Fractionation of R-407C in Brazed Plate Evaporators at Low Mass Flux

摘要

This investigation explored the effects of fractionation and distribution of R-407c in brazed plateevaporators with an emphasis at low mass flux. Experiments were performed in parallel with R-22. A parametricstudy was performed with nominal and reduced mass fluxes, with and without distributor and with variousorientations of the evaporator. Also, operating conditions were varied to investigate the effect of refrigerant inletquality and superheat at the exit of the evaporator. This investigation took an evaporator as part of a systemapproach rather than evaluating local heat transfer.All experiments show that R-407c is a viable replacement for R-22. Overall heat transfer coefficients forR-407c were essentially equal to that of R-22 when operating at nominal conditions. U-values for R-407c at lowmass flux were up to 15% less than R-22. However, R-407c U-values were approximately 10% greater than R-22when operating with a saturated refrigerant outlet condition.Contrary to expectations, only slight fractionation was seen for all test conditions and evaporatorconfigurations. It was concluded that lower inlet quality slightly increases refrigerant composition shift. Also seenin this investigation was greater composition shift at greater mass flux for individual experiments. This conditioncould be related to significant oil hold-up seen at low mass flux.A distributor was developed in this investigation to allow better distribution when operating withrefrigerant flow in the downward direction. While downward flow did not have an effect on refrigerant compositionshift in this investigation, this orientation may be useful for other applications where more severe fractionation isseen.Other issues addressed were superheat instability and oil hold-up in the evaporator. Liquid droplets wereseen at the exit of the evaporator when operating with low superheat. Also, it was found that up to 35% of thevolume of the evaporator could be filled with oil even when operating at nominal capacity.