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Quiz: Basic Fan Anatomy

Test your understanding of the housing, impeller, shaft, bearings, motor, drive train and housing features that every fan shares with these review questions.

1. Which part of a fan is the only one that adds energy to the airstream?

  1. The housing scroll
  2. The inlet funnel
  3. The evasé
  4. The impeller
Show Answer

The correct answer is D. The impeller is the rotating part that does the work on the air, and it is the only part of the fan that adds energy to the airstream. Every other part guides the air, supports the impeller or turns it. The scroll and the evasé convert some of the air's velocity into static pressure, and the inlet funnel smooths the air into the impeller, but none of them adds energy.

Concept Tested: Impeller

See: The Impeller

2. Besides "housing," which names are used in the field for the stationary shell that encloses the impeller?

  1. Rotor, hub or shroud
  2. Side, scroll or casing
  3. Pedestal, framing or base
  4. Funnel, cone or bell
Show Answer

The correct answer is B. FanPedia describes the fan housing as the main structure of the fan, which directs the air through the system and supports the inlet, outlet, framing and structural supports; in the field it is also called the side, scroll or casing. Rotor, hub and shroud are rotating parts or impeller parts. Pedestal and framing are supports. Funnel, cone and bell are other names for the inlet funnel.

Concept Tested: Fan Housing

See: The Fan Housing

3. What distinguishes an inlet collar from an inlet flange?

  1. A collar is bolted to the duct, while a flange is clamped to a flexible sleeve
  2. A collar is used only on axial fans, while a flange is used only on centrifugal fans
  3. A collar is a plain sleeve for a clamped slip-fit connection, while a flange is a drilled ring for a bolted connection
  4. A collar guides air smoothly into the impeller, while a flange holds the impeller on the shaft
Show Answer

The correct answer is C. An inlet collar is a short, plain cylindrical sleeve with no bolt holes: a duct or flexible connector slips over it and is clamped. An inlet flange is a flat ring drilled with a bolt-hole pattern, giving a rigid, sealable joint that mates with a companion flange. When the connection must be bolted, rigid or sealed, specify a flange. Option D describes the inlet funnel and the hub, and option A reverses the two parts.

Concept Tested: Inlet Collar

See: Inlet Collar

4. What does a bearing's L-10 life rating mean?

  1. The operating hours at which 10 percent of a population of identical bearings, at a given speed and load, are expected to have failed
  2. The operating hours that every bearing in the population is guaranteed to reach
  3. The operating hours at which half of a population of identical bearings are expected to have failed
  4. The operating hours a single bearing runs before it loses 10 percent of its load capacity
Show Answer

The correct answer is A. Bearing life is a statistical estimate. FanPedia defines L-10 life as the hours at which 10 percent of a population of bearings, at a given speed and load, are expected to have failed. It is not a guarantee for every bearing. The hours at which 50 percent have failed is L-50 life, the average life, calculated as five times the L-10 life. Most Twin City Fan models offer an L-10 life of 40,000 hours.

Concept Tested: Bearing

See: Bearing Life

5. A fan's bearings are rated L-10 = 60,000 hours. The fan runs 12 hours a day, 6 days a week. What is the expected L-10 life in years?

  1. 6.9 years
  2. 13.7 years
  3. 16.0 years
  4. 80.1 years
Show Answer

The correct answer is C. Hours per year are 12 × 6 × 52 = 3,744. The L-10 life in years is 60,000 ÷ 3,744 = 16.0 years. Option A assumes the fan runs 24 hours a day, 7 days a week (8,736 hours per year). Option B counts 12 hours on every day of the year instead of 6 days a week, and option D is the L-50 life, five times the L-10 figure.

Concept Tested: Bearing

See: Bearing Life

6. A motor turning 1,760 RPM has a 6-inch motor sheave, and the fan sheave is 8 inches (pitch diameters). What is the ideal fan speed?

