Identification and reduction of heat loss in heat exchangers are vital to energy performance and thermal efficiency improvements. This study presents an analytical methodology for determining entropy generation rate using experimental power curves derived for laminar, transitional, and turbulent flow heat transfer in terms of the Colburn j-factor and pressure drop in terms of the friction factor as a function of Reynolds number, viscosity ratio, and respective coefficients and powers. Experimental data obtained using a smooth circular copper tube heat exchanger under a constant heat flux boundary condition of 2 kW/m² and a Reynolds number range of 1330 to 10449 were used. The variables considered include the thermal entropy generation, fluid flow entropy generation, total entropy generation rates, and Bejan number. To enable the identification of each flow regime, these variables were plotted as a function of the Reynolds number on a log-log scale. The results compared well with the Bejan equation with an average deviation of 2%. Entropy generation was dominated by heat transfer, with a minimum contribution from pressure drop. The laminar flow regime produced the highest total entropy generation with a positive gradient as the Reynolds number increased until the commencement of transitional flow regime. The transitional flow irreversibility was lower than laminar flow and was characterized as an intermediate entropy generation with a steep negative gradient between laminar and turbulent. The turbulent flow regime generated the least total entropy generation rate as the Reynolds number increased with a shallow gradient.