Why vapour-compression systems use a throttle valve instead of an expander In a vapour compression refrigeration system, a throttle (expansion) valve is used in place of an ideal expander primarily because:

Introduction / Context:
Real vapour-compression systems expand refrigerant through a throttling device (capillary/TEV/EEV) rather than a turbine-type expander. Understanding why helps clarify practical system design and economics.

Given Data / Assumptions:

• Typical refrigerants at HVAC/R temperature lifts.
• Expansion occurs from condenser pressure to evaporator pressure.
• Two-phase region often encountered during expansion.

Concept / Approach:
While an isentropic expander could, in principle, recover some work and reduce throttling losses, the recoverable shaft work is small for common refrigerants and conditions. The mechanical complexity, cost, reliability issues, and two-phase nature of flow make expanders unattractive in standard systems.

Step-by-Step Solution:
Consider ideal expansion work: W_rec ≈ h_in − h_out (isentropic).For typical refrigerants, this enthalpy drop is modest across the valve.Balance recovered work against added capital, efficiency penalties, and maintenance.Conclusion: throttling is preferred; expander use is limited to specialized cycles.

Verification / Alternative check:
Exergy analysis shows throttling destroys availability, but the absolute value of recoverable work in standard HVAC ranges is too small to justify turbines; contrast with cryogenic plants where expanders are indeed used.

Why Other Options Are Wrong:

• Mass reduction and condenser size claims are not the primary drivers.
• Cost argument alone is incomplete without acknowledging the negligible work potential in typical applications.
• Expander does not increase evaporator pressure.

Common Pitfalls:
Assuming any reduction in throttling loss is automatically economical; ignoring two-phase expansion and erosion risks in micro-turbomachinery.

Final Answer:
The positive work obtainable from isentropic expansion across the valve is very small