ABSTRACT
Through the convective film, the temperature changes in a nonlinear fash-
hot side of the heat exchanger shown in Figure 5.1, Newton’s law of cooling is written as
h h hot s,hQ h A T T ( )= − (5.1) In this equation hh is the convective heat transfer coefficient. This is an unfortunate nomenclature conflict when h is used to represent enthalpy. However, this is a common nomenclature and is used throughout this chapter. The convective heat transfer coefficient has units of Btu/h·ft2·°F or W/m2·K. Its value is meant to help quantify the resistance to heat flow through the convective film. Most heat exchangers utilize forced convection to transfer the heat between the fluids. Typical convective heat transfer coefficients for forced liquid flow are typically between 18 and 3500 Btu/h·ft2·°F (100-20,000 W/m2·K). Heat exchangers where one or both of the fluids are undergoing a phase change can experience much higher convective heat transfer coefficients: upward of 18,000 Btu/h·ft2·°F (100,000 W/m2·K). Heat exchangers that take advantage of the high heat transfer coefficients offered by a phase change are commonly known as boilers, evaporators, or condensers.
