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ACRC Contract Report 41PDF


Title:Fractionation of R-407C in Brazed Plate Evaporators at Low Mass Flux
Author(s):Tallitsch, D.A.; Hrnjak, P.S.
plate evaporators
Abstract:This investigation explored the effects of fractionation and distribution of R-407c in brazed plate evaporators with an emphasis at low mass flux. Experiments were performed in parallel with R-22. A parametric study was performed with nominal and reduced mass fluxes, with and without distributor and with various orientations of the evaporator. Also, operating conditions were varied to investigate the effect of refrigerant inlet quality and superheat at the exit of the evaporator. This investigation took an evaporator as part of a system approach rather than evaluating local heat transfer. All experiments show that R-407c is a viable replacement for R-22. Overall heat transfer coefficients for R-407c were essentially equal to that of R-22 when operating at nominal conditions. U-values for R-407c at low mass flux were up to 15% less than R-22. However, R-407c U-values were approximately 10% greater than R-22 when operating with a saturated refrigerant outlet condition. Contrary to expectations, only slight fractionation was seen for all test conditions and evaporator configurations. It was concluded that lower inlet quality slightly increases refrigerant composition shift. Also seen in this investigation was greater composition shift at greater mass flux for individual experiments. This condition could 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 with refrigerant flow in the downward direction. While downward flow did not have an effect on refrigerant composition shift in this investigation, this orientation may be useful for other applications where more severe fractionation is seen. Other issues addressed were superheat instability and oil hold-up in the evaporator. Liquid droplets were seen at the exit of the evaporator when operating with low superheat. Also, it was found that up to 35% of the volume of the evaporator could be filled with oil even when operating at nominal capacity.
Issue Date:2001-07
Publisher:Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Air Conditioning and Refrigeration Center CR-41
Genre:Technical Report
Date Available in IDEALS:2009-08-06

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