Zeotropic mixtures sometimes in the past have been proposed in order to improve the energy efficiency of refrigeration processes when a fluid need to be cooled in an extended tem¬perature range; moreover they offer also the possibility of a stepless and energy effective capacity modulation, by means of a change in the composition of the operating fluid, through a fractionation process. On the other hand, an unwanted fractionation, as a consequence of leaks, or of the operation of flooded evaporators, is regarded as a disadvantage of zeotropic mixtures and so it is for the deterioration of two phase heat transfer coefficient, due to a mass transfer re-sistance, related to the development of a concentration gradient close the surface where the phase change occurs. Today, zeotropic mixtures appear as the nearly unique possibility of finding a substitute for R22 to be used in retrofitting existing plants. R407C, a blend of R32/125/134a (23/25/52) is presently credited with the best possibilities of being the final choice, but the question is not yet settled. Energy efficiency certainly is among the major points to consider in the search for the best candidate to replace R22; such a comparison is not so easy to carry out, as different criteria can be followed, each of them leading to a different result. A large number of papers are available on this subject; a good insight can be found in /Ref. 1, 2/. This paper is an attempt to rationalise this matter; a general criterion is proposed for evaluating the ability of a zeotropic mixture to achieve a good energy efficiency. This criterion takes into account both the internal efficiency of the refrigeration cycle and the external irre-versibilities arising from the imperfect matching of the temperature curves of the two fluids in-volved in the heat transfer processes for condensation and evaporation. As an example, a com-parison is made between the performance of R22 and R407C for different operating conditions.
Theoretical evaluation of performance of zeotropic mixtures in refrigerating cycles
Bobbo S;Camporese R;
1995
Abstract
Zeotropic mixtures sometimes in the past have been proposed in order to improve the energy efficiency of refrigeration processes when a fluid need to be cooled in an extended tem¬perature range; moreover they offer also the possibility of a stepless and energy effective capacity modulation, by means of a change in the composition of the operating fluid, through a fractionation process. On the other hand, an unwanted fractionation, as a consequence of leaks, or of the operation of flooded evaporators, is regarded as a disadvantage of zeotropic mixtures and so it is for the deterioration of two phase heat transfer coefficient, due to a mass transfer re-sistance, related to the development of a concentration gradient close the surface where the phase change occurs. Today, zeotropic mixtures appear as the nearly unique possibility of finding a substitute for R22 to be used in retrofitting existing plants. R407C, a blend of R32/125/134a (23/25/52) is presently credited with the best possibilities of being the final choice, but the question is not yet settled. Energy efficiency certainly is among the major points to consider in the search for the best candidate to replace R22; such a comparison is not so easy to carry out, as different criteria can be followed, each of them leading to a different result. A large number of papers are available on this subject; a good insight can be found in /Ref. 1, 2/. This paper is an attempt to rationalise this matter; a general criterion is proposed for evaluating the ability of a zeotropic mixture to achieve a good energy efficiency. This criterion takes into account both the internal efficiency of the refrigeration cycle and the external irre-versibilities arising from the imperfect matching of the temperature curves of the two fluids in-volved in the heat transfer processes for condensation and evaporation. As an example, a com-parison is made between the performance of R22 and R407C for different operating conditions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
