Analysis of room temperature magnetic regenerative refrigeration

Results of a room temperature magnetic refrigeration test bed and an analysis using a computational model are presented. A detailed demonstration of the four sequential processes in the transient magnetocaloric regeneration process of a magnetic material is presented. The temperature profile during the transient approach to steady state operation was measured in detail. A 5 °C evolution of the difference of temperature between the hot end and the cold end of the magnetocaloric bed due to regeneration is reported. A model is developed for the heat transfer and fluid mechanics of the four sequential processes in each cycle of thermal wave propagation in the regenerative bed combined with the magnetocaloric effect. The basic equations that can be used in simulation of magnetic refrigeration systems are derived and the design parameters are discussed.     

A design method for mixed gas Joule–Thomson refrigeration cryosurgical probes

A cryosurgical probe is a medical instrument that is designed to kill cancerous tissue by exposure to cryogenic temperatures. The most important figure of merit for a cryosurgical probe of fixed size is the dimension of the cryolesion that can be generated within a specified time during a surgical procedure. The optimization of the cryoprobe, through variation of the geometry, operating conditions, or working fluid composition, should be aimed directly at this clinical goal rather than at maximizing the refrigeration capacity, COP, or minimizing the tip temperature. This unique optimization criterion has created the need for a design method that considers both the probe design and the heat transfer details external to the probe (e.g. the blood perfusion and metabolic heat rate in the patient). This paper outlines a design method that will allow manufacturers of cryosurgical probes to identify an optimum combination of refrigerant mixture, cryoprobe geometry, and other cycle operating conditions. Specifically, a numerical model of the development of a cryolesion has been developed and verified; this model relates the size of the cryolesion, refrigeration required, and tip temperature. A detailed model of the cryoprobe refrigeration cycle relates the refrigeration to the tip temperature that can be achieved. The integration of these models allows the designer to select the optimal cryoprobe based on cryolesion size. This paper focuses on optimizing the mixed gas composition; however, the design technique is generic and can be used to select other system parameters.     

Refrigeration throughout the world

The economic impact of refrigeration technology throughout the world is already very impressive, and more significant than is generally believed. While the yearly investment in machinery and equipment may approach US$100 billion, the value of products treated by refrigeration is perhaps ten times this amount. The importance of refrigeration is bound to increase since it will be an essential factor in solving two major problems of the future: the supply of enough food and the development of new energy sources. The use of low temperatures constitutes a major means of conservation of perishable foods during storage and distribution and it is widely applied in the developed countries. In the Third World, however, the use of low temperatures is mainly limited to food for export and the needs of the ‘well-to-do’ sector of the population. Refrigerated or frozen foods, in their present form, are generally priced too high for the undernourished poor. There is a pressing need for simple and inexpensive cooling methods to enable efficient low cost mass production and distribution under more primitive conditions. The future of man, and his food supply in particular, depends on the availability of sufficient energy. At present, fossil fuels carry the main load, but in the course of the next century new energy sources, such as nuclear fusion, will have to be commissioned. Refrigeration technology will play an important part in this development.     

Control optimisation of CO2 cycles for medium temperature retail food refrigeration systems

This paper describes a detailed procedure into the investigation of optimised control strategies for CO₂ cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO₂ system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.     

A review of numerical models of airflow in refrigerated food applications

Temperature homogeneity in most food refrigeration systems is directly governed by the airflow patterns in the system. Numerical modelling of airflow provides an opportunity to develop improved understanding of the underlying phenomena influencing system performance, which can lead to reduced temperature heterogeneity and increased effectiveness and efficiency of refrigeration systems. With the rapid advances in computational power of recent years, the use of Computational Fluid Dynamics (CFD) techniques in this application has become popular. This paper reviews the application of CFD and other numerical modelling techniques to the prediction of airflow in refrigerated food applications including cool stores, transport equipments and retail display cabinets.     

Experimental verification of a mathematical model for simulation of industrial refrigeration plants

A mathematical model, proposed for simulation of food refrigeration processes, was tested against experimental data collected in a New Zealand meat processing plant which had a total refrigeration capacity of 2.5 MW. Predicted air temperatures in freezing and chilling operations followed the same trends as measured data. Differences were shown to be more attributable to uncertainties in data than to deficiencies in the formulation of the mathematical model. The model is considered suitable for use in simulation of a wide range of food refrigeration processes. Such simulations provide useful information about plant performance that cannot be obtained by steady state analysis. Predictions were worst for short times immediately following step changes in plant operation; should more accurate simulation of these periods be required a different form of mathematical model is needed.     

Unsteady state analysis of the compression cycle of a hermetic reciprocating compressor

In this work, an unsteady state analysis of the compression cycle of a small hermetic reciprocating compressor for domestic refrigeration was carried out. A specific one-dimensional model of the valves was developed and the mass and energy balances were applied to the refrigerant inside the cylinder to determine the mass, pressure and temperature behaviour and the heat and work transfer through the compression process. This analysis was inserted into a traditional steady state model of the compressor to evaluate the efficiency of the compression cycle and the performance of the compressor unit. The whole simulation code was validated against the experimental measurements carried out on a R134a commercial unit in a wide range of operative conditions: a fair agreement was found between predicted and measured performances. The simulation code can be a useful tool for the analysis, the design and the development of small hermetic reciprocating compressors for domestic refrigeration.     

