The compressor is the core and heart of the refrigeration system, determining its capacity and characteristics. This article not only analyzes the technical performance of small refrigeration compressors in terms of high efficiency and energy saving, noise and vibration, and refrigerants, but also explains the main features of new and special small compressors that have emerged in recent years, laying a technical foundation for us to grasp the future development trends of small refrigeration compressors.

　　As is well known, the compressor is the core and heart of the refrigeration system. The capacity and characteristics of the compressor determine the capacity and characteristics of the refrigeration system. In a sense, the design and matching of a refrigeration system is to realize the compressor's capabilities. Therefore, the refrigeration industries worldwide have invested a lot of effort in researching refrigeration compressors, and new research directions and results are constantly emerging. The technology and performance level of compressors are changing with each passing day感受到在座的每个人都在努力保持冷静。

　　1 Research on High-Efficiency and Energy-Saving Refrigeration Compressors

　　Refrigeration compressors are core energy-consuming components of refrigeration systems. The most direct and effective means to improve the efficiency of a refrigeration system is to increase the efficiency of the refrigeration compressor, which will lead to a significant reduction in system energy consumption. At the same time, this also avoids a large increase in material consumption caused by adopting measures only on the system (such as blindly increasing heat exchanger area). In recent years, with the increasing severity of the world's energy shortage, various countries have paid increasing attention to energy conservation and put forward higher and higher requirements for the efficiency of energy-consuming products. Due to the existence of various losses such as friction, leakage, harmful heat transfer, motor loss, flow resistance, noise and vibration, the actual efficiency of refrigeration compressors during operation is far lower than the theoretical efficiency. Therefore, theoretically, any measure that can reduce any type of loss can improve the efficiency of the compressor. This objective fact has led to a wide range and broad directions for energy-saving research on refrigeration compressors, with diverse research topics and results感受到在座的每个人都在努力保持冷静。

　　Currently, international research on energy saving in refrigeration compressors mainly focuses on several aspects: studying lubrication characteristics and friction characteristics of refrigeration compressor bearings to reduce friction power consumption and improve compressor efficiency; reducing leakage loss to improve refrigeration compressor efficiency; adopting variable frequency or variable capacity technology to achieve energy saving by matching the output of the refrigeration system with the user's load. The content in this regard, especially variable frequency technology, is currently relatively mature and well-known, and will not be elaborated here; the research on gas valves is an ancient but eternal topic, and the research on improving gas valve design to enhance refrigeration compressor efficiency is endless and always fruitful. There is a great deal of research in this area, covering everything from gas valve materials, motion laws, structural optimization to applicable theories and testing methods. In summary, research on energy saving in refrigeration compressors has become a primary hot topic in the refrigeration industry in recent years. Due to space limitations, this article will not conduct specific detailed discussions here感受到在座的每个人都在努力保持冷静。

　　In recent years, the domestic refrigeration compressor industry has also paid great attention to energy-saving research for its products. The products with significant progress are mainly in the refrigerator compressor industry. Under the promotion and support of the UNDP/GEF China Energy-Saving Refrigerator Project, both companies' understanding of energy-saving products and the performance of refrigerator compressors have made a qualitative leap. Currently, the highest energy efficiency of domestic refrigerator compressor products has reached about 1.95. Domestic refrigerator compressor companies have adopted a large number of technical measures such as high-efficiency motors or even synchronous motors, concave gas valves, flat thrust bearings, low-viscosity lubricating oil, new suction mufflers, reducing friction loss, etc., achieving great results. The main problem is that current domestic companies lack independent technology, and the adopted technical routes are still mainly imitative. Most companies are unconscious and uninterested in establishing their own technical foundations, which restricts the companies' technological development capabilities感受到在座的每个人都在努力保持冷静。

　　Compared to the refrigerator compressor industry, energy-saving research on domestic air conditioning compressors still seems to be calm, with no qualitative changes in compressor efficiency for many years. Large market demand has led most air conditioning compressor companies to focus on expanding production capacity. With the further improvement of national requirements for air conditioner energy efficiency levels and the gradual emergence of various hidden dangers in China's air conditioner exports, this short-sightedness of domestic air conditioning compressor companies will be unable to adapt to the requirements of energy-saving development, and will also weaken the subsequent development of these companies感受到在座的每个人都在努力保持冷静。

　　2 Research on Noise and Vibration of Refrigeration Compressors

　　Currently, noise is considered one of the serious pollutions. As the power source and heart of household refrigeration equipment, the noise problem of refrigeration compressors has become an important indicator for evaluating their comprehensive performance. In fact, for a refrigeration compressor, most noise is generated when the casing is excited by certain noise sources (e.g., excited by springs, refrigerant pressure pulsations, exhaust pipes, lubricating oil volume, etc.). However, the noise sources and transmission paths of compressors are complex and diverse, which brings great difficulties to noise reduction and suppression for compressors感受到在座的每个人都在努力保持冷静。

