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As a natural working medium, CO2 is a key research orientation of chlorofluoro carbons (CFCs). In this article, we will cover the basics about CO2 refrigeration compressors, the differences between a CO2 compressor and other types of compressors, and the advantages of CO2 as a refrigerant.
Due to the negative impacts of CFCs on our atmosphere, environment protection and research of refrigerants to replace them have become a global concern. To address this, international communities have been making laborious efforts for decades, as outlined below:
Since 1994, after the UNFCCC was adopted, there has been a Conference of Parties (COP) every year in different cities across the world. As we can clearly see from the timeline, the process of CFCs replacement is accelerating all over the world. Phasing out Ozone Depleting Substances (ODS) has become an international responsibility.
The alternative working medium should meet three requirements: Safety, Environmental Acceptability, and Device Applicability . With years of unremitting efforts, scientists have developed many transitional or long-term substitutes for CFCs and hydrochlorofluorocarbons (HCFCs) such as R134a, R407C, R410A, and R290. Relevant technologies and equipment are also under study. Some of them have been widely used in refrigeration and the Heating, Ventilating and Air Conditioning (HVAC) industry already.
The Montreal Protocol requires countries to phase out CFCs, HCFCs, and other substances that deplete the ozone layer within a time limit, and stipulates the use period of these substances for developed and developing countries, respectively. The HCFC refrigerants widely used in industry right now are listed in the greenhouse gases by Kyoto Protocol, because of their obvious greenhouse effect.
Some European countries have banned the use of HCFCs in areas of refrigeration and HVAC. Meanwhile, they further propose to phase out HCFCs from other areas. The legislation of some countries strictly restrict or require the phasing out of the R134a refrigerant in the 2020s, which in fact makes a true dilemma for the refrigeration and HVAC industry—the need to adapt to the elimination of CFCs and HCFCs refrigerants, and to find alternatives.
In the research of environmental protection and refrigerant substitutes, natural refrigerants such as water, ammonia, hydrocarbons, and CO2 are the focus. Gustav Lorentzen of Norway, Former Chairman of the International Refrigeration Society, believed that natural refrigerants are the ultimate solution to environmental problems.
As a natural refrigerant, CO2 has many advantages over other substances, such as:
Due to its preferable performance in heating and cooling, CO2 has become a hot research topic worldwide. Lorentzen, in his research, had the opinion that CO2 was, “an irreplaceable refrigerant.” Indeed, it is one of the most promising natural refrigerants and is expected to become one of the ideal refrigerants of the 21st century.
At present, CO2 refrigeration technology is primarily applied in three areas: Automobile air conditioning, heat pumps, and multi-stage refrigeration systems . Since CO2 is an environmentally friendly refrigerant, CO2 compressors have been designed and manufactured for different uses. There are six types of CO2 compressors, as follows:
Compared with compressors of other refrigerants, CO2 compressors are characterized by their high working pressure; small structure size and pressure ratio; large pressure differences of suction and exhaust; and high cooling and heating efficiency.
More and more enterprise manufacturers have joined in the trend to research and develop CO2 refrigeration compressors. Big manufacturers around the world such as Danfoss, Bitzer, Dorin, and Emerson not only have launched carbon dioxide compressors, but are also providing carbon dioxide solutions for different commercial sectors.
Danfoss’ carbon dioxide gas cooler solutions have the advantage of easy installation, simple debugging, and maximum system operation efficiency. As a positive responder to reduce carbon emission, Danfoss’ CO2 Adaptive Liquid Management Solution won the 2020 AHR Expo Innovation Award.
As the first manufacturer to produce carbon dioxide refrigeration compressors, Bitzer has been keeping close contact with major customers in the world for many years. Together with its clients, the company has been jointly researching and developing carbon dioxide refrigeration technology. At present, the earliest carbon dioxide unit developed by Bitzer and its customers has been operating safely for nearly 20 years.
Dorin is the manufacturer of the world's first semi-hermetic reciprocating compressor (piston compressor) of CO2 refrigerant, which can be used in HPWH and light, medium-sized commercial refrigeration equipment. Compressor motors have power ranging from 0.75KW to 15kw and can operate in medium and high-speed modes (1450 rpm and 2900 rpm). The diameter of the compressor is between 18mm and 34mm.
As the world's leading provider of HVAC and refrigeration solutions, Emerson's subsidiary brand Copeland’s scroll compressor can be used in low-temperature multi-stage refrigeration systems, which is the only compressor in the US that meets the UL requirements for multi-stage systems of high-pressure subcritical carbon dioxide.
