At the suction of a refrigeration compressor, the working fluid should be in which state to avoid liquid slugging and ensure reliable operation?

Introduction / Context:
Compressor reliability depends critically on the phase of the refrigerant at suction. Liquids are essentially incompressible; entrained liquid droplets can damage valves and pistons or erode impellers. Thus, proper superheat at the evaporator outlet/suction line is a standard design and service requirement.

Given Data / Assumptions:

• Vapor-compression system with thermostatic or electronic expansion device.
• Evaporator outlet provides some degrees of superheat for control and compressor protection.

Concept / Approach:
The correct suction state is slightly superheated vapor. This ensures zero liquid carryover and gives the expansion device a controllable signal (superheat) while avoiding excessive discharge temperatures that would result from too much superheat.

Step-by-Step Solution:

Verification / Alternative check:
Service manuals specify minimum superheat (e.g., 5–10 K) at compressor suction to prevent flooding; this corroborates the chosen option.

Why Other Options Are Wrong:

• Dry saturated liquid / subcooled liquid: Liquids at suction would cause slugging and damage.
• Two-phase mixture: Also risky; indicates improper evaporator control or overfeeding.
• Supercritical fluid: Not relevant at typical HVAC pressures/temperatures.

Common Pitfalls:
Over-superheating, which can elevate discharge temperatures excessively; the goal is slight, controlled superheat.

Final Answer:
Superheated vapour refrigerant