Performance of a split-type air conditioner matched with coiled adiabatic capillary tubes using HCFC22 and HC290

This paper experimentally investigated the system performance of a split-type air conditioner matching with different coiled adiabatic capillary tubes for HCFC22 and HC290. Experiments were carried out in a room-type calorimeter. The results have shown that (1) similar cooling effects can be achieved by matching various capillary tubes of different inner diameters; (2) parallel capillary tubes presented better system performance and flow stability with weaker inlet pressure fluctuations than the single capillary tube; (3) with the coil diameter of the capillary tube increasing from 40 mm to 120 mm, the mass flow rate tended to increase slightly. But the cooling capacity, input power and energy efficiency ratio (EER) did not show evident tendency of change; (4) the refrigerant charge and mass flow rate for HC290 were only 44% and 47% of that for HCFC22, respectively, due to the much lower density. And HC290 had 4.7–6.7% lower cooling capacity and 12.1–12.3% lower input power with respect to HCFC22. However, the EER of HC290 can be 8.5% higher than that of HCFC22, which exhibits the advantage of using HC290. In addition, the experimental uncertainties were analyzed and some application concerns of HC290 were discussed.     

Numerical simulation and experimental validation of coiled adiabatic capillary tubes

The objective of this study is to extend and validate the model developed and presented in previous works [O. García-Valladares, C.D. Pérez-Segarra, A. Oliva, Numerical simulation of capillary tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part I: mathematical formulation and numerical model, Applied Thermal Engineering 22 (2) (2002) 173–182; O. García-Valladares, C.D. Pérez-Segarra, A. Oliva, Numerical simulation of capillary tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part II: experimental validation and parametric studies, Applied Thermal Engineering 22 (4) (2002) 379–391] to coiled adiabatic capillary tube expansion devices working with pure and mixed refrigerants. The discretized governing equations are coupled using an implicit step by step method. 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, vapor qualities, velocities, heat fluxes, etc.) together with the thermophysical 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 (pure substances and mixtures), critical or non-critical flow conditions, metastable regions, and transient aspects. Comparison of the numerical simulation with a wide range of experimental data presented in the technical literature will be shown in the present article in order to validate the model developed.     

Simulated and experimental performance of split packaged air conditioner using refrigerant HC-290 as a substitute for HCFC-22

This paper discusses the use of propane (HC-290) as a safe and energy efficient alternative to HCFC-22 in a typical split air conditioner with nominal cooling capacities up to 5.1 kW. Initially split air conditioner performance is simulated for cooling capacity, energy efficiency ratio (EER), and refrigerant charge. Tests were conducted for different test cases in a psychrometric test chamber with HCFC-22 and HC-290. The test conditions considered are as per Indian Standards, IS 1391 (1992) Part I. The various parameters considered were based on simulated performance with the objective to achieve maximum EER for the desired cooling capacity. As the flammability is an issue for HC-290, the reduction of HC-290 charge was another objective. Two different types of condensers, first with smaller size tubing and another parallel flow condenser (PFC) or minichannel condenser were used in order to reduce HC-290 charge. For HC-290, the highest EER achieved was 3.7 for cooling capacity 4.90 kW for a refrigerant charge of 360 g.The important safety aspects of using HC-290 in air conditioner are discussed. The refrigerant charge as per EN 378 for different cooling capacities and room sizes is also considered.     

Evolving an optimal composition of HFC407C/HC290/HC600a mixture as an alternative to HCFC22 in window air conditioners

Countries that have ratified Montreal Protocol have to phase out HCFC22 in the near future due to its ozone depleting potential (ODP) and hence new eco-friendly refrigerants are being evolved as substitutes. At Present HFC407C is one of the promising drop-in substitutes for HCFC22. But it is immiscible with mineral oil and hence polyol ester (POE) oil is recommended. Since POE oil is highly hygroscopic in nature it is not user friendly. However such oil immiscibility issue of HFC134a has been overcome [M. Janssen, F. Engels, The use of HFC134a with mineral oil in hermetic cooling equipment, Report 95403, No. 07, presented in the 19th International Congress of Refrigeration, The Hague, 1995] by the addition of HC blend to it, which also resulted in performance improvements. In the present work an attempt has been made to study the possibility of using HFC407C/HC290/HC600a refrigerant mixture as a substitute for HCFC22 in a window air conditioner and to evolve an optimal composition for the mixture. Experiments were carried out in a room calorimeter setup fitted with 1050 W capacity window air-conditioner. Condenser inlet air temperatures were held constant at 30, 35, 40 and 45 °C, while evaporator inlet air temperatures were varied over a range viz. 21, 23, 25, 27 and 29 °C during the experimentation. The HC percentage was also varied from 10 to 25% in steps of 5%. The new refrigerant mixtures demand longer condenser length to decrease the high discharge pressure matching with HCFC22 systems and hence the length has been increased while testing the mixtures. This also resulted in better heat transfer in condenser. The performance analysis revealed that the new refrigerant mixture performed better than that of HCFC22. It has in fact been found that the new mixture can improve the actual COP by 8 to 11% and hence it can reduce the energy consumption by 5 to 10.5%. The overall performance has proved that the new refrigerant mixture could be an excellent substitute for HCFC22.     

