Quiz: Damper Details and Duct Connections¶
Test your understanding of damper types and details, inlet vanes and pre-spin, inlet box styles, and the connections that join a fan to its ductwork with these review questions.
1. Which description matches a blast gate?¶
- A flat gate that slides across the airway to give manual adjustment on cast aluminum pressure blowers
- A parallel blade, end-pivoted damper that closes by gravity when the fan stops
- A wafer-type butterfly valve on the outlet flange that controls flow to full shutoff, rated to 250°F, with a 125# ASA bolt pattern
- A ring of radial vanes built into the inlet funnel that pre-spins the air
Show Answer
The correct answer is C. FanPedia describes the blast gate as a wafer-type butterfly valve that mounts to the fan's outlet flange and controls flow all the way to full shutoff. It has a maximum temperature of 250°F, is available for automatic control, and its bolt pattern matches a 125# ASA pipe flange. Option A is the slide gate damper, option B a ceiling or wall gravity damper, and option D nested inlet vanes.
Concept Tested: Blast Gate
2. What pressure and temperature rating does FanPedia give for the rubber sleeve connector of the tube adapter and sleeve?¶
- Up to 5 psi and 180°F
- Up to 15 psi and 250°F
- Up to 5 psi and 600°F
- Up to 125 psi and 180°F
Show Answer
The correct answer is A. The tube adapter and rubber sleeve with clamps is a ready-made flexible outlet connection for pressure blowers: a 4-inch metal collar and flange that bolts to the blower, plus a 6-inch, 2-ply molded rubber slip-type connector held by two hose clamps. The connector is rated for pressures up to 5 psi and 180°F. Option B borrows the blast gate's 250°F limit, option C the inlet vane limit, and option D confuses the "125#" flange class with a pressure rating.
Concept Tested: Tube Adapter and Sleeve
3. How does a ceiling type gravity damper differ from a wall type gravity damper?¶
- The ceiling type uses opposed blades, while the wall type uses parallel blades
- The ceiling type needs an actuator, while the wall type closes by gravity
- The wall type keeps out rain and snow, while the ceiling type only prevents backflow
- They share the same technical description and differ only in mounting plane, horizontal for a ceiling and vertical for a wall
Show Answer
The correct answer is D. FanPedia's technical description is the same, word for word, for both: parallel blade, end-pivoted, opening when the fan is energized, closing by gravity when power is off, and blocking backflow and weather. The ceiling type sits in a horizontal plane and the wall type in a vertical plane, so the two are not interchangeable in the field. Both use parallel blades (A), neither needs an actuator (B), and both exclude rain and snow (C).
Concept Tested: Wall Type Gravity Damper
4. What causes blade flutter in a damper?¶
- An actuator driving the blades faster than the airflow can follow
- Airflow alone holding the blades partly open at low flow, so the velocity pressure only partially opens them
- Airfoil blades that are too stiff for the damper frame
- Closing an opposed blade damper past 75% open
Show Answer
The correct answer is B. FanPedia's description of motorized dampers names the cause: in low-flow conditions, the velocity pressure of the airstream may only partially open automatic damper blades, creating blade flutter and potentially more noise. The remedy is a damper actuator that drives the blades to a commanded position, so an actuator is the cure, not the cause (A). Airfoil blades are stiffer for their weight, which is a benefit (C), and the 75% figure applies to parallel blade modulation, not flutter (D).
Concept Tested: Blade Flutter
See: Damper Blades: Flat or Airfoil, and the Flutter Problem
5. What is the main trade-off of choosing nested inlet vanes instead of external inlet vanes?¶
- Nested vanes are limited to clean airstreams, and replacing them means replacing the entire inlet funnel assembly
- Nested vanes cannot be fitted with an actuator, so they are manual only
- Nested vanes do not pre-spin the air, so they save less power
- Nested vanes are limited to 250°F, while external vanes reach 600°F
Show Answer
The correct answer is A. Nested inlet vanes are built into the inlet funnel instead of a separate housing bolted to the inlet. They are applied to clean airstreams up to 600°F, while external vanes are the choice for contaminated air, and replacing nested vanes requires replacing the inlet funnel assembly. Both styles come with a manual handle and can take an actuator (B), both pre-spin the air (C), and both reach 600°F (D).
