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• A novel rotary compressor of multiple vanes and a rotating cylinder is introduced. • The compressor is deemed to be energy efficient yet low in gas pressure pulsations. • Mathematical models are formulated to describe its operational characteristics. • A prototype is instrumented and tested in an R134a refrigeration system. • Good agreement is obtained between the predicted and measured mass flow rates. This paper presents a new variant of the Revolving Vane (RV) compressor, suitable for applications that require high cooling capacity yet low pressure pulsation. By adding supplementary free sliding vanes into the original RV mechanism, the compressor produces multiple discharge cycles per revolution, and hence, it can be configured to produce high volumetric capacity with lower pressure pulsations which are expected to lower the discharge noise level. In this paper, the working principles and the theoretical modelling of the multi-vane RV compressor are presented. A functional prototype has been designed, fabricated, instrumented and tested at the industrial grade compressor performance test bench, using refrigerant R134a as the working fluid. The prototype has operated at 1000 to 2000 rpm and attained pressure ratios of 5.3. The prototype's mass flow rates and volumetric efficiency were successfully measured and correlated with the predictions from numerical simulations. Discrepancies between the predicted and the measured mass flow rates are within ±15%. [ABSTRACT FROM AUTHOR]

• Refrigerant place on the market (POM) in Europe will suffer a significant cut in 2021. • Up to 2020, European refrigerants POM has been respected, despite the sustained demand. • Meeting the 2021 quota will be challenging due to inertia in the adoption of POM mitigation measures. • Industrial, stationary and commercial refrigeration will concentrate more than half of the total refrigerant demand, measured in CO 2 e. The European Union (EU) Regulation on fluorinated greenhouse gases (F-gases) has encouraged to reduce gradually the number of hydrofluorocarbons (HFC) that can be placed on the market (POM) to 21% of the baseline level in 2030. However, to this day, the EU refrigeration, air conditioning and heat pump (RACHP) market is still dominated by these substances. This study describes a methodology to estimate the refrigerant demand by refrigeration, air conditioning, and heat pumps available to the EU customers until 2030. The work is based on the most relevant current statistical data, refrigerant distribution (R134a, R404A, R407C and R410A), and future technology acceptance and trends. The study presents a refrigerant demand grow scenario and provides a basis for a closer market follow-up to facilitate refrigeration industry stakeholders' decision-making. The results indicate that by 2021 will be challenging to accomplish the fluorinated gas quota considering the current HFC phase-down process. However, by 2030, the transition is possible in the EU, assuming the additional measures to mitigate the leakage from already installed equipment will be taken. By that time, natural refrigerants, including CO 2 , ammonia, and hydrocarbons, can dominate the market. However, the share of HFC or HFC/HFO mixtures in operation is still significant (R32 or mixtures with similar behaviour, and R404A low flammability alternatives). Consequently, industrial and commercial refrigeration (large scale applications) will concentrate approximately half of the GWP weighted CO 2 e, negligible direct emissions for domestic refrigeration or mobile air conditioning, dominated by natural refrigerants pure HFO refrigerants, respectively. [ABSTRACT FROM AUTHOR]

Kigali's amendment on Montreal Protocol has recognized the great impact of hydrofluorocarbons (HFCs) on climate change. In the European Union, the Regulation (EU) No. 517/2014 (F-gas Regulation) controls the use of HFCs in several applications. This paper reviews the recent investigations performed because of F-gas Regulation, with focus on lower global warming potential (GWP) synthetic alternatives. The GWP limit and the date of prohibition have an influence on the studies found for each application. The major relevance of the studies has been observed on mobile air conditioners for pure hydrofluoroolefins (HFOs), possibly caused by the earlier control. Additionally, a great number of studies have been found for stationary refrigeration systems using several mixtures and residential air conditioners using R32. An important number of articles investigate synthetic alternatives for domestic refrigerators given the flammability barriers for hydrocarbons in some countries. Despite higher GWP allowance on cascade supermarket systems, few articles are available on this topic. Given the extent of the current studies and the rate of new refrigerant developments, an increase in studies using the new synthetic mixture is expected in the coming years. [ABSTRACT FROM AUTHOR]

