Exergy In Engineering Thermodynamics by PK NAG (Chapter 08)

A: The threshold is ln(p/p0). For ideal gas exergy at equal temperature, e ≈ RT0 ln(p/p0). With R ≈ 0.287 kJ/kg·K, T0 = 300 K, ln(8) ≈ 2.08, giving ~180 kJ/kg. Temperature equality doesn't kill pressure exergy.

A: The controlling idea is invariance of total destruction. If each vendor picks a different T0 and p0, irreversibility appears to move between exchangers and turbines. Standards force a single reference so comparisons mean something.

A: The key number is the temperature approach. Exergy destruction in heat transfer scales with finite ΔT. Smaller approaches mean less entropy generation, even if CAPEX and area rise.

A: The boundary is throttling. Relief keeps pressure within MAWP but destroys pressure exergy almost completely. Safety is addressed; thermodynamic usefulness is sacrificed.

A: The pressure ratio is the threshold. Throttling conserves h but generates entropy, wiping out most pressure exergy. For large ratios like 10:1, destruction is of the same order as inlet exergy.

A: The limiting number is temperature level. High-grade chemical exergy in fuel is downgraded to lower-temperature steam. Energy is conserved; ability to do work is not.

A: The threshold is turbine inlet temperature. Gas turbines operate at far higher T, extracting work before large entropy generation. Rankine cycles suffer exergy loss in the boiler.

A: The controlling parameter is temperature. Poor cooling raises T, increasing irreversibility in compression. Flow and bearings are secondary effects, not the main exergy hit.

A: The hard boundary is the dead state. Reaction exergy destruction includes both thermal and mixing effects. Different reference compositions would distort where losses actually occur.