Numerical analysis of refrigerant flow along non-adiabatic capillary tubes using a two-fluid model

This work presents a numerical model to simulate steady state refrigerant flow along capillary tube-suction line heat exchangers, commonly used in small refrigeration systems. The flow along the straight and horizontal capillary tube is divided into two regions: a single-phase and a two-phase flow region. The flow is taken as one-dimensional and the metastable flow phenomenon is neglected. The two-fluid model is employed for the two-phase flow region, considering the hydrodynamic and the thermodynamic non-equilibrium between the liquid and vapor phases. Comparisons are made with experimental measurements of the mass flow rate and temperature distribution along capillary tube-suction line heat exchangers working with refrigerant R134a in different operating conditions. The results indicate that the present model provides a good estimation of the refrigerant mass flow rate. Moreover, comparisons with a homogeneous model are also made. Some computational results referring to the quality, void fraction and velocities of each phase are also presented and discussed.     

Performance enhancement of a split air conditioner using porous material inside the evaporator pipes

In this experimental study, a porous material is used inside the pipes of the evaporator as the main heat exchanging device in the air conditioning cycle. The used porous material consists of stainless steel balls of different diameters. As a case study, refrigerant R454B, which is a drop-in replacement to refrigerant R410A, is used as a working fluid in the air conditioner thermodynamic cycle. Four different porosities were used during the experimental tests; 100% (empty tube), 46%, 40%, and 33%. This study investigated the influence of variation of porosity as well as outside air temperature and refrigerant evaporation temperature on the cycle coefficient of performance, evaporation capacity, pressure drop, and power consumption during the compression process. Measured evaporation temperatures and indoor temperatures during tests were in the range of 1.5–12°C and 18–25°C, respectively. The use of porous material in the evaporation heat exchanger resulted in a considerable increase in the cycle evaporation capacity and coefficient of performance. Varying porosity from 100% to 33% resulted in an average percent increase of cycle evaporation capacity and coefficient of performance by 48.8% and 84.3%, respectively. Also, decreasing porosity from 100% to 33% resulted in an average percent increase in power consumption during the compression process by about 27%. An average percent increase of power consumption of compressor by about 25.9% is also reported, when evaporation temperature increased from 1.5°C to 12°C. Increasing outside air temperature from 27.1°C to 39.5°C resulted in decreasing evaporation capacity and coefficient of performance by 35.2% and 34.5%, respectively, and in increasing compressor power consumption by about 14.3%. A considerable pressure drop was recorded during the evaporation process when using porous material. The volumetric evaporation capacity, as well as compressor discharge temperature, are increased by increasing evaporating temperature and by decreasing evaporator porosity. The increase in air ambient temperature resulted in a considerable increase in refrigerant mass flow rate.     

Flow characteristics of pure refrigerants and refrigerant mixtures in adiabatic capillary tubes

This paper provides the results of simulations using an adiabatic capillary tube model which is developed to study the flow characteristics in adiabatic capillary tubes used as a refrigerant control device in refrigerating systems. The developed model can be considered as an effective tool of capillary tubes' design and optimization for systems using newer alternative refrigerants. The model is validated by comparing with the experimental data of Li et al. and Mikol for R12 and Melo et al. for R134a. In particular, it has been possible to compare various pairs of refrigerants. It is found that the conventional refrigerants consistently give longer capillary lengths than the alternative refrigerants. For all pairs, the conventional refrigerant consistently give lower pressure drops for both single-phase and two-phase flow which resulted in longer tube lengths. In addition, an example of capillary tube selection chart developed from the present numerical simulation is shown. The chart can be practically used to select the capillary tube size from the flow rate and flow condition or to determine mass flow rate directly from a given capillary tube size and flow condition. The results of this study are of technological importance for the efficient design when systems are assigned to utilize various alternative refrigerants.     

