Which refrigerant is associated with very high pressures in some systems?

• A. Ammonia
• B. R134a
• C. R744
• D. R404A

Answer: (C)

Carbon dioxide (R-744) is known for operating at exceptionally high pressures in some refrigeration systems. Its critical pressure is about 73 bar, and with a relatively low critical temperature (31.1°C), CO2 systems—especially transcritical ones—can push the high-side pressure well above what most other refrigerants experience. That combination means very high pressures are a characteristic feature of many CO2 systems, requiring components and safety devices designed for those pressures.

In contrast, the other refrigerants listed typically operate at much lower high-side pressures in their ordinary applications, so they aren’t associated with those extreme pressure levels.