Impeller Construction and Material Handling¶
Summary¶
This chapter goes deeper into impeller design: airfoil and backward curved blades, double width double inlet impellers, impeller width and airflow path, end rings, gussets and tie rods, axial blade tips, blade count and pitch angle, and fiberglass and Trantorque hubs. It then covers impellers built for dirty airstreams: radial bladed, radial tip, open radial paddle, material handling, and paper handling designs. After completing it, readers can explain why a given impeller suits clean air, high efficiency, or a particulate-laden airstream.
Concepts Covered¶
This chapter covers the following 23 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Impeller End Ring | 2 |
| Impeller Gusset | 1 |
| Axial Blade Tip | 1 |
| Axial Blade Count | 1 |
| Blade Pitch Angle | 1 |
| Double Width Double Inlet | 8 |
| Backward Curved Impeller | 4 |
| Airfoil Impeller | 4 |
| Reverse Bore Hub | 1 |
| Impeller Airflow Path | 1 |
| Impeller Width | 1 |
| Impeller Tie Rod | 1 |
| Fiberglass Hub | 1 |
| Trantorque Hub | 1 |
| Radial Bladed Impeller | 13 |
| High Efficiency Centrifugal Fan | 1 |
| Bearing Structural Support | 1 |
| Setback Outlet Flange | 1 |
| Radial Tip Impeller | 3 |
| Material Handling Impeller | 3 |
| Open Radial Paddle Impeller | 3 |
| Air Handling Impeller | 1 |
| Paper Handling Impeller | 2 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
- Chapter 4: Centrifugal, Axial and Mixed Flow Fans
- Chapter 5: Roof, Wall and Specialty Exhaust Fans
- Chapter 6: Industrial Fans and Application Product Lines
- Chapter 7: Housing Construction and Exploded Views
- Chapter 8: Centrifugal and Axial Impeller Types
Why Impeller Construction Matters¶
Chapter 8 taught you to name an impeller by its blade shape. This chapter looks at how impellers are actually built, which parts hold them together, and why Twin City Fan offers so many radial designs for airstreams that carry sawdust, paper trim, threads or grit. The question behind every section is the one a customer asks on the phone: what is in the airstream, and which wheel will survive it while still meeting the duty point?
From Wheel Shape to Wheel Survival
A selection that looks perfect on the performance curve can be wrapped in shredded paper or worn through in a few months if the impeller is wrong for the airstream. By the end of this chapter you will be able to read a job description and say which impeller belongs in it, and why. Let's follow the airflow.
Reading an Impeller: Airflow Path and Width¶
Impeller Airflow Path¶
The impeller airflow path is the route air takes from the moment it reaches the blades until it leaves them. FanPedia's technical descriptions (p. 186) separate impellers by exactly this: in a centrifugal or radial impeller, air enters axially, along the shaft, is accelerated by the blades and leaves radially, at a right angle to the shaft. In an axial impeller, the blades push air along the shaft with a swirling, tangential motion. A mixed flow impeller sits between the two; FanPedia (p. 32) describes its airfoil blades tapering from the impeller front plate to the hub end, which gives a larger tip width and a diagonal path. The path matters because it decides the housing shape, where ductwork connects, and where material in the air will strike the blades.
The table below collects the three paths you have just read about.
| Impeller family | Air enters | Air leaves | Where to look on a drawing |
|---|---|---|---|
| Centrifugal (radial flow) | Along the shaft, through the inlet | At a right angle to the shaft, into the scroll | Inlet on the side, outlet on the scroll |
| Axial | Along the shaft | Along the shaft, with swirl | Inlet and outlet on the same centerline |
| Mixed flow | Along the shaft | Diagonally, then turned back to the axis | Tubular housing, tapered blades |
Impeller Width¶
Impeller width is the axial distance across the blades of a centrifugal impeller, from back plate to front plate (or inlet side to inlet side on a double inlet wheel). At a fixed diameter and speed, a wider impeller passes more air, while pressure capability is set mostly by tip speed. FanPedia (p. 4) notes that pressure blowers use narrower housings and impeller widths than standard centrifugal fans, and that Arrangement 4 plenum fans "can be selected with varying impeller widths to provide desired performance at direct drive motor speeds" (p. 170). The current Twin City Fan catalog lists special width and diameter impellers as an option on its radial tip and high efficiency fans, for the same reason: width is the factory's way to fit flow to a fixed motor speed.
