Basic Fan Anatomy¶
Summary¶
This chapter introduces the parts that every fan shares: the housing, the impeller, the shaft and its bearings, the motor, the inlet and outlet, and the drive train that connects them. It also covers basic housing features such as welded and bolted construction, access doors, inspection ports, split housings, flanges, screens, backdraft dampers, and stack caps. These parts are the building blocks for the fan families in the next three chapters. After completing it, readers can point to and name each major part of a fan.
Concepts Covered¶
This chapter covers the following 19 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Fan Housing | 1194 |
| Impeller | 5562 |
| Fan Motor | 2380 |
| Fan Inlet | 147 |
| Fan Outlet | 303 |
| Fan Shaft | 2742 |
| Access Door | 18 |
| Welded Housing | 2 |
| Bolted Housing | 1 |
| Bearing | 1755 |
| Inlet Collar | 3 |
| Inlet Flange | 11 |
| Outlet Flange | 9 |
| Backdraft Damper | 16 |
| Bird and Safety Screens | 16 |
| Stack Cap | 3 |
| Split Housing | 5 |
| Inspection Port | 1 |
| Drive Train | 725 |
Prerequisites¶
This chapter builds on concepts from:
Why Fan Anatomy Comes First¶
Every fan order, drawing, and field report is written in the vocabulary of fan parts. A specification asks for "an access door on the scroll," a mechanic reports "the drive-side bearing is running hot," and a contractor asks whether the "inlet is a collar or a flange." If you cannot point to each of those parts, you cannot check the order, answer the question, or catch the mistake before the fan ships.
Meet the Parts Before You Meet the Product Lines
Every Twin City Fan model, from a small panel fan to a large industrial blower, is built from the same short list of parts. Learn those parts here and every exploded view, submittal drawing, and field call in the rest of this book gets easier to read. Let's follow the airflow.
In Chapter 1 you learned that a fan is a machine that moves air by turning at some speed and absorbing some power. This chapter opens that machine up. We will follow a single cubic foot of air from the duct, through the fan, and back into the duct, naming each part it touches. Then we will look at the parts that turn the impeller, the parts that hold everything up, and the smaller features that let people inspect, clean, connect, and protect a fan.
The parts fall into four groups, and the chapter follows them in this order:
- The air path: the housing, the impeller, the inlet, and the outlet, the parts that touch the airstream.
- The rotating support: the shaft and the bearings that carry it.
- The power source and the connection to it: the motor and the drive train.
- The housing features and accessories: welded or bolted construction, split housings, access doors, inspection ports, flanges, collars, screens, backdraft dampers, and stack caps.
Most of the illustrations in this chapter come from the exploded views in FanPedia (pages 9 to 19). An exploded view is a drawing that pulls a machine's parts apart along their assembly lines, so you can see each part and where it goes. Later chapters revisit each group in detail; here the goal is a working map of the whole fan.
The Fan Housing¶
The fan housing is the stationary shell that encloses the impeller and guides the air from the inlet to the outlet. FanPedia describes it as the main structure of the fan: it directs the air (or air carrying particulate) through the system, and it supports the other key components, including the inlet, the outlet, the framing, and the structural supports. Other components may be mounted inside it, outside it, or both. In the field you will also hear the housing called the side, the scroll, or the casing.
The shape of the housing depends on the fan family. A housed centrifugal fan has a snail-shaped housing. A tubeaxial or inline fan has a tubular housing, a straight cylinder with the impeller inside. A square inline fan has a box-shaped housing. Some fans have no housing at all: plenum fans, panel fans, and ring fans do without one, and a plug fan's housing is optional. Those fans still have an inlet and a discharge, but the space around them (an air-handler cabinet, a wall, or a ring) does the job a housing would do.
On a single-width, single-inlet (SWSI) centrifugal fan, the housing is made of several named pieces:
- The housing scroll is the curved outer wrapper. Its spiral shape gives the air leaving the impeller room to slow down and build pressure as it travels toward the outlet.
- The two housing sides are the flat plates on either side of the scroll. The inlet side carries the inlet opening; the drive side carries the opening for the shaft.
- The cutoff is the point where the scroll comes closest to the impeller, just before the outlet. It separates the air that is leaving from the air that is still circling.
- The wiper bar works near the inlet funnel to direct air through the housing, reducing turbulence and improving efficiency.
- The framing stiffens the housing and ties it to its supports.
- The inlet opening is usually round, so it can carry a collar, a funnel, a flange, or a combination of these. The outlet opening, also called the discharge, can be square, rectangular, or round.
The table below reorganizes those pieces by the job each one does, using the terms just defined.
| Housing piece | What it does | Where you see it on a drawing |
|---|---|---|
| Housing scroll | Wraps the impeller and guides air toward the outlet | The curved outline in a side view |
| Housing sides | Close the scroll and carry the inlet and shaft openings | The flat faces in a front view |
| Cutoff | Separates outgoing air from recirculating air | Just upstream of the outlet |
| Wiper bar | Helps direct air near the inlet funnel | Near the inlet funnel |
| Framing | Stiffens the housing and connects it to supports | Angles or bars around the scroll |
| Inlet and outlet openings | Let air in and out; carry flanges or collars | The two ends of the air path |
Worked Example: Which of These Fans Has a Housing?¶
A distributor's quote lists five fans for one building: a SWSI centrifugal fan for a process exhaust, a tubeaxial fan for a parking garage, a plenum fan in an air handler, a plug fan in a second air handler, and a panel fan for a warehouse wall. The customer asks which ones will arrive with a housing that needs space and support.
- SWSI centrifugal: scroll housing. Plan for its footprint, outlet flange, and supports.
- Tubeaxial: tubular housing, connected into the duct run at both ends.
- Plenum fan: no housing; the air-handler cabinet encloses it.
- Plug fan: housing optional. If the order lists none, the cabinet encloses it.
- Panel fan: no housing; it mounts in a panel that fits a wall opening.
Answer: two fans certainly have housings, one may, and two do not.
The Housing Is Part of the Aerodynamics
It is tempting to think of the housing as a protective box, but on a housed centrifugal fan the scroll and cutoff are part of how the fan builds pressure. That is why a housing change, such as adding a split or an inlet box, is never just a sheet-metal question.
Welded and Bolted Housings¶
Housings are joined together in one of two ways. A welded housing has its scroll, sides, and framing joined by welds, so the housing is one permanent piece. Welding gives a stiff housing with no bolted seams to leak. When FanPedia describes nominally leak-tight construction, it calls for continuous welds, with the weld locations varying by model (page 120).
A bolted housing is assembled from sections joined with bolts through mating flanges, so it can be taken apart. Bolted joints let a housing be shipped in pieces, opened for impeller removal, or replaced in parts. The trade-off is that every bolted seam is a possible leak path. For nominally leak-tight service, FanPedia requires closely spaced bolt holes, on 3-inch to 4-inch centers, and gasketing on bolted connections such as flanges, access doors, inlet funnels, and split housings (page 120). Many fans combine the two: a welded housing with bolted access doors, flanges, and inlet funnels.
Split Housings¶
A split housing is a housing built in sections that bolt together along a planned joint, so the housing can be opened or shipped in pieces. FanPedia (pages 156, 157 and 194) names three centrifugal styles. A horizontal split runs along the horizontal centerline; on size 807 and above, the shop may split the housing this way for shipping. A pie split cuts out a wedge of 90 degrees or more so the impeller can be removed without disturbing the inlet or outlet ductwork. A 3-way split divides the housing into three sections, either for shipping or so the fan can pass through an opening of a specific size, and it takes extra drafting and engineering time. Specify a split when the fan must enter a small room or when the impeller will be pulled for service.
The Impeller¶
The impeller is the rotating part that does the work on the air. It is the only part of the fan that adds energy to the airstream; every other part either guides the air, supports the impeller, or turns it. In older literature and on some job sites it is called the wheel, and on axial fans it is sometimes called the propeller or rotor.
A centrifugal impeller is built from a few standard pieces. The blades are the curved or flat vanes that push the air. The back plate is the solid disk on the drive side that the blades are attached to. The impeller front plate (also called the shroud) is the ring on the inlet side that holds the blade tips and matches the inlet funnel. The hub is the center piece that fastens the impeller to the shaft. Air enters at the center of the impeller, turns 90 degrees, and is flung outward between the blades into the housing scroll.
An axial impeller is built differently. Its blades are attached around a hub, and air passes straight through along the shaft line. FanPedia (page 35) labels the axial parts as the blade, the blade tip, the hub, the hub boss, and, on adjustable-pitch impellers, the shank at the root of each blade that lets the blade angle be set.
