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Highlights • Mass fraction difference as the driving force for the mass transfer. • New variable to fit experimental data for absorption chillers. • Linear relation of cooling capacity to mass fraction difference. • Less deviation compared to characteristic temperature function. Abstract Absorption chillers are considered as heat transfer devices and hence most commonly modeled as heat transfer processes. The absorption of the refrigerant vapor though inherently involves mass transfer into the absorbent, which in comparison to the heat transfer usually is a slow transport process. Modeling absorption chillers, however, usually does not include this mass transfer process. This in fact neglects the dominant process in absorption chillers which is addressed in the present paper. [ABSTRACT FROM AUTHOR]

A comparison of absorption and desorption is conducted using a detailed model describing heat and mass transfer. First, the influences of various assumptions have been evaluated. Second, typical conditions for both absorption chillers and heat transformers have been defined. The performance of absorption and desorption processes have been analysed for a flow length of 0.1 m. In an absorption chiller, during desorption, the viscosity is lowered and the mass diffusivity is increased. These circumstances cause a 46% higher transfer rate as compared to absorption. Thus, the overall performance of the process is determined by the absorber component. In a heat transformer, during absorption at an elevated pressure and temperature level, the viscosity is lower and mass diffusivity is higher as compared to desorption. Therefore, the transfer rate of during absorption is 10% higher as compared to desorption. Hence, the desorber performance is somewhat more influential to the overall system performance. [ABSTRACT FROM AUTHOR]

Abstract: Since the late seventies of the 20th century, several analytical models for combined heat and mass transfer in laminar falling film absorption have been proposed. Nevertheless the analytical solutions obtained with the Fourier method for the coupled process are complex and for short flow length a certain instability occurs which have been explained with the inconsistency of the initial and boundary conditions. Therefore boundary layer models have been justified in order to solve the transfer problem for short flow length. Moreover a linear approximation of the phase equilibrium is required. The analytical solutions for heat and mass transfer presented in this paper are obtained by using the Laplace transform to solve the partial differential equations for an isothermal as well as impermeable wall. An originally unknown constant temperature and mass fraction boundary condition at the interface are set. The temperature and mass fraction profile across the film are obtained formally independently. In order to determine the yet unknown interface temperature and mass fraction the phase equilibrium and the interface energy balance are applied, using averaged gradients with regard to the streamwise coordinate. The interface temperature and mass fraction obtained with this procedure are interpreted and treated as mean values. From the known evolution of the mean interface temperature and mass fraction, the local values are derived by inverting the first mean value theorem for integration. The results show very good agreement to the established analytical solutions. The solving procedure does not depend on the input parameters such as the Lewis number for instance, which is needed in order to determine the eigenvalues within the Fourier method. Moreover arbitrary correlations for the phase equilibrium are applicable. The present solution is mathematically stable and offer explicit expressions in order to calculate the mean heat and mass fluxes directly. Therefore this solution is favourable to implement entire absorption process simulation, yet describing the coupled heat and mass transfer process comprehensively. [Copyright &y& Elsevier]

Abstract: The Stefan-number is one out of the multitude of dimensionless numbers which predominantly are used in chemical engineering or heat and mass transfer problems. It relates sensible heat to latent heat and is a key number in solving the problem of heat transfer during solidification or melting. However, numbers similar to this show up also when several thermodynamic problems or relationships in refrigeration engineering are analyzed. Using the Stefan-number it can be distinguished if a refrigerant superheats or condenses during compression, it gives the size of the throttling loss, and it helps to decide if a suction line heat exchanger is beneficial or not. For absorption heat pumping, it allows to quantify the most important loss mechanism, the solution heat exchanger loss. Of course, all this information can be acquired in different ways – and maybe in more precise ways – also, but at least for teaching the basics of refrigeration the Stefan-number is simple to use and very easy to understand. In this paper, the different ways of how to use the Stefan-number in teaching and understanding thermodynamics of refrigerants are presented. [Copyright &y& Elsevier]

Abstract: Heat pumping devices which are operating on sorption effects comprise at least two times two categories, namely liquid or solid sorption cycles on the one hand, and open or closed cycles on the other hand. For the benefit of energy saving and the environment as well as for the further development of the technology it is important to be able to compare and evaluate these options in the context of the respective application. Some ideas to this end are shown in this paper. First the meaning of temperatures and humidity for the difference between open and closed systems is discussed, and there are a lot of similarities. Then some differences between solid and liquid sorption which are especially important for the question of minimum operating temperature and part load behavior are being reviewed, and we find that there is something to learn, mutually. Finally the consequences of solar fraction, auxiliary power consumption, and cogeneration efficiency are being highlighted, in order to check the perception of sorption systems to be environmentally benign. These consequences are important for all kinds of sorption cooling systems. It can be expected that there are significant improvements in the efficiency of power plants and compression cooling systems; consequently we find that sorption technology must improve its performance considerably also in order to stay competitive. [Copyright &y& Elsevier]