In recent years, several emerging technologies in the domain of solid-state physics have been investigated as serious alternatives for future refrigeration, heat pumping, air conditioning, or even power generation applications. These technologies relate to what is called caloric energy conversion, i.e., barocalorics, electrocalorics, magnetocalorics, and elastocalorics. Of these technologies, the greatest progress has been observed in the domain of magnetic refrigeration. However, in the recent few years, significant research efforts have also been made in the field of electrocaloric and elastocaloric refrigeration. Many of these technologies suggest the possibility for improvements in energy efficiency, compactness, noise level, as well as a reduction in environmental impacts, so it seems very probable that they will start to fill particular market niches as a replacement for vapor-compression technology in the future. [ABSTRACT FROM AUTHOR]

The EU Regulation No 517/2014 is going to phase-out most of the refrigerants commonly used in refrigeration and air conditioning systems (R134a, R404A and R410A) because of their extended use and their high GWP values. There are very different options to replace them; however, no refrigerant has yet imposed. In this paper we review and analyze the different mixtures proposed by the AHRI as alternative refrigerants to those employed currently. These mixtures are composed by HFC refrigerants: R32, R125, R152a and R134a; and HFO refrigerants: R1234yf and R1234ze(E). It is concluded, from the theoretical analysis, that most of the new HFO/HFC mixtures perform under the HFC analyzed (although some experimental studies show the contrary) and, in most cases, do not meet the GWP restrictions approved by the European normative. Furthermore, some of the mixtures proposed would have problems due to their flammability. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. The compressor was tested with oil injection mass flow fractions of up to 45%. At an evaporating temperature of −10 °C and condensing temperature of 30 °C, the overall isentropic efficiency was up to 70% at the highest oil injection rate. Overall, over the testing envelope investigated, there are no significantly negative effects experienced for the compressor and the compressor isentropic efficiency and refrigerant mass flow rate improve monotonically as the oil injection rate is increased. [Copyright &y& Elsevier]

Abstract: The specific isobaric heat capacity (cp ) was measured for R125 (pentafluoroethane) in the gas phase by using a flow calorimeter. Twelve measurements for R125 were obtained at temperatures from 313 to 333K and at pressures from 0.8 to 2.4MPa. Some of them are close to the saturation curve. The expanded uncertainty (k =2) of the temperature measurements is estimated to be less than 23mK, and that of the pressure measurements is less than 14kPa. The expanded uncertainty for cp is estimated to range from 12 to 22Jkg−1 K−1. Also, the experimental data were compared with available equations of state. From the results, it became clear that these data will be essential to improve available models so as to represent more reliable thermodynamic properties of R125 and refrigerant mixtures with R125 that are used for refrigeration and air conditioning systems. [ABSTRACT FROM AUTHOR]

Abstract: Vapor compression refrigeration technology has seen great improvement over the last several decades in terms of cycle efficiency through a concerted effort of manufacturers, regulators, and research engineers. As the standard vapor compression systems approach practical limits, cycle modifications should be investigated to increase system efficiency and capacity. One possible means of increasing cycle efficiency is to flood the compressor with a large quantity of oil to achieve a quasi-isothermal compression process, in addition to using a regenerator to increase refrigerant subcooling. In theory, compressor flooding and regeneration can provide a significant increase in system efficiency over the standard vapor compression system. The effectiveness of compressor flooding and regeneration increases as the temperature lift of the system increases. Therefore, this technology is particularly well suited towards lower evaporating temperatures and high ambient temperatures as seen in supermarket refrigeration applications. While predicted increases in cycle efficiency are over 40% for supermarket refrigeration applications, this technology is still very beneficial for typical air-conditioning applications, for which improvements in cycle efficiency greater than 5% are predicted. It has to be noted though that the beneficial effects of compressor flooding can only be realized if a regenerator is used to exchange heat between the refrigerant vapor exiting the evaporator and the liquid exiting the condenser. [ABSTRACT FROM AUTHOR]

