An extensive set of experimental data has been measured for an industrial scale exchanger configured as a countercurrent flow, TEMA(4) ’E’ Shell and as both combining and dividing mode ’J’ Shells, with one and two coolant passes. Steam and steam-air mixtures have been condensed at atmospheric and reduced pressure. Separate measurements of condensate flowrate and pressure drop in each flow path provide direct evidence of the accuracy of the currently used design assumptions. The paper shows the factors which affect condensate temperature, which is usually taken as that of the saturated vapour. It shows also the effect of shell and tube side configurations on performance. Further, evidence is presented of a flow instability in the case of ’J’ shell condensers, which causes an air pocket to develop near the vapour inlet and this has implications for good venting.