6 Conclusions
In the present study, we use k and p to indicate the gains and costs of the flow and heat transfer, exponent n to describe the relative importance between heat transfer and pressure drop, and EEI defined by k/p n is proposed to evaluate and grade the single-phase flow and heat transfer energy efficiency of heat exchangers. EEI is an index of the overall heat transfer coefficient per unit equivalent pressure drop. According to our study on the flow and heat transfer database, exponent n is selected as 0.31 Principle validation of EEI shows a good agreement with the Second Law of Thermodynamics. The impacts of plate heat exchanger structure, thermal conductive resistance, and fluid thermo-physical properties on EEI are analyzed, which concludes that reducing the plate thickness can effectively improve the energy efficiency of plate heat exchangers in water-water flow and heat transfer. The overall distribution f(x) = N(191.2, 22.12 ) of EEI for plate heat exchangers are concluded, and EEI can be used to quantitatively evaluate and grade the energy efficiency of plate heat exchangers. We propose EEI = k/p n and this evaluation method can be a reference for other types of heat exchangers.
