Duct Airflow and Velocity Calculator¶

Run the Duct Airflow and Velocity Calculator MicroSim Fullscreen

To run this sketch in the p5.js editor, first add the kit files it uses (fanpedia-sim.css, fanpedia-core.js, fanpedia-p5.js, fan-models.js and fan-drawings.js from ../shared-libs/, or their published URLs under https://moroshek.github.io/FanTASTIC-textbook/sims/shared-libs/) to the editor's index.html, then paste ch01-duct-airflow-velocity-calculator.js as the sketch.

About This MicroSim¶

This calculator gives practice with the airflow equation \( Q = V \times A \) on the kinds of ducts that appear on drawings. Duct sizes come in inches, but the equation needs the flow area in square feet. A rectangular duct's area is \( (W \times H) \div 144 \) ft², and a round duct's area is \( \pi D^2 \div 576 \) ft².

The sim opens with four fixed challenges. Each one asks for a single number:

#	Given	Asked	Model answer
1	2,400 CFM in a 16 in × 12 in rectangular duct	Velocity	1,800 ft/min
2	5,000 CFM in a 24 in round duct	Velocity	1,592 ft/min
3	A 20 in × 10 in rectangular duct at 1,500 ft/min	Airflow	2,083 CFM
4	10,000 CFM with velocity no higher than 2,000 ft/min	Minimum flow area	5.00 ft²

Challenge 1 uses the machine-shop worked example from Chapter 1. Challenges 2 to 4 are illustrative values, and the sim labels them that way.

An answer within ±2% of the model value is correct. Each challenge allows two attempts. A non-numeric or empty entry gets "Enter a number" and does not use an attempt. A correct answer shows "✓ Correct: model value is …"; a wrong one shows ✗ with the challenge's worked feedback, and a second wrong answer reveals the model value and counts as missed. After every check the sim shows your number beside the model value, lays out the worked calculation, and highlights the step where your number departs from it. It recognizes common slips: forgetting to divide by 144, treating a round duct as a square of side D, using π D² ÷ 144, multiplying where you should divide, and giving the area in square inches. A running count "Challenges correct: n of 4" stays on screen. Mastery is 4 of 4 correct, with at most two attempts each.

Explore mode (the free mode) unlocks after challenge 4 and is never scored. You can change the duct shape, width, height or diameter, and the airflow. The flow area (3 decimals), the velocity (to the nearest 10 ft/min) and the substituted equation update immediately. The first time you enlarge the duct while the airflow stays fixed, the sim points out that the CFM did not change and only the velocity fell. Explore mode also shows your challenge results and the mastery line, and Start over runs the four challenges again.

How to Use¶

Read challenge 1 and calculate the velocity on paper or a calculator.
Type the number in the Your answer box and press Check (or Enter).
Read the feedback and the highlighted step. If you have an attempt left, correct your work and try again.
Press Next challenge to continue. After challenge 4, press Start explore mode and read the mastery line.
In Explore mode, set 2,400 CFM in a 16 in × 12 in duct, then change it to 24 in × 16 in. Velocity falls from 1,800 to 900 ft/min while the airflow stays at 2,400 CFM. Explore mode is never scored.
Press Start over to work the four challenges again.

Keyboard: Enter checks the answer; after a challenge is answered, Enter goes to the next one. Tab reaches every control, and the arrow keys move a focused slider.

Data label: "Values from the chapter's machine-shop worked example" (challenge 1) or "Illustrative values" (challenges 2 to 4), shown at the bottom left of the drawing.

Iframe Embed Code¶

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://moroshek.github.io/FanTASTIC-textbook/sims/ch01-duct-airflow-velocity-calculator/main.html"
        height="672px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan¶

Learning Objective¶

The learner will calculate air velocity, airflow volume or required flow area from the other two quantities for rectangular and round ducts, to within 2 percent of the model value (Bloom: Apply, verb: calculate).

Audience¶

Adult professional development: new application engineers, sales staff, representatives and facilities staff.

Duration¶

10-15 minutes

Prerequisites¶

Airflow volume in cubic feet per minute (CFM)
Air velocity in feet per minute
Flow area in square feet and the equation \( Q = V \times A \)
Converting duct dimensions in inches to square feet

All are covered in the Chapter 1 sections "Airflow Volume and Cubic Feet Per Minute" and "Air Velocity".

Activities¶

Challenges (8 min): Work the four challenges in order. Commit to a number before checking. Two attempts each.
Explore mode (5 min): Hold the airflow at 2,400 CFM and enlarge the duct. Then switch to a round duct and find the diameter whose area is closest to a 16 in square. It is not 16 in.
Discussion (2 min): Why does a bigger duct not move more air when the fan delivers the same CFM?

Common Misconceptions Addressed¶

Dividing CFM by the duct dimensions in inches without converting to square feet (challenge 1).
Believing a bigger duct moves more air at the same fan airflow. In fact the CFM is the same and only the velocity drops (Explore mode note).
Believing a round duct of diameter D has the same area as a square duct of side D (challenge 2).

Assessment¶

Running score shown as "Challenges correct: n of 4"; mastery is 4 of 4 challenges correct within ±2%, with at most two attempts each. The sim shows "Mastery reached (4 of 4 needed)." or what is still needed.
Explore mode is not evidence of mastery. Follow up: why does a bigger duct not move more air at the same fan CFM?

References¶

Chapter 1: Air, Pressure and the Fan - The "Air Velocity" section and its worked example "Velocity in a Rectangular Duct".
FanPedia online, Twin City Fan - The fan and blower encyclopedia this textbook is built from.
Volumetric flow rate, Wikipedia - Defines Q = V × A for flow through a cross-section.
Duct (flow), Wikipedia - Background on rectangular and round ductwork.