Ammonia-water Absorption System Research Articles

Heat integration of ammonia-water absorption refrigeration system through heat-exchanger network analysis

Abstract The heat integration of existing single-effect ammonia-water absorption system is studied. A set of possible heat modifications involving with absorption heat or rectification heat recovery are identified on energy transfer diagram, the performance of each heat modification is studied and compared, and the interaction of heat transfer in generator, absorber, rectifier and solution heat exchanger is investigated through different combinations of heat modifications. There are two approaches of increasing thermal performance, one is aimed to increase the feed temperature of the distillation column by isolating heat exchanger components. The purpose of the other approach is to reduce heat consumption through heat integration by using bridge analysis. The two approaches can both boost the thermal performance effectively. The results demonstrate 22% increases in Coefficient of Performance (COP) compared with traditional single-effect cycle under certain working conditions. In general, obtaining the maximum heat saving capacity in advance under different working conditions is necessary either for retrofit or in preliminary design stage. In this paper, the selection guidance of suitable cycle under different working conditions is provided.

Design of compact microscale geometries for ammonia–water desorption

This article presents the development of two novel compact heat and mass exchange concepts for use as the desorber in small-capacity ammonia–water absorption systems. Both concepts feature a counterflow orientation of the vapor and solution streams, in which the generated vapor rises through small channels within the component, flowing past the heated ammonia–water solution. Heat input is supplied by a hot coupling fluid flowing through a microchannel array adjacent to the vapor/solution channels in a counterflow orientation with the solution stream. This geometry provides high heat transfer coefficients, enabling a significant reduction in component size. This arrangement is also found to produce high-purity ammonia vapor, which leads to reduced rectification loads and a higher coefficient of performance of the system compared to more conventional designs. Additionally, both designs can be configured to include an analyzer and rectification section, leading to more compact integrated system packages and a reduction of external fluid connections and cost. A model of the coupled heat and mass exchange process associated with ammonia–water desorption in these concepts is developed. The proposed design is compact, scalable, versatile, and easily mass-produced. This concept can also be extended for use in other multi-component heat and mass transfer applications.

Magnetic Field Enhancement in Ammonia-Water Absorption Refrigeration Systems

Absorption enhancement has been considered as an effective way of improving coefficient of performance (COP) of refrigeration systems and magnetic enhancement is one of these methods. A model of magnetic field enhancement in ammonia-water absorption systems is presented in this paper. A numerical model using finite difference scheme was developed based on the conservation equations and mass transport relationship. Macroscopic magnetic field force was introduced in the momentum equation. The model was validated using data obtained from the literature. Changes in the physical properties of ammonia solution while absorbing both in the direction of falling film and across its thickness were investigated. The magnetic field was found to have some positive effect on the ammonia-water falling film absorption. The results indicate that absorption performance enhancement increased with magnetic intensity. The COP of simple ammonia solution absorption refrigeration system increased by 1.9% and 3.6% for magnetic induction of 1.4 and 3.0 Tesla respectively.

Experimental investigation on an ammonia–water–lithium bromide absorption refrigeration system without solution pump

Experimental researches were carried out on a novel ammonia–water–lithium bromide ternary solution absorption refrigeration and air-conditioning system without solution pump and distillation equipments. The experiments were conducted by using three kinds of NH 3–H 2O binary solution and 17 kinds of ternary solution with difference in mass fraction of NH 3 and LiBr. The experimental results showed that the vapor pressure of the generator in the system would be lower than that of the generator in an ammonia–water absorption system. In above two situations the same ammonia mass fraction and the same solution temperature were kept. The amplitude of vapor pressure decrease of the system generator would be larger with the increase of the mass fraction of LiBr. The maximum amplitude of decrease would be of 50%. With the increase of the mass fraction of LiBr, the coefficient of performance (COP) of the system would be increased initially, and then decreased later when the mass fraction of LiBr exceeded a certain value. This value was about 23% for the solution with ammonia mass fraction of 50% and 55%, and about 30% for the solution with ammonia mass fraction of 60%. Compared with the ammonia–water system, the COP of the ternary solution system with the same mass fraction of ammonia would increase up to 30%. With the ammonia mass fraction of 60% and LiBr mass fraction of 30% applied, the COP of the ternary solution system was increased up to 0.401. It was 51.89% higher than that when binary ammonia–water solution with ammonia mass fraction of 50% was applied. In above two operating situations, the temperature of hot water, cooling water and chilled water in the system would be kept almost constant, respectively.

