Quiz: Impeller Construction and Material Handling¶
Test your understanding of impeller construction, clean-air and double width impellers, axial blade details, hub variants, and radial impellers for dirty airstreams with these review questions.
1. Which impeller has backward inclined blades with a hollow, wing-shaped cross section and reaches the highest static efficiency of the centrifugal family?¶
- Airfoil impeller
- Backward curved impeller
- Radial tip impeller
- Air handling impeller
Show Answer
The correct answer is A. An airfoil impeller has backward inclined blades with an airfoil cross section: a hollow blade with a rounded leading edge and a tapered trailing edge. Air follows that shape with less separation, so airfoil wheels reach the highest static efficiency of the centrifugal family. The trade-off is that a hollow, formed blade is less forgiving of abrasive or sticky material. Backward curved blades are single thickness plate, and radial tip and air handling impellers are radial designs.
Concept Tested: Airfoil Impeller
2. What is an impeller tie rod?¶
- A triangular plate welded where a blade meets the hub
- A flat ring joining the blade ends at the face of the impeller
- A rod running across the width of the impeller, between the end rings, that resists the blades spreading under load
- A keyless bushing that grips the shaft when tightened
Show Answer
The correct answer is C. A tie rod runs across the width of the impeller, usually between the end rings, tying them together so the blades do not spread under centrifugal load. FanPedia's forward curved drawing calls out both the end ring and the tie rod, because a long, narrow-bladed drum is exactly the shape that needs one. Option A is a gusset, option B an end ring, and option D a Trantorque bushing.
Concept Tested: Impeller Tie Rod
See: Holding an Impeller Together: End Rings, Tie Rods and Gussets
3. How does the airflow of a double width double inlet (DWDI) fan compare with a single width single inlet (SWSI) fan of the same diameter, width per side and speed?¶
- About the same airflow at twice the pressure
- Roughly twice the airflow at about the same pressure
- About half the airflow, because the two inlets interfere
- Twice the airflow and twice the pressure
Show Answer
The correct answer is B. A DWDI impeller is, in effect, two single width impellers back to back, with an inlet on each side, so both halves work at once. That gives roughly twice the airflow of the SWSI fan at about the same pressure, which is why DWDI fans are common in air handlers and duct fans. The chapter stresses that this is a rule of thumb for comparing families, not a rating; real selections come from catalog tables.
Concept Tested: Double Width Double Inlet
See: What DWDI Means
4. A contractor wants a DWDI fan to recirculate 160°F oven air. Which choice follows FanPedia's guidance?¶
- A standard Arrangement 3 DWDI fan, because the airflow fits
- An Arrangement 4 DWDI fan with the impeller on the motor shaft
- A standard Arrangement 3 DWDI fan with back plate fins added
- A DWDI fan in Arrangement 3DI or 7DI, whose integrated inlet boxes put the bearings outside the airstream
Show Answer
The correct answer is D. In a standard Arrangement 3 DWDI fan both bearings sit in the airstream, so FanPedia limits it to clean air at 130°F. At 160°F the bearings would sit in hot air. FanPedia describes Arrangements 3DI and 7DI as using integrated inlet boxes to place the bearings outside the airstream, allowing elevated temperatures with relatively clean air. Arrangement 4 is not offered for double inlet impellers, and fins do not move the bearings.
Concept Tested: Double Width Double Inlet
5. A plant exhausts long, light paper shreddings that tend to hang on and wrap around the hub arms and blades. Which impeller is designed for this?¶
- Open radial paddle impeller (RBO)
- Air handling impeller (RBA)
- High efficiency airfoil impeller (HAF)
- Back plate wool material handling impeller (RBW)
Show Answer
The correct answer is D. The material handling impeller (wool), sold as the back plate wool impeller RBW, is the open radial design with a full back plate. The catalog describes it as designed to keep paper shreddings, threads and similar matter from hanging up on and wrapping around the spiders and blades, for long, stringy or fibrous material. The open paddle suits sticky or abrasive material, the air handling impeller is for relatively clean air, and an airfoil wheel is for clean air only.