Abstract: The advantage of separating sensible and latent cooling (SSLC) via the use of separate cycles is saving energy by raising the evaporating temperature of the sensible cooling process. In this study, the pertinent characteristics of the SSLC system using two parallel vapor compression cycles were investigated. The requirement for high air flow rate through the sensible evaporator was the most important hardware design issue, since it might lead to a high air-side pressure drop. To address this issue, a new air distribution method, which could significantly reduce the pressure drop, was proposed to replace the conventional one. The heat exchanger modeling results demonstrated that sensible evaporators of the SSLC system could provide the equivalent cooling capacity at reduced air-side pressure drop to that of the conventional system without increasing the total heat transfer area. Although initial investment of the SSLC system is projected to be higher than the conventional system, the estimated operating cost of the SSLC system is lower than the conventional one because of the low energy consumption. [Copyright &y& Elsevier]

Abstract: Because of their low thermal conductivity, polymers are not commonly considered as a material to construct heat exchangers, except for specific applications, e.g. heat recovery from solvent laden streams, where exotic alloys are required to prevent corrosion. In this review the material properties of polymers are examined, as well as the current state of the art of polymer matrix composites. It is shown that these materials do hold promise for use in the construction of heat exchangers in HVAC&R applications, but that a considerable amount of research is still required into material properties and life-time behavior. A successful application of polymers or polymer matrix composites is based on careful material selection and modification of the design to fully exploit the material properties, as is demonstrated through a series of examples. [Copyright &y& Elsevier]

Abstract: A simulation model of complex refrigeration system based on two-phase fluid network is developed in part I of the paper [Shao, S.Q., Shi, W.X., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network – part I: model development. International Journal of Refrigeration 31 (3), 490–499.]. One same fluid network is used in the model to describe the refrigeration system in different operating modes with the method of the factual and fictitious branches. When there are many heat exchangers with concurrent cooling and heating, the traditional models are not able to define the flowing directions. The developed model, however, whether the initial flowing direction of the connecting branches is right or not, is able to predict the flow and heat transfer. Three typical complex refrigeration systems such as the multi-unit inverter air conditioner, heat pump with domestic hot water and multi-unit heat pump dehumidifier are simulated with the developed model as demonstrations on how to use it. The model can evaluate the influences of one or more parameters on the system performance, which can be used for the optimization of the system by all-condition performance analysis. It is shown that the two-phase fluid network model is effective and convenient for the simulation of different complex refrigeration system, especially for performance analysis, system evaluation, and optimization based on the performance prediction. [Copyright &y& Elsevier]

Abstract: Some complex refrigeration and heat pump systems with several condensers and evaporators have been developed for different kinds of application. Traditional simulation models were developed for systems in certain operating modes and they failed in modeling the complex refrigeration systems with uncertainties of heat exchangers function and refrigerant flowing direction. In order to predict the performance of complex refrigeration systems, a simulation model is presented based on the two-phase fluid network. The model is consisted of distributed-parameter model of heat exchangers and connecting tubes, map-based model of inverter compressor and electronic expansion valve (EEV). Based on the characteristic of refrigeration system and fluid network, the three conservation equations, i.e. energy, momentum and mass equations, are solved iteratively. This model can deal with the uncertainty of refrigerant flow direction by separating the solving process of the components and the fluid network model, and therefore can simulate different kinds of complex refrigeration systems in different operating modes and conditions. The model is validated by the experimental data of an inverter air conditioner in heating/cooling operating modes and it shows the error of the model is mainly determined by the error of submodels of components in calculating heat transfer and pressure loss. The model is applied for performance analysis of three kinds of complex refrigeration systems in the accompanying article [Shi W.X., Shao, S.Q., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network: part II – model applications, International Journal of Refrigeration 31 (3), 500–509.]. [Copyright &y& Elsevier]

