Thermodynamics of compression: what happens when a saturated vapor is compressed at constant temperature or adiabatically near saturation?

Introduction / Context:A saturated vapor lies on the phase boundary between single-phase vapor and two-phase liquid–vapor. Understanding its response to compression is vital in steam systems, refrigeration, and phase-equilibrium calculations. The correct qualitative outcome distinguishes design-safe assumptions from incorrect ones that can damage equipment.

Given Data / Assumptions:

• Start at saturation (vapor quality = 1.0, no liquid present).
• Compression increases pressure.
• Temperature path may be isothermal (at saturation T) or near-adiabatic but remaining within two-phase region initially.

Concept / Approach:At saturation, any compression without adding heat to superheat will move the state into the two-phase region on a P–v or T–s diagram. The vapor begins condensing, yielding a mixture of liquid and vapor—often referred to as “wet steam.” Therefore, both “it condenses” and “it forms wet steam” describe the same physical outcome of entering the two-phase envelope.

Step-by-Step Solution:

Verification / Alternative check:Steam tables show that at a given saturation temperature, higher pressure corresponds to a saturated liquid state; compression at constant T thus necessitates condensation.

Why Other Options Are Wrong:

• (d) Incorrect: condensation and wetness are expected.
• (e) Incorrect: superheating requires raising temperature above saturation, not mere compression at saturation.

Common Pitfalls:Assuming all compression heats the gas enough to superheat; near saturation, compression pushes the vapor into the two-phase region unless heat is also added to maintain superheat.

Final Answer:Both (a) and (b).