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.     

Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study

A capillary tube is widely used as an expansion device for small refrigeration cycles. In a practical refrigeration cycle, some amount of refrigeration oil is discharged from a compressor and refrigerant/oil mixture flows through the capillary tube. This study investigated experimentally the influence of mixing of the refrigeration oil with the refrigerant on the flow through the capillary tube. The experiments are carried out with not only a miscible combination of refrigerant and oil but also an immiscible combination. In both cases, the mass flow rate through the capillary tube and temperature and pressure distributions along the tube are measured under several conditions of subcooled degree and oil concentration. In the case of miscible combination, the mass flow rate of refrigerant decreases with increasing the oil concentration because the viscosity of liquid phase increases by the mixing of viscous oil. Even in the case of the immiscible combination, the oil droplet is so small that it mixes homogeneously in the liquid phase in the capillary tube and the refrigerant mass flow rate decreases by the mixing of immiscible oil. There is no significant influence of the oil concentration on the underpressure, which means pressure difference between saturation pressure and flash inception pressure, in both miscible and immiscible combinations.     

Experimental investigation on the performance of the refrigeration cycle using a two-phase ejector as an expansion device

In the present study, new experimental data on the performance of a never before seen two-phase ejector refrigeration cycle (TPERC) is presented. In this cycle, a two-phase ejector is used as an expansion device. The TPERC enables the evaporator to operate as in a liquid-recirculation system. The results are compared with those of the conventional refrigeration cycle (CRC). The effects of external parameters, i.e., heat sink and heat source temperatures on the system performance are discussed. The results show that the coefficient of performance of the TPERC is higher than that of the CRC over the whole range of experimental conditions. This is due to a higher refrigerant-side heat transfer coefficient in the evaporator, resulting from the higher refrigerant mass flow rate passing through the evaporator. However, the increase becomes relatively smaller as the heat sink temperature increases.     

Effects of flash gas generation at the expansion device inlet on the dynamic characteristics of a refrigeration system

The flash gas generation at the expansion device inlet in multi-air-conditioners causes rapid reduction of refrigerant mass flow rate and irregular distribution of refrigerant into multi-indoor units. The objective of this study is to evaluate the influence of the flash gas generation on the dynamic characteristics of a refrigeration system. The dynamic characteristics of a refrigeration system with an electronic expansion valve (EEV) were measured with time at various levels of flash gas generation, which were expressed as flash gas ratio. In addition, the averaged operating parameters and system COP were investigated with the variation of flash gas ratio. As the EEV inlet condition changed from the subcooled to the two-phase state, the refrigerant flow rate decreased rapidly due to the flash gas generation at the EEV inlet. At two-phase inlet conditions, the system operating parameters, such as mass flow rate, suction and discharge pressures, fluctuated periodically with time. As the flash gas ratio increased, the average COP decreased and the discharge temperature increased, degrading the system performance and reliability more severely.     

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.     

A theoretical study on a novel combined power and ejector refrigeration cycle

A new combined power and refrigeration cycle is proposed for the cogeneration, which combines the Rankine cycle and the ejector refrigeration cycle by adding an extraction turbine between heat recovery vapor generator (HRVG) and ejector. This combined cycle could produce both power output and refrigeration output simultaneously, and could be driven by the flue gas from gas turbine or engine, solar energy, geothermal energy and industrial waste heats. Parametric analysis and exergy analysis are conducted to examine the effects of thermodynamic parameters on the performance and exergy destruction in each component for the combined cycle. The results show that the condenser temperature, the evaporator temperature, the turbine inlet pressure, the turbine extraction pressure and extraction ratio have significant effects on the turbine power output, refrigeration output, exergy efficiency and exergy destruction in each component in the combined cycle. It is also shown that the biggest exergy destruction occurs in the heat recovery vapor generator, followed by the ejector and turbine.     