Abstract: The potential of winglet type vortex generator (VG) arrays for air-side heat transfer enhancement is experimentally evaluated by full-scale wind-tunnel testing of a compact plain-fin-and-tube heat exchanger. The effectiveness of a 3VG alternate-tube inline array of vortex generators is compared to a single-row vortex generator design and the baseline configuration. The winglets are placed in a common-flow-up orientation for improved tube wake management. The overall heat transfer and pressure drop performance are assessed under dry-surface conditions over a Reynolds number range based on hydraulic diameter of 220≤ Re ≤960. It is found that the air-side heat transfer coefficient increases from 16.5% to 44% for the single-row winglet arrangement with an increase in pressure drop of less than 12%. For the three-row vortex generator array, the enhancement in heat transfer coefficient increases with Reynolds number from 29.9% to 68.8% with a pressure drop penalty from 26% at Re =960 to 87.5% at Re =220. The results indicate that vortex generator arrays can significantly enhance the performance of fin-tube heat exchangers with flow depths and fin densities typical to those used in air-cooling and refrigeration applications. [Copyright &y& Elsevier]

Abstract: In this work a model of a vapour compression refrigeration system with a variable-speed compressor, based on a black-box modelling technique, is presented. The kernel of the model consists of a full customized radial basis function network, which has been developed to accurately predict the performance of the system with low cost data requirement in terms of input variables and training data. The work also presents a steady state validation of the model inside and outside the training data set, finding, in both cases, a good agreement between experimental values and those predicted by the model. These results constitute a first step to go through future research on fault detection and energy optimisation in variable-speed refrigeration systems. [Copyright &y& Elsevier]

Abstract: The applications of electronic expansion valves (EEVs) into multi-type heat pumps and inverter heat pumps in building air-conditioning systems have increased for comfort environmental control and energy conservation. However, test data and mass flow models of EEVs are very limited in open literature. The objectives of this study are to investigate the mass flow characteristics of R22 and R410A through EEVs and to develop an empirical correlation for the prediction of mass flow rates of R22 and R410A through EEVs. Mass flow rates through six EEVs were measured by varying the EEV opening, inlet and outlet pressures, and the subcooling. Mass flow rates of R410A were compared with those of R22 at the same test conditions. Based on the experimental data, an empirical correlation for mass flow predictions in EEVs was developed by modifying the orifice equation. The predictions of the present correlation showed good agreement with the measured data with average and standard deviations of 0.76% and 5.9%, respectively. Approximately 92% of the measured data were within ±10% of the predictions. [Copyright &y& Elsevier]

Abstract: A new model for hermetic reciprocating compressors is presented. This model is able to predict compressor efficiency and volumetric efficiency in terms of a certain number of parameters (10) representing the main sources of losses inside the compressor. The model provides users with helpful information about the way in which the compressor is designed and working. A statistical fitting procedure based on the Monte Carlo method was developed for its adjustment. The model can predict compressor performance at most points with a maximum deviation of 3%. A possible gas condensation on cold spots inside the cylinder during the last part of the compression stroke was also evaluated. [Copyright &y& Elsevier]

Abstract: In this paper, a series of compressors with different capacities and geometries working with propane as refrigerant are analyzed in terms of the compressor model developed by [E. Navarro, E. Granryd, J.F. Urchueguía, J.M. Corberán, A phenomenological model for analyzing reciprocating compressors, International Journal of Refrigeration, in this issue, doi:10.1016/j.irefrig.2007.02.006]. The relative influence of the diverse compressor losses is estimated as a function of the operating conditions. In addition, a comparison study between propane and R407C was carried out for one compressor and the observed differences were analyzed in terms of the compressor model. This study was also useful to verify the model''s goodness with the aim of predicting the compressor performance with an untested refrigerant. [Copyright &y& Elsevier]