Evaluation of standing-wave thermoacoustic cycles for cooling applications

The most promising applications for standing-wave thermoacoustic cooling were investigated from the perspective of the ratio of coefficient of performance (COP) to the reversible COP or COPR. A design optimization program based on the thermoacoustic simulation program known as DELTAE was developed. The program was applied to two standing-wave thermoacoustic cooler configurations in order to determine the best possible COPRs for various temperature spans between hot-side and cold-side stack-end temperatures. It was found that the COPR of standing-wave thermoacoustic coolers increases with temperature span and reaches a maximum for temperature lifts around 80 °C. Analysis of the results and the losses clearly shows that the efficiency of these systems may be good for refrigeration, but not for air-conditioning and cryogenic cooling. The COPRs determined from measurements for various thermoacoustic coolers developed so far show similar trends, and generally support the optimization results.     

Mixing and separation characteristics of isobutane with refrigeration oil

This paper discusses the transient mixing and separation characteristics of isobutane with/from refrigeration oil. The mixing/separation processes are observed and investigated experimentally in a glass cylindrical vessel. Since liquid isobutane is less dense than refrigeration oil, the mixing process proceeds one dimensionally by diffusion from the interface between isobutane gas and refrigeration oil. The progress of mixing, therefore, is very slow compared with a combination of halocarbon refrigerant and refrigeration oil having convection flow during the mixing process. The diffusion process can be analyzed using a one-dimensional diffusion model with an appropriate diffusion coefficient, which increases linearly with temperature. The separation of isobutane from the oil–refrigerant mixture occurs at the interface and the denser oil from which isobutane is separated causes a convective flow. Bubble generation under the depressurized conditions is unstable, but in the most cases, it tends to start when a high super saturation degree is reached. The temperature change during the separation process is estimated using latent heat as the separation heat of refrigerant.     

Assessment of condensation heat transfer correlations in the modelling of fin and tube heat exchangers

This is the second paper of a series that assesses the performance of a refrigeration system model by means of cycle parameters. In this case, the condensation temperature is the parameter to study and it is focused on fin and tube condensers. It also studies the influence of the heat transfer models on the estimation of this refrigeration cycle parameter and different correlations for the heat transfer coefficients have been implemented in order to characterise the heat transfer in the heat exchangers. The flow inside the heat exchangers is considered one-dimensional as in previous works. In the cycle definition, other submodels for all the cycle component have been taken into account to complete the system of equations that characterises the behaviour of the refrigeration cycle. This global system is solved by means of a Newton–Raphson algorithm and a known technique called SEWTLE is used to model the heat exchangers. Some experimental results are employed to compare the condensation temperatures provided by the numerical procedure and to evaluate the performance of each heat transfer coefficient. These experimental results correspond to an air-to-water heat pump and are obtained by using R-22 and R-290 as refrigerants.     

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.     

Simple mathematical model for predicting the transient behaviour of an ice-bank system

The time-variable performance of a Refrigerant 22 ice-bank system was simulated by a dynamic model which was derived by assuming that heat transfer was always the limiting process, and which thus ignored hydrodynamic processes. The model comprised four ordinary differential equations describing the position of the ice front, the water temperature, and the refrigerant evaporation and condensation temperatures, each of which was derived by energy balance, plus a number of algebraic equations. Measured plant performance was accurately predicted except immediately after start-up, and in circumstances in which the assumption that the dynamics of refrigerant flow did not exert any controlling influence on the overall process dynamics was inadequate (for example, when the thermostatic expansion valve operation becomes unstable). The model requires only data that should be readily available or can be easily estimated, and thus it is suitable for analyses in the design of ice bank systems to handle time-varying conditions. Simple dynamic models ignoring hydrodynamics can be adequate in circumstances where the main source of variation arises beyond the refrigeration circuit itself.     

A generalized moving-boundary model for transient simulation of dry-expansion evaporators under larger disturbances

A generalized model based on the moving-boundary approach is developed to describe the transient behavior of dry-expansion evaporators in the vapor-compression refrigeration system. To improve the robustness of the traditional moving-boundary model under larger disturbances, the time-variant mean void fraction is employed instead of the constant. Numerical integration is applied to get the mean properties in the two-phase region and the superheated region. The interface wall temperature between the two-phase and the superheated regions is also evaluated by a new weighted mean. Qualitative case study shows that the present model can well predict the transient behaviors of evaporators under larger disturbances and keep the robustness whenever superheated region appears or disappears.     

