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• Performance of a two circuits CO 2 train AC system has been experimentally tested under different operating conditions. • An introduction of a numerical model of CO 2 train AC system has been presented. • Effects of operating conditions on the train AC system have been discussed in detail. • An error analysis between the test and the numerical has been presented. • A related equation between the gas cooler outlet temperature and the optimum high pressure is presented. This paper aims to investigate the effects of different operating parameters on the performance of a transcritical CO 2 air conditioning system for trains. Similar designs of automotive CO 2 air conditioning system are used in the train system such as internal heat exchanger (IHX) to improve the system efficiency. Experimental results are presented for the performance characteristics of the CO 2 train air conditioning system with two independent circuits under different test conditions such as different refrigerant charge amount, compressor working frequency, indoor air volume flow rate and outdoor chamber temperature. Two different operating modes of the two circuits are also included in this research. For mode I while only one circuit is working, the test results show that the maximum cooling capacity and coefficient of performance (COP) is 24.3 kW and 2.7, during 55 Hz and 40 Hz compressor working frequency conditions respectively. Also, the cooling capacity and COP reach the minimum, 16.2 kW and 1.5, when the outdoor chamber temperature is 45°C. For mode II while both two circuits are working, the results show that the cooling capacity and COP are 42.0 kW and 2.4, respectively. The system performance error between the test and numerical modeling which directs the design of the system is within ±5%. Furthermore, an optimum high pressure control algorithm for the transcritical CO 2 cycle of the train AC system is also presented. [ABSTRACT FROM AUTHOR]

Highlights • The air conditioning systems with/without a vapor–liquid heat exchanger are compared. • EER at cooling mode and COP at heating mode are improved by 9.8% and 7.4%, respectively. • Various benefits are identified at the variable operating conditions. • The enhancement mechanisms of the vapor–liquid separation evaporation are proposed. Abstract Air conditioning system consumes a large percentage of energy and its efficiency becomes important in this energy-conscious era. In this paper, a heat exchanger with vapor–liquid separation is applied to air conditioning system (system-B), it is experimentally compared to the baseline system with a conventional heat exchanger (system-A). The results indicate that at cooling mode the system-B has an improvement of EER by 9.8%, while at heating mode, it exhibits a higher heating capacity and lower power consumption, and COP is enhanced by 7.3%. Under variable operating conditions at heating mode, the system-B gets benefits over system-A from various perspectives. Moreover, the study of automatic defrosting of system-B shows well-functioning and the results meet the corresponding requirements. Finally, the mechanisms of how the vapor–liquid separation enhances heat transfer coefficient during evaporation are proposed. It is suggested that its implementation in air conditioning system brings large potential of performance improvement. [ABSTRACT FROM AUTHOR]

The main purpose of this work is to enhance the energy efficiency of CO 2 automobile air conditioning system. Theoretical analysis demonstrated that the mixture of CO 2 and propane can improve its performance, thus, experiments have been carried out to see effects of various CO 2 -propane mass fractions of 100/0, 90/10, 80/20, 70/30, 60/40, 50/50 on the system performance at different ambient temperatures and gas cooler frontal air velocities. Experimental results show similar trends with those from the theoretical results. It has been shown that under the same compressor speed, system COP reaches highest at 60% of CO 2 mass fraction, which is 29.4% higher than pure CO 2 system and even achieves equal level of the R134a system, the optimum pressure and discharge temperature are reduced up to a maximum of 40% and 47   °C during the research range. Furthermore, comparison was carried out under the same cooling capacity by adjusting compressor speed for different mass fraction of CO 2 , results demonstrate that the use of CO 2 -propane mixtures yields a maximum COP rise of 22%even when cooling capacity is kept constant. A new optimum high pressure control algorithm for the transcritical CO 2 -propane mixture cycle has been developed based on the experimental data within a deviation of 5%. [ABSTRACT FROM AUTHOR]