Abstract: This paper studies refrigeration cycles in which plate heat exchangers are used as either evaporators or condensers. The performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated. This procedure is also used to compare several heat transfer coefficients in the refrigerant side. As in previous works the models of all the cycle components are considered together with the heat exchanger models in such a way that the system of equations they provide is solved by means of a Newton–Raphson algorithm. Calculated and measured values of the evaporation and the condensation temperatures are also compared. The experimental results correspond to the same air-to-water heat pump studied in other papers and they have been obtained by using refrigerants R-22 and R-290. [Copyright &y& Elsevier]

Abstract: This is the second paper of a series that assesses the performance of a refrigeration system model by means of cycle parameters. In this case, the condensation temperature is the parameter to study and it is focused on fin and tube condensers. It also studies the influence of the heat transfer models on the estimation of this refrigeration cycle parameter and different correlations for the heat transfer coefficients have been implemented in order to characterise the heat transfer in the heat exchangers. The flow inside the heat exchangers is considered one-dimensional as in previous works. In the cycle definition, other submodels for all the cycle component have been taken into account to complete the system of equations that characterises the behaviour of the refrigeration cycle. This global system is solved by means of a Newton–Raphson algorithm and a known technique called SEWTLE is used to model the heat exchangers. Some experimental results are employed to compare the condensation temperatures provided by the numerical procedure and to evaluate the performance of each heat transfer coefficient. These experimental results correspond to an air-to-water heat pump and are obtained by using R-22 and R-290 as refrigerants. [Copyright &y& Elsevier]

Abstract: A new way to assess the performance of refrigeration system models is presented in this paper, based on the estimation of cycle parameters, such as the evaporation temperature which will determine the validity of the method. This paper is the first of a series which will also study the influence of the heat transfer coefficient models on the estimation of the refrigeration cycle parameters. It focuses on fin and tube evaporators and includes the dehumidification process of humid air. The flow through the heat exchanger is considered to be steady and the refrigerant flow inside the tubes is considered one-dimensional. The evaporator model is discretised in cells where 1D mass, momentum and energy conservation equations are solved by using an iterative procedure called SEWTLE. This procedure is based on decoupling the calculation of the fluid flows from each other assuming that the tube temperature field is known at each fluid iteration. Special attention is paid to the correlations utilised for the evaluation of heat transfer coefficients as well as the friction factor on the air and on the refrigerant side. A comparison between calculated values and measured results is made on the basis of the evaporation temperature. The experimental results used in this work correspond to an air-to-water heat pump and have been obtained by using R-22 and R-290 as refrigerants. [Copyright &y& Elsevier]

Abstract: An analysis was carried out to study the efficiency of annular fin when subjected to simultaneous heat and mass transfer mechanisms. The temperature and humidity ratio differences are the driving forces for the heat and mass transfer, respectively. Analytical solutions are obtained for the temperature distribution over the fin surface when the fin is fully wet. The effect of the atmospheric pressure on the fin efficiency was also studied, in addition to fin optimum dimensions. It is demonstrated that the closed-form solutions for a dry-fin case presented in many text books are special cases for the solutions presented in this paper. [Copyright &y& Elsevier]

Abstract: This study discusses the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on evaporator performance by adopting updated correlations of EVSIM, a numerical analysis model based on the tube-by-tube method developed by Domanski. The heat exchanger types considered are the cross-counter flow type and cross-parallel flow type. The refrigerants considered for the numerical test as a working fluid are R-134a, R-410A and R-22. For inner tube configuration, enhanced tube and smooth tube cases are considered. For the air side evaporation performance, heat exchangers using plate fins, wavy fins and slit fins are analyzed. Results show that the heat transfer rate of the cross-counter flow type heat exchanger is 3% higher than that of the cross-parallel flow type with R-22. The total heat transfer rate of the evaporator using R-410A is higher than those using R-22 and R-134a, while the total pressure drop of R-410A is lower than those of R-22 and R-134a. The heat transfer rate of the evaporator using enhanced tubes is two times higher than that using smooth tubes, but the pressure drop of the enhanced tube is 45–50% higher than that of the smooth tubes. The evaporation performance of slit fins is superior to that of plate fins by 54%. [Copyright &y& Elsevier]

