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Quiz: Roof, Wall and Specialty Exhaust Fans

Test your understanding of building ventilation fans, roof and wall discharge styles, application specialists and entrained flow with these review questions.

1. What is a gravity relief ventilator?

  1. A downblast roof exhauster fitted with a gravity backdraft damper
  2. A small forward curved blower packaged in a ceiling cabinet
  3. A weather-protected roof opening with no fan that lets air leave or enter under natural pressure differences
  4. A low profile axial roof ventilator whose damper closes by gravity when the fan stops
Show Answer

The correct answer is C. A gravity relief ventilator has no fan at all. It is a weather-protected roof opening that lets air leave (relief) or enter (intake) a building under natural pressure differences, such as warm air rising, wind across the roof, or the slight positive pressure from supply fans. It is often specified beside a supply fan so the air can leave without a second motor. Option B describes a cabinet ventilator.

Concept Tested: Gravity Relief Ventilator

See: Gravity Relief Ventilator

2. Twin City Fan's current catalog rates its smoke and heat exhaust fans to withstand at least which conditions?

  1. 250°F for one hour
  2. 600°F continuously
  3. 1,000°F for four hours
  4. 500°F for four hours or 1,000°F for 15 minutes
Show Answer

The correct answer is D. A smoke and heat exhaust fan must keep running while it moves smoke and hot gas out of a burning building, so occupants can escape and firefighters can enter. The current catalog rates these fans to withstand a minimum of 500°F for four hours or 1,000°F for 15 minutes. Their motors, bearings and materials are chosen to survive temperatures that would stop an ordinary fan.

Concept Tested: Smoke and Heat Exhaust Fan

See: Smoke and Heat Exhaust Fan

3. Why can a plenum fan feed several openings in an air handler cabinet at once, while a housed centrifugal fan feeds only one outlet?

  1. With no scroll, the impeller discharges radially all the way around, and the cabinet decides where the air goes
  2. A plenum fan carries several outlet flanges, one for each duct connection on the cabinet
  3. A plenum fan uses an axial impeller that spreads the air evenly across the cabinet
  4. A plenum fan runs faster than a housed fan, so its discharge reaches every opening
Show Answer

The correct answer is A. A plenum fan is an unhoused centrifugal fan that runs inside an air plenum. With no scroll to collect it, air leaves the impeller radially, 360 degrees around the wheel, and pressurizes the cabinet section. The cabinet does the job a housing would do, so top and side openings all draw from the same pressurized space. A housed fan concentrates all its air into one scroll outlet.

Concept Tested: Plenum Fan

See: Plenum Fan

4. Why is a downblast roof exhauster suited only to relatively clean air?

  1. Its mushroom-shaped motor cover cannot pass large airflows
  2. It lays its discharge back over the roof, where grease collects and fumes can reach outdoor-air intakes
  3. It has no bird screen, so debris can enter the impeller
  4. Its forward curved wheel clogs quickly when the air carries contaminants
Show Answer

The correct answer is B. Downblast discharge turns the air leaving the impeller back down toward the roof and out around the unit. That low silhouette and built-in weather protection suit restroom, office and storage exhaust. For kitchen, fume or hot exhaust, the discharge would land on the roof, where grease collects and fumes find the nearest outdoor-air intake, so an upblast unit that throws the plume up and away is the right choice.

Concept Tested: Downblast Discharge

See: Downblast Discharge

5. Fume hoods exhaust 6,000 CFM of contaminated air at 50 ppm. A bypass damper admits 2,000 CFM of outside air, and the nozzle entrains another 4,000 CFM. What is the contaminant concentration in the plume leaving the stack?

  1. 25 ppm
  2. 37.5 ppm
  3. 50 ppm
  4. 16.7 ppm
Show Answer

The correct answer is A. Fan flow is 6,000 + 2,000 = 8,000 CFM, and total flow is 8,000 + 4,000 = 12,000 CFM. The contaminant amount stays the same while the air volume grows, so the plume concentration is 50 × 6,000 ÷ 12,000 = 25 ppm. Option B divides by the fan flow and forgets the entrained air. Option C ignores dilution entirely, and option D uses the entrained flow in place of the contaminated air.

Concept Tested: Entrained Flow

See: Entrained Flow in Laboratory Exhaust

6. A restaurant needs to exhaust grease-laden air from a cooking hood through the roof. Which choice matches the chapter's guidance?

