Vapour-compression cycle temperature levels In a standard vapour compression refrigeration system, at which process point is the refrigerant at the lowest temperature during the cycle?

Introduction / Context:
The temperature profile around the vapour-compression loop determines component sizing and frosting risk. Recognizing the coldest point helps diagnose evaporator performance and subcooling needs.

Given Data / Assumptions:

• Idealized cycle with throttling (isenthalpic) expansion device.
• Negligible pressure drops in lines and heat exchangers.

Concept / Approach:
In throttling, the refrigerant undergoes a pressure drop with no work and no heat transfer (h_out ≈ h_in). The Joule–Thomson effect causes a drop in temperature when expanding from condenser pressure to evaporator pressure for typical refrigerants, producing the cycle’s lowest temperature at the expansion device outlet.

Step-by-Step Solution:
Compressor discharge is at high pressure and high temperature → not the minimum.Condenser rejects heat at near ambient condensing temperature → warmer than evaporator.Expansion valve outlet: sharp pressure drop yields the coldest state.Evaporator then absorbs heat, raising the refrigerant quality/temperature toward suction.

Verification / Alternative check:
On a T–s diagram, the throttling step is a vertical line with temperature decreasing to the evaporating level, the minimum point of the loop.

Why Other Options Are Wrong:

• Compression raises temperature.
• Condensation occurs near ambient plus approach temperature.
• Within the evaporator, temperature increases slightly along the flow as heat is absorbed.

Common Pitfalls:
Confusing the evaporator outlet (warmer) with the expansion device outlet (coldest).

Final Answer:
Immediately after expansion/throttling (expansion device outlet)