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Flow Control Power Comparator

Run the Flow Control Power Comparator MicroSim Fullscreen

About This MicroSim

Three common ways to turn a fan down are an outlet damper, variable inlet vanes and speed control. All three reduce airflow, but they need very different shaft power to do it. This MicroSim plots shaft power (% of design) against airflow (% of design) from 50% to 100% for the three methods and asks you to predict their order before you see the curves.

All three curves are labeled illustrative. The speed-control curve is the ideal fan-law relation, power = 100 × (airflow / 100)³, for a system curve that passes through zero. The outlet-damper and inlet-vane values are illustrative values written for this chapter to show the typical ordering; they are not Twin City Fan test data. FanPedia supports the ordering: outlet damper horsepower savings are "usually much less than other methods" (FanPedia p. 182), and inlet box dampers and inlet vanes save horsepower at reduced loads by pre-spinning the air (FanPedia pp. 145, 183, 188). Values between table rows are found by straight-line interpolation. Real speed control has small drive losses, and a system with fixed static pressure saves less than the ideal cube value.

Airflow (% of design) Outlet damper (% power) Variable inlet vanes (% power) Speed control (% power)
100 100 100 100.0
90 96 88 72.9
80 91 77 51.2
70 86 67 34.3
60 80 59 21.6
50 74 52 12.5

How to Use

  1. Read the challenge (80%, then 70%, then 50% airflow). The chart shows only its axes.
  2. Choose the three methods in order from lowest to highest shaft power, type the saving of the best method compared with the outlet damper in percentage points, and press Check.
  3. A challenge is correct when the ranking is right and the saving is within ±3 percentage points. You get two attempts (the attempt line shows "Attempt 1 of 2"). When the challenge ends (correct, or missed after the second wrong attempt), the three curves are drawn and the powers at that airflow are marked with their values. Press Next challenge; the curves are hidden again.
  4. After challenge 3 a summary shows your first-attempt score and whether you reached mastery (3 of 3 on the first attempt), the curves stay visible and Explore mode unlocks the airflow selector (50% to 100% in 5% steps, starting at 70%). Move it from 100% down to 50% and watch the readout of the three powers and the two savings. Hover any point for its airflow, method and power. Explore mode is never scored. Press Start over at any time to begin again from challenge 1.

Keyboard: Tab moves between controls; Enter in a ranking list or the saving box checks your answer, and Enter again goes to the next challenge; the arrow keys move the airflow selector.

Data label: "Illustrative values", shown below the chart with the footnote on real speed control. The legend also marks each series "(illustrative)".

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/ch18-flow-control-power-comparator/main.html"
        height="942px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

The learner will differentiate outlet dampers, variable inlet vanes and speed control by the shaft power each needs at reduced airflow, correctly ranking the three methods and estimating the savings gap at a stated airflow between 50% and 100% of design (Bloom level: Analyze; verb: differentiate).

Audience

Adult professional development: new application engineers, inside and outside sales staff, manufacturer's representatives and specifying engineers who discuss flow-control options with customers.

Duration

10-15 minutes

Prerequisites

  • Flow control
  • Operating point
  • Fan curve
  • System curve
  • Outlet damper
  • Variable inlet vanes
  • Speed control
  • Brake horsepower

These are defined in Chapter 18's "What Flow Control Means" section and in Chapter 10.

Activities

  1. Predict (6 min): Complete the three challenges in order. Commit a ranking and a saving before each reveal.
  2. Explore (4 min): With the selector unlocked, move from 100% to 50% airflow. Note that the damper curve is nearly flat while the speed-control curve drops steeply, so the gap between them grows as flow falls.
  3. Connect (3 min): Compare the 70% readout with the chapter's worked example "The Same 70% Flow Three Ways" (damper 86%, vanes 67%, speed control 34%) and the 80% value with the Speed Control worked example (51.2%).

Common Misconceptions Addressed

  • Closing a damper to half flow cuts power roughly in half (at 50% flow the damper still needs about 74%).
  • All flow-control methods save about the same energy.
  • Inlet vanes save as much as speed control.
  • Power falls in proportion to airflow under speed control, rather than with roughly the cube of airflow.

Assessment

  • Running score shown as "Challenges correct: n of 3"; mastery is 3 of 3 on the first attempt, and the summary after challenge 3 shows "First-attempt score: n of 3" with "Mastery reached (3 of 3 needed)." or "Mastery needs 3 of 3 on the first attempt."
  • Hovering points and moving the airflow selector are exploration, not evidence.
# Airflow Correct ranking, lowest to highest Saving of best method vs. damper
1 80% Speed control, inlet vanes, outlet damper 39.8 points (91 − 51.2)
2 70% Speed control, inlet vanes, outlet damper 51.7 points (86 − 34.3)
3 50% Speed control, inlet vanes, outlet damper 61.5 points (74 − 12.5)

References

  1. Twin City Fan & Blower. FanPedia: The Fan & Blower Encyclopedia, pp. 145, 182, 183, 188. PDF
  2. Wikipedia. Affinity laws: the fan-law relation between speed, flow, pressure and power.
  3. Wikipedia. Variable-frequency drive: how motor speed control is applied to fans.
  4. Chart.js. Line chart documentation.