An experimental investigation on performance of bubble pump with lunate channel for absorption refrigeration system

An experimental research on the performance of the bubble pump for absorption refrigeration units was made. The bubble pump provides the drive for the absorption cycle and is a decisive component of the absorption refrigeration unit. The bubble pump's property determines the efficiency of the absorption refrigeration system. A continuous experimental system with different size of bubbles pumps were designed, constructed and successfully worked. The experiments were performed by changing some of the parameters affecting the bubble pump performance. The experimental results shows that the performance of the bubble pump depends mainly on the driving temperature, the solution head and the combining tube diameters. With the suitable size of section area of the pump tubes the net elevating height of solution is 2.5 times as high as the solution submergence. The lunate channel has several outstanding characteristics, such as low starting temperature (minimum 68 °C), wide operating temperature range and lower requirement for vacuum condition (under 10 kPa). Then the elevating capability of the bubble pump with lunate channel is much better than others currently. It would provide well foundation for practical applications.     

A study on the performance of multi-stage condensation heat pumpsEtude sur la performance des pompes à chaleur à condensation en plusieurs étages

In this study, computer simulation programs were developed for multi-stage condensation heat pumps and their performance was examined for CFC11, HCFC123, HCFC141b under the same condition. The results showed that the coefficient of performance (COP) of an optimized ‘non-split type’ three-stage condensation heat pump was 25–42% higher than that of a conventional single-stage heat pump. The increase in COP differed among the fluids examined. The improvement in COP was due largely to the decrease in average temperature difference between the refrigerant and water in the condensers, which resulted in a decrease in thermodynamic irreversibility. For the three-stage heat pump, the highest COP was achieved when the total condenser area was evenly distributed to the three condensers. For the two-stage heat pump, however, the optimum distribution of total condenser area varied with working fluids. For the three-stage system, splitting the condenser cooling water for the use of intermediate and high pressure subcoolers helped increase the COP further. When the individual cooling water for the intermediate and high pressure subcoolers was roughly 10% of the total condenser cooling water, the optimum COP was achieved showing an additional 11% increase in COP as compared to that of the ‘non-split type’ for the three-stage heat pump system.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

Energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption refrigeration systems

In this paper, the energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption systems is presented. A computational model has been developed for the parametric investigation of these systems. Newly developed computationally efficient property equations of water–lithium bromide solution have been used in the computer code. The analysis involves the determination of effects of generator, absorber and evaporator temperatures on the energetic and exergetic performance of these systems. The effects of pressure drop between evaporator and absorber, and effectiveness of heat exchangers are also investigated. The performance parameters computed are coefficient of performance, exergy destruction, efficiency defects and exergetic efficiency. The results indicate that coefficient of performance of the single effect system lies in range of 0.6–0.75 and the corresponding value of coefficient of performance for the series flow double effect system lies in the range of 1–1.28. The effect of parameters such as temperature difference between heat source and generator and evaporator and cold room have also been investigated. Irreversibility is highest in the absorber in both systems when compared to other system components.     

Visualization of bubble behavior and bubble diameter correlation for NH3–H2O bubble absorption

The objectives of this paper are to visualize the bubble behavior for an ammonia–water absorption process, and to study the effect of key parameters on ammonia–water bubble absorption performance. The orifice diameter, orifice number, liquid concentration and vapor velocity are considered as the key parameters. The departing bubbles tend to be spherical for surface tension dominant flow, and the bubbles tend to be hemispherical for inertial force dominant flow. A transition vapor Reynolds number is observed at a balance condition of internal absorption potential (by the concentration difference) and external absorption potential (by the vapor inlet mass flow rate). As the liquid concentration increases, the transition Reynolds number and the initial bubble diameter increase. The initial bubble diameter increases with an increase of the orifice diameter while it is not significantly affected by the number of orifices. Residence time of bubbles increases with an increase in the initial bubble diameter and the liquid concentration. This study presents a correlation of initial bubble diameter with ±20% error band. The correlation can be used to calculate the interfacial area in the design of ammonia-water bubble absorber.     