Finite time thermodynamics study and exergetic analysis of ammonia–water absorption systems

This paper presents an optimization study of a single stage absorption machine operating with an ammonia–water mixture under steady state conditions. The power in the evaporator, the temperatures of the external fluids entering the four external heat exchangers as well as the effectiveness of these heat exchangers and the efficiency of the pump are assumed fixed. The results include the minimum value of the total thermal conductance UA tot as well as the corresponding mean internal temperatures, overall irreversibility and exergetic efficiency for a range of values of the coefficient of performance (COP). They show the existence of three optimum values of the COP: the first minimises UA tot, the second minimises the overall irreversibility and the third maximises the exergetic efficiency. They also show that these three COP values are lower than the maximum COP which corresponds to the convergence of the internal and external temperatures towards a common value. The influence of various parameters on the minimum thermal conductance of the heat exchangers and on the corresponding exergy efficiency has also been evaluated. From an exergetic viewpoint it is interesting to reduce the temperature at the desorber and at the evaporator and to raise the values of that parameter at the condenser and the absorber. However these changes must be accompanied by an important increase in the total UA if it is desired to conserve a constant COP. The internal heat exchangers between the working fluid and the solution improve both the overall exergy efficiency and the coefficient of performance of the absorption apparatus.

Performance analysis and validation on transportation of heat energy over long distance by ammonia-water absorption cycle

This paper presents the thermodynamic and hydrodynamic feasibility of the application of the ammonia-water absorption system for heat or cold transportation over long distance. A model of a long-distance heat energy transportation system is built and analyzed, and it shows satisfactory and attractive results. When a steam heat source at the temperature of 120°C is available, the user site can get hot water output at about 55°C with the thermal COP of about 0.6 and the electric COP of about 100 in winter, and cold water output at about 8°C with the thermal COP of about 0.5 and the electric COP of 50 in summer. A small-size prototype is built to verify the performance analysis. Basically the experimental data show good accordance with the analysis results. The ammonia-water absorption system is a potential prospective solution for the heat or cold transportation over long distance. Copyright © 2009 John Wiley & Sons, Ltd.

Transportation of low-grade thermal energy over long distance by ammonia-water absorption

This paper presents the importance and the cycle choice for long-distance transportation of low-grade thermal energy, and the thermodynamic and hydrodynamic feasibility of single-effect ammonia-water absorption system for heat or cold transportation over long distance are also involved. A model of a long-distance thermal energy transportation system is built and analyzed, which shows satisfactory and attractive results. When a steam heat source at 120°C is available, the user site can get hot water output at about 55°C with the thermal COP of about 0.6 and the electric COP of about 100 in winter, and cold water output at about 10°C with the thermal COP of about 0.5 and the electric COP of 50 in summer. A small-size prototype is built to verify the performance analysis. Basically the experimental data show good accordance with the analysis results. The ammonia-water absorption system is a potential prospective solution for the heat or cold transportation over long distance.

Thermodynamic Modeling of an Ammonia-Water Absorption System Associated with a Microturbine

Thermodynamic modeling and Second Law analysis of a small-scale cogeneration system consisting of a 5 refrigerant ton absorption chiller connected by a thermosyphon heat exchanger to a 28 kWe natural gas microturbine are presented. The proposed configuration changes the heat source of the absorption chiller, replacing the original natural gas burning system. A computational algorithm was programmed to analyze the global efficiency of the combined cooling and power plant and the coefficient of performance of the absorption chiller. The results show the consistency of the proposed model and a good performance of the cogeneration system. The thermal efficiency of the combined cooling and power plant is approximately 41%, which represents a 67% increase relative to a single natural-gas microturbine.

Experimental characterization of the rectification process in ammonia–water absorption systems with a large-specific-area corrugated sheet structured packing

In this paper, the mass transfer performance of a large-specific-area corrugated sheet structured packing for ammonia–water absorption refrigeration systems (AARS) is reported. An experimental facility was used to test the performance of the packing. Experimental results of the temperature, ammonia concentration and mass flow rate of the rectified vapour are presented and discussed for different operating conditions including reflux ratio values from 0.2 to 1. The volumetric vapour phase mass transfer coefficient is calculated from the measured data and compared with different correlations found in the literature. A new correlation is proposed which was fitted from the experimental data. Finally, a comparison is made between the actual packing height used in the experimental setup and the height required to obtain the same ammonia rectification in AARS with different packings previously tested by the authors.

Exergy analysis of an ammonia water absorption system

Exergy analysis of an ammonia?water absorption system for cooling applications is presented in this paper. Exergy destruction rate and heat rate in each component of the system are evaluated. From the results obtained it can be concluded that the evaporator and condenser heat loads and exergy destruction rate are less than those of the absorber and generator. This is due to the heat of mixing in the solution. Furthermore, a simulation program is written and used for the determination of the total exergy destruction rate, the effectiveness and the coefficient of performance of the absorption system under different operating conditions.