Abstract: This paper is the second paper out of two which present the development of a dynamic model for single-effect LiBr/water absorption chillers. The first part describes the model in detail with respect to the heat and mass balances as well as the dynamic terms. This second part presents a more detailed investigation of the model performance, including performance analysis, sensitivity checks and a comparison to experimental data. General model functionality is demonstrated. A sensitivity analysis gives results which agree very well to fundamental expectations: it shows that an increase in both external and internal thermal mass results in a slower response to the step change but also in smaller heat flow oscillations during the transient period. Also, the thermal mass has been found to influence the heat flow transients more significantly if allocated internally. The time shift in the solution cycle has been found to influence both the time to reach steady-state and the transients and oscillations of the heat flow. A smaller time shift leads to significantly faster response. A comparison with experimental data shows that the dynamic agreement between experiment and simulation is very good with dynamic temperature deviations between 10 and 25s. The total time to achieve a new steady-state in hot water temperature after a 10K input temperature step amounts to approximately 15min. Compared to this, the present dynamic deviations are in the magnitude of approximately 1–3%. [Copyright &y& Elsevier]

Abstract: This paper is the first of two which presents the development of a dynamic model for single-effect LiBr/water absorption chillers. The model is based on external and internal steady-state enthalpy balances for each main component. Dynamic behaviour is implemented via mass storage terms in the absorber and generator, thermal heat storage terms in all vessels and a delay time in the solution cycle. A special feature is that the thermal capacity is partly connected to external and partly to internal process temperatures. In this paper, the model is presented in detail. For verification, the model has been compared to experimental data. The dynamic agreement between experiment and simulation is very good with dynamic deviations around 10s. General functionality of the model and a more detailed comparison with experimental data are presented in Part II of this paper. [Copyright &y& Elsevier]

Abstract: We present a model describing simultaneous heat and mass transfer of an absorbing or desorbing laminar liquid film flowing over a vertical isothermal plate. We start with a formulation which is comparable to established models by using simplifying assumptions such as homogeneous velocity and constant film thickness. In contrast to those, we allow for effects like change in properties and differential heat of solution within the bulk of the film. Additionally, enthalpy transport due to interdiffusion is accounted for. The impact of the considered effects are discussed and compared. The numerical solution is obtained by utilising a Newton–Raphson scheme to solve the finite difference formulation of the governing equations. Since the temperature gradients adjacent to wall and phase boundary are expected to be large, we discretise the equations on an irregular grid. The results of the model agree very well with established analytical models. It is found that the influence of released differential heat of solution within the bulk is relatively small. However, the impact on the temperature distribution is in the same order of magnitude as the one of a change in properties. Moreover, when comparing desorption with absorption under equivalent conditions, the mass transfer rate during absorption is higher than during desorption. [Copyright &y& Elsevier]

Abstract: Various researchers have developed models of conventional H2O–LiBr absorption machines with the aim of predicting their performance. In this paper, the methodology of characteristic equations developed by is applied. This model is able to represent the capacity of single effect absorption chillers and heat pumps by means of simple algebraic equations. An extended characteristic equation based on a characteristic temperature difference has been obtained, considering the facility features. As a result, it is concluded that for adiabatic absorbers a subcooling temperature must be specified. The effect of evaporator overflow has been characterized. Its influence on cooling capacity has been included in the extended characteristic equation. Taking into account the particular design and operation features, a good agreement between experimental performance data and those obtained through the extended characteristic equation has been achieved at off-design operation. This allows its use for simulation and control purposes. [Copyright &y& Elsevier]

Abstract: Ammonia vapour refining is a common procedure in ammonia–water absorption machines. A solar assisted single effect absorption machine that uses the pair ammonia–water was developed and tested. Its desorber has a built-in adiabatic refining column constituted by a rich solution spray. The refining method proved its feasibility. The spray provided a more or less constant ammonia vapour enrichment of about 1% which is enough for the working temperature ranges of this type of machine. It was also verified that the refining effect of the spray is almost independent of the refrigerant vapour and solution mass flow rates. [Copyright &y& Elsevier]

The performance of an advanced triple-pressure level (TPL) single-stage absorption cycle with refrigerant R125 and various organic absorbents were studied. In the developed TPL cycle, a jet ejector of a special design is added at the absorber inlet. The device serves two major functions: it facilitates pressure recovery and improves the mixing process between the weak solution and the refrigerant vapor coming from the evaporator. These effects enhance the absorption process of the refrigerant vapor into the solution drops. To facilitate the design of a jet ejector for absorption machines, a numerical model of simultaneous heat and mass transfer between the liquid and the gas phases in the ejector was developed.Based on the computerized simulation program, a parametric study of a TPL cycle was carried out. Comparison was made between the performances of the TPL and the common double pressure level (DPL) absorption cycle with refrigerant R125 and various organic absorbents. In addition, the influence of the jet ejector on the performance of the absorption cycle and the size of the unit was studied. [Copyright &y& Elsevier]