Centrifugal, Axial and Mixed Flow Fans¶
Summary¶
This chapter sorts fans into their three families: centrifugal fans, which turn air through a right angle; axial fans, which move air parallel to the shaft; and mixed flow fans, which combine the two. It introduces scroll and tubular housings, guide vanes, housed and unhoused centrifugal fans, inline centrifugal fans, panel and ring fans, power roof ventilators, and laboratory fume exhaust fans, along with the commercial and industrial duty classes in Twin City Fan's catalogs. After completing it, readers can recognize a fan's family from its shape and airflow path.
Concepts Covered¶
This chapter covers the following 23 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Fan Family | 1085 |
| Centrifugal Fan | 562 |
| Axial Fan | 134 |
| Power Roof Ventilator | 30 |
| Commercial Duty Fan | 22 |
| Industrial Duty Fan | 2 |
| Mixed Flow Fan | 4 |
| Scroll Housing | 263 |
| Unhoused Centrifugal Fan | 80 |
| Inline Centrifugal Fan | 25 |
| Tubular Housing | 32 |
| Guide Vanes | 25 |
| Panel Fan | 2 |
| Ring Fan | 1 |
| Roof Ventilator | 4 |
| Centrifugal Roof Exhauster | 6 |
| Roof Mounted Axial Fan | 1 |
| Wall Mounted Fan | 1 |
| Ceiling Ventilator | 2 |
| Laboratory Fume Exhaust Fan | 35 |
| Weatherhood | 4 |
| Roof Curb | 2 |
| Bypass Air Plenum | 4 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 1: Air, Pressure and the Fan
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
Three Families, One Question¶
Every fan in a Twin City Fan catalog, from a quiet restroom exhauster to a vaneaxial fan in a tunnel, answers one question the same way: which way does the air travel as it passes through the impeller? The answer sorts every fan into one of three groups, and that sorting comes before arrangement, impeller type, construction class or accessories. A sales engineer who gets the family wrong will get every later line of the order wrong too, because each family has its own housings, mounting options and catalog pages.
Sorting the Fan Zoo
Walk onto any rooftop or into any mechanical room and you will find a crowd of fans that look nothing alike. By the end of this chapter you will be able to look at one, or at its drawing, and say which family it belongs to and which part of the catalog to open. Let's follow the airflow.
Fan Families¶
A fan family is a group of fans that share the same airflow path through the impeller. Recall from Chapter 3 that the impeller is the rotating part that does the work, and that the fan shaft is the axis it turns on. Every family is defined by the direction air is moving, relative to that shaft, when it leaves the impeller. FanPedia's technical descriptions put it this way: what sets axial and centrifugal impellers apart is how the air passes through the impeller.
There are three families:
- Centrifugal fans. Air enters the impeller along the shaft (axially), is accelerated by the blades, and leaves the impeller radially, at a right angle to the shaft.
- Axial fans. Air enters along the shaft and leaves along the shaft, with a swirling motion added by the rotating blades. The flow never turns the corner.
- Mixed flow fans. Air enters along the shaft and leaves the impeller on a diagonal, partly axial and partly radial. FanPedia describes the mixed flow impeller as a hybrid axial-centrifugal design.
The family is set by the impeller, not by the box around it. That distinction matters because Twin City Fan builds several families into the same outer shape. A round tube bolted into a duct run can hold an axial impeller, a centrifugal impeller or a mixed flow impeller. A rooftop unit with a hood can be axial or centrifugal. If you classify by silhouette alone, you will misfile fans on every job.
The Impeller Decides
Notice that the housing only guides air the impeller has already pointed somewhere. Think of the impeller as the decision and the housing as the paperwork that follows: to find the family, find out which way the impeller throws the air.
The families also lean toward different jobs. FanPedia describes axial impellers as built for moving large volumes of air against relatively low pressures as economically as possible, while centrifugal impellers come in several blade types, each with its own application range. Twin City Fan's current catalog describes its mixed flow and inline centrifugal lines as giving the performance of a centrifugal fan with the space-saving advantages of an axial-type fan. You will meet the pressure and efficiency differences properly in Chapter 10; for now, the airflow path is the sorting key.
Worked Example: Classifying Three Fans on One Drawing Set¶
A mechanical drawing set for a small manufacturing plant shows three fans. You need to tell your inside sales colleague which catalog section to open for each.
- Fan EF-1 is drawn as a round tube in a straight duct run. The schedule note reads "vaneaxial." Vaneaxial is a type of axial fan, so air goes straight through along the shaft. Family: axial.
- Fan SF-2 is drawn with a snail-shaped housing. Duct enters the side of the housing and leaves out the top at a right angle. A right-angle turn through a snail-shaped housing is the signature of a housed centrifugal fan. Family: centrifugal.
- Fan EF-3 is drawn as a round tube in a straight duct run, just like EF-1, but the schedule note reads "tubular inline centrifugal." The outside shape matches EF-1, yet the impeller throws air outward at a right angle; vanes inside the tube then turn the air back to the duct direction. Family: centrifugal.
The trap is EF-3. It looks like EF-1 from outside, but the impeller decides the family. When a drawing is ambiguous, the schedule's fan type, the model designation, or a call to the engineer settles it.
The table below gathers the three families side by side, using FanPedia's own descriptions of the airflow path.
| Family | Air enters the impeller | Air leaves the impeller | FanPedia's description |
|---|---|---|---|
| Centrifugal | Along the shaft | At a right angle to the shaft (radially) | Air "enters the impeller axially and is accelerated by the blades and discharged radially" |
| Axial | Along the shaft | Along the shaft, with swirl | Air is propelled "in an axial direction with a swirling tangential motion" |
| Mixed flow | Along the shaft | On a diagonal, between the two | A "hybrid axial-centrifugal impeller design" |
Centrifugal Fans¶
A centrifugal fan draws air into the center of the impeller along the shaft and throws it outward, at a right angle to the shaft, off the tips of the blades. FanPedia's description of housed centrifugal fans is short and exact: air is drawn through one side (single inlet) or both sides (double inlet) and is discharged at a right angle to the fan shaft. The impeller does two things to the air at once. Its spinning blades accelerate the air in the direction of rotation, and the outward throw raises its pressure as it moves from the small radius at the inlet to the larger radius at the blade tips.
Two widths cover most housed centrifugal fans:
- Single width, single inlet (SWSI). Air enters one side of the housing through one inlet. The drive (bearings, shaft, belt or motor) sits on the other side, out of the inlet's way.
- Double width, double inlet (DWDI). The impeller is roughly twice as wide and takes air in from both sides. A DWDI fan moves more air from the same impeller diameter, which is why Twin City Fan's catalog describes most centrifugal models as available single-wide or double-wide "for even higher volumes of air." Because both sides are inlets, the shaft passes through an inlet.
The blades can be shaped several ways, and FanPedia lists the options a housed centrifugal fan can carry: backward inclined, airfoil, backward curved, radial tip, forward curved and radial bladed. Those impeller types are the subject of Chapter 8. In this chapter, the point is that every one of them throws the air outward, so every one of them is a centrifugal fan.
Not every centrifugal fan has a housing. A housed centrifugal fan wraps the impeller in a scroll housing that collects the air and delivers it to one outlet. An unhoused centrifugal fan runs the impeller without that scroll, inside a plenum. Both are covered below, and Chapter 5 returns to housed centrifugal and plenum fans as product lines.
