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CO2 as a natural working environment is a key research area for chlorofluorocarbons (CFCs). In this article, we’ll look at the basics of CO2 refrigeration compressors, how they differ from other types of compressors, and the benefits of CO2 as a refrigerant.
Due to the negative impact of freons on our atmosphere, environmental protection and the search for refrigerants to replace them have become a global problem. To solve this problem, the international community has been working hard for decades, as shown below:
Since the adoption of the United Nations Framework Convention on Climate Change in 1994, the Conference of the Parties (COP) has been held annually in different cities around the world. The graph clearly shows that the process of replacing CFCs worldwide is accelerating. The phase-out of ozone-depleting substances (ODS) has become an international obligation.
Alternative working environments must meet three requirements: safety, environmental acceptability and suitability of the equipment. After years of tireless efforts, scientists have developed transitional or long-term replacements for various CFCs and hydrochlorofluorocarbons (HCFCs) such as R134a, R407C, R410A and R290. Related technologies and equipment are also under study. Some of them are already widely used in the refrigeration and heating, ventilation and air conditioning (HVAC) industry.
The Montreal Protocol requires countries to phase out CFCs, HCFCs and other substances that deplete the ozone layer within a set time frame and establishes periods for the use of these substances in developed and developing countries respectively. HCFC refrigerants currently widely used in industry are listed as greenhouse gases by the Kyoto Protocol due to their apparent greenhouse effect.
Some European countries have banned the use of HCFCs in refrigeration and HVAC systems. At the same time, they also propose phasing out HCFCs in other regions. Legislation in some countries severely restricts or requires the phase-out of R134a refrigerants in the 2020s, effectively presenting the refrigeration and HVAC industry with a real dilemma – the need to adapt to the phase-out of CFC and HCFC refrigerants and find new coolants. Alternatives.
Research on environmental protection and refrigerant substitutes focuses on natural refrigerants such as water, ammonia, hydrocarbons and CO2. Former president of the International Refrigeration Society, Norwegian Gustav Lorenzen believes that natural refrigerants are the ultimate solution to environmental problems.
CO2 has become the subject of worldwide research due to its excellent heating and cooling properties. In his study, Lorentzen considered CO2 to be an “essential refrigerant”. Indeed, it is one of the most promising natural refrigerants and is expected to become one of the most sought after refrigerants in the 21st century.
At present, CO2 refrigeration technology is mainly used in three areas: automotive air conditioning, heat pumps, and multi-stage refrigeration systems. Because CO2 is an environmentally friendly refrigerant, CO2 compressors are designed and manufactured for a variety of applications. There are six types of CO2 compressors:
Compared with other refrigerant compressors, CO2 compressors are characterized by high working pressure, small structural size and compression ratio, large suction and discharge pressure difference, high cooling and heating efficiency.
More and more enterprise manufacturers are joining the trend of developing CO2 refrigeration compressors. Danfoss, Bitzer, Doling, Emerson and other global manufacturers have not only produced CO2 compressors, but also provided CO2 solutions for various business areas.
The advantages of the Danfoss carbon dioxide cooler are ease of installation, ease of commissioning and maximum system efficiency. Danfoss’ adaptive CO2 fluid management solution, actively responsive to carbon reduction, has won the AHR Expo Innovation Award 2020.
As the first manufacturer of refrigeration compressors for CO2, BITZER has been in close contact with major customers worldwide for many years. The company is working with customers to jointly develop carbon dioxide refrigeration technology. The earliest CO2 device developed by BITZER in collaboration with customers has now been operating safely for nearly 20 years.
Dorin is the world’s first manufacturer of semi-hermetic reciprocating compressors (piston compressors) powered by CO2 refrigerant, which can be used in HPWH and light and medium commercial refrigeration equipment. The compressor motor power ranges from 0.75kW to 15kW and can operate in medium and high speed modes (1450rpm and 2900rpm). Compressor diameter ranges from 18mm to 34mm.
As the world’s leading provider of HVAC and refrigeration solutions, Emerson’s Copeland scroll compressors can be used in low temperature multistage refrigeration applications and are the only compressors in the US to meet UL requirements for high temperature multistage systems. – Pressure in Asia is critical for carbon dioxide.