Boiling heat transfer coefficient of R22 and an HFC/HC refrigerant mixture in a fin‐and‐tube evaporator of a window air conditioner

The commonly used refrigerant in unitary type air conditioners is R22 and its phase out schedule in developing countries is to commence from 2015. Many alternatives to R22 are found in published literature in which R407C has similar characteristics to those of R22 except for its zeotropic nature. However, R407C which is an HFC is made compatible with the mineral oil lubricant in the system compressor by the addition of 20% of HC. This HFC/HC mixture called the M20 refrigerant mixture is reported to be a retrofit refrigerant for R22. Though its latent heat value is greater than that of R22, its refrigerating capacity is lower when it is used to retrofit R22 window air conditioners. Hence, a heat transfer analysis was conducted in the evaporator of a room air conditioner, for practically realized heat flux conditions during standard performance testing. The tests were conducted as per the BIS and ASHRAE standards. Kattan–Thome–Favrat maps are used to confirm the flow patterns, which prevail inside the fin‐and‐tube evaporator in the tested operating conditions. It is revealed that the heat transfer coefficient/heat fluxes are poorer for M20 because of the lower mass flow rate and higher vapor fraction at the entry of the evaporator than that of R22 in the prevailing operating conditions. The heat transfer coefficients of the M20 refrigerant mixture under various test conditions are lower in the range of 14% to 56% than those of R22. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20299     

Performance variation of an R22 window air conditioner retrofitted with a HFC/HC refrigerant mixture under different ambient conditions over a range of charge quantities

R22 has been generally accepted as the most suitable refrigerant for air conditioners, due to its favorable thermodynamic properties. However, R22 is a controlled substance under the Montreal protocol. M20 is a HFC/HC refrigerant mixture that can be used as a substitute for R22. This paper presents experimental investigation on the performance comparison of a window air conditioner operated with the M20 tested under different refrigerant charge levels and outdoor conditions against that with R22. Experiments were conducted in accordance with BIS procedure in a psychrometric test facility. Refrigerant charge in the air conditioner was systematically varied from 900 to 1600 g in steps of 50 g for R22 and 697 to 1279 g in steps of 39 g [equivalent to 50 g of R22] for the M20. At each charge levels, the outdoor room conditions were changed in accordance with BIS standards. It is observed that R22 is more sensitive to deviations in charge levels as compared to the M20. A decrease in charge level of about 7% reduced the system refrigerating capacity by 11.3% with R22 while with the M20 refrigerant mixture it reduces by 6.9% only. Similarly an overcharge by 7% reduces the refrigerating capacity of the system by 13.8% with R22 while with M20 it reduces by 6.5% only. Thus M20 is less sensitive to charge deviations. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20339     

Prediction of heat transfer and flow characteristics in helically coiled tubes using artificial neural networks

In this study, Artificial Neural Network (ANN) models were developed to predict the heat transfer and friction factor in helically coiled tubes. The experiments were carried out with hot fluid in coiled tubes which placed in a cold bath. Coiled tubes with various curvature ratios and coil pitches (nine Layouts) were used. The output data of the ANNs were Nusselt number and friction factor. The validity of the method was evaluated through a test data set, which were not employed in the training stage of the network. Moreover, the performance of the ANN model for estimating the Nusselt number and friction factor in the coiled tubes was compared with the existing empirical correlations. The results of this comparison show that the ANN models have a superior performance in predicting Nusselt number and friction factor in the coiled tubes.     

Prediction of thermal and fluid flow characteristics in helically coiled tubes using ANFIS and GA based correlations

This study introduces the ability of Adaptive Neuro-Fuzzy Inference System (ANFIS) and genetic algorithm (GA) based correlations for estimating the hydrodynamics and heat transfer characteristics in coiled tubes. The experimental data related to the heat transfer and pressure drop in helically coiled tubes with deferent geometrical parameters (coil diameter and pitch) were used. In the experiments, hot water was passed in the coiled tubes, which were placed in a cold bath. Two ANFIS models were developed for predicting the Nusselt number (Nu) and friction factor (f) in the coiled tubes and the geometric parameters were employed as input data. Moreover, empirical correlations for estimating the Nu and f were developed by a phenomenological argument in the form of classical power–law correlations and their constants were found using the GA technique. The mean relative errors (MRE) of the developed ANFIS models for estimation of Nu and f are 6.24% and 3.54%, respectively. On the other hand, for empirical correlations, a MRE of 8.06% was found for prediction Nu while MRE of 5.03% was obtained for f. The results show that the ANFIS models can predict Nu and f with the higher accuracy than the developed correlations.     