Abstract: The multi-zone model is widely used in evaporator and condenser modeling. The basic two-zone evaporator model and three-zone condenser model might reduce to one-zone or two-zone model depending on the operating conditions. In the traditional multi-zone models, logic constraints are invoked to separate the possible running modes and trigger mode changes from one to another, which will lead to the complicated logic in implementation. A logic unconstrained multi-zone model has been developed in this paper. In the new model, all the logic constraints are transformed into continuously differentiable equations. As a result, all the equations can be solved simultaneously without any logic constraints. Numerical examples show that the new logic unconstrained multi-zone models are in good agreement with the traditional ones. [Copyright &y& Elsevier]

Abstract: A dummy port plays an important role in the porting process and the improvement of the performance of a scroll compressor. This paper documents an investigation on the working mechanism of the dummy port in a scroll compressor. To characterize the dummy port effects on the different parts of the scroll compressor, two scroll compressors, one with and the other without a dummy port, are studied comparatively. The flow through the dummy port is examined in the background of an integrated compressor working process. The assembly of the compressor under investigation includes the upper bearing housing, scrolls, check valve, and discharge plenum. The Navier–Stokes equations with a k–ɛ turbulence model are solved at the standard operating conditions of a scroll compressor. Refrigerant-22 is used as the working fluid. The thermodynamic and transport properties of the refrigerant gas are modeled by the Martin–Hou equation of state and power laws, respectively. Global flow physics is investigated first to lay a foundation to understand the working mechanisms that control the porting process before averaging techniques are applied. The behavior of the gas pockets in the porting process is characterized in both geometric and dynamic nature. The time-dependent variation of volume, mass, energy, and volume-averaged field quantities inside the gas pockets are studied throughout the porting process. The impact of the dummy port on the compressor performance is defined. [Copyright &y& Elsevier]

Abstract: This paper presents simple and thermodynamically realistic models of two types of compressors widely used in domestic heat pumps (reciprocating and scroll compressors). These models calculate the mass flow rate of refrigerant and the power consumption from the knowledge of operating conditions and parameters. Some of these parameters may be found in the technical datasheets of compressors whereas others are determined in such a way that the calculated mass flow rate and electrical power match those given in these datasheets. The two models have been tested on five reciprocating compressors and five scroll compressors. This study has been limited to compressors with a maximum electrical power of 10kW and for the following operating conditions: evaporating temperatures ranging from −20 to 15°C and condensing temperatures ranging from 15 to 60°C. The average discrepancies on mass flow rate and power for reciprocating compressors are 1.10 and 1.69% (for different refrigerants: R134a, R404A, R22, R12 and R407C). For scroll compressors, the average discrepancies on mass flow rate and power are 2.42 and 1.04% (for different refrigerants: R134a, R404A, R407C and R22). [Copyright &y& Elsevier]

Abstract: The objectives of this paper are to develop experimental correlations of heat transfer for enhanced tubes used in a falling film condenser, and to provide a guideline for optimum design of the falling film condenser with a high condensing temperature of 59.8°C. Tests are performed for four different enhanced tubes; a low-fin and three Turbo-C tubes. The working fluid is HFC134a, and the system pressure is 16.0bar. The results show that the heat transfer enhancement of low-fin tube, Turbo-C (1), Turbo-C (2) and Turbo-C (3) ranges 2.8–3.4 times, 3.5–3.8 times, 3.8–4.0 times and 3.6–3.9 times, respectively, compared with the theoretical Nusselt correlation. It was found that the condensation heat transfer coefficient decreased with increasing the falling film Reynolds number and the wall subcooling temperature. It was also found that the enhanced tubes became more effective in the high wall subcooling temperature region than in the low wall subcooling temperature region. This study developed an experimental correlation of the falling film condensation with an error band of ±5%. [Copyright &y& Elsevier]

