Copper alloy tube for use in hvacr system

Abstract

There is provided a copper alloy tube having a chemical composition comprising, by mass, 0.80 - 0.95 % of Zn, 0.50 - 0.65 % of Sn, 0.020 - 0.027 % of P, and the balance Cu and unavoidable impurities. The copper alloy tube achieves a good balance between thermal conductivity, tensile strength and formability during tube production as well as an excellent corrosion resistance to formicary corrosion.

Description

[0001] COPPER. ALLOY TUBE FOR USE IN HVACR SYSTEM

[0002] DESCRIPTION

[0003] Field of Invention

[0004] The present invention relates to a copper alloy tube for use in a HVACR (Heating, Ventilation, Air Conditioning, Refrigeration) system.

[0005] State of the Art

[0006] Copper alloys are not only used in tubes of HVACR systems, but also in other technical fields. See JP H09 31570 A and EP 0 908 526 Al as examples of this.

[0007] JP H09 31570 A discloses a copper alloy for use as a building material, the copper alloy having a chemical composition comprising, by mass, 0.1 - 15 % Zn, 0.05 - 2 % Sn, more than 0.01 to 0.04 % P, and the balance Cu and unavoidable impurities.

[0008] EP 0 908 526 Al discloses a copper alloy for electrical applications, the copper alloy having a chemical composition comprising, by mass, 1.0 - 15 % Zn, 0.1 - 1.5 % Sn, 0.01 - 0.8 % Fe, 0.01 - 0.35 % P, and the balance Cu and unavoidable impurities.

[0009] In HVACR systems, copper alloy tubes are widely used because of their excellent thermal conductivity, which allows for efficient heat exchange. Copper alloy tubes are also resistant to corrosion, which ensures long-term durability and reduces the risk of leaks. Additionally, copper is a malleable material, making it easy to bend and shape the copper alloy tubes during installation. Examples of the use of copper alloy tubes in a heat exchanger can be found in US 4 935 076 A, EP 1 630 240 Al, JP 2003 268467 A, and EP 2 055 795 A2.

[0010] US 4 935 076 A discloses a copper alloy for use as the material of a heat exchanger, the copper alloy having a chemical composition comprising, by mass, 1 - 4.5 % Zn, 1.1 - 2.5 % Sn, 0.005 - 0.05 % P, and the balance Cu and unavoidable impurities. EP 1 630 240 Al discloses a copper alloy for use as a heat exchanger tube, the copper alloy having a chemical composition comprising, by mass, 0.01 - 0.40 % Zn, 0.02 - 0.25 % Sn, 0.15 - 0.33 % Co, 0.041 - 0.089 % P, and the balance Cu and unavoidable impurities.

[0011] JP 2003 268467 A discloses a copper alloy tube for a heat exchanger, the copper alloy tube having a chemical composition comprising, by mass, 0.01 - 1.0 % Zn, 0.1 - 1.0 % Sn, 0.005 - 0.1 % P, 0.005 % or less O, 0.005 % or less H, and the balance Cu and unavoidable impurities.

[0012] EP 2 055 795 A2 discloses a copper alloy tube for a heat exchanger, the copper alloy tube having a chemical composition comprising, by mass, 0.1 - 2.0 % Sn, 0.005 - 0.1 % P, 0.005 % or less S, 0.005 % or less O, 0.0002 % or less H, and the balance Cu and unavoidable impurities.

[0013] In recent years, CO2 (carbon dioxide) has gained popularity in refrigeration systems as an environmentally friendly alternative to conventional refrigerants. One of the main drivers behind the adoption of CO2 in refrigeration systems is their low environmental impact. CO2 is a natural refrigerant with zero ozone depletion potential (ODP) and a minimal global warming potential (GWP). However, one challenge with CO2 systems is that CO2 operates at a much higher pressure compared to conventional refrigerants that typically operate at a pressure below 45 bar. The critical point of CO2 is around 31°C and 73.8 bar, which means that transcritical CO2 systems operate above this pressure to maintain refrigeration temperature. Subcritical CO2 systems operate between 45 bar and 73 bar.

