CSLB C-20 HVAC ContractorVentilation and Air DistributionHard
A duct system operating at 1,200 CFM produces 0.60 in. w.c. of total external static pressure. If airflow is increased to 1,400 CFM through the same unchanged duct system, what is the new approximate static pressure (fan affinity law: pressure varies with the square of the CFM ratio)?
- A0.60 in. w.c.
- B0.70 in. w.c.
- C0.82 in. w.c.
- D1.02 in. w.c.
Show answer & explanationAnswer & explanation
Correct answer: C. 0.82 in. w.c.
Ratio = 1400/1200 = 1.167. Squaring gives 1.361. New pressure = 0.60 × 1.361 ≈ 0.82 in. w.c. Static pressure rises with the square of airflow when the duct system is unchanged.
Why the other options are wrong
- A. Assumes no change, ignoring the affinity relationship.
- B. Uses a linear ratio instead of squaring it.
- D. Overestimates by using the cube relationship instead of the square.
Fan Affinity Laws
For a fixed duct system, static pressure varies with the square of the airflow ratio, while horsepower varies with the cube.
- CFM ratio ∝ RPM ratio.
- Pressure ratio = (CFM ratio)².
- Horsepower ratio = (CFM ratio)³.
Memory trick: Pressure squares, power cubes, speed stays linear.