Holding an Impeller Together: End Rings, Tie Rods and Gussets¶
A spinning impeller carries large centrifugal loads, so every design needs parts whose only job is to keep the blades where they belong. Three of these show up on FanPedia's impeller drawings (pp. 31–32).
An impeller end ring (FanPedia also calls it an end rim) is a flat ring that joins the ends of the blades at the face of the impeller. On a forward curved wheel, end rings at each side hold the many short, shallow blades in a drum shape. On the radial bladed paddle impeller, Twin City Fan adds front and back end rings to the open paddle design, which stiffens the blades without closing off the back of the wheel.
An impeller tie rod is a rod that runs across the width of the impeller, usually between the end rings, to tie them together and resist the blades spreading under load. FanPedia's forward curved drawing calls out both the end ring and the tie rod; a long, narrow-bladed drum is exactly the shape that needs one.
An impeller gusset is a triangular or trapezoidal plate welded where a blade meets the hub or back plate, bracing the blade against bending. The open radial paddle impeller uses support gussets at the hub, and the paper handling impeller uses trapezoidal gussets on its full back plate "for extra rugged durability" (FanPedia p. 32).
| Part | Joins | Where FanPedia shows it |
|---|---|---|
| End ring (end rim) | Blade ends at the impeller face | Forward curved; radial paddle impeller |
| Tie rod | One end ring to the other, across the width | Forward curved |
| Gusset | Blade to hub or back plate | Open radial paddle (support gussets); paper handling (trapezoidal gussets) |
Clean-Air Impellers Built for Efficiency¶
Backward Curved and Airfoil Impellers¶
Both designs below are refinements of the backward inclined impeller from Chapter 8: the blades lean away from the direction of rotation, which keeps the power curve from rising without limit as flow increases.
A backward curved impeller has single thickness blades that are curved, rather than flat, with the curve leaning back from the direction of rotation (FanPedia p. 31). The curve gives smoother air passage than a flat blade, while the single thickness plate stays simple, solid and tolerant of some dust.
An airfoil impeller has backward inclined blades with an airfoil cross section: a hollow, wing-shaped blade with a rounded leading edge and a tapered trailing edge (FanPedia p. 31). The airfoil shape lets air follow the blade surface with less separation, which is why 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 than a solid plate.
Efficiency and Ruggedness Pull in Opposite Directions
Notice the pattern that runs through this whole chapter: the shapes that guide air most smoothly are the shapes that dust can erode, coat or fill. Every step from airfoil to backward curved to radial tip to radial bladed trades some efficiency for the ability to survive a dirtier airstream.
High Efficiency Centrifugal Fans¶
A high efficiency centrifugal fan is a housed industrial centrifugal fan built around an airfoil or backward curved impeller and meant for high airflow at moderate to high pressure where power cost matters. Twin City Fan's current catalog groups two such models under "Radial Tip & High Efficiency Fans," describing the design as non-overloading:
- HAF High Efficiency Airfoil Fan: a heavy-gauge, all-welded, non-overloading airfoil impeller for clean air and high pressure; typical uses include forced draft on fluid boilers, dryers, solvent recovery, moisture blow-off, product cooling and recirculation.
- HIB High Efficiency Industrial Backward Curved Fan: a backward curved impeller for clean air or air with light dust loading; widely used on the clean side of baghouses, in high efficiency filtration, forced draft and high-pressure process supply.
The split between the two is the efficiency-ruggedness trade in miniature: choose the airfoil when the air is truly clean, and the backward curved wheel when some dust will get through.
Double Width Double Inlet Fans¶
What DWDI Means¶
A double width double inlet (DWDI) fan uses an impeller that is, in effect, two single width impellers placed back to back on one hub, inside a housing about twice as wide, with an inlet on each side. Air enters from both sides, turns, and leaves through a single scroll outlet. FanPedia's housed centrifugal page (p. 4) shows both forms side by side, and its discharge charts (pp. 48–49) list DWDI fans with forward curved, backward inclined and airfoil impellers.
Because both sides of the wheel work at once, a DWDI fan moves roughly twice the air of a single width single inlet (SWSI) fan of the same diameter and speed, at about the same pressure. That is why DWDI fans are common in air handlers and duct fans, where a lot of air must move through a limited footprint.
