Small-tonnage hermetic refrigeration units In small commercial hermetically sealed machines (historical textbook context), which refrigerant has been most widely used?

Introduction / Context:
Refrigerant choice for compact hermetic systems balances safety, pressure levels, lubricant compatibility, and compressor design. Historically, one refrigerant dominated small sealed units in textbooks and past exams.

Given Data / Assumptions:

• Legacy perspective (pre-phaseout): domestic and light commercial hermetic units.
• Focus on commonly cited exam answers rather than today’s low-GWP replacements.

Concept / Approach:
R-12 provided moderate pressures, chemical stability, non-corrosiveness to most metals, and good miscibility with mineral oils—ideal for small hermetic compressors. Toxicity and very high operating pressures excluded ammonia and CO₂ from widespread domestic/small commercial hermetics of that era.

Step-by-Step Solution:
Compare safety/pressure: R-12 benign and moderate pressure; NH3 toxic; CO₂ very high pressure; SO₂ toxic/corrosive.Match to hermetic design constraints → R-12 fits best.

Verification / Alternative check:
Historical manufacturer catalogs and standard texts list R-12 for small sealed systems; many modern systems have shifted to R-600a or HFC/HFOs for environmental reasons.

Why Other Options Are Wrong:

• Ammonia and sulphur dioxide: safety/material issues in small hermetics.
• CO₂: high pressure hardware, not typical for small hermetics historically.
• Mixtures listed are nonstandard for such units.

Common Pitfalls:
Applying current refrigerant trends to legacy exam questions without noting the time frame.

Final Answer:
R-12 (dichlorodifluoromethane)