[0014] The higher operating pressure of CO2 necessitates the use of specialized components and materials capable of withstanding the increased stress. This includes high-pressure valves, fittings, specially designed CC -compatible compressors and copper alloy tubes, which have to be thicker to handle the high pressure. This poses a challenge for conventional copper alloy tubes because higher thickness increases the cost of raw material and influences formability during tube production. Examples of how this challenge can be mastered can be found in WO 2011 / 066345 Al, US 2009 / 0301701 Al, JP 2008 240128 A, and JP 2011 042825 A.

[0015] WO 2011 / 066345 Al discloses a copper alloy tube for heat exchangers in high pressure applications with cooling media such as CO2, the copper alloy tube having a chemical composition comprising, by mass, 0.07 - 1.0 % Sn, 0.02 - 0.2 % Fe, optionally 0.01 - 0.07 % P, and the balance Cu and unavoidable impurities.

[0016] US 2009 / 0301701 Al discloses a copper alloy tube for use in a refrigerator or heat pump operating with CO2, the copper alloy tube having a chemical composition comprising, by mass, 0.05 - 3 % Fe, 0.01 - 0.15 % P, and optionally 0.05 - 0.2 % Zn, 0.02 - 0.05 % Sn, and the balance Cu and unavoidable impurities.

[0017] JP 2008 240128 A discloses a copper alloy tube for a heat exchanger using CO2, the copper alloy tube having a chemical composition comprising, by mass, 0.03 - 0.15 % Co, 0.1 - 1.0 % Zn, 0.1 - 1.0 % Sn, 0.004 - 0.08 % P, 0.005 % or less S, 0.005 % or less O, 0.0002 % or less H, and the balance Cu and unavoidable impurities.

[0018] JP 2011 042825 A discloses a copper alloy tube for a heat exchanger using CO2, the copper alloy tube having a chemical composition comprising, by mass, 0.70 % Sn, 0.03 % P, 1.000 % Zn, and the balance Cu with unavoidable impurities.

[0019] As mentioned above, copper alloy tubes generally exhibit excellent corrosion resistance in HVACR systems. However, a specific type of corrosion that can affect copper tubes in HVACR systems, especially in systems operating with CO2, is formicary corrosion, also known as "ant-nest corrosion". Formicary corrosion is a localized corrosion phenomenon that occurs in the presence of organic acids, such as formic acid, acetic acid, or other volatile organic compounds. These acids can be generated by the degradation of certain materials, including insulation, adhesives, and cleaning agents used in HVACR systems. Formicary corrosion typically manifests as tiny, thread-like channels or tunnels on the inner and outer surfaces of the copper alloy tubes. These channels can penetrate the tube walls, leading to leaks and system inefficiencies. EP 1 769 211 Al discloses a formicary corrosion resistant heat transfer tube constructed of a tin brass alloy comprising, by mass, 86.0 - 90.0 % Cu, 0.8 - 1.4 % Sn, no more than 0.05 % Pb, no more than 0.05 % Fe, no more than 0.35 % P, and the balance Zn.

[0020] Summary of Invention

[0021] The copper alloy tubes of WO 2011 / 066345 Al and US 2009 / 0301701 Al offer high strength and improved resistance to stress corrosion cracking. However, their high strength results in a lack of formability during tube production and in a spring- back effect. In addition, the copper alloy tubes are not resistant to formicary corrosion.

[0022] The brass alloy tube of EP 1 769 211 Al also suffers under a lack of formability during tube production and has a poor thermal conductivity.

[0023] The object of the present invention is to provide a copper alloy tube, which is suitable for use in a HVACR system, especially in a system operating with CO2 as a refrigerant, and which has high thermal conductivity, high tensile strength and high formability during tube production as well as excellent corrosion resistance to formicary corrosion (ant-nest corrosion).

[0024] The aforementioned object is achieved by a copper alloy tube as specified in claim 1.

[0025] The copper alloy tube of the present invention has a chemical composition comprising, by mass, 0.80 - 0.95 % Zn (zinc), 0.50 - 0.65 % Sn (tin), 0.020 - 0.027 % P (phosphorus), and the balance Cu (copper) and unavoidable impurities. With this composition, the copper alloy tube achieves a good balance between thermal conductivity, tensile strength and formability as well as excellent corrosion resistance to formicary corrosion.

[0026] Zn is preferably 0.90 % or less, more preferably 0.85 % or less. Sn is preferably 0.60 % or less, more preferably 0.55 % or less.

[0027] P is preferably 0.025 % or less, more preferably 0.023 % or less.

