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[EN] In this work the possibilities of enhancing the energy performance of CO2 transcritical refrigeration systems using a dedicated mechanical subcooling cycle are analysed theoretically. Using simplified models of the cycles, the modification of the optimum operating conditions of the CO2 transcritical cycle by the use of the mechanical subcooling are analysed and discussed. Next, for the optimum conditions, the possibilities of improving the energy performance of the transcritical cycle with the mechanical subcooling are evaluated for three evaporating levels (5, 5 and 30 C) for environment temperatures from 20 to 35 C using propane as refrigerant for the subcooling cycle. It has been observed that the cycle combination will allow increasing the COP up to a maximum of 20% and the cooling capacity up to a maximum of 28.8%, being both increments higher at high evaporating levels. Furthermore, the results indicate that this cycle is more convenient for environment temperatures above 25 C. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed., The authors gratefully acknowledge Jaume I University of Spain, who financed the present study through the research project P1.B2013-10.

An experimental comparison has been performed of a cascade refrigeration facility working with the refrigerant pairs R134a/R744 and R152a/R744. This kind of facility is suitable for industrial and commercial refrigeration applications. The high GWP refrigerant R134a has been substituted with the low GWP refrigerant R152a, in accordance with the new environmental regulations aimed at mitigating the Greenhouse effect. As both refrigerants belong to the family of HFC fluids, the replacement has been carried out as a drop-in. Apart from safety considerations, as R152a is included in the A2 group, the results of the wide range of tests conducted show that no special energy improvement or worsening is achieved, and that the replacement of R134a with R152a is technically and energetically feasible. The authors gratefully acknowledge the financial support for the present work provided by the Spanish Ministry of Economy and Competitiveness (research project ENE2014-53760-R).

We present the experimental evaluation of a R134a/CO2 cascade refrigeration plant designed for low evaporation temperature in commercial refrigeration applications. The test bench incorporates two single-stage vapour compression cycles driven by semi hermetic compressors coupled thermally through two brazed plate cascade heat exchangers working in parallel and controlled by electronic expansion valves. The experimental evaluation (45 steady-states) covers evaporating temperatures from 40 to 30 C and condensing from 30 to 50 C. In each steady-state, we conducted a sweep of the condensing temperature of the low temperature cycle with speed variation of the high temperature compressor. Here, the energy performance of the plant is analysed, focussing on the compressors' performance, temperature difference in the cascade heat exchanger, cooling capacity, COP and compressors discharge temperatures., The authors gratefully acknowledge Jaume I University of Spain, who financed the present study through the research project P1.B2013-10.

The aim of the present paper is to describe a general methodology suitable for analysing any intermediate configuration considered in staged vapour compression refrigeration cycles. This general methodology only depends on two basic parameters related to the subcooling and desuperheating obtained in the inter-stage system. A COP expression based on the two basic parameters was obtained from the general configuration. By means of its particularization to seven common configurations, it was then possible to make an energy comparison of the various configurations working with two fluids appropriated for the low temperature domain such as ammonia and R-404A., The authors are indebted to the Spanish Ministry of Education and Science (CTM2008-06468-C02-02/TECNO) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (200800050084716) for their economic support to this work.

[EN] The performance of an Internal Heat Exchanger (IHX) operating in a CO 2 transcritical refrigeration plant is analysed, from an energetic point of view, in this work. The evaluation is based on experimental data by contrasting the performance of the plant working with (44 tests) and without the IHX (46 tests) at the same operating conditions. The experimental evaluation covers three evaporating levels (-5, -10 and -15 °C), at two different gas-cooler outlet temperatures each (31, 34 °C), for a wide range of gas-cooler operating pressures (74.5-105.9 bar). The thermal effectiveness of the IHX is empirically analysed for the different operating conditions in the first part of the paper. Moreover, the relation of its effectiveness with the operating parameters is presented. The second part is devoted to analyse the modification of the energetic performance of the plant caused by the IHX. The results show a maximum increment on cooling capacity of 12%, an increment of the efficiency of the plant up to 12% and a maximum increase on discharge temperature of 10 °C at -15 °C of evaporating temperature. © 2010 Elsevier Ltd and IIR. All rights reserved., The authors are indebted to the Spanish Ministry of Education and Science (CTM2008-06468-C02-02/TECNO) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (200800050084716) for their economic support to the present work.

