IMPROVED PERFORMANCE CHARACTERISTICS OF A FIN-TUBE HEAT EXCHANGER USING HIGH VAPOR FRACTION AND TURBULENT (HVFT) ENTERING FLUID

摘要

Thermal expansion valves have long dominated commercial air conditioning and refrigeration systems largely due to a system price-to-performance advantage over alternative methods. Significant development has evolved refrigerant-to-air heat exchanger design in the past decade and refrigerant metering devices have improved to include electronic expansion valves to enhance coil performance by reducing superheat requirements at the exit of evaporator coils. Nevertheless, thermal expansion valves remain the dominant technology used in commercial equipment. Research over the past few years in bi-phase fluid flow visualization entering refrigerant-to-air evaporator coils fostered interest in testing the affects of radically different liquid/vapor fractions in combination fundamentally changing refrigerant flow regimes. Field tests of several refrigeration systems operating with thermal expansion valves were compared to the same systems with a device added to alter the vapor fraction and flow regime of entering refrigerant to the evaporators were completed over the past year. The results show improved heat transfer coefficients and redistribution of frost on the evaporator coils permitting longer periods between defrost. Comparative psychometric chamber testing of a refrigeration coil first fed with a conventional thermal expansion valve and subsequently fed with a device added to alter the vapor fraction and flow regime resulted in improved coil performance of approximately 20%. These results indicate that strategically changing the vapor fraction and flow regime of two-phase refrigerant flow entering refrigerant-to-air evaporators can significantly increase heat transfer performance and expand operating times before defrosting is needed.