The blade shape names the impeller type. FanPedia shows the centrifugal and mixed flow types on pages 31 and 32; you only need their names here, since Chapter 8 compares them in detail.
| Impeller type | Blade shape in one line |
|---|---|
| Backward inclined | Flat, single-thickness blades tilted back from the direction of rotation |
| Backward curved | Single-thickness blades curved back from the direction of rotation |
| Airfoil | Backward-inclined blades with a wing-shaped (airfoil) cross section |
| Forward curved | Single-thickness blades curved forward, toward the direction of rotation |
| Radial tip | Curved forward at the entering edge and radial at the tip of the leaving edge |
| Radial bladed | Straight paddle blades that point out from the center |
| Mixed flow | Airfoil blades that taper from the front plate to the hub end |
The impeller also has a position on the shaft. An overhung impeller is mounted on the end of the shaft, with both bearings on one side of it. A center-hung impeller sits between the two bearings. FanPedia (page 30) shows Arrangements 1, 4, 8, 9 and 10 with overhung impellers and Arrangements 3 and 7 with center-hung impellers. We will return to this when we reach the shaft.
How Hard Is the Impeller Working?¶
One quick way to compare how hard two impellers are working is tip speed, the speed of the outer edge of the blades as they sweep around. Because the tip travels one circumference per revolution, tip speed grows with both the diameter and the rotational speed. Tip speed matters because both the pressure the impeller can develop and the stress in its metal rise as the tips move faster.
Impeller Tip Speed¶
where:
- \( V_{tip} \) is the tip speed in feet per minute (ft/min)
- \( D \) is the impeller diameter in inches
- \( N \) is the rotational speed in revolutions per minute (RPM)
- 12 converts inches to feet
Worked Example: Tip Speed of a Centrifugal Impeller¶
An illustrative 30-inch impeller turns at 1,200 RPM.
- Multiply the diameter by \( \pi \): \( \pi \times 30 = 94.25 \) inches per revolution.
- Multiply by the speed: \( 94.25 \times 1{,}200 = 113{,}097 \) inches per minute.
- Divide by 12 to convert to feet: \( 113{,}097 / 12 = 9{,}425 \) ft/min.
If the same impeller were sped up to 2,400 RPM, the tip speed would double to 18,850 ft/min. Speed changes performance quickly, which is why a catalog lists the speed limits for each impeller rather than leaving them to judgment. Chapter 25 covers the fan laws that describe exactly how airflow, pressure, and power follow speed.
The Fan Inlet¶
The fan inlet is the opening where air enters the fan, together with the parts that guide the air into the impeller. On a housed centrifugal fan the inlet is a round opening in the housing side. On an inline or axial fan it is the open upstream end of the tube. On a plenum or plug fan it is the inlet funnel itself, open to the cabinet.
Air arriving at the impeller should be smooth and evenly spread across the opening. If it enters the impeller crowded to one side or spinning, part of the impeller works harder than the rest, and the fan delivers less air and makes more noise than its rating. Three parts at the inlet prevent that:
- The inlet funnel is an aerodynamically shaped ring that funnels air into the impeller. FanPedia (page 188) says it provides high efficiency and smooth airflow through the fan, is flanged on one end to mount to the housing, and is also called the inlet cone, funnel, or inlet bell. It reaches close to the impeller front plate, so the gap between them stays small.
- The inlet collar is a short sleeve at the inlet for a slip-fit duct connection.
- The inlet flange is a flat ring with bolt holes for a bolted duct connection.
The collar and the flange are covered in their own sections below. Many inlets carry more than one of these: FanPedia notes that the round inlet opening supports a collar, a funnel, a flange, or any combination (page 59). Inlet accessories, such as inlet boxes, inlet vanes, silencers, and flex connectors, attach at this same opening and are covered in Chapter 18.
Inlet velocity is the number that tells you whether the inlet connection is likely to cause trouble. You met air velocity and cubic feet per minute (CFM) in Chapter 1: velocity is airflow divided by the area it passes through.
Air Velocity Through an Opening¶
where:
- \( V \) is the average air velocity in feet per minute (ft/min)
- \( Q \) is the airflow in cubic feet per minute (CFM)
- \( A \) is the open area in square feet (ft²)
Worked Example: Velocity at an Inlet Collar¶
An illustrative fan moves 6,000 CFM through a round inlet collar 20 inches in diameter.
- Convert the diameter to feet and find the radius: 20 in = 1.667 ft, so the radius is 0.833 ft.
- Find the area: \( A = \pi r^2 = \pi \times 0.833^2 = 2.18 \) ft².
- Find the velocity: \( V = 6{,}000 / 2.18 = 2{,}750 \) ft/min.
At 2,750 ft/min the air is moving fast enough that an elbow bolted right at the collar would push the air to one side of the impeller. That is the practical reason inlet duct layout matters: the inlet funnel can smooth air that arrives straight, but it cannot fix air that arrives spinning or crowded against one wall.
An Elbow at the Inlet Costs Performance
A common trap is accepting a duct layout with an elbow bolted directly to the inlet collar or flange, because the fan's catalog rating assumes smooth, straight air at the inlet. Flag the layout before the order is placed and ask for a straight duct run, turning vanes, or an inlet box designed for the job.
Inlet Collar¶
An inlet collar is a short, plain cylindrical sleeve attached to the fan inlet so a duct or flexible connector can slip over it and be clamped. It has no bolt holes. In the FanPedia exploded views of centrifugal fans (pages 9 and 10), the inlet collar is drawn as one inlet option and the flanged inlet as the other. Collars suit lighter, slip-fit duct connections, such as a flexible sleeve held with clamps. When the connection must be bolted, rigid, or sealed, specify an inlet flange instead.
Inlet Flange¶
An inlet flange is a flat ring or frame around the fan inlet, drilled with a bolt-hole pattern, so a duct or accessory can be bolted to the fan face to face. A flange gives a rigid, sealable joint that can carry load. The matching part on the duct side is the companion flange. FanPedia (page 180) explains that companion flanges are connected to the ductwork in the field to guarantee a matching connection to the fan, and they are shipped loose for field mounting. Companion flanges come round or rectangular.
The inlet flange can also be structural. On Arrangements 4VI and 4HI, direct-drive fans mounted by their inlet, FanPedia notes that the entire fan is supported by the inlet flange only (page 21).
Worked Example: Connecting Ductwork to an Inlet Flange¶
A mechanical contractor will connect a round duct to the inlet of a SWSI centrifugal fan and asks what to order so the bolt holes line up.
- Confirm the fan's inlet connection on the submittal drawing. It shows a flanged inlet, not a collar, so the connection is bolted.
- Read the inlet flange's bolt-hole count, bolt circle, and hole size from the drawing. These are model-specific; never guess them.
- Add a round inlet companion flange to the order. It is drilled to match the fan's inlet flange and is shipped loose.
- The contractor attaches the companion flange to the duct in the field, then bolts it to the fan's inlet flange with a gasket between them.
Result: the duct and fan meet with matching bolt patterns, and nobody drills holes on the job site. If the fan were an Arrangement 4HI hung by its inlet, step 4 would also need the contractor to confirm that the duct-side structure can carry the fan's full weight.
The Fan Outlet¶
The fan outlet, also called the discharge, is the opening where air leaves the fan, together with the parts that connect it to the downstream duct or to the open air. On a housed centrifugal fan the outlet is the rectangular or square opening at the end of the scroll, just past the cutoff. On an inline or axial fan it is the downstream end of the tube. The outlet's position on a centrifugal fan, such as top horizontal or up blast, is designated with a discharge code; that system is covered in Chapter 12.
Because the air has just been accelerated by the impeller, it usually leaves faster than it entered and is not yet evenly spread across the opening. Three kinds of parts deal with that:
- The outlet flange is a flat frame with a bolt-hole pattern for a bolted duct connection (covered in its own section below). On double-width, double-inlet (DWDI) fans, FanPedia shows a setback outlet flange, set back from the end of the outlet (page 61).
- A transition is a duct piece that changes the opening from one shape to another, such as rectangular to round, or keeps the shape but enlarges or reduces it. FanPedia (page 67) notes a transition can be welded to the fan or bolted on as a separate piece.
- An evasé is a transition that enlarges the outlet. It is attached, usually bolted, to the discharge, and its outlet is larger than its inlet. FanPedia (page 146) gives its purpose: to reduce the fan's outlet velocity and increase its static pressure capability. For some fans the published ratings include the evasé.
