LiBr-H2O Solution Research Articles

Experimental investigation of heat and mass transfer in a LiBr-H2O solution falling film absorber on horizontal tubes: Comprehensive effects of tube types and surfactants

This paper studied the effects of various surface geometries and surfactants on the LiBr-H2O solution falling film absorption, and the heat and mass transfer during the absorption process. Three different tubes (plain tube, floral tube and floral finned tube) were tested for the LiBr-H2O solution with two different surfactants (2-Ethyl-1-hexanol and 1-Octanol). The results indicate that the heat and mass transfer coefficients of each tube gradually decrease with solution falling down the tube bundle. It is also showed that the effect of surfactant on heat and mass transfer is obviously greater than that of tube surface geometry. Furthermore, with 2-Ethyl-1-hexanol as the surfactant, the heat and mass transfer can be improved by 400% and 350%, respectively. The results of comparative study show that the heat and mass transfer performance of floral tubes with 2-Ethyl-1-hexanol is more stable and reliable than others.

Thermoeconomic evaluation and optimization of LiBr-H2O double absorption heat transformer driven by flat plate collector

In this paper, a solar double absorption heat transformer (SDAHT) operating with the LiBr-H2O solution is proposed to provide high temperature energy. The flat plate collector (FPC) is used to supply the demanded heat input in the present configuration, which will greatly broaden the application scope of the FPCs. The thermoeconomic concept is applied to the evaluation and optimization of the SDAHT, aimed at minimizing its annual capital cost per kilowatt heat capacity (CPK) and payback period (PP). A dedicated computer model in the software Engineering Equation Solver was developed to conduct the study by means of a parametric analysis. The results show that there exists an optimum absorber/evaporator temperature at which CPK and PP can obtain the minimum values. With solar radiation intensity at 600 W/m2, generation (evaporation) temperature at 75 °C, condensation temperature at 38 °C, absorption temperature at 135 °C and economizer efficiency set to 0.8, the optimum absorber/evaporator temperature and corresponding CPK and PP are 104.0 °C, 928.1 euros/kW and 4.0 years, respectively. The cost of the FPCs takes the major part of CPK. Moreover, the effects of the operating parameters such as the generation (evaporation), condensation, absorption temperatures and design parameters including the first and second economizer efficiencies on the optimum absorber/evaporator temperature and corresponding CPK and PP have been analyzed in detail. Besides, some suggestions derived from the results are also given to assist the engineers in estimation of the economic performance.

Entropy parameters for falling film absorber optimization

A local entropy generation analysis, for water vapor absorption in LiBr-H2O solution, is performed referring to velocity, temperature and concentration fields obtained from the numerical solution of mass and energy transport equations. The hydrodynamic description is based on Nusselt boundary layer assumption and the actual amount of irreversibility introduced is determined for an absorptive falling film over a cooled horizontal tube inside the absorber. Results are explored in different operative conditions in order to examine the impact of the various irreversibility sources in a wide operative range. A least irreversible solution mass flow-rate can always be identified. Furthermore, a simple and general thermodynamic analysis, carried out regarding a refrigerating and a heat boosting applications, makes evidence of a dimensionless group “Q/σT” that separates the weight of the irreversibilities and gives the way to an optimization criterion applied to the absorber in order to improve the whole system efficiency. Both thermodynamic equilibrium and sub-cooling conditions of the solution at the inlet are considered for typical temperature and concentration of refrigerators' absorbers and heat transformers' absorbers. Results suggest the importance to work at reduced mass flow-rates with a thin uniform film. In practice, tension-active additives are required to realize this condition. Also, it is highlighted that the two parameters defined with reference to the dimensionless group “Q/σT” can be maximized by specific values of the tube radius, operative Reynolds number, solution sub-cooling and temperature difference between the wall and the inlet solution.

Development and experimental validation of a simulation model to reproduce the performance of a 17.6 kW LiBr–water absorption chiller

In recent years, the use of air conditioning systems based on cycles driven by thermal energy has increased. These cycles utilize renewable energy or waste heat energy from industrial processes. However, the complexity associated with such facilities makes the simulation of these systems with regard to sizing and performance advisable in many cases. This paper develops a simulation model under the EES environment for a 17.6 kW Yazaki WFC-SC5 simple effect LiBr–H2O absorption chiller. In this study, the heat transfer areas and coefficients of each heat exchanger, the circulating solution flow rate and the properties of the LiBr–H2O solution have been considered. Once the model is created, real data are used to validate the obtained results, providing a high level of correlation between real and simulated data.

Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application

This paper presents a dynamic modeling of a single-effect absorption chiller working with LiBr–H2O solution used in a solar cooling installation operating without any backup systems (hot or cold). In this case, the absorption machine is powered only by a solar collector field. Given the highly variable nature of solar radiation and the building loads, the range of the three source temperatures of the chiller can vary widely since there is no backup system. These fluctuating source temperatures mean that the chiller does not operate in steady state phase during the day. The dynamic modeling of the absorption chiller is therefore very important to predict its performance, taking into account both the transient and steady state phases. The numerical model presented in this paper is based on the mass and energy balances of each component, equations of state and equations of heat transfers. In the first part, this article presents the dynamic modeling of a LiBr/H2O absorption chiller. Then, experimental validation elements are proposed to validate pressures and temperatures of the chiller. Finally, a method is presented to optimize the thermal COP according to different levels of refrigerating capacities.

Irreversibility analysis of falling film absorption over a cooled horizontal tube

Based on a numerical study of the water vapour absorption process in LiBr–H2O solution, for a laminar, gravity driven, viscous, incompressible liquid film, flowing over a horizontal cooled tube, irreversibilities related to fluid friction, heat transfer, mass transfer and their coupling effects have been locally and globally examined. The hydrodynamic description is based on Nusselt boundary layer assumptions. The tangential and normal velocity components, respectively obtained from momentum and continuity equations, have been used for the numerical solution of mass and energy transport equations in the two-dimensional domain defined by the film thickness and the position along the tube surface. Local entropy generation calculation can be performed referring to the calculated velocity, temperature and concentration fields. Results have been explored in different operative conditions, in order to examine comprehensively the impact of the various irreversibility sources and to identify the least irreversible solution mass flow-rate for the absorber. As a parallel, a refined understanding of the absorption process can be obtained. Considering absorption at the film interface and cooling effect at the tube wall, the analysis thermodynamically characterises the absorption process which occurs inside actual falling film heat exchangers and establishes a criterion for their thermodynamic optimisation. Results suggest the importance to operate at reduced mass flow rates with a thin uniform film. Meanwhile, tension-active additives are required to realise this condition.

Experimental study and correlation on the falling column adiabatic absorption of water vapor into LiBr–H2O solution

A new kind of LiBr–H2O absorption process, the falling column adiabatic absorption, is presented and discussed in this paper. Different from the traditional absorption, this process can separate heat and mass transfer process with external cooling; besides, the solution flows through orifices by gravity and does not need any mechanical energy for solution distribution. Experiments were carried out to study the mass transfer process of the falling column absorption. Mass transfer coefficients for different conditions were defined and calculated by an analysis model. The mass transfer coefficients that were found range from 1 × 10−4 to 3 × 10−4 m s−1 under experimental conditions. An experimental correlation of mass transfer coefficient was developed as a function of the mass diffusivity and column diameter with an error band of 15%. Falling column configurations were proved to have good performance and can be an alternative to existing ones.

Energy, exergy, and economic analysis of single and double effect LiBr–H2O absorption chillers

In this study different water-cooled LiBr–H2O absorption systems under different operating and climatic conditions are analyzed and compared, and the effect of considering the chemical exergy of the LiBr–H2O solution on the second law analysis of such systems is probed as well. Results show that the EUF increases with increasing the generator and evaporator temperatures and decreases with the increase in the temperature and relative humidity of the ambient air. The total exergy efficiency increases with increasing the generator or ambient air temperature, decreases with increasing the evaporator temperature, and changes slightly with relative humidity. In Tamb=35°C and φ=30%, the system including direct-fired double-effect chiller has a EUF=1.20 and ψ=7.86% both of which are respectively 11% and 82% higher than the systems including hot-water double-effect and single-effect chillers. The economic analysis reveals that for the operating conditions presented here, the single-effect system is uneconomical and the payback period in the case of utilizing the direct-fired double-effect chiller instead of the hot-water one is about 3.5 years. Finally, the CO2 emission of the single-effect system is respectively about 1.9 and 1.7 times higher than direct-fired and hot-water double-effect ones.

Dynamic simulation of a single-effect LiBr–H2O absorption refrigeration cycle considering the effects of thermal masses

A lumped-parameter dynamic simulation of a single-effect LiBr–H2O absorption chiller considering the effects of thermal masses of main components is presented in this paper. The continuity of species constituting the LiBr–H2O solution, momentum equations and energy balances are solved simultaneously. Six various cases are considered to evaluate the effect of thermal masses of all and some components on the key parameters of an absorption chiller. To eliminate the numerical errors, thermodynamic properties are taken from the EES software. Since the performance of the cycle should be evaluated in each time step, a link is made between EES and MATLAB softwares. The fourth order Runge–Kutta method is selected to solve the simultaneous differential equations. The results show that the heat transfer rate of high-pressure components (generator and condenser) are highly dependent of thermal mass of the condenser whereas the heat transfer rate of low-pressure components (evaporator and absorber) are hardly affected by thermal masses. Furthermore, the major components except condenser show approximately the same behavior when thermal masses are ignored.

