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Quiz: Fan Laws, System Effect and Sound

Test your understanding of fan curve landmarks, efficiency, the fan laws, system effect, combined fans and fan sound with these review questions.

1. What is a fan's shutoff pressure?

  1. The static pressure at which the fan reaches its peak static efficiency
  2. The static pressure the fan develops at zero airflow, with the inlet or outlet blocked
  3. The highest static pressure anywhere on the fan curve, at the boundary of the stall region
  4. The static pressure at which the motor must be shut off to avoid overload
Show Answer

The correct answer is B. Shutoff pressure is the static pressure a fan develops at zero airflow, when the inlet or outlet is completely blocked; Curve A's is 4.6 in. wg. It is not a zero-power condition, since the fan still absorbs power churning the trapped air. The highest pressure on the curve is the peak pressure point (C), which on Curve A is 4.9 in. wg at 4,000 CFM, not the shutoff value. Peak efficiency (A) occurs far from shutoff.

Concept Tested: Shutoff Pressure

See: Landmarks on the Fan Curve

2. Why is a fan's static efficiency zero at free delivery, even though the fan is moving the most air it can?

  1. Because the motor draws no power when the fan discharges into open air
  2. Because the impeller stalls at free delivery and stops doing useful work
  3. Because free delivery is always to the left of the peak pressure point
  4. Because static pressure is zero there, so air power based on static pressure is zero
Show Answer

The correct answer is D. Static efficiency is airflow times fan static pressure divided by 6356 times BHP. At free delivery the fan discharges against zero static pressure, so the numerator is zero even though airflow is at its maximum. The fan still absorbs power (Curve A takes 8.8 BHP near 16,000 CFM), so option A is wrong. Free delivery sits at the far right of the curve, not left of the peak (C), and the fan is not stalled there (B).

Concept Tested: Free Delivery

See: The Efficiency Curve

3. A fan runs at 1,000 RPM and delivers 8,000 CFM at 2.0 in. wg, absorbing 5.0 BHP, on a system whose curve passes through zero. The owner wants 9,200 CFM through the same system. About what brake horsepower will the fan need?

  1. 5.75 BHP
  2. 6.61 BHP
  3. 7.60 BHP
  4. 8.75 BHP
Show Answer

The correct answer is C. Airflow goes with speed, so the speed ratio is 9,200 ÷ 8,000 = 1.15. Power goes with speed cubed: 5.0 × 1.15³ = 5.0 × 1.521 = 7.60 BHP. Option A scales power linearly with speed, option B uses the square (the pressure law), and option D raises the ratio to the fourth power. A 15% airflow increase costing about 52% more power is the same result as the chapter's Curve A speed-up example.

Concept Tested: Power Varies With Speed Cubed

See: Speed Change Calculation

4. A fan selected for 600°F air absorbs 20 BHP at its operating temperature. At the same speed, about how much will it absorb when it starts on 70°F air?

  1. 40 BHP
  2. 20 BHP
  3. 30 BHP
  4. 10 BHP
Show Answer

The correct answer is A. At the same speed the fan moves the same CFM, but BHP rises in proportion to air density. The chapter gives a temperature factor of 0.500 for 600°F air, so 70°F air is twice as dense and the cold start horsepower is 20 ÷ 0.500 = 40 BHP. Option B assumes density does not affect power, and option D applies the density factor in the wrong direction. The motor must be sized for this cold start, or the start-up load must be managed.

Concept Tested: Cold Start Horsepower

See: Cold Start Horsepower

5. A centrifugal fan draws air through a duct from a process hood and discharges directly outdoors with no outlet duct. Which AMCA installation type describes this job?

  1. Type A
  2. Type B
  3. Type C
  4. Type D
Show Answer

The correct answer is C. Two questions decide the type: is the inlet ducted, and is the outlet ducted? Here the inlet is ducted and the outlet is free, which is Type C. Type A is free on both sides, Type B is a free inlet with a ducted outlet, and Type D is ducted on both sides. The installation type depends on the connections, not on the fan type, and a rating made for one type does not describe a job installed as another.

Concept Tested: AMCA Installation Type

See: AMCA Installation Type

6. On the chapter's steep system (4.0 in. wg at 10,000 CFM), adding a second Curve A fan in parallel raises airflow only from 10,000 CFM to about 11,000 CFM. What is the underlying reason?

