Ejector expansion refrigeration system: Ejector design and performance evaluation

Use of a two-phase flow ejector as an expansion device in vapor compression refrigeration systems is one of the efficient ways to enhance its performance. The present work aims to design a constant-area two phase flow ejector and to evaluate performance characteristics of the ejector expansion refrigeration system working with R134a. In order to achieve these objectives, a simulation program is developed and effects of operating conditions and ejector internal efficiencies on the system performance are investigated using EES software. Comparison between present results and published experimental data revealed that the developed model can predict the system COP with a maximum error of 2.3%. The system COP increased by 87.5% as evaporation temperature changed from −10 °C to 10 °C. Finally, correlations to size ejector main diameters as a function of operating conditions, system cooling capacity and ejector internal efficiencies are reported.     

Experimental results with a variable geometry ejector using R600a as working fluid

Experimental results with the first laboratory scale variable geometry ejector (VGE) using isobutane (R600a) are presented. Two geometrical factors, the area ratio and the nozzle exit position, can be actively controlled. The control of the area ratio is achieved by a movable spindle installed in the primary nozzle. The influence of the spindle position (SP) and condenser pressure on ejector performance are studied. The results indicate very good ejector performance for a generator and evaporator temperature of 83 °C and 9 °C, respectively. COP varied between 0.4 and 0.8, depending on operating conditions. The existence of an optimal SP, depending on the back pressure, is identified. A comparison of the benefit of applying the variable geometry design over a fixed geometry configuration is assessed. For example, for a condenser pressure of 3 bar, an 80% increase in the COP was obtained when compared to the performance of a fixed geometry ejector.     

Experimental investigation on R134a vapour ejector refrigeration system

The experimental investigation of the performance of a vapour ejector refrigeration system is described. The system uses R134a as working fluid and has a rated cooling capacity of 0.5 kW. The influence of generator, evaporator and condenser temperatures on the system performance is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The operating conditions are chosen accordingly as, generator temperature between 338 K and 363 K, condenser temperature between 299 K and 310.5 K, and evaporator temperature between 275 K and 285.5 K. Six configurations of ejectors of different geometrical dimensions are selected for the parametric study. The performance of the refrigeration system at different operating temperatures is presented.     

Experimental evaluation of an ejector as liquid re-circulator in a falling-film water chiller

To experimentally evaluate the performance of an ejector working as a liquid re-circulator in a horizontal-tube falling-film evaporator with R134a, experimental tests are performed using a horizontal-tube falling-film water chiller prototype. Experimental observations on intertube liquid flow pattern of tube bundle validate the feasibility of the liquid re-circulation system using a liquid–liquid ejector. The analysis results show that the influence of the motive flow rate on the entrainment ratio of the ejector is small, and the average entrainment ratio of the ejector is about 2.03. With the increase of the valve opening of the regulating valve, the evaporating capacity of the falling-film water chiller rises 4.8%, from 940.2 kW with the re-circulation ratio of one, to 985.5 kW with the re-circulation ratio of 1.135. The COP of the falling-film water chiller reaches a maximum and then drops down with the increase of the re-circulation ratio, and the optimal re-circulation ratio is 1.135.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

A novel linear electromagnetic-drive oil-free refrigeration compressor using R134a

A new type of oil-free moving magnet linear compressor with clearance seals and flexure springs has been designed for incorporation into a vapour compression refrigeration system with compact heat exchangers for applications such as electronics cooling. A linear compressor prototype was built with a maximum stroke of 14 mm and a piston diameter of 19 mm. An experimental apparatus was built to measure the compressor efficiencies and coefficient of performance (COP) of a refrigeration system with the linear compressor, using R134a. The resonant frequency for each operating condition was predicted using the discharge pressure, suction pressure and stroke. Refrigeration measurements were conducted for different strokes under each pressure ratio with a fixed condenser outlet temperature of 50 °C and evaporator temperature ranging from 6 °C to 27 °C. The results show that the COPs are around 3.0 for tests with a pressure ratio of 2.5 (evaporator temperature of 20 °C).     