Outlets that discharge to the open air carry their own accessories: backdraft dampers, screens, and stack caps, which are covered near the end of this chapter. The table below summarizes the outlet parts just described.
| Outlet part | Purpose | Typical connection |
|---|---|---|
| Outlet flange | Bolted connection to the duct or an accessory | Bolted, with a companion flange |
| Setback outlet flange | Outlet flange set back from the end of the outlet (DWDI fans) | Bolted |
| Transition | Changes the shape or size of the opening to match the duct | Welded or bolted |
| Evasé | Enlarges the outlet to lower velocity and raise static pressure capability | Usually bolted |
Worked Example: What an Evasé Does to Outlet Velocity¶
An illustrative centrifugal fan delivers 9,000 CFM through an outlet 18 inches by 24 inches. The engineer is considering an evasé that enlarges the opening to 24 inches by 30 inches.
- Outlet area without the evasé: \( (18/12) \times (24/12) = 1.5 \times 2.0 = 3.0 \) ft².
- Outlet velocity without the evasé: \( 9{,}000 / 3.0 = 3{,}000 \) ft/min.
- Outlet area with the evasé: \( (24/12) \times (30/12) = 2.0 \times 2.5 = 5.0 \) ft².
- Velocity leaving the evasé: \( 9{,}000 / 5.0 = 1{,}800 \) ft/min.
The evasé cuts the leaving velocity by 40 percent. Air that leaves more slowly carries less of the fan's energy away as velocity, which is how an evasé lets the fan deliver more useful static pressure. Chapter 2 explains the pressure side of this trade.
Outlet Flange¶
An outlet flange, also called a discharge flange, is a flat frame around the fan outlet with a bolt-hole pattern for a bolted connection to the downstream duct or to an outlet accessory such as a damper, silencer, or evasé. Like the inlet flange, it mates with a companion flange shipped loose for field mounting. On a centrifugal fan the outlet flange is usually rectangular; on an inline or axial fan it is round.
The bolt pattern is where outlet flanges cause trouble. On cast aluminum pressure blowers, FanPedia (pages 125 and 180) shows a blast gate, a wafer-type butterfly valve, that mounts to the outlet flange, and both the blast gate and the flange bolt pattern match the 125# ASA pipe flange standard. For nominally leak-tight fans, every bolted connection, outlet flange included, needs closely spaced holes on 3-inch to 4-inch centers and gaskets (page 120).
Worked Example: Matching an Outlet Accessory to the Outlet Flange¶
A plant engineer wants to add a blast gate to the discharge of a pressure blower and pipe it to process equipment.
- Identify the outlet connection on the blower's drawing: a flanged outlet with a 125# ASA pipe flange bolt pattern.
- Select the blast gate. Its bolt pattern also matches 125# ASA, so it bolts directly to the outlet flange with no adapter.
- Check the downstream pipe. If it ends in a 125# ASA flange of the same size, it bolts to the blast gate; if not, add a transition or companion flange that matches.
- Note the blast gate's limit from FanPedia (page 180): maximum temperature 250°F. If the process air is hotter, the blast gate is the wrong accessory.
Result: three parts that bolt together with one shared flange standard, plus one temperature check that a quick read of the outlet flange alone would have missed.
Now that you have seen the inlet, impeller, housing, and outlet, you can trace the complete air path. The MicroSim below asks you to put the parts in the order a cubic foot of air meets them.
Diagram: Follow the Airflow Through a Centrifugal Fan¶
Run Follow the Airflow Through a Centrifugal Fan Fullscreen
Follow the Airflow Through a Centrifugal Fan
Type: microsim
sim-id: ch03-follow-the-airflow
Library: p5.js
Status: Specified
Bloom Level: Remember
Bloom Verb: list
Learning Objective: The learner will list, in airflow order, the eight parts that air passes through in a ducted, belt-driven SWSI centrifugal fan, and set aside the three parts that stay out of the air path.
Prerequisites: fan housing, housing scroll, cutoff, impeller, inlet funnel, inlet flange, outlet flange, ductwork, shaft, bearing, motor (all defined in "The Fan Housing", "The Impeller", "The Fan Inlet" and "The Fan Outlet" above; shaft, bearing and motor are named in the chapter introduction).
Evidence of Mastery: The learner places all 11 cards: 8 into numbered air-path slots 1 to 8, and 3 into a "Not in the air path" bin, then submits. A placement is correct when it matches the Content table. Mastery is all 11 cards correct within two submissions. Dragging cards around before submitting is exploration and is not evidence.
Misconceptions: (1) Air enters the impeller at the blade tips (it enters at the center, through the inlet funnel). (2) The cutoff is at the inlet (it is just upstream of the outlet). (3) Every part of a fan touches the air (in a belt-driven Arrangement 9 fan the bearings and motor sit outside the airstream).
Instructional Rationale: Ordering the parts by the path of the air forces the learner to recall each part's position relative to the others, which is the Remember-level skill an exploded view or field call requires. Including three non-air-path cards makes the learner decide what touches the air, not just recite a list.
Content:
| Card | Correct place | Why (shown as feedback) |
|---|---|---|
| Inlet ductwork | Slot 1 | Air arrives from the upstream duct. |
| Inlet flange (with companion flange) | Slot 2 | The bolted joint where the duct meets the fan inlet. |
| Inlet funnel | Slot 3 | Funnels the air smoothly into the eye of the impeller. |
| Impeller | Slot 4 | The only part that adds energy to the air; air enters at the center and is thrown outward between the blades. |
| Housing scroll | Slot 5 | Collects the air leaving the impeller and guides it toward the outlet. |
| Cutoff | Slot 6 | The point of the scroll closest to the impeller, just before the outlet; it separates leaving air from circling air. |
| Outlet flange | Slot 7 | The bolted joint where the fan outlet meets the downstream duct. |
| Outlet ductwork | Slot 8 | Air leaves the fan system through the downstream duct. |
| Shaft | Not in the air path | It passes through the housing side to carry the impeller, but it is not a stage the air flows through. |
| Bearings | Not in the air path | In an Arrangement 9 fan the bearings sit on the pedestal outside the airstream. |
| Motor | Not in the air path | In a belt-driven Arrangement 9 fan the motor is mounted outside the housing and drives through belts. |
Provenance: Part names and positions from FanPedia, page 10 (exploded view, belt-driven centrifugal fan, Arrangement 9) and page 60 (SWSI housing construction). Bearings on the pedestal out of the airstream from FanPedia page 174 (Arrangement 1 technical description); Arrangement 9 shares that overhung pedestal layout (page 30) and adds the motor slide base on the pedestal side (page 175).
Rules: Eleven cards, eight numbered slots, one bin. Each slot holds exactly one card; the bin holds any number. A submission is accepted only when all 11 cards are placed. Correctness is exact match to the Content table; there are no ties.
Learner Activity:
- The learner sees the 11 cards in a shuffled pile and an outline of a fan with eight empty numbered slots along the air path and one bin labeled "Not in the air path".
- The learner places each card into a slot or the bin. Placing a card into an occupied slot returns the previous card to the pile.
- The learner presses Submit. Correct cards lock in place; incorrect cards return to the pile with their feedback text shown.
- After the final submission, the full air path animates once from Slot 1 to Slot 8, highlighting each part as the air reaches it.
Feedback: One set of 11 placements, shuffled order, two submissions. Correct placement: the card locks and shows "Correct:" followed by its Why text. Hint after a first wrong attempt (first submission): the card returns to the pile with a hint: the misconception it reflects, put as a question, and a question about the part, never its slot (cutoff: "The cutoff is not at the inlet. Look for it on the scroll. The cutoff is part of the scroll. Is it near the inlet or near the outlet?"). After the final attempt (second submission): the card's Why text and the misconception it reflects, and all correct positions are revealed. A score "Correct: n of 11" is shown after each submission.
Starting State: All 11 cards are in the pile and all slots are empty. The prompt reads "A cubic foot of air enters the inlet duct. In what order does it meet the fan's parts?"
Chapter Anchors: eight air-path parts and three non-air-path parts; the fan is a belt-driven SWSI centrifugal fan, Arrangement 9.
The Fan Shaft¶
The fan shaft is the steel bar that carries the impeller and transmits the turning force, or torque, from the drive to it. FanPedia (page 79) calls the shaft the core of the rotor assembly, the impeller plus the shaft, and states that the shaft is supported by two bearings in one of two basic layouts: the overhung impeller or the center-hung impeller described in the impeller section. The diameter is chosen to meet the fan's structural, vibration, and balance requirements, and the material depends on the environment the fan runs in. Mild steel is typical; stainless steel and special alloys are used when the airstream or a spark-resistant construction calls for them (page 80).
A shaft is not a plain bar. It is machined with features that connect it to the parts it turns and the instruments that check it. FanPedia (page 80) names four:
- Keystock is the bar of steel, the key, that locks a rotating part (impeller hub, bushing, coupling, or sheave) to the shaft so it cannot slip. A square key is typical; a stepped key is used only when a square key is not possible.
- A keyway is the groove cut into the shaft to hold the key.
- A tachometer (tach) hole is a dimple machined in the shaft end, so a mechanical tachometer can read the shaft's RPM while it runs.
