Drives and Standard Fan Arrangements

Summary

This chapter explains how the motor turns the impeller: direct drive, belt drive with sheaves and V-belts and its drive service factor, and shaft couplings. It introduces overhung and center-hung impellers and the standard numbered fan arrangements that combine them: Arrangements 1, 2, 3, 4, and 7, with the plug, plenum, vertical, axial, inline, and lab exhaust arrangement families. It also explains why some arrangements keep the bearings out of a hot airstream. After completing it, readers can name a fan's arrangement from a drawing.

Concepts Covered

This chapter covers the following 22 concepts from the learning graph:

Concept	Concept Impact Score
Direct Drive	497
Shaft Coupling	28
Overhung Impeller	470
Center Hung Impeller	456
Motor Support Base	1
Belt Drive	713
Sheave	24
Drive Service Factor	23
Fan Arrangement	445
V-Belt	23
Arrangement 1	83
Arrangement 3	10
Arrangement 4	28
Plug Fan Arrangements	68
Plenum Fan Arrangements	5
Vertical Arrangement	3
Axial and Inline Arrangements	15
Lab Exhaust Arrangements	22
Bearings Out of Airstream	22
Arrangement 2	1
Arrangement 3 SI and DI	3
Arrangement 7	2

Prerequisites

This chapter builds on concepts from:

• Chapter 2: Airstreams, Total Pressure and Standard Air
• Chapter 3: Basic Fan Anatomy
• Chapter 4: Centrifugal, Axial and Mixed Flow Fans
• Chapter 5: Roof, Wall and Specialty Exhaust Fans
• Chapter 6: Industrial Fans and Application Product Lines
• Chapter 9: Impeller Construction and Material Handling

---

From Motor Shaft to Impeller

Every quote, drawing and field call eventually asks the same question: how is this fan driven, and where do its bearings sit? By the end of this chapter you will look at an outline drawing and say "that's an Arrangement 9 with the motor on the pedestal" before anyone points at the title block. Let's follow the airflow.

A motor turns, and an impeller turns. Everything between those two shafts decides how fast the fan runs, how easily its speed can change later, how much maintenance it needs, where the bearings live and what temperature of air the fan can handle. This chapter starts with the two basic ways to connect a motor to an impeller, direct drive and belt drive, then looks at the two ways an impeller can sit on its shaft, overhung and center hung. Those two choices, combined with where the motor and bearings are mounted, are what a numbered fan arrangement describes.

You already know the parts from Chapter 3: the impeller, the housing, the shaft, the bearings and the motor. You also met the fan families in Chapter 4 through Chapter 6. Here those parts get assembled into the standard layouts that appear on every Twin City Fan drawing and order.

Two Ways to Turn an Impeller

A drive is the mechanical connection that carries power from the motor shaft to the fan impeller. There are two families. In a direct drive fan, the impeller turns at the motor's speed. In a belt drive fan, belts and pulleys sit between the motor and the fan shaft, so the impeller can turn faster or slower than the motor. Many fan types are offered both ways. FanPedia's exploded views, for example, show a direct drive tubeaxial fan beside a belt driven one, and a direct drive inline centrifugal beside a belt driven version (FanPedia pp. 15–17).

Direct Drive

A direct drive fan has no speed change between the motor and the impeller: the impeller turns at the motor's speed. There are two ways to build one.

• Impeller on the motor shaft. The impeller hub is keyed or clamped straight onto the motor shaft. There is no separate fan shaft and no fan bearings; the motor's own bearings carry the impeller. FanPedia describes Arrangement 4 this way: "the impeller is mounted directly to the motor shaft," and the arrangement "offers low maintenance since there are no fan bearings, fan shaft or drive parts to maintain" (FanPedia p. 164).
• Motor coupled to a fan shaft. The fan has its own shaft and bearings, and a shaft coupling (covered next) joins that shaft end to end with the motor shaft. FanPedia's Arrangements 7 and 8 are built this way (FanPedia p. 21).

The first form is compact and simple. The second keeps the impeller's weight and the shaft's loads on fan bearings sized for the job, which matters on larger and heavier rotors.

Because the impeller turns at motor speed, a direct drive fan can only run at the speeds motors actually come in, unless a variable frequency drive (VFD) changes the motor speed electrically. An AC induction motor's synchronous speed is set by the supply frequency and the number of magnetic poles in its winding:

\[ N_s = \frac{120\, f}{p} \]

where:

• \( N_s \) is the synchronous speed in revolutions per minute (rpm)
• \( f \) is the supply frequency in hertz (60 Hz in North America, 50 Hz in much of the world)
• \( p \) is the number of poles in the motor winding

A loaded induction motor runs a little below its synchronous speed; the difference is called slip, and the nameplate shows the actual full-load speed. Chapter 14 covers motors in detail. For now, the table below lists the synchronous speeds the formula gives for common pole counts.

Poles	Synchronous speed at 60 Hz	Synchronous speed at 50 Hz
2	3,600 rpm	3,000 rpm
4	1,800 rpm	1,500 rpm
6	1,200 rpm	1,000 rpm
8	900 rpm	750 rpm

Worked example: can this duty be direct driven? A selection program shows that a centrifugal fan meets its airflow and pressure at 1,140 rpm. On 60 Hz power, a 6-pole motor (1,200 rpm synchronous, a little less at full load) lands near that speed, so a direct drive selection is realistic. If the same fan instead needed 1,450 rpm, no fixed motor speed matches: a 4-pole motor would spin the fan far too fast, and a 6-pole motor too slow. The application engineer then has three choices. Pick a different fan size or impeller width that meets the duty at a standard motor speed; drive the fan with a VFD that slows a faster motor; or use a belt drive. FanPedia notes this exact trade on direct drive Arrangement 4 plenum fans: "Fans can be selected with varying impeller widths to provide desired performance at direct drive motor speeds," and "performance changes in the field are usually achieved by means of variable inlet vanes or VFD" (FanPedia p. 170). On direct drive axial fans, adjusting the blade angle does the same fine-tuning job (FanPedia p. 172).