Development and performance of steam ejector refrigeration system operated in real application in Thailand

This paper presents the design and construction of a prototype steam ejector refrigeration system which can be operated under the actual condition of Thai environment, which is rather hot and humid. The prototype refrigerator was designed to produce a cooling capacity of approximately 3 kW. Water was selected to be used as the working fluid. The steam boiler used was a vertical fire tube type and it was designed to be used with LPG compact gas burner. The condenser was cooled by water obtained from a conventional cooling tower. The prototype refrigerator was used to produce chilled water which was used to cool a small tested room. It was observed that the room temperature of 24.2 °C was obtained at the cooling load of 3000 W. The cooling water was supplied to the condenser at about 30 °C. The COP obtained was 0.45. This prototype refrigerator is proven to be practical and can be used in actual environment of Thailand.     

Experimental and numerical analysis of an air-cooled double-lift NH3–H2O absorption refrigeration system

The prototype of an air-cooled double-lift NH₃–H₂O absorption chiller driven by hot water at low temperature is presented. The main objective of the study is to illustrate the experimental performances of the prototype under different operating conditions. A mathematical model of the cycle is developed, along with a procedure for the identification of otherwise difficult to measure data, with the purpose of providing the complete picture of the internal thermodynamic cycle. The combined experimental and numerical data allowed assessing the effects on the thermodynamic cycle with varying operating conditions. The unit operated steadily with chilled water inlet 12 °C, outlet 7 °C, air temperature between 22 °C and 38 °C, and hot water driving temperatures between 80 °C and 90 °C. The reference cooling capacity at air temperature of 30 °C is 2.5 kW, with thermal COP about 0.3 and electrical COP about 10.     

Application of an ejector in autocascade refrigeration cycle for the performance improvement

This paper presented a novel autocascade refrigeration cycle (NARC) with an ejector. In the NARC, the ejector is used to recover some available work to increase the compressor suction pressure. The NARC enables the compressor to operate at lower pressure ratio, which in turn improves the cycle performance. Theoretical computation model based on the constant pressure-mixing model for the ejector is used to perform a thermodynamic cycle analysis for the NARC with the refrigerant mixture of R23/R134a. The effects of some main parameters on cycle performance were investigated. The results show the NARC has an outstanding merit in decreasing the pressure ratio of compressor as well as increasing the COP. For NARC operated at the condenser outlet temperature of 40 °C, the evaporator inlet temperature of −40.3 °C, and the mass fraction of R23 is 0.15, the pressure ratio of the ejector reaches to 1.35, the pressure ratio of compressor is reduced by 25.8% and the COP is improved by 19.1% over the conventional autocascade refrigeration cycle.     

Performance of ejector cooling systems using low ecological impact refrigerants

The theoretical behaviour of an ejector cooling system, using as working fluids propane, butane, isobutane, R152a and R134a, is obtained. The ejector works as a thermo-compressor that is simulated with a validated one-dimensional mathematical model, whose errors are lower than 6%. For a system unitary cooling capacity, a parametric study is carried out varying the generation, condensation and evaporation temperatures. From the obtained data, a complete analysis of the system performance can be achieved when the ejector and system operation parameters are considered. The best performance corresponds to the system using propane, because has the highest system coefficient of performance and its ejector has the maximum entrainment ratio value, the least area ratio value and the highest efficiency value. The considered generation temperature ranging from 70 °C to 95 °C is appropriate for low-grade energy sources assisting thermal cooling systems. After this system performance, come those in which R152a and R134a are employed, with isobutane and butane at the end. The obtained results represent potential design points of an efficient ejector cooling system operation, because to each combination of the above mentioned temperatures corresponds one and only one ejector geometry.     

Investigation of an experimental ejector refrigeration machine operating with refrigerant R245fa at design and off-design working conditions. Part 2. Theoretical and experimental results

The main results of a theoretical and experimental investigation of the performance characteristics of an ejector and an ejector refrigeration machine (ERM) operating with refrigerant R245fa at design and off-design working conditions are presented. The ejector and ERM were explored theoretically using improved 1D model and the calculated results were validated experimentally on ejector test rig that has been assembled and operated at National Taiwan University. For typical cases, the performance characteristics variation with condensing, generating and evaporating temperatures along with performance maps are presented. The theoretical results are compared with the results of a set of experiments and good qualitative and quantitative agreement is observed.     

