Auto-cascade Refrigeration Research Articles

High phase-separation efficiency auto-cascade system working with a blend of carbon dioxide for low-temperature isothermal refrigeration

A high phase-separation efficiency auto-cascade refrigeration (ACR) system working with a zeotropic mixture of CO2 is proposed in this study. The selection of a high boiling-point component is analysed. A novel zeotropic mixture consisting of a high CO2 (R744) mass fraction together with isobutane (R600a) as a carrier fluid is proposed. An experimental setup was designed, built, and verified to study the ability of the ACR to provide isothermal refrigeration at temperatures lower than the R744 triple-point temperature. The setup employs a recuperative heat exchanger (RHX) for the maximum phase-separation efficiency. The effect of the refrigerant mass charge on the ACR operation was examined. The temperature distribution across the internal heat exchangers (IHXs) was studied as well. The results revealed that the system was very sensitive to the refrigerant mass charge. With a low mass charge, the ACR did not operate correctly owing to the pinch points occurring between the hot and cold ends of the IHXs. High mass charge resulted in increased power consumption and a decreased coefficient of performance (COP). Moreover, tests were conducted to examine the ability of the ACR system, working with a binary mixture of R744/R600a, to provide isothermal refrigeration with different evaporator heat loads. The ACR system was compared in terms of the evaporation pressure and mass flow to a cascade system working with trifluoromethane (R23) in the low stage. The ACR system can be a promising alternative to cascade systems working with R23. The mixture of R744/R600a presented a significant decrease in the global warming potential (GWP) compared with R23. Near-isothermal evaporation was possible with this mixture over a range of vapour quality values. The application range of CO2 was extended to temperatures lower than the CO2 triple-point temperature, using isobutane as a solvent for dry ice. The enthalpy–mass fraction diagrams incorporating a solid fraction area were constructed for the graphical representation of the ACR system.

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

An auto-cascade absorption refrigeration (ACAR) system could achieve a −60°C 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°C in this context, which used R23-R134a-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, R134a 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°C to −60°C.

Performance of condensing and evaporating characteristics of an auto-refrigerating cascade system

The condensing and evaporating characteristics of an auto-refrigerating cascade cycle employing ternary zeotropic mixture R600a/R23/R14 are studied in this article. When the outlet temperature of c...

Experimental investigation on the concentration distribution behaviors of mixture in an ejector enhanced auto-cascade refrigeration system

In this paper, the concentration distribution behaviors of the mixture R134a/R23 in an ejector enhanced auto-cascade refrigeration cycle (EARC) were experimentally investigated. Relationships between circulating concentration and charged concentration were explored, and the effects of the heat load and ambient temperature on the composition shift characteristics were analyzed. The results indicated that large composition shift occurred at the operation conditions, and the circulating concentration of the low boiling temperature component R23 at the condenser inlet was 25.9–38.1% higher than that in charge due to the differential hold-up of individual refrigerant components in two-phase region. The concentrations of the component in charge and circulation had a linear relationship in EARC, which was similar to those in conventional auto-cascade refrigeration cycle and Joule–Thomson refrigerator, even though these cycles were different in cycle configurations, mixture and operation conditions. R23 showed higher circulation concentration at higher ambient temperature. As the increasing heat load, the circulating concentration of R23 at the condenser inlet first increased and then decreased. In addition, different preferential solubility of individual components in the lubricating oil led to 15.4% richer R23 before the system start-up than that charged. The circulating concentration varied drastically at the early stage of the pull-down process and then tended to be stable over time. The largest shift of the circulation concentration from the charged concentration in the pull-down process reached 32.6%. This work could provide an in-depth understanding of the behaviors of the mixture concentration in the ejector enhanced auto-cascade refrigeration system.