ENERGY AND EXERGY THERMODYNAMIC ANALYSIS OF A TWO-STAGE COMPRESSION REFRIGERATION SYSTEM INTEGRATED WITH AN ABSORPTION SYSTEM (NH3+H2O)

This work proposes an energetic and exergetic thermodynamic analysis of two refrigeration systems: one is a conventional two stages cooling system by steam compression of ammonia and the other is named integrated refrigeration system. The conventional system, used as reference, is largely employed in cooling fish industry. The integrated refrigeration system is similar to the conventional one, although it uses in the intermediate cooling, between the stages of high and low pressure, cold water in closed circuit. The cold water is supplied by ammonia-water absorption system integrated to the conventional compression system. The calorific energy supplied is obtained from waste exceeding of the fish meal production thus the energy delivered tothe integrated refrigeration system is considered of zero cost. Numeric simulation is employed to compare the behavior of both systems. The results obtained in this comparison show that the integrated refrigeration system operates with a reduction of up to 19.73 % in COP. However, the integratedrefrigeration system presented an increase of up to 25.57% in exergetic efficiency and 33.09% in frigorific capacity in relation to the conventionalsystem. These results, added to the decrease of operational cost which will bequantified in a further study, will make very attractive the use of the integrated refrigeration system.

Modeling of simultaneous heat and mass transfer processes in ammonia–water absorption systems from general correlations

This paper presents a general differential mathematical model to analyze the simultaneous heat and mass transfer processes that occur in different components of an ammonia–water absorption system: absorber, desorber, rectifier, distillation column, condenser and evaporator. Heat and mass transfer equations are considered, taking into account the heat and mass transfer resistances in the liquid and vapour phases. The model considers the different regions: vapour phase, liquid phase and an external heating or cooling medium. A finite difference numerical method has been considered to solve the resulting set of nonlinear differential equations and an iterative algorithm is proposed for its solution. A map of possible solutions of the mass transferred composition z is presented when varying the interface temperature, which enables to establish a robust implementation code. The analysis is focused on the processes presented in ammonia–water absorption systems. The model is applied to analyze the ammonia purification process in an adiabatic packed rectification column and the numerical results show good agreement with experimental data.

Effect of pressure on mass absorption in an ammonia–water absorption system

Absorption phenomenon of ammonia vapor into ammonia water solution has been investigated experimentally, by inserting superheated ammonia vapor into a test cell containing a stagnant pool of ammonia water solution. Before commencing the experiment, the pressure in the test cell corresponds to the equilibrium vapor of the ammonia–water system at room temperature. When the valve is opened, mechanical equilibrium is established quickly and the pressure in the test cell becomes equal to that of the ammonia vapor cylinder. The difference between the initial pressure in the vapor cylinder and the initial pressure in the test cell is found to have a major influence on the absorption rate. The main objective of this study is to investigate the effect of this initial pressure difference on the absorption rate of ammonia vapor. A correlation which gives the total absorbed mass of ammonia as a function of the initial concentration, the initial pressure difference and time is derived. In addition the absorbed mass at no pressure difference could be estimated from the absorbed mass at initial pressure difference.

The importance of the ammonia purification process in ammonia–water absorption systems

Practical experience in working with ammonia–water absorption systems shows that the ammonia purification process is a crucial issue in order to obtain an efficient and reliable system. In this paper, the detrimental effects of the residual water content in the vapour refrigerant are described and quantified based on the system design variables that determine the effectiveness of the purification process. The study has been performed considering a single stage system with a distillation column with complete condensation. The ammonia purification effectiveness of the column is analysed in terms of the efficiencies in the stripping and rectifying sections and the reflux ratio. By varying the efficiencies from 0 to 1, systems with neither the rectifying nor stripping section, with either the rectifying or stripping section, or with both sections can be considered. The impact of the ammonia purification process on the absorption system performance is studied based on the column efficiencies and reflux ratio; and its effects on refrigerant concentration, system COP, system pressures and main system mass flow rates and concentrations are analysed. When the highest efficiency rectifying sections are used a combination of generation temperature and reflux ratio which leads to optimum COP values is found. The analysis covers different operating conditions with air and water cooled systems from refrigeration to air conditioning applications by changing the evaporation temperature. The importance of rectification in each kind of application is evaluated.

Thermodynamic Investigation of Two-Stage Absorption Refrigeration System Connected by a Compressor

The present work is to analyze a two-stage cycle based on the ammonia-water absorption system, with intermediate compression. The two generators of the system are heated by geothermal energy at low temperature. The study shows that this system makes it possible at lower generator temperature, under the limits permitted by the systems suggested up to now. For Tg = 335 K, Tc = Ta = 308 K and Te = 263 K, based on the electric consumption, the system efficiency is 8.2. The comparative study of the hybrid system and vapor compression systems shows the superiority of the proposed system. Supplied by the geothermal sources of the Tunisian south, the system makes it possible to obtain for a pilot geothermal station, a production of 75 tons of ice per day. The greenhouse gas emissions should thus be reduced by about 2.38 tons of CO2 per day. Therefore, based on the typical geothermal energy sources in Tunisia which present a global refrigeration potential of 4.4 MW, the daily quantity of ice that could be produced is about 865 tons. The greenhouse gas emissions should thus be reduced by about 10,000 tons of CO2 per year.

