This article deals with a new steady-state formulation of temperatures along a double-pipe heat exchanger in counterflow configuration when the mass flow rate is submitted to step change. The steady-state method is based on estimation of the exponential factor of temperature profile. This method, compared to results obtained from correlations, gives an alternative approach to the well-known procedures for sizing and rating heat exchangers in industrial applications, such as the epsilon-NTU and LMTD methods. It is based on the average convective heat transfer coefficients of the hot and cold sides, while the other methods are based on the overall heat transfer coefficient.