The present work aims to evaluate the performance characteristics of a vapor compression refrigeration system using R-438A as a retrofit refrigerant for R-22. In order to achieve this objective, a test facility is developed and experiments are performed over a wide range of chilled water inlet temperature (11:20 °C), condenser water inlet temperature (25:35 °C) and condenser water mass flow rate (363:543 kg h −1 ). Results showed that as the chilled water inlet temperature changes from 11.5 to 20.5 °C, system COP increases from 1.78 to 2.07 at constant condenser water inlet temperature of 25.5 °C. Cooling capacity and COP of the system using R-438A are lower than R-22 by 11% and 12.5%, respectively. However, compressor discharge temperature using R-438A is slightly lower than R-22 which confirms that R-438A can be used as a retrofit refrigerant for R-22 to complete the remaining life time of the existing plants. [ABSTRACT FROM AUTHOR]

Abstract: Environmental issues such as stratospheric ozone depletion and global warming have been the primary motivations for recent research in the air-conditioning and refrigeration fields. The CO2 air-conditioning system forms a transcritical cycle which shows slightly degraded performance compared to the conventional system. To overcome this disadvantage, a cycle adopting an ejector is suggested. Experimental results of a CO2 air-conditioning system using an ejector were compared for various outdoor temperatures and inverter frequencies. The COPs of an air-conditioning system using an ejector and conventional system were calculated and compared based on the pressures and temperatures of experiments. The cooling capacity and COP in the air-conditioning system using an ejector are higher than those in the conventional system at an entrainment ratio greater than 0.76. In addition, better performance is shown at entrainment ratio greater than a certain value in other test conditions. [Copyright &y& Elsevier]

Abstract: In this research, the feasibility of using non-homogeneous, chemically-modified aluminum surfaces to more effectively manage condensate formation on a round-tube, plain-fin heat exchanger is explored. Surfaces having an anisotropic, patterned wettability may be preferred in HVAC&R applications since droplet motion can be restricted to one direction (i.e. downward with gravity) thereby mitigating condensate carryover into the occupied space. In this work, micro-grooved aluminum surfaces were created and then used to construct two prototype heat exchangers for testing. The results have shown that the micro-grooved structure of the fin surface in combination with an alkyl silane coating can reduce water retention by more than 27 percent. The new fin surface design was also shown to decrease the air-side core pressure drop of the prototype heat exchangers during wet operation by an average of 30–36 percent over the baseline heat exchanger, with the actual percentage strongly dependent on the flow rate. [Copyright &y& Elsevier]

Abstract: In the present paper, a mathematical model is developed to design R134a ejector and to predict the performance characteristics of a vapor jet refrigeration system over a wide range of the investigated parameters. These parameters include boiling temperature (65–85°C), condensing temperature (25–40°C), evaporating temperature (0–10°C), degrees of superheat (0–15°C), nozzle efficiency (0.75–0.95) and diffuser efficiency (0.75–0.95). Simulated results showed that the present model data are in good agreement with experimental data in the literature with an average error of 6%. It is found that the ejector area ratio at boiling temperature of 85°C is about double that at boiling temperature of 65°C for various evaporating and condensing temperatures. The present results confirm that waste heat sources of temperature ranging from 65 to 85°C are adequate to operate vapor jet refrigeration system for air-conditioning applications. [Copyright &y& Elsevier]

Abstract: Several preceding researches have evidenced that the transcritical air conditioning system using CO2 as a refrigerant has an inherent inefficiency resulting in degraded steady-state system performance of a CO2 air conditioning system compared with that of a conventional air conditioning system. As a practical improvement, two-phase ejector was considered in place of expansion device in this study. The two-phase ejector for CO2 air conditioning system was designed and developed considering the non-equilibrium state for evaluating the sonic velocity and the critical mass flux. The experiments of performance with respect to variation of ejector geometry such as the motive nozzle throat diameter, mixing section diameter and the distance between motive nozzle and diffuser were carried out. There exist optimum design parameters in each test. Experiments showed that the coefficient of performance of the system using an ejector was about 15% higher than that of the conventional system. [Copyright &y& Elsevier]

Abstract: This paper gives an overview of historical and present developments on how ejectors can be utilized to improve the performance of air-conditioning and refrigeration systems. Research on ejector refrigeration cycles that utilize low-grade energy sources to produce cooling is summarized. Another major class of ejector refrigeration cycles that is described tries to recover expansion work by means of a two-phase ejector. This particular approach appears to be very promising for transcritical carbon dioxide (CO2, R744) systems with inherently large throttling losses. The paper further presents the latest analytical and experimental results of a comprehensive study carried out to investigate possible performance improvements of transcritical R744 two-phase ejector systems. Relevant operational parameters were varied and effects on performance resulting from different ejector geometries were studied as well. Two-phase mixing shock waves inside the ejector were detected by recoding static wall pressure distributions. [Copyright &y& Elsevier]

