Duct design & air distribution on the PE Mechanical exam

Once you've got the load and the airflow, the air has to get there — and duct problems test whether you can size systems and account for the pressures a fan has to overcome. It's a bounded, formula-driven topic that rewards keeping static, velocity, and total pressure straight.

What you actually need to own

• Total pressure = static pressure + velocity pressure. Velocity pressure (in. w.g., standard air): VP = (V / 4005)^2, with V in ft/min. Keep these three pressures distinct — most mistakes start by blurring them.
• Friction loss along a duct run — from the duct friction chart (or Darcy with an equivalent diameter), driven by airflow, duct size, and length. Equivalent diameter converts a rectangular duct to a round one of equal friction.
• The two sizing methods:
  • Equal friction — hold a constant friction rate (in. w.g. per 100 ft) along the system; simple and common.
  • Static regain — size so that the static pressure recovered as velocity drops offsets downstream losses; gives more balanced pressures in larger systems.
• Fitting and dynamic losses (elbows, transitions, dampers) via loss coefficients applied to velocity pressure.
• System total pressure = the sum along the critical (longest/highest-loss) path — that's what sets the fan selection. (See pumps & fans for the fan side.)

Where people lose points

• Confusing static, velocity, and total pressure — the single most common error.
• Forgetting fitting/dynamic losses and sizing only on straight-duct friction.
• Adding losses on the wrong (non-critical) path instead of the governing run.
• Unit slips in velocity pressure (ft/min vs the 4005 constant for standard air).

How to study it

Practice tracing the critical path of a system and summing static + velocity + fitting losses to a total — that's the skill the exam rewards. Do enough equal-friction and a couple of static-regain problems that the method choice and the pressure bookkeeping are automatic, then connect the result to the fan curve.

Part of the HVAC & Refrigeration exam. → Start your 7-day free trial — no card.