Analysis and optimization of a dual-load vapor compression cycle using non-azeotropic refrigerant mixtures
Smith, Mark Kennedy
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https://hdl.handle.net/2142/21225
Description
Title
Analysis and optimization of a dual-load vapor compression cycle using non-azeotropic refrigerant mixtures
Author(s)
Smith, Mark Kennedy
Issue Date
1994
Doctoral Committee Chair(s)
Newell, Ty A.
Department of Study
Mechanical Science and Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Language
eng
Abstract
Some Non-Azeotropic Refrigerant Mixtures (NARMs) have been identified as potential replacements for R12 because of their low ozone depletion potential, low global warming potential and promising thermodynamic characteristics which could improve cycle efficiency. A NARM experiences a variable temperature glide during a constant-pressure phase change process, making it a logical candidate for the two-temperature level cooling found in refrigerators. There are two fundamental thermodynamic benefits to using a NARM over a pure refrigerant: (1) mixture and air-temperature glides can be better matched to improve the system performance, and (2) lower refrigerant temperatures can be achieved through the use of intercooling with no decrease in evaporating pressure. The objective of this research was to investigate optimal pure refrigerant (R12 and R134a) and NARM (65% R22/35% R123 and 80% R22/20% R141b) refrigerator system configurations that minimized life-cycle cost.
A two-evaporator flow loop was constructed to help develop an evaporator heat transfer model and take NARM heat transfer data. For the mass flux range of 25-45 kg/m$\sp2$-s, the mixture heat transfer coefficients were on the order of 50% less than those of R12. For higher mass fluxes, the mixture coefficients rose rapidly, and approached the R12 values.
A steady-state optimization model was used to minimize the life-cycle cost of each system configuration studied. The optimized system configuration with the lowest life-cycle cost was a R22/R123 system with both high and low-temperature intercoolers. This system used 5.7% less energy, 23% more evaporator area, and its life-cycle cost was 2.1% less than that of an optimized R134a single-evaporator system. Furthermore, this system used 10.5% less energy, 46% more evaporator area, and its life-cycle cost was 4.5% less than that of a modeled R12 base-case system. The optimized R22/R123 systems performed better than the equivalent R22/R141b systems. The high-temperature intercooler mixture systems performed as well as the two intercooler systems. Mixture heat transfer coefficient enhancement had a limited impact on life-cycle cost.
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