CO₂-R134a용 2단 팽창식 오토 캐스케이드 냉동기의 성능특성 연구

CO₂-R134a용 2단 팽창식 오토 캐스케이드 냉동기의 성능특성 연구

Performance analysis of an advanced ejector-expansion autocascade refrigeration cycle

In this paper, an advanced ejector-expansion autocascade refrigeration cycle (AEARC) using hydrocarbon mixture R290/R170 for applications in low-temperature freezers is proposed. In the AEARC, a two-phase flow driven ejector is introduced with new cycle configuration to reduce the thermodynamic loss in throttling process and lift the suction pressure of compressor significantly. The performances of the AEARC and traditional autocascade refrigeration cycle (ARC) are compared using energy and exergy analysis methods, and several important parameters for AEARC are also discussed in detail. The results indicate that AEARC is feasible and there are obvious performances improvements in theCOP, volumetric refrigeration capacity and exergic efficiency. Especially, in AEARC, the COP and volumetric refrigeration capacity increase by 80.0% and 78.5% at most compared to that of ARC, respectively. In general, the AEARC can provide significant performance improvement and produce better actual operation benefit. The potential practical advantages may be worth further attention in future research.

SECOND LAW ANALYSIS OF AUTO CASCADE REFRIGERATION CYCLE USING MIXED HYDROCARBON REFRIGERANT R-600A /R-290/ R170

Experimental and theoretical analysis investigations are achieved on the performance of the three-stage auto cascade refrigeration system. Energy and exergy analysis of auto cascade system is considered using zeotropic mixed hydrocarbon refrigerant R-600a/R-290/R-170 at different mass fractions of (25.5/42.5/32), (24.25/42.75/33), (23/43/34) and (20.5/43.5/36) %. The experimental work was performed on test rig for three stages auto cascade refrigeration system of one ton capacity which is designed and constructed for the present study. The theoretical analysis was carried out using a simulation software PROII based on EES and REFPROP software. The investigated results of the mixed refrigerant R600a/R-290/R170 have showed an enhancement in COP and cycle capacity by about 12.39% and 15% respectively, and the evaporator temperature of the system has approached a relatively lower value of -60.3°Cat mass fraction (23/43/34) compared to the average value of the other mass ratios. The higher values of the exergy efficiency observed for the condenser, evaporator and compressor were 0.92, 0.87 and 0.7 respectively. Comparison of the auto-cascade performance with R-600a/R-290/R-170 at mass fraction (23/43/34) has displayed an enhancement around 45.3 % in the cycle capacity and 39% reduction in the evaporator temperature compared to the mixed refrigerant R-134a/R-410A at mass fraction (70/30). The theoretical results have displayed a reasonable agreement compared to the experimental results with deviations 33%, 22%, and 18% in COP, cycle capacity and overall exergy efficiency respectively.

Theoretical analysis of a double ejector-expansion autocascade refrigeration cycle using hydrocarbon mixture R290/R170

This paper proposed a double ejector-expansion autocascade refrigeration cycle (DEARC) using hydrocarbon mixture R290/R170 for applications in low-temperature freezers. In DEARC, the two ejectors are introduced to reduce the thermodynamic loss in throttling process. The performances of the traditional autocascade refrigeration cycle (ARC), single ejector-expansion autocascade refrigeration cycle (EARC) and DEARC are compared, and several important parameters for DEARC are also discussed in detail. The results indicate that DEARC is feasible and there are obvious performances improvements on theCOP, volumetric refrigeration capacity and exergic efficiency. Especially, in DEARC, the COP increases by 97.9% and 29.6% at most compared to that of ARC and EARC, respectively, the volumetric refrigeration capacity is 334.8% and 188.6% higher than that of the two cycles. In general, the DEARC provides obvious performance improvement and produces better actual operation benefit. The potential practical advantages may be worth further attention in future studies.

Hybrid auto-cascade refrigeration system coupled with a heat-driven ejector cooling cycle

The hybrid auto-cascade refrigeration system with an integrated ejector cooling cycle (HACRS) driven by high-grade power and low-grade heat simultaneously is developed in this paper. The working fluid applied in the system is a zeotropic refrigerant mixture of R170/R600a. The heat-driven ejector cooling cycle is employed to the auto-cascade refrigeration cycle to form a novel hybrid auto-cascade refrigeration system coupled with an ejector cycle (HACRS). The ejector is applied to increase the suction pressure of the compressor, and cooling capacity from the ejector cycle is also utilized by the evaporative-condenser and dephlegmator in the HACRS. The system performance is evaluated, based on the mathematical model of the system from the principle of mass and energy conservation. The results indicate that energy consumption of the compressor in HACRS could be reduced by 50% as compared to that in the conventional auto-cascade refrigeration cycle (ACRC). The HACRS can use the heat-driven ejector cycle and the recovery of exhaust waste heat of the compressor to improve its mechanical coefficient of performance (COPme) effectively.