[EN] The aim of the present paper is to detail an analysis, based on experimental data, of the inter-stage working conditions of a two-stage vapour compression facility equipped with a compound compressor, which operates with the most usual inter-stage configurations (two-stage with direct liquid injection and two-stage with subcooler) in medium- and low-capacity commercial refrigeration applications. The experimental analysis is performed in an evaporating temperature range between -36 and -20 degrees C, and in a condensing temperature range between 30 and 47 degrees C, using one of the fluids most widely-used in Europe for low-temperature applications, the R-404a. The inter-stage working temperature/pressure obtained in the tests has been contrasted with the two usual criterion of the optimum working conditions definition: the arithmetical mean of the refrigerant condensing and evaporating temperatures and the criterion of equal pressure ratios in both stages. This paper presents the differences and affinities with the criterion and analyses the influence of the intermediate systems (direct liquid injection and subcooler) on the inter-stage operating conditions., The authors are indebted to the Generalitat Valenciana for partial support under project GV05/091.

International agreements will restrict in the near future the use of high-GWP refrigerants in Europe. These restrictions will favour the implantation of refrigeration systems with low-GWP fluids, especially in applications with high leakage rate. To clarify possible solutions that accomplish the forthcoming F-Gas Regulation, we present simplified models of five two-stage vapour compression refrigeration systems and evaluate them with low-GWP refrigerants (HFC, HFO and naturals). We analyse the energy performance over a wide range of evaporating and environment temperatures and present the TEWI analysis under a same scenario, typical of a centralized commercial refrigeration application. We conclude that, for high-GWP refrigerants, direct emissions have greater weight in TEWI than the indirect ones, so future solutions might be based on low-GWP fluids, in some cases with risk of toxicity or flammability. We observe the indirect two-stage systems (cascades) with CO2 as low temperature fluid are promising solutions, especially for warm regions., The authors gratefully acknowledge Jaume I University of Spain, who financed the present study through the research project P1.B2013-10.

[EN] The EU Regulation 517/2014 has recently been approved in a further attempt to curb the effects of GlobalWarming. As a consequence, the refrigeration sector is moving towards refrigerants with a low GlobalWarming Potential (GWP100) in accordance with the limit fixed by these regulations (150). In this regard, the old refrigerant R152a attracts renewed interest due to its low GWP (138) and its similarity to R134a. The present work shows the results of using R152a in a vapour compression plant equipped with a hermetic compressor and an IHX designed for R134a. The refrigerant was replaced by a conventional drop-in process in order to carry out an energy comparison. The results have revealed an improvement in the COP with R152a up to 13% despite a reduction in the cooling capacity of about 10%. During the test campaign, R134a hermetic compressors have been shown to be capable of operating with R152a., The authors acknowledge Jaume I University of Spain, who financed partially the present study through the research project P1.B2013-10.

Unlike other refrigerants, R744 (or carbon dioxide) has a very low critical temperature (30.98 C) which hampers working in subcritical conditions especially when the environment is used as a heat rejection sink. Accordingly, CO2 refrigeration plants are designed to work in supercritical conditions. Notwithstanding, the heat rejection temperature may decrease below the critical one during long periods of time (for example in winter or autumn), thereby offering the possibility of operating in subcritical conditions. According with this and in order to give useful information for related system design and operation, the aim of this paper is to analyse the behaviour of a CO2 refrigeration plant designed for supercritical conditions when it works in subcritical conditions. To achieve this objective a series of experimental assays has been performed. The results obtained shown a decrease in the coefficient of performance (COP) and cooling capacity ðQ_ evÞ if the heat exchanger designed as a gas-cooler operates as a condenser in some cases. To avoid this, a possible solution is proposed consists on using an inverter drive to reduce the refrigerant mass flow rate in order to enhance the gas-cooler/condenser thermal effectiveness. From the experimental results obtained, it could be remarked that if the heat exchanger is designed as a condenser and operates as a gas-cooler, the energy performance of the refrigeration plant could improve under certain subcritical and supercritical conditions., The authors are indebted to the Spanish Ministry of Education and Science (IESCO2) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (ENE2011-22968) for the economic support given to the present work.

