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Quiz: Housing Construction and Exploded Views

Test your understanding of scroll housing parts, split housings, tubular housing parts, guards, plug fan hardware and rooftop hardware with these review questions.

1. On a housed centrifugal fan, what is the cutoff?

  1. The flat side of the scroll housing that carries the inlet opening
  2. The structural steel that stiffens the housing and gives the fan its feet
  3. The point near the outlet where the scroll wrapper comes closest to the impeller
  4. The bolted joint along the housing's horizontal centerline
Show Answer

The correct answer is C. The cutoff sits at the discharge end of the scroll, where the wrapper comes closest to the impeller. Like a tongue, it separates air leaving through the discharge from air that would otherwise keep circling the housing, and FanPedia groups it with the wiper bar as a feature that helps prevent turbulence and increase efficiency. Option A describes a housing side plate, option B housing framing, and option D a horizontal split.

Concept Tested: Cutoff

See: The Scroll Housing: Side Plates, Wrapper and Cutoff

2. According to FanPedia, why is a three-way split housing normally required?

  1. For shipping, or to let the fan pass through a specific sized opening
  2. To let the impeller come out without disturbing inlet or outlet ductwork
  3. To make the housing nominally leak-tight
  4. To reduce the drafting and engineering time on a quote
Show Answer

The correct answer is A. FanPedia says the three-way split, which cuts the housing into three sections up to 180 degrees, is "normally required either for shipping or to enable the fan to enter a specific sized opening," such as a doorway or roof hatch on a retrofit. Option B is the purpose of a pie split. Splits are not recommended for leak-tight construction, and a three-way split adds drafting and engineering time rather than saving it, so it should be identified early in a quote.

Concept Tested: Three-Way Split Housing

See: Split Housings

3. Why does a scroll wrapper give the space between the impeller and the housing a gradually growing cross-section?

  1. To make room for the bearings inside the housing
  2. To let the housing be split into sections for shipping
  3. To hold the inlet collar and inlet funnel in place
  4. To collect the air the impeller throws off, slow it down and turn part of its velocity into static pressure
Show Answer

The correct answer is D. The scroll wrapper is the curved band of sheet metal between the two side plates that runs out to the discharge. Its spiral shape gives the air a gradually growing passage, so it collects the air leaving the impeller, slows it, and converts part of its velocity into static pressure before the air leaves through the outlet. The inlet collar and funnel are carried by the side plate, and splitting is a separate housing feature.

Concept Tested: Scroll Wrapper

See: The Scroll Housing: Side Plates, Wrapper and Cutoff

4. A plant wants to be able to pull the impeller out of a housed centrifugal fan without disconnecting the inlet or outlet ductwork. Which housing feature meets that need?

  1. A horizontal split housing
  2. A pie split housing
  3. A three-way split housing
  4. A bolted access door
Show Answer

The correct answer is B. A pie split housing removes a wedge of the housing, cut at 90 degrees or greater, so the impeller can come out "without disturbing inlet or outlet" ductwork. FanPedia builds pie splits into Arrangements 3SI, 7SI and 7DI for easy impeller removal. A horizontal split is the standard split, a three-way split serves shipping or tight openings, and an access door is only large enough to inspect and clean the impeller.

Concept Tested: Pie Split Housing

See: Split Housings

5. What is the difference between a belt tube and a belt guard on a belt driven fan?

  1. They are two names for the same part, used by different manufacturers
  2. A belt tube shields belts that cross the airstream; a belt guard keeps people away from belts outside the housing
  3. A belt tube is used only on centrifugal roof exhausters; a belt guard is used only on plug fans
  4. A belt guard protects the belts from the airstream; a belt tube protects people from the moving drive
Show Answer

The correct answer is B. In a belt driven axial or inline fan, the motor sits outside the tube while the impeller shaft is inside it, so the belts must cross the airstream; the belt tube (also called a belt fairing) shields them from the air and whatever it carries. A belt guard encloses belts and sheaves outside the housing so people cannot reach the moving drive. Option D reverses the two jobs. A belt driven inline fan has both.

Concept Tested: Belt Tube

See: Guards and Covers

6. A technician must reach the impeller, motor and drive of a square inline fan in a tight ceiling space. Which housing feature gives the most direct access?