- A turndown is a section where the shaft is machined to a smaller diameter, on one or both ends, typically at the hub, the shaft seals, the bearings, or the coupling.
Several shaft accessories work alongside these features. A shaft collar (locking collar) helps hold the shaft in place so it cannot shift into a bearing or the impeller. A shaft sleeve shields the shaft from the airstream when the shaft and airstream materials differ. A shaft cooler sheds heat from the shaft in high-temperature service. A zero speed switch detects when the shaft slows or stops (pages 128 and 129). Chapter 16 covers these in detail.
The arrangement of the fan decides where the shaft's bearings sit relative to the impeller. The table below restates the two layouts from FanPedia page 79 with one more case: in Arrangement 4 there is no separate fan shaft at all.
| Layout | Where the impeller sits | Arrangements | Who supplies the shaft and bearings |
|---|---|---|---|
| Overhung impeller | On the end of the shaft, both bearings on one side | 1, 8, 9, 10 | Fan shaft on two fan bearings |
| Center-hung impeller | Between the two bearings | 3, 7 | Fan shaft on two fan bearings |
| Impeller on motor shaft | On the end of the motor shaft | 4 | Shaft is integral to the motor; its two bearings are inside the motor frame |
Worked Example: Locating the Shaft and Bearings from the Arrangement¶
A service manager has three fans to inspect and wants to know where the bearings are before sending a mechanic. The fans are a belt-driven Arrangement 9 centrifugal exhaust fan, a belt-driven Arrangement 3 double-width supply fan, and an Arrangement 4 direct-drive centrifugal fan.
- Arrangement 9: overhung impeller. The shaft runs from the impeller, through the housing side, to two bearings on the pedestal outside the housing. The mechanic can grease both bearings from outside the fan.
- Arrangement 3: center-hung impeller. One bearing is on each side of the impeller, mounted on the housing framing. On a double-width fan, both bearings sit in or near the inlet openings, so the mechanic should expect to work at both inlets.
- Arrangement 4: impeller on the motor shaft. There is no fan shaft and no fan bearings to service; the only bearings are inside the motor.
Result: three different service plans from one piece of information on the nameplate. Chapter 11 walks through every arrangement.
Check It From the Drive Side
When you need to find the bearings on an unfamiliar fan, find the arrangement number on the nameplate or submittal first, then stand on the drive side, the end of the shaft where the belts or coupling connect. Check it from the drive side: that one habit also sets you up to read rotation and discharge codes correctly in Chapter 12.
Bearings¶
A bearing is the part that supports a rotating shaft, lets it turn with little friction, and holds it in position. Every fan with its own shaft has two fan bearings. FanPedia (page 97) explains that the two bearings support and locate the shaft both radially (holding it on its centerline) and axially (keeping it from sliding end to end) in relation to the stationary housing. One bearing is fixed: it controls the shaft's axial position. The other is floating: it lets the shaft grow and shrink lengthwise with temperature. For horizontally mounted fans the fixed bearing is on the drive side.
Inside a bearing, the rolling elements, balls or rollers, ride between an inner ring on the shaft and an outer ring held by the bearing housing. Fans mostly use pillow block bearings, in which the bearing sits in a housing bolted to a mounting surface parallel to the shaft, such as a pedestal top. FanPedia (pages 92 and 178) distinguishes three common types, summarized in this table:
| Bearing type | Rolling element | What it is good for |
|---|---|---|
| Solid pillow block, ball type | Balls | Smooth, low-friction rotation |
| Solid pillow block, spherical roller | Crowned (spherical) rollers | High loads and shock loads; limited speed |
| Split pillow block | Ball or spherical roller | Housing splits open, so internal parts can be serviced without fully removing the shaft |
A flange bearing is mounted in a flanged housing when the mounting surface is perpendicular to the shaft; FanPedia notes these are used mostly on some axial fans and special applications (page 177). Lubrication systems, positioners, and sensors are covered in Chapter 17.
Bearing Life¶
All bearings eventually wear out. Under controlled laboratory loads with proper lubrication, a bearing fails by metal fatigue, and bearing life is a statistical estimate of when a share of a large group of identical bearings will have failed. FanPedia (page 90) defines two ratings:
- L-10 life is the number of operating hours at which 10 percent of a population of bearings, at a given speed and load, are expected to have failed.
- L-50 life, or average life, is the hours at which 50 percent are expected to have failed. It is calculated as five times the L-10 life.
Most Twin City Fan models offer an L-10 life of 40,000 hours; some are offered at 20,000, 60,000, 80,000 or 100,000 hours. The bearing life for each model is in its product catalog. A bearing rating in hours becomes useful to a customer only when it is converted to years of their actual running schedule.
Bearing Life in Years¶
where:
- \( Y \) is the expected L-10 life in years
- \( L_{10} \) is the L-10 bearing life in hours
- \( h \) is the operating hours per day
- \( d \) is the operating days per week
- 52 is the number of weeks per year
Worked Example: Converting L-10 Hours to Years¶
A school runs its supply fan 16 hours a day, 5 days a week. The fan's bearings are rated L-10 = 40,000 hours.
- Hours per year: \( 16 \times 5 \times 52 = 4{,}160 \) hours.
- L-10 life in years: \( 40{,}000 / 4{,}160 = 9.6 \) years.
- L-50 life: \( 5 \times 40{,}000 = 200{,}000 \) hours, or \( 200{,}000 / 4{,}160 = 48 \) years.
The same bearing on a process fan running 24 hours a day, 7 days a week, uses \( 24 \times 7 \times 52 = 8{,}736 \) hours per year, so its L-10 life is only 4.6 years. These results match the table on FanPedia page 90. Read the L-10 figure as "by about 9.6 years, roughly 1 in 10 of these bearings will have failed," not as "every bearing lasts 9.6 years."
The chart below lets you test that conversion on any rating and schedule.
Diagram: Bearing Life in Years by Operating Schedule¶
Run Bearing Life in Years by Operating Schedule Fullscreen
Bearing Life in Years by Operating Schedule
Type: chart
sim-id: ch03-bearing-life-years
Library: Chart.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the L-10 and L-50 bearing life in years from a rated L-10 life in hours and an operating schedule, to within 0.1 year.
Prerequisites: bearing, L-10 life, L-50 life, operating schedule (defined in "Bearing Life" above).
Evidence of Mastery: For three challenge cases the learner types the L-10 life in years before the chart reveals it. An answer is correct when it is within ±0.1 year of the model value. Mastery is 3 of 3 correct. Changing the rating or schedule and reading the chart is exploration, not evidence.
Misconceptions: (1) L-10 life is the life of every bearing (it is when 10 percent are expected to have failed). (2) L-50 life is twice L-10 (it is five times L-10). (3) A bearing rated in hours lasts the same number of years for every customer (years depend on the schedule).
Instructional Rationale: Converting hours to years for a stated schedule is an Apply-level procedure, so the learner commits a number before the chart shows the answer. The bar chart then lets the learner see across schedules how the same rating spans from a few years to many decades.
Content:
L-10 ratings the learner can choose: 20,000; 40,000; 60,000; 80,000; 100,000 hours. Default: 40,000 hours.
Operating schedules shown as five bars, in this order:
| Schedule label | Hours per day | Days per week | Hours per year |
|---|---|---|---|
| 24 h/day, 7 days/week | 24 | 7 | 8,736 |
| 24 h/day, 5 days/week | 24 | 5 | 6,240 |
| 16 h/day, 5 days/week | 16 | 5 | 4,160 |
| 8 h/day, 5 days/week | 8 | 5 | 2,080 |
| 2 h/day, 5 days/week | 2 | 5 | 520 |
Each bar shows L-10 years; a second series shows L-50 years. Hovering a bar shows the rating, the schedule, the hours per year, the L-10 years and the L-50 years.
Challenge cases, in this order:
| # | L-10 rating (h) | Schedule | Model L-10 years | Feedback when wrong |
|---|---|---|---|---|
| 1 | 40,000 | 16 h/day, 5 days/week | 9.6 | 16 × 5 × 52 = 4,160 h per year; 40,000 / 4,160 = 9.6 years. |
| 2 | 40,000 | 24 h/day, 7 days/week | 4.6 | Around-the-clock service is 8,736 h per year; 40,000 / 8,736 = 4.6 years. |
| 3 | 20,000 | 8 h/day, 5 days/week | 9.6 | 8 × 5 × 52 = 2,080 h per year; 20,000 / 2,080 = 9.6 years. Half the rating at a quarter of the hours of around-the-clock service still gives about ten years. |
Provenance: L-10 and L-50 definitions, the L-50 = 5 × L-10 rule, the available TCF ratings, and the example values for 40,000 h and 20,000 h from FanPedia, page 90. Values for 60,000, 80,000 and 100,000 h are computed with the Rules.