[0028] Resistance to formicary corrosion can be assessed using the formicary corrosion test specified in the detailed description below. The copper alloy tube is sufficiently resistant to formicary corrosion if the average depth of the ten deepest pits in a sample subjected to the formicary corrosion test is less than 150 pm after 14 days of exposure. The average depth of the ten deepest pits is more preferably less than 100 pm after 14 days of exposure and even more preferably less than 100 pm after 21 days of exposure.

[0029] The copper alloy tube preferably has a grain size between 5 and 30 pm.

[0030] The copper alloy tube preferably has a tensile strength of at least 270 MPa, preferably of at least 290 MPa, as measured in accordance with ISO 6892-1:2019.

[0031] The total content of the unavoidable impurities is preferably 0.10 % or less, more preferably 0.05 % or less.

[0032] The respective content of all unavoidable impurities other than Fe may be less than 0.010 % and the content of Fe may be less than 0.050 %.

[0033] The respective content of all unavoidable impurities other than Fe, Pb, S and Sb may be even less than 0.0010 %.

[0034] The copper alloy tube of the present invention is suitable for use in a HVACR system. The copper alloy tube is particularly suitable for use in a HVACR system operating with CO2, including a system operating above the critical point of CO2 which is around 31°C and 73.8 bar.

[0035] Brief Description of Drawings Fig. 1 shows a phase diagram calculated by the inventors for a CuZnSnP alloy having different contents of Zn and fixed contents of 0.65 % by mass Sn and 0.025 % by mass P.

[0036] Fig. 2 shows a precipitation volume fraction diagram derived from the phase diagram shown in Fig. 1.

[0037] Fig. 3 shows a phase diagram calculated by the inventors for a CuZnSnP alloy having different contents of Sn and fixed contents of 1.0 % by mass Zn and 0.025 % by mass P.

[0038] Fig. 4 shows a precipitation volume fraction diagram derived from the phase diagram shown in Fig. 3.

[0039] Fig. 5 shows the microstructure of a plain copper alloy tube according to a first embodiment of the present invention.

[0040] Fig. 6 shows the microstructure of an inner-grooved copper alloy tube according to a second embodiment of the present invention.

[0041] Fig. 7 shows the corrosion morphology of a copper alloy tube according to the first embodiment of the present invention and a DHP copper tube after 14 days of exposure.

[0042] Fig. 8 shows the corrosion morphology of a copper alloy tube according to the first embodiment of the present invention and a DHP copper tube after 21 days of exposure.

[0043] Fig. 9 shows the corrosion morphology of a copper alloy tube according to the second embodiment of the present invention and a DHP copper tube after 14 days of exposure. Fig. 10 shows the corrosion morphology of a copper alloy tube according to the second embodiment of the present invention and a DHP copper tube after 21 days of exposure.

[0044] Detailed Description

[0045] In the following detailed description, the percentages of the chemical composition are by mass.

[0046] As explained above, the copper alloy tube of the present invention has a chemical composition comprising 0.80 - 0.95 % Zn, 0.50 - 0.65 % Sn, 0.020 - 0.027 % P, and the balance Cu and unavoidable impurities.

[0047] Hereinafter, the reasons for adding Zn, Sn and P as alloying elements and the reasons for limiting their content will be described.

[0048] Zn: 0.80 - 0.95 %

[0049] The phase diagrams in Figs. 1 and 3 suggest that, during solidification of the molten copper alloy, a solid Cu phase including P2Zn3particles is initially formed, and then an intermetallic Cu3Sn phase is formed. The inventors were able to confirm the presence of the intermetallic Cu3Sn phase in the microstructure by TEM analysis.

[0050] As can be seen from the precipitation volume fraction diagram in Fig. 2, the volume fraction of intermetallic Cu3Sn increases for a content of Zn in the range 0.1 % or more as the content of Zn increases.

[0051] The higher the volume fraction of intermetallic Cu3Sn is, the higher is tensile strength and the lower is formability during tube production. When Zn is less than 0.80 %, it is difficult to achieve a tensile strength of at least 270 MPa that is needed for high-pressure applications. When Zn is more than 0.95 %, formability becomes a problem. Since Zn has a lower thermal conductivity than Cu, thermal conductivity may also no longer be acceptable in HVACR applications. To improve formability and thermal conductivity, Zn is preferably 0.90 % or less, more preferably 0.85 % or less.

