A review of linear compressors for refrigeration

Linear compressor has no crank mechanism compared with conventional reciprocating compressor. This allows higher efficiency, oil-free operation, lower cost and smaller size when linear compressors are used for vapour compression refrigeration (VCR) system. Typically, a linear compressor consists of a linear motor (connected to a piston) and suspension springs, operated at resonant frequency. This paper presents a review of linear compressors for refrigeration system. Different designs and modelling of linear compressors for both domestic refrigeration and electronics cooling (miniature VCR system) are discussed. Key characteristics of linear compressor are also described, including motor type, compressor loss, piston sensing and control, piston drift and resonance. The challenges associated with the linear compressors are also discussed to provide a comprehensive review of the technology for research and development in future.     

Thermodynamic model for reciprocating compressors with the focus on fluid dependent efficiencies

Fluids with high global warming potential, which are used in existing refrigeration cycles and heat pumps, will have to be replaced soon by less harmful fluids, but the fluid selection is difficult especially due to the unknown compressor performance. In this work a differential compressor model for reciprocating compressors is introduced which predicts volumetric and isentropic efficiencies quickly and can be easily fitted with measured data at only one operation point of an existing compressor. In order to characterise the influence of different fluids two semi-physical correlations for the valve flows are fitted here, and a procedure of transferring them to different compressors is shown. The model is validated on, in total, 63 measured points based on numerous fluids from one semi-hermetic reciprocating compressor which is part of a heat pump cycle. The calculations lead to mean prediction errors of 3.0% for the isentropic and 2.3% for the volumetric efficiency.     

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.     

无油线性压缩机的频率特性和活塞偏移特性研究

针对线性压缩机随工况和热负荷变化而引起共振频率和活塞中心位置偏移量改变的问题,建立了关于压缩机共振频率的线性等效模型和傅里叶变换计算模型。两种模型用来预测线性压缩机的共振频率,后者同时用来计算活塞中心位置偏移量,模型中考虑了不同行程、不同排气压力对共振频率的影响以及不同行程、不同排气压力和不同运行频率对活塞中心位置偏移量的影响。为了验证模型的有效性,搭建了用来测量线性压缩机共振频率和活塞中心位置偏移量的测试系统,并将测量结果与模型所计算结果进行对比,结果表明,线性等效模型相比于傅里叶变换计算模型能更加准确的预测线性压缩机的共振频率;线性压缩机的共振频率随着行程的增加而逐渐减小,随着排气压力的增加而逐渐增加。实验表明,线性压缩机活塞中心位置偏移量随行程的增加而几乎不变,随着排气压力的增加而逐渐增加。傅里叶变换计算模型针对不同行程下活塞中心位置偏移量预测值高于实际值;但在不同排气压力或不同行程下的活塞偏移量可用该模型计算压缩机在上止点位置的偏移量来进行预测,误差不大于10%。     

A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor

Lubricating oil improves the reliability of compressors and systems, whereas increases the system complexity. Compared with other types of compressors that have oil-free models, a rolling piston compressor has more leakage paths and bigger leakage loss. Therefore, the leakage is an important problem to be solved in order to develop an oil-free rolling piston compressor. The paper put forward a new structure of rotary compressor adopting a low pressure shell, connecting the cavities within piston and behind vane to the cavity at suction pressure and using radial compliance mechanisms. Then the leakage models were developed to calculate the mass flow rates within both the present rolling piston compressor without any oil as sealant and the new structure of oil free compressor. Results showed that by the new structure, the influences of leakage on the performance of a R290 oil free rolling piston compressor can be largely decreased.     

A comprehensive model of a miniature-scale linear compressor for electronics cooling

A comprehensive model of a miniature-scale linear compressor for electronics cooling is presented. Linear compressors are appealing for refrigeration applications in electronics cooling. A small number of moving components translate to less theoretical frictional losses and the possibility that this technology could scale to smaller physical sizes better than conventional compressors. The model developed here incorporates all of the major components of the linear compressor including dynamics associated with the piston motion. The results of the compressor model were validated using experimental data from a prototype linear compressor. The prototype compressor has an overall displacement of approximately 3 cm³, an average stroke of 0.6 cm. The prototype compressor was custom built for this work and utilizes custom parts with the exception of the mechanical springs and the linear motor. The model results showed good agreement when validated against the experimental results. The piston stroke is predicted within 1.3% MAE. The volumetric and overall isentropic efficiencies are predicted within 24% and 31%, MAE respectively.     

Testing and modelling of an automotive wobble plate compressor

Wobble plate compressors are well used in air conditioning for high-class automobiles. They allow continuous control by automatic adjustment of the piston stroke, to keep the low pressure above a certain limit. Here an externally controlled wobble plate compressor is analyzed experimentally through its isentropic and volumetric effectivenesses and control characteristics. Compressor effectivenesses depend mainly on the compressor speed and pressure ratio: there is obtained, for example, isentropic and volumetric effectivenesses of 0.65 and 0.8 for a pressure ratio of 4 at 1000 rpm and 0.4 and 0.35 for the same pressure ratio at 4000 rpm. This degradation is attributed to the increasing of the supply pressure drop. The “lubricant” (oil + dissolved refrigerant) mass flow rate is obtained by minimization of the residuals of the thermal balances on the compressor, condenser and evaporator. Here an important oil-flow circulation is obtained: between 9.5% and 12.5% of the refrigerant flow rate. A special displacement sensor is used to measure instantaneous piston stroke and to relate it to overall compressor performance. This measurement is then compared with the results obtained with a semi-empirical model, which is able to predict, in part load, the compressor displacement. The model predicts the displacement ratio with deviations that vary between −14.5% and +8.1%.     

