is in a heat transfer system, the temperature of the main stream of heat transfer fluid in the heating unit, as distinct from film temperature, which is the temperature of the slower moving layer of fluid in contact with the surface of the heating tube or coil. Film temperature may be far higher than bulk temperature and may exceed the maximum temperature for which the fluid is recommended. The differential between the two temperatures depends primarily on flow velocity, which is a function of the design of the installation and the flow properties of the fluid. These factors must be taken into account when selecting the proper heat transfer fluid.