RACA Journal June 2016 - Page 92

Technical Part 11: C02 in mobile air-conditioning systems *Permission to reprint from Bitzer Kühlmaschinenbau #BitzerReport The following report deals with potential measures of a short- to mediumterm change towards technologies with a reduced environmental impact in medium and large-sized commercial HVAC&R systems. W ithin the scope of the long-discussed measures for reducing direct refrigerant emissions, and the ban on the use of R134a in mobile air-conditioning (MAC) systems within the EU, the development of CO2 systems has been pursued intensively since several years. At the first glance, efficiency and therefore the indirect emissions from CO2 systems under typical ambient conditions appear to be relatively unfavourable, but it must be considered that present R134a systems are less efficient than stationary plants of the same capacity. The reasons for this lies in the specific installation conditions and the relatively high pressure losses in pipework and heat exchangers. With CO2, pressure losses have significantly less influence. Moreover, system efficiency is further improved by the high heat transfer coefficients in the heat exchangers. This is why optimised CO2 air-conditioning systems are able to achieve efficiencies that are comparable to those of R134a. Regarding the usual leakage rates of such systems, a more favourable balance is obtained in terms of total equivalent warming impact (TEWI). From today’s viewpoint, it is not yet possible to make a prediction as to whether the CO2 technology can in the long run prevail in this application. Certainly, this also depends on experiences with low global warming potential (GWP) refrigerants (previously covered) which in the meantime are partially introduced by the automotive industry. Hereby, other aspects such as operating safety, costs, and global logistics will play an important role. R124 and R142b as substitutes for R114and R12B1 Instead of the refrigerants R114 and R12B1 predominantly found in the past in high temperature heat pumps and crane cabin A/C installations, HCFC R124 and R142b can be used as alternatives in new installations. 90 RACA Journal I June 2016 With these gases it is also possible to use long proven lubricants, preferably mineral oils and alkyl benzenes with high viscosity. Because of the Ozone Depleting Potential, the use of these refrigerants must only be regarded as an interim solution. In the EU member states, the application of HCFCs is no longer allowed. For R124 and R142b the same restrictions are valid as for R22 (previously covered in the report). The flammability of R142b should also be considered with the resulting safety implications (safety group A2). Resulting design criteria/ Converting existing plants In comparison to R114 the boiling temperatures of the alternatives are lower (approximately 10°C) which results in larger differences in the pressure levels and volumetric refrigerating capacities. This leads to stricter limitations in the application range concerning high evaporation and condensing temperatures. A conversion of an existing installation will in most cases necessitate the exchanging of the compressor and regulating devices. Owing to the lower volume flow (higher volumetric refrigerating capacity), possible adjustments to the evaporator and the suc­tion line will be required. Over the previous years, Bitzer compressors have been found to be well suited with R124 and R142b in actual installations. Depending on performance data and compressor type modifications are necessary, however. Performance data including further design instructions are available on request. Chlorine free substitutes for special applications Due to the limited markets for systems with extra high and low temperature applica­tions, the requirements for the development www.hvacronline.co.za