The trade-off, then, is simplicity against flexibility. A direct drive fan has fewer wearing parts (no belts to tension or replace), no belt dust in the airstream, and no belt losses. FanPedia calls out the cleanliness benefit for Arrangement 4 plenum fans: the compact fan and motor unit "eliminates belt residue and requires less maintenance than other arrangements," which is why they are "widely used in cleanroom, pharmaceutical and other critical applications" (FanPedia p. 170). The cost is that the fan speed is locked to the motor speed unless a VFD is added, and on impeller-on-motor-shaft designs the motor sits close to the airstream, which limits the air temperature (FanPedia p. 171).

Shaft Coupling

A shaft coupling connects two shafts end to end so that one drives the other. FanPedia lists the coupling among the shaft accessories and defines its two jobs on a fan: it is "used to connect the fan shaft to the motor shaft or fan shaft to fan shaft (i.e. twin fan assembly)" (FanPedia p. 129). Fans use a flexible coupling, which transmits torque while tolerating a small amount of misalignment between the two shafts. Even so, the motor and fan shafts must be aligned carefully when the fan is installed, and they must stay aligned while it runs. FanPedia explains that concrete inertia bases are typically used on longer direct drive fans "to mitigate assembly deflection, maintaining proper alignment between the motor, coupling, shaft and bearings" (FanPedia p. 175). Chapter 22 covers those bases.

When you see the word "Coupling" on an arrangement drawing, it tells you three things at once:

• the fan is direct driven, so fan speed equals motor speed;
• the fan has its own shaft and bearings, so it is not an impeller-on-motor-shaft Arrangement 4;
• the motor sits in line with the fan shaft, not beside it as on a belt drive.

Worked example: reading the coupling clue. A customer emails a photo of a single inlet centrifugal fan. The motor sits on an extension of the bearing pedestal, in line with the fan shaft, and a short guarded section between them joins the two shafts. There is no belt guard. Reasoning from the coupling: the fan is direct driven, but not Arrangement 4, because Arrangement 4 has no fan shaft to couple to. FanPedia describes exactly this layout as Arrangement 8, "a modified version of Arrangement 1 used for direct drive. The Arrangement 1 bearing pedestal is extended to accommodate the motor. A flexible coupling connects the fan and motor shaft" (FanPedia p. 165). If the impeller had instead sat between two bearings with the coupled motor on a pedestal, the answer would be Arrangement 7. If a motor is ever replaced on a coupled fan, the coupling alignment must be checked again before restart.

Motor Support Base

On a direct drive tubular fan, the motor often sits inside the tube, in line with the airflow, with the impeller on its shaft. The motor support base is the structure inside the tubular housing that carries that motor. FanPedia labels it "Motor Support Base" in the exploded view of a direct drive inline centrifugal and mixed flow fan (FanPedia p. 17); the direct drive axial exploded views show the matching support structure under the motor (FanPedia pp. 15–16). Compare the belt driven versions on the same pages, where the motor sits outside the tube on a motor base and saddle and the drive runs inside a belt tube. The motor support base is a structural part, not an adjustable base; there are no belts to tension on a direct drive fan.

Belt Drive

A belt drive transmits power from the motor shaft to the fan shaft through belts running on grooved pulleys. Because the two pulleys can be different sizes, the fan can turn at almost any speed, not just at a motor speed. FanPedia summarizes the benefit plainly: "Belt driven configurations offer performance flexibility" (FanPedia p. 162). On axial and inline fans, "any future change in rating can be accomplished through a simple sheave change or blade angle adjustment" (FanPedia p. 173). A belt drive also lets the motor sit outside the airstream, beside or above the fan instead of in line with it.

The price of that flexibility is maintenance and a little efficiency. Belts stretch and wear, so they need periodic tensioning and eventual replacement, which is why belt driven motors sit on adjustable mounts such as slide bases (Chapter 15). Belts shed dust, and some power is lost in the drive. The table below summarizes the two drive families described so far.

Question	Direct drive	Belt drive
Fan speed	Fixed at motor speed (unless a VFD is used)	Set by the sheave ratio; changeable with a sheave change
Parts between motor and impeller	None, or a flexible coupling	Two sheaves, belts, often bushings
Routine maintenance	Lowest; no belts	Belt tension checks and belt replacement
Belt residue in the air	None	Possible; belt dust
Where the motor goes	On the fan shaft line	Beside, above or on the pedestal; can be out of the airstream
FanPedia examples	Arrangements 4, 7, 8	Arrangements 1, 3, 9, 10

Sheave

A sheave is the grooved pulley that a V-belt runs on; FanPedia writes "sheave/pulley" on its exploded views (FanPedia p. 15). A belt drive has two: the motor sheave (also called the drive sheave) on the motor shaft and the fan sheave (the driven sheave) on the fan shaft. FanPedia's shaft accessories page describes drive sheaves as "sheave mounted to fan shaft and motor shaft," which "can be with or without a bushing," a bushing being the tapered sleeve that clamps a sheave to its shaft (FanPedia p. 128). A sheave's working size is its pitch diameter, the diameter at which the belt effectively rides in the groove; drive calculations use pitch diameter, not the outside diameter.

Because the belt does not slip appreciably, the belt surface moves at the same speed over both sheaves. That makes the speed ratio the inverse of the diameter ratio:

\[ N_{fan} = N_{motor} \times \frac{D_{motor}}{D_{fan}} \]

where:

• \( N_{fan} \) is the fan shaft speed in rpm
• \( N_{motor} \) is the motor's full-load speed in rpm (from the nameplate)
• \( D_{motor} \) is the pitch diameter of the motor sheave in inches
• \( D_{fan} \) is the pitch diameter of the fan sheave in inches

A larger fan sheave slows the fan; a larger motor sheave speeds it up.