  1. 2,347 RPM
  2. 1,760 RPM
  3. 1,100 RPM
  4. 1,320 RPM
Show Answer

The correct answer is D. In a belt drive, fan speed equals motor speed times motor sheave pitch diameter divided by fan sheave pitch diameter: 1,760 × 6 ÷ 8 = 1,320 RPM. Option A inverts the ratio, option B is the direct-drive case where the impeller turns at motor speed, and option C uses the chapter's 5-inch motor sheave instead of 6 inches. Real drives lose a little speed to belt slip, so the formula gives the ideal value.

Concept Tested: Drive Train

See: The Drive Train

7. On an Arrangement 4 direct-drive fan, where are the bearings that carry the rotating impeller?

  1. On a bearing pedestal outside the housing, both on one side of the impeller
  2. Inside the motor frame, because the impeller is mounted on the motor shaft
  3. One on each side of the impeller, mounted on the housing framing
  4. In the inlet funnel, where they are cooled by the incoming air
Show Answer

The correct answer is B. In Arrangement 4, the impeller mounts on the end of the motor shaft, so there is no separate fan shaft and no fan bearings to service; the only bearings are the motor bearings inside the motor frame. Option A describes overhung arrangements such as 1 and 9, with two fan bearings on a pedestal. Option C describes center-hung Arrangements 3 and 7. FanPedia notes that Arrangement 4 offers low maintenance for this reason.

Concept Tested: Fan Shaft

See: The Fan Shaft

8. What does an evasé do at the fan outlet?

  1. It enlarges the outlet to reduce the outlet velocity and increase static pressure capability
  2. It changes a round outlet to a square one without changing the outlet area
  3. It closes the outlet to stop air flowing backward when the fan is off
  4. It sets the outlet flange back from the end of the outlet on double-width fans
Show Answer

The correct answer is A. An evasé is a transition that enlarges the outlet; its outlet is larger than its inlet. FanPedia gives its purpose as reducing the fan's outlet velocity and increasing its static pressure capability. In the chapter's example, enlarging a 3.0 ft² outlet to 5.0 ft² cuts the leaving velocity from 3,000 to 1,800 ft/min. Option B describes a transition, option C a backdraft damper, and option D a setback outlet flange.

Concept Tested: Fan Outlet

See: The Fan Outlet

9. What does the service factor on a motor nameplate mean, and how should it be used when sizing a motor for a fan?

  1. It is spare horsepower meant to cover a fan brake horsepower above the nameplate rating
  2. It is the ratio of the fan's brake horsepower to its air horsepower at the design point
  3. It is always 1.15 for fan motors, whether or not they run on a variable frequency drive
  4. It is the multiplier above rated horsepower at which the motor may be used, but the fan's BHP should still be at or below nameplate horsepower
Show Answer

The correct answer is D. The service factor is the multiplier above the rated horsepower at which a motor may be used; FanPedia's example is a 100 HP motor with a 1.15 service factor operated at 115 HP. It is not spare horsepower: size the motor so the fan's BHP at the design point is at or below nameplate horsepower. Common values include 1.0 for a motor on a variable frequency drive and 1.25, so 1.15 is not universal.

Concept Tested: Fan Motor

See: Service Factor

10. A customer orders three centrifugal fans: a clean-air supply fan inspected about once a year, a bakery exhaust fan cleaned every week, and an insulated fan on a hot dryer exhaust. Which access door choices fit, in that order?

  1. Quick-open door, bolted door, raised door
  2. Raised door, quick-open door, bolted door
  3. Bolted door, quick-open door, raised door
  4. Bolted door, raised door, quick-open door
Show Answer

The correct answer is C. The choice follows how often someone must reach inside and what surrounds the door. Rare inspection of clean air suits a bolted door, which seals well. Weekly cleaning favors a quick-open door held with latches, so nobody removes a ring of bolts each visit. On an insulated fan a flush door would be buried, so a raised door, usually 2 inches above the insulation, can be opened without cutting the insulation.

Concept Tested: Access Door

See: Access Door