Experimental research on the pull-down performance of an ejector enhanced auto-cascade refrigeration system for low-temperature freezer

This paper presents an experimental investigation on pull-down performance of an ejector enhanced auto-cascade refrigeration cycle for low-temperature freezer application. The pull-down performances of the freezer established on the ejector enhanced cycle and conventional auto-cascade refrigeration cycle were compared. Additionally, operation behaviors of the ejector and the new cycle based freezer at different mixture concentrations and throttle valve openings were investigated. The results indicated that ejector enhanced system exhibited shorter pull-down time and lower freezer air temperature after continuous operation in comparison with the conventional system. And the pull-down time was saved by 34.4% and the energy consumption of the compressor was reduced by 29.6% at the desired freezing temperature of −40 °C.The largest time average values of the pressure lift ratio and entrainment ratio reached up to 2.854 and 1.340, and the time average compression ratio of the compressor was reduced by 11.6% due to the effective pressure lifting effect of the ejector. The optimal mass fraction ratio of 30%/70% for the mixture R23/R134a was proposed with respect to the shortest pull-down time. This research may offer good guidelines for further improving the performance of the auto-cascade refrigeration system in low-temperature freezer.

Experimental investigation of an ejector-enhanced auto-cascade refrigeration system

This study presents the experimental investigation of an ejector-enhanced auto-cascade refrigeration cycle (EARC) with zeotropic refrigerant R134a/R23. Performance comparisons among the EARC and two conventional cycles were conducted at selected operating conditions. The effects of ambient temperature, charged mass fraction ratio of the mixture, throttle valve opening, and heat load on the performance characteristics of the EARC were investigated. The results indicated that the EARC had more advantages in terms of lower refrigeration temperature and higher energy utilization efficiency over the conventional cycles, and the coefficient of performance (COP) and exergy efficiency improvements of the EARC reached up to 9.6% and 25.1%, respectively. The throttle valve opening was optimal with respect to the maximum system exergy efficiency determination. The refrigerant R134a/R23 with the optimal mass fraction ratio of 0.70/0.30 was proposed.

Investigation on vapor liquid equilibrium for strongly-zeotropic ternary mixture of 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) + trifluoromethane (R23) + tetrafluoromethane (R14)

The cubic equation of state/excess Gibbs free energy (CEoS/GE) model is a new idea to predict non-ideal systems. In this study, the Peng-Robinson (PR) EoS combined with the Wong-Sandler (WS) mixing rule and the modified UNIFAC group contribution activity model was used to calculate the vapor liquid equilibrium (VLE) data. The average absolute relative deviations (AARD) of pressure and the average absolute deviations (AAD) of the vapor phase mass fraction between the calculated results and experimental data are 0.87% and 0.005 for fluoroethane (R161)+2,3,3,3-tetrafluoroprop-1-ene (R1234yf) over the temperature range of (283.15–323.15K) and at pressures varying from 0.4MPa to 1.8MPa, respectively. The liquid and vapor phase mass fraction deviations are within ±0.028 and ±0.025 for difluoromethane (R32)+pentafluoroethane (R125)+1,1,1,2-tetrafluoroethane (R134a) system at temperatures of (205–350K), respectively. The VLE behavior for the strongly-zeotropic mixture of 2,3,3,3-tetrafluoroprop-1-ene (R1234yf)+trifluoromethane (R23)+tetrafluoromethane (R14) which is suitable for the three-stage auto cascade refrigeration system was calculated at pressures of (0.2–2.2MPa) and the corresponding three-dimensional phase diagrams were constructed. It was found that the sliding temperature increases with the decrease of pressure and the maximum predicted sliding temperature reached 73K.

