Refrigeration engineering: Among the common refrigerants R-11, R-12, R-22, and ammonia (NH3), which substance has the lowest freezing point (i.e., can remain fluid at the coldest temperature before solidifying)?

Introduction / Context:
When selecting a refrigerant, one important property is its freezing point. A lower freezing point reduces the risk of the refrigerant solidifying in very low-temperature evaporators, which would block passages and damage equipment.

Given Data / Assumptions:

• Candidates considered: R-11 (CCl3F), R-12 (CCl2F2), R-22 (CHClF2), and ammonia (NH3).
• We compare typical freezing points reported in standard property tables.
• No blending effects; pure substances are compared.

Concept / Approach:
The refrigerant with the most negative (lowest) freezing temperature offers the widest margin against solidification. Halocarbon refrigerants such as R-12 and R-22 have extremely low freezing points, often far below those of common natural refrigerants like ammonia.

Step-by-Step Solution:

Verification / Alternative check:
Cross-checking multiple handbooks shows R-22’s freezing point a few degrees below R-12 and far below ammonia’s −77.7 °C, confirming the selection.

Why Other Options Are Wrong:

• R-11: Significantly higher freezing point; not suitable for ultra-low evaporators.
• R-12: Very low, but still a few degrees higher than R-22.
• Ammonia: Excellent refrigerant but freezes near −78 °C, much higher than halocarbons.

Common Pitfalls:
Confusing boiling points with freezing points, or assuming ammonia must always be “best” because it is natural and efficient. Property selection must match the temperature range.

Final Answer:
R-22