Effects of accumulator heat exchangers on the performance of a refrigeration system

An accumulator heat exchanger (AHX) consists of an accumulator and an inner heat exchanger (IHX) contained in a shell. The AHX has been used in multi-air-conditioners to obtain system reliability and high performance by providing liquid refrigerant into expansion devices and preventing wet-compression. Energy is exchanged between the evaporator exit and the condenser exit in the AHX. In this study, the heat transfer characteristics of the AHX were investigated experimentally, and the effects of the AHX on the performance of a refrigeration system using R22 were measured. The operating characteristics of the refrigeration system with the AHX were considerably different from those without the AHX. The AHX system showed higher refrigerant flow rate than the non-AHX system at a constant EEV (electronic expansion valve) opening because of higher subcooling, resulting in better performance and reliability of the refrigeration system. At 50% EEV opening, the cooling capacity and COP of the AHX system were higher than those of the non-AHX system by 7.5% and 3.2%, respectively.     

Experimental investigation of mass recovery adsorption refrigeration cycle

The study investigates the performance of silica gel–water adsorption refrigeration cycle with mass recovery process by experimental prototype machine. In an adsorption refrigeration cycle, the pressures in adsorber and desorber are different. The mass recovery cycle utilizes the pressure difference to enhance the refrigerant mass circulation. Moreover, novel cycle was proposed for improvement of cooling output. In our previous study, simulation analysis shows that mass recovery cycle has the advantage over conventional single-stage. Experiments with prototype machine were conducted to investigate the performance improvement of mass recovery cycle in the present paper. Specific cooling power (SCP) and coefficient of performance (COP) were calculated with experimental data to analyze the influences of operating conditions. The proposed cycle was compared with the single-stage cycle in terms of SCP and COP. The results show that SCP of mass recovery cycle is superior to that of conventional cycle and mass recovery cycle is effective with low temperature heat source.     

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.     

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.     

A theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.     

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.     

Simulation of a solar driven aqua-ammonia absorption refrigeration system Part 1: mathematical description and system optimization

A theoretical thermodynamic analysis was performed on an aqua-ammonia refrigeration (AAR) cycle coupled to a solar water heating system using CPC-collectors and augmented with auxialiary energy. Thirteen AAR cycles were considered in the search for the maximum coefficient of performance. Also investigated was the exergetic performance of the AAR cycle. The adopted temperature and mass flow rate control strategy on the storage tank-auxiliary heater-AAR generator loop ensured operation of the refrigeration systems at design conditions. Consequently, the ratio between the pumps' electrical power and the designed evaporator heat transfer rate was kept to a minimum.     

Calculation of basic parameters for gravity-fed evaporators for refrigeration and heat pump systems

A method for calculating the basic parameters for gravity-fed evaporators has been developed based on the calculation of the pressure drop of two-phase flow of refrigerant over pipes and pipe components. Gravity-fed evaporators have a unique self-regulation ability and are among the most efficient and reliable refrigeration and heat pump systems, provided that they are correctly designed.     

Real-time measurement of mixing ratio of refrigerant/refrigeration oil mixture

The mixing of refrigeration oil with refrigerant in a refrigeration cycle has great influence on cycle performance. A sampling method is the most general way to measure the mixing ratio of refrigerant and refrigeration oil. Since the sampling method is time-consuming and reduces the amount of refrigerant and oil in the cycle, a real-time measurement is desirable. In this study, a refractive index measurement was applied to measure the mixing ratio of refrigerant/oil mixture. A laser displacement sensor was used to detect any change in optical path which results from changes of the refractive index of refrigerant/oil mixture. For the practical application of real-time measurement of the oil circulation ratio (OCR) in the refrigeration cycle, a correlation between the refractive index and the mixing ratio was developed. In addition, the changes of the refractive index in a range of a few percentages of the oil concentration and under subcooled conditions were measured. Finally, a transient measurement of the OCR in a practically operating refrigeration cycle was carried out successfully.     