A comparative study on exergetic, exergoeconomic and exergoenvironmental assessments of two internal auto-cascade refrigeration cycles

This research makes a comparative study of an internal auto-cascade refrigeration system (IACRS), i.e. system 1, and a novel ejector enhanced IACRS, or system 2, employing a pressure-constant ejector at the entrance of the phase separator based on exergy, exergoeconomic and exergoenvironmental analyses. R600 is used as a convenient refrigerant in the proposed systems. Compressor mass flow rate (ṁ1), condenser inlet temperature (T3), refrigeration evaporator inlet temperature (T6) and freezer evaporator inlet temperature (T11) are chosen as substantial design parameters to evaluate the performance of both proposed systems. The simulation and comparative study show that at all ranges of selected parameters, the average energy and exergy coefficient of performance, i.e. COPEn and COPEx, respectively, obtained for system 2 have a drastic increment relative to system 1. Moreover, outcomes identify system 2 as a more economic system due to its low total product cost rate (ĊP,tot) at which ṁ1 with 20.95% decrement of ĊP,tot has a significant positive effect. Further results show that the environmental impact is improved by applying system 2 so that the total product environmental impact rate associated with exergy (ḂP,tot) for ṁ1, T3, T6 and T11 decreases within 21.52%, 10.59%, 8.96% and 8.35% respectively, as compared with system 1.

Parametric assessment and multi-objective optimization of an internal auto-cascade refrigeration cycle based on advanced exergy and exergoeconomic concepts

This research deals with the advanced exergy and exergoeconomic analyses and multi-objective optimization of an internal auto-cascade refrigeration cycle. Butane is used as the refrigerant and all heat exchangers are modeled by considering pressure drops. Sensitivity study is carried out to assess the variation of exergetic and economic improvement potentials; namely, total avoidable exergy destruction, total avoidable exergy destruction cost and total avoidable investment cost rates to the compressor mass flow rate, condenser, refrigerator evaporator and freezer evaporator inlet temperatures. Parametric study indicates that the condenser inlet temperature growth improves the total avoidable exergy destruction within 88.19%, the total avoidable investment cost rate increases by about 126.92% and 3.68% as compressor inlet mass and refrigerator evaporator inlet temperature rise, respectively and the increment of refrigerator evaporator inlet temperature shows a positive effect on the total avoidable exergy destruction cost rate. In addition, improvement potentials are maximized by applying Non-dominated Sort Genetic Algorithm-II. The multi-objective optimization indicates 76.78%, 38.66% and 103.38% improvements in total avoidable exergy destruction rate, total avoidable investment and total avoidable exergy destruction cost rates, respectively relative to the base design point.

The study of thermodynamic properties of zeotropic mixtures of R600a/R23/R14

Thermodynamic properties of zeotropic mixtures of R600a/R23/R14 have been investigated and calculated based on the established mathematical model. The deviations of bubble pressure and vapor composition between the simulation results and the experimental data were −0.16% and −1.22% for R600a/R23 and −0.14% and 0.5% for R134a/R23, respectively. The simulation results of R32/R125/R134a were compared with the experimental values of the component, and the average error was 0.31%, and most deviations were within ±5%. The average error in enthalpy value and entropy value between the simulation and National Institute of Standard Technology data was −0.25% and 3.33%, respectively. This suggests that the simulation results meet the requirements of engineering calculation. Based on the experimental results, the thermodynamic properties of the auto-cascade refrigeration system were calculated and analyzed in detail. The pressure–enthalpy diagrams of the auto-refrigerating cascade system with R600a/R23/R14 under the operating conditions were given, and the cycle of this system can be clearly described by the diagram of spatial pressure–enthalpy, which indicates that this simulation program could analyze the system cycle effectively and provide a direction for improvement of the auto-refrigerating cascade system.

Theoretical Investigation on the Properties of R744/R290 Mixtures

At present, an interesting research is to find promising environmentally-friendly refrigerants in auto-cascade refrigeration cycle. This paper proposed a refrigerant mixture of R744 and R290 due to its zero ozone depletion potential and negligible global warming potential. The properties of R744, R290 and the mixtures refrigerant were analyzed. And performances comparisons between R744, R290, R22 and R134a were investigated based on the simulation data. Then, the vapor-liquid phase equilibrium properties of mixture were calculated. And the conversion relations of total mole fraction, vapor phase mole fraction, liquid phase mole fraction, quality, equilibrium temperature and pressure were systematically analyzed. The results show that R744 and R290 are potential refrigerant. But each pure refrigerant has obvious disadvantage, it is a good choice that R744 and R290 are mixed together into non-azeotropic mixture, which is suitable for auto-cascade refrigeration cycle due to a large temperature glide during the phase changing.

