In refrigeration engineering, what is the normal atmospheric-pressure boiling point of ammonia (NH3) used as a refrigerant?

Introduction / Context:
Ammonia (NH3) is a classic refrigerant used in industrial refrigeration because of its high latent heat and excellent thermodynamic performance. Knowing its saturation (boiling) temperature at standard atmospheric pressure is a foundational property needed for cycle calculations, selection of evaporating pressures, and safety considerations.

Given Data / Assumptions:

• Pressure is approximately 1 atm (101.325 kPa).
• Boiling point refers to saturation temperature where liquid and vapour coexist.
• Pure anhydrous ammonia is considered (no mixture effects).

Concept / Approach:
The boiling point at a specified pressure is the saturation temperature at which the liquid begins to change phase to vapour. For refrigerants, property tables or p–T charts provide this value. Engineers use these data to pick evaporator temperatures and to estimate compressor suction conditions.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check multiple standard references (refrigerant tables or ASHRAE data) which consistently list ~ -33°C at 1 atm for ammonia.

Why Other Options Are Wrong:

• -10.5°C and -30°C are too warm; ammonia would still be saturated vapour at 1 atm around these temperatures.
• -77.7°C is closer to some cryogenic substances, not ammonia.
• 0°C is the freezing/boiling reference for water, not NH3.

Common Pitfalls:
Confusing boiling point at 1 atm with saturation temperature at the system’s operating pressure; in closed cycles the boiling point shifts with pressure.

Final Answer:
-33.3°C