The table below gathers the centrifugal configurations this chapter discusses, so you can compare where air enters and leaves each one.
| Configuration | Housing | Where air enters | Where air leaves the fan | Example current Twin City Fan lines |
|---|---|---|---|---|
| Housed centrifugal, SWSI | Scroll | One side, along the shaft | One outlet, at a right angle to the shaft | BC-SW, BAF-SW, NFC-SW |
| Housed centrifugal, DWDI | Scroll | Both sides, along the shaft | One outlet, at a right angle to the shaft | BC-DW, BAF-DW, NFC-DW |
| Plenum fan (unhoused) | None; sits in a plenum | Inlet funnel, along the shaft | All around the impeller, into the plenum | Plenum fan lines |
| Plug fan (unhoused) | None; mounted through a plenum wall | Inlet funnel, along the shaft | All around the impeller, into the plenum | Plug fan lines |
| Tubular inline centrifugal | Tubular | One end of the tube | Other end of the tube, along the shaft | TSL |
| Square inline centrifugal | Square, removable side panels | One end | Straight through or at a 90-degree turn, depending on the ductwork | BSI, DSI |
| Centrifugal roof exhauster | Roof unit with shroud or hood | Roof opening below | Upblast, downblast, louvered or hooded | VC, DCRD, BCRD, BCRU |
Worked Example: Letting the Fan Make the Turn¶
A plant engineer has a horizontal exhaust duct that runs along the ceiling of a paint-mixing room and must turn straight up through the roof. The engineer asks whether the fan or a duct elbow should make the 90-degree turn.
- Read the geometry. Air arrives horizontally and must leave vertically: a right-angle turn.
- Match it to a family. A housed centrifugal fan takes air in along its shaft and discharges it at a right angle to the shaft. Mount the fan with its shaft horizontal, in line with the incoming duct, and choose a discharge that points straight up.
- Check the alternative. An axial fan would discharge in the same direction the air entered, so the design would need an elbow before or after it. Every elbow costs pressure and space.
- Choose the width. If the airflow is large and both sides of the fan are open to the room or a plenum, a DWDI fan can take air in from both sides. Here the air arrives through a single duct, so an SWSI fan with its inlet on the duct end is the natural fit.
- Pass it on. The order still needs a discharge position, a rotation and an arrangement. Those are designated from the drive side and are covered in Chapter 11 and Chapter 12.
The housed centrifugal fan does the turn and the pressure rise in one component. That combination is the most common reason it ends up on the drawing.
Scroll Housings¶
The scroll housing is the snail-shaped casing around the impeller of a housed centrifugal fan. FanPedia notes that the housing is also called the side, scroll or casing, and that it is the main structure supporting the inlet, outlet, framing and structural supports. Its shape has a job: the gap between the impeller tips and the housing wall starts narrow and grows steadily as the scroll wraps around the impeller, ending at the outlet.
Why grow? Each blade throws a little more air into the gap as it passes. Further around the scroll, more air has joined the flow, so the passage must be larger to carry it. The expanding passage also slows the air down. Recall from Chapter 2 that total pressure is the sum of static pressure and velocity pressure. Air leaving the blade tips is moving fast and carries much of its energy as velocity pressure. As the scroll widens, the air slows and part of that velocity pressure is recovered as static pressure, the pressure that pushes air through ductwork, filters and coils.
The parts of a scroll housing have names you will see on drawings and in FanPedia's exploded views (pp. 9-10) and its SWSI housing construction drawing (p. 60):
- The housing scroll is the curved wrapper sheet that forms the spiral.
- The housing sides are the flat side sheets that close the spiral; the inlet is cut into a side.
- The cutoff is the point near the outlet where the scroll passes closest to the impeller tips. It splits the air leaving through the outlet from the air that would otherwise keep circling.
- The wiper bar, which FanPedia groups with the cutoff, helps direct air through the housing in the inlet funnel area to prevent turbulence and increase efficiency.
- The inlet funnel (also called the inlet cone) guides air smoothly into the impeller eye.
- The inlet collar and inlet flange are the duct connections on the inlet side: a collar is a short round sleeve, and a flange is a bolting rim.
- The outlet (discharge) flange is the bolting rim around the outlet.
- The framing is the structural steel that stiffens the housing and supports it.
- The pedestal is the structure that carries the bearings and drive on some arrangements.
Fast Air In, Useful Pressure Out
The scroll is not just a cover; it is a pressure converter. Think of it like a funnel run backwards: the passage widens, the air slows, and the speed it gave up shows up as static pressure the duct system can use.
Worked Example: How Much Pressure Can the Scroll Recover?¶
The numbers in this example are illustrative, chosen to show the idea; they are not ratings for any Twin City Fan model. Use the velocity pressure equation from Chapter 2 for standard air, where \( V \) is velocity in feet per minute and \( VP \) is velocity pressure in inches of water gauge:
Suppose air leaves the impeller tips at about 4,000 fpm, and the scroll and outlet slow it to about 2,000 fpm by the time it reaches the outlet flange.
Halving the velocity cuts velocity pressure to one quarter, because velocity pressure goes with the square of velocity. Up to about \( 1.00 - 0.25 = 0.75 \) in. wg of velocity pressure is available to become static pressure. A real scroll recovers only part of it, because friction and turbulence take a share, which is one reason details such as the cutoff and the wiper bar matter.
Worked Example: Reading Rotation From the Scroll¶
A field technician sends you a photo of an unlabeled housed centrifugal fan, taken from the drive side, and asks which way the impeller turns. The scroll answers the question. The impeller carries air around the scroll in its direction of rotation, and the passage grows in that same direction. Start at the cutoff, follow the scroll the way the gap gets wider, and you arrive at the outlet. The direction you just traced is the direction of rotation. If that path runs clockwise in the photo, the impeller turns clockwise as viewed from the drive side. Chapter 12 and Chapter 13 turn this observation into the formal rotation and discharge designation.
Trace It From the Cutoff
Here's a shortcut for any scroll you can see: put a finger on the cutoff and slide it along the housing toward the wide end. The direction your finger moves is the impeller's rotation. Check it from the drive side.
Before trying the interactive diagram, make sure each of the ten housing terms in the list above is familiar. The diagram shows a single width, single inlet housing based on FanPedia's construction drawing and asks you to locate each part.
Diagram: Parts of a Centrifugal Scroll Housing¶
Run Parts of a Centrifugal Scroll Housing Fullscreen
Parts of a Centrifugal Scroll Housing
Type: infographic
sim-id: ch04-scroll-housing-parts
Library: html
Status: Specified
Bloom Level: Remember
Bloom Verb: identify
Learning Objective: The learner will identify the ten named parts of a single width, single inlet centrifugal scroll housing on a drawing of the fan.
Prerequisites: scroll housing, housing scroll, housing side, cutoff, wiper bar, inlet funnel, inlet collar, inlet flange, outlet (discharge) flange, framing, pedestal (all defined in the section "Scroll Housings" above).
Evidence of Mastery: In quiz mode the learner is given each of the 10 part names, one at a time, and clicks the region of the drawing where that part is. A click is correct when it lands inside the named part's hotspot. Mastery is 9 of 10 correct on the first attempt. Hovering and opening infoboxes in explore mode are exploration, not evidence.
