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Modern Engineering Thermodynamics

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554 CHAPTER 14: Vapor and Gas Refrigeration Cycles<br />

CRITICAL THINKING<br />

Replacing refrigerants R-11 and R-12 with R-123 and R-134a in existing equipment is not a matter of simply draining out<br />

the old refrigerant and adding the new, because they have different physical and thermodynamic properties. This conversion<br />

is very expensive and the owners of the equipment must bear the costs. This is why the final phase-out of all CFCs is<br />

not scheduled until the year 2030, when the equipment existing today would be obsolete and need to be replaced anyway.<br />

If the CFCs are as dangerous as we think they are, then why are we waiting so long to eliminate them? Who else should<br />

share in the conversion costs?<br />

although it still contains chlorine, it is 20 times less detrimental to the ozone layer than R-11 or R-12. However,<br />

after January 1, 2010, no virgin R-22 can be used in existing systems, and after January 1, 2015, no recycled<br />

refrigerant R-22 can be used in existing systems.<br />

14.9 CASCADE AND MULTISTAGE VAPOR-COMPRESSION SYSTEMS<br />

Refrigeration applications like the quick freezing of processed food or the production of liquefied gases such as<br />

liquefied natural gas (LNG, methane) and liquefied petroleum gas (LPG, propane and butane) require moderately<br />

cold refrigeration temperatures in the range of −30.°C to−180°C (−22°F to−290°F) with an outside<br />

ambient temperature near 20.°C (68°F). This temperature range is too large for a single vapor-compression<br />

refrigeration cycle, because it requires a very large pressure ratio across the compressor. To solve this problem,<br />

we can connect (or cascade) two or more cycles together to form a cascade vapor-compression refrigeration cycle<br />

with lower individual compressor pressure ratios, as shown in Figure 14.17. This figure shows a double-cascade<br />

Q H<br />

3A<br />

Condenser A<br />

2A<br />

Cycle A<br />

W C − A<br />

4A<br />

3B<br />

Evaporator A<br />

Condenser B<br />

1A<br />

2B<br />

Cycle B<br />

W C − B<br />

4B<br />

Condenser B<br />

1B<br />

Q L<br />

T<br />

3B<br />

3A<br />

4 h A<br />

2 s A<br />

1A<br />

2A<br />

2 s B<br />

2B<br />

4 h B<br />

1B<br />

FIGURE 14.17<br />

A dual-cascade, vapor-compression refrigeration system with the same refrigerant used in each cycle.<br />

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