05/00385 Second law based thermodynamic analysis of ammonia—water absorption systems: Adewusi, S. A. and Zubair, S. M. Energy Conversion and Management, 2004, 45, (15–16), 2355–2369

05/00385 Second law based thermodynamic analysis of ammonia—water absorption systems: Adewusi, S. A. and Zubair, S. M. Energy Conversion and Management, 2004, 45, (15–16), 2355–2369

Monomethylamine–water vapour absorption refrigeration system

Thermodynamic equations for the performance evaluation of the monomethylamine–water vapour absorption refrigeration system have been obtained, analysed and reported in this paper. These equations have been derived from experimental data with a good agreement with the expected values and have been expressed in polynomial form. These equations were utilized on the energy and mass balances for obtaining the coefficient of performance (COP) with this pair and compared with the ammonia–water absorption system. It has been observed that this refrigerant–absorbent pair has a greater coefficient of performance at low generation temperatures and moderate condenser, absorber and evaporator temperatures.

Second law based thermodynamic analysis of ammonia–water absorption systems

Abstract The second law of thermodynamics is used to study the performance of single-stage and two-stage ammonia–water absorption refrigeration systems (ARSs) when some input design parameters are varied. The entropy generation of each component and the total entropy generation ( S tot ) of all the system components as well as the coefficient of performance (COP) of the ARSs are calculated from the thermodynamic properties of the working fluids at various operating conditions. The results show that the two-stage system has a higher S tot and COP, while the single-stage system has a lower S tot and COP. This is a paradox since one would expect that an efficient thermal system should have a higher COP and lower S tot . This anomaly is explained with respect to the performance results for both single and two-stage systems. The trend in COP and S tot with the change in heat exchangers effectiveness, absorbers temperatures and condenser and evaporator temperatures for both systems investigated conform with expectation, i.e. an increase in COP corresponds to a decrease in S tot .

Heat and mass transfer analysis of a helical coil rectifier in an ammonia–water absorption system

This paper presents a detailed study on the ammonia–water vapour rectification process in absorption systems using a helical coil rectifier. A differential mathematical model has been developed on the basis of mass and energy balances and heat and mass transfer equations. The differential volume has been defined in each coil turn by a differential angle on the turn and a second differential angle on the coiled tube cross section. It contains the corresponding differential portion of coolant, coiled tube wall, condensate film and vapour. Simultaneous heat and mass transfer processes have been taken into account in the vapour and liquid phases. The model equations have been solved using the finite-difference method. Results have been obtained for characteristic data from an ammonia–water absorption refrigeration system. Most significant calculated variable profiles along the coil height as well as in the coiled tube cross section are presented and discussed. The influence of the heat and mass transfer coefficients on the rectifier performance has also been considered.

Combined Heat and Mass Transfer Under Different Inlet Subcooling Modes During NH3-H2O Falling Film Absorption Process

Experiments were conducted for ammonia-water falling film absorption in a plate heat exchanger with offset strip fins. The objectives of this paper were to analyze combined heat and mass transfer during the ammonia-water absorption process under different inlet subcooling modes, and to obtain heat transfer coefficients (Nusselt number). This paper examined the effects of the inlet subcooling modes, the inlet concentration difference, liquid Reynolds number, and vapor Reynolds number on the heat transfer performance. Inlet liquid concentrations were set at 0, 5, 10, and 15 percent in mass of ammonia, while inlet vapor concentration ranged from 64.7 to 83.6 percent. Experiments were conducted in three ways according to the inlet subcooling conditions, i.e., Case A Tv>Tl, Case B Tv∼Tl, and Case C Tv<Tl. In Case A, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption was confirmed in the experiments, which resulted in a lower absorption performance. The heat transfer coefficient increased as the inlet subcooling increased in all cases. The effect of inlet subcooling on heat transfer performance was more significant in Case A than in Cases B and C. The inlet subcooling had more significant effect on the heat transfer performance than the inlet concentration difference. Nusselt number increased as liquid and vapor Reynolds numbers increased. The vapor velocity should be maximized to increase absorption performance in cocurrent ammonia-water absorption process. The parametric analysis provides fundamental understandings of the ammonia-water absorption process, and thus gives a guideline for heat exchanger compactness in ammonia-water absorption systems.