D1 · HVAC and Refrigeration Distribution and Systems

Foundation lesson · 25 minute study session

Follow the controlling air path

Duct branches do not all carry the same air or require their losses to be added together. Trace the actual route and define the pressure quantity before selecting a fan.

By the end of this lesson

Use continuity to size a duct cross section.
Distinguish parallel branches from one pressure-loss route.
Separate static, velocity and total pressure.

Continuity comes first

Duct volume flow equals cross-sectional area times mean velocity. Use consistent length units. A rectangular duct dimension in inches must be converted before area is compared with a velocity in ft/min. Flows add where branches merge; the same steady flow passes along a series segment without leakage. Convert dimensions before forming $A$ , then use $v=\dot V/A$ .

Build a pressure budget

A route contains duct friction, fittings, dampers, terminals and other components. For parallel branches, calculate each complete route and use the controlling requirement; summing all parallel branch losses does not represent a real air path. Balance lower-resistance branches as required by the system design.

Match the fan pressure basis

Total pressure is static pressure plus velocity pressure. Fan static and fan total pressure are distinct rating quantities. Fan selection must use the pressure definition and test configuration supplied by the question. A higher-efficiency filter can change pressure loss, shifting the operating point.

A decision map for this lesson

Read the route, then size the section

A shared loss belongs to every route. Parallel branch losses are compared, rather than added as one series path.

A realistic fan and duct assembly shows a common supply trunk dividing into three separate branches with balancing dampers. — **Find the controlling route**Connect the equipment to the calculation

A shared duct loss of 0.30 inches water serves three parallel routes with losses 0.60, 0.85 and 1.10. Complete-route losses are 0.90, 1.15 and 1.40 inches water; route C controls at the stated design flows. — **Find the controlling route**Connect the equipment to the calculation

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
A duct cross section is requested	Use $A=\dot V/v$ , then convert dimensions to one consistent length basis.	Putting an inch dimension directly into a square-foot area calculation.
The fan must serve several parallel routes	Compare each complete route with its shared upstream/downstream losses included.	Adding every parallel branch drop in series.
Total and velocity pressure are supplied	Use $p_s=p_t-p_v$ on the same pressure datum.	Adding velocity pressure to total pressure to obtain static pressure.
An inlet elbow distorts the fan flow	Check installation system effect against the catalog inlet conditions.	Assuming a catalog curve remains valid after the inlet conditions change.

On a narrow screen, scroll across the table to compare the decision and the error.

Relationships and their limits

Duct continuity

\dot V=Av;\qquad A=\dot V/v

V in ft³/min, A in ft², v in ft/min.

Pressure relationship

p_t=p_s+p_v

Use matching pressure units and the specified measurement location.

Symbols and unit basis
Symbol	Meaning	Units or basis
$\dot V$	Volumetric airflow	cfm
$A$	Duct cross-sectional area	ft²
$v$	Mean air velocity	ft/min
$p_t$	Total pressure	stated pressure units
$p_s$	Static pressure	same pressure units
$p_v$	Velocity pressure	same pressure units

One decision, fully worked

Apply the method.

A duct carries 3,000 cfm. The design velocity limit is 1,000 ft/min. A rectangular section has a fixed internal width of 24 in. Find the minimum internal height ignoring available stock sizes.

Divide airflow by allowable velocity to obtain required area.
Convert the fixed width to 2 ft.
Divide area by width, then convert height to inches.

A=3{,}000/1{,}000=3\ \mathrm{ft^2}

H=3/2=1.5\ \mathrm{ft}=18\ \mathrm{in}

Decision

The minimum idealized internal duct size is 24 in by 18 in.

Independent check

The area is 432 in², or 3 ft²; 3,000/3 returns 1,000 ft/min.

Specialist extension

Translate ideal diffuser data into a usable check

A flow-area calculation establishes velocity through the specified area, not occupant-zone comfort or throw. Distinguish nominal face area, free area and the manufacturer’s effective area. Check the supplied throw, terminal velocity, layout and acoustic criteria independently rather than inferring them from a velocity alone.

A new applied case

A diffuser has 2.0 ft² face area and a supplied free-area fraction of 0.60. It handles 720 cfm. Find average velocity through free area.

A_f=2.0(0.60)=1.2\ \mathrm{ft^2}

v_f=720/1.2=600\ \mathrm{ft/min}

Decision. The free-area velocity is 600 ft/min.

Cross-check. Using face area alone gives 360 ft/min and addresses a different area basis. Neither number alone proves acceptable throw.

Unit basis. Area is ft²; free-area fraction is dimensionless; volume is cfm; velocity is ft/min.

NCEES PE Mechanical Reference Handbook v2.1; see the module reference path.

Open targeted practice in the study studio →

Transfer the method

HVAC and Refrigeration Distribution and Systems · HV-LC-D1

Try a new decision.

Select one

A circular duct carries 3,600 cfm at an average velocity of 1,800 ft/min. Ignore wall thickness. The required inside diameter is most nearly:

A12.0 in.B27.1 in.C19.2 in.D19.2 ft

A lesson check supports practice, not a readiness score. Your response stays in this page session and clears on reload.

Catch the common errors

Treating square inches as square feet.
Adding losses of separate parallel branches together.
Substituting fan static pressure for total pressure without checking the definition.

Handbook navigation drill

Find it, then explain it.

Search: Duct Design

Locate duct friction and fitting information, then identify the input flow, geometry and pressure units.

Check your navigation

You can trace one complete route and distinguish its losses from those of other branches.

Reference path: §9.3.6–9.3.7, pp. 489–515. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Theory sources

NCEES handbook v2.1, §§9.3.6–9.3.8.
Grondzik and Kwok, Mechanical and Electrical Equipment for Buildings, 12th ed., §§12.9 and 12.11.

Worked examples and lesson prose are original ReasonSpan^™ material. Obtain authorized publications separately; this preview does not reproduce the supplied books or handbook.