Misconceptions: (1) The scroll housing is only a protective cover. (2) The cutoff and the outlet flange are the same thing. (3) The inlet collar and inlet funnel are the same part (the collar is the outside duct connection; the funnel guides air into the impeller eye).
Instructional Rationale: Remember-level recall of part names is built by first exploring labeled hotspots, then retrieving each name against an unlabeled drawing. The click-to-locate quiz produces a checkable record of which part names the learner can retrieve.
Content: A side view of an SWSI housed centrifugal fan, drawn from the inlet side and cut away enough to show the impeller inside the scroll, with an inset view showing the inlet side features. Ten hotspots:
| # | Part | Infobox text (explore mode) |
|---|---|---|
| 1 | Housing scroll | The curved wrapper that forms the spiral. Its passage grows from the cutoff to the outlet, slowing the air and recovering static pressure. |
| 2 | Housing side | A flat side sheet that closes the spiral. The inlet opening is cut into it. FanPedia notes the whole housing is also called the side, scroll or casing. |
| 3 | Cutoff | The point near the outlet where the scroll passes closest to the impeller tips; it separates air leaving through the outlet from air that would keep circling. |
| 4 | Wiper bar | Helps direct air through the housing in the inlet funnel area to prevent turbulence and increase efficiency. |
| 5 | Inlet funnel | Guides air smoothly into the eye of the impeller. Also called the inlet cone. |
| 6 | Inlet collar | A short round sleeve on the inlet side for connecting duct. |
| 7 | Inlet flange | A bolting rim on the inlet side for connecting duct or accessories. |
| 8 | Outlet (discharge) flange | The bolting rim around the outlet, where the discharge duct attaches. |
| 9 | Framing | Structural members that stiffen and support the housing. |
| 10 | Pedestal | Structure that carries the bearings and drive; present on some arrangements only. |
Quiz prompt format: "Click the
Provenance: Part names and the wiper bar description from FanPedia (Twin City Fan, 2021) p 59 (housing features) and p 60 (SWSI centrifugal housing construction drawing). The cutoff description and the velocity-to-static-pressure explanation are standard fan engineering, as stated in this chapter's "Scroll Housings" section. The drawing is a simplified illustration, not a dimensioned model.
Rules: Explore mode: hovering a hotspot highlights it and shows the part name; clicking opens the infobox. Quiz mode: the 10 parts are asked in a shuffled order, each once. The learner gets two attempts per part. A part answered correctly on either attempt is highlighted with its label shown for the rest of the quiz. Only first-attempt answers count toward the score.
Learner Activity:
- In explore mode, the learner hovers and clicks each of the 10 hotspots and reads the infoboxes.
- The learner switches to quiz mode. All labels disappear.
- For each prompt, the learner clicks a region. The sim marks it right or wrong.
- After all 10 parts, a summary shows the first-attempt score and lists any parts missed, each with its infobox text.
Feedback: 10 items, shuffled, two attempts each. Correct: "Correct:
Starting State: Explore mode with all 10 hotspots outlined but unlabeled, and the prompt "Hover over each part of the scroll housing to learn its name, then take the quiz."
Chapter Anchors: The chapter lists ten housing terms and states that the diagram is based on FanPedia's SWSI construction drawing. The chapter states that the scroll passage grows from the cutoff to the outlet.
Unhoused Centrifugal Fans¶
An unhoused centrifugal fan is a centrifugal impeller and inlet funnel run without a scroll housing. FanPedia states plainly that not every fan needs a housing, and lists plenum fans and plug fans among the fans that run without one. With no scroll to collect it, air leaves the impeller all the way around its circumference and fills the space the fan sits in. That space is a plenum, which FanPedia defines as an air-filled space within a structure that the fan pressurizes. The plenum, not a scroll, then delivers air to wherever it goes next.
FanPedia describes two types:
- A plenum type unhoused fan is designed for air handling applications where the fan operates within an air plenum, such as inside an air handling unit cabinet.
- A plug type unhoused fan is designed for circulating or recirculating air within a plenum. The fan is "plugged" into the plenum wall: the mounting panel attaches to the outside of the wall and the impeller sits inside. FanPedia lists high-temperature ovens and OEM paint booths among the common industrial uses.
Worked Example: An Oven Builder's Fan¶
An OEM oven builder needs a fan to recirculate hot air inside an oven chamber and wants the motor and bearings outside the hot zone where they can be reached for service.
- Family. The air only needs to be stirred and pressurized inside the chamber; no ductwork leaves the fan. An unhoused centrifugal fan pressurizes the space it sits in, which is exactly the job.
- Type. The impeller must be inside the chamber while the drive stays outside. That is the plug type: FanPedia's exploded views of plug fans (pp. 13-14) show a mounting panel and an insulation plug between the impeller on one side and the shaft, bearings, pedestal and motor on the other.
- What to confirm. Oven temperature affects construction, bearings and materials. High-temperature construction is covered in Chapter 21; for now, record the operating temperature on the inquiry so the selection can account for it.
A plenum type fan would suit the same builder if the job were an air handler cabinet instead of an oven wall. The difference is where the drive lives relative to the wall.
Axial Fans¶
An axial fan moves air parallel to the shaft. Its impeller looks like a propeller: blades are attached to a central hub, and as they turn they push air along the axis of rotation. FanPedia describes the result as air propelled in an axial direction with a swirling tangential motion created by the rotating blades. That swirl is important. It is energy the impeller put into the air that is moving around the duct rather than down it, so it does no useful work unless something recovers it.
Axial impellers come with many blade profiles and blade counts. FanPedia states that the emphasis in axial design is on moving large volumes of air against relatively low pressures as economically as possible. Some axial impellers have fixed blade pitch, and some have adjustable pitch, meaning the blade angle can be reset to change performance. The impeller types, including the hub position, appear in Chapter 8.
Axial fans take several forms in Twin City Fan's lines:
- An inline axial fan sits in a tubular housing in a duct run. FanPedia notes that inline axial fans can be mounted horizontally or vertically, ducted or unducted.
- A tubeaxial fan is an inline axial fan with a plain tubular housing.
- A vaneaxial fan is an inline axial fan whose housing includes a vane section, guide vanes that straighten the airflow as it moves through the fan.
- Panel fans and ring fans center an axial impeller in a flat panel or a ring, usually mounted in a wall.
- Roof mounted axial fans start from a tubeaxial fan or a panel fan and add roof accessories such as a hood or stack cap.
Twin City Fan's current catalog describes the two inline types by the job they do. Tubeaxial fans cover a wide range of requirements "from general ventilation to process air supply." Vaneaxial fans are for applications where "large volumes of air are required at moderate to high pressures." The table summarizes the axial forms just described.
| Axial form | Housing | Vanes | Typical use described in FanPedia or the current catalog | Example current lines |
|---|---|---|---|---|
| Tubeaxial | Plain tube | None | General ventilation to process air supply | TD, TB, TCTA, TCTS |
| Vaneaxial | Tube with vane section | Guide vanes | Large volumes at moderate to high pressures | TCVA, TCVX, TCVS |
| Panel fan | Flat panel | None | Wall mounted, straight-through flow | TCWPF, WPD, WPB |
| Ring fan | Ring | None | Wall mounted, straight-through flow | (FanPedia type) |
| Roof mounted axial | Tubeaxial or panel fan plus hood or stack cap | None | Rooftop exhaust or supply | TUD, TUB, THD, THB |
Worked Example: Tubeaxial or Vaneaxial?¶
A specifying engineer is choosing an axial fan for a straight duct run and asks a representative to explain the difference between Twin City Fan's tubeaxial and vaneaxial lines.
