A method and apparatus for determining heat transfer effectiveness comprises measuring coolant flow rate and input temperature for coolant to the reactor, utilizing temperature values for reactant and effluent temperatures in a relationship to obtain a value for the log means temperature difference across the reactor and calculating the actual heat transfer coefficient for heat transfer surfaces of the reactor as a function of the log mean temperature difference, the heat transfer area, the coolant flow rate and additional constant factors which are characteristics of the coolant itself.