Difficulty: Easy
Correct Answer: high
Explanation:
Introduction / Context:
CO₂ (R-744) has attractive environmental credentials (ODP = 0, very low GWP for transcritical venting applications), but its thermodynamic characteristics impose design and efficiency challenges compared to many halocarbon refrigerants in traditional cycles.
Given Data / Assumptions:
Concept / Approach:
At common ambient temperatures, CO₂ systems often run transcritically with a gas cooler rather than a condenser. The absence of a condensation plateau and high pressure levels lead to higher compressor discharge temperatures and significant compressor power for a given cooling capacity.
Step-by-Step Solution:
Consider a medium-temperature application at 35–40 °C ambient.CO₂ discharge pressures can exceed 90 bar; optimal gas-cooler outlet temperature control is critical.For the same cooling load, compressor input tends to be higher than many conventional refrigerants unless advanced cycles (parallel compression, ejectors) are used.
Verification / Alternative check:
Performance maps show improved efficiency in cooler climates or with system enhancements; however, baseline systems typically exhibit higher kW/TR than legacy HFCs at warm ambients.
Why Other Options Are Wrong:
Common Pitfalls:
Assuming CO₂ is always inefficient; in cold climates or with modern architectures, efficiency can be competitive. The question asks about general non-use historically.
Final Answer:
high
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