Field study using the ground as a heat sink for the condensing unit of an air conditioner in Thailand

This paper reports on an investigation of the feasibility of using earth to absorb the heat normally rejected into the atmosphere by the condensing unit of a conventional air conditioner. To this end, a copper tube of about 67 m in length was buried at a depth of 1 m underground, where the temperature was constant at about 27°C year round. The copper coil of an air type condenser is about 22 m long. For the buried condenser, the R-22 refrigerant requirement was 5.8 kg as compared with 1.2 kg for the air condenser system. It was found that with this modified condensing unit, the coefficient of performance (COP) was much higher than that of a conventional one: it varied between 7.1 (daytime) and 8.1 (nighttime), compared to 2.8 and 3.1, respectively. The ground temperature near the buried copper coil did not increase, thus demonstrating the ability of the soil to dissipate the absorbed heat into the ground. Consequently, there is a high potential for contributing to environmental protection by using the ground as a heat sink. The elimination of the condensing fan is an additional advantage of the buried condenser system.     

An assessment of friction factor and viscosity correlations for model prediction of refrigerant flow in capillary tubes

In this paper, a homogeneous model including the metastable liquid and metastable two‐phase region is presented to assess the effects of various friction factor equations and two‐phase viscosity correlations on simulating the behaviour of capillary tubes. Both straight and coiled capillary tubes are considered and R‐22 is used for comparison. The predicted pressure distribution, tube lengths or mass flow rates are compared with experimental data reported in literature. It is confirmed that the predicting accuracy with homogeneous model can be improved by employing the suitable correlations of friction factor and two‐phase viscosity. For straight capillaries, the Churchill and Colebrook friction factor correlations give almost the same simulating results. However, the numerical results show that the optimum combination of correlations of friction factor and two‐phase viscosity may be different when compared with different experimental data. For coiled capillaries, the Mori and Nakayama friction factor correlation agrees well with Ito's formula for single liquid‐phase flow. Together with Giri's friction factor equation for two‐phase flow, Cicchitti viscosity model best predicts the measured mass flow rate with an average error of 4.88%. Copyright © 2004 John Wiley & Sons, Ltd.     

Performance of R433A for replacing HCFC22 used in residential air-conditioners and heat pumps

In this study, thermodynamic performance of R433A and HCFC22 is measured in a heat pump bench tester under air-conditioning and heat pumping conditions. R433A has no ozone depletion potential and very low greenhouse warming potential of less than 5. R433A also offers a similar vapor pressure to HCFC22 for possible ‘drop-in’ replacement. Test results showed that the coefficient of performance of R433A is 4.9–7.6% higher than that of HCFC22 while the capacity of R433A is 1.0–5.5% lower than that of HCFC22 for both conditions. The compressor discharge temperature of R433A is 22.6–27.9 °C lower than that of HCFC22 while the amount of charge for R433A is 57.0–57.7% lower than that of HCFC22 due to its low density. Overall, R433A is a good long term environmentally friendly alternative to replace HCFC22 in residential air-conditioners and heat pumps due to its excellent thermodynamic and environmental properties with minor adjustments.     

Construction and testing of a wet-compression absorption carbon dioxide refrigeration system for vehicle air conditioner

The environmental benefits of the transcritical carbon dioxide (CO₂) refrigeration cycle are considerable. But its application is greatly challenged by the high operation pressure, which could be as high as 120 bar. A wet-compression absorption (WCA) CO₂ refrigeration cycle was constructed by adding a non-volatile liquid into a CO₂ refrigeration cycle. CO₂ is highly soluble in the liquid and easily absorbed and desorbed by it. In the WCA CO₂ refrigeration cycle, the high side pressure was less than 35 bar, which was tremendously reduced compared to the transcritical CO₂ refrigeration cycle.In this paper, following a thermodynamic analysis of working fluid, a WCA CO₂ refrigeration demonstrator plant was constructed within the restricted physical and operational envelope of an existing vehicle refrigeration unit. This unique plant operated satisfactorily, delivering sustainable cooling for refrigerated vehicle. The relationship between system performance and the cycle ratio and IHX (internal heat exchanger) efficiency was tested. The components used in the demonstrator were entirely based on existing components and not optimized and considerable potential exists for efficiency improvements.     