Table 1 (Click to view in high-res): Performance comparison of different types of compressors. Note: Due to limited information, the symbol “-” indicates that we couldn’t find the data.
CO2 scroll refrigeration compressors are playing an irreplaceable role among other types of compressors. In view of its features of low vibration, low noise, longevity, reliability, and high efficiency, CO2 scroll compressors are promised to occupy a large share in the market of small and medium cooling capacity compressors. Thus, we believe since scroll compressors are more conducive to improve efficiency of the refrigeration system, its development prospect is brighter than other types.
For a complete refrigeration system, one cannot work with only a compressor and other mechanical components. Refrigerants, or cooling mediums, play a vital role too. Refrigerant is the working medium in the refrigeration equipment. It circulates in the refrigeration system, and by the change of its own thermodynamic state, refrigerant can complete the work of energy conversion and transfer with the outside environment to achieve the purpose of refrigeration.
CO2 refrigerant (R744) is a kind of natural working medium, colorless and tasteless at normal atmospheric temperatures. The critical temperature and pressure are 31.1 ℃ and 7.37 MPa, respectively.
The physical properties of CO2 are as follows:
As a refrigerant, CO2 has the following three advantages over other refrigerants:
The disadvantages of CO2 as a refrigerant are its low critical temperature (31.1 ℃) and high critical pressure (7.37 MPa). Especially for the latter, if a trans-critical cycle is adopted in a refrigeration system, the maximum working pressure of CO2 will be 10 MPa, which requires higher standards for the design of refrigeration systems and components. See Table 2.
Table 2 (Click to view in high-res): Comparison of CO2 with other types of refrigerants. Note: Due to limited materials, symbol “-” in the table indicates that we couldn’t find the data. The relative price is relative to R12. We assume the price of R12 to be 1.
Energy-saving, environment protection and low carbon life are the development direction of modern technologies. As a natural working medium, CO2 has been widely used because of its advantages of environment friendliness, non-toxicity, and easy access.
According to Table 2, we can conclude that, compared with common refrigerants, the refrigerating capacity per unit volume of CO2 is 5.2 times that of NH3; 7.9 times that of R134a at 0-10 ℃; 4 times that of R410A; 8.4 times that of R600a; 8.2 times that of R152a, and 5.8 times that of R290.
From the perspective of thermodynamics, CO2 is in the state of supercritical fluid, with high specific heat, high thermal conductivity, and low kinematic viscosity, which makes it have better transmission performance, heat transfer characteristics, and higher heat transfer efficiency than liquid and other gases.
Compared with other refrigerants, the adiabatic index of CO2 is higher, the compression ratio of trans-critical refrigeration cycle is smaller (about 2-4), and the compression process is closer to an isentropic process. All these factors help to improve the indication efficiency of refrigeration.
From the perspective of environment protection, except for CO2 and NH3, other refrigerants have higher GWP. The GWP index of R410A is 2100 times that of CO2, and R134a is 1300 times that of CO2. Both can be put in the category of greenhouse gas that needs to be reduced. As a result, refrigerants like R410A and R134a can only be used as an alternative working medium and should not be used for a long time. As a natural gas in the earth's biosphere, CO2 has the lowest ODP and GWP, which are respectively 0 and 1.
Finally, considering usage safety, CO2 is non-toxic and nonflammable as a natural working fluid, making it a very safe substance. On the contrary, NH3, R600a, R152a, and R290 are all combustible, thus much more dangerous than CO2. By now, the industry hasn’t solved the safety problems caused by these four refrigerants yet.
CO2 refrigerants have better properties than other refrigerants and is gradually used in refrigeration compressors. Though strictly speaking, CO2 is not perfect either. With its low critical temperature (31.1 ℃) and high critical pressure (7.37 MPa), the requirements for compressors and pipelines are particularly high, which will surely increase the costs of product manufacturing. Nevertheless, the world’s top compressor manufacturers are studying improvement methods, such as the use of trans-critical circulation systems. We believe that the development prospect of CO2 refrigerant is bright, and it will play a leading role in the refrigeration compressor market in the future. As a part of natural refrigerants, there’s a promising future and market for CO2 in the immediate future when the voices of environmental protection are rising.
The International Society of Automation is a non-profit professional association founded in 1945 to create a better world through automation. ISA advances technical competence by connecting the automation community to achieve operational excellence. The organization develops widely-used global standards; certifies industry professionals; provides education and training; publishes books and technical articles; hosts conferences and exhibits; and provides networking and career development programs for its 40,000 members and 400,000 customers around the world.