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52 Ionel Oprea and low viscosity are desirable. No single fluid satisfies all the attributes desired of a refrigerant; consequently, various refrigerants are used. Refrigerant Table 1. Refrigerant Atmospheric Lifetime, ODP and GWP values Atmospheric Lifetime, years a ODP b GWP c 100 R-600 0.018 d 0 ~20 d R-717 0.01 d 0
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 53 system efficiencies should be expected to be equal to, or higher than, those of R404a, R717, and R134a systems. The risks represented by the flammability of the hydrocarbons must be seriously taken into account. The risks can be reduced by designing the systems for a minimum charge of refrigerant, careful leak detection during production, hermetic design with a minimum number of connections, the use of spark-proof electric components and ventilation of confined spaces. Fig. 1 – Representation of refrigerant cycle for a zeotropic refrigerant. The following matters are taken into account in this multicriteria analysis: 1.COP - coefficient of performance: refrigerants with a high critical temperature give the best COP; the natural refrigerants are superior in this respect. 2. qk: in HP cycles involving condensation, a refrigerant must be chosen so that this change of state will occur at a temperature somewhat below the critical temperature; 3. q0: since the evaporation of the liquid is the only step in the HP cycle which produces heating, the latent heat of a refrigerant should be as high as possible. Considering condensation and evaporation, ammonia (R717) is a better heat conductor, compared with the synthetic refrigerants. 4. V - swept volume flow: compressors are often some of the most critical and expensive systems at a production facility, and deserve special attention. 5. W - compressor work: for the efficiency of the process the compressor work is also of interest. The compressor work of isentropic compression from the temperature of 0°C to 70°C, 60°C, 52°C, 45°C to the condensing temperature, is given for the different fluids.
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BULETINUL INSTITUTULUI POLITEHNIC D
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VICTOR COTOROS şi EMIL BUDESCU, As
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ALEXANDRU RADU, CONSTANTIN PANĂ an
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