Concept Tested: Nested Inlet Vanes
6. On a fan running at its wide-open operating point, an outlet damper is closed so that the resistance multiplier m rises from 1.0 to 2.0. What happens to the operating point?¶
- Airflow falls to half, and static pressure stays the same
- The fan curve drops, so both airflow and static pressure fall
- The steeper system curve meets the unchanged fan curve at lower airflow and higher static pressure
- Airflow stays the same, and static pressure doubles
Show Answer
The correct answer is C. Throttling multiplies the system's resistance in P = m k Q², making the system curve steeper. The fan curve does not move (B is wrong), so the new intersection lies to the left along the fan curve, at lower airflow and, on a typical curve, higher static pressure. Airflow does not fall in proportion to m (A): in the chapter's illustrative explorer, doubling m drops airflow from 10,000 to about 7,385 CFM while pressure rises from 5.00 to 5.45 in. wg. Option D ignores the airflow change.
Concept Tested: Damper Throttling
See: Damper Throttling
7. A pressure blower discharges into a pipe line at 3 psi, but the airstream is 220°F. The pipe has a 125# ASA flange of the same nominal size as the blower outlet. Which connection fits?¶
- Tube adapter and rubber sleeve, because the pressure is below 5 psi
- Slip connection with a rubber sleeve and clamps
- Free-standing inlet box with an outlet collar
- Bolted flange connection to the matching 125# ASA pipe flange
Show Answer
The correct answer is D. The tube adapter's rubber sleeve is rated to 5 psi and 180°F. The pressure passes, but 220°F exceeds the temperature rating, so the sleeve is ruled out (A), and a rubber-sleeve slip connection (B) has the same weakness. Because a 125# ASA bolt pattern bolts directly to a standard pipe flange of the same nominal size and class, a bolted flange connection is the fit. An inlet box (C) does not apply to a discharge pipe line.
Concept Tested: ASA Pipe Flange Bolt Pattern
8. An order shows a centrifugal fan on spring isolators with ductwork hard-bolted at both the inlet and the outlet. What should be added?¶
- Companion flanges at both ends so the bolt holes line up
- Flexible connectors at the inlet and outlet
- A blast gate at the outlet
- Bearing dowel pins to stop the fan from moving on its springs
Show Answer
The correct answer is B. Rigid duct ties an isolated fan back to the structure, defeating the isolators and loading the housing as the springs move. FanPedia lists flexible connectors as required on all isolated fans; they reduce vibration transmission to the ductwork and allow some misalignment. Companion flanges (A) still make a rigid bolted joint. A blast gate (C) controls flow, and dowel pins (D) restrain bearings on a pedestal, not the fan on its isolators.
Concept Tested: Flexible Connector
9. On a laboratory exhaust system, fume hood sashes close and building exhaust demand drops, but the fan must keep its airflow and plume height. Which damper responds, and how?¶
- The isolation damper closes partway to throttle the fan
- An outlet damper on the fan discharge closes to raise discharge velocity
- The isolation damper on the standby fan opens to share the load
- The bypass damper opens to admit outside air through the bypass air plenum, making up the lost flow
Show Answer
The correct answer is D. The bypass damper controls outside air admitted through the bypass air plenum. As building demand drops, it opens to make up the difference, so the fan keeps moving a nearly constant volume and holds the discharge velocity that carries the plume clear of the building. The isolation damper's job is different: it closes behind an idle fan so running fans cannot pull air backward through it, so options A and C confuse the two. Throttling the discharge (B) would reduce airflow, not hold it.
Concept Tested: Lab Exhaust Bypass Damper
10. Using Euler's turbomachinery equation, w = U2·Vθ2 − U1·Vθ1, why does pre-spin in the direction of impeller rotation reduce the power a fan draws?¶
- Pre-spin raises the blade speed U1 at the inlet, so the impeller turns faster
- Pre-spin removes the tangential velocity at the outlet, so the first term becomes zero
- Pre-spin makes the inlet swirl Vθ1 positive, so the second term subtracts from the work done on each unit mass of air
- Pre-spin adds resistance in the inlet, which moves the operating point left on the same fan curve
Show Answer
The correct answer is C. With no pre-spin, the inlet swirl Vθ1 is zero and the second term vanishes. Inlet vanes or an inlet box damper turn the incoming air in the direction of rotation, making Vθ1 positive, so U1·Vθ1 subtracts from the work. The fan develops less pressure, moves less air and draws less power. Blade speed depends on fan speed, not on the vanes (A), the outlet swirl is not removed (B), and option D describes outlet damper throttling, which saves less power.
Concept Tested: Pre-Spin