Abstract: Solar desiccant-based air-conditioning has the potential to significantly reduce cost and/or greenhouse gas emissions associated with cooling of buildings. Parasitic energy consumption for the operation of supply fans has been identified as a major hindrance to achieving these savings. The cooling performance is governed by the trade-off between supplying larger flow-rates of cool air or lower flow-rates of cold air. The performance of a combined solid desiccant-indirect evaporative cooler system is analysed by solving the heat and mass transfer equations for both components simultaneously. Focus is placed on varying the desiccant wheel supply/regeneration and indirect cooler secondary/primary air-flow ratios. Results show that for an ambient reference condition, and 70 °C regeneration temperature, a supply/regeneration flow ratio of 0.67 and an indirect cooler secondary/primary flow ratio of 0.3 gives the best performance with COP e > 20. The proposed cooling system thus has potential to achieve substantial energy and greenhouse gas emission savings. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the theoretical details of the journal bearing design for Revolving Vane (RV) rotary type compressor. The mathematical models, force analysis, and the theoretical design analysis for the dynamically loaded journal bearing of the compressor are presented. The variations of the minimum oil film thickness and the maximum film pressure developed are shown and the influences of design parameters, compressor configurations, and operating conditions on the journal bearing performance are described. For an air-conditioning compressor of 1.5kW cooling capacity, the lowest minimum oil film thickness for lower and upper journal bearings is 6.6μm and 4.3μm respectively, when the cylinder is simply supported and the rotor is cantilever-supported. The lowest minimum oil film thickness for the journal bearing, which supports the cylinder, reduces to 4.0μm if the cantilever-type support is used. Besides that, a short-fat configuration of RV compressor is favorable in the interest of lowering bearing forces. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the experimental performance of an automotive air conditioning (AAC) system for the cases of employing fixed and variable capacity compressors (FCC and VCC). For this aim, an experimental system consisting of original components from an HFC134a AAC system has been set up and instrumented. For each compressor case, the system has been tested under steady-state operating conditions by varying the compressor speed, temperatures of the air streams entering the condenser and evaporator as well as the velocities of these air streams. The energy and exergy analysis has been applied to the experimental system, and its performance for both compressor operations has been evaluated. The results show that the operation with the VCC usually yields a higher COP than the operation with the FCC in expense of a lower cooling capacity. Furthermore, the cooling capacity and the rate of total exergy destruction in the VCC operations remain almost constant after a certain compressor speed, while both parameters increase continually with the compressor speed in the FCC operations. [Copyright &y& Elsevier]

Abstract: A desiccant dehumidification system with air can decrease energy consumption because it can be driven by low-grade waste heat below 80°C. If this system can be driven by low-temperature heat sources whose temperature is below 50°C, exhausted heat from fuel cells or air conditioners that exist everywhere can be used as heat sources. This could lead to considerable energy saving. This study provides a detailed evaluation of the performance of a four-partition desiccant wheel to make a low-temperature driving heat source possible and achieve considerable energy saving by the simulation and experiment. Further, the study investigates the in-depth performance of a hybrid air-conditioning system with a four-partition desiccant wheel by simulation. As a result, it was clear that there exists an optimum rotational speed to maximize the dehumidification performance and that the hybrid air-conditioning system improves COP by approximately 94% as compared to the conventional vapour compression-type refrigerator. [Copyright &y& Elsevier]

Abstract: This paper deals with the performance evaluation of an innovative, waste heat driven dual evaporator type three-bed adsorption cycle for cooling application. The innovative adsorption chiller has two evaporators, three adsorbent beds, and a condenser, and the evaporators work at different pressure levels. The effects of hot water inlet temperature, chilled water inlet and outlet temperatures, and cycle time on the specific cooling capacity (SCC) and coefficient of performance (COP) were predicted by simulation. For the same operating condition, the SCC and COP of the dual evaporator type three-bed adsorption chiller were found to be 1.5 and 1.7 times higher than those of the two-bed single-stage adsorption chiller, respectively. [Copyright &y& Elsevier]