A study on numerical simulations and experiments for mass transfer in bubble mode absorber of ammonia and water

An absorber is a major component in the absorption refrigeration systems, and its performance greatly affects the overall system performance. In this study, both the numerical and experimental analyses in the absorption process of a bubble mode absorber were performed. Gas was injected into the bottom of the absorber at a constant solution flow rate. The region of gas absorption was estimated by both numerical and experimental analyses. A higher gas flow rate increases the region of gas absorption. As the temperature and concentration of the input solution decrease, the region of gas absorption decreases. In addition, the absorption performance of the countercurrent flow was superior to that of cocurrent. Mathematical modeling equations were derived from the material balance for the gas and liquid phases based on neglecting the heat and mass transfer of water from liquid to gas phase. A comparison of the model simulation and experimental results shows similar values. This means that this numerical model can be applied for design of a bubble mode absorber.     

A study on the performance of multi-stage heat pumps using mixturesEtude sur la performance de pompes à chaleur multi-étagées utilisant des mélanges

Multi-stage heat pumps composed of a condenser, evaporator, compressor, suction line heat exchanger, and low and/or high stage economizers are studied by computer simulation. Their thermodynamic performance and design options are examined for various working fluids. In the simulation, HCFC22/HCFC142b and HFC134a are studied as an interim and long term alternatives for CFC12 while HFC32/HFC134a and HFC125/HFC134a are studied as long term alternatives for HCFC22. The results indicate that the three-stage super heat pump with appropriate mixtures is up to 27.3% more energy efficient than the conventional single-stage system with pure fluids. While many factors contribute to the performance increase of a super heat pump, the most important factor is found to be the temperature matching between the secondary heat transfer fluid and refrigerant mixture, which is followed by the use of a low stage economizer and suction line heat exchanger. The contribution resulting from the use of a high stage economizer, however, is not significant. With the suction line heat exchanger, the system efficiency increases more with the fluids of larger molar liquid specific heats. From the view point of volumetric capacity and energy efficiency, a 40%HCFC22/60%HCFC142b mixture is proposed as an interim alternative for CFC12 while a 25%HFC32/75%HFC134a mixture is proposed as a long term alternative for HCFC22.     

Distillation column configurations in ammonia–water absorption refrigeration systems

In ammonia–water absorption refrigeration systems a purification process to reduce the water content in the vapour leaving the generator is required. During this process the water content in the vapour must be reduced to a minimum, otherwise it tends to accumulate in the evaporator and strongly deteriorates the efficiency of the system. The vapour purification can be carried out by partial condensation, by establishing a liquid–vapour counter flow or by combining both methods. In systems with partial condensation, the distillation column can be composed of one or more rectifiers using different cooling mediums, and the rectifying and stripping sections. In complete condensation systems only the rectifying and stripping sections can be used. Therefore different distillation column arrangements should be considered. This paper presents a study of several distillation column configurations for single stage ammonia–water absorption refrigeration systems with partial and complete condensation. In order to evaluate and compare the different configurations, a parameter that indicates the ratio of the ammonia vapour concentration increase in each part of the column to the total ammonia purification has been defined. The analysis has been based on the system COP. Finally the efficiency in each part of the column has been calculated to estimate its design requirements.     

Carbon dioxide as a working fluid in drying heat pumpsLe dioxyde de carbone comme fluide actif dans des séchoirs à pompe à chaleur

Carbon dioxide as a working fluid in refrigeration and heat pump systems is increasingly important in view of the CFC substitution problem. It is both under ecological and economical aspects an attractive alternative to the HFC working fluids at present in practical use. The thermophysical properties and characteristics of carbon dioxide are quite different from those of refrigerants used in conventional vapour compression cycles. Its application in conventional vapour compression refrigerating systems is limited by its critical parameters (t_c=31.1°C and p_c=73.8 bar). The possibility to use carbon dioxide also beyond these limits in high temperature processes, e.g. heat pumps, is given by the application of a trans-critical process. The design and construction of a commercial drying heat pump system (batch type cabinet dryer with 12 kW heating capacity and closed air circuit) using the natural working fluid carbon dioxide is shown and experimental results of investigations carried out are presented. Energy savings are given compared to manufacturer's data of energy consumption.     

Experimental correlation of combined heat and mass transfer for NH3–H2O falling film absorptionCorrélation expérimentale entre le transfert de chaleur et de masse pour de l'absorption au NH3–H2O à film tombant

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re₁, vapor Reynolds number Re_v, inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.     

A theoretical study of an innovative ejector powered absorption–recompression cycle refrigerator

This paper describes a novel cycle which uses a steam ejector to enhance the concentration process by compressing the vapour from the lithium bromide solution to a state that it can be used to re-heat the solution from which it came. The energy efficiency and the performance characteristics of the novel cycle are theoretically investigated in this paper. The theoretical results show that the coefficient of performance (COP) of the novel cycle is better than the conventional single-effect absorption cycle. The characteristics of the cycle performance show its promise in using high temperature heat source at low cost.     