Rules: Hours per year = hours per day × days per week × 52. L-10 years = L-10 hours / hours per year. L-50 hours = 5 × L-10 hours; L-50 years = L-50 hours / hours per year. Display years rounded to 0.1 year. The rating is chosen from the five listed values only; schedules are the five listed only.
Learner Activity:
- The learner reads challenge 1 and types the L-10 life in years.
- The learner presses Check. The chart switches to that rating, highlights the matching schedule bar, and shows its value next to the learner's answer.
- After all three challenges, the learner can choose any of the five ratings. All ten bars (five L-10, five L-50) update at once. The learner should notice that every bar scales in proportion to the rating, and that the 2 h/day bar is about 17 times the 24/7 bar.
Feedback: Three challenges, fixed order, two attempts each. Correct: "Correct:
Starting State: Challenge 1 is shown with an empty answer box. The chart shows the five schedule labels with the bars hidden. The question reads "A 40,000-hour L-10 bearing runs 16 hours a day, 5 days a week. How many years is its L-10 life?"
Chapter Anchors: L-10 = 40,000 h at 16 h/day, 5 days/week gives 4,160 h/year and 9.6 years; L-50 is 200,000 h and 48 years; 24/7 service is 8,736 h/year and 4.6 years; L-50 = 5 × L-10; ratings 20,000, 40,000, 60,000, 80,000 and 100,000 h.
The Fan Motor¶
The fan motor is the machine that converts electrical energy into the mechanical rotation that turns the impeller. FanPedia (page 68) notes that the electric motor is by far the most common way to power a ventilating fan, and that two technologies are used.
An induction motor is an alternating-current (AC) motor in which the current in the rotor, needed to produce torque, is created by electromagnetic induction from the magnetic field of the stator winding, so no electrical connection to the rotor is needed. An electronically commutated (EC) motor uses on-board electronics to run and control the motor: a rectifier converts the AC supply to direct current (DC), and a controller sends the right amount of current, in the right direction, at the right time, through each winding. The EC motor's speed can be set by its own speed controller or by a 0-10 V signal from an outside source, and it runs efficiently even at reduced speed.
FanPedia (page 69) names the parts of an induction motor, the more common of the two. You will see them on drawings and nameplates:
- The stator is the stationary part; its windings carry current and produce the magnetic field.
- The rotor assembly (rotor and motor shaft) is the rotating part that delivers power to the fan.
- The motor bearings support the rotor at each end, inside the endplates.
- The conduit box is where the power wiring connects, usually in position F1 or F2 on the side of the motor (F3 is on top). F1 or F3 is the standard location; F2 is standard on IEC frames but a special modification on NEMA motors (FanPedia page 75).
- The nameplate identifies the manufacturer, serial number, and ratings.
Diagram: Induction Motor Parts¶
Run Induction Motor Parts Fullscreen
Induction Motor Parts
sim-id: ch03-induction-motor-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the ten labeled parts of an electric induction motor on an exploded view, given each part's function.
Content: Ten numbered markers on the exploded view: fan cover, endplates, nameplate, stator, rotor assembly, external fan, conduit box, frame, motor bearings, and motor shaft.
Provenance: FanPedia page 69, "Motors: Typical Electric Induction Motor Components" (exploded view), with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 69.
The table below compares the two technologies on the points just described.
| Feature | Induction motor | EC motor |
|---|---|---|
| Supply | AC | AC, converted to DC on board |
| How the rotor gets its magnetism | Current induced from the stator field | Permanent magnets in the rotor |
| How speed is varied | External speed control | Built-in electronics, own controller or 0-10 V signal |
| Where it is common | Most fans in the industry | Smaller fans where efficient speed control matters |
Service Factor¶
Two nameplate ratings matter most when you match a motor to a fan. The horsepower (HP) rating is the power the motor can deliver continuously. The fan's brake horsepower (BHP), from Chapter 1, is the power the fan actually draws at its operating point. The service factor (S.F.) is the multiplier above the rated horsepower at which the motor may be used. FanPedia gives the example of a 100 HP motor with a 1.15 service factor, which may be operated at 115 HP, and lists 1.0 (when operated on a variable frequency drive, VFD) and 1.25 as other common values (page 69).
Worked Example: Is the Motor Large Enough?¶
A fan selection shows 53 BHP at the design point. The proposed motor is 50 HP with a 1.15 service factor.
- Across the line, the motor may be loaded to \( 50 \times 1.15 = 57.5 \) HP. 53 BHP is within that limit, but it uses the service factor every hour the fan runs, leaving no margin for the motor to run cooler or handle denser air.
- The specification then adds a VFD. On a VFD the service factor is 1.0, so the limit becomes \( 50 \times 1.0 = 50 \) HP. 53 BHP is now above the limit.
- The next standard size, a 60 HP motor, carries 53 BHP with margin under either condition.
Result: change the motor to 60 HP before the order is entered. Chapter 14 covers motor enclosures and insulation classes, and Chapter 15 covers speed control.
Service Factor Is Not Spare Horsepower
A frequent mistake is selecting a motor whose nameplate horsepower is below the fan's brake horsepower and counting on the service factor to cover the gap. Size the motor so the fan's BHP at the design point is at or below the nameplate horsepower, and remember that a motor on a VFD usually has a service factor of 1.0.
The Drive Train¶
The drive train is the set of parts that carries power from the motor shaft to the impeller. It decides whether the impeller turns at the motor's speed or at some other speed, and whether the motor sits in line with the fan or beside it. There are three basic ways to connect them:
- Direct drive, impeller on the motor shaft. The impeller mounts straight onto the motor shaft, as in Arrangement 4. The drive train is just the motor and the impeller hub. FanPedia (page 174) notes that Arrangement 4 offers low maintenance since there are no fan bearings, fan shaft, or drive parts to maintain.
- Direct drive through a coupling. The motor shaft and the fan shaft are joined end to end by a coupling, a device that connects two shafts so they turn together. FanPedia (page 174) describes Arrangement 8 as Arrangement 1 with the bearing pedestal extended to carry the motor, and a flexible coupling connecting the fan and motor shafts. Arrangement 7 is the coupled version of Arrangement 3.
- Belt drive. A sheave (also called a pulley), a grooved wheel, is mounted on the motor shaft and another on the fan shaft, and V-belts run in the grooves between them. A bushing may connect a sheave to its shaft. The motor sits on a motor base or slide base that moves to tension the belts. A belt guard encloses the belts and sheaves.
Diagram: Direct Drive Centrifugal Fan Parts¶
Run Direct Drive Centrifugal Fan Parts Fullscreen
Direct Drive Centrifugal Fan Parts
sim-id: ch03-direct-drive-centrifugal-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the twelve labeled parts of an Arrangement 8 direct drive centrifugal fan on an exploded view, given each part's function.
Content: Twelve numbered markers on the exploded view: inlet collar, inlet funnel, impeller, housing, outlet flange, access hole for mounting bolts, pedestal, bearings, shaft, coupling, motor, and inlet flange (in the inset).
Provenance: FanPedia page 9, "Exploded Views: Centrifugal Fans", Direct Drive Centrifugal Fans (Arrangement 8), with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 9.
In a direct drive the impeller turns at motor speed. In a belt drive, the ratio of the two sheave diameters sets the fan speed, so changing a sheave changes the fan's speed and, with it, the fan's airflow. FanPedia notes that belt-driven fans can reach the exact point of rating through a simple sheave change (page 173).
Belt Drive Speed Ratio¶
where:
- \( N_{fan} \) is the fan shaft speed in RPM
- \( N_{motor} \) is the motor speed in RPM
- \( D_{motor} \) is the pitch diameter of the motor sheave in inches
- \( D_{fan} \) is the pitch diameter of the fan sheave in inches
The pitch diameter is the diameter at which the belt actually rides in the sheave groove, slightly smaller than the outside diameter. Real drives lose a small amount of speed to belt slip; the formula gives the ideal value.
Worked Example: Fan Speed on a Belt Drive¶
An illustrative motor runs at 1,760 RPM. Its sheave has a 5.0-inch pitch diameter, and the fan sheave has an 8.0-inch pitch diameter.
- Find the ratio: \( 5.0 / 8.0 = 0.625 \).
- Find the fan speed: \( 1{,}760 \times 0.625 = 1{,}100 \) RPM.
- Now suppose the system needs a bit more air. Replacing the motor sheave with a 5.4-inch sheave gives \( 1{,}760 \times 5.4 / 8.0 = 1{,}188 \) RPM, about 8 percent faster.
A larger motor sheave, or a smaller fan sheave, speeds the fan up. Because the fan's power draw rises much faster than its speed, a sheave change must always be followed by a check that the motor can still carry the load.
