Effect of superheating in a vapour-compression refrigeration cycle How does adding suction superheat (beyond dry saturation at evaporator exit) generally affect the coefficient of performance (C.O.P.)?

Introduction / Context:
Superheating in vapour-compression systems can occur intentionally in the evaporator or unintentionally in the suction line. Its effect on performance depends on how it influences refrigerating effect and compressor work.

Given Data / Assumptions:

• Suction superheat occurs after the refrigerant is dry saturated at the evaporator exit.
• Pressure drops are modest; comparison made at the same evaporating and condensing pressures.
• Compressor is idealized with isentropic compression for the conceptual discussion.

Concept / Approach:
Superheating generally increases the refrigerant enthalpy at the compressor inlet, thereby increasing the refrigerating effect per kg (h1 − h4) because h1 rises while h4 (post-expansion enthalpy) is almost unchanged. Compressor work per kg (h2 − h1) also rises, but in many refrigerant–condition combinations the percentage increase in refrigerating effect is greater than the percentage increase in work, leading to a net increase in C.O.P. = (refrigerating effect)/(compressor work).

Step-by-Step Solution:

Verification / Alternative check:
Plot points on a p-h diagram for a common refrigerant (e.g., R-134a). Moving the suction point into the superheat region increases the area representing RE more than the increase in W for many practical cases.

Why Other Options Are Wrong:

No change: contradicts enthalpy changes.Decreases: can happen for extreme suction-line superheat with large pressure drops, but not as a general rule under controlled evaporator superheating.“Work to zero” is impossible; compression requires work.

Common Pitfalls:
Confusing evaporator superheat (beneficial capacity gain) with suction-line heat pickup (which may hurt C.O.P.). Proper design ensures controlled superheat at the evaporator outlet with minimal suction-line heat gain.

Final Answer:

Increases C.O.P.