Reciprocating compressors for refrigeration and heat pump application

Reciprocating compressors have been used in refrigeration for about 150 years. At first they operated at a maximum of 100 r.p.m. and so were very large. Much of the development has been directed to increasing speed and hence reducing size and cost. Recent improvements have been directed mostly to valve design because valve performance largely controls the indicator diagrams and hence the isentropic efficiency. Present studies are directed to improving the mechanical efficiency. Despite all this progress, further improvement is still possible.     

Flow visualization applied to a small refrigeration compressor

This paper describes a flow visualization technique that was used to evaluate qualitatively the gas flow pattern inside a small, hermetically sealed, reciprocating refrigeration compressor. The applicable compressor designs are those in which the suction gas from the evaporator is dumped into the compressor shell, and is then drawn through a muffler into the suction plenum of the compressor. The physical separation of the muffler inlet from the suction gas inlet serves to reduce compressor noise and also provides an easy and convenient means of separating any liquid (compressor oil or liquid refrigerant) from the refrigerant gas. For the flow visualization studies the compressor housing was replaced by a clear plastic shell. Atmospheric air seeded with white smoke was the working fluid. The suction inlet and muffler were parts from a commercial compressor. The flow pulsations were modelled by connecting the muffler outlet to the input plenum of an auxiliary compressor. The flow patterns near the muffler inlet were recorded with a video camera. The mixing of the inlet gas with the gas circulating inside the muffler was studied. The effect of alignment and offset of the muffler inlet relative to the suction inlet, the effect of muffler size, and the effect of a shroud around the muffler were studied. The results were used to guide a companion study of detailed temperature and pressure measurements inside a working compressor.     

Enhanced ejector refrigeration cycles powered by low grade heat. Part 2. Design procedures

In Part 1 of this work, the conventional and compression enhanced ejector refrigeration cycles were discussed from a thermodynamic and conceptual point of view. This paper describes the development of procedures which will enable system design, optimization and control of operation. Special attention is given to the ejector design and the recommended modification of conventional system components such as the evaporator and generator. A multi-ejector system is introduced in order to expand the range in which the system may operate efficiently.     

Neutron radiography study of pulsed boiling in a water-filled heat pipe

The boiling and condensation processes in a water-filled industrial heat pipe have been investigated by dynamic neutron radiography. Pulsed boiling was visualized and analysed up to a characteristic temperature depending on the filling quantity. Three typical regions were distinguished in the heat pipe: that constantly filled by the liquid; a periodically wetted zone; and a region constantly filled by the vapour. Pulsation was found to be not strictly periodic and its mean amplitude and frequency were determined. It is concluded that any models based on the equilibrium state are incorrect.     

Predicting heat pump performance by thermodynamic generalizations of fluid flow and heat transfer data

Thermodynamic generalizations based on reduced pressure proposed in the 1960s are reviewed and updated to reflect the current state of the art. The application of the method is illustrated by analytical and numerical examples and an assessment made of its value in heat exchanger design practice with special emphasis on two-phase forced convection refrigeration cycle applications. It is shown that this thermodynamic approach provides the heat exchanger designer, and to some extent the system engineer with an additional tool which is simple, effective and above all more reliable, particularly in evaporator and condenser design practice, than current conventional semi-empirical correlations.     

Measuring the convective heat transfer coefficient while chilling carcasses

The importance of the heat transfer coefficient when chilling carcasses justifies its re-examination and work to its magnitude and variation. While the air velocity at the surface of carcass (with its rather irregular configuration) is an elusive quantity, the author demonstrates that the measurement of the transfer coefficient and its variation should be based on the rate of air flow over unit mass of carcass and on the rate of weight loss through evaporation.     

