is a circulating medium (often a
petroleum oil) that absorbs heat in one part of a system (e.g., a solar
heating system or a remote oil-fired system) and releases it to another part
of the system. Heat transfer fluids require high resistance to cracking
(molecular breakdown) when used in systems with fluid temperatures above
260°C (500°F). See bulk temperature.
Systems can be either closed or open to the atmosphere. To prevent
oxidation in a closed system an inert
gas is sometimes used in the expansion tank (or reservoir) to exclude air
(oxygen). See gas blanket. If the system is
open and the fluid is exposed simultaneously to air and to temperatures
above 66°C (150°F), the fluid must also have good
oxidation stability, since a
protective gas blanket cannot be contained.