Which refrigerant is known for very high pressures and used in transcritical cycles?

• A. Ammonia
• B. R22
• C. R744
• D. R290

Answer: (C)

Transcritical cycles hinge on operating the high side above the refrigerant’s critical point, so the refrigerant never fully condenses in a conventional condenser. Carbon dioxide has a very low critical temperature (about 31°C) and reaches the high, very high pressures required in hot climates, so these systems use a gas cooler and run in transcritical mode. That combination—very high pressures plus transcritical operation—is why this refrigerant is known for both features. The other options don’t routinely operate under transcritical conditions: they have higher critical temperatures and standard subcritical cycles, with pressures that stay within typical ranges for their conventional condensers.