Heat transfer fluids are substances used to transfer heat from one location to another in various applications, such as heating systems, cooling systems, and industrial processes. These fluids can be liquids or gases and are chosen based on their thermal properties, such as specific heat capacity, thermal conductivity, and viscosity. Common examples include water, oils, and specialized fluids like glycol mixtures. In many systems, heat transfer fluids help maintain optimal temperatures, improve energy efficiency, and protect equipment from overheating. They are essential in applications like solar thermal energy, geothermal systems, and chemical processing, where effective heat management is crucial for performance and safety.