- Same family, same path. Both are axial fans in tubular housings; air enters and leaves along the shaft. Both catalogs list direct-drive and belt-driven models and fixed or adjustable pitch impellers.
- The difference is the vane section. FanPedia's exploded views of direct drive tubeaxial and vaneaxial fans (pp. 15-16) show the same housing, impeller, shaft and motor; the vaneaxial adds guide vanes.
- What the vanes buy. The tubeaxial discharges air that is still swirling. The vaneaxial's guide vanes turn that swirl into straight flow, recovering part of the energy as static pressure. That is why the catalog positions the vaneaxial for moderate to high pressures and the tubeaxial for general ventilation and process air supply.
- The recommendation. If the duct system's pressure requirement is modest and cost matters most, start with the tubeaxial section. If the system needs more pressure or the downstream equipment is sensitive to swirling air, start with the vaneaxial section. Either way, the final selection is made from the published performance, as shown in Chapter 10.
Tubular Housings¶
A tubular housing is a round, straight-through cylinder that holds the impeller and lets air enter one end and leave the other, in line with the duct. FanPedia's housing drawings place the tubular fan next to the centrifugal scroll, and its fan type pages show tubular housings on three different families: inline axial fans (tubeaxial and vaneaxial), tubular inline centrifugal fans, and mixed flow fans. A tubular housing usually carries flanges at each end, an impeller access door, and a motor support or, for belt drive, a motor base and a belt tube (also called a belt fairing) that shelters the belt as it passes through the airstream to the shaft.
Diagram: Vaneaxial Fan Parts¶
Run Vaneaxial Fan Parts Fullscreen
Vaneaxial Fan Parts
sim-id: ch04-vaneaxial-fan-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the eleven labeled parts of direct drive and belt driven vaneaxial fans on FanPedia's exploded views, given each part's function or appearance.
Content: Eleven numbered markers on the exploded views: tubular housing, guide vanes, shaft, impeller, motor, motor support, motor base, impeller access door, sheave, belt tube, and bearing housing.
Provenance: FanPedia page 16, "Exploded Views: Axial Fans, 'Direct Drive Vaneaxial Fans' (left pair) and 'Belt Driven Vaneaxial Fans' (right pair)", with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 16.
The tubular housing is the reason silhouettes mislead. Three fans with identical tubes can belong to two different families.
Worked Example: What Is Inside the Tube?¶
A facilities technician finds three tubular fans in a mechanical room, none with a readable nameplate, and is asked to report which family each belongs to. FanPedia's exploded views (pp. 15-17) give the clues, and an impeller access door lets the technician look inside with the fan locked out.
| What the technician sees through the access door | Family | Reason |
|---|---|---|
| Blades fixed around a central hub like a propeller, with a ring of stationary vanes downstream | Axial (vaneaxial) | Propeller-style blades push air along the shaft; the stationary vanes are guide vanes. |
| An inlet funnel feeding a wheel with a back plate and blades that throw air outward, then stationary vanes | Centrifugal (tubular inline centrifugal) | The wheel throws air radially; the straightening vanes turn it back along the tube. |
| An inlet funnel feeding an impeller whose blades taper from the front plate to the hub end, then stationary vanes | Mixed flow | FanPedia describes the mixed flow impeller's airfoil blades as tapering from the front plate to the hub end, resulting in a larger tip width. |
A Tube Is Not a Family
A common trap is writing "axial" on an order because the fan is a round tube in a duct. Tubular housings carry axial, inline centrifugal and mixed flow impellers, so confirm the impeller from the model designation or the access door before you classify.
Guide Vanes¶
Guide vanes are fixed, curved blades mounted in the housing downstream of an axial impeller. They take the swirling air that leaves the impeller and turn it to flow straight along the duct. FanPedia describes the vaneaxial housing as built with vane sections "to help straighten the airflow as it moves through the fan," and its exploded views label them GUIDE VANES (p. 16). Removing the swirl does two useful things: the rotational energy is partly recovered as static pressure, and the air leaves in a cleaner, straighter pattern for whatever is downstream.
The same idea appears in tubular inline centrifugal and mixed flow fans, where FanPedia labels the stationary vanes straightening vanes. There, the vanes turn air that the impeller sent outward or diagonally back into the duct direction.
Worked Example: Which Guide Vanes Does the Customer Mean?¶
An inside sales engineer receives two requests on the same day.
- Request A: "Vaneaxial fan, guide vanes required." This is consistent. Every vaneaxial fan has guide vanes by definition; the line item simply confirms the vane section that makes it a vaneaxial rather than a tubeaxial.
- Request B: "Housed centrifugal fan with inlet guide vanes for capacity control." This is a different part. FanPedia lists "inlet guide vanes" as another name for the inlet vane accessory: radial vanes at the fan inlet that pre-spin the air entering the fan to control the flow, available in nested and external styles. They are a flow-control accessory, covered in Chapter 18, not straightening vanes.
The engineer quotes Request A from the vaneaxial line and adds an inlet vane accessory to Request B, then confirms with the customer that "inlet guide vanes" means inlet vanes for flow control.
Same Words, Different Parts
Watch out for the phrase "guide vanes" on a centrifugal fan: it almost always means inlet vanes, the flow-control accessory, not straightening vanes. Ask which one the customer means before the order is entered, because the two are different parts in different catalog sections.
Inline Centrifugal Fans¶
An inline centrifugal fan puts a centrifugal impeller in a housing that lets air go straight through, in line with the duct. FanPedia describes two forms. The tubular inline centrifugal fan uses a tubular housing: the impeller throws air outward against the tube wall, and straightening vanes turn it back to the duct direction. The square inline centrifugal fan uses a square housing with removable side panels, and FanPedia notes that it can move air straight through or through a 90-degree turn, depending on how the ductwork is connected. Twin City Fan's current catalog describes inline centrifugal fans as offering the performance of a centrifugal fan with the space-saving advantages of an axial-type fan, and lists the tubular TSL and the square BSI (belt driven) and DSI (direct drive) among them.
Worked Example: A Fan That Fits the Ceiling¶
A contractor is replacing a fan in a straight run of supply duct above an office ceiling. The space is tight, the existing duct is straight on both sides, and the system needs the pressure capability the engineer associates with a centrifugal impeller.
- Housing first. A housed centrifugal fan would force a right-angle turn and need duct transitions on both sides. An inline housing keeps the duct straight.
- Impeller second. An axial fan would also keep the duct straight, but the engineer wants a centrifugal impeller. The inline centrifugal fan provides both.
- Tubular or square. If the duct is round and the run is straight, the tubular inline centrifugal fits the duct directly. If the duct is rectangular, or if a 90-degree turn at the fan would save an elbow, the square inline centrifugal can be connected for either path, and its removable side panels allow service from below the ceiling grid.
