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HKIE Professional Assessment - Chemical Discipline study materials
HKIE Professional Assessment - Chemical Discipline study resources

HKIE Professional Assessment - Chemical Discipline: Transport Phenomena

HKIE Professional Assessment - Chemical Discipline · Work through your exam, one topic at a time.

All guide sections

Transport Phenomena

Scope

Transport phenomena questions test the analogy between momentum, heat, and mass transfer, and whether you can select the right correlation and boundary condition. Expect pipe pressure drop, pump power, heat-exchanger duty, drying or absorption mass-transfer coefficients, and dimensionless-group reasoning.

Core rules

  • Reynolds number Re = rho u D / mu determines regime. Friction factor f comes from the Moody chart or Colebrook equation. For a circular pipe, pressure drop is Delta P = 4f (L/D)(rho u^2/2) when f is the Fanning factor; state which friction factor you use.
  • Heat transfer: Q = U A Delta T_lm. For counter-current flow, Delta T_lm = (Delta T1 - Delta T2) / ln(Delta T1 / Delta T2). Correct with F for multi-pass exchangers.
  • Mass transfer: N_A = k_c A (C_A - C_Ai) or k_y A (y_A - y_Ai). Dimensionless groups are Sh = k_c L / D_AB, Sc = mu / (rho D_AB), and Pr = Cp mu / k.
  • The Chilton-Colburn analogy j_D = j_H = f/2 links the three transport coefficients. It is a design aid, not a universal law.
  • For packed beds, the Ergun equation combines viscous and inertial losses; using a pipe correlation for a packed bed is a common error.
  • Boundary layers and entry lengths matter. Fully developed correlations fail in short pipes or near distributors.

Worked example

A counter-current exchanger has a hot stream cooling from 150 C to 90 C and a cold stream heating from 30 C to 70 C. Delta T1 = 150 - 70 = 80 K; Delta T2 = 90 - 30 = 60 K. Delta T_lm = (80 - 60) / ln(80 / 60) = 20 / 0.2877 = 69.5 K. If Q = 500 kW and U = 400 W/m2K, A = 500,000 / (400 x 69.5) = 18.0 m2. The log mean is closer to the smaller terminal difference. Using the arithmetic mean of 70 K would slightly under-size the exchanger. In co-current flow the same terminal temperatures cannot be achieved because the cold outlet is limited by the hot outlet.

Traps

  • Mixing Fanning and Darcy friction factors; a factor-of-four error follows.
  • Using LMTD when a phase change gives isothermal duty, or when the correction factor F is far below 1.
  • Ignoring fouling and wall resistance. U is not a material property.
  • Applying a correlation outside its Re, Pr, or L/D range.

Always carry a hand check on pressure drop or duty; simulation results without a scale check are weak evidence.