Thermodynamics analysis on a heat exchanger unit during the transient processes based on the second law

Diminishing fossil fuel resources have intensified the need for energy saving. Heat transfer is a basic method of energy delivery and convention, and heat transfer during the transient processes may be affected by the dynamic performances of heaters. Analysis of the heater performances during the transient process based on the second law of thermodynamics may show the room for the improvement in energy saving. Variations of boundaries, such as the flow rates and temperatures of the work fluids, may affect the dynamic behaviors of heaters. The variation rates, formats and ranges of the inlet work fluids flow rates and temperatures on the irreversibility and exergy delivery characteristics are discussed in this paper. The average exergy efficiency (η_E,avg) of the heater during the transient process with different operational parameters are presented and compared. The results show that, in the identical variation range, the maximum difference in η_E,avg with different flow rates and temperature variation rates of the cold work fluid are 0.3% and 0.4%, respectively. With step variation format, the maximum difference in η_E,avg for the different variation ranges of the cold work fluid is 0.85%.