Worked example: sizing a fan sheave. A belt driven fan must run at 1,250 rpm. The motor runs at 1,760 rpm at full load (an illustrative 4-pole nameplate value), and the motor sheave has a 6.0 in pitch diameter. Rearranging the formula:

\[ D_{fan} = D_{motor} \times \frac{N_{motor}}{N_{fan}} = 6.0 \times \frac{1760}{1250} = 8.45 \text{ in} \]

Stock sheaves come in discrete sizes. If the nearest available pitch diameter is 8.4 in (illustrative), the fan runs at \( 1760 \times 6.0 / 8.4 = 1{,}257 \) rpm, about half a percent fast, which is normally acceptable; the selection should be rechecked at that speed. If the customer later needs more air, a smaller fan sheave or a larger motor sheave raises the fan speed, but horsepower rises steeply with speed, so the motor must be checked before any speed increase (Chapter 25 covers the fan laws that predict it).

Sanity-Check the Ratio in Your Head

Before trusting a drive calculation, ask which sheave is bigger. If the fan must turn slower than the motor, the fan sheave must be the larger one; if your answer says otherwise, the ratio is upside down.

V-Belt

A V-belt is a drive belt with a trapezoidal cross-section that wedges into the matching V-shaped groove of a sheave. The wedging action multiplies the grip between belt and sheave, so a V-belt can carry useful power at modest tension. Fan drives commonly use several V-belts side by side on multi-groove sheaves; each belt carries a share of the load. FanPedia refers to "V-belt drive" throughout its arrangement descriptions; for example, double width fans "are generally supplied in Arrangement 3 for V-belt drive" (FanPedia p. 167), and belt driven axial fans use "either a fixed or adjustable V-belt drive system" (FanPedia p. 173). A fixed drive has sheaves of set pitch diameter; an adjustable drive uses a motor sheave whose groove width can be changed to vary its pitch diameter, which lets a technician trim the fan speed during balancing.

The speed at which the belt travels matters because a belt's power capacity depends on it. Belt speed in feet per minute follows from the motor sheave's circumference:

\[ V = \frac{\pi \, D_{motor} \, N_{motor}}{12} \]

where:

• \( V \) is the belt speed in feet per minute (ft/min)
• \( D_{motor} \) is the motor sheave pitch diameter in inches (divide by 12 to get feet)
• \( N_{motor} \) is the motor speed in rpm

Worked example: belt speed and belt count. A drive uses a 5.0 in motor sheave on a 1,760 rpm motor. Belt speed is \( \pi \times 5.0 \times 1760 / 12 = 2{,}304 \) ft/min. The drive manufacturer's rating table gives the horsepower one belt of the chosen section can carry at that belt speed and sheave size; suppose it is 6.0 hp per belt (illustrative). If the drive must carry a design load of 19.5 hp (computed in the next section), the number of belts is \( 19.5 / 6.0 = 3.25 \), rounded up to 4 belts, because a fraction of a belt cannot share the load. Rounding down would overload every belt. The real per-belt rating must always come from the current drive manufacturer's data.

Drive Service Factor

A belt drive is not sized for the motor's nameplate horsepower alone. The drive service factor is a multiplier, at least 1.0, that the drive designer applies to the motor's power to allow for the duty the drive will see: starting loads, hours of operation per day, and the character of the driven machine. The result is the design horsepower the belts and sheaves are selected to carry:

\[ HP_{design} = HP_{motor} \times SF_{drive} \]

where:

• \( HP_{design} \) is the design horsepower used to select belts and sheaves
• \( HP_{motor} \) is the motor nameplate horsepower
• \( SF_{drive} \) is the drive service factor from the belt drive manufacturer's table for the driven machine and duty

Worked example: design horsepower. A belt driven fan has a 15 hp motor and runs around the clock. Suppose the belt maker's table gives a drive service factor of 1.3 for a fan in continuous service (illustrative; use the current table for the actual value). Design horsepower is \( 15 \times 1.3 = 19.5 \) hp, the figure used in the belt count example above. A drive selected for only 15 hp would run its belts near their limit every hour of every day and wear them out early.

Two Different Service Factors

A common trap is reading the motor nameplate's service factor and treating it as the drive service factor. The motor service factor says how far above rated horsepower the motor may run (FanPedia's example on page 69: 1.15 on a 100 HP motor allows 115 HP), while the drive service factor is a sizing multiplier for belts and sheaves from the drive manufacturer's table. Keep them in separate lines of your calculation.

The microsim below puts the three belt drive formulas side by side, so you can see how sheave sizes, fan speed, belt speed and belt count move together.

Diagram: Belt Drive Speed Calculator

Run Belt Drive Speed Calculator Fullscreen

Belt Drive Speed Calculator

Type: microsim sim-id: ch11-belt-drive-speed-calculator
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the fan speed, belt speed and required belt count of a V-belt fan drive from the motor speed, the two sheave pitch diameters, the motor horsepower and the drive service factor, with fan speed within ±5 rpm and belt count exact.

Prerequisites: belt drive, sheave, pitch diameter, motor sheave, fan sheave, V-belt, belt speed, drive service factor, design horsepower (all defined in the "Belt Drive" section of this chapter).

Evidence of Mastery: For four challenge drives, the learner types the fan speed in rpm and the number of belts before the answer is revealed. A fan speed is correct when it is within ±5 rpm of the model value; a belt count is correct only when it equals the model value. Mastery is 4 of 4 challenges with both answers correct. Moving the free-exploration controls is exploration, not evidence.

Misconceptions: (1) A larger fan sheave makes the fan run faster. (2) The speed ratio equals the diameter ratio the same way up (fan/motor) rather than inverted. (3) A belt count of 3.25 rounds down to 3. (4) The motor's nameplate service factor is the drive service factor.

Instructional Rationale: Calculating a drive is an Apply-level procedure, so the learner commits numbers for fixed challenges and gets the worked calculation as feedback; free exploration afterward lets them see the inverse diameter relationship directly.