Abstract: In the present paper, two retrofitted compact and high efficient microchannel heat exchangers were proposed. The new microchannel heat exchangers have advantages in compactness (17.2% and 15.1% volume reduction for evaporator and condenser, respectively), weight (2.8% and 14.9% lighter for evaporator and condenser, respectively), heat transfer characteristics compared with the currently used heat exchangers in mobile air conditioning (MAC) industry. One enhanced and one baseline R134a MAC systems were established including the new microchannel heat exchangers and the traditional MAC heat exchangers, respectively. The system performances have been experimentally carried out under variable ambient conditions in psychrometric calorimeter test bench. The optimal system refrigerant charge amounts for both systems were tested and the results showed that the enhanced system with more compact heat exchangers could reduce system charge amount and the minimum effective charge amount was less than that of the baseline system. The enhanced system could supply more cooling capacity to car compartment under all test conditions because of higher performance heat exchangers. The coefficient of performance (COP) of the enhanced system was slightly lower than that of the baseline system under idle conditions but higher under all the other test conditions. Cooling capacity and COP of the enhanced system was increased by about 5% and 8% under high vehicle speed condition. [Copyright &y& Elsevier]

Abstract: This paper reports a performance analysis for a hybrid air conditioning system. In this system, the sensible heat load is primarily treated by a vapor compression heat hump; the latent heat load is treated by a liquid dehumidification system that uses a lithium chloride solution as a desiccant. In addition, by decreasing the humidity ratio of air flowing through the outdoor heat exchanger of the vapor compression heat pump, frosting can be retarded. The overall system performance was evaluated by a cycle simulation conducted both in summer and winter modes. Compared to a traditional air conditioning system, the hybrid air conditioning system improves the coefficient of performance (COP) by approximately 20% and 100% in summer and winter, respectively. [Copyright &y& Elsevier]

Abstract: A new type of compressor, called an orbiting compressor, is introduced in this paper. The orbiting compressor is characterized by an orbiting piston, and the piston or orbiter consists of a circular base plate and a ring type vane protruding vertically from the base plate. The orbiter is made to orbit in an annular space formed between two concentric circular walls via an Oldham-ring mechanism, producing two sealed gas pockets on both sides of the vane wrap with a 180° phase difference. This operating mechanism leads to alternating compression and discharge processes, which results in low torque variation. The orbiting compressor has been designed for an R410A residential air conditioner with a cooling capacity of 10.0 kW. The performance of the orbiting compressor model has been analytically investigated, where the volumetric, adiabatic and mechanical efficiencies were calculated to be 94.8%, 90.4% and 93.4%, respectively for the ARI condition. The EER was estimated to be about 10.86 with a motor efficiency of 89%. [Copyright &y& Elsevier]

Abstract: A thermodynamic cycle model is used to select an optimum adsorbent-refrigerant pair in respect of a chosen figure of merit that could be the cooling production (MJm−3), the heating production (MJm−3) or the coefficient of performance (COP). This model is based mainly on the adsorption equilibrium equations of the adsorbent–refrigerant pair and heat flows. The simulation results of 26 various activated carbon–ammonia pairs for three cycles (single bed, two-bed and infinite number of beds) are presented at typical conditions for ice making, air conditioning and heat pumping applications. The driving temperature varies from 80°C to 200°C. The carbon absorbents investigated are mainly coconut shell and coal based types in multiple forms: monolithic, granular, compacted granular, fibre, compacted fibre, cloth, compacted cloth and powder. Considering a two-bed cycle, the best thermal performances based on power density are obtained with the monolithic carbon KOH-AC, with a driving temperature of 100°C; the cooling production is about 66MJm−3 (COP=0.45) and 151MJm−3 (COP=0.61) for ice making and air conditioning respectively; the heating production is about 236MJm−3 (COP=1.50). [Copyright &y& Elsevier]

Abstract: In this paper the problem of designing advanced control systems for increasing the performances of low capacity HVAC system with single scroll compressor is addressed. In particular, a simulation environment based on Matlab/Simulink that has been validated on a state-of-the-art experimental facility and used to design an adaptive controller for single scroll compressor, packaged air-cooled water chillers is presented. The capability of the controller to substantially increase the energy performance of the system, as well as to achieve excellent regulation performances in process applications is demonstrated. [Copyright &y& Elsevier]

