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140 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 (inner diameter). For the same reasons as above, the compressor displacement is reduced by 80 – 85% for a given capacity. Compressor and heat exchanger size and weight reductions seem possible due to the reduced refrigerant-side volumes and cross-sections. 6.4. High-pressure safety issues In general, the hazards of refrigerants and vapor compression systems are associated with the physical and chemical characteristics of the refrigerant as well as with the pressures and temperatures occurring in the system. Factors discussed by Pettersen et al. [77] in relation to CO2 were Flammability. CO2 is non-flammable, and it is even used as a fire-fighting or fire-preventing gas. Thus, flammability is not an issue. Inhalation safety. Although CO2 is usually regarded as non-toxic there are physiological effects from breathing air with a CO2-concentration above a few percent. At 2–3% concentration by volume, the breathing rate will increase, and headache may be experienced after some time. The IDLH (Immediate Danger to Life and Health) concentration is set to 4% [78], and the lowest reported lethal concentration is 10% [79]. In practice, a maximum allowable concentration of about 5% by volume seems to be a reasonable limit [79,80]. In the design and operation of CO2 systems, this will be the maximum acceptable concentration as a result of sudden release or prolonged leakage of CO2 into occupied space. The assessment of hazards resulting from accidental leakage of CO2 into occupied space should consider factors like † rate of CO2 outflow at decreasing system pressure, † formation of dry ice inside system at 0.52 MPa system pressure, † amount of CO2 dissolved in lubricant, † room/cabin ventilation rate, and † stratification (CO2 is heavier than air). Frost burn is probably not a problem with CO2 since the triple point is at 0.52 MPa, i.e. there will be no boiling refrigerant at atmospheric pressure. Also, the toxic or irritating effects from decomposition products known to occur when fluorocarbon refrigerants contact flames or hot surfaces will not occur with CO2. Explosion or rupture of a pressurized component or vessel. The hazards may include blast effects and shocks, as well as flying fragments. Such incidents may be caused by a number of factors, such as malfunctioning safety device, overheating, over-charging, incorrect operation, construc- tion weakness/corrosion, mechanical impact, etc. Low-side pressures in CO2 systems are typically 3 – 4 MPa, and high-side pressures may be as high as 12 – 14 MPa. These pressures are 5 – 10 times higher than in traditional fluorocarbon systems. High pressure is not a safety issue in itself, since the equipment will be designed for this. In case of a component rupture, however, the explosion energy (stored energy) may characterize the extent of potential damage. The explosion energy can be estimated based on component (refrigerant-side) volumes, pressures and refrigerant property data. The possible occurrence of a BLEVE (Boiling Liquid Expanding Vapor Explosions) may create a more severe blast effect than by an ordinary refrigerant expansion. The following sections will address the two last issues—explosion energy and BLEVE—based on calculations and experimental data. 6.4.1. Explosion energy The explosion energy can be estimated as the energy released by expansion of the refrigerant contained in a component or system The expansion process will be very rapid, with little or no time for heat transfer between the ambient air and the expanding gas, and the explosion energy can therefore be estimated as the reversible adiabatic (isentropic) work of expansion. A detailed analysis of explosion energies should consider the refrigerant charge inside each component, as well as the local pressure, temperature and vapor fraction. In the current analysis, pressures are assumed to be equalized and temperatures uniform throughout the system. As a consequence, the charge and volume are assumed to reflect the refrigerant condition inside the entire system. When the temperature of the system is varied, the charge/volume ratio will remain constant (constant average specific volume), and pressure will vary either as saturation pressure, or as pressure along a constant-volume (isochoric) line in the gas region. Pettersen [81] calculated and compared the explosion energies of equal-capacity (7 kW) ductless split residential air conditioning systems with R-22 and CO2 as refrigerant. The R-22 system was based on flat-fin/round-tube heat exchangers, while the CO2 prototype system had all-aluminum microchannel heat exchangers. Even though pressures were higher in the CO2 system, reductions in internal volume and refrigerant charge gave comparable energy levels in the two systems. At room temperature, the CO2 system energy was higher, while at elevated tempera- ture that may occur in a fire the R-22 energy was highest. For the R-22 system, the total volume occupied by refrigerant was approximately 11.4 l, of which 0.7 l (6%) was in the indoor unit and 8.5 l (75%) in the outdoor unit. The remaining volume was in the 10 m of piping between the two units. The average refrigerant density in the system was then 300 kg/m3 (3.5 kg charge/11.4 l refrigerant volume). Corresponding data for the CO2 prototype system were: 4.2 l total volume, 0.27 l (6%) in the indoor unit and 3.3 l (78%) in the outdoor unit, and an average charge density of 260 kg/m3 (1.1 kg/4.2 l). Fig. 29 shows the total explosion energies (in kJ) for the two systems at varying initial temperature. The energies are equal around 60 8C,PDF Image | CO2 Vapor Compression Systems
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