Basic Concept

There are three mechanisms of heat transfer: conduction, convection, and radiation. In most heat exchangers, convection will be the dominant mechanism. Conduction and radiation will generally be negligible in large heat exchangers, but radiation will be important in fired heaters.

Heat transfer across a surface by convection is given by the equation: �=���Δ��

Where: � = heat transferred per unit time (energy/time)

� = overall heat-transfer coefficient (energy/time-area-temperature)

� = heat-exchange area (area)

� = correction factor (unitless)

Δ�� = log mean temperature difference or the temperature driving force, (temperature)

Δ�� is the driving force for a pure countercurrent contact pattern in a tubular system. the correction factor � is used because most heat exchangers do not implement true countercurrent contact. Design equations for heat exchangers will use generally use some form of this equation with the appropriate modifications to account for different configurations and approximations. (Towler)