One Fan, Many Speeds
Notice what the belt drive buys you: the same fan, impeller, and motor can be set to many different speeds just by choosing two sheaves. That is why belt drive is the flexible choice for matching a fan to a system exactly, and direct drive is the low-maintenance choice when the motor speed already fits.
The MicroSim below lets you practice the speed-ratio calculation and see how each sheave moves the fan speed.
Diagram: Belt Drive Speed Calculator¶
Run Belt Drive Speed Calculator Fullscreen
Belt Drive Speed Calculator
Type: microsim
sim-id: ch03-belt-drive-speed-calculator
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the fan shaft speed of a belt drive from the motor speed and the two sheave pitch diameters, to within ±10 RPM.
Prerequisites: drive train, sheave, V-belt, pitch diameter, motor speed, direct drive, belt drive (defined in "The Drive Train" above).
Evidence of Mastery: For three challenge drives, the learner types the fan speed in RPM before the drive runs. An answer is correct when it is within ±10 RPM of the model value. Mastery is 3 of 3 correct. Changing sheaves in free exploration is not evidence.
Misconceptions: (1) A larger fan sheave makes the fan turn faster (it makes it turn slower). (2) A belt drive always slows the fan down (if the motor sheave is larger than the fan sheave, the fan turns faster than the motor). (3) Fan speed depends on the difference between the sheave sizes rather than their ratio.
Instructional Rationale: Calculating a speed from a ratio is an Apply-level procedure, so the learner commits a number before the drive turns and the sim shows the result. Free exploration afterward confronts misconception 1 and 2 directly, because the learner can watch the fan speed fall as the fan sheave grows.
Content:
Quantities the learner can change in free exploration:
| Quantity | Min | Max | Step | Default | Unit |
|---|---|---|---|---|---|
| Motor sheave pitch diameter | 3.0 | 12.0 | 0.2 | 5.0 | in |
| Fan sheave pitch diameter | 4.0 | 24.0 | 0.2 | 8.0 | in |
Motor speeds the learner can choose: 1,160 RPM; 1,760 RPM; 3,500 RPM. Default: 1,760 RPM. The sim labels these "illustrative nominal motor speeds".
Challenge drives, in this order:
| # | Motor speed (RPM) | Motor sheave (in) | Fan sheave (in) | Model fan speed (RPM) | Feedback when wrong |
|---|---|---|---|---|---|
| 1 | 1,760 | 5.0 | 8.0 | 1,100 | Multiply motor speed by motor sheave over fan sheave: 1,760 × 5.0 / 8.0 = 1,100 RPM. |
| 2 | 1,760 | 6.0 | 12.0 | 880 | The fan sheave is twice the motor sheave, so the fan turns at half the motor speed: 1,760 / 2 = 880 RPM. |
| 3 | 1,160 | 9.0 | 6.0 | 1,740 | Here the motor sheave is larger, so the fan turns faster than the motor: 1,160 × 9.0 / 6.0 = 1,740 RPM. |
The display always shows the ratio (motor sheave / fan sheave) to three decimals and the fan speed in RPM, and labels the drive "fan slower than motor", "fan same as motor" or "fan faster than motor".
Provenance: The speed-ratio formula is standard belt-drive kinematics, ignoring belt slip. Motor speeds and sheave sizes are illustrative and labeled as such. The idea that a sheave change sets the exact point of rating comes from FanPedia, page 173.
Rules: Fan speed = motor speed × motor sheave / fan sheave, rounded to the nearest 1 RPM. Ratio label: "fan slower than motor" when ratio < 1.000; "fan same as motor" when ratio = 1.000; "fan faster than motor" when ratio > 1.000. Belt slip is not modeled, and the sim says so in a note. The largest possible fan speed is 3,500 × 12.0 / 4.0 = 10,500 RPM and the smallest is 1,160 × 3.0 / 24.0 = 145 RPM; the display covers that whole range.
Learner Activity:
- The learner reads challenge 1 (motor speed and both sheaves, shown as labeled numbers and drawn to scale) and types the fan speed.
- The learner presses Check. The drive turns, and the fan speed is shown next to the learner's answer.
- After all three challenges, free exploration unlocks. The learner changes either sheave or the motor speed; the drawing, ratio, label and fan speed update immediately.
- The first time the learner enlarges the fan sheave, the sim notes "Bigger fan sheave: fan speed went down."
Feedback: Three challenges, fixed order, two attempts each. Correct: "Correct:
Starting State: Challenge 1 is shown with an empty answer box and the drive at rest. The question reads "The motor turns at 1,760 RPM with a 5.0-inch sheave driving an 8.0-inch fan sheave. How fast does the fan turn?"
Chapter Anchors: 1,760 RPM with a 5.0-inch motor sheave and 8.0-inch fan sheave gives a ratio of 0.625 and 1,100 RPM; a 5.4-inch motor sheave gives 1,188 RPM, about 8 percent faster. The 5.4-inch value lies on the motor sheave step grid.
With the drive train in place, every major part of the fan has been named. The interactive exploded view below brings them back together in the drawing you will meet on submittals and in FanPedia.
Diagram: Exploded View of a Belt-Driven Centrifugal Fan¶
Run Exploded View of a Belt-Driven Centrifugal Fan Fullscreen
Exploded View of a Belt-Driven Centrifugal Fan
Type: infographic
sim-id: ch03-exploded-centrifugal-fan
Library: html
Status: Specified
Bloom Level: Remember
Bloom Verb: identify
Learning Objective: The learner will identify each of the 14 labeled parts in an exploded view of a belt-driven Arrangement 9 centrifugal fan when given the part's name or function.
Prerequisites: exploded view, fan housing, impeller, inlet funnel, inlet collar, inlet flange, outlet flange, shaft, bearing, sheave, V-belt, motor, slide base, pedestal (all defined in this chapter above).
Evidence of Mastery: In quiz mode the learner answers 10 prompts by selecting a part on the drawing. A selection is correct when it is the part named in the Content quiz table. Mastery is 9 of 10 correct on the first attempt. Opening infoboxes in explore mode is exploration, not evidence.
Misconceptions: (1) The inlet funnel and inlet collar are the same part (the funnel shapes the air into the impeller; the collar is a duct connection). (2) The pedestal is part of the housing (it is a separate support for the bearings and shaft). (3) The sheave on the fan shaft is the same size as the motor sheave (their ratio sets fan speed).
Instructional Rationale: Identifying parts on the same kind of drawing used in submittals and in FanPedia is the Remember-level skill this chapter promises. Explore mode lets the learner attach each name to a location and a function; quiz mode requires the learner to retrieve the part from a name or a function rather than read a label.
Content:
The drawing shows a belt-driven SWSI centrifugal fan, Arrangement 9, exploded along the shaft line, with these 14 parts. Each part, when selected in explore mode, opens an infobox with the text below.
| # | Part | Infobox text |
|---|---|---|
| 1 | Inlet collar | Short sleeve at the inlet for a slip-fit duct connection, clamped, with no bolt holes. |
| 2 | Flanged inlet (inlet flange) | Flat ring with a bolt-hole pattern for a bolted duct connection; mates with a companion flange shipped loose. |
| 3 | Inlet funnel | Aerodynamically shaped ring that funnels air smoothly into the impeller; also called inlet cone or inlet bell. |
| 4 | Impeller | The rotating part that adds energy to the air; air enters at its center and leaves at the blade tips. |
| 5 | Housing | The scroll-shaped shell that encloses the impeller and guides air to the outlet; also called side, scroll or casing. |
| 6 | Access hole for mounting bolts | Opening in the housing used to reach the bolts that mount the housing. |
| 7 | Outlet flange | Flat frame around the outlet with a bolt-hole pattern for the downstream duct or accessory. |
| 8 | Shaft | Steel bar that carries the impeller and transmits torque from the drive; with the impeller it forms the rotor assembly. |
| 9 | Bearings | Two bearings on the pedestal that support and locate the shaft, outside the airstream in Arrangement 9. |
| 10 | Sheave (pulley) | Grooved wheel on the fan shaft; with the motor sheave and belts it sets the fan speed. |
| 11 | Belt | V-belts that carry power from the motor sheave to the fan sheave. |
| 12 | Motor | Converts electrical energy into rotation; here it sits beside the fan and drives through belts. |
| 13 | Motor slide base | Adjustable base that slides the motor to tension the belts. |
| 14 | Pedestal | Support structure that carries the bearings and shaft, and in Arrangement 9 the motor slide base on its side. |
Quiz prompts, in this order:
| # | Prompt | Correct part | Why (shown as feedback) |
|---|---|---|---|
| 1 | Select the part that adds energy to the air. | Impeller | It is the only rotating part in the air path. |
| 2 | Select the part that funnels air smoothly into the impeller. | Inlet funnel | The funnel shapes the inlet air; the collar and flange are duct connections. |
| 3 | Select the part a duct slips over and is clamped to. | Inlet collar | A collar has no bolt holes; a flanged inlet is bolted. |
| 4 | Select the part that the downstream duct is bolted to. | Outlet flange | The outlet flange carries the bolt pattern for the discharge duct. |
| 5 | Select the part that carries the impeller and transmits torque to it. | Shaft | The shaft and impeller form the rotor assembly. |
| 6 | Select the parts that support and locate the shaft. | Bearings | Two bearings on the pedestal hold the shaft radially and axially. |
| 7 | Select the part that you would change to change fan speed. | Sheave (pulley) | The ratio of the motor and fan sheaves sets fan speed. |
| 8 | Select the part that moves to tension the belts. | Motor slide base | Sliding the motor away from the fan tightens the belts. |
| 9 | Select the part also called the scroll or casing. | Housing | Side, scroll and casing are field names for the housing. |
| 10 | Select the structure that holds the bearings and the motor slide base. | Pedestal | The pedestal is a separate support, not part of the housing. |
Provenance: Part list and arrangement from FanPedia, page 10 (exploded view, belt-driven centrifugal fans, Arrangement 9). Infobox definitions from this chapter's sections, which cite FanPedia pages 59, 79, 90 and 188.