DWDI Flow Approximation¶
where:
- \( Q_{\text{DWDI}} \) is the airflow of the double width double inlet fan, in CFM
- \( Q_{\text{SWSI}} \) is the airflow of a single width single inlet fan with the same impeller diameter, the same width per side and the same speed, at the same pressure, in CFM
This is a sizing rule of thumb for comparing families, not a rating. Real selections always come from the catalog's performance tables, because the inlet conditions on each side are never perfectly identical.
Bearings, Supports and the Outlet Flange¶
Two parts on a DWDI housing exist because of the double inlet. FanPedia's technical description of Arrangement 3 DWDI (p. 167) explains that the impeller is mounted between the bearings and supported by the fan housing, and since both bearings are located in the airstream, standard DWDI fans should be used for clean air applications with air temperatures limited to 130°F.
The bearing structural support is the welded steel frame that carries each bearing in front of an inlet and transfers the rotor's weight into the housing. FanPedia's DWDI housing drawing (p. 61) shows one at each inlet, outboard of the inlet funnel. Because the support crosses the inlet opening, it sits in the incoming airstream; that is part of why the inlet conditions matter.
The setback outlet flange is the bolted flange frame around the DWDI discharge opening, labeled separately on FanPedia's DWDI housing drawing (p. 61). The duct connection is made to this flange, so its position and bolt pattern come from the certified dimensional drawing, not from an assumption that it matches an SWSI housing.
The drawing also repeats parts you met in Chapter 7: the housing side and housing scroll (the curved wrapper that collects air from the impeller), the cutoff (the tongue of the scroll at the outlet that separates discharging air from air that would recirculate), a wiper bar at each inlet funnel (FanPedia p. 59: it helps direct air through the housing in the inlet funnel area to prevent turbulence and increase efficiency), the two inlet funnels that direct air into the impeller, and the framing that stiffens the housing.
Diagram: DWDI Housing Parts Explorer¶
Run DWDI Housing Parts Explorer Fullscreen
DWDI Housing Parts Explorer
Type: infographic
sim-id: ch09-dwdi-housing-parts-explorer
Library: html
Status: Specified
Bloom Level: Remember
Bloom Verb: identify
Learning Objective: The learner will identify the eight labeled parts of a double width double inlet housing from a description of each part's job, with at least 7 of 8 correct.
Prerequisites: double width double inlet, impeller, bearing, inlet funnel, housing scroll, housing side, cutoff, wiper bar, framing, bearing structural support, setback outlet flange (all defined in the section "Double Width Double Inlet Fans" above).
Evidence of Mastery: In the check phase the learner reads eight function descriptions, one at a time, and clicks the part on the exploded DWDI housing that matches. A click is correct when it lands on the part named in the Content table's "Correct part" column. Mastery is 7 of 8 correct on the first attempt. Opening infoboxes in the explore phase is exploration, not evidence.
Misconceptions: (1) The bearing structural support is part of the motor base. (2) The cutoff and the wiper bar are the same part. (3) A DWDI fan has one inlet funnel, like an SWSI fan.
Instructional Rationale: Remember-level recognition of parts is built by pairing each label with its job and then retrieving the label from the job alone. Click-to-identify on the real exploded view ties the vocabulary to the picture a learner will see on a submittal drawing.
