ESDU 83038 gives a simplified calculation procedure for estimating the flow rate of the shellside fluid through the various leak and by-pass paths of a shell-and-tube exchanger. The reduced flow rate over the tube bundle is thus estimated. The method then predicts both the overall pressure loss and heat transfer coefficient for the shellside fluid. The procedure applies to the turbulent flow of a gas or liquid, heated or cooled without phase change, on the shellside of an exchanger consisting of a bundle of plain tubes in a square or triangular array with segmental baffles. It can be used for: (i) rating the performance of an existing heat exchanger, when the rates and inlet temperatures are known and the outlet temperatures and heat transfer rate are required; (ii) evaluating the results of tests on a heat exchanger, where the measured heat transfer rate and pressure loss must be compared with predictions; (iii) designing a new heat exchanger, where the thermal duty and maximum pressure loss are specified and a detailed design must be produced. A comprehensive practical worked example illustrates the use of the method.