Refrigerant property check: What is the freezing (melting) point of sulphur dioxide (SO₂) used in legacy refrigeration systems?
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A-56.6°C
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B-75.2°C
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C-77.7°C
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D-135.8°C
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E-10.0°C
Answer
Correct Answer: -75.2°C
Explanation
Introduction / Context:Sulphur dioxide (SO₂) is an older inorganic refrigerant historically used before modern halo-carbons and ammonia dominated. Knowing key thermophysical points like freezing (melting) temperature is important for safe operation and to avoid blockages in expansion devices at low temperatures.
Given Data / Assumptions:
- Standard atmospheric pressure conditions when quoting freezing point.
- Pure SO₂ (no mixture effects).
- Freezing point refers to the solid–liquid equilibrium temperature.
Concept / Approach:The freezing point is a fixed material property at 1 atm and is obtained from standard thermophysical tables. Below this temperature, SO₂ solidifies, which is hazardous for valves and capillary tubes in refrigeration circuits.
Step-by-Step Solution:
Consult reliable property data: SO₂ freezing point ≈ −75.2°C.Interpretation: Systems approaching this temperature risk solid formation if local pressure and composition bring the state to the solid–liquid curve.Operational takeaway: Select evaporator temperatures safely above the freezing point or use refrigerants suited to the required low temperature.Verification / Alternative check:Cross-check with chemical handbooks and refrigeration property charts listing SO₂ melt point near −75°C.
Why Other Options Are Wrong:
- −56.6°C and −77.7°C are not the standard melt points for SO₂ at 1 atm.
- −135.8°C is more characteristic of far colder substances; not SO₂.
- −10°C is close to SO₂ normal boiling point (about −10°C), not the freezing point.
Common Pitfalls:Confusing boiling and freezing points; both are negative for SO₂ but differ by tens of degrees.
Final Answer:-75.2°C