The contractor's model choice then comes from the inline centrifugal section of the catalog, not the axial section, because the impeller is centrifugal.
Mixed Flow Fans¶
A mixed flow fan uses a hybrid axial-centrifugal impeller, so air leaves the impeller on a diagonal, partly along the shaft and partly outward. FanPedia groups mixed flow fans with tubular inline centrifugal fans: both are built with tubular housings, both move air straight through the fan, and both use straightening vanes to return the air to the duct direction. FanPedia describes the mixed flow impeller's airfoil blades as tapering from the impeller front plate to the hub end, resulting in a larger tip width. Twin City Fan's current catalog describes its mixed flow fans as giving the performance of a centrifugal fan with the space-saving advantages of an axial-type fan, for large volumes of clean air at low to moderate pressures, and splits the line into high-pressure models such as QSL and low-pressure models such as QCLB. For classification, remember that a mixed flow fan is its own family even though its tube looks like a vaneaxial's.
With the three families and their housings and vanes defined, you can test the impeller-decides rule on seven fan configurations. For each one, you predict two directions: the direction air leaves the impeller, and the direction air leaves the whole fan.
Diagram: Airflow Path Explorer¶
Run Airflow Path Explorer Fullscreen
Airflow Path Explorer
Type: microsim
sim-id: ch04-airflow-path-explorer
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify seven fan configurations by the direction air leaves the impeller and the direction air leaves the fan, relative to the fan shaft.
Prerequisites: impeller, fan shaft, fan housing (Chapter 3); fan family, centrifugal fan, axial fan, mixed flow fan, scroll housing, unhoused centrifugal fan, plenum fan, tubular housing, guide vanes, straightening vanes, inline centrifugal fan (all defined in this chapter above this block).
Evidence of Mastery: For each of the 7 configurations, the learner commits two predictions before the airflow animation plays: (a) impeller exit direction and (b) fan exit direction. Each prediction is correct when it matches the answer columns in Content. Mastery is 12 of 14 predictions correct on the first attempt. Watching the animation and replaying it are exploration, not evidence.
Misconceptions: (1) A fan in a round, straight-through tube must be an axial fan. (2) Air leaves a centrifugal fan in the same direction it leaves the impeller in every case. (3) Guide vanes add pressure by speeding the air up. (4) A mixed flow fan discharges at a diagonal out of the fan.
Instructional Rationale: Classifying by airflow direction is an Understand-level task, so the learner must commit a prediction before the answer is revealed, then see a slowed step-through of the actual path. Separating impeller exit from fan exit forces the learner to see that the housing and vanes, not the impeller, decide where air finally leaves.
Content:
Answer choices for impeller exit direction: "Parallel to the shaft", "At a right angle to the shaft", "Diagonal (between parallel and right angle)".
Answer choices for fan exit direction: "Parallel to the shaft", "At a right angle to the shaft, through one outlet", "Outward in all directions around the impeller".
| # | Configuration shown (side view, shaft drawn horizontally) | Impeller exit | Fan exit | Why (feedback) |
|---|---|---|---|---|
| 1 | Housed centrifugal fan, single width single inlet (SWSI), in a scroll housing | At a right angle to the shaft | At a right angle to the shaft, through one outlet | The centrifugal impeller throws air outward; the scroll gathers it and sends it out one outlet at a right angle to the shaft. |
| 2 | Housed centrifugal fan, double width double inlet (DWDI), in a scroll housing | At a right angle to the shaft | At a right angle to the shaft, through one outlet | Air enters from both sides, but the impeller still throws it outward and the scroll still collects it to one outlet. |
| 3 | Plenum fan (unhoused centrifugal) inside an air handler cabinet | At a right angle to the shaft | Outward in all directions around the impeller | With no housing, nothing collects the air; it leaves the impeller all the way around and pressurizes the surrounding plenum. |
| 4 | Tubular inline centrifugal fan with straightening vanes | At a right angle to the shaft | Parallel to the shaft | The impeller is centrifugal, so air leaves it radially; the tube and straightening vanes turn the air back to flow along the duct. |
| 5 | Tubeaxial fan (no guide vanes) | Parallel to the shaft | Parallel to the shaft | An axial impeller pushes air along the shaft; it leaves the fan still swirling because there are no vanes to straighten it. |
| 6 | Vaneaxial fan (guide vanes downstream of the impeller) | Parallel to the shaft | Parallel to the shaft | Same axial path as the tubeaxial; the guide vanes remove the swirl but do not change the main direction. |
| 7 | Mixed flow fan in a tubular housing with straightening vanes | Diagonal (between parallel and right angle) | Parallel to the shaft | The hybrid impeller sends air out on a diagonal; the tubular housing and vanes return it to the duct direction. |
After the two predictions for an item are committed, the animation shows particles entering along the shaft, following the configuration's path through the impeller, and leaving the fan. Swirl is shown as a rotating component of particle motion for items 5 and 6; for item 6 the swirl visibly stops after the guide vanes.
Labels the learner sees on the drawings: "Shaft", "Impeller", "Inlet", "Outlet" (every item except 3: the plenum fan has no single outlet, because air leaves all around the impeller), plus "Scroll housing" (items 1 and 2), "Plenum wall" (item 3), "Tubular housing" (items 4 to 7), "Straightening vanes" (items 4 and 7), "Guide vanes" (item 6).
Provenance: Airflow paths from FanPedia (Twin City Fan, 2021) pp 4-7 (fan types) and p 186 (technical description of axial versus centrifugal impellers); vane locations from the exploded views on pp 15-17. The animation speed and particle counts are illustrative.
Rules: Items are presented in the fixed order 1 to 7. An item's animation does not play until both predictions are committed. The learner may not change a committed prediction. A replay button replays the current item's animation any number of times. The score counts only first-attempt predictions.
Learner Activity:
- The learner reads the configuration name and studies the side-view drawing with the impeller blades shown but no airflow.
- The learner chooses an impeller exit direction and a fan exit direction, then commits.
- The animation plays. Each prediction is marked correct or incorrect, and the "Why" text appears.
- The learner advances to the next item. After item 7 a summary shows the score and lists the items missed.
- In the summary, the learner should notice that items 4, 6 and 7 all have a tube with stationary vanes and all leave the fan parallel to the shaft, but of those three only item 6 is an axial fan (item 5, the tubeaxial, is axial too but has no vanes).
Feedback: 7 items, fixed order, one attempt per prediction. Correct prediction: "Correct." followed by the "Why" text. Incorrect prediction: "Not quite. The answer is
Starting State: Item 1 is shown with no airflow drawn and both prediction choices empty. The prompt reads "Which way will the air leave the impeller, and which way will it leave the fan?"
Chapter Anchors: The chapter names three families: centrifugal, axial and mixed flow. The chapter states that seven configurations are included and that the learner predicts two directions for each. The chapter's Fan Families worked example names a tubular inline centrifugal fan as the look-alike trap for an axial fan.