Numerical and experimental investigation of two phase flow during boiling in a coiled tube

A numerical simulation, using the VOF multiphase flow model, and the corresponding experiments were conducted to investigate the boiling flow of R141B in a horizontal coiled tube. The numerical predictions of phase evolution were in a good agreement with the experimental observations, and the two phase flow in the tube bends was much more complicated due to the influence of liquid–vapor interaction with the interface evolution. The associated heat transfer was also considered. It was found that the temperature profile in the two phase flow was significantly affected by the phase distribution and higher temperature always appears in the vapor region.     

高湿度地区某整理车间蒸发冷却空调系统设计分析

江阴市某纺织品整理车间设计空调系统,对车间进行降温,改善工人工作环境。针对达到规范所要求的室内温湿度和适当放宽室内温湿度(在人体舒适区域内)时,提出蒸发冷却与机械制冷相结合的空调方案及蒸发冷却空调方案,并根据各自方案的特点、风量及制冷量等情况,得出应用蒸发冷却空调的可行性,最后根据此车间的特点,对方案进行优化,使得蒸发冷却与工位送风相结合,初投资和运行费用更低、节能效果更加显著。     

Numerical simulation of adiabatic and isothermal cracks in functionally graded materials using optimized element-free Galerkin method

In the present work, element-free Galerkin method (EFGM) is modified and implemented to simulate thermoelastic fracture in functionally graded materials (FGMs). By solving the simple heat transfer problem, the temperature distribution over the domain can be obtained which is later used as an input to determine the displacement and stress fields. The crack surfaces are modeled under adiabatic and isothermal conditions. To capture stress fields around the crack tip, intrinsic enrichment criterion is used. A modified conservative M-integral technique has been used to extract the stress intensity factors (SIFs) for the simulated problems. A new algorithm to ensure equal number of nodes in support domain has been suggested. The optimum size of support domain is derived by performing an optimization of predefined EFGM parameters, namely, total number of nodes in problem geometry, Gauss quadrature, and number of nodes in support domain. Taguchi L-16 orthogonal array is used to obtain optimized values of these parameters. The results of analysis by optimized EFGM (OEFG) show about 80% reduction in computational time and an improvement in accuracy over EFGM. The present analysis shows that the results obtained by OEFG are in good agreement with those available in the literature.     

Numerical study on high-temperature diluted air combustion for the turbulent jet flame in crossflow using an unsteady flamelet model

The turbulent jet flame in a crossflow with highly preheated diluted air has been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the flamelet concept coupled with the standard k–ε turbulence model. The NO formation is estimated by using the Eulerian particle transport equations in a postprocessing mode. For methane and propane with various conditions of inlet air temperature and oxygen concentration, the three-dimensional characteristics of the flame are successfully captured. The jet-flame trajectory is in remarkably good agreement with the existing cold-flow correlations. When the oxygen concentration is high, the maximum flame temperature becomes high and the two fuels show quite different characteristics in the downstream region. On the other hand, for low oxygen concentrations, the temperature difference between the two fuels is relatively small and remains fairly constant throughout the combustion chamber. The propane gives a higher NO formation compared to the methane especially when the oxygen concentration is high. A higher temperature, longer residence time of the combustion gases may be responsible for the higher thermal NO formation.     

Flow boiling heat transfer coefficient of R-134a/R-290/R-600a mixture in smooth horizontal tubes using varied heat flux method

An experimental study on in-tube flow boiling heat transfer of R-134a/R-290/R-600a refrigerant mixture has been carried out under varied heat flux test conditions. The heat transfer coefficients are experimentally measured at temperatures between ?8 and 5 °C for mass flow rates of 3–5 g s^?1. Acetone is used as a hot fluid which flows in the outer tube of diameter 28.57 mm while the refrigerant mixture flows in the inner tube of diameters 9.52 and 12.7 mm. By regulating the acetone flow conditions, the heat flux is maintained between 2 and 8 kW/m² and the pressure of the refrigerant is maintained between 3.2 and 5 bar. The comparison of experimental results with the familiar correlations shows that the correlations over predict the heat transfer coefficients for this mixture when stratified and stratified-wavy flow prevail. Multiple regression technique is used to evolve and modify existing correlations to predict the heat transfer coefficient of the refrigerant mixture. It is found that the modified version of Lavin–Young correlation (1965) predicts the heat transfer coefficient of the considered mixture within an average deviation of ±20.5 %.