The outside film coefficient for a process coil in a liquid bath heater is the result of natural or free convection. Temperature variations in the fluid cause density variations. These density variations in turn cause the fluid to circulate, which produces the free convective heat transfer. For horizontal pipes and tubes spaced more than one diameter apart, the following equation, based on empirical studies, may be used to determine the film coefficient.
The fluid properties can be determined as before, except for the bath fluid coefficient of thermal expansion, which is given in Table 2-5. The density of water is one divided by the specific volume given in steam tables .