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Quiz: Fan Performance, Curves and Ratings

Test your understanding of fan pressures, power and efficiency, fan class, fan and system curves, density corrections, AMCA 210 ratings and fan sound with these review questions.

1. Under the AMCA definition used in North American fan catalogs, what is fan static pressure?

  1. The static pressure reading on a gauge at the fan outlet
  2. Fan total pressure minus the fan outlet velocity pressure
  3. Fan total pressure plus the fan outlet velocity pressure
  4. The total pressure at the fan inlet
Show Answer

The correct answer is B. Fan total pressure is the rise in total pressure across the fan, outlet minus inlet. Fan static pressure is fan total pressure minus the outlet velocity pressure: all the energy the fan adds except the kinetic energy of the air leaving the outlet. The chapter warns that a gauge reading across the fan is not the same thing, because the definition is built from total pressures, not gauge readings.

Concept Tested: Fan Static Pressure

See: Fan Total Pressure and Fan Static Pressure

2. Why is "10,000 CFM" by itself not a usable fan performance rating?

  1. CFM is not a recognized unit of airflow in fan catalogs or AMCA ratings
  2. Airflow is always stated in pounds per minute in a fan rating, never in CFM
  3. A fan has only one possible airflow, so stating the number is redundant
  4. Without the pressure, speed, power and density it assumes, the airflow cannot be checked or ordered
Show Answer

The correct answer is D. Fan performance is the complete statement of what a fan does at one set of conditions: airflow, pressure, speed, brake horsepower and the air density those numbers assume. The same fan at the same speed can deliver a whole family of airflow and pressure pairs, and the system picks which one. Remove any qualifier and the number cannot be checked. Option C is the misconception the chapter corrects: a fan has a curve, not a single airflow.

Concept Tested: Fan Performance

See: What a Fan Performance Rating Says

3. A fan moves 12,000 CFM through an outlet with an inside area of 3.0 ft² against 3.0 in. wg fan static pressure, at standard air. What is the fan total pressure?

  1. About 4.0 in. wg
  2. About 3.0 in. wg
  3. About 2.0 in. wg
  4. About 3.5 in. wg
Show Answer

The correct answer is A. Outlet velocity is 12,000 / 3.0 = 4,000 FPM. Outlet velocity pressure at standard air is (4,000 / 4005)² ≈ 1.00 in. wg. Fan total pressure is fan static pressure plus outlet velocity pressure: 3.0 + 1.0 = 4.0 in. wg. Option B ignores the velocity pressure, and option C subtracts it instead of adding it. On high-velocity fans, confusing the two pressures can shift a selection by a full inch of water gauge.

Concept Tested: Outlet Velocity Pressure

See: The Fan Outlet: Area, Velocity and Velocity Pressure

4. A specification requires a minimum static efficiency of 75%. At the selected point, the fan's static efficiency is 70% and its total efficiency is 77%. What is the correct conclusion?

  1. The fan complies, because its total efficiency exceeds 75%
  2. The fan complies, because the average of the two efficiencies is above 73%
  3. The fan does not comply; total efficiency is always the larger of the two and cannot be substituted for static efficiency
  4. The two efficiencies cannot both apply to the same point, so the data is wrong
Show Answer

The correct answer is C. Static efficiency uses fan static pressure and total efficiency uses fan total pressure, so total efficiency is always the larger at a given point. Both are valid at once; the chapter's worked example has 70% static and about 77% total at the same point. A specification that names static efficiency must be met on static efficiency, so 70% fails a 75% requirement even though the total efficiency would pass a total-efficiency requirement.

Concept Tested: Fan Efficiency

See: Brake Horsepower and Fan Efficiency

5. Where on a backward inclined fan's pressure curve is the stall region?

  1. To the left of the peak of the pressure curve, toward zero flow
  2. At the far right end of the curve, near maximum airflow
  3. At the point of peak efficiency
  4. Wherever the brake horsepower curve reaches its maximum
Show Answer

The correct answer is A. For a backward inclined or airfoil fan, the stall region begins at the airflow of peak pressure and extends left toward zero flow. There the airflow is too low for the blade angles, air separates from the blades and recirculates, and the fan may pulse, surge, vibrate and rumble. Catalog tables usually stop short of this region, which is why some cells at low airflow and high pressure are blank. Never select a fan left of the peak.

Concept Tested: Stall Region

See: The Stall Region

6. Why must the motor on a forward curved fan be sized for the highest airflow the fan could realistically see, not just the design point?

  1. Forward curved fans always run in the stall region
  2. Forward curved fans cannot use belt drives
  3. Its brake horsepower keeps rising with airflow, so lower-than-designed system resistance pushes power above the design value
  4. Its maximum safe speed is lower than any other fan's
Show Answer

The correct answer is C. Forward curved and radial bladed fans have an overloading power curve: brake horsepower keeps rising as airflow increases, all the way to the right end of the curve. If the duct system has less resistance than calculated, an access door is left open, or a damper is removed, the fan moves more air and draws more power than at the design point. A motor sized only for the design point can then trip. Non-overloading curves have a ceiling instead.