Abstract: The performance of a once-through solar desiccant cooling system, for air-conditioning a commercial office space, was modelled using the TRNSYS computer simulation software package. The study particularly focused on the potential for designing and operating a desiccant cooling system, without any thermal backup provided to mitigate for intermittent solar availability in three Australian cities. Increasing (i) indirect evaporative cooler effectiveness, (ii) air flow to the office space, and (iii) solar collector area were all shown to reduce the frequency of high temperature events inside the building occupied space. In the warm temperate climate of Melbourne (and to a lesser extent Sydney), high ventilation rates enabled comfort conditions to be maintained at or near acceptable levels in the occupied space, without the use of a backup thermal source. The synergy between evaporative cooling and solar desiccant cooling, observed in the warm temperate climates, was not evident in the tropical Darwin climate, suggesting that the selected ventilation desiccant cooling cycle is not appropriate for tropical climates. [Copyright &y& Elsevier]

Abstract: This paper presents new mass transfer performance data of a cross-flow liquid desiccant dehumidification system using a structured packed tower. The structured packing consists of cross-corrugated cellulose paper sheets with a surface area per unit volume ratio of 608m2 m−3. The liquid desiccant, viz. calcium chloride, flows through the pad from top to bottom, while the air flows horizontally making it a cross-flow configuration. The experimental dehumidification effectiveness from the present study was compared with the widely used Chung''s correlation (although developed for counter flow arrangement, as opposed to cross-flow in the current study) and Liu et al.''s correlation. A new empirical correlation was developed for the dehumidification effectiveness, which fitted the experimental data to within ±10%. The effect of varying air and solution inlet conditions and flow rates on the system performance was also quantified in the paper. [Copyright &y& Elsevier]

Abstract: The objectives of this study were to evaluate the performance of a novel two-stage rotary desiccant cooling (TSRDC) system and to obtain useful data and experiences for practical application. Newly developed compound desiccant (silica gel–haloids) was adopted in the system. An experimental set-up was built and used to test the system performance under three typical environmental conditions. System performances were evaluated in terms of moisture removal D and thermal coefficient of performance COPth. It has found that the required regeneration temperature of TSRDC system is low and COPth of the system is high. Regeneration temperatures from 65°C to 80°C, 65°C to 75°C and 80°C to 90°C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study. [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 growing interest in Italy for its cultural heritage and the necessity for better conservation usually require that museums be provided with appropriate HVAC systems. Control of indoor relative humidity must ensure that the vapour transfer between the materials and the ambient is such as to avoid mechanical, chemical and biological damage. Thus, it is necessary to balance the main thermal loads, that one due to the occupants in exhibition areas, subject to rapid change and directly proportional to the degree of overcrowding, and thermal load related to outdoor air. A case study concerning a simulated modern museum is presented in this paper. Using the dynamic simulation code DOE 2.1 E and hourly climatic data (TRY), the operating costs of different all-air systems (also with dehumidification by adsorption) have been evaluated for exhibition areas and stock spaces, as well as system performance in controlling thermal-hygrometric ambient parameters. [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 discusses the conservation of energy in a cogeneration system. A steam power cycle (Rankine) produces electrical power 2MW and steam is bleeded off from the turbine at 7bar to warm a factory or units of buildings during the winter or to supply a steam ejector refrigeration cycle to air-conditioning the same area during the summer. In the summer this system can be as alternative solution instead of absorption. Certainly the ejector refrigeration unit is more economical than absorption unit. The ratio of electrical power/heat is varied into the region (0.1–0.4) and the evaporator temperature of the ejector cycle is varied into the region (10–16°C). A computer program has been developed for the study of performance parameters of the cogeneration system. [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: 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: Desiccant systems find applications in a very large variety of industrial and daily usage products including the new HVAC installations. An overview of liquid desiccant technology has been presented in this paper along with a compilation of experimental performance data of liquid desiccant dehumidifiers, empirical dehumidification effectiveness and mass transfer correlations in a useful and easy to read tabular format. The latest trends in this area suggest that hybrid systems are of current interest to HVAC industry, not only for high latent load applications but also for improving indoor air quality. The paper presents a comprehensive comparative parametric analysis of packed bed dehumidifiers for three commonly used desiccant materials viz. triethylene glycol, lithium chloride and calcium chloride, using empirical correlations for dehumidification effectiveness from the literature. The analysis reveals significant variations and anomalies in trends between the predictions by various correlations for the same operating conditions, and highlights the need for benchmarking the performance of desiccant dehumidifiers. [Copyright &y& Elsevier]

