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142 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 and Ramier [88] for a 2.4 l R-22 vessel, for instance, have a quite similar shape to graphs from the present tests before the top pressure sensor was insulated. The results have thus not given any reason to expect BLEVE in CO2 system accumulators or receivers. In real systems the presence of compressor lubricant, particles and contaminants, as well as unstable pressure/temperature, would make homogeneous nucleation even less likely. Graz and Stenzel [89] outlined a new draft SAE safety standard (J639) for mobile air conditioning and heat pump systems, also including CO2. Proposed rules for CO2 (R- 744) included: † Carbon dioxide (CO2) air-conditioning systems and integrated systems for heating and cooling (heat pumps) shall have a pressure blow-out disk as the overpressure relief device on both the high and low-pressure sides of the system when operating in the heating or cooling modes. The blow-out disc on the high-pressure side shall have a maximum allowable release pressure of 16 MPa, when the system is operating in the cooling mode and a maximum allowable release pressure of 12 MPa when the system is operating in the heating mode. The blow-out disc on the low-pressure side shall have a maximum allowable release pressure of 12 MPa, when the system is operating in the heating or cooling modes. † Components on the high-pressure side of the system shall have an ultimate burst pressure, when tested at the highest temperature reached by said component under typical operating conditions, which is not less than two times the release pressure of the Pressure Relief Device. In addition, components on the high-pressure side of the system shall have an ultimate burst pressure, when tested at the highest temperature reached by said component under typical operating conditions and after appropriate exposure to the system operating conditions (temperature, pressure, pressure cycling, vibration, corrosion), that is not less than 1.5 times the release pressure of the Pressure Relief Device. † Components on the low-pressure side of the system shall have an ultimate burst pressure, that is not less than two times the release pressure of the Pressure Relief Device. † For safety relevant pressurized components of air- conditioning systems and integrated systems a proof of integrity shall be carried out (by calculations, experiments or by a combination of both). Basis is the knowledge about the operational loadings and demanded lifetime of the components as well as of the specific behavior of the used structural material and its process treatment. The proof of integrity shall be carried out against (i) bursting, (ii) failure by fatigue, respectively, creep fatigue due to cycling pressure (pressure cycles between the conditions: system ‘in operation’—‘out of operation’, and (iii) failure by cruising vibrations. Creep fatigue instead of fatigue only has to be considered in case of some structural materials (e.g. aluminum alloys) which point out a time and temperature dependent behavior yielding to a decrease in strength over the time at elevated temperatures. 7. Component design 7.1. Compressors The vapor pressure of CO2 is higher than conventional refrigerants and the transcritical CO2 cycle operates at much higher pressures than the conventional vapor compression systems. Higher pressure gives special requirements regarding the design of suitable components, especially compressors for the CO2 systems. As the compressor is one of major components of air-conditioning and refrigeration systems and has an important effect on the system performance, compressor technology for the CO2 transcri- tical systems has reached an advanced level after years of development. Some examples are recent compressor models shown by Parsch [90], and Bullard [91]. Parsch [90] exploited the potential for a compact design with CO2 as shown in Fig. 30. Fig. 31 shows a 155 cm3 R-134a compressor next to a 21 cm3 CO2 compressor [91]. The relationship between a compressor’s mass and its displacement rate is not an obvious one, and will depend on specific design tradeoffs involving piston diameter and stroke and number, rpm, materials, etc. There is no evidence at the present time to suggest that Fig. 30. Mobile air-conditioning compressors with variable displacement. State-of-the-art R-134a design (left) and a recent design for CO2 (right) [90].PDF Image | CO2 Vapor Compression Systems
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