Flow and thermal convection in heat exchangers can be affected by variations in viscosity. In circular pipes the significance of this effect was expressed by Sieder and Tate's empirical formula. Similar correction can be derived for other heat transfer problems. Exact solutions for heat transfer during the non-isoviscous laminar flow of Newtonian and non-Newtonian liquids in circular and flat ducts under conditions of constant wall temperature or constant wall heat flux are analyzed. Simple generalized procedures for heat transfer coefficient and friction estimate are recommended. (C) 2003 Elsevier B.V. All rights reserved.