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Quiz: Discharge Positions and Rotation

Test your understanding of the drive-side viewing rule, rotation, discharge positions and angles, inlet box positions and motor positions with these review questions.

1. From which side is a centrifugal fan viewed when its rotation, discharge position and inlet box position are named?

  1. The inlet side
  2. The discharge side
  3. The drive side
  4. Whichever side the installer faces
Show Answer

The correct answer is C. Centrifugal fan orientation is described viewed from the drive side, the side where the sheave and belts, the coupling, or the motor is located. A housing seen from the other side is a mirror image, so a fixed viewpoint is the only way to make orientation words unambiguous. The inlet side (A) gives a mirrored view, the discharge side (B) is the axial convention, and an installer's side (D) changes from job to job.

Concept Tested: Viewed From Drive Side

See: Viewed From the Drive Side

2. What is the discharge angle of a Top Horizontal (THD) discharge?

  1. 45 degrees
  2. 90 degrees
  3. 135 degrees
  4. 270 degrees
Show Answer

The correct answer is B. The discharge angle counts from upblast (360) in the direction of rotation in 45-degree steps, so Top Horizontal is 90, written for example as CW 90. Forty-five degrees (A) is Top Angular Up, 135 degrees (C) is Top Angular Down, and 270 degrees (D) is Bottom Horizontal.

Concept Tested: Discharge Angle

See: Discharge Angle

3. Why is "Top Horizontal" on its own not a complete discharge description?

  1. A top horizontal outlet points right on a clockwise fan and left on a counterclockwise fan, so the rotation must be stated too
  2. Top horizontal outlets are only offered on double width fans, so the fan width must be stated too
  3. The name does not say whether the fan is belt driven or direct driven
  4. The discharge angle for top horizontal changes with the motor position
Show Answer

The correct answer is A. On a clockwise fan the air at the top of the wheel moves to the right, so a CW top horizontal outlet points right; on a counterclockwise fan the same outlet points left. The two housings are mirror images and cannot be swapped, so a discharge description always pairs a rotation with a position. Top horizontal is available on single width fans (B), the drive type does not change the outlet direction (C), and the angle does not depend on motor position (D).

Concept Tested: Discharge Position

See: Discharge Position

4. How does the inlet box angle differ from the discharge angle?

  1. The inlet box angle is read from the inlet side, while the discharge angle is read from the drive side
  2. The inlet box angle uses 30-degree steps, while the discharge angle uses 45-degree steps
  3. The inlet box angle counts in the direction of rotation, while the discharge angle is a fixed clock
  4. The inlet box angle is always counted clockwise from the top as seen from the drive side, whatever the fan's rotation
Show Answer

The correct answer is D. The inlet box angle is a fixed clock around the housing: 45-degree steps from 45 to 360, with 360 at the top and the angle increasing clockwise, as seen from the drive side. It does not change with rotation, so a CCW fan's inlet box at 90 is still on the right. Option C reverses the two rules: it is the discharge angle that counts in the direction of rotation. Both angles are read from the drive side (A) in 45-degree steps (B).

Concept Tested: Inlet Box Position

See: Inlet Box Position

5. Why must you know a fan's arrangement before you can interpret a motor position letter?

  1. The motor position letter is the same for all arrangements but changes meaning with rotation only
  2. The arrangement tells you the discharge angle, which the motor position letter repeats
  3. Each arrangement family has its own motor position system, such as W, X, Y and Z for Arrangements 1, 3 and 3F, L and R for Arrangement 9, and A through H for axial Arrangement 9 fans
  4. Motor position letters are used only on direct drive arrangements, so the arrangement shows whether a letter is needed
Show Answer

The correct answer is C. Different fan families use different naming systems: W, X, Y and Z for Arrangements 1, 3 and 3F; L and R for Arrangements 9 and 9F; clock positions for K mount motors; and A through H for axial and inline Arrangement 9 fans. A letter means nothing until you know which system applies. Motor positions describe belt-drive motors, not direct drive ones (D), and they are separate from the discharge angle (B).