General method for the evaluation of complex heat pump cycles

Whenever the fractional temperature lift ΔT/T_c of a heat pump is 0.15, simple cycles with one-stage throttling exhibit unsatisfactory energy performance. The adoption of multi-stage throttling, both in non-regenerative and regenerative cycles, is the most direct way of improving the cycle coefficient of performance (COP). The performance of these complex cycles is found to be a function of the molecular complexity of the working fluid, the reduced evaporation temperature, the fractional temperature lift and the number of stages of throttling. Furthermore, complex cycles are shown to be equivalent to a combination of simple cycles and their performance may be directly inferred by this route. Such calculations show that for a given fractional temperature lift an optimum molecular complexity (between that of R12 and n-butane) exists. Fluids with simpler molecules exhibit excessive vapour superheating during compression, and those with more complex molecules have excessive throttling losses. Also, with complex cycles, regeneration should be applied only to the cycle at the lowest temperature in order to improve the cycle COP and to prevent condensation during compression. As a general trend, however, complex cycles suffer a significant loss in performance compared to optimized simple cycles due to the adverse area of the two-phase diagram in which they work.     

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.     

Enhanced ejector refrigeration cycles powered by low grade heat. Part 1. Systems characterization

Conventional and improved ejector refrigeration cycles are discussed. Special emphasis is given to the adaptation of these cycles for the utilization of low grade or waste heat. A compression enhanced ejector system is suggested as a mechanically efficient way to improve the ejector cycle. It is demonstrated that a combination of mechanical and thermal energies may provide a wide range of design alternatives which should yield a competitive refrigeration system. The paper provides an over-all view of the systems by discussing their principle of operation, expected performance and design considerations.     

Thermo-economic analysis of refrigeration and heat pump systems

This Paper is based on a lecture presented the Austrian Society of Refrigeration Engineers. The optimization of refrigeration and heat pump systems discussed and a thermo-economic analysis is outlined using the Second Law of Thermodynamics.     

Refrigerated vehicles — what next?

The successful operation of a thin wall refrigerated vehicle is dependent upon a number of operational procedures some of which are difficult to implement. Although vehicles of this type have been in use for several years, it is only recently that full-scale laboratory and transit temperature measurements have been made in their cargo. The results of such research are discussed and the important parameters of equipment design and operation are reviewed. The effects of ageing of insulation, air circulation, cargo temperature and refrigeration equipment are all considered.     

Development of advanced heat transformers utilizing new working fluids

Based on experience from an industrial-scale heat transformer, which is now commercially available, GEA has embarked on a new research and development programme: the development of an advanced heat transformer and investigation into the combination heat transformer/absorption heat pump. Advanced heat transformers are heat transformers utilizing new working fluids and/or a multi-stage absorption plant. A brief report is presented on an experimental research programme at the University of Essen which aims to discover new working fluids for sorption plants. Some of the most important results for heat transformer application are presented in the form of graphs or tables and the suitability of the new working fluids is compared with some well-known working couples. The Industrial requirements for an advanced heat transformer are also considered along with the possibility of meeting these requirements. Finally, the heat transformer is discussed in the context of other heat recovery systems on the basis of two fictitious industrial applications.     

A general dynamic simulation model for evaporators and condensers in refrigeration. Part II: simulation and control of an evaporator

A mathematical model of an evaporator based on one-dimensional partial differential equations representing mass conservation, and tube wall energy has been formulated. These equations are then restructured and linked to a program data base of all major refrigerants and refrigerant mixtures. The result is a simulation model of an evaporator that is general and flexible. The model is tested over a wide range of operating conditions and a simple controller is implemented to demonstrate the effectiveness of the model for controller and systems design.

Résumé

On a établi un modèle mathématique d'un évaporateur basé sur des équations aux dérivés partielles unidimensionnelles qui représentent la conservation de masse et l'énergie de la paroi du tube. Ces équations ont été restructurées ensuite, puis reliées à une base de données sur les principaux frigorigènes purs et en mélanges. De cette manière, on obtient un modèle d'évaporateur d'application générale et souple. Ce modèle a été éprouvé dans des conditions de fonctionnement très variées et on a employé un système de régulation simple pour montrer l'efficacité du modèle pour la conception et la régulation des systèmes.     

