Mahmoud Bourouis

Absorption of water vapour in the falling film of water–lithium bromide inside a vertical tube at air-cooling thermal conditions

Abstract In the absorbers of air-cooled water–lithium bromide absorption chillers, the absorption process usually takes place inside vertical tubes with external fins. In this paper we have carried out an experimental study of the absorption of water vapour over a wavy laminar falling film of water–lithium bromide on the inner wall of a smooth vertical tube. The control variables for the experimental study were; absorber pressure, solution mass flow rate, solution concentration and cooling water temperature. Relatively high cooling water temperatures were selected to simulate air-cooling thermal conditions. The parameters considered to assess the performance of the absorber were; the mass absorption flux, the outlet solution degree of subcooling and the falling film heat transfer coefficient. The results indicate that in water cooling thermal conditions the mass absorption fluxes are in the range 0.001–0.0015 kg·m−2·s−1, whereas in air-cooling thermal conditions the range of mass absorption values decreases to 0.00030–0.00075 kg·m−2·s−1.

Double-lift absorption refrigeration cycles driven by low-temperature heat sources using organic fluid mixtures as working pairs

At present, much interest is being shown in absorption refrigeration cycles driven by low temperature heat sources, such as solar energy or low-grade waste-heat. Double-lift absorption cycles working with ammonia-water have been recommended for refrigeration applications which require cold at 0°C and which are activated by waste heat between 70 and 100°C. This paper discusses the potential of the organic fluid mixtures trifluoroethanol (TFE)-tetraethylenglycol dimethylether (TEGDME or E181) and methanol-TEGDME as working pairs in series flow and vapour exchange double-lift absorption cycles. The ammonia-water mixture was used for comparison purposes. The results show that the performances of these cycles improve significantly when they have the above mentioned organic fluid mixtures as working pairs. For example, the coefficient of performance of the vapour exchange cycle working with TFE-TEGDME is 15% higher than with ammonia-water. In this study, we used a modular software package, which we developed for the thermodynamic properties and cycles simulation of absorption systems.

Industrial heat recovery by absorption/compression heat pump using TFE–H2O–TEGDME working mixture

Abstract The thermodynamic performance of a single-stage absorption/compression heat pump using the ternary working fluid Trifluoroethanol–Water–Tetraethylenglycol dimethylether (TFE–H2O–TEGDME) for upgrading waste heat has been studied. A simulation program has been developed using a mathematical model based on mass and energy balances in all components of the cycle and thermodynamic equilibrium considerations. In order to establish the optimum operating conditions of the cycle for various thermal conditions, sensitivity studies of the coefficient of performance (COP), the flow rate of the weak solution and the compressor volumetric displacement, both per unit of upgraded energy, were carried out versus of water content in the vapour phase. The results obtained show that the operation of the cycle with this ternary system is still more advantageous than the TFE–TEGDME binary working pair. So, it is possible to upgrade thermal waste heat from 80 to 120°C, with a COP of about 6.4, with a compression pressure ratio of 4 at a low pressure of 100 kPa, the water mole fraction in the vapour being 42%. At these operating conditions, the necessary weak solution mass flow rate is about three times lower than the corresponding binary one. The performance comparison of such a cycle with other absorption cycles like the heat transformer or the single-effect heat pump, both of them using the ternary system, shows its interest and potential.

Absorption of organic fluid mixtures in plate heat exchangers

It is well known that the absorber is the key component in energy conversion systems that are based on absorption cycles. This paper describes an experimental investigation into the absorption process of organic fluid mixtures in an absorption system which has a spray and a plate heat exchanger. The absorber consists of an adiabatic mixing chamber with a spray, where the solution that is weak in refrigerant is sprayed into the refrigerant vapour. A two-phase mixture is formed and enters a plate heat exchanger, where the solution is cooled to complete the absorption process. We carried out experiments with different types of spray nozzles using the organic fluid mixtures methanol–tetraethyleneglycol dimethylether (TEGDME) and trifluoroethanol (TFE)–TEGDME. We analyse how the solution mass flow rate, absorber pressure and cooling water temperature affected the absorber performance and we discuss the results in terms of the absorber load, absorbed mass flux, degree of subcooling of the solution at the absorber outlet, solution film heat and mass transfer coefficients. The results indicate that the absorption system proposed is suitable for relatively low pressures. For water temperatures of 30 °C and absorber pressures between 2 and 6 kPa, the absorption rates for TFE–TEGDME were 1 to 2.5 g·s−1·m−2. The corresponding values for methanol–TEGDME with absorber pressures between 10 and 15 kPa were 0.4 to 1.2 g·s−1·m−2.

Simulation and data validation in multistage flash desalination plants

Abstract This paper describes the main concepts of a specific Fortran program tackling steady-state simulation and data validation (gross error detection and data reconciliation) for multistage flash desalination plants. The process simulation was carried out using an equation-oriented approach in which the decomposition of the system leads to a sensitivity matrix which is diagonal by blocks with extra diagonal blocks. The topology of this matrix is conserved according to a procedure which can be extended to an unsteady-state simulation of the process, justifying the interest of the adopted methodology. The validation of the raw process data was achieved by the implementation of a simultaneous strategy of gross error detection and data reconciliation, based on the bivariate method. In this approach we minimize an objective function using the maximum likelihood principle, which takes into account both contributions from random and gross errors. The complete mathematical model describing the process behaviour at steady state was considered as constraints in the process optimization. Results corresponding to the structural and operating conditions of the distiller No 5 of Umm Al Nar East plant which was put into service in 1987/1988 in the Emirate of Abu Dhabi are presented. This package was developed at the School of Chemical Engineers in Toulouse (France), and the project was sponsored by the International Centre for Water and Energy Systems (ICWES) located in Abu Dhabi (United Arabs Emirates).