[0052] Sn: 0.50 - 0.65 %

[0053] As can be seen from the precipitation volume fraction diagram in Fig. 4, intermetallic CusSn forms at a content of Sn in the range of 0.5 % or more.

[0054] As discussed above, the volume fraction of intermetallic CusSn is related to tensile strength and formability during tube production. When Sn is less than 0.50 %, it is difficult to achieve a tensile strength of at least 270 MPa that is needed for high- pressure applications. When Sn is more than 0.65 %, formability becomes a problem. Since Sn has a lower thermal conductivity than Cu, thermal conductivity also suffers.

[0055] To improve formability and thermal conductivity, Sn is preferably 0.60 % or less, more preferably 0.55 % or less.

[0056] P: 0.020 - 0.027 %

[0057] The phase diagrams in Figs. 1 and 3 suggest that PzZna particles are formed during solidification of the molten copper alloy. Further, the precipitation volume fraction diagrams in Figs. 2 and 4, which have been derived from the phase diagrams in Figs. 1 and 3, suggest that the volume fraction of P2Zn3 particles is substantially constant at different contents of Zn and Sn for a fixed content of 0.025% P. It is therefore reasonable to assume that the content of P determines the volume fraction of P2Zn3 particles. It is also reasonable to assume that the P2Zn3 particles serve as nucleation points in the formation of the microstructure.

[0058] Presence of P2Zn3particles in the microstructure was not confirmed by TEM analysis. What exactly happens to the P content during solidification, i.e. whether P2Zn3particles really form as suggested by the phase diagrams or if a content of P remains in solid solution, therefore remains to a certain extent speculative, since some P has been found to be in solid solution.

[0059] Regardless of this, the inventors were surprised to find out through extensive testing that a content of P in a range 0.020 - 0.027 % suppresses formicary corrosion when the contents Zn and Sn are in the claimed ranges. The inventors' design contradicts the prevailing opinion expressed by Kozo Kawano et al. in "Influence of P concentration on Ant's Nest Corrosion in Copper Tubes", Materials Science, 2018 and by Tambang Manik et al. in "Effect of phosphorus on the ant nest corrosion mechanism", Materials Today Communications, vol. 36, 2023, 106560.

[0060] According to the prevailing opinion, at a content of 0 % P, the thin passive layer of CU2O formed on the surface of a copper alloy tube is rapidly dissolved in the presence of organic acids, and local discontinuity of the CU2O layer allows dissolution of metallic copper and the onset of formicary corrosion (ant-nest corrosion). At a content of 0.02 wt% P, the presence of P on the surface of the copper alloy tube causes release of H2PO4, which leads to a widening of the pit mouth at an early stage and to a corrosion morphology of both formicary corrosion and wide shallow pit corrosion. Finally, at a content of 0.2 wt% P, the amount of H2PO4 increases to such an extent that it has an impact on oxygen consumption and suppresses the development of CU2O, leading to a corrosion morphology of micro pitting corrosion.

[0061] The inventors believe that a preferential dissolution of Zn and Sn occurs in the copper alloy tube of the present invention and results in the development of Sn oxides that suppress the development of CU2O at a much lower content of P and much lower levels of H2PO4 than expected by the prevailing opinion.

[0062] When P is less than 0.020 %, suppression of formicary corrosion is less noticeable or not noticeable at all. When P is more than 0.027 %, thermal conductivity suffers.

[0063] To improve thermal conductivity, P is preferably 0.025 % or less, more preferably 0.023 % or less. Elements other than Cu, Zn, Sn, and P are considered impurities that should be avoided as they could have an undesirable effect on thermal conductivity, tensile strength, formability during tube production, and corrosion morphology. The total content of unavoidable impurities is preferably 0.10 % or less, more preferably 0.05 % or less.

[0064] The unavoidable impurities include elements such as Pb (lead), Fe (iron), Ni (nickel), Al (aluminum), Si (silicon), Mn (manganese), S (sulfur), Cd (cadmium), Bi (bismuth), Cr (chromium), Sb (antimony), Mg (magnesium), As (arsenic), Se (selenium), Te (tellurium), Ag (silver), Co (cobalt), and Zr (zirconium).

