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Operating Point Finder

Run the Operating Point Finder MicroSim Fullscreen

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About This MicroSim

The chart shows Curve A, the illustrative backward inclined fan from Chapter 10 at one fixed speed and standard air (solid indigo line, straight segments between the table points), and a duct system curve (dashed orange parabola):

  • System curve: P = R × (Q ÷ 10,000)², where R is the system resistance in in. wg at 10,000 CFM.
  • Operating point: the airflow, searched in 1 CFM steps from 0 to 16,000 CFM, where the fan pressure and the system pressure are closest.
  • The hatched band left of 4,000 CFM is Curve A's stall region (left of its pressure peak).

For each of three challenge systems, you predict the operating point before it is drawn. The systems come from the chapter section "The Operating Point":

# System Model operating point BHP
1 As designed: 4.0 in. wg at 10,000 CFM (R = 4.0) 10,000 CFM, 4.00 in. wg 9.0
2 Filters loaded: 4.0 in. wg at 8,000 CFM (R = 6.25) 8,390 CFM, 4.40 in. wg 8.3
3 Resistance overestimated: 2.5 in. wg at 10,000 CFM (R = 2.5) 11,600 CFM, 3.36 in. wg 9.4

A prediction is correct when its airflow is within ±300 CFM and its pressure within ±0.15 in. wg of the model intersection. You get two attempts per challenge; after the second wrong attempt the intersection is revealed and the challenge counts as missed. Each reveal marks the operating point (orange diamond) and shows the operating airflow (nearest 10 CFM), pressure (two decimals), BHP (one decimal, interpolated from the table) and whether the point is in the stall region.

Mastery is 3 of 3 on the first attempt. After challenge 3, Explore mode unlocks a slider for the system resistance at 10,000 CFM (1.0 to 35.0 in. wg). The system curve, the operating point and the readouts update at once; above about R = 30.6 in. wg the point falls below 4,000 CFM and the sim flags "In the stall region: select a different fan." Explore mode is never scored.

How to Use

  1. Read the system for challenge 1. Type your predicted airflow (CFM) and pressure (in. wg) in the two boxes, or click the chart where you think the curves cross; the click fills both boxes.
  2. Press Check (or Enter). A correct prediction reads "✓ Correct: the curves cross at …" in a solid green box. A wrong one reads "✗ Not quite." with an explanation in a dashed red box; use the second attempt.
  3. Press Next challenge (or Enter) for systems 2 and 3. "Challenges correct: n of 3" keeps the running score.
  4. After challenge 3, press Start exploring and move the resistance slider. Watch the point move left as the system gets steeper and right as it gets flatter, and find where it enters the stall region. Start over clears the score and returns to challenge 1.

Keyboard: Tab reaches the two prediction boxes, every button and the slider. Enter checks the prediction, and Enter again goes to the next challenge; the arrow keys move the focused slider. Clicking the chart is optional; typing gives the same result.

Data label: "Illustrative fan curve · Curve A, standard air, one fixed speed", 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/ch10-operating-point-finder/main.html"
        height="607px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

The learner will solve for the operating point of an illustrative fan in a stated duct system by predicting where the fan curve and system curve intersect, to within ±300 CFM and ±0.15 in. wg (Bloom: Apply, verb: solve).

Audience

Adult professional development: new application engineers, sales staff, representatives and facilities staff who have read the sections on the fan curve, the system curve and the operating point in Chapter 10.

Duration

10-15 minutes

Prerequisites

  • Fan curve and stall region
  • System resistance and the system curve (square law)
  • Operating point
  • Brake horsepower

Activities

  1. Challenges (5-8 min): For each system, compute the system pressure at two or three airflows (for example 8,000, 9,000 and 10,000 CFM), compare with the fan curve, and commit a prediction.
  2. Review (2 min): For any miss, find the airflow where the system's need passes the fan's offer, as the feedback describes.
  3. Explore (3-5 min): Find the resistance at which the operating point first enters the stall region, and the BHP at R = 1.0 in. wg. Explain why a non-overloading fan still protects its motor there.

Common Misconceptions Addressed

  • A fan always delivers its rated airflow, whatever the system: three systems give three different airflows from the same fan at the same speed.
  • A dirtier, more resistant system makes the fan move more air "because it works harder": the loaded-filter system crosses further left, at less air.
  • The system curve is a straight line: the dashed curve is a parabola, so 20% more air needs 44% more pressure.

Assessment

  • The running line "Challenges correct: n of 3" counts challenges solved on either attempt. At the end the sim shows "First-attempt score: n of 3" and the mastery line; mastery is 3 of 3 on the first attempt.
  • Moving the slider in Explore mode is not evidence of mastery. Ask the learner what would change the operating point if the system curve stayed the same (a change in fan speed).

References

  1. Fan (machine) - Wikipedia overview of fan types and fan performance.
  2. Affinity laws - Wikipedia overview of how fan curves move with speed, the other way to move an operating point.
  3. FanPedia: The Fan & Blower Encyclopedia - Twin City Fan's reference on fan curves, system resistance and flow control.