Content: An exploded view of a DWDI centrifugal housing with these labeled parts, each with an infobox:
| # | Part | Infobox text (explore phase) |
|---|---|---|
| 1 | Housing side | One of the two flat side sheets of the housing; on a DWDI fan each side carries an inlet. |
| 2 | Housing scroll | The curved wrapper that collects air leaving the impeller and guides it to the outlet. |
| 3 | Cutoff | The tongue of the scroll at the outlet that separates air leaving the fan from air that would recirculate around the housing. |
| 4 | Wiper bar (one at each inlet) | Helps direct air through the housing in the inlet funnel area to prevent turbulence and increase efficiency. |
| 5 | Inlet funnel (one at each inlet) | Aerodynamically shaped piece that directs air into the impeller. |
| 6 | Bearing structural support (one at each inlet) | Steel frame that carries a bearing in front of an inlet and transfers rotor weight into the housing; it sits in the airstream. |
| 7 | Setback outlet flange | Bolted flange frame around the discharge opening; the duct connects here, and its location comes from the dimensional drawing. |
| 8 | Framing | Structural members that stiffen the housing and carry it to its mounting. |
Check-phase descriptions, in this order:
| # | Description shown | Correct part | Feedback when wrong |
|---|---|---|---|
| 1 | Carries a bearing in front of an inlet, in the airstream. | Bearing structural support | This frame holds the bearing; it is not a motor base, and on a DWDI fan there is one at each inlet. |
| 2 | Where the discharge duct is bolted to the fan. | Setback outlet flange | The duct connects at the outlet flange around the discharge opening. |
| 3 | Separates air leaving the fan from air that would recirculate. | Cutoff | The cutoff is the scroll tongue at the outlet; the wiper bar sits at the inlet funnel. |
| 4 | Directs air into the impeller from each side. | Inlet funnel | A DWDI fan has an inlet funnel on each side. |
| 5 | Collects air leaving the impeller and guides it toward the outlet. | Housing scroll | The scroll is the curved wrapper around the impeller. |
| 6 | Reduces turbulence in the inlet funnel area. | Wiper bar | The wiper bar works at the inlet funnel; the cutoff works at the outlet. |
| 7 | Flat sheet that carries an inlet. | Housing side | Each side sheet of a DWDI housing carries one inlet. |
| 8 | Stiffens the housing and carries it to its mounting. | Framing | Framing is the structural steel around the housing, not part of the airflow path. |
Provenance: Part list and layout from FanPedia p. 61 (Housing Construction, Centrifugal Fans (DWDI)); wiper bar and inlet funnel wording from FanPedia p. 59 and p. 188; bearing location in the airstream from FanPedia p. 167 (Arrangement 3 DWDI).
Rules: In the check phase, clicking either of a duplicated part (wiper bar, inlet funnel, bearing structural support) counts as that part. Each description accepts one click per attempt.
Learner Activity:
- Explore phase: the learner clicks any labeled part and reads its infobox. A list shows which of the 8 parts have been opened.
- After opening all 8, or choosing to skip ahead, the learner starts the check phase; the labels are hidden.
- For each of the 8 descriptions, the learner clicks the part that matches. The clicked part is marked correct or incorrect, and the feedback text appears.
- At the end the learner sees the score and can restart the check with labels hidden again.
Feedback: Eight descriptions, fixed order, two attempts each. Correct: "Correct:
Starting State: The exploded DWDI housing with all eight labels visible and no infobox open. Prompt: "A DWDI housing has parts an SWSI housing does not. Click each one to see what it does."
Chapter Anchors: Eight labeled parts. Standard Arrangement 3 DWDI fans are for clean air with air temperatures limited to 130°F because both bearings are in the airstream.
The 130°F Line on a DWDI Quote
A common trap is quoting a standard Arrangement 3 DWDI fan for warm process air or dusty exhaust because the airflow fits; its bearings sit in the airstream, so the standard fan is limited to clean air at 130°F or below. When the air is hotter, FanPedia points to Arrangements 3DI and 7DI, whose integrated inlet boxes put the bearings outside the airstream on independent pedestals.
Worked Example: Sizing Up a DWDI Selection¶
An application engineer receives two inquiries from the same mechanical contractor. The numbers below are illustrative, chosen to show the reasoning; real ratings come from the catalog.
Inquiry 1. An air handler needs 24,000 CFM of filtered, 70°F supply air. The contractor's first idea is a single width fan, but the SWSI size that fits the cabinet delivers only about 12,000 CFM at the required pressure and a reasonable speed (illustrative).
- Check the flow ratio. The job needs twice what the SWSI wheel can deliver. By the DWDI flow approximation, \( Q_{\text{DWDI}} \approx 2 \times 12{,}000 = 24{,}000 \) CFM for a DWDI fan of the same diameter, width per side and speed.
- Check the airstream. The air is filtered and at 70°F, so it is clean and below the 130°F limit for a standard Arrangement 3 DWDI fan with bearings in the airstream.
- Check the discharge. FanPedia's DWDI discharge charts (pp. 48–49) list seven positions (UBD, TAU, THD, TAD, DBD, BHD and BAU) and no bottom angular down, so the engineer confirms the cabinet layout uses one of the seven.
- Result: a DWDI fan in Arrangement 3 is a sound starting point, to be confirmed against the catalog performance table.
Inquiry 2. The same contractor wants a similar fan to recirculate 160°F oven air.
- Check the airstream. 160°F is above the 130°F limit for a standard DWDI fan, because the bearings would sit in the hot air.
- Look for an arrangement that moves the bearings. FanPedia (p. 167) describes Arrangements 3DI and 7DI as using integrated inlet boxes to locate the bearings outside the airstream, which allows elevated operating temperatures with relatively clean air.