  1. Parallel fans add pressure at the same airflow, and the steep system needs airflow instead
  2. The combined curve doubles airflow only at equal pressure, and on a steep system more airflow costs much more pressure
  3. The second fan always stalls when two identical fans share one system
  4. Each fan loses half its efficiency when it shares an inlet plenum with another fan
Show Answer

The correct answer is B. Parallel fans add airflow at the same pressure, but the operating point must still land on the system curve. On a steep system, extra airflow requires much more pressure, so the operating point climbs to about 4.8 in. wg, where each fan delivers far less than at 4.0 in. wg. Option A describes series fans. Stall (C) is a risk only if each fan runs left of its peak pressure point. On a flat system the same parallel pair adds over 40% more air.

Concept Tested: Fans in Parallel

See: Fans in Parallel and in Series

7. A VAV system must hold 1.5 in. wg of its 4.0 in. wg design pressure at every flow. At 70% airflow, air power falls to about 48% of design with speed control, not the ideal 34%. What causes the difference?

  1. The fixed pressure component keeps system pressure from falling with airflow squared, so the fan must still develop more pressure than the cube law assumes
  2. Variable frequency drives lose about 14% of their input power at any reduced speed
  3. The fan laws apply only to speed changes of less than 20%, so the cube law fails at 70% airflow
  4. Static efficiency always rises at reduced speed, which increases the power the fan draws
Show Answer

The correct answer is A. The ideal cube law holds only when the system curve passes through zero. With 1.5 in. wg held constant, the system needs 1.5 + 2.5 × 0.7² = 2.73 in. wg at 70% airflow instead of 1.96 in. wg, so air power is 0.7 × 2.73 ÷ 4.0 = 48% of design. Speed control still saves the most, but by less. Drive and motor losses (B) reduce savings further, yet they are not the cause of the 48% figure.

Concept Tested: Flow Control Efficiency

See: Controlling Flow Efficiently

8. A fan has an FEI of 1.21 at one duty point but 0.88 at the project's actual design point, which lies far to the right on its curve. The energy code requires a minimum FEI of 1.00 at the design point. Which response is best?

  1. Accept the fan, because FEI is judged at the fan's best operating point
  2. Fit a larger motor, because more motor capacity raises the fan's FEI
  3. Keep the fan and add an outlet damper to move the duty to the left
  4. Choose a different fan size so the design point lands closer to peak efficiency
Show Answer

The correct answer is D. FEI compares the electrical input power of a reference fan with that of the actual fan at the same duty point, so one fan model has a different FEI at every point on its curve. The code judges the design point, not the best point (A). A larger motor (B) does not reduce the power the fan needs, and a damper (C) adds pressure loss rather than meeting the duty more efficiently. The chapter's remedy for a failing selection is a different size.

Concept Tested: Fan Energy Index

See: Fan Energy Index

9. An elbow bolted to Curve A's inlet adds about 0.5 in. wg of system effect, so the fan delivers about 9,560 CFM instead of the 10,000 CFM duty. Speeding up to 1,444 RPM would need about 10.08 BHP, more than the 10 hp motor. Which redesign restores the duty point without changing the motor?

  1. Speed the fan up to 1,444 RPM and rely on the motor's service factor to carry the extra load
  2. Add a second identical fan in series immediately downstream of the elbow
  3. Replace the inlet elbow with an inlet box or straight inlet duct and keep 1,380 RPM and the 10 hp motor
  4. Throttle an outlet damper so the operating point moves back to the design airflow
Show Answer

The correct answer is C. The shortfall comes from inlet system effect, so removing its cause restores the original effective system curve, and the fan returns to 10,000 CFM at 1,380 RPM on its 10 hp motor. Service factor is margin, not capacity, so option A is not a sound design. A series fan (B) adds cost and does not remove the loss, and a damper (D) adds resistance and reduces airflow further. FanPedia describes the inlet box as designed to minimize the system effect of a 90 degree turn into the fan.

Concept Tested: Inlet System Effect

See: Restoring the Duty Point After System Effect

10. A fan has a 10-blade impeller running at 1,770 RPM, giving a blade pass frequency of 295 Hz. In which octave band will this tone appear in the fan's sound data?

  1. The 250 Hz band
  2. The 125 Hz band
  3. The 500 Hz band
  4. The 1,000 Hz band
Show Answer

The correct answer is A. Blade pass frequency is the number of blades times revolutions per second: 10 × 1,770 ÷ 60 = 295 Hz. The 250 Hz octave band covers roughly 177 to 354 Hz, so 295 Hz falls inside it and that band will usually stand out. The 125 Hz band ends at about 177 Hz, and the 500 Hz band starts at about 354 Hz. Octave band data lets an acoustical engineer choose a silencer that attacks the right frequencies.

Concept Tested: Octave Band

See: Octave Band