Panel Fans and Ring Fans¶
A panel fan is an axial fan whose impeller is centered in a flat, square panel; a ring fan centers an axial impeller in a ring-shaped housing. FanPedia notes both are typically wall mounted and move air in a straight line through the fan. They are the simplest axial forms: no tube, little or no ductwork, and air simply passes through an opening in a wall. FanPedia's exploded view of a panel fan (p. 18) shows the accessories that usually come with one: a mounting adapter, an inlet safety screen, a backdraft shutter, a weatherhood and a bird screen. Panel fans also serve as the base for some roof units, described below.
Wall Mounted Fans and Weatherhoods¶
A wall mounted fan is a fan designed to be installed in or on a wall to supply or exhaust air directly through it, usually without ductwork. Twin City Fan's current catalog describes its wall mounted fans as built for cost-effective, general ventilation, offered direct and belt driven, with aluminum or steel impellers and fixed or adjustable pitch. The catalog sorts them by construction:
- Heavy-duty construction: TCWPX (adjustable pitch blades) and TCWPZ (fabricated steel impeller)
- Medium-duty construction: WPD (direct drive) and WPB (belt driven)
- Light-duty construction: TCPE (direct drive)
- Fiberglass construction: TCWPF (panel fan, direct drive)
Their fixed and adjustable pitch impellers mark most of them as panel fans in the axial family.
A weatherhood is a hood mounted over the outdoor side of a wall fan to keep rain and snow out of the opening while letting air pass. In FanPedia's panel fan exploded view (p. 18), the weatherhood sits on the outside of the wall, with a bird screen across its open side to keep birds out.
Power Roof Ventilators¶
Many building fans live on the roof, where they exhaust air from the spaces below or bring outside air in. A power roof ventilator (PRV) is a motor-driven fan unit built to mount on a roof opening. The word "power" separates it from a gravity ventilator, which has no fan. A PRV can be centrifugal or axial. FanPedia's Centrifugal Power Roof Ventilators page shows five rooftop configurations: upblast and side wall units exhaust air in a straight line through the fan, while downblast, louvered and hooded units move air up through the fan and deflect it down and outward.
Twin City Fan's current catalog splits rooftop products across two sections that are easy to confuse. Centrifugal Roof & Wall Exhausters are the centrifugal PRVs. Roof Ventilators are axial PRVs built on tubeaxial impellers, such as upblast TUD and TUB units, low profile LUD and LUB units, and hooded THD and THB units, along with filtered supply and smoke and heat versions.
Worked Example: Decoding "PRV, Hooded, Belt Driven"¶
A specification schedule lists one exhaust fan as "PRV, hooded, belt driven." A new representative asks which catalog page to open.
- PRV does not name a family. Both centrifugal and axial PRVs exist, and both have hooded versions.
- Look for the family clue. The current catalog's hooded centrifugal roof exhausters include BCLH (belt driven); the hooded axial roof ventilators include THB (tubeaxial, belt driven).
- Use the job to narrow it. A hooded axial roof ventilator suits high-volume, low-pressure general exhaust, or supply in its filtered form. A centrifugal roof exhauster handles more duct resistance and, in the catalog's words, uses backward inclined, non-overloading impellers "for maximum efficiency and quiet operation."
- Ask before guessing. If the schedule gives an airflow and a static pressure, the selection can be checked against both lines. If it does not, the representative asks the engineer whether the unit is ducted and what static pressure it must overcome.
The lesson for every rooftop line is the same: "PRV" tells you where the fan sits, not how the air moves through it. Chapter 5 covers the rooftop product lines in detail.
Roof Ventilators¶
In Twin City Fan's current catalog, roof ventilators are the axial power roof ventilators: tubeaxial-based units in upblast, low profile upblast, hooded and filtered supply forms. The catalog describes them as providing cost-effective, general purpose exhaust ventilation of commercial buildings as well as a large variety of industrial applications, with direct and belt driven models and cast aluminum, adjustable pitch or fabricated steel, fixed pitch impellers. The term is narrower in the catalog than in everyday speech: on a job site, "roof ventilator" may be said of any roof fan, so confirm the family before quoting.
Roof Mounted Axial Fans¶
A roof mounted axial fan is a tubeaxial fan, used as the base model, with roof accessories added. FanPedia shows two forms, and air moves in a straight line through the fan in both:
- With a stack cap and curb cap, the air discharges straight up.
- With a hood and curb cap, the air is deflected down and outward for exhaust; for supply, the flow runs in the reverse direction.
Roof mounted versions of panel fans follow the same pattern, built from a panel fan with a stack cap or a hood.
Centrifugal Roof Exhausters¶
A centrifugal roof exhauster is a power roof ventilator with a centrifugal impeller. Air enters from the roof opening below, turns outward through the impeller, and leaves through the unit's shroud, hood or louvers. FanPedia's exploded views (p. 19) show the parts of the upblast and downblast versions: a motor cover and motor housing (downblast), the motor, a bird screen, the impeller, a shroud (upblast) or skirt (downblast), and a curb cap that sits on the roof curb. Twin City Fan's current catalog describes its centrifugal roof and wall exhausters as direct and belt driven, with backward inclined, non-overloading centrifugal impellers, in configurations for relatively clean air and for grease-laden air. Upblast units throw the exhaust up and away from the roof; downblast units turn it down and outward toward the roof surface.
Worked Example: Reading the Configuration From the Model List¶
A distributor is asked for a belt-driven centrifugal exhauster that discharges straight up, for a ducted general exhaust from a warehouse office. The distributor uses the configuration names in the current catalog to narrow the choices.
| Requirement | Matches | Rules out |
|---|---|---|
| Centrifugal impeller | Centrifugal Roof & Wall Exhausters section | Axial roof ventilators (TUD, TUB) |
| Discharges straight up | Upblast roof exhausters | Downblast (DCRD, BCRD, VC), louvered (DCLP, BCLP), hooded (DCLH, BCLH) |
| Belt driven | BCRU, VCUB | VCU (direct drive) |
| General exhaust, not a kitchen | BCRU, VCUB | BCRUR (kitchen), BCRUSH (smoke and heat) |
The remaining candidates, BCRU and VCUB, are then compared on performance and features. Upblast discharge versus downblast discharge, and why kitchen exhaust uses upblast, are developed in Chapter 5.
Roof Curbs¶
A roof curb is a raised frame built into the roof around the opening, flashed into the roofing so water cannot get in, on which a rooftop fan sits. In FanPedia's centrifugal roof exhauster exploded view (p. 19), the fan's curb cap, a flat base plate at the bottom of the fan, rests on top of the roof curb. The curb lifts the fan above standing water and snow and gives the roofer a weatherproof seam to work to. When quoting a roof fan, confirm two things:
- Whether the curb is supplied with the fan or by others.
- That the curb fits the fan's curb cap, so the fan seats and seals on it.
Ceiling Ventilators¶
A ceiling ventilator is a small exhaust fan mounted in or above a ceiling, drawing air from the room through a ceiling grille and discharging it into ductwork. Twin City Fan's current catalog groups ceiling and cabinet ventilators together and describes them as designed for commercial applications requiring quiet, continuous and reliable operation, including bathrooms, generator rooms, data centers and general HVAC exhaust. The current lines include T ceiling ventilators and TL inline/ceiling ventilators, which can be mounted in more than one configuration. Ceiling and cabinet ventilators appear only in the catalog's commercial product list, which leads to the next topic.
