Heat transfer coefficients for thermal design of heat exchangers are taken from equations in laminar or turbulent flow. The equations used to calculate the Nusselt numbers in tubes do not agree in the transition region between laminar and turbulent flow. There is a gap at Re = 2300. A linear interpolation between the Nusselt numbers at Re = 2300 and Re = 4000, each resulting from well-known equations is presented. The linear interpolation is based on the results of a paper published recently. New experimental findings about the Nusselt numbers in tubes at turbulent flow again confirm the respective equation. (C) 2013 Elsevier Ltd. All rights reserved.