Chemical Engineering - MCQ Practice Questions

The Peclet number Pe = Re·Pr = (velocity × characteristic length × ρ·cp)/k represents the ratio of convection to conduction. When Pe << 1, conduction dominates over convection because advective transport is very slow compared to thermal diffusion.

The correction factor F (applied to LMTD) for 1-2 STHE typically ranges from 0.5 to 0.9 depending on temperature ratios and heat capacity ratios. F is always ≤ 1.

Grashof number (Gr = gβΔT L³/ν²) directly represents buoyancy to viscous force ratio. Rayleigh number (Ra = Gr·Pr) combines Grashof and Prandtl numbers for natural convection.

Radiative heat transfer follows Stefan-Boltzmann law: Q ∝ T⁴. If T increases by 10%, new flux = (1.1T)⁴ = 1.464T⁴ ≈ 46.4% increase.

Overall heat transfer coefficient is the reciprocal of total thermal resistance: 1/U = 1/(h₁A) + L/(kA) + 1/(h₂A). This accounts for series arrangement of convection and conduction resistances.

Critical heat flux (CHF) is the maximum heat flux in nucleate boiling. Beyond this point, further heat input causes transition to film boiling with lower heat transfer coefficient, leading to surface temperature rise (burnout).

Prandtl number = ν/α where ν = μ/ρ (momentum diffusivity) and α = k/(ρCp) (thermal diffusivity). Pr << 1 means heat diffuses faster than momentum; Pr >> 1 means momentum diffuses faster.

Adding shell passes (1-2 or 2-4 configuration) brings the temperature distribution closer to counterflow arrangement, increasing the correction factor F (reducing mismatch with LMTD). This increases effective heat transfer driving force without changing h significantly.