- Result: the engineer quotes a DWDI fan in Arrangement 3DI or 7DI, or moves to an SWSI arrangement, rather than a standard Arrangement 3 DWDI. Inlet boxes and arrangement details are covered in Chapter 11.
Axial Impeller Details: Tips, Blade Count and Pitch¶
FanPedia's axial impeller overview (p. 35) names five parts: blade, blade tip, hub, hub boss and shank. Three properties of those blades shape axial performance.
The axial blade tip is the outer end of an axial blade, the part closest to the fan housing. The tip moves fastest of any point on the blade, so it does the most work, and the gap between the tip and the housing (tip clearance) controls how much air leaks back from the discharge side to the intake side around the blade. A worn or bent tip, or a housing that is out of round, opens that gap and lowers pressure capability.
Axial blade count is the number of blades on an axial impeller. FanPedia's technical description (p. 186) notes that axial impellers come with "many variations of blade profile and number of blades." As a general rule, more blades let an impeller develop more pressure at a given diameter and speed, while fewer blades suit high volume at low pressure and lower cost.
The blade pitch angle is the angle between the blade and the plane in which it rotates. A steeper pitch takes a bigger bite of air on each revolution, so at a fixed speed it moves more air and draws more power. Fixed pitch impellers have their angle set when they are cast or fabricated; adjustable pitch impellers let the blade angle be reset. For direct drive Arrangement 4 axial fans, FanPedia (p. 172) notes that "where exact performance of the system is required, the user can adjust the blade angle to fine-tune the system."
Pitch Up Means Horsepower Up
Watch out for a field request to "just add a few degrees of pitch" to get more air: at the same speed, more pitch also means more brake horsepower, and the motor can be overloaded. Before anyone changes the blade angle, check the new operating point against the catalog curve for that pitch and confirm the motor has the margin.
Hub Variants: Reverse Bore, Fiberglass and Trantorque¶
Chapter 8 introduced the straight bore and taper lock hubs. Three more variants appear on FanPedia's axial pages.
A reverse bore hub is a hub bored from the opposite face so the impeller can be mounted on the shaft facing the other way. FanPedia's airflow and rotation pages (pp. 42–43) pair a standard bore hub with MP airflow (air passes over the motor, then through the impeller; hub on discharge) and a reverse bore hub with PM airflow (air passes through the impeller, then over the motor; hub on intake), for left hand and right hand impellers, both adjustable and fixed cast. The practical point: airflow direction is decided by how the hub is bored, so it must be right on the order.
A fiberglass hub is an axial impeller hub molded from fiberglass instead of cast aluminum. FanPedia (p. 34) shows the adjustable F4/F6 impellers with an aluminum hub alongside the FG4/FG6 impellers with a fiberglass hub; the fiberglass hub matches fiberglass blades in corrosive airstreams where metal would be attacked.
A Trantorque hub uses a Trantorque bushing, a keyless compression bushing that grips the shaft when tightened. FanPedia's axial hub types (p. 44) show it next to the taper lock hub and bushing; the taper lock version is drawn with a key and hardware, while the Trantorque version is not.
| Hub | Locks to shaft by | What to check on the order |
|---|---|---|
| Reverse bore | Same as standard, bored from the opposite face | Airflow PM (reverse bore) or MP (standard bore), and LH or RH |
| Fiberglass | Molded hub matching fiberglass blades | Airstream chemistry, and whether a fiberglass impeller is offered for the model |
| Trantorque | Keyless compression bushing | Shaft diameter and bushing size |
Impellers for Dirty Airstreams¶
Radial Bladed Impellers¶
A radial bladed impeller has flat blades that point straight out from the hub, along radii of the wheel, with no backward or forward lean. That simple geometry is the key to its toughness. Material in the air strikes a flat, heavy blade and is flung straight off the tip rather than sliding along a curved surface and building up, and the blades carry their centrifugal load in straight tension. Twin City Fan's current catalog calls its heavy-gauge, all-welded radial bladed fan "the 'workhorse' of the industry," proven in "dirty, abrasive, sticky or bulky particulate-laden airstreams."
The price is efficiency. Radial blades guide air less smoothly than backward curved or airfoil blades, so a radial bladed fan needs more power for the same duty. That is acceptable when the alternative is a high efficiency wheel that wears out, plugs or wraps with material.