The impact of work input to sorption cyclesImpact de l'apport en énergie sur les cycles à sorption

In recent years the interest in cooling machines or heat pumps combining the principles of compression and sorption technology is increasing. The reason is that both technologies have specific drawbacks which can be overcome by the combination. Our discussion is centred around absorption cycles which use a compressor, and, consequently, an input of a significant amount of mechanical work in addition to heat. In most publications cycles of this kind are discussed in terms of one single COP as usual in the refrigeration industry. This, however, is wrong from a thermodynamic, and misleading from a technical and economical point of view. In order to highlight the need for a strict thermodynamic approach, a fundamental difference between distinct kinds of work input, namely “recoverable work”, “dissipative work” and “heat transformation work” is discussed in the first part of the paper. In the second part it is shown how the input of both work and heat into a energy conversion system has to be handled with both mechanical and thermal COP. The method is thermodynamically sound and straightforward, technically feasible and easy to apply, and most quickly transferred into economical terms. In the third part, a practical example of a compression–absorption hybrid is investigated.     

Performance of a packed bed absorber for aqua ammonia absorption refrigeration systemPerformance d''un matelas dispersant dans un système frigorifique à absorption à ammoniac/eau

A mathematical model of a packed bed absorber for aqua-ammonia absorption refrigeration system is presented. The model is used to predict the performance of the bed at various design and operating conditions. The governing equations and the boundary conditions are derived to predict the bed performance. A numerical integral method and an iteration scheme are used to solve the governing one dimensional, non-linear simultaneous differential equations which are subjected to three point boundary value problem. A computer program is prepared and carefully debugged to solve the governing equations with the help of some supporting equations to describe the properties of the working fluids and the heat and mass transfer coefficients in the bed. The analysis show that the absorption process is affected by the following parameters: the volumetric heat rejection model, bed height, vapor and solution flow rates to the bed and the inlet conditions; and packing material type. The effect of changing each of those parameters on the performance of the bed is studied after suggesting a model for the volumetric heat rejection from the bed. The results showed that changing the bed pressure and/or the vapor inlet temperature have negligible effect on the performance of the bed. Changing other parameters are found to affect the performance of the bed by different degrees. Also, the results show that within the present range of parameters, a bed height less than 0.7 m guarantees an absorption efficiency better than 91%.

Résumé

On présente un modèle mathématique d'un matelas dispersant dans un système frigorifique à absorption à ammoniac/eau. On utilise ce modèle pour prévoir la performance du matelas utilisant diverses conceptions et sous diverses conditions de fonctionnement. On établit des équations qui décrivent ce processus et les conditions limites afin de prévoir la performance du matelas. On utilise une méthode numérique intégrale et un schéma d'itération afin de résoudre les équations unidimensionnelles, non-linéaires, simultanées et différentielles, qui sont soumises au problème des limites à trois points. Un programme informatique est préparé et débogué afin de résoudre les équations qui gouvernent le processus étudié, avec l'aide de quelques équations supplémentaires qui décrivent les caractéristiques des fluides actifs et les coefficients de transmission thermique et de transfert d'énergie massique du matelas. L'analyse montre que le processus d'absorption est influencé par des paramètres suivants: le modèle de rejet de chaleur volumétrique, la hauteur du matelas, les débits d'écoulement de la vapeur et de la solution vers le matelas, les conditions d'entrée et le matériau dispersant utilisé. On étudie également l'influence de la variation de chacun de ces paramètres sur la performance du matelas apres avoir proposé un modèle de rejet de chaleur volumétrique par le matelas. Les résultats montrent que si on change la pression dans le matelas et/ou la température de la vapeur à l'arrivée, de tels changements ont un effet negligeable sur le matelas. Suite au changement d'autres paramètres, la performance du matelas a été modifée de diverses façons. Les résultats montrent également qu'avec les paramètres adoptés ici, une hauteur du matelas inférieure à 0,7 m assure un taux d'absorption supérieur à 91%.     

