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The compressor technology is particularly demanding. The special requirements result in a completely independent approach. For example, this involves design, materials (bursting resistance), displacement, crank gear, working valves, lubrication system, as well as compressor and motor cooling. Hereby, the high thermal load severely lim- its the application for single-stage compres- sion. Low temperature cooling requires 2-stage operation, whereby separate high and low pressure compressors are particularly advantageous with parallel compounded systems. The criteria mentioned above in connection with subcritical systems apply to an even higher degree for lubricants. For further information to lubricants see page 41 and Fig. 37, page 45. Further development is necessary in vari- ous areas, and transcritical CO2 technology cannot in general be regarded as state-of- the-art. For transcritical CO2 applications, BITZER offers a wide range of special compressors. Their use is aimed at spe- cific applications, therefore individual examination and assessment are required. Suplementary BITZER information con- cerning compressor selection for trans- critical CO2 systems (see also https://www.bitzer.de) CO2 in mobile air-conditioning systems Within the scope of the long-discussed measures for reducing direct refrigerant emissions, and the ban on the use of R134a in MAC systems within the EU, the development of CO2 systems has been pursued intensively since several years. At first glance, efficiency and therefore indi- rect emissions from CO2 systems under typical ambient conditions appear to be unfavourable. But it must be considered that previous R134a systems are less effi- cient than stationary plants of the same capacity, because of specific installation conditions and high pressure losses in pipework and heat exchangers. With CO2, pressure losses have significantly less influ- ence. Moreover, system efficiency is further improved by the high heat transfer coeffi- cients in the heat exchangers. This is why optimized CO2 air conditioning systems are able to achieve efficiencies comparable to those of R134a. Regarding the usual leakage rates of such systems, a more favourable balance is obtained in terms of TEWI. From today's viewpoint, it is not yet possible to make a prediction as to whether CO2 can in the long run prevail in this application. It certainly also depends on the experience with "low GWP" refrigerants that have meanwhile been introduced by the automo- tive industry (see chapter R1234yf, page 11). Among others, operating safety, costs, and global logistics will play an important role. Fig. 32 36 Example of a transcritical CO2 Booster system o Brochure KP-130 Semi-hermetic reciprocating compres- sorsfortranscriticalCO applications 2 o Additional publications upon request Halogen free (natural) refrigerants Simplified sketchPDF Image | REFRIGERANT REPORT 21
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