Foaming of refrigerating oil in a rolling piston type rotary compressor

This paper is concerned with foaming of refrigerating oil in a rolling piston type rotary compressor for an air conditioner. Factors causing foaming during the starting operation were investigated experimentally. It was proved that foaming was caused by the stirring of the refrigerant-oil mixture with a motor rotor, a vane and a discharge refrigerant flow. The foaming was correlated with the solubility of refrigerant in oil and with the amount of refrigerant contained in the stirred region of the liquid mixture.     

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.     

Design considerations for refrigeration cycles

The Carnot COP, which assumes a thermodynamically ideal cycle in which no irreversibilities exist, is often considered to be a design goal for actual cycles. However, the Carnot COP does not consider heat transfer mechanisms. Heat transfer at a finite rate is necessarily an irreversible process and unavoidable in a refrigeration cycle. The lack of consideration of rate processes reduces the usefulness of the Carnot COP as a realistic design goal. In this paper, the limitations of both thermodynamics and heat transfer are considered to identify a more realistic design goal for the COP of refrigeration cycles. The consideration of heat transfer limitations leads to a design rule for the optimum distribution of heat exchange area between the low- and high-temperature heat exchangers.     

An aerodynamic investigation of a centrifugal compressor for HCFC123

A centrifugal compressor was investigated experimentally to improve HCFC123 compression performance. A CFC11 compressor working with HCFC123 shows lower adiabatic head rise, efficiency and volume flow. Another CFC11 compressor working with HCFC123 and a modified impeller shows a result comparable to that when CFC11 is compressed. A tandem impeller is designed to compensate for the volume flow increase for HCFC123. This impeller sufficiently absorbs the volume flow increase and also improves efficiency.     

On reducing the size of liquid separators for injector circulation plate freezers

Liquid separators for injector plate freezers are large because the liquid level rises towards the end of the freezing process. To calculate the volume of liquid being collected in the separator during freezing the freezing time and the heat removed must be evaluated. A simple method of freezing time estimation based on the progression of a phase change front is proposed. The size of separators can be reduced considerably by letting part of the liquid feed by-pass the injector during initial freezing. With this arrangement the injector dimensions are based upon a refrigerating capacity lower than the maximum.     

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.     

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.     

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.     

Comparative study of irreversibilities in an aqua-ammonia absorption refrigeration system

Irreversibilities in components of an aqua-ammonia absorption refrigeratio system (ARS) have been determined by second law analysis. The components of the ARS are as follows: condenser, evaporator, absorber, generator, pump, expansion valves, mixture heat exchanger and refrigerant heat exchanger. It is assumed that the ammonia concentration at the generator exit is, independent of the other parameters, equal to 0.999 and at the evaporator exit the gas is saturated vapour. Pressrre losses between the generator and condenser, and the evaporator and absorber are taken into consideration. In the results the dimensionless exergy loss of each component, the exergetic coefficient of performance, the coefficient of performance and the circulation ratio are given graphically for each different generator, evaporator, condenser and absorber temperature.     

An investigation of the performance of a scroll compressor under liquid refrigerant injection

In this study, fundamental and practical influence of liquid refrigerant injection on the performance of a refrigerant scroll compressor has been investigated experimentally and theoretically. In the theoretical analysis, a compression model of vapor/liquid mixture is developed by taking account of heat transfer from the cylinder wall to suction, compression and injection refrigerant. An experiment has been done under the condition of keeping the oil temperature constant in order to investigate the fundamental influence of the liquid refrigerant injection on the compressor performance, and the results were compared with the theoretical ones. It was found that the injection basically increases the compression power and decreases the compressor efficiency, though the situation depends on the condition of the heat transfer to the injection refrigerant. And furthermore, the performance of the liquid refrigerant injection compressor under practical operating condition without controlling the oil temperature has been investigated. Under this condition, the compressor showed recovery and slight improvement of performance due to the decrease of the oil and cylinder temperatures by the injection. In addition, influence of the refrigerant injection on the oil viscosity and refrigerant solubility in the oil, which relate mechanical loss and reliability of the compressor, have been discussed.     

