buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

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.