Abstract: In this paper, the performance of the solar-driven ejector refrigeration system with iso-butane (R600a) as the refrigerant is studied. The effects that both the operating conditions and the solar collector types have on the system''s performance are also examined by dynamic simulation. The TRNSYS and EES simulation tools are used to model and analyze the performance of a solar-driven ejector refrigeration system. The whole system is modelled under the TRNSYS environment, but the model of the ejector refrigeration subsystem is developed in the Engineering Equations Solver (EES) program. A solar fraction of 75% is obtained when using the evacuated tube solar collector. In the very hot environment, the system requires relatively high generator temperature, thus a flat plate solar collector is not economically competitive because the high amount of auxiliary heat needed to boost up the generator temperature. The results from the simulation indicate that an efficient ejector system can only work in a region with decent solar radiation and where a sufficiently low condenser temperature can be kept. The average yearly system thermal ratio (STR) is about 0.22, the COP of the cooling subsystem is about 0.48, and the solar collector efficiency is about 0.47 at T e 15°C, T c 5°C above the ambient temperature, evacuated collector area 50m2 and hot storage tank volume 2m3. [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: The paper is aimed to develop a high performance rotary solid desiccant cooling system using a novel compound desiccant wheel (DW). The unique feature of the desiccant wheel is that it can work well under a lower regeneration temperature and have a higher dehumidification capacity due to the contribution of the new compound desiccant materials. Experimental results indicate that the novel desiccant wheel under practical operation can remove more moisture from the process air by about 20–40% over the desiccant wheel employing regular silica gel. A mathematical model that is used to predict the system performance has been validated with the test results. By integrating the desiccant wheel with evaporative cooling, heat recovery and heating for regeneration sections, a solid desiccant cooling system can be formed. Simulation results show that because of the use of the new compound desiccant, the desiccant cooling system can work under much lower regeneration temperature and have a relative high COP, thus low grade thermal energy resources, such as solar energy, waste heat, etc., can be efficiently utilized to drive such a cooling cycle. [Copyright &y& Elsevier]

Abstract: Air conditioning is a non-critical application for fault detection and diagnosis (FDD) where decisions about servicing faults should involve the use of economics. Existing methods for evaluating impacts of faults on equipment performance only consider some individual factors such as the equipment coefficient of performance (COP) or cooling capacity. This paper develops an overall economic performance degradation index (EPDI) for air conditioning equipment that includes the combined effects of degradations in COP, cooling capacity, and sensible heat ratio (SHR). EPDI quantifies the performance degradation caused by faults based on economics so it can be used as part of the decision making process in an overall FDD system. Furthermore, EPDI can be used along with estimates of typical field performance degradations to assess the economic benefits associated with the application of automated FDD. A case study is presented where EPDI was applied to measurements for an existing unit where faults were artificially introduced. [Copyright &y& Elsevier]

Abstract: An accumulator heat exchanger (AHX) consists of an accumulator and an inner heat exchanger (IHX) contained in a shell. The AHX has been used in multi-air-conditioners to obtain system reliability and high performance by providing liquid refrigerant into expansion devices and preventing wet-compression. Energy is exchanged between the evaporator exit and the condenser exit in the AHX. In this study, the heat transfer characteristics of the AHX were investigated experimentally, and the effects of the AHX on the performance of a refrigeration system using R22 were measured. The operating characteristics of the refrigeration system with the AHX were considerably different from those without the AHX. The AHX system showed higher refrigerant flow rate than the non-AHX system at a constant EEV (electronic expansion valve) opening because of higher subcooling, resulting in better performance and reliability of the refrigeration system. At 50% EEV opening, the cooling capacity and COP of the AHX system were higher than those of the non-AHX system by 7.5% and 3.2%, respectively. [Copyright &y& Elsevier]