Concept Tested: Overloading Power Curve

See: Overloading and Non-Overloading Power Curves

7. An exhaust fan must handle 300°F air (temperature correction factor 0.697) against 2.5 in. wg actual static pressure at sea level. At what static pressure should it be looked up in a standard-air catalog?

  1. About 1.74 in. wg
  2. 2.5 in. wg
  3. About 3.08 in. wg
  4. About 3.59 in. wg
Show Answer

The correct answer is D. To enter a standard-air catalog, divide the actual required pressure by the density correction factor: 2.5 / 0.697 ≈ 3.59 in. wg. The fan is selected at the actual CFM and that equivalent pressure, and the catalog BHP is then multiplied by 0.697 to get the BHP when running hot. Option A multiplies instead of dividing, the most common direction error, and option B skips the correction, which would leave the fan short on pressure at the job.

Concept Tested: Temperature Correction

See: Temperature Correction

8. Which statement best describes system effect?

  1. The pressure loss of the ductwork, elbows and filters, which is already in the system curve
  2. A loss of fan performance caused by the way the fan is connected to the system, which appears in neither the fan curve nor the duct calculation
  3. The reduction in fan pressure caused by hot, low-density air
  4. The extra airflow a fan delivers when the system resistance is overestimated
Show Answer

The correct answer is B. Catalog ratings are measured with ideal test ductwork. An elbow on the inlet, a short or missing discharge duct, or an abrupt transition makes the fan perform worse than its rating. System effect is that loss, expressed as extra pressure the fan must overcome; it exists only because the fan's inlet or outlet airflow is disturbed. Because it is in neither the fan curve nor the system curve, it must be added to the design pressure before selection.

Concept Tested: System Effect

See: System Effect

9. A selection needs 1,500 RPM. The catalog lists maximum safe speeds of 1,300 RPM (Class I), 1,650 RPM (Class II) and 2,000 RPM (Class III) for this size, and the job temperature carries a derate factor of 0.95. Which class should be ordered?

  1. Class I
  2. Class III
  3. Class II
  4. None of these classes; a larger fan size is required
Show Answer

The correct answer is C. Apply the derate first: Class I becomes 1,300 × 0.95 = 1,235 RPM, Class II becomes 1,650 × 0.95 ≈ 1,568 RPM, and Class III becomes 1,900 RPM. The selection speed of 1,500 RPM exceeds the derated Class I limit but not the derated Class II limit. The rule is to choose the lightest class whose derated limit is at or above the selection speed, so Class II is ordered. Class III would work but is heavier than needed.

Concept Tested: Fan Class

See: Fan Class

10. A fan's airflow has dropped since start-up. Its speed has not changed and nothing in the fan is damaged. Which explanation fits the fan curve and system curve picture?

  1. The fan curve has shifted to the right because the impeller has worn in
  2. The system curve has become steeper, for example from a loaded filter or a closed damper, moving the operating point left along the unchanged fan curve
  3. The system curve has become flatter, moving the operating point to the left
  4. The air has become denser, which lowers the airflow at a fixed speed
Show Answer

The correct answer is B. The operating point is where the fan curve and system curve cross. With speed unchanged, the fan curve stays the same, so the change must be in the system. A loaded filter, a closed damper or an iced coil makes the system curve steeper, and the crossing moves left to less airflow. A flatter system curve would move the point right. Density does not change CFM at a fixed speed; it changes pressure and horsepower.

Concept Tested: Operating Point

See: The Operating Point

11. What does a sone rating measure?

  1. Perceived loudness on a linear scale, where 2 sones is judged twice as loud as 1 sone
  2. The total acoustic power a fan radiates, in decibels relative to 10⁻¹² watts
  3. The rate at which impeller blades pass the housing cutoff
  4. The sound pressure a microphone measures at one location
Show Answer

The correct answer is A. A sone is a unit of perceived loudness on a linear scale; 1 sone is the loudness of a 1,000 Hz tone at 40 dB sound pressure level, and a 10-sone fan sounds about twice as loud as a 5-sone fan. That makes sones easier for non-specialists to compare than decibels. Option B describes sound power level, option C blade pass frequency, and option D sound pressure level.

Concept Tested: Sone Rating

See: Sone Rating

12. Why do fan manufacturers publish sound power level rather than sound pressure level?

  1. Sound power is always the lower number, so it looks better in a catalog
  2. Sound power can be added arithmetically when several fans run together
  3. Sound pressure cannot be measured in decibels
  4. Sound power is a property of the fan at its operating point and does not depend on the room or the distance from the fan
Show Answer

The correct answer is D. Sound power level is the total acoustic power a fan radiates, a property of the fan at its operating point, like brake horsepower. Sound pressure level is what a microphone measures at one spot and depends on distance, room size and reflections. Decibels never add arithmetically: two identical 90 dB fans together produce 93 dB, not 180 dB. A specification asking for "85 dB" without saying power or pressure, and at what distance, cannot be checked.

Concept Tested: Sound Power Level

See: Sound Power Level