Modelling of reciprocating and scroll compressors

This paper presents simple and thermodynamically realistic models of two types of compressors widely used in domestic heat pumps (reciprocating and scroll compressors). These models calculate the mass flow rate of refrigerant and the power consumption from the knowledge of operating conditions and parameters. Some of these parameters may be found in the technical datasheets of compressors whereas others are determined in such a way that the calculated mass flow rate and electrical power match those given in these datasheets.The two models have been tested on five reciprocating compressors and five scroll compressors. This study has been limited to compressors with a maximum electrical power of 10 kW and for the following operating conditions: evaporating temperatures ranging from −20 to 15 °C and condensing temperatures ranging from 15 to 60 °C.The average discrepancies on mass flow rate and power for reciprocating compressors are 1.10 and 1.69% (for different refrigerants: R134a, R404A, R22, R12 and R407C). For scroll compressors, the average discrepancies on mass flow rate and power are 2.42 and 1.04% (for different refrigerants: R134a, R404A, R407C and R22).     

活塞式制冷压缩机技术现状及发展趋势

活塞式制冷压缩机在制冷空调行业有着广泛的应用,本文介绍现有活塞式制冷压缩机产品的应用及技术现状,对特殊型式的活塞式制冷压缩机产品性能等进行阐述,指出活塞式制冷压缩机技术发展趋势。     

Semi-empirical modeling and analysis of oil flooded R410A scroll compressors with liquid injection for use in vapor compression systems

Oil flooding is a technique that can be utilized in compression systems to achieve near-isothermal compression. This can lead to a boost in system efficiency and a reduction in compressor power consumption. In this paper a semi-empirical model for oil flooded compressors using liquid injection was developed. The model is validated with experimental data and integrated into a thermodynamic model of a vapor compression system with oil flooding and regeneration. The performance of the heat pump system is predicted and the semi-empirical model is used to identify and estimate the magnitude of the irreversibilities during the compression process. A method for generalizing the model for different working fluids is also presented. Using this model, design recommendations are made to improve the efficiency of the studied liquid flooded compressors for heat pump applications.     

Object-oriented simulation of reciprocating compressors: Numerical verification and experimental comparison

Numerical simulation of reciprocating compressors is important for the design, development, improvement and optimization of the elements constituting the compressor circuit. In this work, an object-oriented unstructured modular numerical simulation of reciprocating compressors is presented. Pressure correction approach is applied for the resolution of tubes, chambers and compression chambers, while valve dynamics are modelled assuming a spring-mass system having single degree of freedom. The modular approach offers advantages of handling complex circuitry (e.g. parallel paths, multiple compressor chambers, etc.), coupling different simulation models for each element and adaptability to different configurations without changing the program. The code has been verified with some basic tests for assuring asymptotic behaviour to guarantee error free code and physically realistic results. Cases with different compressor configurations and working fluids (R134a, R600a and R744) have also been worked out. Numerical results are compared with experimental data and illustrative cases of multi-stage compression are also presented.     

A Simulation Study on the Transient Motion of a Reciprocating Compressor Suction Valve Under Complicated Conditions

Stepless capacity control technology for reciprocating compressors is a key contributor to energy saving for the petroleum and petrochemical industries. Devices called “unloaders” are utilized to control the capacity of the compressor by forcibly holding the suction valves open during a variable portion of the compression stroke to control the compressor output. This approach can also lead to various faults of the suction valve. This paper describes the simulation and experimental studies of the transient motion of suction valves under stepless capacity control. Beginning with mathematical models for the normal cycle, improved models of a reciprocating compressor under stepless capacity control have been built. A simulation study of the working process of a double-acting reciprocating compressor has been completed. Theoretical formulas for the transient motion of the valve plate under complicated conditions and the dynamic pressure in the cylinder are compared with the experimental results. Based on the above simulations, a finite element analysis of the valve plate and valve seat has been completed. The experiment results showed that the vibration of the compressor cylinder under complicated conditions was consistent with numerical simulation results. Research presented in this paper is significant in providing tools for diagnosing faults in order to optimize the design of reciprocating compressors that utilize a stepless capacity control system.     

Performance analysis of a series of hermetic reciprocating compressors working with R290 (propane) and R407C

In this paper, a series of compressors with different capacities and geometries working with propane as refrigerant are analyzed in terms of the compressor model developed by [E. Navarro, E. Granryd, J.F. Urchueguía, J.M. Corberán, A phenomenological model for analyzing reciprocating compressors, International Journal of Refrigeration, in this issue, doi:10.1016/j.irefrig.2007.02.006]. The relative influence of the diverse compressor losses is estimated as a function of the operating conditions.In addition, a comparison study between propane and R407C was carried out for one compressor and the observed differences were analyzed in terms of the compressor model. This study was also useful to verify the model's goodness with the aim of predicting the compressor performance with an untested refrigerant.     

