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Duct System Pressure Explorer

Run the Duct System Pressure Explorer MicroSim Fullscreen

This p5.js sketch is built on the shared FanPedia MicroSim kit. To run it in the p5.js editor, add the kit files it loads (fanpedia-sim.css, fanpedia-core.js, fanpedia-p5.js, fan-models.js, 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 before pasting ch02-duct-system-pressure-explorer.js.

About This MicroSim

The sim draws one exhaust system in flow order: a hood, a 24 in. × 24 in. inlet duct (4.00 ft²), Station 1 at the fan inlet, the fan, Station 2 at the fan outlet, a 20 in. × 20 in. outlet duct (2.78 ft²) and a discharge stack. Three readout panels show velocity V, velocity pressure VP, static pressure SP and total pressure TP at each station, and fan total pressure FTP, fan static pressure FSP and air power AHP for the fan. Click any label in the schematic, or pick it under Definition of…, to read what that part is.

The system is illustrative, chosen so that 10,000 CFM reproduces the chapter's worked example. Air is standard air (0.075 lb/ft³). The model:

  • V1 = Q ÷ 4.00 and V2 = Q ÷ 2.78 (fpm); VP = (V ÷ 4005)² (in. wg)
  • TP1 = −0.81 × (Q ÷ 10,000)² and TP2 = +2.81 × (Q ÷ 10,000)² (the system resistance relation)
  • SP = TP − VP at each station; FTP = TP2 − TP1; FSP = FTP − VP2; AHP = Q × FTP ÷ 6356

Velocities are shown to the nearest 10 fpm, pressures to 0.01 in. wg and air power to 0.01 hp, all computed from unrounded intermediates.

Five challenges come first, in this order. The readout each one asks for (and any readout that would give it away) shows "?" until you check an answer:

# Question Answer Tolerance
1 At 10,000 CFM, what is the velocity in the inlet duct (fpm)? 2,500 fpm ±25
2 At 10,000 CFM the outlet velocity is 3,600 fpm. What is the outlet velocity pressure? 0.81 in. wg ±0.02
3 Inlet SP −1.20, inlet VP 0.39, outlet SP +2.00, outlet VP 0.81. What is the fan total pressure? 3.62 in. wg ±0.03
4 What is the air power at 10,000 CFM and 3.62 in. wg fan total pressure? 5.70 hp ±0.05
5 The airflow is raised to 12,000 CFM in the same system. What fan total pressure is now required? 5.21 in. wg ±0.05

You get two attempts at each. A wrong first answer shows the worked calculation and leaves one attempt; a second wrong answer shows the answer with the calculation, and the challenge counts as missed. Before challenge 5 the airflow moves to 12,000 CFM. After challenge 5, Explore mode unlocks the airflow slider (4,000 to 14,000 CFM in 500 CFM steps) and every value updates at once. Mastery is 4 of 5 on the first attempt. Explore mode is never scored.

How to Use

  1. Read challenge 1, type a number in Your answer and press Check (or Enter).
  2. A correct answer reads "✓ Correct: value unit." in a solid green box, followed by the worked calculation. A wrong one reads "✗ Not quite." with the worked calculation in a dashed red box; use the second attempt.
  3. Press Next challenge (or Enter) for the next one. "Challenges correct: n of 5" counts first-attempt answers only.
  4. After challenge 5, press Start exploring. Move the airflow and check that SP1 is always negative, that TP1 is always smaller in magnitude than SP1, and that going from 5,000 to 10,000 CFM multiplies FTP by 4 and air power by 8. The panel shows the airflow, FTP and air-power ratios against 10,000 CFM. Start over clears the score and returns to challenge 1.

Keyboard: Enter checks the answer, and Enter again goes to the next challenge. Tab reaches the answer box, the buttons, the Definition of… list and the airflow slider; the arrow keys move a focused slider.

Data label: "Illustrative system · standard air, 0.075 lb/ft³", 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/ch02-duct-system-pressure-explorer/main.html"
        height="677px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

The learner will calculate duct velocity, velocity pressure, total pressure, fan total pressure and air power for a simple exhaust duct system, to within the tolerance stated for each challenge (Bloom: Apply, verb: calculate).

Audience

Adult professional development: new application engineers, sales staff, representatives and facilities staff who have read the first half of Chapter 2.

Duration

10-15 minutes

Prerequisites

  • Airflow continuity equation and feet per minute
  • Velocity pressure and the velocity pressure equation
  • Static pressure, total pressure, fan total pressure and fan static pressure
  • Air power, duct system and the system resistance relation

Activities

  1. Challenges (6-8 min): Work each challenge on paper before typing an answer. Write every pressure with its sign.
  2. Review (2 min): For any missed challenge, compare your working with the worked calculation the sim shows.
  3. Explore (3-5 min): Set the airflow to 5,000 and then 10,000 CFM and record FTP and air power at each. Confirm the ratios are 4 and 8. Then sweep the airflow and watch SP1 and TP1.

Common Misconceptions Addressed

  • Inlet static pressure is added as a positive number: challenge 3 and its feedback keep the sign, and the Station 1 panel always shows a negative SP.
  • Velocity pressure is proportional to velocity: challenge 2 squares the velocity ratio, and the outlet VP is about double the inlet VP although the area is only 30% smaller.
  • A 20% airflow increase needs 20% more pressure: challenge 5's feedback names 3.62 × 1.20 as the wrong step, and Explore mode shows FTP × 1.44 at 12,000 CFM.
  • Static pressure always falls in the direction of flow: the readouts show SP rising from −1.20 to +2.00 across the fan while total pressure carries the energy budget.

Assessment

  • First-attempt score shown as "Challenges correct: n of 5"; mastery is 4 of 5 on the first attempt. Moving the airflow slider is exploration, not evidence.
  • Ask the learner: a customer wants 30% more air from this system. By how much do the pressure and the air power rise? (1.69 and 2.20 times.)

References

  1. Dynamic pressure - Wikipedia article on the pressure of a moving fluid brought to rest, the basis of velocity pressure.
  2. Stagnation pressure - Wikipedia article on stagnation (total) pressure as the sum of static and dynamic pressure.
  3. FanPedia: The Fan & Blower Encyclopedia - Twin City Fan's reference on fan performance, pressure and system terms.
  4. AMCA International - publisher of the fan application manuals and test standards that define fan total and fan static pressure.