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Blends containing unsaturated fluorinated hydrocarbons Some zeotropic blends with R-1234ze(E) were considered as possible candidates in vehicle air conditioning systems by many researchers (see for example, Schulze, 2015). Two mildly flammable A2L blends, R-444A and R-445A, were introduced by a large chemical company, and some application studies were conducted. However, even though the two blends are equivalent to HFO-1234yf with respect to flammability and performance, they have a high temperature glide which poses additional challenges in servicing. Therefore, these blends have not received any further interest and have not been introduced in MAC systems. Hydrocarbons and blends containing hydrocarbons In Australia and the USA, hydrocarbon blends, sold under various trade names, have been used as refrigerants to replace CFC-12 and to a lesser extent HFC-134a. Retrofits with HCs are legal in some Australian states, however, illegal in others, and in the USA. The US EPA has forbidden the use of HCs for retrofit. The use of HCs for new MAC systems could in principle be possible but requires proof that safety issues have been mitigated (USEPA, 1994). HCs or HC-blends, when correctly chosen, present suitable thermodynamic properties for the vapour compression cycle and allow achieving high energy efficiencies. Similar to HFC-152a, HCs would most likely be employed in indirect (secondary loop) systems only. Nevertheless, even with the use of indirect systems, the vast majority of vehicle manufacturers does not consider HCs as replacement fluids for mass-produced MAC systems due to safety concerns. 10.4.5 Bus air conditioning Mass transit vehicles include buses and coaches (discussed in this chapter) and rail cars (discussed in Chapter 6). As compared to passenger cars, their air conditioning systems are larger in size, have a higher cooling capacity, and generally use modified commercial components. Usually, they are specifically packaged for each application. The mass transit vehicle fleet is smaller than that of passenger cars. Available statistical data for Europe show that, per 1000 inhabitants, there are approximately 1.6 buses and coaches, and 477 passenger cars (Statistical Handbook, 2012). The number is assumed to be different elsewhere (in the North America and Japan, there are more passenger cars per person and less mass transit vehicles per person; in many developing countries it is the reverse). It can be assumed that, on average, at least 50% of the current EU mass transit vehicle fleet is air-conditioned. Both climate and economic conditions would determine the likelihood of air conditioning in transit vehicles in other regions. At present, most buses and coaches have the entire air-conditioning system mounted on the roof, except for the compressor, which is driven by the vehicle engine. Bus MAC systems may also be self-contained and electrically powered. The self-contained concept reduces both the charge and the leakage rate. The predominant refrigerant used in new buses and coaches is HFC-134a, although R-407C has also been employed and may constitute a significant portion of the market in some regions. Several years ago the refrigerant charge used to be in excess of 10 kg per unit (Schwarz, 2007), however, the introduction of microchannel condensers as well as component downsizing has reduced the current refrigerant charge to less than 5 kg in some new systems. In China, heat pump systems are used for thousands of electric buses, which exclusively use R- 410A. Older systems in Article 5 countries still utilize HCFC-22. According to the European statistics, the average age of a European bus or coach is 16.5 years, indicating that the stock of HCFC-22 MAC systems in buses is likely to remain for decades to come. Although limited by the volumes, the mass transit vehicle industry closely follows the developments in passenger cars and other fields. Since 2003, a large German manufacturer has on-going fleet tests of R-744 systems in buses (see e.g., Eberwein, 2011, Schirra, 2011, and Sonnekalb, 2012). In the year 2012, a 2018 TOC Refrigeration, A/C and Heat Pumps Assessment Report 195PDF Image | Heat Pumps Technical Options
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