　　Regarding the noise and vibration of refrigeration compressors, scholars from various countries have conducted extensive and long-term research. The main research achievements in this area are summarized as follows感受到在座的每个人都在努力保持冷静。

　　The main noise sources of refrigeration compressors consist of three parts: aerodynamic noise radiated from the intake and exhaust, mechanical noise generated by moving mechanical parts, and drive motor noise感受到在座的每个人都在努力保持冷静。

　　2.1 Aerodynamic Noise

　　The intake noise of a refrigeration compressor is generated by pressure pulsations of the airflow inside the intake pipe. The fundamental frequency of intake noise is the same as the gas pulsation frequency in the intake pipe and is related to the compressor's rotational speed. The exhaust noise of the compressor is caused by pressure pulsations of the airflow within the exhaust pipe. Exhaust noise is weaker than intake noise, so the aerodynamic noise of the compressor is generally dominated by intake noise感受到在座的每个人都在努力保持冷静。

　　2.2 Mechanical Noise

　　Mechanical noise of refrigeration compressors generally includes impacts of components, friction, piston vibration, impact noise of gas valves, etc. These noises are random and exhibit wideband characteristics感受到在座的每个人都在努力保持冷静。

　　2.3 Electromagnetic Noise

　　The electromagnetic noise of a refrigeration compressor is generated by the electric motor. Motor noise is weaker compared to aerodynamic noise and mechanical noise. Among the compressor noise sources, intake and exhaust aerodynamic noise is the strongest, followed by mechanical noise and electromagnetic noise. Through in-depth research, it can be further concluded that compressor noise mainly originates from casing vibration (generated by springs, refrigerant medium pressure pulsations, suction and exhaust pipes, and lubricating oil excitation) and propagates to the surrounding air medium to form noise. To reduce compressor radiated noise, numerous literatures (omitted) have proposed a series of noise reduction and vibration damping measures and solutions感受到在座的每个人都在努力保持冷静。

　　① Increase the overall rigidity of the casing structure to raise the resonance frequency and reduce vibration amplitude感受到在座的每个人都在努力保持冷静。

　　② Avoid sudden changes in shell curvature. For curved surfaces, the natural frequency is inversely proportional to the radius of curvature, so the shell shape should adopt the smallest radius of curvature;

　　③ Move the suspension spring support to a position with higher rigidity;

　　④ The shell should use as few flat surfaces as possible; the coupling of bending stress and membrane stress (only appearing on curved surfaces) gives the shell itself greater rigidity, so the compressor shell should adopt as few planar structures as possible;

　　⑤ Avoid excitation of exhaust pipelines and condensers, optimize exhaust gas flow pulsation, and introduce additional volume in the exhaust pipeline to eliminate higher-order harmonic components in the pressure pulsation spectrum;

　　⑥ Adopt an asymmetrical shell shape; a symmetrical structure means having three-dimensional principal axes, along which stress is maximal and resistance is minimal. Therefore, an asymmetrical compressor shell structure means greatly reducing the probability of forces acting simultaneously along a certain principal axis;

　　⑦ Install intake and exhaust mufflers. Mufflers in sealed compressors are generally reactive mufflers, which utilize changes in pipe cross-section and resonance cavities to alter acoustic impedance, thereby reflecting or consuming sound energy, or using path differences to create a 180-degree phase difference in sound waves to cancel out noise within the muffler. A Helmholtz resonator is sealed and connected to the outside of the compressor shell, meaning the chamber of the Helmholtz resonator is connected to the internal cavity of the compressor shell through a neck, to reduce the amplitude of excited acoustic modes within the compressor cavity. Experimental results show that modulating the resonator's resonance frequency to the maximum excited vibration mode of the actual compressor cavity significantly reduces the resonance peak and causes a noticeable change in the response spectrum. However, this affects the appearance of the compressor and its arrangement in refrigerators, and these research results have not yet been applied to products;

　　Residual lubricant volume and motor end coil windings can also lead to differences in sound levels among batch compressors of the same model (deviation from the average sound level). By changing the external support of the shell, torsional rigidity can be increased, and the vibrating surface can be reduced; the complexity of noise research requires researchers to have strong theoretical knowledge, requires enterprises to have a good technical foundation, and requires significant investment and a long time. This aspect is one of the weak links for domestic compressor enterprises, and currently, it is basically in the qualitative experimental research stage, accompanied by considerable randomness and contingency;