[0065] The content of Fe is typically less than 0.050 % and preferably less than 0.020 %. The respective content of all unavoidable impurities other than Fe is typically less than 0.010 % and preferably less than 0.005 %. The respective content of all unavoidable impurities other than Fe, Pb, S and Sb may be even less than 0.0010 %.

[0066] The copper alloy tube of the present invention may have a grain size between 5 and 30 pm. When the grain size exceeds 30 pm, fatigue strength can be a problem. It is noted that good fatigue strength is crucial for HVACR components.

[0067] The copper alloy tube of the present invention preferably has a tensile strength of at least 270 MPa as measured in accordance with ISO 6892-1:2019, more preferably of at least 290 MPa.

[0068] The copper alloy tube of the present invention may have a yield strength Rp0.2 of at least 60 MPa and an elongation of at least 40 %.

[0069] The copper alloy tube of the present invention may have an outer diameter in a range of 3 - 16 mm and a wall thickness in a range of 0.15 - 1 mm.

[0070] For example, for a copper alloy tube with an outer diameter of 9.52 mm and an operating pressure of 130 bar, the necessary wall thickness is 0.66 mm. Compared to an equivalent conventional tube made of deoxidized high phosphorus (DHP) copper, which has a wall thickness of 0.84 mm, a material saving of 21 % is achieved.

[0071] Preferably, the copper alloy tube of the present invention may be seamless produced by hot extrusion and cold drawing.

[0072] In a first embodiment of the present invention, the copper alloy tube is a plain tube having an inner surface without grooves as shown in Fig. 5. Fig. 5 shows an example of the microstructure of the plain tube in a longitudinal cross-section.

[0073] In a second embodiment of the present invention, the copper alloy tube is an inner- grooved tube having grooves on the inner surface as shown in Fig. 6. The grooves enhance heat transfer performance. Fig. 6 shows an example of the microstructure of the inner-grooved tube in a transversal cross-section.

[0074] The formicary corrosion test used for evaluating the resistance to formicary corrosion is as follows.

[0075] Specimens are cut to a desired length. Specimen length is selected based on the available total sample dimensions, but should not be lower than 50 mm. Three specimens are tested from each sample category, in each interval. Prior to exposure, the exposed area of each specimen is calculated, and specimen weight and macroscopic and stereoscopic appearance are recorded. In case of inner- grooved samples, the inner groove is included in the calculated area.

[0076] The specimens are suspended in a formic acid solution having a concentration of 1000 ppm, in glass jars, with 45° inclination, using PTFE threads or a similar material able to resist the exposure conditions. The number of specimens placed in each glass jar should allow for uniform exposure of each specimen. The specimens should not come in contact with adjacent specimens or with the wall of the glass jar. The volume of the formic acid solution to the exposed area ratio is 0.03 ml / mm2. For example, in case of 05x70mm specimens, 2 specimens per jar, 125 ml formic acid solution are placed in a glass jar.

[0077] The rubber band inserts of each glass jar are covered with PTFE tape.

[0078] The glass jars are placed in an environmental chamber that performs daily thermal cycles 16 h: 40°C and 8 h: 25°C.

[0079] The samples are examined and evaluated at different exposure periods.

[0080] The following evaluation criteria can be applied to assess the resistance to formicary corrosion. a) Max pit: Deepest single value per sample category b) AVE-10: Average depth of the ten deepest pits per sample category c) Pitting Factor: deepest penetration / average penetration

[0081] (A pitting factor of one represents uniform corrosion. The larger the number, the greater the depth of maximum penetration.) d) CAI (Corrosion Attack Index): Each point of the 10 deepest pits / wall thickness

[0082] (CAI is a comparative assessment of tube failure tendency and not of absolute corrosion evolution. CAI ranges from 0: no attack to 100: tube failure.)

[0083] Comparative tests

[0084] In a first comparative test, a copper alloy tube according to the first embodiment of the present invention (i.e., a plain tube) was compared with a plain DHP copper tube in terms of resistance to formicary corrosion.

[0085] The material of the copper alloy tube according to the first embodiment of the present invention (in the following referred to as "invention alloy" or "CuZnSnP alloy") had a chemical composition comprising 0.9 % Zn, 0.6 % Sn, 0.025 % P, and the balance Cu and unavoidable impurities. The material of the DHP copper tube had a chemical composition comprising 0.02 % P, and the balance Cu and unavoidable impurities.