Performance analysis of a modified autocascade refrigeration cycle with an additional evaporating subcooler

A modified autocascade refrigeration cycle with an additional evaporating subcooler is proposed, which can make good use of the temperature glide characteristic of zeotropic mixtures. The energy and exergy performance comparison between the modified cycle and a basic autocascade refrigeration cycle using the zeotropic mixture of R23/R134a is carried out by the simulation method. Under the considered operating conditions, the coefficient of performance (COP), volumetric cooling capacity and exergy efficiency of the modified cycle can be improved by up to an average of 37.5%, 42.3% and 34.3% compared to that of the basic cycle, respectively. Within a certain subcooling range, the performance of the modified cycle will be improved with increasing the subcooling degree in the evaporating subcooler. In addition, the increase of the vapor quality at the condenser outlet not only leads to the performance improvements of two cycles but also results in the increase of the working pressure ratio and discharge temperature of the compressor. A similar situation also occurs for the performance variations of two cycles with the mass fraction of the zeotropic mixture.

Energy and exergy analysis of a new ejector enhanced auto-cascade refrigeration cycle

A new ejector enhanced auto-cascade refrigeration cycle using R134a/R23 refrigerant mixture is proposed in this paper. In the new cycle, an ejector is used to recover part of the work that would otherwise be lost in the throttling processes. The performance comparison between the new cycle and a basic auto-cascade refrigeration cycle is carried out based on the first and second laws of thermodynamics. The simulation results show that both the coefficient of performance and exergy efficiency of the new cycle can be improved by 8.42–18.02% compared with those of the basic cycle at the same operation conditions as the ejector has achieved pressure lift ratios of 1.12–1.23. It is found that in the new cycle, the highest exergy destruction occurs in the compressor followed by the condenser, cascade condenser, expansion valve, ejector and evaporator. The effect of some main parameters on the cycle performance is further investigated. The results show that for the new cycle, the achieved performance improvement over the basic cycle is also dependent on the mixture composition and the vapor quality at the condenser outlet. The coefficient of performance improvement of the new cycle over the basic cycle degrades with increasing vapor quality. In addition, there exists an optimum mixture composition to obtain the maximum coefficient of performance for the new cycle when other operation conditions are given. The optimum mixture composition of both cycles may be fixed at about 0.5 under the given evaporating temperature.

Experimental and theoretical investigation on the performance of CO2/propane auto-cascade refrigerator with a fractionation heat exchanger

This paper proposed a small-sized auto-cascade refrigeration cycle using CO2/propane as the refrigerant. A fractionation heat exchanger (FHEX) was introduced into this cycle to improve the separation efficiency of CO2 and propane. The effect of FHEX on the performance of the cycle was analyzed. And the comparison between with and without FHEX was presented based on the experiment and numerical simulation data at the cooling water inlet temperature of 18.3 °C. Results show that the evaporating temperatures of the cycle with FHEX are 9.1–20.3 °C lower than that of the cycle without FHEX with the increase of CO2 mass fraction from 0.22 to 0.31. The corresponding refrigeration capacity, power input and COP have 37.1–61.7%, 24.7–36.8% and 16.4–42.7% reduction, respectively. The further computation results of different vapor quality in FHEX at the condensing temperature of 23.3 °C and 37.0 °C indicate that reducing the vapor quality causes an enhancement in the separation of CO2 and propane, and thus decreases the evaporating temperature. Moreover, all of the refrigeration capacity, power input and COP are decreased. It is concluded that the cycle performance would be improved by increasing CO2 mass fraction or decreasing the cooling water temperature.

Experimental investigation of the performance of a single-stage auto-cascade refrigerator

Auto-refrigerating cascade (ARC) systems possess many advantages comparing with traditional cascade refrigeration systems. This work proposed a novel ternary mixture, R600a/R23/R14, for ARC systems for 190 K applications. The performance of the ternary mixture and the influences of compositional ratio and bypass scheme were assessed in a modified domestic cooler. The results demonstrated the feasibility of the proposed R600a/R23/R14 ternary mixture as an environmental benign alternative for ARC systems. The performance varied little within a certain composition range and a mass ratio of 35/30/35 for R600a/R23/R14 mixture was recommended. It also showed that the two bypass schemes, which can regulate more effectively the refrigerant compositions, were better than the conventional hot-gas bypass approach. The variation of the evaporator temperature suggested the presence of local dryout at high heat loads (i.e., larger than the design value), which should be carefully prevented.