Thermodynamic modelling of a double-effect LiBr-H2O absorption refrigeration cycle

The goal of this paper is to estimate the conductance of components required to achieve the approach temperatures, and gain insights into a double-effect absorption chiller using LiBr-H2O solution as the working fluid. An in-house computer program is developed to simulate the cycle. Conductance of all components is evaluated based on the approach temperatures assumed as input parameters. The effect of input data on the cycle performance and the exergetic efficiency are investigated.

Comparison of Corrosion Resistance of Carbon Steel and Some Stainless Steels Exposed to LiBr-H2O Solution at low Temperatures

Comparison of Corrosion Resistance of Carbon Steel and Some Stainless Steels Exposed to LiBr-H2O Solution at low Temperatures

Modeling of Artificial Neural Network for Predicting Specific Heat capacity of working fluid LiBr-H2O used in Vapor Absorption Refrigeration System

The objective of this work is to model an artificial neural network (ANN) to predict the value of specific heat capacity of working fluid LiBr-H2O used in vapour absorption refrigeration systems. A feed forward back propagation algorithm is used for the network, which is most popular for ANN. The consistence between experimental and ANNâ??s approach result was achieved by a mean relative error -0.00573, sum of the squares due to error0.00321, coefficient of multiple determination R-square 0.99961and root mean square error 0.01573 for test data. These results had been achieved in Matlab environment and the use of derived equations in any programmable language for deriving the specific heat capacity of LiBr-H2O solution.

COUPLED HEAT AND MASS TRANSFER DURING ABSORPTION OF WATER VAPOR INTO LIBR-H2O SOLUTION FAN SHEETS

COUPLED HEAT AND MASS TRANSFER DURING ABSORPTION OF WATER VAPOR INTO LIBR-H2O SOLUTION FAN SHEETS

Corrosion study of SS-316L exposed to LiBr–H2O solution applying electrochemical techniques and weight loss method

Corrosion evaluation of SS-316L exposed to LiBr–H2O solution of 50 wt-% was investigated applying the electrochemical noise technique (ENT), potentiodynamic polarisation curves, and the weight loss method (WLM) at 25, 50, 60, 70 and 80°C. The current and potential noise signals were obtained each four hours during 15 days. These data were analysed in the time and frequency domains. A statistical analysis was made to obtain the localisation index and the resistance noise. Also, the Stern–Geary equation and the Faraday's law were used to determine the corrosion rates. A comparison of the corrosion rates obtained from ENT and WLM showed the same tendency with temperature, but a difference of one order of magnitude was seen. The results of the corrosion rates from ENT and WLM presented a good agreement with the inverse noise resistance results. SEM analyses of the corroded samples together with atomic absorption analysis of corrosion products are also presented.

Exergy analysis of double effect vapor absorption refrigeration system

Energy and exergy analyses previously performed by the authors for a single effect absorption refrigeration system have been extended to double effect vapor absorption refrigeration system with the expectation of reducing energy supply as well as an interest in the diversification of the motive power employed by HVAC technologies. The total exergy destruction in the system as a percentage of the exergy input from a generator heating water over a range of operating temperatures is examined for a system operating on LiBr–H2O solution. The exergy destruction in each component, the coefficient of performance (COP) and the exergetic COP of the system are determined. It is shown that exergy destructions occur significantly in generators, absorbers, evaporator2 and heat exchangers while the exergy destructions in condenser1, evaporator1, throttling valves, and expansion valves are relatively smaller within the range of 1–5%. The results further indicate that with an increase in the generator1 temperature the COP and ECOP increase, but there is a significant reduction in total exergy destruction of the system for the same. On the other hand, the COP and ECOP decrease with an increase in the absorber1 temperature while the total exergy destruction of the system increases significantly with a small increase in the absorber1 temperature. The results show that the exergy method can be used as an effective criterion in designing an irreversible double effect absorption refrigeration system and may be a good tool for the determination of the optimum working conditions of such systems. Copyright © 2007 John Wiley & Sons, Ltd.

Corrosion Performance of SS-304L Exposed to LiBr-H2O Solution at 70{degree sign}C Applying the Electrochemical Noise Technique, Polarization Curves and Optical Transmitance Measurements.

