Analysis of heat transfer characteristics of tube-in-tube internal heat exchangers for HFO-1234yf and HFC-134a refrigeration systems

Abstract

This paper presents the experimental and theoretical evaluation of heat transfer characteristics of a tube-in-tube internal heat exchanger for R1234yf and R134a refrigerants. The relationship between COP and overall heat transfer coefficient, influence on length and effectiveness of the heat exchanger, annular space, and pressure drop was investigated under operating conditions between −6.4 °C and 6.4 °C evaporation and 46 °C condensation temperature. The result showed that, under the same conditions, the COP of R1234yf increases by 3.78% and that of R134a increases by 2.11% when an internal heat exchanger is used. The system performance, especially for the R1234yf system, is sensitive to overall heat transfer coefficient, effectiveness, heat capacity, and latent heat of the refrigerant. The overall heat transfer coefficient of the tube-in-tube internal heat exchanger for R1234yf is lower than that of R134a by 11–17%. For both refrigerants, the large annular space of the heat exchangers tends to provide low heat transfer coefficients due to laminar flows through the annular space, which is not suitable for enhancing COP. Finally, the exergetic COP increases with the effectiveness of the internal heat exchanger, that is 4.4% for R1234yf and 1.35% for R134a.