[0086] The invention alloy tube and the DHP copper tube were subjected to the formicary corrosion test described above. Fig. 7 shows the corrosion morphology of the samples after 14 days of exposure, and Fig. 8 shows the corrosion morphology of the samples after 21 days of exposure. Figs. 7 and 8 show X-ray diffraction (XRD) images of the samples at different magnifications.

[0087] As can be seen from Figs. 7 and 8, the DHP copper tube shows significantly more formicary corrosion than the invention alloy tube.

[0088] In a second comparative test, a copper alloy tube according to the second embodiment of the present invention (i.e., an inner-grooved CuZnSnP alloy tube) was compared with an inner-grooved DHP copper tube in terms of resistance to formicary corrosion. The chemical compositions of the tubes were the same as in the first comparative test.

[0089] The CuZnSnP alloy tube and the DHP copper tube were subjected to the formicary corrosion test described above. Figs. 9 and 10 are XRD images of the samples at different magnifications, showing the corrosion morphology of the samples after 14 days of exposure and 21 days of exposure, respectively.

[0090] The aforementioned evaluation criteria b) and d) were applied to assess the resistance of the samples to formicary corrosion. The evaluation results are shown in Table 1.

[0091] Table 1 As can be seen from Figs. 9, 10 and Table 1, the DHP copper tube shows significantly more formicary corrosion than the CuZnSnP alloy tube. This is particularly evident from the criterion Ave- 10, i.e. the average depth of the ten deepest pits per sample category. The average depth of the ten deepest pits for the CuZnSnP alloy tube is significantly lower than for the DHP copper tube.

[0092] A sample is assessed as having insufficient resistance to formicary corrosion if the average depth of the ten deepest pits in the sample is 150 pm or more after 14 days of exposure. The sample is assessed as sufficiently resistant to formicary corrosion if the average depth of the ten deepest pits in the sample is less than 150 pm after 14 days of exposure. The sample is assessed as having good resistance to formicary corrosion if the average depth of the ten deepest pits is 100 pm after 14 days of exposure. The sample is assessed as having excellent resistance to formicary corrosion if the average depth of the ten deepest pits is less than 100 pm after 21 days of exposure. Based on this classification, the DHP copper tube is assessed as having insufficient resistance to formicary corrosion, whereas the CuZnSnP alloy tube is assessed as having excellent resistance to formicary corrosion.

Claims

CLAIMS1. A copper alloy tube having a chemical composition comprising, by mass, 0.80 - 0.95 % Zn, 0.50 - 0.65 % Sn, 0.020 - 0.027 % P, and the balance Cu and unavoidable impurities.

2. The copper alloy tube according to claim 1, wherein Zn is 0.90 % or less, preferably 0.85 % or less.

3. The copper alloy tube according to claim 1 or 2, wherein Sn is 0.60 % or less, preferably 0.55 % or less.

4. The copper alloy tube according to any one of claims 1 to 3, wherein P is 0.025 % or less, preferably 0.023 % or less.

5. The copper alloy tube according to any one of claims 1 to 4, wherein the average depth of the ten deepest pits in a sample subjected to the formicary corrosion test specified in the description is less than 150 pm after 14 days of exposure, preferably less than 100 pm after 14 days of exposure, and more preferably less than 100 pm after 21 days of exposure.

6. The copper alloy tube according to any one of claims 1 to 5, having a grain size between 5 and 30 pm.

7. The copper alloy tube according to any one of claims 1 to 6, having a tensile strength of at least 270 MPa, preferably at least 290 MPa, as measured in accordance with ISO 6892-1 :2019.

8. The copper alloy tube according to any one of claims 1 to 7, wherein the total content of the unavoidable impurities is 0.10 % or less, preferably 0.05 % or less.

9. The copper alloy tube according to any one of claims 1 to 8, wherein the respective content of all unavoidable impurities other than Fe is less than 0.010 % and the content of Fe is less than 0.050 %.

10. The copper alloy tube according to any one of claims 1 to 9, wherein the respective content of all unavoidable impurities other than Fe, Pb, S and Sb is less than 0.0010 %.

11. Use of the copper alloy tube according to any one of claims 1 to 10 in a HVACR system.

12. The use according to claim 11, wherein the HVACR. system is a system operating with CO2 as a refrigerant.

13. The use according to claim 12, wherein the system operates above the critical point of CO2.

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