ESDU 95027 presents methods, based on existing correlations of experimental data, that apply in laminar, transition and turbulent flow. A full derivation of the expressions for friction factor and Nusselt number is given and sketches comparing prediction with experiment show the accuracy of the correlations to be within 15 per cent. A detailed calculation procedure is provided and its use illustrated with a worked example. Compared with a plain tube of the same internal diameter, tubes with twisted tape inserts give a greater heat transfer coefficient (particularly in laminar flow) for similar pressure loss. They are particularly useful in retrofit situations but their use in design will result in a more compact exchanger.