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.     

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.     

Numerical optimisation of a two-stage ejector refrigeration plant

Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.     

A 1-D analysis of ejector performanceAnalyse unidimensionnelle de la performance d'un éjecteur

A 1-D analysis for the prediction of ejector performance at critical-mode operation is carried out in the present study. Constant-pressure mixing is assumed to occur inside the constant-area section of the ejector and the entrained flow at choking condition is analyzed. We also carried out an experiment using 11 ejectors and R141b as the working fluid to verify the analytical results. The test results are used to determine the coefficients, η_p, η_s, φ_p and φ_m defined in the 1-D model by matching the test data with the analytical results. It is shown that the1-D analysis using the empirical coefficients can accurately predict the performance of the ejectors.     

Performance parameters for the design of a combined refrigeration and electrical power cogeneration system

This paper discusses the conservation of energy in a cogeneration system. A steam power cycle (Rankine) produces electrical power 2 MW and steam is bleeded off from the turbine at 7 bar to warm a factory or units of buildings during the winter or to supply a steam ejector refrigeration cycle to air-conditioning the same area during the summer. In the summer this system can be as alternative solution instead of absorption. Certainly the ejector refrigeration unit is more economical than absorption unit. The ratio of electrical power/heat is varied into the region (0.1–0.4) and the evaporator temperature of the ejector cycle is varied into the region (10–16 °C). A computer program has been developed for the study of performance parameters of the cogeneration system.     

Effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger

Liquid refrigerant injection technique can be a very effective method for controlling subcooling and the compressor discharge temperature of a refrigeration system at high ambient temperatures. In this study, the effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger were investigated by varying the liquid injection rate at the conditions of constant expansion valve opening in the evaporator and constant total flow rate. During the tests, the ambient temperature was maintained at 43 °C. With the increase of the liquid injection rate, the subcooling at the inner heat exchanger outlet increased and the superheat at the accumulator outlet decreased. However, unacceptable results such as the increase of the compressor discharge pressure and decrease of the system performance were also observed depending on the control method applied. To obtain high system performance and reliability, optimum control methods for liquid injection in the accumulator heat exchanger are suggested. The liquid injection technique for the refrigeration system with an accumulator heat exchanger was found to be an effective method for controlling adequate subcooling and the compressor discharge temperature of the refrigeration system at high ambient temperatures.     

CFD analysis of ejector in a combined ejector cooling system

One-dimensional ejector analyses often use coefficients derived from experimental data for a set of operating conditions with limited functionality. In this study, several ejector designs were modelled using finite volume CFD techniques to resolve the flow dynamics in the ejectors. The CFD results were validated with available experimental data. Flow field analyses and predictions of ejector performance outside the experimental range were also carried out. During validation, data from CFD predicted the entrainment ratios with greater accuracy on definite area ratios, although no shock was recorded in the ejector. Predictions outside the experimental range—at operating conditions in a combined ejector–vapour compression system—and flow conditions resulting from ejector geometry variations are discussed. It is found that the maximum entrainment ratio happens in the ejector just before a shock occurs and that the position of the nozzle is an important ejector design parameter.     

Comprehensive investigation of numerical methods in simulating a steady-state vapor compression system

Computer simulation has become a required tool in the design phase of vapor compression systems; however with relatively few exceptions most simulations focus on the basic four component systems. With an increasing focus being placed on energy efficiency, the simulation of multi-component vapor compression systems (having multiple evaporators, condenser or compressors) will become essential to assist in the design of these more complicated systems. The implementation of a component-based framework will facilitate the simulation of multi-component systems. This paper describes three algorithms used to simulate a component-based vapor compression system. A test matrix of 6174 sample runs covering a wide range of operating conditions was constructed to determine the robustness and speed of each method when using three different types of nonlinear equation solvers. Each method was tested by simulating a basic four component cycle and a more advanced multiple evaporator system. The results are presented in such a format as to describe the reasons that contribute to any instability of the solvers and the computational efficiency of each method is discussed.     

