Desirable refrigerant properties for vapour-compression systems include which of the following?
-
ALow normal boiling point (to allow low evaporating temperatures)
-
BHigh critical temperature (to ease condensation at moderate pressures)
-
CHigh latent heat of vaporization (to reduce mass flow rate)
-
DAll of these
-
ENone of these
Answer
Correct Answer: All of these
Explanation
Introduction / Context:Selecting a refrigerant involves balancing thermodynamic, environmental, safety, and materials considerations. Thermodynamically favorable properties help reduce compressor work, heat-exchanger size, and operating pressures.
Given Data / Assumptions:
- Single-stage vapour-compression cycle context.
- Ambient condenser cooling near 25–45°C.
- Safe and compatible materials assumed separately from thermodynamics here.
Concept / Approach:Low boiling point permits low evaporator temperatures without excessive vacuum. A high critical temperature allows condensation at practical temperatures and pressures. High latent heat reduces required mass flow for a given capacity, improving efficiency and component sizes.
Step-by-Step Solution:
Relate low boiling point to attainable coil temperatures and pressure levels.Relate high critical temperature to condenser approach temperature and head pressure.Relate high latent heat to smaller compressors and heat exchangers for the same tonnage.Combine these to see that each property is beneficial; hence “All of these.”Verification / Alternative check:Compare common refrigerants: ammonia has high latent heat; R-134a has favorable pressures and critical temperature; CO₂ has very low critical temperature making transcritical operation necessary in warm climates—illustrating the importance of T_critical.
Why Other Options Are Wrong:Each single item is indeed desirable; choosing any one misses the holistic requirement. “None” contradicts standard design guidance.
Common Pitfalls:Ignoring non-thermodynamic factors (toxicity, flammability, GWP, ODP) which also affect refrigerant choice.
Final Answer:All of these