Rules: Explore mode: selecting any part opens its infobox; one infobox is open at a time. Quiz mode: one selection per prompt per attempt; a prompt is correct only when the exact part in the Correct part column is selected.
Learner Activity:
- The learner starts in explore mode and selects parts to read their infoboxes. A part that has been opened is marked as visited.
- The learner can collapse the drawing to the assembled view and expand it again to see how each part moves into place.
- The learner switches to quiz mode. Labels are hidden. Each prompt appears in turn and the learner selects a part.
- After the tenth prompt, a summary shows which prompts were missed, with their Why text.
Feedback: Ten prompts, fixed order, two attempts each. Correct: "Correct:" followed by the Why text, and the part's label appears. Hint after a first wrong attempt: "That is the
Starting State: The exploded drawing in explore mode with all 14 labels visible and no infobox open. The prompt reads "Select any part to see what it does. When you are ready, switch to quiz mode."
Chapter Anchors: 14 labeled parts; Arrangement 9, belt-driven SWSI centrifugal fan; 10 quiz prompts; the bearings sit outside the airstream in Arrangement 9.
Lots of Names, One Machine
If the part names are starting to blur together, that is normal; most people need a couple of passes through an exploded view before the names stick. Go back to the airflow MicroSim and say each part's name out loud as the air reaches it, then try the exploded view quiz again.
Housing Features for Access and Service¶
Fans need to be inspected, cleaned, and repaired, often without removing the ductwork. Three housing features make that possible. The split housing, covered earlier, opens the housing for impeller removal. The other two are the access door and the inspection port.
Access Door¶
An access door is a removable or hinged cover over an opening in the fan housing, large enough to let a person reach or see into the housing to inspect, clean, or service the impeller and interior. Access doors appear on centrifugal housings, on the tubes of axial and inline fans (where FanPedia labels them impeller access doors, page 63), and on most inlet boxes, where FanPedia lists the access door as standard (page 145). FanPedia (page 125) shows three common styles:
- A bolted access door is a plate bolted to the housing, either on the housing face or wrapped over the edges of the housing. It seals well but takes time to open.
- A raised access door is built up above the housing surface. On insulated fans FanPedia notes it is usually raised 2 inches above the insulation (page 117), so the door can be opened without cutting through the insulation.
- A quick-open access door is held with latches or handles, so it can be opened without tools for frequent inspection or cleaning.
Worked Example: Choosing the Access Door¶
A customer is ordering three centrifugal fans: one for a clean-air supply system, one for a bakery exhaust that must be cleaned weekly, and one insulated fan for a hot dryer exhaust.
- Clean-air supply: inspection is rare, so a standard bolted access door on the scroll is enough and seals well.
- Bakery exhaust: buildup must be removed every week. A quick-open access door saves removing and replacing a ring of bolts each visit.
- Insulated dryer exhaust: a flush door would be buried in insulation. Specify a raised access door, usually raised 2 inches above the insulation.
Result: three fans, three access doors, each chosen by how often the inside of the fan will be reached. For a fan that must be cleaned more often than a door allows, FanPedia describes swingout and clamshell constructions, covered in Chapter 24.
Ask How Often the Fan Gets Cleaned
Here is a shortcut for any access question: ask the customer how often someone will need to see the impeller. Once a year points to a bolted door; every week points to a quick-open door or even swingout construction.
Inspection Port¶
An inspection port is a small, closable opening in the fan housing that lets a technician look into the fan, or insert a light, gauge, or probe, without opening an access door. FanPedia lists the inspection port as a standard housing accessory alongside the access door and the split housing (page 59), and a belt-driven tubeaxial fan in its exploded views carries a separate inspection door near the bearing housing (page 15). Because the port is smaller than an access door, it is quicker to open and adds less leakage risk, but it allows viewing only, not cleaning or impeller removal. Specify one when the owner wants to check impeller buildup or wear between full inspections.
Accessories That Protect the Openings¶
Fans that draw air from, or discharge it to, the open air need protection at their openings: to keep people and objects out of the impeller, to keep birds and weather out, and to keep air from flowing backward when the fan is off. Three accessories handle those jobs.
Bird and Safety Screens¶
A screen is a wire mesh or grille fastened across a fan opening. The two common kinds do different jobs. A bird screen keeps birds and large debris out of an opening that faces the outdoors, such as the discharge of a roof exhauster or the hood of a wall fan. A safety screen guards an opening against a person's hand, tools, or loose objects reaching the impeller, and is used on any opening someone could reach.
FanPedia's exploded views show both. Upblast and downblast centrifugal roof exhausters carry a bird screen (page 19). A panel fan carries an inlet safety screen on the side people can reach and a bird screen on its outdoor weatherhood (page 18). High-temperature centrifugal fans include a safety screen as part of their general construction (page 110).
Worked Example: Screens for a Wall-Mounted Exhaust Fan¶
A warehouse will have a panel fan in the outside wall, 8 feet above a walkway, exhausting through a weatherhood.
- Inside opening (the inlet): it faces the warehouse and is within reach of a ladder or a forklift mast. Specify an inlet safety screen.
- Outside opening (the weatherhood): it faces the outdoors, where birds nest in sheltered openings. Specify a bird screen on the weatherhood.
- When the fan is off: cold outside air will blow back through the opening. That calls for a backdraft damper, covered next.
Result: two screens with two different jobs, one on each side of the same fan. A screen also adds a small resistance to airflow, which the fan selection should account for.
A Bird Screen Is Not a Safety Screen
Watch out for specifications that list "screen" without saying which kind, because a mesh sized to stop birds is not designed to keep fingers out of an impeller. Write "bird screen" or "safety screen" and name the opening each one covers.
Backdraft Damper¶
A backdraft damper is a damper that lets air flow out through the fan in the normal direction and closes to stop air from flowing back in when the fan stops. A damper is a set of movable blades that can open or close an opening. The most common backdraft damper is the gravity damper, which needs no actuator. FanPedia (page 181) describes three types:
- A butterfly-type gravity damper is built into roof-mounted upblast fans. It prevents a backdraft of outside air and provides weather protection when the fan is not running.
- Ceiling-type and wall-type gravity dampers have parallel, end-pivoted blades that open automatically when the fan is energized and close by gravity when power is turned off. They prevent air from flowing back through the system and also keep out rain and snow.
Panel fans use a similar part called a backdraft shutter, mounted between the fan and its weatherhood (page 18).
Worked Example: Stopping Winter Backdraft¶
The warehouse in the previous example runs its wall exhaust fan only during the day shift. At night, wind pushes cold air in through the idle fan, and the heating bill climbs.
- Diagnose the flow: with the fan off, nothing holds the opening shut, so wind pressure drives air backward through the fan.
- Choose the damper: the fan is wall-mounted, so a wall-type gravity damper fits. When the fan starts, the airflow pushes its blades open; when the fan stops, the blades fall shut by gravity.
- Check for flutter: if the fan runs at low airflow, a gravity damper may only partly open. FanPedia (page 182) notes that low flow can leave automatic damper blades partly open, causing flutter and noise, and that a motorized damper, opened by an actuator, avoids this. For this fan, which runs at full speed, the gravity damper is fine.
Result: one accessory that closes the opening every night with no wiring or controls.
