Refrigerant selection — desirable thermophysical properties Which combination best characterizes a good refrigerant for vapor-compression systems from a thermodynamic and practical standpoint?

Introduction / Context:
Refrigerant choice influences system size, efficiency, safety, and environmental impact. Desired properties simplify compressor design, reduce mass flow, and ensure reliable evaporator performance without risk of freezing under low-temperature duty.

Given Data / Assumptions:

• Focus on thermodynamic desirables; safety and environmental criteria are acknowledged but not the subject of the options.
• Single-component refrigerants as conventional examples.

Concept / Approach:
A high latent heat allows a given cooling capacity with a smaller mass flow rate, reducing compressor displacement and piping size. A low freezing point ensures liquid refrigerant and brine mixtures do not solidify at the evaporator temperatures. Moderate pressures, adequate critical temperature, chemical stability, and compatibility are also important, but not offered among the options.

Step-by-Step Solution:
Assess latent heat: higher is better for capacity per unit mass.Assess freezing point: lower is better to avoid solidification in low-temperature applications.Reject high specific volume because it increases compressor size for the same mass flow.Reject “low COP” as undesirable; engineers seek high COP.

Verification / Alternative check:
Capacity equation Q_dot = m_dot * h_fg shows direct benefit from large h_fg; practical refrigerant tables corroborate that higher latent heat often correlates with lower volumetric flow for a given duty.

Why Other Options Are Wrong:
(b) High operating pressure stresses components and increases leakage risk. (c) High specific volume increases compressor swept volume. (d) Low COP is never desirable. (e) Low latent heat yields poor capacity per mass flow; “high critical temperature” alone does not offset that.

Common Pitfalls:
Focusing on a single property; real selection balances thermodynamics, safety, environmental impact, and availability.

Final Answer:
high latent heat of vaporisation and low freezing point