Content:

Adjustable quantities in free-exploration mode:

Quantity	Min	Max	Step	Default	Unit
Motor full-load speed	860	3,540	10	1,760	rpm
Motor sheave pitch diameter	3.0	12.0	0.2	5.0	in
Fan sheave pitch diameter	3.0	24.0	0.2	8.0	in
Motor horsepower	1	100	0.5	15	hp
Drive service factor	1.0	1.8	0.1	1.3	(multiplier)
Rating per belt	1.0	20.0	0.5	6.0	hp per belt

Challenge drives, in this order:

#	Motor rpm	Motor sheave (in)	Fan sheave (in)	Motor hp	Drive SF	Rating per belt (hp)	Model fan rpm	Model belt speed (ft/min)	Model belts	Feedback when wrong
1	1,760	5.0	8.0	15	1.3	6.0	1,100	2,304	4	Fan rpm = 1,760 × 5.0 / 8.0 = 1,100. Design hp = 15 × 1.3 = 19.5; 19.5 / 6.0 = 3.25 belts, rounded up to 4.
2	1,760	6.0	8.4	10	1.2	5.0	1,257	2,765	3	Fan rpm = 1,760 × 6.0 / 8.4 = 1,257. Design hp = 10 × 1.2 = 12.0; 12.0 / 5.0 = 2.4, rounded up to 3.
3	1,170	9.0	6.0	20	1.4	8.5	1,755	2,757	4	The motor sheave is larger, so the fan runs faster than the motor: 1,170 × 9.0 / 6.0 = 1,755. Design hp = 20 × 1.4 = 28.0; 28.0 / 8.5 = 3.29, rounded up to 4.
4	3,500	4.0	10.0	7.5	1.2	4.5	1,400	3,665	2	Fan rpm = 3,500 × 4.0 / 10.0 = 1,400. Design hp = 7.5 × 1.2 = 9.0; 9.0 / 4.5 = 2.0 exactly, so 2 belts.

Displayed outputs: fan speed (rpm, rounded to the nearest whole rpm), speed ratio (motor rpm ÷ fan rpm, to two decimals), belt speed (ft/min, nearest whole number), design horsepower (to one decimal), belts required (whole number). The two sheaves are drawn to scale relative to each other, joined by a belt, and each rotates at a rate proportional to its computed speed; the relative size of the two sheaves is itself content being taught.

A label reads: "Motor speeds, per-belt ratings and service factors are illustrative. Use the current drive manufacturer's tables for real selections."

Provenance: Formulas: standard belt drive engineering, as stated in this chapter's "Sheave", "V-Belt" and "Drive Service Factor" sections. All numeric values are illustrative; challenge 1 matches the chapter's worked examples. The motor service factor contrast is from FanPedia p. 69.

Rules:

• Fan rpm = motor rpm × motor sheave ÷ fan sheave.
• Belt speed (ft/min) = π × motor sheave (in) × motor rpm ÷ 12.
• Design hp = motor hp × drive service factor.
• Belts required = ceiling(design hp ÷ rating per belt). When the quotient is a whole number, it is not rounded up (challenge 4).
• A belt speed > 6,500 ft/min shows the note "Very high belt speed: check the drive manufacturer's limits for this belt section."
• Fan rpm checks: correct when |entered − model| <= 5. Belt count: correct only when equal.

Learner Activity:

1. The learner reads challenge 1's motor speed, sheave sizes, horsepower, service factor and per-belt rating, and types a fan rpm and a belt count.
2. The learner presses Check. The sheaves turn at the model speeds, and the sim shows the model values beside the learner's entries.
3. The learner completes challenges 2, 3 and 4. In challenge 3 the learner should notice the fan turning faster than the motor.
4. After challenge 4, free-exploration mode unlocks. The learner changes any quantity and every output updates at once. The first time the fan sheave is increased, the sim notes "Bigger fan sheave, slower fan."

Feedback: Four challenges, fixed order, two attempts each; fan rpm and belt count are judged separately. Correct: "Correct: ." Hint after a first wrong attempt, never the model rpm, belt count or worked sums: which field is off ("Your fan speed is right; the belt count is not." and the reverse, or "Both the fan speed and the belt count are off."), then for a wrong fan speed "Fan rpm = motor rpm × motor sheave ÷ fan sheave." (replaced by "Your ratio is inverted: the larger sheave turns slower." when the fan rpm equals motor rpm × fan sheave ÷ motor sheave within ±5 rpm; challenge 3 adds "Here the motor sheave is the larger one."), and for a wrong belt count "Belts = design hp (motor hp × drive service factor) ÷ rating per belt, rounded up." (replaced by "Round belt counts up; a partial belt cannot carry its share." when the count equals the quotient rounded down, or by "A whole-number quotient is not rounded up." when it is one more than an exact whole-number quotient). After the final attempt: the "Feedback when wrong" text for that challenge, with the inverted-ratio and rounded-down notes when they apply; the model value and worked calculation are shown, and the item counts as missed. A running count "Challenges fully correct: n of 4" is shown.

Starting State: Challenge 1 is shown with empty answer boxes and the sheaves at rest. The question on screen is "How fast will the fan turn, and how many belts does this drive need?"

Chapter Anchors: The chapter's worked examples use a 1,760 rpm motor, a 5.0 in motor sheave and an 8.0 in fan sheave (challenge 1: 1,100 rpm); a belt speed of 2,304 ft/min; a 15 hp motor with a drive service factor of 1.3 giving 19.5 design hp; 6.0 hp per belt giving 3.25, rounded up to 4 belts; and a 6.0 in motor sheave with an 8.4 in fan sheave giving 1,257 rpm. The chapter states that a larger fan sheave slows the fan and that the motor S.F. example on FanPedia p. 69 is 1.15 on a 100 HP motor.

Where the Impeller Sits on the Shaft

The drive answers how the impeller is turned. The next question is where the impeller sits relative to the bearings that carry it. FanPedia's shaft overview states it directly: the shaft "is supported by two bearings in one of two basic mounting arrangements: 1. Overhung impeller 2. Center hung impeller" (FanPedia p. 79). Every centrifugal arrangement number is built on one of these two.

Overhung Impeller

An overhung impeller is mounted on the end of the shaft, beyond both bearings. FanPedia's arrangement descriptions put it the same way: "The impeller is overhung on the shaft, i.e., mounted at the end of the shaft" (FanPedia p. 162). Picture a diving board: the board is clamped at two points near one end and the load sits out past the end. The two bearings sit together on a bearing pedestal, the stand outside the housing that carries the bearings, and the shaft reaches from the pedestal through the housing side into the impeller.