A simulation approach for hermetic reciprocating compressors including electrical motor modeling

The design of a high-efficiency reciprocating compressor requires an understanding of the interactions between different phenomena occurring inside the compressor. This paper describes a comprehensive simulation approach for hermetic reciprocating compressors including modeling of the electrical motor. The simulation of the compression cycle follows an integral control volume formulation for mass and energy conservation. A thermal model is adopted with steady-state thermal energy balances applied to the compressor components via global thermal conductances. The equivalent circuit method is employed to form a steady-state model of a single-phase induction motor. The coupling between the three models provides the motor slip and mean compressor speed, which are seen to affect the compressor efficiency. The simulation model is validated through comparisons between predictions and measurements of the parameters associated with the compressor efficiency, temperature distribution and motor performance. A parametric analysis is carried out to investigate the dependence of the motor temperature on the input voltage and the results are discussed.     

Development of a high efficiency refrigeration compressor

In order to improve the energy efficiency ratio of small reciprocating hermetic compressors the following factors are investigated: clearance volume, bore and stroke, rod length ratio, geometric dimensions of a piston and valves and superheat of suction gas.A prototype machine is developed based on this investigation. The reliability of this new type of compressor is confirmed and its energy efficiency ratio reaches 3.02 J W⁻¹ s⁻¹, which is 25% higher than the average value of our current machines.     

New non-dimensional performance parameters for the characterization of refrigeration compressors

Pierre (1982) developed general correlations of compressor performance for a wide range of open piston compressors based on a vast amount of data provided by the most important open compressors manufacturers at that time. Based on a wide experimental test campaign with 6 different hermetic and semihermetic compressors, this paper assesses the potential of Pierre's correlations to describe the performance of modern compressors. A full review of the functionals proposed by Pierre in order to improve their accuracy was then performed and as a result the authors found that it is possible to improve the characterization by employing a nondimensional definition of the compressor performance instead of using the efficiencies. These new parameters provide a much more consistent estimation at the borders of the correlation range than the usual correlation obtained following the ARI standard, and could be even employed to extrapolate the performance of the compressor to non-tested conditions with reasonable accuracy.     

跨临界二氧化碳压缩机热力性能仿真与分析

针对跨临界二氧化碳半封闭式往复式活塞压缩机建立了一个通用数学模型,既包括热力学模块,也包括机械模块。热力学模块主要描述气缸内部的气体压缩过程。机械模块包括运动学模型和曲轴连杆机构模型,考虑了轴承上的功耗损失。采用一台压缩机样机对模型进行了不同运行工况下的实验验证,结果显示压缩机流量和耗功的最大误差分别不超过5%和8%。通过仿真分析了变结构和变工况条件下的压缩机性能,结果表明:在不同的运行工况下,存在最佳缸径行程比;容积效率和等熵效率都随着转速的增加而下降;吸排气阀门内径存在最佳值;对于容积效率的影响,吸气阀间隙比排气阀间隙更大,活塞与汽缸间隙比活塞环与汽缸间隙更大。     

基于MLE阈值规则的小波特征提取技术在气阀故障诊断中的应用

气阀故障是往复压缩机最常见的故障类型之一,占故障总数的60%以上,如果不及时发现并解决,往复压缩机的压缩效率将大大降低。针对目前往复压缩机气阀故障诊断中存在的问题,结合小波降噪技术,提出了采用基于最大似然估计（MLE：Maximum Likelihood Estimation）阈值规则对气阀早期故障弱冲击变化信号进行特征提取的方法,实现了气阀故障的早期预警。     

Linear compressors for electronics cooling: Energy recovery and its benefits

A comprehensive model of a linear compressor for electronics cooling was previously presented by Bradshaw et al. (2011) then enhanced and used for a sensitivity analysis of the leakage gap, eccentricity, and piston geometry by Bradshaw et al. (2013). The current work utilizes the previously developed model to explore the energy recovery characteristics of a linear compressor as compared to those of a reciprocating compressor. The impact of dead (clearance) volume on both a linear and reciprocating compressor is analyzed. In contrast to a reciprocating compressor the overall isentropic efficiency of the linear compressor remains relatively unaffected by an increase in dead volume up to a certain point. This behavior is attributed to the ability of the linear compressor to recapture the energy of the compressed gas during the expansion process. This characteristic behavior allows a linear compressor to be used for efficient capacity control from roughly 35–100%.     

冰箱压缩机消声器声学特性的数值分析

分析全封闭往复活塞冰箱压缩机的消声器消声性能。利用软件建立不同结构形状进气消声器的有限元模型,通过导入声学软件进行计算,获得一腔室,两腔室消声器和复杂形状消声器内部声场的声压分布及传递损失。分析结构参数对消声性能的影响。