Twin City Fan builds the radial bladed impeller in several forms (FanPedia p. 32; current catalog), each matched to a kind of material:
- An open radial paddle impeller has single thickness, radial paddle blades on the hub, braced with support gussets, and no back plate or rings. With nothing for material to catch on, it suits coarse, sticky, heavy or abrasive material; the catalog's RBO uses this open type paddle impeller. A second paddle impeller adds front and back end rings for stiffness; the catalog lists the RBR as a paddle impeller fan as well.
- A material handling impeller is a radial impeller for conveying solids rather than air. Twin City Fan's version, which FanPedia calls the material handling impeller (wool) and the catalog calls 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 from buffing system exhaust and similar matter from hanging up on and wrapping around the spiders (the hub arms that carry the blades) and blades, for "long, stringy or fibrous materials."
- A paper handling impeller is a material handling impeller made extra rugged for high-impact loading: a full back plate with trapezoidal gussets (FanPedia p. 32) and a paper deflector cone over the face of the hub. The catalog's RBP handles corrugated paper trims, fibrous material and metal trim, and the deflector cone helps keep paper from wrapping around the hub or the leading edges of the blades. All RBP fans are supplied with bearing stop blocks and a relieved inlet transition, which smooths the flow of trim into the fan.
- An air handling impeller is the radial family's version for relatively clean air: heavy-gauge blades welded to both the back plate and the impeller front plate (FanPedia p. 32). The catalog's RBA uses it for relatively clean air, fumes, vapor exhaust and air with a light contaminant, and describes its blades as backwardly inclined, so where the two sources differ, follow the current catalog.
Ask What the Particles Do, Not Just What They Are
Here's a shortcut for the first call: ask whether the material will stick, wrap, strike or just pass through. Sticky or abrasive points to an open paddle, wrapping points to the back plate wool design, hard impacts point to paper handling, and "passes through" lets you consider an air handling or radial tip wheel.
Radial Tip Impellers¶
A radial tip impeller is a hybrid: its blades curve forward at the entering edge and become radial at the tip of the leaving edge (FanPedia p. 31). The curved entry turns air onto the blade more smoothly, which raises efficiency, while the radial tip keeps a self-cleaning outer edge. Twin City Fan's catalog says radial tip designs are better suited for dirty, particulate-laden air than the high efficiency wheels and that, at high airflows, the radial tip fan is much more efficient than the comparable radial blade fan. The RTF is described for large volumes of clean or dirty gas at moderate to high pressure, such as exhausting bag-type collectors, precipitators, scrubbers and cyclones, and induced draft on boilers, incinerators and kilns; the HRT is a high specific speed version.
The table below collects the dirty-air family you have just read about, with the clean-air wheels for comparison.
| Impeller | Blade form | Airstream it is built for | Current TCF model(s) |
|---|---|---|---|
| Airfoil | Hollow airfoil, backward inclined | Clean air, high pressure, best efficiency | HAF |
| Backward curved | Single thickness, curved back | Clean air or light dust loading | HIB |
| Radial tip | Curved forward at entry, radial at tip | Large volumes of clean or dirty gas | RTF, HRT |
| Air handling (radial) | Heavy blades welded to back plate and front plate | Relatively clean air, fumes, light contaminant | RBA |
| Open radial paddle | Radial paddles, support gussets, no plates | Coarse, sticky, heavy or abrasive material | RBO (open type), RBR (paddle) |
| Material handling (wool) | Radial blades on a full back plate | Long, stringy or fibrous material | RBW |
| Paper handling | Full back plate, trapezoidal gussets, deflector cone | Paper trim, fibrous material, metal trim, high impact | RBP |
Worked Example: Matching Radial Impellers to a Plant¶
A distributor walks a packaging plant with the maintenance manager, who lists four fans to replace. The engineer works each one by asking what the material does in the airstream.
- Corrugated box trim line. Strips of corrugated trim are blown from the slitters to a baler. The trim strikes hard and can wrap around a hub. Choice: paper handling impeller (RBP), with its deflector cone to stop wrapping and its relieved inlet transition to smooth trim into the fan. Not chosen: an open paddle, which has no cone to keep trim off the hub.
- Shredder dust and paper shreddings. Long, light shreds that tend to hang on and wrap around the spiders and blades. Choice: back plate wool material handling impeller (RBW), designed for exactly this. Not chosen: the paper handling wheel, whose extra ruggedness is for high-impact loading rather than wrapping.
