Introduction to Heat Exchangers

A: The hard boundary here is the log term. ΔT_hot-in/cold-out is 160−55 = 105°C, while ΔT_hot-out/cold-in is 90−30 = 60°C. LMTD = (105−60)/ln(105/60) ≈ 71°C. Arithmetic averaging ignores the logarithmic temperature driving force, and the correction factor comes later, not inside the base LMTD.

A: The number that bites is the duty scale. Q ≈ 50,000/3600×2,200×80 ≈ 2.4 MW. Area ≈ 2.4×10⁶ /(250×30) ≈ 320 m², so a few hundred square metres fits. Dropping the hour-to-second conversion or misusing the sensible rise as LMTD skews the estimate by an order.

A: The non-negotiable limit is end-of-run performance. API 660 assumes refinery services foul unpredictably, and the exchanger still has to meet duty before turnaround. ASME VIII governs pressure integrity, not heat transfer, and TEMA class affects mechanical robustness, not mandatory fouling allowances.

A: The threshold here is chloride plus oxygen at moderate temperature. Austenitic SS fails by SCC under tensile stress long before uniform wall loss appears. Sulfidation and naphthenic acid attack need sulfur species or much higher metal temperatures.