Published online by Cambridge University Press: 15 October 1999
A method is proposed to determine the thermal effectiveness of heat exchangers in situ, when one of the fluids is submitted to any kind of flow rate variations. It leads to the definition of the average effectiveness in unsteady state, which forms an extension of the classical effectiveness used in steady state. It requires an unsophisticated equipment of measurement and should lead to an easy and continuous control of the fouling of heat exchangers.