Three consequences follow from that geometry.

• The bearings stay outside the housing. Air flows through the housing; the pedestal stands beside it. FanPedia says of Arrangement 1 that the two fan bearings "are mounted on the bearing pedestal, out of the airstream, which makes them ideal for high temperature or contaminated air applications" (FanPedia p. 162).
• The bearing nearest the impeller carries the most load. Like the support nearest the end of a diving board, it reacts to the impeller's weight plus the leverage of the overhang, so the overhang distance and the bearing spacing matter in the design.
• The impeller can come out from one side. With nothing on the far side of the impeller, the rotor can be serviced without disturbing a second bearing on the inlet side.

FanPedia's impeller orientation page lists the overhung arrangements as Arrangements 1, 8, 9 and 10, plus Arrangement 4 (FanPedia p. 30). Arrangement 4 is a special case: the impeller is overhung on the motor shaft, and the shaft's "two bearings [are] located within motor frame" (FanPedia p. 79).

Worked example: tracing the load path of an Arrangement 1 fan. Start at the impeller and follow the weight to the floor. The impeller hangs on the end of the shaft inside the housing. The shaft passes out through the housing side, usually through a shaft seal, to the two pillow block bearings bolted to the top of the pedestal. The pedestal carries the bearing loads to the fan base. The housing carries no shaft load at all; it only contains the air. The motor sits separately on the floor or fan base and drives the shaft through belts. If this fan moves 450°F oven exhaust, nothing in the load path from bearing to floor is in that air, which is why FanPedia lists Arrangements 1, 8, 9, 9F and 10 for its high temperature centrifugal construction, for airstream temperatures from 276°F to 1,000+°F (FanPedia p. 110).

Center Hung Impeller

A center hung impeller is mounted between the two bearings, one on each side of the impeller. FanPedia: "the impeller center hung on the shaft, i.e., mounted between the bearings making it structurally sound and compact" (FanPedia p. 163). Picture a bridge span supported at both ends: the load sits between the supports, so each bearing carries roughly its share of the impeller weight and there is no overhang leverage. The shaft can be shorter and stiffer for a given impeller, which suits large and heavy rotors and double width impellers.

The catch is that one bearing has to sit on the inlet side of the impeller, and air enters the fan there. On a single width fan, that inlet-side bearing is usually carried by a bracket in the inlet, in the airstream. FanPedia states that "Arrangement 3 has one bearing located in the airstream" (FanPedia p. 163). On a double width, double inlet (DWDI) fan, both sides are inlets, so "both bearings are located in the airstream," and FanPedia limits standard DWDI fans to "clean air applications with air temperatures limited to 130°F" (FanPedia p. 167).

FanPedia's impeller orientation page lists the center hung arrangements as Arrangement 3 and Arrangement 7, each in single and double width (FanPedia p. 30).

The table below sets the two impeller positions side by side.

Feature	Overhung impeller	Center hung impeller
Impeller location	At the end of the shaft, beyond both bearings	Between the two bearings
Load on the bearings	Nearest bearing carries weight plus overhang leverage	Shared between the bearings, no overhang
Bearings in the airstream	No; both on a pedestal outside the housing (or inside the motor, Arrangement 4)	Inlet-side bearing in the airstream (both on DWDI), unless inlet boxes move them out
FanPedia arrangements (p. 30)	1, 8, 9, 10 and 4	3 and 7, single and double width

Worked example: why a center hung fan for a large clean-air supply? An air handling job needs a very large DWDI fan moving outdoor air at ambient temperature. The rotor is wide and heavy. Center hanging it between two bearings gives a short, stiff shaft span with no overhang leverage, and the 130°F limit for bearings in the airstream is not a concern at ambient temperature, so a standard DWDI Arrangement 3 fits. Change one fact: the same fan must handle 300°F process air. Now the in-airstream bearings rule out the standard Arrangement 3, and the engineer needs either an overhung arrangement or a center hung design that moves its bearings out of the air, which is what the SI and DI arrangements covered below do.

Diving Board or Bridge

Notice that the overhung-versus-center-hung choice is really a choice about where the bearings must stand. An overhung impeller keeps both bearings on one side, out of the air but carrying leverage; a center hung impeller shares the load evenly but has to put a bearing on the inlet side, where the air is.

The sorter below asks you to classify arrangements by impeller position and to say where the bearings are.

Diagram: Overhung or Center Hung Sorter

Run Overhung or Center Hung Sorter Fullscreen

Overhung or Center Hung Sorter

Type: microsim sim-id: ch11-overhung-center-hung-sorter
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify twelve fan arrangements as overhung or center hung and state whether each arrangement's bearings sit in or out of the airstream, from a one-line description of each arrangement.

Prerequisites: overhung impeller, center hung impeller, bearing pedestal, airstream, single width single inlet (SWSI), double width double inlet (DWDI), inlet box, direct drive, belt drive, shaft coupling (defined in this chapter and in Chapters 3 and 4).

Evidence of Mastery: For each of 12 cards, the learner commits two choices: impeller position (Overhung or Center hung) and bearing location (All out of airstream, At least one in airstream, or No fan bearings). A card is correct only when both choices match the answer columns below. Mastery is 10 of 12 cards correct on the first attempt.

Misconceptions: (1) Direct drive means center hung. (2) Arrangement 4 has fan bearings in the airstream. (3) Every Arrangement 3 has its bearings out of the airstream. (4) Center hung fans always have a bearing in the airstream, even with inlet boxes.

Instructional Rationale: Classifying described arrangements into a two-way scheme makes the learner apply the defining feature (where the impeller sits relative to the bearings) rather than memorize numbers, and the second choice links that geometry to bearing exposure, which is why it matters.

