In a standard mechanical vapor-compression refrigeration system, the refrigerant in the liquid line between the condenser outlet and the expansion valve is predominantly in which phase?

Introduction / Context:
Understanding the phase of refrigerant at different points in the vapor-compression cycle is essential for diagnostics, design, and safety. The line from the condenser to the expansion device is commonly called the liquid line. This question verifies the correct phase there in normal operation.

Given Data / Assumptions:

• System is operating properly with a condenser that rejects heat to ambient.
• There may be a small amount of subcooling at the condenser exit to ensure pure liquid reaches the expansion device.
• No flash gas in the liquid line under normal conditions.

Concept / Approach:
In the condenser, refrigerant rejects latent heat and condenses from vapor to liquid. Additional sensible cooling below saturation temperature (subcooling) improves system reliability by preventing premature flashing before the expansion valve. Therefore, the correct description is “subcooled (or saturated) liquid.”

Step-by-Step Solution:

Verification / Alternative check:
Field practice uses sight glasses and subcooling measurements to confirm liquid quality at the expansion valve inlet. Presence of flash gas indicates insufficient subcooling or excessive pressure drop, confirming that the normal/ideal state is pure liquid.

Why Other Options Are Wrong:

• Superheated vapor / dry saturated vapor: These occur at compressor outlet or evaporator outlet, not in the liquid line.
• Wet mixture: Undesirable in the liquid line; leads to maldistribution and expansion valve instability.
• Supercritical fluid: Requires pressures above the critical point (not typical for common HVAC conditions).

Common Pitfalls:
Confusing evaporator outlet superheat with liquid-line conditions; assuming two-phase flow is normal before the valve—it is not ideal.

Final Answer:
Subcooled (or saturated) liquid