Central to the problem of heat exchangers design is the prediction of pressure drop and heat transfer in the exchanger passages. In order to make such predictions for non-Newtonian fluids, it is necessary to know the relation between the viscous properties of the fluid and the wall shear rate. This is done by assuming a constitutive equation which relates the local apparent viscosity of the fluid to the local shear rate. An often used constitutive equation because of its simplicity is the power law equation which however is only valid for particular fluids over a limited wall shear rate (or duct Reynolds number) range.This study concerns the limits of applicability of the power law equation. The method involves a considerations of a more general equation which has power law and Newtonian behavior as asymptotes. The more general equation contains an operating and property parameter whose values specify the operating shear rate range and thus the appropriate viscous properties. It is concluded that using the power law equation outside of its applicability range can lead to serious errors in predicting the pressure drop and heat transfer. This conclusion is illustrated by specific examples for both forced and free convection in heat exchanger ducts (Brewster and Irvine , Irvine ).