Performance of a single-stage auto-cascade refrigerator operating with a rectifying column at the temperature level of -60°C

This paper proposes a new approach to the performance optimization of an auto-cascade refrigerator(ACR) operating with a rectifying column and six types of binary refrigerants(R23/R134a,R23/R227ea,R23/R236fa,R170/R290,R170/R600a,and R170/R600) at a temperature level of -60 °C. Half of the six binary refrigerants are nonflammable,of which the 0.5 and the 0.6 mole fractions of R23 for the R23/R236fa possess the most prospective composition for the medium and low suction pressure compressors,respectively. The remaining three binary refrigerants are flammable but with low global warming potentials,of which the 0.6 mole fraction of R170 for the R170/R600 is the most prospective one. The results show that the overall matching as well as local matching of heat capacity rates of hot and cold refrigerants in the recuperators are important for the improvement of coefficient of performance of the cycle,which can be adjusted through the simultaneous optimization of the pressure level and composition. The new approach proposed also offers a wider range of applications to the optimization in performance of the cycle using multi-component refrigerants.

Performance Analysis of an Ejector Enhanced Two-Stage Auto-Cascade Refrigeration Cycle for Low Temperature Freezer

In this paper,an ejector enhanced two-stage auto-cascade refrigeration cycle(EARC)using ternary mixture R600a/R32/R1150 is proposed for application of-80℃freezing.In EARC cycle,an ejector was employed to recover the expansion work in the throttling processes and lifted the suction pressure of the compressor.The performances of the ejector enhanced two-stage auto-cascade refrigeration cycle and conventional auto-cascade refrigeration cycle(CARC)were compared using thermodynamic analysis methods.The influences of the important operation parameters including mass fraction ratio of the mixture,fluid quality at the second separator inlet,condensation temperature,evaporation temperature,and expansion ratio of expansion valve on the performances of EARC cycle were discussed in detail.The results indicate that ternary mixture R600a/R32/R1150 has the optimal mass fraction ratio of 0.45/0.2/0.35 with respect to the maximum COP.The EARC cycle yields higher performance than the CARC cycle in terms of COP,exergy efficiency and volumetric refrigeration capacity.And 4.9%-36.5%improvement in COP and 6.9%-34.3%higher exergy efficiency could be obtained in EARC cycle comparing with CARC cycle.The finding of this study suggests that the EARC cycle has a promising application potential for low temperature freezing.

Equivalent Cycle and Optimization of Auto-Cascade Absorption Refrigeration Systems

Auto-cascade absorption refrigeration(ACAR) systems are a class of new cycles that can achieve low refrigeration temperatures by utilizing low-quality thermal energy. In this study, the equivalent thermodynamic processes of a reversible ACAR system are established, and illustrated in a T-s diagram. The formula of the coefficient of performance for the reversible ACAR system is derived from the first and second thermodynamic laws. And then, the equivalent cycle of an irreversible ACAR system is established. The irreversible ACAR system is optimized by minimizing entropy generation of the thermodynamic processes. As a result, the optimum distribution ratio of heat fluxes at cascade process, which is defined as a ratio of heat fluxes between a condensing reservoir and cascade reservoir, and the optimum cascade temperature are obtained. Finally, its coefficient of performance and thermodynamic perfect degree are determined with minimum entropy generation.

Analyses of an Improved Double-Absorber Absorption Refrigeration System at Low Temperatures

An auto-cascade absorption refrigeration(ACAR)system could achieve a-60℃ refrigeration temperature by low-grade heat.For an ACAR system,its performance is mainly affected by energy and mass coupling of the auto-cascade processes.A novel ACAR system with double-absorber was proposed to get higher-efficient refrigeration as low as-60℃ in this context,which used R23-R134 a-DMF(N,N-Dimethylformamide)as its working fluids.Theoretical calculation and analyses were conducted under different working conditions.From the calculated results,the new system gained a COP value 20%higher than that of an ACAR system with single-absorber under the same generating,condensing,absorbing and refrigerating temperatures.Compositions of a refrigerant mixture showed key influences on energy and mass coupling of the auto-cascade processes,and an optimal composition of the mixed refrigerants was obtained for the new ACAR system.In addition,it was clearly found that absorbing processes of the new system had great effects on energy and mass coupling of the auto-cascade processes.Based on the difference of absorbing characteristics among R23,R134 a and DMF,the absorbing processes were intensified under the different absorbing pressures.As a result,an optimal matching pressure was obtained for the new ACAR system.Energy and mass coupling of the auto-cascade processes were further optimized,and the highest COP value was obtained.The theoretical analyses showed that performance of the innovative ACAR system could be superior to that of an ACAR system with single-absorber at a refrigeration temperature from -55℃ to -60℃.