Optimising the refrigeration cycle with a two-stage centrifugal compressor and a flash intercooler

The optimisation of a refrigeration process with a two-stage centrifugal compressor and flash intercooler is presented in this paper. The two-stage centrifugal compressor stages are on the same shaft and the electric motor is cooled with the refrigerant. The performance of the centrifugal compressor is evaluated based on semi-empirical specific-speed curves and the effect of the Reynolds number, surface roughness and tip clearance have also been taken into account. The thermodynamic and transport properties of the working fluids are modelled with a real-gas model. The condensing and evaporation temperatures, the temperature after the flash intercooler, and cooling power have been chosen as fixed values in the process. The aim is to gain a maximum coefficient of performance (COP). The method of optimisation, the operation of the compressor and flash intercooler, and the method for estimating the electric motor cooling are also discussed in the article.     

Development of a circulating system for a jet refrigeration cycle

This paper proposed a workless-generator-feeding (WGF) system for a jet refrigeration cycle, using R141b. This feeding system does not require any mechanical power. The liquid refrigerant from the condenser was fed to the vapour-generator by means of the generator pressure and gravitational force. The system was tested and compared with a conventional system using a mechanical pump. It was found that this system was workable. The heat input to the generator was slightly higher than that for a system using a mechanical pump. The jet refrigeration cycle employing this new feeding system provided a slightly lower coefficient of performance (COP) compared to a system using a mechanical pump. However, this new system did not require any mechanical energy. Therefore, the jet refrigeration system employing this WGF system is truly a heat-power refrigeration cycle.     

Stress corrosion cracking in refrigeration systems

It is generally understood that stress corrosion cracking can occasionally affect the high pressure vessels in ammonia refrigeration systems, but the nature of the problem is often mis-represented in design codes, safety standards and textbooks and ways in which the initiation of stress corrosion can be prevented are ignored. It is also not appreciated that stress corrosion can also affect low pressure vessels and pipework in ammonia systems, and copper pipework in fluorocarbon refrigeration systems.This paper provides an overview of published literature on stress corrosion cracking and then presents some recent case study material illustrating various ways in which stress corrosion has arisen in practice. The conclusions will provide guidance for design engineers and equipment owners on eliminating stress corrosion cracking and on dealing with it if it arises.     

Maximum mass flow ratio due to secondary flow choking in an ejector refrigeration system

The occurrence of flow choking in an ejector of an ejector refrigeration system (ERS) was analysed and a model for predicting the maximum flow ratio of the ejector was developed. The multi-parameter equation to calculate the mass flow ratio takes into account the performance of the primary nozzle, the flow entrainment and mixing relating to ejector geometry and operating conditions. We validated the model using the reported experimental data of refrigerant R113, R141b and steam ERS. The present model was shown to provide better accuracy compared with results obtained by applying the existing 1-D ejector theory. We discussed the application of the model and highlighted the significance of the parameters for future work.     

Recent developments in simulation techniques for vapour-compression refrigeration systems

Simulation has been widely used for performance prediction and optimum design of refrigeration systems. A brief review on history of simulation for vapour-compression refrigeration systems is done. The models for evaporator, condenser, compressor, capillary tube and envelop structure are summarized. Some developing simulation techniques, including implicit regression and explicit calculation method for refrigerant thermodynamic properties, model-based intelligent simulation methodology and graph-theory based simulation method, are presented. Prospective methods for future simulation of refrigeration systems, such as noise-field simulation, simulation with knowledge engineering methodology and calculation methods for nanofluid properties, are introduced briefly.     

Experimental results for a hydraulic refrigeration system using n-butane

The hydraulic refrigeration system (HRS) is a vapor-compression system that accomplishes the compression and condensation of the refrigerant in a unique manner, by entraining refrigerant vapor in a down-flowing stream of water and utilizing the pressure head of the water to compress and condense the refrigerant. A multi-stage HRS was designed, fabricated, and tested using n-butane as the refrigerant. In general, both the refrigeration rate and the coefficient of performance (COP) increased with a corresponding decrease in the compression fluid temperature of the third and final stage. The refrigeration rate and COP were also found to increase with a corresponding increase in evaporator temperature. The predictions of an enhanced model incorporating two-phase hydraulic losses show excellent agreement with the experimental data with a maximum error of ±20%. The results of the experimental investigation indicate that the HRS offers an attractive and feasible alternative to conventional vapor-compression systems, especially in applications where direct-contact heat exchange in the evaporator is desirable.     

