The heat-exchanger effectiveness is the ratio of the actual rate of heat transfer in the given exchanger to the hypothetical rate of heat transfer if the area of the exchanger were infinite. This effectiveness is given by

,

where and are the inlet and outlet temperatures of the hot fluid, is the inlet temperature of the cold fluid, and is the ratio of heat capacities of the cold (low) and hot (high) fluids, given by

.

The Demonstration plots the effectiveness of a heat exchanger for both parallel flow and counter-flow configurations versus the number of transfer units for various values of .