A contribution to the evaluation of the economic perspectives of absorption chillers: Contribution à l'évaluation des perspectives économiques des refroidisseurs à adsorption

The development of new working pairs and cycles extends the field of application of absorption systems with corresponding environmental benefits. The performance of standard cycles can be enhanced, e.g. by multi-staging. For each application the suitable working pair and cycle can be chosen regarding thermodynamical and economical aspects. Still, the performance strongly depends on the given external conditions. In this paper, basic thermodynamic constraints stemming from those conditions and valid for all sorption cycles are derived using the concept of endo-reversibility. Fundamental economic conclusions can be drawn. Subsequently, real machines are analysed. A comparison with manufacturers data and experimental data is made. The working pairs lithium bromide/water and binary hydroxide solution/water are discussed.     

Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.     

Twin screw oil-free wet compressor for compression–absorption cycle

The performance of a twin screw compressor operating under wet (two-phase) compression conditions in an ammonia–water compression absorption heat pump cycle is investigated both theoretically and experimentally. The paper reports on the influence of the location of liquid intake or, depending what applies, injection angle and mass flow rate of the injected liquid on compressor performance. Labyrinth seals separate the oil-free process side from oil lubricated bearing housing. Labyrinth seals leakage is modelled and its impact on performance is theoretically and experimentally investigated. The need for liquid injection from the discharge side to obtain acceptable performance is discussed based on experimental results.     

Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.     

Dynamic simulation of a thermostatically controlled counter-flow evaporatorSimulation dynamique d'un évaporateur à détendeur thermostatique avec débit à contre-courant

A numerical model is developed to simulate the transient behaviour of a counter-flow water cooling evaporator controlled by a thermostatic expansion valve (TEV) in a vapour compression refrigeration system. The liquid–vapour slip in the two-phase region of the evaporator is accounted for by a void fraction model (VFM). The thermal capacitance of the TEV is included in the analysis. For the purpose of comparison with predictions of the model, experimental data available are filtered to obtain the best estimate of the mean variation of the liquid–vapour transition plane. The predictions are in good trend-wise agreement with the filtered experimental data. The results of the transient simulations demonstrate the dependence of the stability of the evaporator–TEV system on the characteristics of the TEV, the thermal capacitance of the bulb, thermal conductance between the bulb and wall and on the nature of the input disturbance.     

Economie d'énergie: pompe à chaleur à absorption

One of the largest fields of research is energy recovery, particularly the increase of the energy level by using a heat pump.An absorption heat pump allows an upper level of about 140°C to be reached with a low consumption of electrical energy.The first part of this work establishes an expression for the COP from an enthalpy balance taking into account some hypotheses. From this expression a selection of the couple solute-solvant is possible according to operating conditions. A computation of the performance coefficient has been made as a function of various parameters such as the condenser, evaporator, absorber and separator temperature, and also the solution flow.Some results obtained with a small pilot plant are presented. They provide confirmation of the hypothesis and show that an absorption heat pump is able to increase the energy level from 60–80°C to 120–140°C with a satisfactory COP.

Résumé

L'un des domaines de recherche les plus étendus est celui de la récupération d'énergie et principalement l'augmentation du niveau d'énergie par l'utilisation d'une pompe à chaleur.On a choisi une pompe à chaleur à absorption parce qu'elle permet d'atteindre un niveau supérieur de 140°C environ avec une faible consommation d'énergie électrique.Dans le première partie de cette étude on a établi une expression du coefficient de performance à partir du bilan enthalpique en s'appuyant sur certaines hypothèses. A partir de cette expression un choix du couple soluté-solvant est possible suivant les conditions de travail. On a calculé le coefficient de performance en fonction de divers paramètres tels que la température au condenseur, à l'évaporateur, à l'absorbeur et au séparateur ainsi que le flux de solution.On présente quelques uns des résultats obtenus dans une petite installation pilote: ils permettent de confirmer l'hypothèse. Ils montrent qu'une pompe à chaleur à absorption peut faire passer le niveau d'énergie de 60–80°C à 120–140°C avec un coefficient de performance satisfaisant.     

Comparative study of irreversibilities in an aqua-ammonia absorption refrigeration system

Irreversibilities in components of an aqua-ammonia absorption refrigeratio system (ARS) have been determined by second law analysis. The components of the ARS are as follows: condenser, evaporator, absorber, generator, pump, expansion valves, mixture heat exchanger and refrigerant heat exchanger. It is assumed that the ammonia concentration at the generator exit is, independent of the other parameters, equal to 0.999 and at the evaporator exit the gas is saturated vapour. Pressrre losses between the generator and condenser, and the evaporator and absorber are taken into consideration. In the results the dimensionless exergy loss of each component, the exergetic coefficient of performance, the coefficient of performance and the circulation ratio are given graphically for each different generator, evaporator, condenser and absorber temperature.