Lakdar Kairouani

Performance of a Water Ammonia Absorption System Operating at Three Pressure Levels

The present study deals with a compression-absorption machine. The proposed hybrid cooling system uses water-ammonia as a working fluid and operates at three pressure levels. The absorber is at an intermediate pressure (Pint) taken between the evaporator pressure (PEV) and the condenser pressure (PCD), unlike the single stage machine which works between two pressure levels. The proposed new system is studied and compared to the conventional machine. In order to evaluate the performance of the invoked machine, a procedure based on the MAPLE software is set up to compute accurately the thermodynamic properties of the working fluid. The analyses of the numerical results highlight that the performance of the novel proposed configuration is better than that relative to the conventional system. The study reveals the great impact of the intermediate pressure on the performance improvement and on reducing the generator temperature allowing the system to work at low enthalpy. In fact, for an evaporator temperature and a condenser temperature fixed respectively at -10°C and 40°C, the proposed hybrid refrigeration cycle operates at a generator temperature TGE = 75°C and the installation’s COP is about 0.56. While for the same conditions, the single stage machine COP cannot exceed 0.51 with a generator temperature of about 135°C. Consequently, our enhanced novel configuration presents the opportunity to operate at low enthalpy sources.

Exergetic Analyses of a Particular Absorption Cooling System

The objective of this work is to present an exergy analysis of a novel absorption configuration using water-ammonia as working fluid. The proposed configuration operates at three pressure levels. The absorber is at an intermediate pressure (Pint). A thermodynamic model based on the mass energy and exergy balances is developed for this purpose. The parameters analyzed are the refrigeration systems performance (COP), the exergy efficiency, the global exergy destruction in the system, the exergy destruction and the irreversibility in different components. The effects of generator, absorber, condenser and evaporator on the performance of the system are examined. Numerical results highlight the great importance of the intermediate pressure on the performance of the system and specially on reducing the operating generator temperature. Consequently, the intermediate pressure is directly responsible on the adaptability of the proposed cooling absorption cycle to low enthalpy sources.

Use of ejectors in cascade refrigeration systems for exergetic performance improvement

In this paper an expansion cascade refrigeration system are compared to cascade systems using ejectors. This comparison is made for a variety of fluids. Natural fluids and hydrofluorocarbons (HFCs) are used in the two loops. Theoretical models are written using FORTRAN. The thermodynamic parameters of fluids are given using the software REFPROP 8.0. For fixed working conditions, using CO2, R23 or R32 in LPL and R152a in HPL in an expansion cascade system gives the best Coefficient of Performance (COP) which attains 1.57 for Ti = −15°C. A comparison between the different systems with ejectors showed that a cascade system with two ejectors is the most powerful. It allows an optimum COP equal to 1.8 and a relative performance equal to 19.8% when propane is used in HPL and CO2 in LPL for Ti = −25°C. But the best relative performance is given when R125 is used in HPL; it attains 24.2% when CO2 is used in LPL. An exergetic analysis shows that introduction of two ejectors in cascade system ameliorate the exergy efficiency by 27.3% when CO2 is used in LPL and R152a in HPL for Ti = −30°C.

Performance evaluation of a solar ejector-vapour compression cycle for cooling application

This study deals with the performance of the ejector-vapour compression cycle assisted by solar. The effect of operating conditions on the combined cycle performance is examined. Also, a comparison of the system performance with environment friendly refrigerants (R134a, R600, R123, R141b, R142b, R152a, R290, and R245fa) is made. This performance is calculated using an empirical correlation. Thermodynamic properties of functioning fluids are obtained with package REFPROP 8. Using the typical meteorological year file containing the weather data of the city of Tunis, the system performance is computed for three collector types. The theoretical results show that the R290 offers the highest coefficient of performance, COP=3.75, for generator temperature TB = 78°C, condenser temperature Tc = 30°C and the intercooler temperature Te = 15°C.

Solving equations describing the steady-state model of MSF desalination process using Solver Optimization Tool of MATLAB software

AbstractThis work presents an algorithm for solving the large system of non-linear equations describing the steady state model of the two main layouts for the MSF process (the multistage flashing with brine recirculation and the once-through multistage flash desalination (MSF-OT)). The model accounts for the geometry of the stage, the mechanism of heat transfer, and the variation of the thermophysical properties of various fluids with temperature and salinity. In addition, the overall heat transfer coefficient was evaluated at each stage using convective heat transfer coefficient for internal and external flows, tube thermal conductivity and fouling resistance. Furthermore, the model takes into consideration the dependence of thermodynamic losses on stream flow rate, temperature, and the brine salinity. The system of equations was solved through an iterative procedure by using Solver Optimization Tool of MATLAB software. For each stage, the developed model was then used for evaluating the temperatures of ...