Content:

#	Card text shown to the learner	Impeller position	Bearings	Why (shown as feedback)
1	Arrangement 1 SWSI: belt driven; two bearings on a pedestal beside the housing	Overhung	All out of airstream	The impeller is at the shaft end; both bearings sit on the pedestal, out of the airstream (FanPedia p. 162).
2	Arrangement 3 SWSI: usually belt driven; one bearing on each side of the impeller	Center hung	At least one in airstream	Arrangement 3 has one bearing in the airstream, on the inlet side (FanPedia p. 163).
3	Arrangement 3 DWDI: V-belt drive; double width impeller with inlets on both sides	Center hung	At least one in airstream	Both bearings are in the airstream, which limits standard DWDI fans to clean air at 130°F (FanPedia p. 167).
4	Arrangement 3SI: single width fan with an attached inlet box; bearings on independent pedestals	Center hung	All out of airstream	The inlet box lets the inlet-side bearing stand outside the airstream (FanPedia p. 164).
5	Arrangement 3DI: double width fan with integrated inlet boxes; bearings on independent pedestals	Center hung	All out of airstream	Integrated inlet boxes put both bearings outside the airstream (FanPedia p. 167).
6	Arrangement 4 SWSI: impeller mounted directly to the motor shaft; motor on a pedestal	Overhung	No fan bearings	The impeller hangs on the motor shaft; the only bearings are inside the motor (FanPedia pp. 79, 164).
7	Arrangement 7 DWDI: direct drive; flexible coupling; pedestal carries the motor, the coupling and one bearing	Center hung	At least one in airstream	Like Arrangement 3 DWDI, the impeller sits between the bearings with an inlet on each side, so both bearings sit in the airstream; only the 7DI's inlet boxes move the bearings out (FanPedia pp. 167–168).
8	Arrangement 7SI: direct drive; single width fan with an integrated inlet box; flexible coupling	Center hung	All out of airstream	The inlet box locates the bearing outside the airstream (FanPedia p. 165).
9	Arrangement 7DI: direct drive; double width fan with integrated inlet boxes; flexible coupling	Center hung	All out of airstream	Integrated inlet boxes locate both bearings outside the airstream (FanPedia p. 168).
10	Arrangement 8 SWSI: direct drive; the Arrangement 1 pedestal extended to carry the motor; flexible coupling	Overhung	All out of airstream	Arrangement 8 is a modified Arrangement 1, so the impeller stays at the shaft end with both bearings on the pedestal (FanPedia pp. 79, 165).
11	Arrangement 9 SWSI: belt driven; motor on a slide base on the side of the bearing pedestal	Overhung	All out of airstream	Arrangement 9 is in the overhung group with Arrangements 1, 8 and 10 (FanPedia p. 30).
12	Arrangement 10 SWSI: belt driven; motor on an adjustable base inside the bearing pedestal	Overhung	All out of airstream	Arrangement 10 is in the overhung group, with its bearings on the pedestal (FanPedia pp. 30, 167).

Provenance: FanPedia p. 30 (impeller orientation: overhung for Arrangements 1, 8, 9, 10 and 4; center hung for Arrangements 3 and 7, single and double width), p. 79 (shaft overview), and pp. 162–168 (technical descriptions cited per card).

Rules: Each card has two answer fields; both must be chosen before Check is enabled. A card is correct only when both fields match. There is no partial credit in the score, but feedback states which field was wrong.

Learner Activity:

1. A card appears with its text. The learner predicts the impeller position, then the bearing location.
2. The learner presses Check. A simple side-view schematic of that arrangement appears, showing the impeller, the shaft, the bearings and the edge of the airstream, with the correct answers.
3. The learner moves to the next card. Cards already answered cannot be re-answered in the same round.
4. After 12 cards, the learner sees the score and the list of missed cards and may start a new shuffled round.

Feedback: 12 cards, shuffled, one attempt each per round. Correct: "Correct: , ." Incorrect: names the wrong field and shows the "Why" text. If card 6 or card 10 is marked center hung, the feedback adds: "Direct drive does not decide impeller position; Arrangements 4 and 8 are direct drive and overhung." The schematic and answer appear after each check. A running count "Correct: n of 12" is shown.

Starting State: Card 1 of a shuffled deck is shown with both answer fields empty and the question "Where is the impeller, and where are the bearings?"

Chapter Anchors: The chapter states that FanPedia p. 30 lists Arrangements 1, 8, 9, 10 and 4 as overhung and Arrangements 3 and 7 as center hung; that Arrangement 3 SWSI has one bearing in the airstream; that standard DWDI fans have both bearings in the airstream and are limited to 130°F clean air; and that Arrangement 4's only bearings are in the motor frame.

The Fan Arrangement

A fan arrangement is a standard numbered description of how a fan's drive, impeller, bearings and motor are put together. The number tells the reader, in one or two characters, whether the fan is direct or belt driven, whether the impeller is overhung or center hung, what carries the bearings, and where the motor is mounted. The numbering comes from the Air Movement and Control Association (AMCA) arrangement standard, and Twin City Fan, like other manufacturers, uses those base numbers on its drawings and catalogs. Suffix letters then mark variants: SI for a single inlet fan with an integral inlet box, DI for a double inlet fan with integral inlet boxes, F for an extended frame that carries the motor, P for a pedestal (grade mounted) plug fan, and so on (FanPedia pp. 20–29).

Arrangements are always defined for a fan type. FanPedia groups its arrangement drawings by family: centrifugal fans (SWSI on pp. 20–22 and DWDI on p. 23), plenum fans (pp. 24–25), plug fans (pp. 26–27), axial and inline centrifugal fans (p. 28), and fume exhaust fans (p. 29). The same base number can look different from one family to the next. An Arrangement 4 centrifugal fan has its motor on a pedestal beside the housing, while an Arrangement 4 axial fan has its motor inside the tube; both are "impeller mounted to motor shaft."

The table below collects the base arrangements this chapter covers and the defining features of each; the sections that follow explain each one. The table reinforces what you already know about drives and impeller positions.