A corrosion study of SS-304L was carried out exposing the material to LiBr-H2O solution 50% (wt.) at 70{degree sign}C for 20 days, applying the electrochemical noise technique and polarization curves. The aim of this work was to determine the corrosion rate and the type of corrosion of SS-304L. The corrosion evaluation was supported by optical observations of corroded specimens and EDX analysis of corrosion products. The dissolved corrosion products in LiBr solution were also analyzed by the atomic absorption technique. To obtain the corrosion rates, the noise resistance from ENT was calculated, and then the Stern-Geary equation and the Faraday Law were used. In general, the results of corrosion rates showed an oscillatory behavior, obtaining a cumulative mass loss of 6.11 mg/cm2. The electro-chemical signals showed important evidences of localized corrosion, showing significant transients of high magnitude, but also low amplitude and high frequency noise. The pitting indexes mainly lie in the mixed corrosion zone, being in agreement with visual observations of corroded samples. Optical transmittance measurements of corrosive solution with dissolved Fe, Cr, and Ni species are also presented.

Exergetic analysis of solar powered absorption refrigeration system using LiBr-H<SUB align=right>2O and NH<SUB align=right>3-H<SUB align=right>2O as working fluids

This study provides an easy to follow description of the second law (of thermodynamics) method as applied to a single effect absorption refrigeration cycle. Two different sets of working fluids: LiBr-H2O and NH3-H2O solution are considered, and a method to calculate the entropy of both working fluids is offered. The total exergy destruction in the system as a percentage of the exergy input from the generator heating water over a range of operating temperature is examined for the system operates on both the working fluids separately. It is found that the total exergy destruction for NH3-H2O solution is significantly higher than the LiBr-H2O solution for a given generator temperature. The exergy destruction in each component and the exergetic efficiency of system operates on LiBr-H2O and NH3-H2O solution is also determined. It is shown that the exergy destructions are significantly occur in generator and absorber, and the exergetic efficiency increases significantly as absorber temperature increases.

Thermodynamic modelling of a single-effect LiBr-H2O absorption refrigeration cycle

The objective of this paper is to develop a computer program for design and thermodynamic analysis of a single effect absorption chiller using LiBr-H2O solution as working fluid. The conditions of hot water entering and leaving the desorber, cooling water entering the absorber and leaving the condenser, chilled water entering and leaving the evaporator, as well as the approach temperatures in condenser, evaporator, desorber, and absorber, the effectiveness of solution heat exchanger, the chiller refrigeration power, and the ambient temperature are used as input data. The program then gives the thermodynamic properties of all state points, the design information of all heat exchangers in the cycle and the overall cycle performance. The results deduced from the computer program are used to study the effect of design parameters on cycle performance. For example, increasing the evaporator and generator temperatures or decreasing the condenser and desorber temperatures can improve the second-law efficiency of the cycle. It is also noticed that the temperatures of hot water, cooling water, and chilled water, respectively, at the inlet of the desorber, condenser, and evaporator have a great effect on cycle coefficient of performance. The results of this program can be used either for sizing a new refrigeration cycle or rating an existing system. It can also be used for optimization purposes. The predictions of the present program are compared with other simulating programs and qualitative agreement is achieved.

Coupled heat and mass transfer in absorption of water vapor into Libr-H2o solution flowing on finned inclined surfaces

The absorption characteristics of water vapor into a LiBr-H₂O solution flowing down on finned inclined surfaces are numerically investigated in order to study the absorbing performances of different surface shapes of finned tubes as an absorber element. A three -dimensional numerical model is developed. The momentum, energy, and diffusion equations are solved simultaneously using a finite difference method. In order to obtain the temperature and concentration distributions, the Runge-Kutta and the Successive over relaxation methods are used. The flat, circular, elliptic, and parabolic shapes of the tube surfaces are considered in order to find the optimal surface shapes for absorption. In addition, the effects of the fin intervals and Reynolds numbers are studied. The results show that the absorption mainly happens near the fin tip due to the temperature and concentration gradient, and the absorbing performance of the parabolic surface is better than those of the other surfaces.

Non-absorbable gas effects on heat and mass transfer in falling film absorption

Film absorption involves simultaneous heat and mass transfer in the gas-liquid system. While the non-absorbable gas does not participate directly in the absorption process, its presence does affect the overall heat and mass transfer. An experimental study was performed to investigate the heat and mass transfer characteristics of LiBr-H₂O solution flowing over 6-row horizontal tubes with the water vapor absorption in the presence of non-absorbable gases. The volumetric concentration of non-absorbable gas, air, was varied from 0.17 to 10.0%. The combined effects of the solution flow rate and its concentration on the heat and mass transfer coefficients were also examined. The presence of 2% volumetric concentration of air resulted in a 25% reduction in the Nusselt number and 41 % reduction in the Sherwood number. Optimum film Reynolds number was found to exist at which the heat and mass transfer reach their maximum value independent of air contents. Reduced Nusselt and Sherwood numbers, defined as the ratio of Nusselt and Sherwood numbers at given non-absorbable gas content to that with pure water vapor, were correlated to account for the reduction in the heat and mass transfer due to non-absorbable gases in a falling film absorption process.