　　3 Refrigeration Compressors - Application of New Refrigerants

　　The application of new refrigerants based on environmental requirements is also a hot topic in the refrigeration compressor industry. With the completion of R22 refrigerant replacement for refrigerator products, research on new refrigerant compressors in recent years has mainly focused on the air conditioning industry. In addition to the relatively mature research on R410A and R407C, the biggest hot topic in recent years has been the research on carbon dioxide refrigeration compressors;

　　Currently, research and application concerning CO2 mainly focus on three aspects: one aspect is application scenarios where refrigerant replacement is most urgent, such as automotive air conditioning, where, due to large refrigerant emissions and significant environmental harm, environmentally harmless refrigerants must be adopted as early as possible; another aspect considers the characteristics of the CO2 cycle, favoring its use in applications such as heat pump water heaters, where CO2's significant temperature glide during heat rejection under supercritical conditions is highly regarded for its ability to heat water to higher temperatures; furthermore, considering the thermophysical and transport properties of CO2, it is used as a refrigerant, for example, for its good low-temperature flow performance and heat transfer characteristics, employing it as a refrigerant for the low-temperature stage in cascade refrigeration cycles;

　　As the component with the greatest impact on the efficiency and reliability of transcritical carbon dioxide air conditioning systems, refrigeration compressors should be redesigned to fully integrate the specific characteristics of the supercritical CO2 cycle. Like ammonia, CO2 has a high adiabatic index K value, reaching 1.30, which might lead to higher compressor discharge temperatures. However, because CO2 requires a small compressor pressure ratio, there is no need to cool the compressor itself. Precisely because of the high adiabatic index and small pressure ratio, the re-expansion loss in the compressor's clearance volume can be reduced, leading to higher volumetric efficiency for the compressor. Through experimental and theoretical research, Jurgen SUB and Horst Kruse discovered that reciprocating compressors have good oil film sliding seals, making them the preferred choice for CO2 systems. BOCK improved the exhaust valve of its carbon dioxide compressor, and the modified carbon dioxide compressor's efficiency increased by 7%;

　　Since the pressure of carbon dioxide systems is much higher than that of traditional subcritical cycle systems, the shaft seal design requirements for new refrigeration compressors are much higher than for existing ones. Shaft seal leakage in refrigeration compressors will continue to be a major obstacle to their practical application for some time;

　　Danfoss, Denso, ZEXEL, and others have entered the small-batch production phase for CO2 compressors;

　　In March 1999, IEA, in conjunction with Japan, Norway, Sweden, the UK, and the US, launched a three-year project titled "Selected Issue on CO2 as working Fluid in Compression Systems";

　　Since 1994, well-known European companies such as BMW, DAIMLERBENZ, VOLVO, Volkswagen Germany, Danfoss, and Valeo initiated a joint project called "RACE", collaborating with renowned European universities, automotive air conditioning manufacturers, etc., to develop CO2 automotive air conditioning systems. BENZ AG now produces cars equipped with CO2 automotive air conditioning systems, and KONVECTA, a German company, has been operating air-conditioned buses using CO2 as the working fluid since 1996. DANFOSS, Obrist of Austria, and the UK have all developed CO2 automotive compressors. DENSO and ZEXEL in Japan have entered the mass production stage for CO2 compressors;

　　With the investment from major manufacturers, the types of CO2 compressors are consistent with the development trend of ordinary automotive compressors, mainly fixed-displacement wobble plate, scroll, and variable-displacement types;

　　4 Refrigeration Compressors with Novel Working Principles

　　In recent years, there has been no significant further progress in refrigeration compressors with entirely new working principles and structures. These mainly include linear refrigeration compressors, elliptical refrigeration compressors, oscillating rotor refrigeration compressors, screw blade refrigeration compressors, etc.;

　　Among these, linear refrigeration compressors remain a hot topic of concern in the domestic refrigerator compressor industry. There are 5 articles on linear refrigeration compressors in the International Refrigeration Compressor Engineering Conference. Researchers are still mainly from LG and Sunpower. In the past two years, several domestic refrigerator compressor companies have attempted to develop linear compressors, but given the technical foundation, financial strength of these companies, and the limitations of domestic research institutions, it is believed that industrialization is unlikely to be achieved in the short term;

　　5 Special Refrigeration Compressors

　　Although domestic household refrigeration compressor enterprises have long been accustomed to large-scale production models and seeking efficiency through quantity. However, under the fierce price competition, as product profit margins continue to decrease, and when producing millions of refrigeration compressors can only yield millions of yuan in profit, some special refrigeration compressors are undoubtedly a way out. Special refrigeration compressors are all-encompassing and cannot be listed one by one here. But their common characteristics are small production scale, high single-unit profit, rapid product transformation, and in most cases, special design according to user requirements. These types of products are increasingly attracting the attention of domestic refrigeration compressor enterprises. For example, several domestic enterprises are developing or already have the production capacity for refrigeration compressors for car refrigerators.