This work focuses on the investigation of new drop-in or long-term refrigerants for low evaporating temperatures that can be used as substitutes for HCFC-22, which is gradually being phased-out. This process is already in progress in European countries (Regulation CE-1005/2009) and has been accelerated in Article 5 Countries of the Montreal Protocol. Specifically, in this work the energy influence of the suction line/liquid line heat exchanger on the new substitute fluids is addressed from an experimental approach. The study has been based on experimental measurements in a single-stage vapour compression refrigeration plant, which has been tested in the same external conditions operating with and without an Internal Heat Exchanger (IHX). The energy influence of the IHX working with R22 and three potential substitutes for low temperature applications, the chlorine-free drop-in fluids R417B and R422A and the chlorine-free long-term substitute R404A has been analysed. From the experimental results, reductions in capacity and COP have been observed when R22 is replaced by the drop-in fluids, although the presence of an IHX can help to lessen these reductions. Furthermore, the criteria that are usually employed to determine the energy advantage or disadvantage of the use of the IHX with the new refrigerants have been tested, the results confirming that they remain valid for them.

The gas-cooler heat exchanger is a characteristic device of the refrigeration plants that use CO2 as the working fluid in transcritical conditions. The proper gas-cooler sizing, allows getting the maximum effectiveness in the heat exchange, thus permitting to reduce the temperature approach and improving the heat recovered in the coolant (secondary) fluid and the COP of this kind of systems. This work deals with the development of a steady-state coaxial gas-cooler model that uses water as coolant fluid. The model is based on the finite-volume methodology and is validated with experimental data. An uncertainty lower than ±10% was obtained in the calculation of the heat transfer rates and an uncertainty lower than ±3 °C in the outlet temperatures of both fluids. Several correlations for estimating the CO2 heat convection coefficient have been studied to select the one that best fits the model. Using the model, the influence of different parameters on the gas-cooler thermal effectiveness has been analyzed and presented in this work. Finally, a comparison between the traditional ε-NTU method and the developed model has been done., The authors are indebted to the Spanish Ministry of Education and Science (CTM2008-06468-C02-02/TECNO) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (200800050084716) for the economic support given to the present work.

This work presents the experimental evaluation of the energy performance of a modified single-stage CO2 transcritical refrigeration plant with an internal heat exchanger (IHX) based on vapour injection in suction line. This modification, which is only applicable to refrigeration plants with an expansion process divided in two stages with a liquid receiver between them, consists of extracting saturated vapour from the liquid receiver in order to: decrease the vapour quality at the evaporator inlet, and reduce the superheating degree at the compressor suction by means of the expansion and following injection of the extracted refrigerant. Three different injection points have been evaluated experimentally: before the IHX, after the IHX and just before the suction chamber of the compressor. The experimental measurements show that the cooling capacity and COP can be enhanced in 9.81% and 7.01%, respectively. Furthermore, a reduction in the discharge temperature of the compressor up to 14.7 C has been measured inside the evaluation range., The authors are indebted to the Spanish Ministry of Education and Science (CTM2008-06468-C02-02/TECNO) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (200800050084716) for the economic support given to the present work.

[EN] In this paper, a simplified model for the study of shell-and-tubes heat exchangers (HXs) is proposed. The model aims to agree with the HXs when they are working as condensers or evaporators. Despite its simplicity, the model proves to be useful to the pre-designment and correct selection of shell-and-tubes HXs working at full and complex refrigeration systems. The heat transfer coefficient and pressure drop correlations are specially selected and treated to implement them into the shell-and-tubes HXs presented. The model is implemented and tested in the modellization of a general refrigeration cycle and the results are compared with data obtained from a specific test bench for the analysis of shell-and-tubes HXs., This research has been partially financed by Spanish Ministry of Education and Culture (MEC), under the Project DPI2005-09262-C02-01/02 and by Fundacion Seneca (CARM) with the research project with Reference Number 088766/PI/08.

