Air Distribution
This domain tests how air actually moves through the system: fan performance, static pressure budgets, and the delivery characteristics of terminals and diffusers.
Core rules
- Fan laws: CFM varies with RPM, static pressure with RPM squared, brake horsepower with RPM cubed, at constant air density.
- The operating point is where the fan curve crosses the system curve. Adding restriction moves along the fan curve to lower flow.
- External static pressure includes ductwork, filters, coils, terminal boxes and accessories, but not the internal cabinet losses listed by the manufacturer.
- Diffuser performance is described by throw, drop, spread, terminal velocity and noise criteria (NC). High face velocity means noise and drafts.
- Balance by adjusting dampers proportionally, measuring at the terminal, not by guesswork.
Worked example
A fan delivers 10,000 CFM at 1.0 in. w.c. external static and 3.0 brake horsepower at 1,000 RPM. Speed is reduced to 900 RPM. New CFM = 10,000 x 0.9 = 9,000. New static = 1.0 x 0.9 squared = 0.81 in. w.c. New power = 3.0 x 0.9 cubed = 2.19 bhp. This is why variable-speed control saves so much energy: a 10 percent speed reduction cuts power by about 27 percent.
Traps that cost marks
- Adding the static drops of parallel branches instead of using the highest-resistance path.
- Comparing measured external static to a total static rating.
- Taking static readings in turbulent airflow right at the fan outlet.
- Ignoring filter loading, dirty coils and closed dampers when diagnosing low airflow.
- Balancing by closing one branch hard, which shifts noise and flow elsewhere.