The author rejects, momentarily, actual experimentation with real heat exchangers for logical cogitation and arm-chair experiments. Observed as mental images or models, these heat exchangers reveal their basic characteristics—characteristics that will be partially obscured in an actual experiment by secondary effects inherent in the equipment. These basic characteristics, once perceived, may be used to correlate by means of frequency-response diagrams many results heretofore diverse and seemingly unrelated, that have been reported in the literature. Included among such results are not only the dynamics of various types of heat exchangers but also the thermal dynamic interaction between fluids and the confining pipe used for their transport. In addition, the author warns that all too frequently the use of the average temperature in heat-exchanger dynamics is based on faulty logic.