- Adhesive-laden dust from a glue station, conveyed to a collector. The material is sticky and some of it is abrasive filler. Choice: open radial paddle impeller (RBO), whose open blades give the material nowhere to build up. Not chosen: a backward curved or airfoil wheel, where sticky deposits would build on curved surfaces and unbalance the impeller.
- Exhaust from the dust collector's clean side, a large volume of air with only light dust remaining. Choice: move out of the radial family to a backward curved high efficiency fan (HIB), which the catalog lists for the clean side of baghouses. Not chosen: a radial bladed fan, which would work but would waste power on a duty that does not need its ruggedness.
The engineer's notes for the order record the impeller type next to each model, because the impeller, not just the model family, is what the plant is buying.
Diagram: Impeller Recommender for Real Airstreams¶
Run Impeller Recommender for Real Airstreams Fullscreen
Impeller Recommender for Real Airstreams
Type: microsim
sim-id: ch09-impeller-recommender
Library: p5.js
Status: Specified
Bloom Level: Evaluate
Bloom Verb: recommend
Learning Objective: The learner will recommend one of seven impeller types for each of ten airstream descriptions, judged against Twin City Fan's catalog application statements, with at least 8 of 10 correct.
Prerequisites: airfoil impeller, backward curved impeller, high efficiency centrifugal fan, radial tip impeller, radial bladed impeller, air handling impeller, open radial paddle impeller, material handling impeller, paper handling impeller (all defined in this chapter above).
Evidence of Mastery: For each of the 10 job cards the learner places the card in one of seven impeller bins and commits. A placement is correct when it matches the "Correct impeller" column in Content. Mastery is 8 of 10 correct on the first attempt. Reading bin descriptions before committing is exploration, not evidence.
Misconceptions: (1) The most efficient impeller is always the best choice. (2) Any radial bladed impeller handles any dirty airstream. (3) Paper shreddings and corrugated trim need the same impeller. (4) Radial tip fans are only for clean air.
Instructional Rationale: Evaluate-level judgment requires weighing efficiency against ruggedness for a specific airstream, so the learner commits a recommendation for each realistic job before seeing the catalog reasoning. Sorting with feedback that names the deciding property of the airstream builds the "what does the material do" habit the chapter teaches.
Content: Seven bins, each with a one-line description the learner can open:
| Bin | Description |
|---|---|
| Airfoil (HAF) | Hollow airfoil blades; highest efficiency; clean air, high pressure. |
| Backward curved (HIB) | Single thickness curved blades; clean air or light dust loading. |
| Radial tip (RTF) | Curved at entry, radial at tip; large volumes of clean or dirty gas. |
| Air handling radial (RBA) | Heavy blades welded to back plate and front plate; relatively clean air, fumes, light contaminant. |
| Open radial paddle (RBO) | Open paddles, no plates; coarse, sticky, heavy or abrasive material. |
| Back plate wool, material handling (RBW) | Radial blades on a full back plate; long, stringy or fibrous material. |
| Paper handling (RBP) | Full back plate with gussets and a deflector cone; paper trim, fibrous material, metal trim, high impact. |
Job cards, shown in a shuffled order:
| # | Job card | Correct impeller | Why (shown as feedback) |
|---|---|---|---|
| 1 | Forced draft fan for a fluid boiler; clean air at high pressure; the owner wants the lowest power bill. | Airfoil (HAF) | Clean air and high pressure are the airfoil's home ground, and it has the highest efficiency. |
| 2 | Moisture blow-off on a product line; clean air; high pressure. | Airfoil (HAF) | The catalog lists moisture blow-off for the HAF; with clean air there is no reason to give up efficiency. |
| 3 | Fan on the clean side of a baghouse; air carries light dust. | Backward curved (HIB) | The catalog lists the clean side of baghouses for the HIB; solid curved blades tolerate light dust. |
| 4 | Induced draft fan on an incinerator; large volume of dirty gas; moderate to high pressure. | Radial tip (RTF) | Radial tip fans handle large volumes of dirty gas and are more efficient than radial blade fans at high airflow. |
| 5 | Exhaust from a cyclone; high airflow; the gas still carries particulate. | Radial tip (RTF) | The catalog lists cyclone exhaust for the RTF; the radial tip sheds particulate while keeping efficiency at high flow. |
| 6 | Vapor exhaust from a process tank; air with a light contaminant; a rugged fan is wanted. | Air handling radial (RBA) | The air handling impeller is the radial family's choice for fumes, vapor and light contaminant. |
| 7 | Pneumatic conveying of coarse, sticky and abrasive material. | Open radial paddle (RBO) | Open paddles give sticky, abrasive material nothing to build up on or catch. |
| 8 | Exhaust from a buffing system that throws off threads. | Back plate wool, material handling (RBW) | The back plate wool impeller is designed to keep threads from wrapping around the spiders and blades. |
| 9 | Trim removal on a corrugated box line; strips of paper trim strike the wheel. | Paper handling (RBP) | Paper trim with high-impact loading calls for the paper handling impeller, whose deflector cone prevents wrapping at the hub. |
| 10 | Removal of metal trim from a stamping line. | Paper handling (RBP) | The catalog lists metal trim and other high-impact loading for the paper handling impeller. |
Provenance: Bin descriptions and every job card are taken from the application statements on Twin City Fan's current product pages (tcf.com, Radial Bladed Fans: RBA, RBO, RBW, RBP; Radial Tip & High Efficiency Fans: RTF, HAF, HIB), retrieved 2026-10-08, and from FanPedia pp. 31–32. Job settings such as "process tank" and "stamping line" are illustrative wrappers around the catalog statements.
