Refrigerant property recall — normal boiling point of R-12 (dichlorodifluoromethane) Is the approximate normal boiling point of R-12 about −30 °C under 1 bar atmospheric pressure?
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ATrue
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BFalse
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CTrue only at reduced pressure
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DFalse, it is about −10 °C
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EFalse, it is above 0 °C
Answer
Correct Answer: True
Explanation
Introduction / Context:Older vapor-compression systems frequently referenced R-12 (CCl2F2) in teaching materials. Knowing its normal boiling point helps understand why it was suitable for subzero evaporator temperatures at moderate suction pressures.
Given Data / Assumptions:
- Normal boiling point refers to saturation at approximately 1 bar (standard atmospheric pressure).
- R-12 property value commonly cited near −29.8 °C.
Concept / Approach:A refrigerant with a low normal boiling point can evaporate at low temperatures while maintaining manageable pressures, enabling efficient refrigeration for comfort and medium-temperature applications.
Step-by-Step Solution:
Recall property: T_b,normal for R-12 ≈ −29.8 °C.Compare to the statement “about −30 °C.”Conclusion: the statement is correct within engineering rounding.Verification / Alternative check:Reference pressure–temperature charts for R-12 show saturation around −30 °C at approximately 1 bar absolute.
Why Other Options Are Wrong:Values like −10 °C or above 0 °C would imply much higher saturation temperatures inconsistent with known R-12 data; restricting to “reduced pressure only” misunderstands the definition of normal boiling point.
Common Pitfalls:Confusing R-12 with R-22 or R-134a, which have different boiling points; always confirm the specific refrigerant.
Final Answer:
True