Quiz: Centrifugal and Axial Impeller Types¶
Test your understanding of impeller parts, centrifugal and axial impeller types, airflow codes and hub position, hub types, and impeller balancing with these review questions.
1. What does FanPedia's centrifugal impeller drawing call the ring-shaped plate on the inlet side of the impeller, also known as the impeller shroud?¶
- The hub boss
- The back plate
- The wiper bar
- The impeller front plate
Show Answer
The correct answer is D. The impeller shroud is the ring-shaped plate on the inlet side of a centrifugal impeller, with a large central hole where air enters. FanPedia labels it the impeller front plate, and both names are common in the trade. Together with the back plate on the closed side, it sandwiches the blades and guides air outward. The hub boss is the raised center of an axial hub, and the wiper bar is a housing part near the inlet funnel.
Concept Tested: Impeller Shroud
2. According to FanPedia, what do back plate fins (also called thrust fins or anti-thrust vanes) do?¶
- They add airflow by acting as a second set of blades
- They reduce the thrust load on the bearings and help limit airstream leakage through the shaft hole
- They hold the blades of a forward curved impeller in a drum shape
- They lock the hub to the shaft without a key
Show Answer
The correct answer is B. Back plate fins are small radial vanes on the outside of the back plate. FanPedia says they "reduce the thrust load on the bearings" and can create a negative pressure behind the impeller that draws air in through the shaft hole, which helps reduce leakage of the airstream to atmosphere. They are not working blades. End rings hold forward curved blades, and a keyless hub lock is a different part entirely.
Concept Tested: Back Plate Fins
See: Back Plate Fins
3. What does it mean that a backward inclined fan has a non-overloading power curve?¶
- Its brake horsepower is the same at every airflow
- Its brake horsepower keeps rising as airflow increases
- At a fixed speed, its brake horsepower rises to a peak and then falls, so a motor sized for the peak cannot be overloaded at that speed
- It can never draw more than its design-point horsepower, whatever the speed
Show Answer
The correct answer is C. On a backward inclined fan's curve, brake horsepower at a fixed speed rises to a peak and then falls as airflow keeps increasing. A motor sized for that peak cannot be overloaded at that speed, wherever the fan ends up operating on its curve. Option B describes the overloading curve of a forward curved impeller. Option D overstates the idea: the protection holds only at that speed, not at any speed.
Concept Tested: Backward Inclined Impeller
4. A 24-inch centrifugal impeller runs at 1,750 rpm. Using V = πDN / 12, what is its tip speed?¶
- About 11,000 ft/min
- About 132,000 ft/min
- About 3,500 ft/min
- About 5,500 ft/min
Show Answer
The correct answer is A. Tip speed is π × 24 × 1,750 / 12 = π × 3,500 ≈ 10,996 ft/min, about 11,000 ft/min. Option B forgets to divide by 12 to convert inches to feet, option C leaves out π, and option D uses the radius instead of the diameter. Tip speed matters because higher tip speed lets the blades build more pressure and also puts more stress on the impeller.
Concept Tested: Centrifugal Impeller
5. A tubeaxial fan order specifies airflow = PM. Which hub position and hub bore go with that build?¶
- Hub on intake side, with a reverse bore hub
- Hub on discharge side, with a reverse bore hub
- Hub on discharge side, with a standard bore hub
- Hub on intake side, with a standard bore hub
Show Answer
The correct answer is A. Read the code as the air's path: PM means through the impeller first, then over the motor. FanPedia calls that build hub on intake side, and its left hand and right hand pages pair it with a reverse bore hub. The opposite build, MP (over the motor, then through the impeller), is hub on discharge side with a standard bore hub. Options B and D mix the two builds.
Concept Tested: Hub on Intake Side
6. A contractor asks for a direct drive fan with a double inlet impeller mounted on the motor shaft (Arrangement 4). Why is that request inconsistent?¶
- Arrangement 4 is a belt drive arrangement
- Double inlet impellers are used only on axial fans
- A double inlet impeller takes air from both sides, so a motor shaft ending inside it would block one inlet; double width fans are center hung in Arrangements 3 and 7
- Arrangement 4 requires a forward curved impeller
Show Answer
The correct answer is C. A single width single inlet impeller has one closed side, so a shaft can enter from that side and the impeller can hang on the end of the motor shaft. A double inlet impeller takes air from both sides, so a shaft cannot end inside it without blocking one inlet. FanPedia's double width, double inlet page lists only Arrangement 3 and 7 variants, with the impeller between two bearings. Arrangement 4 is direct drive, not belt drive.
Concept Tested: Single Width Single Inlet
7. Why does a taper lock hub make impeller change-outs easier than a straight bore hub?¶
- It needs no bushing at all
- Loosening the bushing screws releases the taper's grip, so the hub does not have to be forced off a close-fitting bore
- It is welded to the shaft, so the shaft and impeller are replaced together
- Its bore is machined to match one shaft diameter exactly
Show Answer
The correct answer is B. In a taper lock hub, a tapered bushing with a straight bore is drawn into the tapered hub by its screws, clamping it onto the shaft. Backing the screws out, using the bushing's removal holes, releases the grip, so change-outs do not depend on pulling a hub off a close-fitting bore. Option D describes the straight bore hub, which must be machined to one shaft diameter and is the type that can seize.
Concept Tested: Taper Lock Hub
See: Taper Lock Hub
8. A 100 lb impeller with a maximum service speed of 1,800 rpm is balanced to grade G6.3. Using U = 6.015 G W / N, what is the permissible residual unbalance?¶
- About 0.21 oz·in
- About 21.1 oz·in
- About 6.32 oz·in
- About 2.11 oz·in
Show Answer
The correct answer is D. Multiply 6.015 × 6.3 × 100 = 3,790, then divide by 1,800 rpm to get about 2.11 oz·in. Options A and B are off by a factor of ten, a decimal slip. Option C is the chapter's worked example for a 200 lb impeller at 1,200 rpm. A higher speed or a tighter grade (smaller G) lowers the permissible unbalance.
Concept Tested: Impeller Balancing
See: Impeller Balancing
9. An impeller balanced for 1,200 rpm is later run at 2,400 rpm with the same residual unbalance. How does the rotating force from that unbalance change?¶
- It stays the same, because the unbalance has not changed
- It becomes about four times as large
- It doubles, in proportion to speed
- It falls by half, because the impeller is better centered at speed
Show Answer
The correct answer is B. The force from a given unbalance grows with the square of speed, F ≈ 1.77 U (N/1000)². Doubling the speed multiplies the force by 2² = 4; in the chapter's example, about 16 lbf at 1,200 rpm becomes about 64 lbf at 2,400 rpm. Option C assumes a straight proportion. This is why an impeller balanced for one speed is not automatically acceptable at a higher one, and why speed increases are checked against the catalog.
Concept Tested: Impeller Balancing
See: Impeller Balancing
10. A colleague calls a wheel "forward curved" because it has many narrow blades. What should be checked before that impeller type goes on the order?¶
- The impeller diameter, since forward curved wheels are always larger
- Whether the hub is straight bore or taper lock
- Whether the blade tips point in the direction of rotation or trail it, using the rotation arrow
- Whether the fan has back plate fins
Show Answer
The correct answer is C. Blade count and width do not identify the type; the direction of the blade curve against rotation does. Forward curved blade tips point in the direction of rotation, while backward inclined tips trail it. Find the rotation arrow and confirm the curve before writing the impeller type. Forward curved wheels actually move a given airflow in a smaller size, and hub type and back plate fins say nothing about blade shape.
Concept Tested: Forward Curved Impeller