Arrangement	Drive	Impeller position	Bearings carried by	Motor mounted
1	Belt (direct drive possible)	Overhung	Pedestal, out of airstream	On floor or fan base, separate from fan
2	Belt or direct	Overhung	Bracket on the fan housing	Separate from fan
3	Usually belt	Center hung	Housing; one bearing on each side of impeller	On floor or fan base
4	Direct	Overhung on motor shaft	No fan bearings (motor bearings only)	On a pedestal; impeller on motor shaft
7	Direct, coupled	Center hung	Housing and a pedestal that also carries the motor	On the pedestal, coupled to fan shaft
8	Direct, coupled	Overhung	Extended Arrangement 1 pedestal	On the extended pedestal, coupled
9	Belt only	Overhung	Pedestal	On the side of the pedestal
10	Belt only	Overhung	Pedestal	Inside the pedestal

Arrangements 8, 9 and 10, and the many lettered variants, are developed in Chapter 24. They appear here so you can see the whole numbering pattern.

Worked example: naming an arrangement from a drawing. An outline drawing shows a single inlet centrifugal fan. Working through it systematically:

1. Find the drive. There is a belt guard and two sheaves, so the fan is belt driven. That rules out 4, 7 and 8.
2. Find the impeller relative to the bearings. Both bearings sit together on a pedestal on the drive side; nothing is on the inlet side. The impeller is overhung. That rules out 3 (and 7).
3. Find the motor. The motor is bolted to a slide base on the side of the bearing pedestal, so the fan ships as one assembly with motor and drive mounted.

Overhung, belt driven, motor on the side of the pedestal: Arrangement 9. Had the motor stood on its own base on the floor beside the fan, it would be Arrangement 1; had it been tucked inside the pedestal, Arrangement 10 (FanPedia pp. 165–167).

Three Questions Make the Number

Think of an arrangement number as a compressed answer to three questions: how is it driven, where is the impeller relative to the bearings, and what carries the motor? Once you can answer those from a drawing, the number almost names itself.

Arrangement 1

Arrangement 1 is an overhung impeller fan with both bearings on a pedestal and the motor mounted separately on the floor or fan base. FanPedia's drawing caption reads "Direct Drive or Belt Driven; Motor Mounted on Floor or Fan Base" (FanPedia p. 20), and the technical description adds that it "is usually belt driven" and that its pedestal bearings sit "out of the airstream, which makes them ideal for high temperature or contaminated air applications" (FanPedia p. 162). With belt drive, "the motor can be mounted in any of the four AMCA standard motor positions: W, X, Y or Z"; motor positions are defined in Chapter 12.

Arrangement 1 is also the parent of other arrangements. Arrangement 8 is "a modified version of Arrangement 1 used for direct drive," with the pedestal extended to carry a coupled motor (FanPedia p. 165), and Arrangement 9F extends the frame to carry a motor "similar to an Arrangement 1 fan" (FanPedia p. 166).

Worked example: checking a motor position. A customer orders an Arrangement 1 fan with clockwise (CW) rotation and a bottom horizontal discharge (BHD), and asks for the motor in position Z. FanPedia lists these motor position restrictions for Arrangement 1, all because the motor would interfere with the housing outlet or not fit (FanPedia p. 162):

• BHD discharge: CW rotation, position Z not allowed; CCW rotation, position W not allowed.
• TAD (top angular down) discharge: CW rotation, position W not allowed; CCW rotation, position Z not allowed.
• THD (top horizontal) discharge: CW rotation, position W, and CCW rotation, position Z, face a height restriction because the motor may not fit below the discharge.

The requested combination, BHD with CW rotation and motor position Z, is on the list. The application engineer offers W, X or Y instead, or a different discharge. The same check applies to Arrangements 3 and 3F, which carry the identical restriction list (FanPedia p. 163). For the THD cases, FanPedia adds that "on Arrangement 1 fan the motor will fit if pedestal is lengthened to accommodate motor" (FanPedia p. 162).

Arrangement 2

Arrangement 2 is defined in the AMCA arrangement standard as an overhung impeller fan, belt or direct driven, whose bearings are mounted in a bracket supported by the fan housing rather than on a separate pedestal. It is essentially Arrangement 1 with the pedestal replaced by a bracket on the housing. FanPedia's arrangement drawings (pp. 20–29) do not include an Arrangement 2, so treat it as a code you may meet on older drawings or other manufacturers' equipment. If a customer asks for one, check what the current Twin City Fan catalog offers before quoting; an Arrangement 1 or 9 usually meets the same need.

Arrangement 3

Arrangement 3 is a center hung impeller fan with one bearing on each side of the impeller, the bearings supported by the fan housing, and the motor mounted separately on the floor or fan base. FanPedia shows it for single width fans as "Belt Driven; Motor Mounted on Floor or Fan Base" (FanPedia p. 20) and for double width fans as "Direct Drive or Belt Driven" (FanPedia p. 23). The description calls it "structurally sound and compact" and notes that the single width version "has one bearing located in the airstream" (FanPedia p. 163). Double width fans "are generally supplied in Arrangement 3 for V-belt drive" (FanPedia p. 167), and Arrangement 3 is also "the most common plenum fan arrangement," frequently used in OEM and site-built air handlers (FanPedia p. 168).

Worked example: matching Arrangement 3 to the air. Two DWDI fan requests arrive on the same day. The first is a supply fan for an office air handler moving filtered outdoor air. The second exhausts 200°F air from a dryer. For the first, a standard Arrangement 3 DWDI is a natural choice: it is compact, belt driven for easy speed adjustment, and the air is clean and well under 130°F. For the second, the same arrangement fails on FanPedia's own limit: both bearings sit in the airstream, and standard DWDI fans are for clean air "limited to 130°F" (FanPedia p. 167). The engineer looks instead at Arrangement 3DI, which uses integrated inlet boxes to move the bearings out of the airstream (next section), or at an overhung arrangement.

Arrangement 3 Is Not Automatically Hot-Air Ready

A common trap is assuming that because Arrangement 3 is common and robust, it suits any temperature. Its inlet-side bearing (both bearings on a DWDI) sits in the air; check the air temperature and cleanliness first, and move to an SI/DI or overhung arrangement when the air is hot or dirty.