Laboratory Fume Exhaust Fans¶
A laboratory fume exhaust fan removes contaminated air from fume hoods and discharges it high enough above the building that it does not re-enter through air intakes, windows or doors. Twin City Fan's current catalog explains the requirement: laboratory fumes can be hazardous to human life in concentrated form, and lab exhaust fans must move air at high velocities to achieve a high plume height while entraining clean ambient air to dilute the chemical concentration. The fans are often built from special materials to withstand the fumes.
FanPedia describes how these fans dilute the exhaust. Lab exhaust configurations draw in fresh outside air in two ways: through a bypass air plenum, which produces bypass flow, and through the fan's nozzle or windband, which produces entrained flow. The fresh air mixes with the contaminated air to dilute it and carry it up and away from the building. FanPedia shows lab exhaust configurations in two families: housed centrifugal lab exhaust fans with scroll housings, and inline centrifugal and mixed flow lab exhaust fans with tubular housings.
The current catalog's lab exhaust lines reflect those families:
- Induced flow, mixed flow: TVIFE (direct drive) and QIFE (belt driven)
- Induced flow, centrifugal: BAIFE (airfoil) and BCIFE (backward inclined)
- Venturi nozzle: QFE (mixed flow) and TFE (inline centrifugal)
Worked Example: Following the Flows¶
The numbers here are illustrative, chosen to show how the flows add up; they are not ratings for any Twin City Fan model. FanPedia's lab exhaust drawings label five flows: contaminated air, bypass flow, fan flow, entrained flow and total flow.
A lab building's fume hoods draw 5,000 CFM of contaminated air. The bypass air plenum adds 1,000 CFM of outside air ahead of the fan, and the nozzle entrains another 3,000 CFM of outside air into the discharge plume.
| Flow | How it is found | Illustrative value |
|---|---|---|
| Contaminated air | From the fume hoods | 5,000 CFM |
| Bypass flow | Outside air through the bypass air plenum | 1,000 CFM |
| Fan flow | Contaminated air + bypass flow | 6,000 CFM |
| Entrained flow | Outside air drawn in at the nozzle or windband | 3,000 CFM |
| Total flow | Fan flow + entrained flow | 9,000 CFM |
The fan itself handles only the fan flow, 6,000 CFM, so that is the airflow it is selected for. The plume leaving the nozzle carries 9,000 CFM, which is why the discharge is more dilute than the air in the hood ducts: in this example the contaminated air is 5,000 of the 9,000 CFM, or about 56 percent of the plume. How bypass dampers and plume dilution are controlled on real jobs is covered in Chapter 6.
Size the Fan for Fan Flow
Want to save yourself a re-selection? When a lab exhaust schedule lists several airflows, find out which one is the fan flow before you look up a model. The fan is selected for the air that actually passes through the impeller; entrained air joins the plume after it.
Bypass Air Plenums¶
A bypass air plenum is a chamber on the inlet side of a laboratory exhaust fan that admits fresh outside air into the fan's inlet alongside the contaminated air from the fume hoods. FanPedia identifies it as the source of bypass flow, and its accessory drawings for lab exhaust models show bypass dampers and isolation dampers with actuators on these units. The bypass air adds to the fan flow, dilutes the contaminated air before it reaches the impeller, and helps keep the discharge moving fast enough for a high plume.
Commercial and Industrial Duty¶
Twin City Fan's current website sorts its product lines into two lists: Commercial Solutions and Industrial Solutions. Many families appear in both lists, including centrifugal fans, axial fans, inline centrifugal fans, mixed flow fans, plenum fans, roof ventilators, wall mounted fans and lab exhaust fans. A few appear in only one. Ceiling and cabinet ventilators and kitchen and restaurant exhaust fans are listed only under commercial. Air kits, plug fans, and radial tip and high efficiency fans are listed only under industrial.
A commercial duty fan is a fan built and rated for building ventilation and HVAC service: comfort air handling, restroom and general exhaust, and similar jobs in offices, schools, hospitals and retail buildings, usually with clean air at normal temperatures, quiet operation and standard construction. The catalog's descriptions of commercial products repeat those priorities: ceiling ventilators for "quiet, continuous and reliable operation," wall mounted fans for "cost-effective, general ventilation." Commercial lines are often offered in standard sizes, with standard accessories, from published performance.
Worked Example: Two Calls, Two Duty Classes¶
A distributor takes two calls in one morning.
- Call 1: a school district needs restroom exhaust for a new classroom wing: clean room air, a ceiling grille in each restroom, ducted to the roof. The air is clean and at room temperature, the setting is a building, and quiet matters. This is commercial duty. The distributor looks at ceiling ventilators for the restrooms, or a centrifugal roof exhauster on the roof, both in the commercial product list.
- Call 2: a foundry needs a fan to pull hot, dusty air from a process hood to a dust collector. The airstream is hot and particulate-laden, and the fan will run as part of a production process. This is industrial duty. The distributor starts from industrial centrifugal fans and expects the inquiry to need construction options such as the high-temperature, spark-resistant or special-material choices the catalog lists as optional construction.
The two calls might both end in a centrifugal fan, but the duty class changes which lines, construction and accessories are worth considering.
An industrial duty fan is a fan built for process and plant service, where the airstream may be hot, dusty, wet, corrosive or explosive, and the fan is part of a production process that must keep running. Recall these airstream conditions from Chapter 2. Industrial fans are typically heavier in construction and are offered with options matched to the airstream, such as the spark-resistant, high-temperature, nominally leak-tight and special-material constructions on the current catalog's centrifugal fan pages. The construction classes behind those options are covered in Chapter 20 and Chapter 21.
Duty Is About the Airstream and the Building
Notice that duty class and fan family are two separate questions. The family comes from the airflow path through the impeller; the duty comes from the airstream, the setting and how hard the fan must work, so a centrifugal fan can be commercial or industrial.
Putting It Together: Sorting Real Product Lines¶
You now have all the pieces: the three families, the housings that can disguise them, and the product lines built on each. The MicroSim below asks you to sort fifteen current Twin City Fan product lines by family, using the clues in each card's description.
Diagram: Fan Family Sorter¶
Run Fan Family Sorter Fullscreen
Fan Family Sorter
Type: microsim
sim-id: ch04-fan-family-sorter
Library: p5.js
Status: Specified
Bloom Level: Analyze
Bloom Verb: differentiate
Learning Objective: The learner will differentiate fifteen Twin City Fan product lines into the centrifugal, axial and mixed flow families from a short description of each, correctly placing at least 13 of 15.
Prerequisites: fan family, centrifugal fan, axial fan, mixed flow fan, scroll housing, tubular housing, guide vanes, unhoused centrifugal fan (plenum and plug types), inline centrifugal fan, panel fan, power roof ventilator, centrifugal roof exhauster, roof ventilator, wall mounted fan, laboratory fume exhaust fan (all defined in this chapter).
Evidence of Mastery: The learner places each of the 15 cards into one of three bins: Centrifugal, Axial, Mixed flow. A placement is correct when it matches the "Correct bin" column in Content. Mastery is 13 of 15 correct on the first placement.
Misconceptions: (1) Any fan in a tubular housing is axial. (2) Any roof fan, or any "PRV," is centrifugal. (3) Lab exhaust fans are their own family rather than centrifugal or mixed flow fans. (4) Fans without a housing cannot be centrifugal.