Abstract: A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behavior of capillary tube–suction line heat exchangers considering metastable region and separated flow has been developed in Part I of this paper. The developed numerical model allows analysis of aspects such as geometry, type of fluid, critical or non-critical flow conditions and metastable region. The accuracy of the detailed simulation model is demonstrated in this part (Part II) of the paper by comparing simulation results with a wide range of steady state experimental data from the technical literature, which include the refrigerant mass flow rate, outlet suction line temperature, and temperature profile along concentric and lateral capillary tube–suction line heat exchangers. Of the 196 data points evaluated for mass flow rate 96.4% are within an error of ±15%, 81.1% are within ±10% with a mean deviation of ±6.3%. Of the 143 data points evaluated for outlet suction line temperature 89.5% are within an error of ±2°C, with a mean deviation of ±0.98°C. The numerical results obtained are used to understand the refrigerant flow behavior inside non-adiabatic capillary tubes. Some divergence problems in the numerical solution process is found to be the discontinuity in non-adiabatic capillary tube flow characteristics caused by re-condensation of the refrigerant within the heat exchanger zone; this aspect needs special attention while modeling the non-adiabatic capillary tube flow. Other important parameter to be evaluated experimentally with special care is the capillary tube internal diameter due to its strong influence on the refrigerant flow results (results of any study based on the nominal diameter are to be used with caution). [Copyright &y& Elsevier]

Abstract: The cooling performance of a CO2 cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO2 cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO2 system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO2 cycle with the IHX was reduced by 0.5MPa. The IHX increased the cooling capacity and COP of the CO2 cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60Hz. [Copyright &y& Elsevier]

Abstract: A capillary tube is a common expansion device widely used in small-scale refrigeration and air-conditioning systems. A generalized correlation of refrigerant mass flow rate through adiabatic capillary tubes covering both subcooled and two-phase inlet conditions is expected for multiple purposes. Based on the homogeneous equilibrium flow model, a new group of dimensionless parameters has been proposed. To express the nonlinear relationship between the mass flow rate and the associated parameters, the multi-layer perceptron neural network is employed as a universal function approximator. Simulated data from a validated homogeneous equilibrium model are used for the neural network training and testing. A 5-6-1 network trained with the simulated data of R600a and R407C shows good generality in predicting the simulated data of R12, R22, R134a, R290, R410A, and R404A. Also, the deviations between the trained neural network and the experimental data from the open literature fall into ±10%. [Copyright &y& Elsevier]

Abstract: A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behavior of capillary tube–suction line heat exchangers has been carried out. The governing equations (continuity, momentum, energy and entropy) for fluid flows, together with the energy equation in solids, are solved iteratively in a segregated manner. The discretized governing equations in the zones with fluid flow are coupled using a fully implicit step-by-step method. An implicit central difference numerical scheme and a line-by-line solver were used in solids. A special treatment has been implemented in order to consider transitions (subcooled liquid region, metastable liquid region, metastable two-phase region and equilibrium two-phase region). All the flow variables (enthalpies, temperatures, pressures, mass fractions, heat fluxes, etc.) together with the thermophysical and transport properties are evaluated at each point of the grid in which the domain is discretized. The numerical model allows analysis of aspects such as geometry, type of fluid, critical or non-critical flow conditions, metastable regions and transient cases. Comparison of the numerical simulation with experimental data presented in the technical literature will be shown in Part II of the present paper. [Copyright &y& Elsevier]

Abstract: This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40°C with mass fluxes ranging from 400 to 800kgm−2 s−1, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%. [Copyright &y& Elsevier]

Abstract: China has become one of the largest refrigerants market in the world with the rapid economy development. This paper details the status of natural working fluid research and application developments in China as an essential element for the sustainable economic development and environmental protection. The natural working fluids examined include carbon dioxide, water, ammonia, air and hydrocarbon. Prestigious universities and enterprises in China have made significant achievements in recent years, especially on the application of carbon dioxide, water, and ammonia. Application of absorption and adsorption systems using water and ammonia as refrigerants are promising technically and economically, and various commercialized products have been developed for utilization in industry as well as commercial building. Evaporative/desiccant cooling markets are also expanding quickly in China. However, commercialized products with carbon dioxide as working fluid are still not available in China. Academic and industrial sectors should work together promote the application of carbon dioxide. With the strong support from government and society, it can be expected that more achievements could be obtained in near future. [Copyright &y& Elsevier]