[EN] As a result of the ban of the CFC fluids in 2000 and the future limitation of HCFC fluids, which manufacture limit is the year 2010, refrigerant producers have been working to develop different mixtures, mainly based on HFC fluids, which emulate the properties of the former refrigerants used in refrigeration plants. At the moment, the HFC-404A and the HFC-507A are the more widely used fluids in Europe as replacement of the CFC-502 in plants operating at low temperatures. This work presents the experimental evaluation, from an energy point of view, of both refrigerants in the same refrigeration plant, which corresponds to a double-stage vapour compression plant driven by a compound compressor. The characterization was made for an evaporating temperature range between ¿36 and ¿20 °C for a constant condensing temperature of 40 °C., The authors are indebted to 'Fundacion Caixa Castello-Bancaixa' (P1.1B2008-12) and to the 'Ministerio de Medioambiente y Medio Rural y Marino' (200800050084716) for the support given to the present study.

[EN] This paper presents a detailed experimental analysis of the energetic effects of using a subcooler system in a two-stage refrigeration facility driven by a compound compressor. The analysis is carried out by comparing the experimental performance of the two-stage cycle working with the subcooler system to the experimental performance of the two-state cycle working without the subcooler system in an evaporating temperature range between -36 degrees C and -21 degrees C and in a condensing temperature range between 31 degrees C and 49 degrees C using R-404A as the working fluid. Experimental results show that the amount of refrigerant used to obtain the subcooling, which is injected between the compression stages, causes an increment in the interstage working pressure, in power consumption, and in the cooling capacity of the cycle. This paper discusses and analyzes these modifications.

[EN] This work presents a mathematical model of a shell-and-tube condenser based on mass continuity, energy conservation and heat transfer physical fundamentals, whose methodology can be easily adapted for modelling any type of condenser. The model is formulated as a combination of control volumes that represents all the refrigerant states in the condenser and the liquid receiver function, which is carried out by the condenser of the experimental plant. Model validation is performed by using steady-state data and transient tests from an experimental vapour compression plant; the prediction error of the model is lower than 5% and a good representation of the dynamic performance of the condenser is achieved. A theoretical comparison involving the importance of the dynamic responses of the evaporator and the condenser at the plant is also presented., The authors are indebted to the Generalitat Valenciana for partial support under programme Formación de Personal Investigador (FPI) CTBPRB/2005/105 and project GV05/091.

This work presents a mathematical model of a shell-and-tube evaporator based on mass continuity, energy conservation and heat transfer physical fundamentals. The model is formulated as a control volume combination that represents the different refrigerant states along the evaporator. Since the model is based on refrigerant and secondary fluid states prediction, it can be easily adapted for modelling any type of evaporator. The strategy of working with physical fundamentals allows the steady- and dynamic-state analysis of any of its performance variables. The paper presents a steady-state validation made with two pure refrigerants (HCFC-22, HFC-134a) and with a zeotropic blend (HFC-407C), and a dynamic validation with transient experimental tests using HCFC-22. The model prediction error is lower than 5% and it is well in accordance with actual dynamic behaviour. Copyright © 2006 John Wiley & Sons, Ltd.

[EN] Based on a previously developed and validated steady-state model of a single-stage vapour compression plant, the present paper describes an in-depth study directed to examine the consequences of changes in the operating conditions in such a plant. These include modifications of condensing and evaporating temperatures and the total superheating degree at compressor suction, which are the main energy parameters in a refrigeration plant. The aim of this work is not to determine the final energy state when the operating conditions are modified, which could be achieved using the steady-state model, but to study the mechanism that explains the variation. The pursuit of this goal implies conducting a detailed analysis of the performance and influence of several intermediate variables, the evolution of which has a direct effect on the final cooling capacity, compression power and COP values. The results obtained show the significant influence of the suction pressure (through the specific suction volume) on the compression power and cooling capacity, while the condensing and evaporating temperatures have a similar influence on the COP., The authors are indebted to the Generalitat Valenciana for partial support under project GV05/091

[EN] This paper describes a Second Law Analysis based on experimental data of a two-stage vapour compression facility driven by a compound compressor for medium and low-capacity refrigeration applications, which operates with the most usual inter-stage configurations (direct liquid injection and subcooler). The experimental analysis is performed for an evaporating temperature range between 36 degrees C and 20 degrees C and for a condensing temperature range between 30 degrees C and 47 degrees C using the refrigerant R-404A. The final results are compared with energy analysis from previous works. Additionally, a new criterion of equivalence between the simple vapour compression cycle and the two-stage compression cycle is given., The authors are indebted to the Spanish Ministry of Education and Science (CTM2008-06468-C02-02/TECNO) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (200800050084716) for their economic support to this work.