Abstract: CO2 and propane mixtures are considered as alternative refrigerants due to their negligible direct global warming potentials and favorable thermodynamic properties. To properly evaluate the system performance using zeotropic mixtures, the circulation concentration was measured and the cause for its shift from the charged concentration was discussed. The circulation concentration of CO2/propane mixtures has increased CO2 fraction than its charged concentration. In addition, the effect of refrigerant charge on the cooling performance was tested for the transcritical cycle of CO2 and the subcritical cycle of CO2/propane mixtures of 75/25 and 60/40 by the charged mass percentage. It is shown that CO2 refrigeration system could operate without a significant impact on its COP over a relatively wider range from the optimum charge. [Copyright &y& Elsevier]

Abstract: A stream analysis model was developed to simulate the behavior of accumulators and their influence on the automotive air conditioning (A/C) systems. It allows a comprehensive steady state simulation with a set of input conditions such as refrigerant vapor mass flow rate and pressure at the inlet of an accumulator. In this study, the refrigerant/oil mixture is R134a/PAG oil which are totally miscible, but could be any air conditioning refrigerant/oil, including carbon dioxide (CO2)/oil. The model accounts for all major effects inside the accumulator, such as friction, bends, sudden expansion, sudden contraction and heat exchange. The outputs are vapor quality, pressure and temperature at various positions of accumulator. In order to verify the mathematical model, experiments are performed in an experimental setup made up of real size automotive air conditioning components. The simulated results agree well with the experimental data. The simulation and experimental results show an important function of accumulators that is to determine the vapor quality into compressor, and thus has influence on the performance of whole automotive A/C systems. [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: The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7gkg−1 dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20°C for the summer design conditions of 32°C, 40% relative humidity. With air volume flow rates of 200m3 h−1 the system can provide 886W of cooling power. A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved. [Copyright &y& Elsevier]

Abstract: In this study, a CO2 automotive air conditioner prototype was designed and constructed. The compressor was of swash plate design; the gas cooler and evaporator were made of fin-tubes; a manual expansion valve and an internal heat exchanger accumulator were used. The lubricant, the CO2 charge, the evaporator outlet pressure, the compressor speed, the air inlet temperature and flow rate of the gas cooler and the air flow rate of the evaporator were varied and the performance of the prototype was experimentally investigated in detail. The cooling capacity, compressor power consumption, CO2 mass flow rate, and COP value were analyzed. The experimental results showed that the CO2 system performance was greatly affected by different lubricants; the CO2 system performance was sensitive to the mass charge; the high side pressure affected the system performance greatly and a high side pressure controller was needed. [Copyright &y& Elsevier]

Abstract: In order to determine the conservative stable region (CSR) proposed in part 1 of this paper, a dynamic model of the automotive air conditioning system with a variable displacement compressor and a thermal expansion valve is developed. The sensitivity analysis is done to show how uncertainties in input parameters influence the predicted results of system performance. The dynamic simulation is verified and the simulated results agree well with the experimental data. The CSR of automotive air conditioning system with a variable displacement compressor and a thermal expansion valve is simulated and analyzed when the vehicle speed or the air flow rate through the evaporator changes. The size of CSR is influenced by the direction and amplitude of external disturbance and the initial system state. The system is stable and the whole region is the CSR when the vehicle speed decreases or the air flow rate through evaporator increases, and the system sometimes is outside the CSR when the vehicle speed increases or the air flow rate through evaporator decreases suddenly. The influence of the initial state, change rate of compressor rotary speed and air temperature at the condenser inlet on the CSR when the vehicle speed increases is simulated. And the influence of the air temperature at the condenser inlet on the CSR when the air flow rate through evaporator decreases is analyzed, too. [Copyright &y& Elsevier]

Abstract: A detailed mathematical model of a compressor driven metal hydride (CDMH) cooling system is presented. The model takes into account the transient characteristics of the coupled metal hydride reactors with external fins, reciprocating compressor and conditioned space. The model can be used for predicting the system performance during the initial pull-down as well as the stable cycling periods. Results are obtained for a 1kW capacity air conditioning system using optimized reactors packed with Zr0.9Ti0.1Cr0.55Fe1.45 hydride. The COP''s obtained are in the range of 1.7–2.2 depending on operating conditions while the specific cooling power obtained is about 600kJkgalloy−1 h−1. It is shown that in order to make these systems commercially viable it is important to select suitable materials with fast reaction kinetics. Since, system performance parameters can be predicted more effectively, the model can be used for design of optimized systems. [Copyright &y& Elsevier]

