Heat Transfer and Heat Exchange Equipment
Heat transfer spans conduction, convection and radiation plus exchanger design. Key topics: Fourier's law and thermal resistance networks (plane wall, cylindrical shell); convective coefficients (Dittus-Boelter, Nu = hD/k); natural vs forced convection; radiation (Stefan-Boltzmann, emissivity, view factors); overall heat transfer coefficient U (including fouling and wall resistance); log mean temperature difference (LMTD); effectiveness-NTU method; and exchanger types (shell-and-tube, plate, double-pipe).
Traps: using arithmetic mean ΔT instead of LMTD when the temperature difference varies significantly; forgetting fouling factors in U; confusing parallel vs counterflow (counterflow gives higher effectiveness); and misidentifying which stream controls the overall coefficient. Questions often test whether you know that the controlling resistance dominates U.
Worked example (rule of thumb): 1/U = 1/h_i + R_f,i + t/k + R_f,o + 1/h_o. If one term is 10× larger than the others, U ≈ 1/(that term). For LMTD: ΔT_lm = (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂). For a quick check, if ΔT₁/ΔT₂ < 2, the arithmetic mean is a fair approximation.