Liquid refrigerant cooled below its saturation temperature after condensation but before throttling is said to be undergoing:

Introduction / Context:
Sub-cooling is a common enhancement in vapour-compression systems. Cooling the condensed liquid refrigerant below the saturation temperature at condenser pressure improves system capacity and reduces throttling losses.

Given Data / Assumptions:

• Refrigerant exits condenser as saturated liquid at condenser pressure.
• A sub-cooler or liquid line heat exchanger removes additional sensible heat.
• Expansion device follows (valve/capillary).

Concept / Approach:
Sub-cooling means T_liquid < T_sat at the same pressure. This ensures the refrigerant entering the expansion device is fully liquid, minimizing flash vapour before the evaporator and increasing the refrigeration effect per unit mass.

Step-by-Step Solution:

Verification / Alternative check:
h_subcooled < h_sat, so refrigeration effect (h_evap.out − h_exp.out) increases. P-h diagrams clearly show the benefit.

Why Other Options Are Wrong:
“Super-cooling” is non-standard wording; “normal cooling” is vague; “flash gas formation” occurs after throttling, which sub-cooling helps to reduce; “wet compression” refers to liquid carryover into the compressor and is undesirable.

Common Pitfalls:
Confusing sub-cooling (liquid below T_sat) with superheating (vapour above T_sat) and misplacing the process relative to the expansion valve.

Final Answer:
sub-cooling or under-cooling