Concept Tested: Motor Position

See: Motor Position

6. A submittal lists a fan as CCW 135. Which way does the air leave, as seen from the drive side?

  1. Down and to the right, Top Angular Down
  2. Down and to the left, Top Angular Down
  3. Up and to the left, Top Angular Up
  4. Down and to the left, Bottom Angular Down
Show Answer

The correct answer is B. Forty-five-degree steps from upblast give 135 degrees as Top Angular Down. For a counterclockwise fan the compass direction is 360 − 135 = 225 degrees, which is down and to the left. Option A is the direction for CW 135, the mirror-image housing. Option D gives the right direction but the wrong position: Bottom Angular Down is 225 on the discharge-angle scale, not 135.

Concept Tested: Discharge Angle

See: Discharge Angle

7. A technician sends a photo taken from the inlet of an Arrangement 1 single inlet fan. The outlet is at the top pointing to the technician's left, and the housing's rotation arrow looks counterclockwise. The order says CW Top Horizontal. What is the correct conclusion?

  1. The fan was built CCW Top Horizontal and must be replaced
  2. The fan was built CW Bottom Horizontal, because the outlet appears on the left
  3. The fan is CCW, but rewiring the motor will make it match the order
  4. Mirrored to the drive side, the fan is CW with a top outlet pointing right, which matches the order
Show Answer

The correct answer is D. On this fan the inlet side is opposite the drive side, so the photo is a mirror image. Flipping it left to right makes the outlet point right and the arrow read clockwise: CW Top Horizontal, exactly as ordered. Options A and C take the inlet-side view at face value. The chapter calls viewpoint the most common cause of a false "wrong fan" report.

Concept Tested: Viewed From Drive Side

See: Viewed From the Drive Side

8. An inlet duct arrives horizontally from the left of an observer standing on the drive side. Which inlet box position meets it?

  1. 270, Horizontal Left Intake
  2. 90, Horizontal Right Intake
  3. 315, Angular Down Intake
  4. 225, Angular Up Intake
Show Answer

The correct answer is A. The inlet box angle is counted clockwise from the top as seen from the drive side, so the box on the left is position 270, Horizontal Left Intake. Position 90 (B) is the box on the right. Position 315 (C) is at the upper left, for a duct coming down at an angle, and position 225 (D) is at the lower left, for a duct rising at an angle.

Concept Tested: Inlet Box Position

See: Inlet Box Position

9. A mezzanine fan must discharge straight down through a floor sleeve already cast in the slab. Seen from the drive side, the sleeve centerline is to the right of the fan shaft. Which order is correct, and why?

  1. CCW 180, because a counterclockwise wheel moves air downward on its right side
  2. Either CW 180 or CCW 180, because downblast points the same way for both rotations
  3. CW 180, because a clockwise wheel moves air downward on its right side, so its downblast outlet sits on the right of the housing bottom
  4. CW 270, because the outlet must be at the bottom of the housing
Show Answer

The correct answer is C. Downblast points straight down for both rotations, but the housings differ. A clockwise wheel moves air downward on its right side, so a CW downblast outlet sits on the right of the housing bottom, over the sleeve. A CCW downblast outlet sits on the left (A), which would miss the sleeve, so the hands are not interchangeable (B). CW 270 (D) is Bottom Horizontal, which points sideways.

Concept Tested: Down Blast Discharge Position

See: Down Blast Discharge Position

10. After a motor replacement on a belt-driven SWSI fan ordered CW, a technician standing at the sheave guard bumps the motor and sees the wheel turning counterclockwise. What is the best analysis?

  1. The technician is on the wrong side; mirrored, the wheel is really turning clockwise
  2. The housing was built to the wrong hand and must be replaced
  3. A backward wheel moves no air at all, so the fan cannot have been run this way before
  4. The observer is already on the drive side, so the wheel really is turning backward; swapping any two of the three supply leads reverses it
Show Answer

The correct answer is D. The sheave guard is on the drive side, so no mirroring is needed (A): the wheel is turning CCW when CW is required. The housing matches the order, so the motor wiring is the fault, not the housing (B). On a three-phase motor, interchanging any two supply leads reverses rotation. A backward centrifugal wheel still moves some air at reduced performance (C), which is why the fault is easy to miss from the inlet side.

Concept Tested: Rotation

See: Rotation