  1. A direct drive downblast roof exhauster, such as the DCRD, for its low profile
  2. A gravity relief ventilator, such as the GRV, to avoid a second motor
  3. A listed kitchen exhaust fan with upblast discharge, such as the BCRUR
  4. A fiberglass roof exhauster, such as the WA, for its resistance to moisture
Show Answer

The correct answer is C. Grease is the deciding factor: it coats any surface it lands on and is a fire hazard. Kitchen fans therefore use upblast or wall discharge that throws the plume clear of the roof, and they carry a safety listing for grease-laden air; Twin City Fan lists its kitchen fans to UL 705 Supplement SC. A downblast unit would lay grease on the roof, a gravity ventilator has no fan, and fiberglass answers corrosion rather than grease.

Concept Tested: Kitchen Exhaust Fan

See: Kitchen Exhaust Fan

7. A wastewater treatment plant needs to exhaust humid air carrying corrosive vapors. Which choice does the chapter point to?

  1. A smoke and heat exhaust fan, built to survive high temperatures
  2. A fiberglass fan, molded from FRP for moisture-laden and corrosive air
  3. A standard steel housed fan with a downblast roof discharge
  4. A stainless steel fan, described as the most economical corrosion-resistant choice
Show Answer

The correct answer is B. A fiberglass fan has its housing, and often its impeller, molded from fiberglass-reinforced plastic so it can exhaust moisture-laden, corrosive or chemically contaminated air that would eat through carbon steel. Wastewater treatment is one of the typical jobs. The current catalog describes FRP as more economical than stainless steel or other exotic alloys, which rules out option D. Options for resin type, surface veil and a stainless shaft match the chemistry.

Concept Tested: Fiberglass Fan

See: Fiberglass Fan

8. What does a mancooler do?

  1. It exhausts hot air from a foundry through the roof
  2. It supplies filtered make-up air through a hooded roof unit
  3. It cools motors and bearings that sit in a hot airstream
  4. It blows air directly on workers in a hot space to help them cool
Show Answer

The correct answer is D. A mancooler is an axial fan packaged to blow air directly on workers in a hot industrial space, such as a foundry pour station or a loading dock. It does not exhaust anything; it moves air across the body to speed evaporative and convective cooling. Because it runs within reach of people, inlet and outlet screens fabricated to meet OSHA safety requirements are standard on both the movable and stationary forms.

Concept Tested: Mancooler

See: Mancooler

9. A contractor has a rectangular duct above an office ceiling with little space, needs the air to turn 90 degrees at the fan, and wants service access without removing the fan from the duct. Which fan style fits best?

  1. A tubeaxial fan, because it moves air straight through a compact tube
  2. A housed centrifugal fan, because it discharges at a right angle to its shaft
  3. A plenum fan, because it discharges in every direction around the impeller
  4. A square inline centrifugal fan, because it fits rectangular duct, can turn the air 90 degrees and has removable side panels
Show Answer

The correct answer is D. A square inline centrifugal fan puts a centrifugal impeller in a square housing with removable side panels. It drops into rectangular duct runs and above-ceiling spaces, can move air straight through or through a 90-degree turn depending on the duct connections, and can be serviced without being taken out of the duct. A tubeaxial cannot make the turn, a housed fan needs more space and transitions, and a plenum fan needs a cabinet.

Concept Tested: Square Inline Centrifugal Fan

See: Square Inline Centrifugal Fan

10. Why does a tubeaxial fan discharge air that still carries some rotational swirl?

  1. Its plain cylindrical housing has no guide vanes to straighten the air
  2. Its impeller turns faster than the impeller of a vaneaxial fan
  3. It uses a centrifugal impeller that throws air against the tube wall
  4. Its belt tube deflects the air as the air passes the drive
Show Answer

The correct answer is A. A tubeaxial fan is an axial fan in a plain cylindrical housing with no guide vanes. Air enters one end, passes through the impeller and leaves the other end in a straight line, but with some rotational swirl, because nothing straightens it. A vaneaxial fan adds a vane section that straightens the airflow, which suits it to higher pressures. The belt tube only shields the belt where it crosses the airstream.

Concept Tested: Tubeaxial Fan

See: Tubeaxial Fan