Start-up and shut-down operation in a reciprocating compressor refrigeration system with capillary tubes

Refrigerant migration during start-up and shut-down cycles can affect energy losses. Two reciprocating compressor refrigeration systems were studied: (1) a conventional cycle with refrigerant migration through the capillary tube after compressor shut-down and (2) a system in which a magnetic cut-off valve was closed on compressor shut-down. It was found that by preventing the refrigerant from migrating during shut-down, energy losses during start-up could be reduced, resulting in a 4% decrease in motor power input at this time. A computer program has been developed to calculate energy losses due to refrigerant migration.     

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.     

Numerical study on the lubrication oil distribution in a refrigeration rotary compressor

For a rolling piston rotary compressor, oil supply into various lubrication elements has been analytically studied. The lubrication system, consisting of centrifugal shaft pump, radial oil feeding holes, bearings with grooves and some other sliding surfaces has been modeled by employing equivalent electric circuit network. A computer simulation program has been developed to solve the network model of the lubrication system. Its numerical solutions include total oil flow rate into the shaft inlet, oil flow rates in the main and sub journal bearings and in the eccentric bearing, and oil leakages through roller end clearances into suction and compression chambers. Validation of the numerical simulation has been made by the measurement of the total oil flow rate into the shaft. With the aid of computer simulation, parametric studies have also been carried to investigate the effects of the bearing groove shape and groove inclination angle on the bearing oil flow rates.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

Performance of an HCFC22-based vapour absorption refrigeration system

A single-stage vapour absorption refrigeration system (VARS) is tested with monochlorodifluoromethane (HCF22) as refrigerant and different absorbents: dimethylether of tetraethylene glycol (DMETEG) and dimethyl acetamide (DMA). The influence of generator temperatures in the range 75–95°C, which represents low-grade heat sources, is studied. Cooling water temperatures were varied between 20 and 30°C. Two cases of cooling water flow paths are considered, i.e. water entering either absorber or condenser, which are connected in series. For HCFC22-DMETEG, COP values in the range 0.2–0.36 and evaporator temperatures between 0 and 10°C are obtained. For HCF22-DMA, COP values in the range 0.3–0.45 and evaporator temperatures between −10 and 10°C are obtained. It is observed that HCFC22-DMETEG can work at lower heat source temperatures than HCFC22-DMA. However, at the same operating conditions HCFC22-DMA is better from the viewpoints of circulation ratio and COP. Experiments also show that at low heat source temperature, cooling water temperature has strong influence on circulation ratio but does not affect COP significantly. Preferably, cooling water should first flow through the condenser and then through the absorber in order to achieve improved overall performance.     

A preliminary evaluation of the Groll rotary vane compressor

The principles of operation and design of the prototype of a novel design of vane compressor with significant advantages over the conventional sliding vane type are described. Its performance is given and compared to that of equivalent commercially available sliding vane and reciprocating compressors used in refrigeration systems. Its volumetric efficiency on average was 3% greater than that of the sliding vane compressor and 23% more than that of the reciprocating machine. The adiabatic efficiency was on average 15% greater than that of the sliding vane and about equal to that of the reciprocating compressor. Owing to its high rotational speed it is more compact than the reciprocating compressor. Since the vanes are rigidly retained about their axis of rotation they cannot chatter as in a sliding vane compressor. It is therefore smooth and quiet in operation and no mechanical faults were encountered in 432 h of operation.     

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 reliable one-dimensional method applied to heat-transfer problems associated with insulated rectangular tanks in refrigeration systems

In this study, the heat-transfer characteristics of insulated cubes or rectangular tanks are analyzed by using the one-dimensional solid square-top wedge thermal resistance (SSWT) model and plate thermal resistance (PTR) model. It is found that, in cases of insulated tanks, the errors generated by the SSWT model are all positive and those generated by the PTR model are all negative. Thus, a combined PTR and SSWT (CPSWT) model, resulted by mixing SSWT and PTR models with the proportion factors of 0.6 vs. 0.4 (64-CPSWT model), can neutralize the positive and negative errors and obtain very accurate results in comparison with the three-dimensional numerical solutions. The errors returned by the 64-CPSWT model are within ±1% when applied to refrigeration systems with practical sizes and insulated thickness. Suggestions on how to obtain reliable heat-transfer results for an insulated rectangular tank in a refrigeration system are also presented.     

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.