Rules: One bin per card. A card is correct only for the bin in the "Correct impeller" column. Cards 1 and 2 share a bin, as do 4 and 5 and 9 and 10; bins may receive any number of cards, and the learner is not told how many cards belong in each bin.
Learner Activity:
- The learner reads a job card and may open any bin's description.
- The learner places the card in a bin and commits. The card is marked correct or incorrect and the "Why" text appears.
- On a wrong first attempt, the card returns and the learner tries again.
- After all 10 cards, a summary lists each card, the learner's first choice and the correct impeller, so the learner can see which misconception caused each miss.
Feedback: Ten cards, shuffled order, two attempts per card. Correct: "Correct:
Starting State: Card 1 of 10 is shown with the seven bins and no card placed. Prompt: "Which impeller would you put in this fan?"
Chapter Anchors: Seven impeller types; ten job cards; HAF, HIB, RTF, RBA, RBO, RBW and RBP as the current Twin City Fan models. The chapter states that at high airflows the radial tip fan is much more efficient than the comparable radial blade fan.
The Impeller Is the Product
Think of the radial family as one housing and drive platform carrying different wheels: RBA, RBO, RBR, RBW and RBP share Catalog 902, but each name stands for a different impeller. When a customer orders "a radial blade fan," the real decision is still open until the impeller is named.
Key Takeaways¶
- The impeller airflow path is axial-in, radial-out for centrifugal wheels, axial-through for axial wheels, and diagonal for mixed flow wheels; impeller width sets flow at a given diameter and speed.
- End rings join the blade ends at the impeller face, tie rods tie the end rings together across a forward curved wheel, and gussets brace blades against the hub or back plate.
- Airfoil wheels give the highest efficiency for clean air; backward curved wheels trade a little efficiency for tolerance of light dust. Twin City Fan's high efficiency centrifugal fans are the HAF (airfoil) and HIB (backward curved).
- A double width double inlet fan moves roughly twice the air of an SWSI fan of the same size and speed. Standard Arrangement 3 DWDI fans have both bearings in the airstream, carried on bearing structural supports, and are limited to clean air at 130°F; the duct connects at the setback outlet flange.
- On axial impellers, the blade tip and its clearance control leakage, blade count shifts the pressure-volume balance, and blade pitch angle sets flow and power at a given speed.
- A reverse bore hub reverses the impeller's mounting for PM airflow; fiberglass hubs suit corrosive airstreams; a Trantorque hub locks to the shaft with a keyless bushing.
- The radial bladed impeller is the rugged, self-cleaning workhorse for dirty airstreams, at a cost in efficiency. Its variants are the open radial paddle (sticky, abrasive), material handling back plate wool (stringy, fibrous), paper handling (trim and high impact) and air handling (relatively clean air, fumes) impellers.
- The radial tip impeller combines a curved entry with a radial tip, handling large volumes of dirty gas more efficiently than a radial blade fan at high airflow.
You Can Read the Airstream Now
You can now look at a job description, decide whether the material will stick, wrap, strike or pass through, and name the impeller that survives it, from airfoil all the way to paper handling. That judgment is what separates a fan that is selected from a fan that is applied.