Hybrid vapor compression refrigeration system with an integrated ejector cooling cycle

A refrigeration system was developed which combines a basic vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling cycle is driven by the waste heat from the condenser in the vapor compression refrigeration cycle. The additional cooling capacity from the ejector cycle is directly input into the evaporator of the vapor compression refrigeration cycle. The governing equations are derived based on energy and mass conservation in each component including the compressor, ejector, generator, booster and heat exchangers. The system performance is first analyzed for the on-design conditions. The results show that the COP is improved by 9.1% for R22 system. The system is then compared with a basic refrigeration system for variations of five important variables. The system analysis shows that this refrigeration system can effectively improve the COP by the ejector cycle with the refrigerant which has high compressor discharge temperature.     

Mathematical modeling and thermodynamic investigation of the use of two-phase ejectors for work recovery and liquid recirculation in refrigeration cycles

This paper presents the results of a numerical investigation on the performance of ejector cycles in which the work recovered is used to recirculate liquid through the evaporator. The ejector recirculation cycle, in which the ejector is only used to recirculate liquid and improve evaporator performance, and the standard ejector cycle, in which the ejector can be used to both recirculate liquid and directly unload the compressor, are investigated. The analysis uses a microchannel evaporator and refrigerants R134a, R410A, and CO₂. It is seen that fluids that have large throttling loss but gain little benefit from liquid recirculation (CO₂) should use the ejector to directly unload the compressor, while fluids that have lower throttling loss but gain significant benefit from liquid recirculation (R134a) should use the ejector to improve evaporator performance through liquid recirculation. It is also seen that the ejector recirculation cycle is better suited for ejector off-design operation.     

Performance analysis of the ejector-expansion refrigeration cycle using zeotropic mixtures

To evaluate the performance of the ejector-expansion refrigeration cycle (EERC) using zeotropic mixtures, a numerical study is conducted. A constant-pressure two-phase ejector model for zeotropic mixtures is established. The effects of both the fluid composition and the working conditions are investigated. Mixture R134a/R143a is selected as the working and the simulation results reveal that, the cycle COP increases first and then decreases as MF_t (the mass fraction of R134a) increases in the researched condition. The COP gets a maximum value of 4.18 with MF_t of 0.9 and yields a minimum value of 3.66 with MF_t of 0.5. With mixture 0.9/0.1, the COP improvement reaches a maximum value of 10.47%. This improvement rises at high condensing temperature or low evaporating temperature. The exergy analysis shows that the compressor and ejector contribute the most exergy destruction, and the cycle exergy efficiency achieves a maximum value with MF_t of 0.7.     

Thermodynamic performance of air conditioners working with R417A and R424A as alternatives to R22

The energy and exergy parameters of R417A and R424A gases which can be used instead of R22 were experimentally investigated for a split-type air conditioner. Although GWP amounts of the available alternative refrigerants are higher compared to R22, their ODP values are zero. The experiments were accomplished for three different ambient temperature values of 25 °C, 30 °C and 35 °C. The covered test conditions were carried out for steady-state case while keeping the inside medium temperature at constant temperature of 22 °C. The cooling capacity, COP, exergy destruction of components in the unit (i.e., compressor, condenser, evaporator, and expansion device), exergetic efficiency and some other parameters of the system were determined. COP values for the refrigerants of R417A and R424A were noted to be smaller compared to R22. Similarly, both isentropic efficiency of the compressor and exergetic efficiency of the system were higher for R22. The use of R424A will be more suitable rather than R417A since COP values of R417A are lower about 5–16% compared to R424A. The COP value of R22 is greater than that of R417A and R424A by amounts of 17–23% and 4–18%, respectively. At the greater evaporation temperatures (0 °C to +5 °C as in the air-conditioners) it can be stated that R424A is more preferable than R417A as an alternative refrigerant to R22.     

Assessment of R-438A as a retrofit refrigerant for R-22 in direct expansion water chiller

The present work aims to evaluate the performance characteristics of a vapor compression refrigeration system using R-438A as a retrofit refrigerant for R-22. In order to achieve this objective, a test facility is developed and experiments are performed over a wide range of chilled water inlet temperature (11:20 °C), condenser water inlet temperature (25:35 °C) and condenser water mass flow rate (363:543 kg h⁻¹). Results showed that as the chilled water inlet temperature changes from 11.5 to 20.5 °C, system COP increases from 1.78 to 2.07 at constant condenser water inlet temperature of 25.5 °C. Cooling capacity and COP of the system using R-438A are lower than R-22 by 11% and 12.5%, respectively. However, compressor discharge temperature using R-438A is slightly lower than R-22 which confirms that R-438A can be used as a retrofit refrigerant for R-22 to complete the remaining life time of the existing plants.     

