscous fluids can be safely used in heat transfer systems if they meet other performance criteria such as thermal conductivity and stability. Therefore, identifying a heat transfer fluid as flammable is crucial in ensuring that mechanical systems promote a safe working environment while maintaining efficiency. This is why flammable substances are explicitly excluded from acceptable heat transfer fluids in accordance with safety codes and regulations.