  1. A quick-open access door cut into the housing
  2. The cutoff at the discharge
  3. A three-way split housing
  4. The removable housing access panels, which take off a whole side of the housing
Show Answer

The correct answer is D. A housing access panel is a whole removable side of the housing rather than a door cut into it. FanPedia describes square inline fans as built with square housings and removable side panels, and its view labels the access panel "removable, 3 sides." Removing a panel exposes the impeller, motor and drive at once, which is why these fans are easy to service in tight ceiling spaces. An access door is smaller, and splits and cutoffs do not provide service access.

Concept Tested: Housing Access Panel

See: Getting Inside: Housing Access Panels

7. What is the job of the insulation plug on a plug fan?

  1. It fills the opening in the plenum wall around the fan so the wall keeps its insulating value
  2. It seals the shaft where it passes through the housing side plate
  3. It replaces the mounting panel on grade-mounted arrangements
  4. It insulates the motor from the weather on outdoor installations
Show Answer

The correct answer is A. The insulation plug is an insulated section mounted with the panel that fills the opening in the plenum wall where the fan passes through, so the wall keeps its insulating value. It appears on both FanPedia plug fan exploded views and on the Arrangement 9P "Mounting Panel with Insulation Plug" detail. Because plug fans often serve hot equipment such as ovens, the plug matters in high-temperature service. A motor cover, not an insulation plug, protects the motor from weather.

Concept Tested: Insulation Plug

See: Plug Fan Hardware: Mounting Panel and Insulation Plug

8. A customer orders a nominally leak-tight fan and also asks for a split housing so it can be shipped in sections. What should you do before the quote goes out?

  1. Quote the split as requested, since splitting a housing does not add any leakage paths
  2. Drop the leak-tight requirement, because leak-tight construction and split housings can never be combined
  3. Flag the conflict: splits are not recommended for leak-tight fans, and one used there needs gasketing and close bolt spacing
  4. Replace the split with a pie split, because a pie split joint is sealed and never leaks
Show Answer

The correct answer is C. Every split is another joint that can leak. FanPedia says split housings are "not recommended" for nominally leak-tight construction, and when a split is used there it needs gasketing, close bolt spacing and centering plates to seal around the shaft seal and inlet. The right move is to flag the requirement before quoting, not to ignore it or abandon the leak-tight construction. A pie split is still a split joint.

Concept Tested: Horizontal Split Housing

See: Split Housings

9. What is the underlying reason that inline centrifugal and mixed flow fans carry straightening vanes downstream of the impeller?

  1. The air leaving the impeller swirls, and the vanes turn it back into straight-through flow so the swirl is recovered as useful pressure
  2. The vanes carry the bearings in the middle of the airstream
  3. The vanes block reverse airflow when the fan is shut down
  4. The vanes protect the belts from the airstream
Show Answer

The correct answer is A. Air leaving the impeller of a tubular fan carries a swirl. Straightening vanes are fixed vanes downstream of the impeller that take the swirl out and turn the air back into straight-through flow. Recovering that swirl as useful pressure is why a vaned tubular fan can develop more pressure than a plain tube, the same job guide vanes do on a vaneaxial fan. The vanes do not carry bearings, act as a backdraft damper, or shield belts; the belt tube does that last job.

Concept Tested: Straightening Vanes

See: Tubular Housings: Straightening Vanes, Discharge Cone and Belt Tube

10. An exploded view labels a MOTOR COVER at the top, a SHROUD below the impeller, a CURB CAP, and a ROOF CURB at the bottom. Which fan family's drawing is this?

  1. A housed centrifugal fan with a scroll housing
  2. A belt driven plug fan
  3. A square inline fan
  4. A centrifugal roof exhauster
Show Answer

The correct answer is D. FanPedia's centrifugal roof exhauster view stacks the curb cap directly on the roof curb, places a shroud around the impeller to guide the air it throws off, and labels a motor cover above the motor housing. A housed centrifugal would show side plates, a scroll and a cutoff; a plug fan would show a mounting panel and insulation plug; and a square inline fan would show removable access panels. Sorting labels this way is how an exploded view becomes a parts list.

Concept Tested: Roof Exhauster Shroud

See: Rooftop Hardware: Shroud, Curb Cap, Nozzle and Windband