A year-round dynamic simulation of a solar-driven ejector refrigeration system with iso-butane as a refrigerant

In this paper, the performance of the solar-driven ejector refrigeration system with iso-butane (R600a) as the refrigerant is studied. The effects that both the operating conditions and the solar collector types have on the system's performance are also examined by dynamic simulation. The TRNSYS and EES simulation tools are used to model and analyze the performance of a solar-driven ejector refrigeration system. The whole system is modelled under the TRNSYS environment, but the model of the ejector refrigeration subsystem is developed in the Engineering Equations Solver (EES) program. A solar fraction of 75% is obtained when using the evacuated tube solar collector. In the very hot environment, the system requires relatively high generator temperature, thus a flat plate solar collector is not economically competitive because the high amount of auxiliary heat needed to boost up the generator temperature. The results from the simulation indicate that an efficient ejector system can only work in a region with decent solar radiation and where a sufficiently low condenser temperature can be kept. The average yearly system thermal ratio (STR) is about 0.22, the COP of the cooling subsystem is about 0.48, and the solar collector efficiency is about 0.47 at T_e 15 °C, T_c 5 °C above the ambient temperature, evacuated collector area 50 m² and hot storage tank volume 2 m³.     

Experimental investigation on NH3–H2O compression-assisted absorption heat pump (CAHP) for low temperature heating in colder conditions

The coefficient of performance (COP) and heating capacity of the absorption heat pump (AHP) decreased obviously as the evaporator inlet temperature dropped. Compression-assisted AHP (CAHP) could operate efficiently in colder conditions, and a prototype was constructed for experimental investigation. At a generator inlet of 130 °C, as the evaporator inlet decreases from −5 °C to −25 °C, the COP drops from 1.513 to 1.372, while the heating capacity deteriorates from 77.26 kW to 47.11 kW. Comparisons between CAHP and normal AHP indicated that CAHP can extend the lower limit of evaporator inlet temperature from −10 °C to −25 °C. Besides, CAHP can enhance the heating capacity by approximately 55.5–85.0% even when AHP can operate normally. Moreover, the improvement contributed by CAHP is greater under much colder conditions. The heating COP and capacity of CAHP are improved in all the conditions, while the primary energy efficiency is advantageous under lower evaporator inlet temperatures.     

Performance analysis of a supersonic ejector cycle working with R245fa

A supersonic ejector chiller for industrial use is currently being developed and tested as part of a project cooperation between Frigel s.p.a and DIEF (Department of Industrial Engineering, University of Florence). The refrigerator was built following a “ready to market” setup criterion and is intended for applications on the industrial refrigeration market or in air conditioning. The plant has a nominal cooling power of 40 kW and is powered by low temperature heat (from 90 up to 100 °C). The ejector is equipped with a movable primary nozzle and 9 static pressure probes along the mixing chamber/diffuser duct. The working fluid is R245fa. An extensive numerical campaign was performed to analyze the internal dynamics of the ejector. All the simulations were carried out by accounting for the real gas properties of the refrigerant. Comparison with experimental data resulted in close agreement both in terms of global and local parameters. Analyses showed that in order to achieve an accurate matching with the experimental data, it is necessary to correctly account for the surface roughness of the ejector. This is especially true for off-design operating conditions.     

The effect of operating conditions on the performance of a supersonic ejector for refrigeration

A previously developed one-dimensional model, based on a forward marching solution technique of the conservation equations has been used to study ejector operation and performance in a large range of refrigeration working conditions. Several important features of ejector operation characteristics were simulated. Global parameter values, their local distributions along the ejector including the temperature, the pressure and the Mach number were calculated for design and off design conditions. Operation parameters such as the entrainment ratio ω, compression ratios P_exit/P_ev, P_g/P_exit and the geometric ratio (D/D_c)² were found to significantly affect performance. The impact of the generator, the evaporator, the condenser and related thermodynamic parameters, which have been assessed in this study, are summarized as:

Fluid mixing conditions dictated by the fluid type, the mixing chamber geometry, the inlet and outlet constraints, may lead to off design operation with related stability and performance deterioration

Internal superheat generation, due to inefficient mixing and normal shock waves is very important in off design operation

Some degree of inlet superheat (around 5 °C) is necessary to prevent internal condensation but excess superheat is detrimental to the condenser efficiency at exit

Generator pressure conditions and the evaporator temperature significantly affect ejector performance.