Concept Tested: Material Handling Impeller
6. A field technician wants to "just add a few degrees of pitch" to an adjustable pitch axial impeller to get more air at the same speed. What else changes?¶
- Nothing; pitch affects only airflow
- Brake horsepower rises too, so the motor could be overloaded
- The impeller diameter increases
- The airflow direction reverses
Show Answer
The correct answer is B. Blade pitch angle is the angle between the blade and its plane of rotation. A steeper pitch takes a bigger bite of air each revolution, so at a fixed speed it moves more air and draws more power. Before anyone changes the blade angle, the new operating point should be checked against the catalog curve for that pitch and the motor's margin confirmed. Pitch does not change the diameter, and airflow direction is set by the hub bore and rotation.
Concept Tested: Blade Pitch Angle
7. Adhesive-laden dust with some abrasive filler is conveyed from a glue station to a collector, passing through the fan. Which impeller gives the material the least to build up on?¶
- Open radial paddle impeller (RBO)
- Backward curved impeller (HIB)
- Airfoil impeller (HAF)
- Forward curved impeller
Show Answer
The correct answer is A. An open radial paddle impeller has single thickness radial paddle blades on the hub, braced with support gussets, and no back plate or rings, so sticky or abrasive material has nothing to catch on. The catalog's RBO uses this open type paddle impeller. On backward curved or airfoil wheels, sticky deposits would build up on curved surfaces and unbalance the impeller, and forward curved impellers have shallow blade pockets that collect dust.
Concept Tested: Open Radial Paddle Impeller
8. Why does a radial bladed impeller survive dirty, sticky or abrasive airstreams better than curved-blade designs?¶
- Its blades are hollow, so material passes through them
- It runs at much lower tip speed than any other impeller
- Material strikes the flat, heavy blades and is flung off the tip instead of sliding along a curved surface and building up
- Its blades curve forward, which scoops material away from the hub
Show Answer
The correct answer is C. A radial bladed impeller's flat blades point straight out along radii of the wheel. Material strikes a flat, heavy blade and is flung straight off the tip instead of sliding along a curved surface and building up, and the blades carry their centrifugal load in straight tension. The price is efficiency: radial blades guide air less smoothly than backward curved or airfoil blades. Hollow blades belong to airfoil impellers, which are less tolerant of dirty air.
Concept Tested: Radial Bladed Impeller
9. A fan exhausts a large volume of air from the clean side of a baghouse, where only light dust remains. A radial bladed fan would work. Why is a backward curved high efficiency fan (HIB) the better choice?¶
- The radial bladed fan cannot move a large volume of air
- Backward curved wheels are more rugged than radial bladed wheels
- The HIB is the only fan that can be mounted on a baghouse
- The duty does not need radial ruggedness, so the radial fan would waste power; the HIB handles light dust loading at higher efficiency
Show Answer
The correct answer is D. Once the dust has been filtered out, the airstream no longer needs the rugged, self-cleaning radial design, and that design costs efficiency. The catalog lists the HIB backward curved fan for clean air or light dust loading and for the clean side of baghouses. A radial bladed fan would work but would waste power for the life of the fan. Option B reverses the efficiency-ruggedness trade-off.
Concept Tested: High Efficiency Centrifugal Fan
10. Which ordering runs from the impeller best suited to truly clean air at the highest efficiency to the one most able to survive a dirty airstream?¶
- Airfoil, backward curved, radial tip, radial bladed
- Radial bladed, radial tip, backward curved, airfoil
- Backward curved, airfoil, radial bladed, radial tip
- Radial tip, airfoil, radial bladed, backward curved
Show Answer
The correct answer is A. The chapter traces one pattern: shapes that guide air most smoothly are the shapes dust can erode, coat or fill. Airfoil wheels give the highest efficiency for clean air; backward curved trades a little efficiency for tolerance of light dust; radial tip handles dirty gas while staying more efficient than a radial blade at high airflow; and radial bladed is the rugged workhorse. Option B is the same list reversed, from most rugged to most efficient.
Concept Tested: Backward Curved Impeller