Cooling loads analysis of an endoreversible solar absorption refrigerator cycle

This paper deals with the analysis of the Cooling Loads (CL) of a solar absorption refrigerator. For this purpose, a new Process Conceptual Design (PCD) approach based on equivalent endoreversible model and hierarchical decomposition is developed. The influences of many cooling load aspects on the model performances are investigated. A sensitive study is made: performance coefficient (i.e. its inverse) vs. different cooling load aspects. New relationships between technical and economical criteria and optimal parameters stand out. An endoreversible new bound for the Inverse Specific Cooling Load (ISCL) is achieved. For every case, functional and practical domains are defined. Academic interest domains are also notified.

Performance of air-cooled absorption air-conditioning systems working with water-(LiBr + Lil + LiNO3 + LiCl)

Abstract The potential of the fluid mixture water-(LiBr + Lil+ LiNO3 +LiCl) (5:1:1:2 molar) is studied for air-cooled absorption air-conditioning systems. This multicomponent system shows a considerably higher solubility than that of water-LiBr and is also less corrosive. It is, therefore, one of the potential alternatives to replace water-LiBr in future air-cooled absorption chillers. A comparative study based on thermodynamic simulation of the single-and double-effect cycles with water-LiBr and the new fluid mixture is first reported. Once the operating conditions were established in terms of temperature and concentration in the generator and the absorber, the absorption process of a falling film flowing on the inner surface of a vertical tube at typical air cooling thermal operating conditions of the absorber was modelled in order to compare the absorption rates of the multicomponent salt mixture with those of water-LiBr. This study was completed by an experimental characterization of the absorption process in a vertical falling film tube. The results show that the multicomponent salt solution is more suitable than water-LiBr if the temperatures in the generator and absorber/condenser are low and high, respectively. Therefore this new working fluid can be recommended for air-cooled absorption air-conditioning systems driven by low temperature heat sources.

Numerical Prediction of Flow Patterns in Bubble Pumps

In the present study, the ammonia-water mixing flow in a bubble pump is numerically simulated. The flow patterns of a two-phase flow in a bubble pump were studied under different conditions of heat flux and tube diameter. A one-dimensional two-fluid model was developed under constant heat flux. This model was used to predict the variations in void fraction and liquid and vapor velocities throughout the tube. Then, the void fraction profile and the curve of liquid velocity versus vapor velocity were used to predict the flow patterns along the tube length. It was found that at heat fluxes below 15 kW m ―2 , bubbly, slug, and churn flows are the dominating regimes, and the length of these flow regimes depends on the tube diameter. For heat fluxes higher than 15 kW m ―2 , the bubble pump operates under the churn and annular regimes, and the bubble pump performance is improved when the tube diameter increases.

Experiments on the characteristics of saturated boiling heat transfer in a plate heat exchanger for ammonia/lithium nitrate and ammonia/(lithium nitrate+water)

The new developments in absorption systems for air-conditioning applications respond to the new energetic and environmental situation that requires systems more efficient, use of renewable energy sources, such as thermal solar energy, and integration in polygeneration systems for the energy supply of buildings. Among these new developments, its of great interest to mention absorption systems designed specifically for solar applications and direct heat recovery from exhaust gases of micro-generation systems. Some of these new systems are already air-cooled. Ammonia-water mixture is one of the most used working pairs in absorption refrigeration systems; nevertheless, several authors have proposed the use of lithium nitrate as an absorbent instead of water. Ammonia/lithium nitrate refrigeration systems do not require a rectifier to remove the absorbent from the vapor stream leaving the desorber, and the driving temperature is lower than that required for ammonia-water systems. The main disadvantage of this working fluid is related with its high viscosity, which penalizes heat and mass transfer processes in the absorber and generator. To overcome this limitation a double solution is proposed. First, the use of plate heat exchangers in the design of the main components of the cycle and, second, the addition of a small amount of water to the ammonia/lithium nitrate mixture in order to reduce the viscosity but avoiding the need of rectification. In this work, we experimentally investigated saturated flow boiling heat transfer of the ammonia/lithium nitrate and ammonia/ (lithium nitrate + water) mixtures with water content in the absorbent of 20 % by weight, flowing in a vertical plate heat exchanger. The test section consists of four commercial plates with a chevron angle of 60°, referred to the vertical axis of the plate, forming three channels. The effects of heat flux ranging from 5 to 20 kW/m2, mass fluxes from 50 to 100 kg/s.m2 and mean vapor quality from 0 to 0.2 on the boiling coefficient and total pressure drop were analyzed. The results show that, at the considered operating conditions, boiling heat transfer coefficient increases with increasing the heat flux and mass flux, whereas the vapor quality slightly influences the boiling heat transfer coefficient. The addition of water increases the boiling heat transfer coefficient more than 30 % when compared with the binary mixture.

Thermophysical Properties and Heat and Mass Transfer of New Working Fluids in Plate Heat Exchangers for Absorption Refrigeration Systems

Ammonia/lithium nitrate and ammonia/(lithium nitrate + water) have been proposed as suitable working pairs for absorption refrigeration systems driven by low-temperature heat sources. The use of water in the absorbent of the ternary mixture reduces the viscosity of the binary mixture and increases the affinity between the refrigerant (ammonia) and the absorbent, which should have a positive effect on the absorption process. In this paper, a brief review on the thermodynamic and transport properties available in the open literature for NH3/LiNO3 and NH3/(LiNO3 + H2O) mixtures is presented, and the most significant results of an experimental characterization of absorption and desorption processes taking place in plate heat exchangers are discussed.