Table 1 (click for high resolution view): Comparison of the performance of different types of compressors. Note. Due to limited information, the “-” symbol means that no matching data was found.
CO2 scroll refrigeration compressors play an indispensable role in other types of compressors. With low vibration, low noise, long life, high reliability and high efficiency, CO2 scroll compressors are expected to take a large share in the small and medium compressor market. Therefore, we believe that scroll compressors contribute more to the efficiency of refrigeration systems, and their development prospects are brighter than other types.
In a complete refrigeration system, compressors and other mechanical components cannot be used individually. The refrigerant or cooling medium also plays a vital role. The refrigerant is the working medium in refrigeration equipment. It circulates in the refrigeration system. By changing its own thermodynamic state, the refrigerant completes energy conversion and transfer work with the external environment to achieve the purpose of cooling.
The refrigerant CO2 (R744) is a natural operating medium that is colorless and odorless at room temperature. The critical temperature and pressure are 31.1℃ and 7.37MPa, respectively.
The disadvantages of CO2 as a refrigerant are the low critical temperature (31.1°C) and high critical pressure (7.37 MPa). Especially for the latter, if the cooling system uses a transcritical cycle, the maximum operating pressure of CO2 will reach 10 MPa, which requires higher requirements for the design of the cooling system and its components. See Table 2.
Table 2 (click for larger resolution): Comparison of CO2 with other types of refrigerants. Note. Due to limited information, the symbol “-” in the table means that the corresponding data was not found. Relative prices are relative to R12. Let’s assume that the price of R12 is 1.
Energy saving, environmental protection and low-carbon life are the direction of development of modern science and technology. As a natural working medium, CO2 has the advantages of being environmentally friendly, non-toxic and readily available, and is therefore widely used.
According to Table 2, we can conclude that, compared with conventional refrigerants, the cooling capacity per unit volume of CO2 is 5.2 times greater than that of NH3, at 0-10 ° C 7.9 times greater than that of R134a, 4 times greater than NH3. R410A, 8.4 times more than R600a, R152a 8.2 times more than R290, and 5.8 times more than R290.
From a thermodynamic point of view, CO2 is in a supercritical fluid state with high specific heat capacity, high thermal conductivity and low kinematic viscosity, which makes it more efficient in transportation, heat transfer performance and higher heat transfer than other substances such as the efficiency of liquids. . gas.
Compared to other refrigerants, the CO2 adiabatic index is higher, the compression ratio of the transcritical refrigeration cycle is lower (about 2-4), and the compression process is closer to an isentropic process. All these factors contribute to the said cooling efficiency.
In terms of environmental protection, except for CO2 and NH3, the GWP values ​​of other refrigerants are relatively high. The GWP index of R410A is 2100 times that of CO2 and R134a is 1300 times that of CO2. Both can be included in the category of greenhouse gases that need to be reduced. Therefore, refrigerants such as R410A and R134a can only be used as alternative operating fluids and should not be used for long periods of time. As a natural gas in the Earth’s biosphere, CO2 has the lowest ODP and GWP values ​​of 0 and 1 respectively.
Finally, in terms of safety of use, CO2, as a natural working fluid, is non-toxic and non-flammable, and is also a very safe substance. In contrast, NH3, R600a, R152a and R290 are flammable and therefore much more dangerous than CO2. So far, the industry has not considered the safety issues associated with these four refrigerants.
CO2 refrigerant has better performance than other refrigerants and is gradually being used in refrigeration compressors. Although, strictly speaking, CO2 is not perfect. It has a low critical temperature (31.1°C) and a high critical pressure (7.37 MPa), which makes particularly high demands on compressors and pipelines, which will inevitably lead to an increase in production costs. However, the world’s leading compressor manufacturers are exploring improvements such as transcritical circulation systems. We believe that the outlook for CO2 refrigerant is bright and will play a leading role in the refrigeration compressor market in the future. As a part of natural refrigerants, CO2 will have a broad future and a broad market in the near future as calls for environmental protection grow.