Abstract: Sensitivity analysis can be used to identify important model parameters, in particular, normalized sensitivity coefficients; by allowing a one-on-one comparison. Regarding design of evaporative coolers, the sensitivity analysis shows that all sensitivities are unaffected by varying the mass flow ratio and that outlet process fluid temperature is the most important factor. In rating evaporative coolers, effectiveness is found to be most sensitive to the process fluid flow rate. Also, the process fluid outlet temperature is most sensitive to the process fluid inlet temperature. For evaporative condensers, the normalized sensitivity coefficient values indicate that the condensing temperature is the most sensitive parameter and that these are not affected by the value of the mass flow ratio. For evaporative condenser design, it was seen that, for a 53% increase in the inlet relative humidity, the normalized sensitivity of the surface area increased 1.8 times in value and, for a 15°C increase in the condenser temperature, the sensitivity increased by 3.5 times. The performance study of evaporative condensers show that, for a 72% increase in the inlet relative humidity, the normalized sensitivity coefficient for effectiveness increased 2.4 times and, for a 15°C increase in the condenser temperature, it doubled in value. [Copyright &y& Elsevier]

Abstract: The mathematical models of evaporative fluid coolers and evaporative condensers are studied in detail to perform a comprehensive design and rating analysis. The mathematical models are validated using experimental as well as numerical data reported in the literature. These models are integrated with the fouling model presented in an earlier paper, using the experimental data on tube fouling. In this paper, we use the fouling model to investigate the risk based thermal performance of these evaporative heat exchangers. It is demonstrated that thermal effectiveness of the evaporative heat exchangers degrades significantly with time indicating that, for a low risk level (p=0.01), there is about 66.7% decrease in effectiveness for the given fouling model. Furthermore, it is noted that there is about 4.7% increase in outlet process fluid temperature of the evaporative fluid cooler. Also, a parametric study is performed to evaluate the effect of elevation and mass flow rate ratio on typical performance parameters such as effectiveness for rating calculations while surface area for design calculations. [Copyright &y& Elsevier]

Abstract: A general distributed model with two-phase flow for refrigerant coupled with a frost model is developed for studying the dynamic behavior of an evaporator. The equations are derived in non-steady-state manner for the refrigerant and a quasi-steady state model with permeation for the frost. The complex flow and geometry of the finned tube evaporator lead to uneven wall and air temperature distributions, which in turn affect the rate of frost growth and densification along the coil depth. Results include frost accumulation and its effect on energy transfer, air off-coil temperature, refrigerant liquid dry-out position and propagation of frost formation along the coil. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator, respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO2 cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8–11MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated. A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle. [Copyright &y& Elsevier]

Abstract: Carbon dioxide has been used as a refrigerant in vapour compression systems of many types for over 130 years, but it is only in the last decade that inventive minds and modern techniques have found new ways to exploit the uniquely beneficial properties of this remarkable substance. This paper traces the development of the old carbon dioxide systems, considers the technical, commercial and social reasons for their slow development and subsequent decline and examines the recent renaissance across a surprisingly broad range of applications, from trans-critical car air conditioners to low temperature industrial freezer plants. The paper then concentrates on industrial refrigeration systems, which were the basis of early developments in the period 1865–1885, but which have been somewhat overlooked in the current renaissance. The paper concludes with a review of possible future developments, indicating the areas of research and product development required to maximise the potential of the only non-toxic, non-flammable, non-ozone-depleting, non-global-warming refrigerant available for Rankine cycle vapour compression systems in the 21st century. [Copyright &y& Elsevier]