Evaluation of the mean ice ratio as a function of temperature in a heterogeneous food: Application to the determination of the target temperature at the end of freezing

The freezing process is widely used in the food industry. In the 70s, French regulation authorities have created in collaboration with the food industry the concept of «surgélation» process with the objective of improving the image of high quality frozen foods. The process of “surgélation” which could be translated as “super freezing” corresponds to a freezing process for which a final temperature of −18 °C must be reached “as fast as possible”. This concept was proposed in opposition to a conventionally “freezing” process for which no specific freezing rate is expected and the final storage temperature can be of −12 °C only. The objective of this work is to propose a methodology to evaluate the mean amount of frozen ice in a complex food as a function of temperature and to deduce a target temperature that must be considered as the temperature for which the food may be considered as “frozen”. Based on the definition proposed by the IIF-IIR red book, this target temperature has been defined as the temperature for which 80% of the freezable water is frozen. A case study is proposed with a model food made of two constituents.     

CFD evaluating the influence of airflow on the thermocouple-measured temperature data during air-blasting chilling

A numerical investigation using a computational fluid dynamics (CFD) code is carried out to examine the influence of airflow on the thermocouple-measured temperature during air-blast chilling. A CFD model is developed to predict the temperature distribution and weight loss of the foods with and without the presence of thermocouples; the simulated temperature data are compared with experimental results. The comparison shows significant differences in the temperature results, especially near the surface of the foods being chilled. The study demonstrates that the temperature error depends not only on the length of the thermocouple remaining outside the product, but also the depth of the thermocouple inserted into the product. This study reveals a hidden source of experimental error and may help process engineers to better understand the phenomenon, and to correctly evaluate and examine their experimental and/or simulated temperature results for the air-blast chilling process.     

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.     

Mass recovery adsorption refrigeration cycle—improving cooling capacity

The study investigates the performance of two-bed, silica gel-water adsorption refrigeration cycle with mass recovery process. The cycle with mass recovery can be driven by the relatively low temperature heat source. In an adsorption refrigeration cycle, the pressures in adsorber and desorber are different. The chiller with mass recovery process utilizes the pressure difference to enhance the refrigerant mass circulation. Cooling capacity and coefficient of performance (COP) were calculated by cycle simulation computer program to analyze the influences of operating conditions. The mass recovery cycle was compared with conventional cycle such as the single stage adsorption cycle in terms of cooling capacity and COP. The results show that the cooling capacity of mass recovery cycle is superior to that of conventional cycle and the mass recovery process is more effective for low regenerating temperature.     

Characteristics of Voorhees refrigerating machine with hermetic piston compressor producing refrigeration at one or two temperature levels

Research on the operation of the refrigerating machine working on the Voorhees cycle which permits two-stage compression in a single-cylinder compressor has been carried out. The purpose of this research was to study the possibilities of using the Voorhees machine in a domestic refrigerator for production of refrigeration at one or two temperature levels. The experiments were carried out on the basis of a small hermetic lubricated compressor with a low refrigerating capacity operating on a commonly used R12 and natural refrigerant isobutane R600a. The improved refrigerating capacity in the Voorhees cycle with isobutane makes the latter an alternative substitute for conventional refrigerants. Some peculiarities in the operation of a hermetic piston compressor as part of the Voorhees refrigerating machine have been revealed. They require the use of a compressor developed specially for the Voorhees cycle. The method of optimizing the cycle parameters for a one temperature refrigerating system is suggested in this paper. The research carried out proved that the optimum intermediate pressures of the Voorhees refrigerating machine producing refrigeration either at one or two temperature levels are different.     

Modelling of food transportation systems – a review

In 2002, over a million refrigerated road vehicles, 400,000 refrigerated containers and many thousands of other forms of refrigerated transport systems are used to distribute chilled and frozen foods throughout the world. All these transportation systems are expected to maintain the temperature of the food within close limits to ensure its optimum safety and high quality shelf life.Increasingly, modelling is being used to aid the design and optimisation of food refrigeration systems. Much of this effort has concentrated on the modelling of refrigeration processes that change the temperature of the food such as chilling, freezing and thawing. The purpose of a refrigerated transport system is to maintain the temperature of the food and appears to have attracted less attention from modellers. This paper reviews the work that has been carried out specifically on the modelling of food temperature, microbial growth and other parameters in the transportation of food.     

Two- and three-dimensional CFD applied to vertical display cabinets simulation

A commercial CFD code has been employed to simulate the air flow pattern and the temperature distribution in a frozen food vertical display cabinet. At first the choice of solver parameters has been investigated in a 2D modelisation. 3D simulations have been then performed, and the effects of the cabinet length, of the warm air curtain and of longitudinal ambient air movement have been investigated. The results show that, in short cabinets, 3D secondary vortices at the side walls provide the most important mechanism for hot air entrainment. Comparison with experimental results shows that a 2D simulation is totally inadequate for such configurations, while 3D computations predict refrigeration power within engineering accuracy. Furthermore, the computed refrigerating power shows that even low room air velocity, due to its interaction with the end-wall vortices, has a significant impact on cabinet performance.