Numerical simulation of a variable speed refrigeration system

This work presents two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system. The condenser and the evaporator were divided into a number of control volumes. Time dependent partial differential equations system was obtained from the mass, energy and momentum balances for each control volume. As the expansion valve and the compressor both have very small thermal inertia, the steady state models were applied for these components. Transient and steady state models numerical predictions were compared and good agreement was found. Further simulations were performed with the objective of verifying the possibility of controlling the refrigeration system and the superheating of the refrigerant in the evaporator outlet by varying the compressor speed and the throttling valve sectional area. The results indicate that the proposed models can be used to formulate an algorithm for controlling a refrigeration system.     

The effect of oil in refrigeration: Current research issues and critical review of thermodynamic aspects

A lubrication agent is necessary in almost all the refrigeration vapour compression systems, particularly for the correct operation of the compressor. However, a certain portion of the oil always circulates with the refrigerant through the cycle. This circulation is at the origin of a deviation from the theoretical behaviour (i.e. based on pure refrigerant) of the components. This article aims at reviewing the oil-related researches in the field of refrigeration. Previous reviews in the literature focused on the thermo-hydraulic consequences of the presence of oil; we will analyse here its thermodynamical consequences. In a first part, a brief literature review will give an overview of current scientific and technological issues concerning the impact of oil on components or on whole refrigeration systems. The typical approaches and methods employed to address this problem will be described. These researches require sound tools for the evaluation of thermodynamic properties of refrigerant–oil mixtures. The second part of this article is hence a critical review of these tools, and focuses particularly on liquid–vapour equilibrium, absorption–diffusion, and mixture enthalpy calculation.     

Thermoeconomic optimization of a two stage combined refrigeration system: a finite-time approach

A finite-time thermoeconomic performance analysis based on a new kind of optimization criterion has been carried out for a two-stage endoreversible combined refrigeration cycle model. The optimal performances and design parameters that maximize the objective function (cooling load per total cost) are investigated. In this context, the optimal temperatures of the working fluids, the optimum performance coefficient, the optimum specific cooling load and the optimal distribution of the heat exchanger areas are determined in terms of technical and economical parameters. The effects of the economical parameter that characterizes the investment and energy consumption costs on the general and the optimal performances have been discussed.     

Simulation and measurement on the full-load performance of a refrigeration system in a shipping containerSimulation et mesures de la performance d'un système frigorifique d'un conteneur maritime entièrement chargé

A mathematical model of a refrigeration system in a shipping container has been developed to allow for full-load simulation of its thermal performance. Sub-models are created on the key components: compressor, evaporator, condenser, and thermostatic expansion valve. The sub-models are then coupled by appropriate mass and energy transfer relations to form the full model. Comparison with a series of cooling capacity tests conducted on a 2.2 m (40 ft) fullscale container housed in a temperature-controlled environmental test chamber indicates good agreement, with simulation results being within ±10% uncertainty of measurements.     

Ammonia in low capacity refrigeration and heat pump systems

Ammonia has been used as refrigerant in large vapour compression systems continuously since the beginning of the era of refrigeration. In small systems, it has hardly been used at all since the introduction of the halogenated hydrocarbons around 1930. Lately, with the search for alternatives with less influence on global warming, the use of ammonia in small systems has come into focus again.In the present paper, the work done at the Royal Institute of Technology (KTH) with the aim of developing a prototype of a domestic water to water heat pump with a heating capacity of 9 kW is presented. It has been shown that such a system can be designed to operate with about 100 g of ammonia.Crucial problems in the development of the direct expansion system were to arrange for oil return, and to achieve good heat transfer in the evaporator. These problems were solved by use of an oil which is soluble in ammonia.The main obstacle for introducing this technology commercially is the limited supply of components. Particularly, there are no hermetic or semi-hermetic compressors for ammonia available in this size range.     

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.