Radiation and natural convection heat transfer from wire-and-tube heat exchangers in refrigeration appliances

The air-side heat transfer from wire-and-tube heat exchangers of the kind widely used in small refrigeration appliances has been studied. Radiation and free-convection components have been separately investigated. The radiation component was theoretically computed using a diffuse, gray-body network with interactions between each part of the heat exchanger and the surroundings. For the free-convection heat transfer component, a semiempirical correlation was developed on the basis of experimental tests conducted on a set of 42 low-emittance exchangers with various geometrical characteristics. Comparisons between overall heat transfer predictions and a second, independent set of experiments on eight high-emittance exchangers showed satisfactory agreement. The proposed analysis is suitable either to determine the heat transfer performance of an existing (already sized) exchanger or to design a new one for prescribed heat duty and working temperatures.     

Thermodynamics of magnetic refrigeration

A comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes is presented. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, and a list of the thermodynamic potentials is given. These are based on the very recent discovery of an alternative Kelvin force. It is shown that this force is compatible with the internal energy proposed by Landau and Lifshitz. New formulas for the specific enthalpies are presented. Cyclic processes are discussed in detail, e.g. the Brayton, Ericsson and Carnot cycles. Magnetic refrigeration and magnetic heat pump cycles are preferably designed by applying the cascade or/and regeneration principle. Cascade systems allow wider temperature ranges to be obtained. The main objective of this article is to yield a theoretical basis for an optimal design of new magnetic refrigeration and heat pump devices.     

Electronic energy saving in refrigeration equipment

Electronic technology is now available for use on most types of refrigeration equipment. Loss reduced motors (LRMs), using development of technologgy originally invented for the NASA space programme, tackle a major area of inefficiency. Refrigeration engineers are well aware of the inefficiency of running the compressor drive motor at part load, and in central plant have minimized the problem by using several motors with load switching to match the actual load requirement. However, in smaller single compressor equipment no solution has yet been proposed. LRMs can provide the solution and, moreover, are etremely cost-effective with installation paybacks well within two years.     

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.     

Thermophysical properties of the trifluoroethanol-pyrrolidone system for absorption heat transformers

Thermophysical properties of the 2,2,2-trifluoroethanol-2-pyrrolidone system have been investigated. The scope of the study includes density, viscosity, thermal conductivity, the equilibrium vapour pressure-concentration-temperature relationship, specific heat capacity, heat of mixing and enthalpy. Experimental measurements were carried out. Mathematical correlations for various properties were mainly derived from measured data. The good combination of physical and thermal properties of trifluoroethanol and pyrrolidone shows that they can be used as a working pair for absorption heat transformers.     

Pressure, flow and temperature transients in refrigeration systems

The pressure, flow and temperature transients which occur in simple refrigeration systems (incorporating both dry expansion and flooded evaporators), when subjected to disturbances such as control and load inputs and when defrosting, are described. The effects of oil in such systems are also considered. It is concluded that such transients have a significant influence on system reliability and that system design still contains a significant element of art as well as technology.     

Study of different internal vapour transports for adsorption cycles with heat regeneration

A ‘three-temperature’ model of the adsorption cycles with heat regeneration is used for investigating and analysing the influence of different parameters on the performance of such cycles. The influence of the heat source temperature on the thermodynamic efficiency (COP/COP_Carnot) is investigated. The result is that the thermodynamic efficiency of the cycle is always limited. In order to reduce the internal irreversibilities, different internal vapour transports for pressurising (depressurisation) the adsorber are investigated: first, adiabatic direct pressurisation (depressurisation) with the condenser (evaporator) instead of pressure changes by heat transfer; second, adiabatic internal vapour recovery between the adsorbers (partial pressurisation/depressurisation); third, separation of the adsorber into separate compartments between which vapour cannot be redistributed during pressurisation or depressurisation. Results show that the first process significantly reduces the COP, while the second one enhances the cooling power, and the third one does not change the performance. Analysis gives satisfactory explanation of these results.