Instructional Rationale: Differentiating requires the learner to separate the relevant clue (the impeller's airflow path) from irrelevant ones (housing shape, mounting location, application). Mixing look-alike cards, such as a tubular inline centrifugal next to a tubeaxial, forces that separation, and committed placements give a checkable record.
Content: Fifteen cards, each showing the model designation and the description below. Card text must not contain the words "centrifugal," "axial," "mixed flow" or "mixed-flow"; those words appear only in the feedback.
| # | Card shown to the learner | Correct bin | Why (feedback) |
|---|---|---|---|
| 1 | BC-SW: single width, single inlet fan in a snail-shaped housing; air leaves at a right angle to the shaft | Centrifugal | A scroll housing with a right-angle discharge is a housed centrifugal fan. |
| 2 | NFC-DW: double width, double inlet fan with a forward curved wheel in a snail-shaped housing | Centrifugal | Forward curved wheels throw air outward; DWDI housed fans are centrifugal. |
| 3 | Plenum fan: wheel and inlet funnel with no housing, running inside an air handler cabinet | Centrifugal | An unhoused fan is still centrifugal; the plenum collects the air instead of a scroll. |
| 4 | Plug fan: wheel inside an oven wall, mounting panel and motor outside | Centrifugal | Plug fans are unhoused centrifugal fans plugged into a plenum wall. |
| 5 | TSL: airfoil wheel in a round tube, with straightening vanes; air goes straight through | Centrifugal | A tubular inline centrifugal fan: the wheel throws air outward and the vanes turn it back along the tube. |
| 6 | BSI: belt-driven fan in a square housing with removable side panels; straight-through or 90-degree flow | Centrifugal | A square inline centrifugal fan uses a backward inclined centrifugal wheel. |
| 7 | DCRD: direct-drive downblast roof exhauster | Centrifugal | DCRD is in the catalog's Centrifugal Roof & Wall Exhausters section. |
| 8 | BCIFE: induced flow laboratory exhaust fan with a backward inclined wheel | Centrifugal | Backward inclined wheels are centrifugal; lab exhaust is an application, not a family. |
| 9 | TD: direct-drive fan in a plain round tube, propeller-style blades on a hub, no vanes | Axial | A tubeaxial fan: air goes straight along the shaft and leaves swirling. |
| 10 | TCVA: cast aluminum propeller-style impeller in a round tube, with a vane section downstream | Axial | A vaneaxial fan: the guide vanes straighten the swirl but the path is still along the shaft. |
| 11 | WPB: belt-driven wall mounted fan, impeller centered in a flat panel | Axial | A panel fan centers an axial impeller in a panel and moves air straight through the wall. |
| 12 | TUD: direct-drive upblast roof ventilator built on a tube with a propeller-style impeller | Axial | The catalog's Roof Ventilators are tubeaxial-based PRVs; not every roof fan is centrifugal. |
| 13 | QSL: high-pressure airfoil fan in a round tube with straightening vanes; blades taper from the front plate to the hub end | Mixed flow | Blades that taper from the front plate to the hub end mark the hybrid impeller of a mixed flow fan. |
| 14 | QCLB: low-pressure, single-surface-blade fan in a round tube; air leaves the impeller on a diagonal | Mixed flow | A diagonal impeller exit is the mixed flow signature. |
| 15 | TVIFE: direct-drive induced flow laboratory exhaust fan with a hybrid impeller whose air leaves on a diagonal | Mixed flow | TVIFE is listed as an induced flow, mixed flow lab exhaust fan. |
Provenance: Model designations and their family groupings from Twin City Fan's current product catalog pages on tcf.com (products: centrifugal fans, plenum fans, plug fans, inline centrifugal fans, axial fans, wall mounted fans, roof ventilators, centrifugal roof and wall exhausters, mixed flow fans, laboratory and fume exhaust fans), retrieved 2026-10-08. Housing and impeller descriptions from FanPedia (Twin City Fan, 2021) pp 4-8, 17 and 32.
Rules: The 15 cards are presented in a shuffled order. Each card can be placed once; after placement it is locked and marked correct or incorrect. The score counts first placements only. After all 15 cards are placed, the learner may press "Sort again," which reshuffles all cards and resets the score.
Learner Activity:
- The learner reads the top card and places it into Centrifugal, Axial or Mixed flow.
- The card locks into the bin and is marked correct or incorrect, and the "Why" text appears beneath it.
- The learner continues until all 15 cards are placed.
- A summary shows the score and lists each misplaced card with its correct bin and "Why" text. The learner should notice which clue misled them on each miss: housing shape, roof location or application name.
Feedback: 15 items, shuffled, one attempt each. Correct: "Correct:
Starting State: Three empty bins labeled Centrifugal, Axial and Mixed flow, and the first card face up. The prompt reads "Which family does this product line belong to? Look for the airflow path, not the housing."
Chapter Anchors: The chapter states that the sorter holds fifteen current Twin City Fan product lines and three families. The chapter names TSL, BSI and DSI as inline centrifugal lines; TD, TB, TCTA and TCTS as tubeaxial lines; TCVA, TCVX and TCVS as vaneaxial lines; QSL and QCLB as mixed flow lines; TVIFE, QIFE, BAIFE, BCIFE, QFE and TFE as lab exhaust lines; and DCRD, BCRD, VC and BCRU as centrifugal roof exhausters.
Key Takeaways¶
- A fan family is defined by the airflow path through the impeller: centrifugal (in along the shaft, out at a right angle), axial (in and out along the shaft, with swirl) or mixed flow (out on a diagonal).
- The impeller decides the family; the housing does not. Tubular housings carry axial, inline centrifugal and mixed flow impellers alike.
- A scroll housing collects air from a centrifugal impeller and, by widening from the cutoff to the outlet, slows the air and recovers velocity pressure as static pressure. Tracing the scroll from the cutoff toward the outlet gives the direction of rotation.
- Unhoused centrifugal fans run without a scroll: plenum types sit inside a plenum, and plug types mount through a plenum wall with the drive outside.
- Guide vanes straighten the swirl leaving an axial impeller and make a tubeaxial into a vaneaxial. "Inlet guide vanes" on a centrifugal fan usually means the inlet vane flow-control accessory, a different part.
- Inline centrifugal fans and mixed flow fans keep the duct straight while using centrifugal or hybrid impellers.
- Panel fans, ring fans and most wall mounted fans are axial; weatherhoods protect their outdoor openings.
- A power roof ventilator can be centrifugal (centrifugal roof exhausters) or axial (the catalog's roof ventilators and roof mounted axial fans); roof-mounted units normally sit on a roof curb through their curb cap, while wall units mount on the wall instead.
- Ceiling ventilators serve quiet, continuous commercial exhaust.
- Laboratory fume exhaust fans dilute contaminated air with bypass flow from a bypass air plenum and entrained flow at the nozzle; the fan is sized for the fan flow, not the total plume flow.
- Commercial duty and industrial duty describe the service, not the family: the airstream, the setting and the construction options separate them.
You Can Read the Fan Zoo
You can now look at a fan, a drawing or a model list and name its family, its housing and the catalog section it lives in, even when a tubular housing or a rooftop hood tries to hide it. That sorting step is the first line of every fan order you will ever write.