Abstract: The psychrometric chart is a useful tool for calculating the energy change between any two states and visually identifying the contribution of ‘sensible’ and ‘latent’ effects. However, the amount of work needed to move from one state to another is not read directly from the chart. In this paper, temperature and vapor pressure changes are cast into terms of minimum work relative to a dead state, and work contours are plotted on the psychrometric chart. These contours show the minimum work needed to move the state of the air further from the ambient conditions or the maximum work produced by letting a state approach the ambient conditions. A dividing line between heat pump and refrigeration operation is set on the basis total energy of the conditioned space relative to the ambient. Finally, an example is used to illustrate the implementation of the analysis. [Copyright &y& Elsevier]

A numerical simulation using a validated computational fluid dynamics model is used to investigate and compare the performances of displacement and mixing ventilations (MVs), under different boundary conditions. A companion paper will report the results in terms of thermal comfort, while this paper investigates the indoor air quality (IAQ). The IAQ will be measured in terms of carbon monoxide, mean age of air and the three volatile organic compounds, toluene, benzene and formaldehyde, which are commonly found in indoor room furnishings. The case studies include offices, classrooms, retail shops and industrial workshops, under typical Hong Kong thermal boundary conditions. Compared with conventional MV, displacement ventilation may provide better IAQ in the occupied zone. [Copyright &y& Elsevier]

A numerical simulation using a validated Computational Fluid Dynamics (CFD) model of several types of buildings was used to investigate the difference between mixing and displacement ventilation (DV). The comparison of performances of displacement and mixing ventilations (MV) will be reported in two parts, one on thermal comfort and the other one on indoor air quality. This paper, i.e., Part I, compares the performances of floor-supply DV systems with traditional MV systems for offices, classrooms, retail shops and industrial workshops under a wide range of Hong Kong thermal and flow boundary conditions, such as a very high cooling load. Through proper design, DV can maintain a thermally comfortable environment that has a low air velocity, a small temperature difference between the head and ankle level, and a low percentage of dissatisfied people. [Copyright &y& Elsevier]

Abstract: Indoor space requires heating, cooling and ventilating for maintaining human occupant space to a comfortable level. Heat pump system is now widely used since it has the capabilities of providing both cooling and heating with a single unit. Ventilation, which exhausts the contaminated indoor air and brings in the fresh outdoor air is essential for maintaining pleasant indoor air quality. Ventilation, however, causes energy loss since air-conditioning is necessary to change the state of outdoor air to that of indoor. When outdoor air is introduced into the interior space, it must be cooled or heated to bring it to the indoor space condition. In this work, three methods of recovering sensible heat during heating and ventilation process of heat pump have been studied experimentally. Those methods are by a separate sensible heat exchanger, introduction of indoor air to the evaporator (single heat recovery), and finally a combination of fore-mentioned two methods (double heat recovery). An air-source heat pump system with none, single and double heat recovery capabilities has been built and tested in two constant-temperature and constant-humidity thermal chambers that simulate the indoor and outdoor environments. From the experiment performed under standard heating condition with a ventilation ratio of 23.1%, coefficient of performance for none, sensible heat exchanger, single and double heat recoveries were 2.88, 3.20, 3.18 and 3.28, respectively. Double heat recovery heat pump that has the ventilation and double heat recovery functions integrated into a single unit showed the best COP performance. [Copyright &y& Elsevier]

A steady state computer simulation model has been developed for refrigeration circuits of automobile air conditioning systems. The simulation model includes a variable capacity compressor and a thermostatic expansion valve in addition to the evaporator and micro channel parallel flow condenser. An experimental bench made up of original components from the air conditioning system of a compact passenger vehicle has been developed in order to check results from the model. The refrigeration circuit was equipped with a variable capacity compressor run by an electric motor controlled by a frequency converter. Effects on system performance of such operational parameters as compressor speed, return air in the evaporator and condensing air temperatures have been experimentally evaluated and simulated by means of developed model. Model results deviate from the experimentally obtained within a 20% range though most of them are within a 10% range. Effects of the refrigerant inventory have also been experimentally evaluated with results showing no effects on system performance over a wide range of refrigerant charges. [Copyright &y& Elsevier]