Simulation model for complex refrigeration systems based on two-phase fluid network – Part I: Model development

Some complex refrigeration and heat pump systems with several condensers and evaporators have been developed for different kinds of application. Traditional simulation models were developed for systems in certain operating modes and they failed in modeling the complex refrigeration systems with uncertainties of heat exchangers function and refrigerant flowing direction. In order to predict the performance of complex refrigeration systems, a simulation model is presented based on the two-phase fluid network. The model is consisted of distributed-parameter model of heat exchangers and connecting tubes, map-based model of inverter compressor and electronic expansion valve (EEV). Based on the characteristic of refrigeration system and fluid network, the three conservation equations, i.e. energy, momentum and mass equations, are solved iteratively. This model can deal with the uncertainty of refrigerant flow direction by separating the solving process of the components and the fluid network model, and therefore can simulate different kinds of complex refrigeration systems in different operating modes and conditions. The model is validated by the experimental data of an inverter air conditioner in heating/cooling operating modes and it shows the error of the model is mainly determined by the error of submodels of components in calculating heat transfer and pressure loss. The model is applied for performance analysis of three kinds of complex refrigeration systems in the accompanying article [Shi W.X., Shao, S.Q., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network: part II – model applications, International Journal of Refrigeration 31 (3), 500–509.].     

Experimental investigation of a vapor absorption refrigeration system

An experimental investigation of the performance of a commercially available vapor absorption refrigeration (VAR) system is described. The natural gas-fired VAR system uses aqua-ammonia solution with ammonia as the refrigerant and water as the absorbent and has a rated cooling capacity of 10 kW. The unit was extensively modified to allow fluid pressures and temperatures to be measured at strategic points in the system. The mass flow rates of refrigerant, weak solution, and strong solution were also measured. The system as supplied incorporates air-cooled condenser and absorber units. Water-cooled absorber and condenser units were fitted to extend the VAR unit's range of operating conditions by varying the cooling water inlet temperature and/or flow rates to these units. The response of the refrigeration system to variations in chilled water inlet temperature, chilled water level in the evaporator drum, chilled water flow rate, and variable heat input are presented.     

Performance of compression/absorption hybrid refrigeration cycle with propane/mineral oil combination

This paper discusses the feasibility of a vapor compression/absorption hybrid refrigeration cycle for energy saving and utilization of waste heat. The cycle employs propane as a natural refrigerant and a refrigeration oil as an absorbent. A prototype of the cycle is constructed, in which a compressor and an absorption unit are combined in series. The performance of the cycle is examined both theoretically and experimentally. Although the solubility of the propane with the oil is not enough as a working pair in the absorption unit, the theoretical calculation shows that the hybrid cycle has a potential to achieve a higher performance in comparison with a simple vapor compression cycle by using the waste heat. In the experiment, the prototype cycle is operated successfully and it is found that an improvement of an absorber is necessary to achieve the good performance close to the theoretical one. The application of an AHE (absorber heat exchanger) can reduce the heat input to a generator. Further examinations on some other combinations of refrigerant/refrigeration oil and additives are desirable.     

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

A new way to assess the performance of refrigeration system models is presented in this paper, based on the estimation of cycle parameters, such as the evaporation temperature which will determine the validity of the method. This paper is the first of a series which will also study the influence of the heat transfer coefficient models on the estimation of the refrigeration cycle parameters. It focuses on fin and tube evaporators and includes the dehumidification process of humid air. The flow through the heat exchanger is considered to be steady and the refrigerant flow inside the tubes is considered one-dimensional. The evaporator model is discretised in cells where 1D mass, momentum and energy conservation equations are solved by using an iterative procedure called SEWTLE. This procedure is based on decoupling the calculation of the fluid flows from each other assuming that the tube temperature field is known at each fluid iteration. Special attention is paid to the correlations utilised for the evaluation of heat transfer coefficients as well as the friction factor on the air and on the refrigerant side. A comparison between calculated values and measured results is made on the basis of the evaporation temperature. The experimental results used in this work correspond to an air-to-water heat pump and have been obtained by using R-22 and R-290 as refrigerants.     

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.