Stack Cap¶
A stack cap is a short, open-topped discharge stack fitted to the outlet of a roof-mounted or vertically mounted fan, so the air leaves straight up in a vertical jet. FanPedia (pages 7 and 8) contrasts it with a hood: a fan with a stack cap moves air in a straight line through the fan, while a hooded fan deflects the discharge down and outward. Roof-mounted axial fans and panel fans are offered with either. For vertical inline, mixed flow, and axial fans, FanPedia's discharge codes include "VUS", vertical up with stack cap (page 51). Choose a stack cap when the exhaust should be thrown up and away from the roof, as in many fume and heat exhaust applications, and a hood when the opening needs protection from rain more than a high discharge.
Putting It All Together¶
You now have names for every major part of a fan and the features around it. The last activity asks you to apply them to short job-site requests.
Diagram: Which Housing Feature Does the Job?¶
Run Which Housing Feature Does the Job? Fullscreen
Which Housing Feature Does the Job?
Type: microsim
sim-id: ch03-housing-feature-classifier
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify 12 job-site requests by the housing feature or opening accessory that meets each one, choosing from four options per request.
Prerequisites: welded housing, bolted housing, split housing, access door, inspection port, inlet collar, inlet flange, outlet flange, companion flange, bird screen, safety screen, backdraft damper, gravity damper, stack cap, hood (all defined in this chapter above).
Evidence of Mastery: The learner answers all 12 requests by choosing one of four options and committing the answer. An answer is correct when it matches the Correct option column in Content. Mastery is 10 of 12 correct on the first attempt.
Misconceptions: (1) Bird screens and safety screens are interchangeable. (2) An inspection port allows cleaning and impeller removal (it allows viewing only). (3) A collar and a flange are the same connection. (4) A backdraft damper controls airflow while the fan runs (it only stops reverse flow when the fan is off). (5) A split housing is only for maintenance (it is also used for shipping and tight openings).
Instructional Rationale: Classifying realistic requests requires the learner to connect each feature's function to a need, which is the Understand-level step between naming a part and selecting one. The distractors are features that share a location or a word with the right answer, so a correct choice shows the learner knows what each feature does.
Content:
Requests, in this order, each with four options in the order shown:
| # | Request shown to the learner | Options | Correct option | Why (shown as feedback) |
|---|---|---|---|---|
| 1 | "We need to look at the impeller for buildup between cleanings without opening a door." | Access door; Inspection port; Pie split housing; Safety screen | Inspection port | A port is a small, closable opening for viewing; an access door is for reaching in. |
| 2 | "The impeller must be pulled out without disconnecting the inlet or outlet duct." | Horizontal split housing; Pie split housing; Raised access door; Inlet collar | Pie split housing | A pie split cuts a wedge of 90 degrees or more so the impeller comes out without disturbing inlet or outlet. |
| 3 | "The fan has to fit through a doorway smaller than the assembled housing." | 3-way split housing; Quick-open access door; Welded housing; Stack cap | 3-way split housing | A 3-way split divides the housing into three sections for shipping or for passing through a specific opening. |
| 4 | "The fan is insulated, and we need to open the door without cutting the insulation." | Bolted access door; Raised access door; Inspection port; Inlet flange | Raised access door | A raised access door is built up above the insulation, usually by 2 inches. |
| 5 | "Our flexible sleeve slips over the inlet and is held with clamps." | Inlet flange; Inlet collar; Companion flange; Outlet flange | Inlet collar | A collar is a plain sleeve for slip-fit connections; a flange is bolted. |
| 6 | "The contractor needs a part to attach to the duct in the field so the bolt holes match the fan inlet." | Companion flange; Inlet collar; Evasé; Inspection port | Companion flange | Companion flanges are attached to the duct in the field and are shipped loose to match the fan flange. |
| 7 | "The downstream duct must bolt rigidly to the fan discharge." | Outlet flange; Inlet collar; Stack cap; Bird screen | Outlet flange | The outlet flange carries the bolt pattern for the discharge duct. |
| 8 | "Birds keep nesting in the roof exhauster's discharge." | Safety screen; Bird screen; Backdraft damper; Inspection port | Bird screen | A bird screen keeps birds and debris out of outdoor-facing openings. |
| 9 | "A panel fan inlet is within reach of people on a ladder." | Bird screen; Safety screen; Stack cap; Gravity damper | Safety screen | A safety screen keeps hands and objects away from the impeller. |
| 10 | "Cold air blows in through the wall fan every night when it is off." | Backdraft damper; Safety screen; Stack cap; Quick-open access door | Backdraft damper | A gravity backdraft damper opens when the fan runs and closes by gravity when it stops. |
| 11 | "The fume exhaust must discharge straight up, away from the roof." | Hood; Stack cap; Backdraft damper; Bird screen | Stack cap | A stack cap moves air straight through and up; a hood deflects it down and outward. |
| 12 | "The housing seams must not leak, so we want no bolted seams in the scroll." | Bolted housing; Welded housing; 3-way split housing; Inspection port | Welded housing | Continuous welds join the housing permanently with no bolted seams to leak. |
Provenance: Feature definitions from this chapter's sections, which cite FanPedia pages 7, 8, 18, 19, 59, 117, 120, 125, 156, 157, 180, 181 and 194. Requests are illustrative job-site wording written for this chapter.
Rules: Twelve requests, fixed order, four options each in the order listed. One option is correct per request. Answers are committed one request at a time and cannot be changed after the second attempt.
Learner Activity:
- The learner reads request 1 and its four options.
- The learner chooses one option and commits it. Feedback appears immediately.
- The learner moves to the next request. After request 12, a summary lists every request with the learner's choice, the correct option and the Why text.
- The learner can retry only the missed requests in a second round; the first-attempt score is kept.
Feedback: Two attempts per request. Correct: "Correct:" followed by the Why text. Incorrect on the first attempt: the Why text for the option the learner chose is not shown; instead, a hint names the job of the correct feature without naming it (for request 1: "Think of the smallest opening that still lets you see inside."). Hints for requests 2 to 12, in order: "Which split keeps the inlet and outlet in place?"; "Which split divides the housing for shipping?"; "Which door sits above the insulation?"; "Which connection has no bolt holes?"; "Which part goes on the duct, not the fan?"; "Which part has the bolt pattern at the discharge?"; "Which mesh is meant for animals?"; "Which mesh is meant for people?"; "Which part acts only when the fan stops?"; "Which part throws air straight up?"; "Which joining method leaves no seams to gasket?". Incorrect on the second attempt: the correct option is revealed with its Why text and the request counts as missed. Score "Correct on first attempt: n of 12" is shown at the end.
Starting State: Request 1 is shown with its four options and no selection. The heading reads "A customer calls with a request. Which feature does the job?"
Chapter Anchors: 12 requests; pie splits are 90 degrees or more; raised access doors are usually raised 2 inches above insulation; companion flanges are shipped loose; gravity dampers close by gravity when power is turned off.
Key Takeaways¶
- The fan housing (side, scroll, or casing) encloses the impeller, guides the air from inlet to outlet, and supports the other components. Plenum, panel, and ring fans have no housing; a plug fan's housing is optional.
- A welded housing is one permanent, stiff piece with no bolted seams to leak; a bolted housing comes apart for shipping and service but needs close bolt spacing and gaskets for leak-tight service. Split housings (horizontal, pie, 3-way) are planned bolted joints for shipping, tight openings, or impeller removal.
- The impeller is the only part that adds energy to the air. Its blade shape names its type, and it is either overhung (Arrangements 1, 4, 8, 9, 10) or center-hung (Arrangements 3 and 7). Tip speed is \( \pi D N / 12 \) in ft/min.
- The fan inlet brings air in through an inlet funnel, with an inlet collar (slip fit) or an inlet flange (bolted). The fan outlet connects through an outlet flange, a transition, or an evasé that lowers outlet velocity.
- The fan shaft carries the impeller and transmits torque; it has keyways, keystock, a tach hole, and turndowns. Two bearings, one fixed and one floating, support and locate it. L-10 life in years is the hours rating divided by hours run per year; L-50 is five times L-10.
- The fan motor is an induction or EC motor. Size it so the fan's BHP is at or below nameplate horsepower; service factor is 1.0 on a VFD.
- The drive train is direct (impeller on the motor shaft, or a coupling) or belt drive, where fan speed equals motor speed times motor sheave over fan sheave.
- Access doors (bolted, raised, quick-open) let people reach inside; inspection ports let them look inside. Bird screens keep animals out, safety screens keep people out, backdraft dampers stop reverse flow when the fan is off, and stack caps throw the discharge straight up.
You Can Name Every Part of a Fan
You can now trace the air from inlet flange to outlet flange, find the shaft and bearings from the arrangement number, and convert a bearing's L-10 hours into years for a real schedule. That vocabulary is the foundation for the centrifugal, axial, and mixed flow fan families in Chapter 4.