Arrangement 3 SI and DI

Arrangement 3SI and Arrangement 3DI are center hung arrangements that add integral inlet boxes so that the bearings can stand outside the airstream. In 3SI (single inlet), a single width fan "utilizes an attached inlet box to locate the bearing outside of the airstream on independent bearing pedestals that allows for elevated operating temperatures and relatively clean air" (FanPedia p. 164). In 3DI (double inlet), a double width fan uses "integrated inlet boxes to locate the bearings outside of the airstream on independent bearing pedestals" (FanPedia p. 167). Both are usually direct drive: "the motor is located by the customer off the fan assembly and direct-coupled to the shaft," and both include a pie split housing for easy impeller removal (FanPedia pp. 164, 167). The inlet box turns the incoming air through a right angle, which is what clears a path for the shaft to pass out of the box to a bearing that the air never touches.

Arrangement 4

Arrangement 4 is a direct drive fan with the impeller mounted on the motor shaft and the motor mounted on a pedestal. FanPedia's caption: "Direct Drive; Impeller Mounted to Motor Shaft" (FanPedia p. 20). It has no fan shaft and no fan bearings, which gives it the lowest maintenance of any arrangement: "there are no fan bearings, fan shaft or drive parts to maintain" (FanPedia p. 164). It is also compact. The limits come from the same simplicity. The motor's bearings carry the whole impeller, which limits size: FanPedia notes that "Arrangement 4 fans are typically limited up to size 365" (FanPedia p. 164). And the motor sits close to the air, so "high airstream temperatures may limit the use of this arrangement" (FanPedia p. 171). FanPedia's high temperature page for Arrangement 4 centrifugal fans describes a shaft seal, a shaft cooler, a cooler guard and a hub for airstream temperatures from 181°F to 1,000+°F (FanPedia p. 111). Current practice is more conservative: Twin City Fan engineering says Arrangement 4 is not used for high-temperature applications, and its standard ranges are typically up to about 180°F (for example EPQN 0-180°F, QSL 0-160°F, TCTA 0-105°F, from Twin City Fan's model temperature tables). Other ranges are special order.

Several variants build on it (FanPedia pp. 20–21):

• 4S: swingout construction for easy access to the impeller and housing interior.
• 4VI and 4HI: mounted directly on the fan inlet, vertical or horizontal; "the entire fan is supported by the inlet flange only."
• Plenum, plug, axial, inline and lab exhaust Arrangement 4: covered later in this chapter.

Worked example: when Arrangement 4 fits. A small exhaust fan serves a clean, room-temperature process with no one available for belt maintenance. Direct drive at a standard motor speed meets the duty, and the selected fan size is well under the size where Arrangement 4 is typically limited. Arrangement 4 is the natural choice: no belts, no fan bearings to grease, compact footprint. Now raise the air temperature to 600°F. Arrangement 4 is no longer the right choice: Twin City Fan engineering does not use it for high-temperature applications. An overhung Arrangement 1, 8, 9 or 10 with the bearings and motor well away from the heat is the answer (FanPedia pp. 110–111).

Arrangement 7

Arrangement 7 is a direct drive, center hung fan whose motor is coupled to the fan shaft and mounted on a pedestal. FanPedia captions it "similar to Arr. 3 but with motor pedestal" (FanPedia p. 21) and describes it as an arrangement in which "the impeller is mounted between the bearings, but the Arrangement 7 incorporates a pedestal designed to accommodate the motor, flexible coupling and one bearing" (FanPedia p. 168). The SI and DI versions, 7SI and 7DI, add integrated inlet boxes to move the bearings out of the airstream and are mounted on a unitary base as standard, with an inertia base available as an option (FanPedia pp. 165, 168). Choose Arrangement 7 over 3 when the duty suits a direct drive motor speed and you want the motor carried on the fan's own pedestal instead of driving through belts.

Bearings Out of the Airstream

You have now seen one idea several times: some arrangements are chosen because they keep the bearings out of the airstream. Bearings are lubricated precision parts. Hot air thins and breaks down their grease; dusty, corrosive or wet air attacks their seals; and in spark-resistant construction, a bearing in the airstream is a potential ignition source. FanPedia's spark-resistant construction pages state flatly: "Bearings not allowed in airstream" (FanPedia p. 104). Keeping the bearings outside the housing also makes them easier to reach for lubrication and inspection.

An arrangement keeps its bearings out of the airstream in one of three ways:

1. Overhang the impeller so both bearings sit on a pedestal outside the housing: Arrangements 1, 8, 9 and 10 (FanPedia pp. 30, 162).
2. Add inlet boxes to a center hung fan so the inlet-side bearing stands outside the box: Arrangements 3SI, 3DI, 7SI and 7DI (FanPedia pp. 164–168).
3. Put the whole rotating assembly outside the air path, as plug fans do: in Arrangement 9 plug fans, "motor, shaft and bearings are outside of the process airstream" (FanPedia p. 172), and belt driven axial and inline Arrangement 9 fans are "the perfect choice for applications that require the motor to be out of the airstream" (FanPedia p. 173).

Worked example: screening arrangements for a hot, dirty exhaust. A foundry needs a belt driven centrifugal exhaust fan for 350°F air carrying dust. The application engineer screens the arrangements:

Candidate	Bearings in the airstream?	Verdict
Arrangement 3 SWSI	Yes, one (FanPedia p. 163)	Reject: hot, dusty air at a bearing
Arrangement 3 DWDI	Yes, both; limited to 130°F clean air (FanPedia p. 167)	Reject
Arrangement 4	No fan bearings, but the motor is close to the air (FanPedia p. 171)	Reject: Twin City Fan engineering does not use Arrangement 4 for high-temperature applications, and it is not belt driven anyway
Arrangement 1	No; both on the pedestal (FanPedia p. 162)	Keep
Arrangement 9 or 10	No; overhung, bearings on the pedestal (FanPedia p. 30)	Keep; choose by where the motor should go

The shortlist is Arrangements 1, 9 and 10, which also appear in FanPedia's high temperature centrifugal construction list, for 276°F to 1,000+°F (FanPedia p. 110). The final choice comes down to motor location and footprint, the subject of Chapter 24.

The Code Alphabet Takes a Few Passes

If 3SI, 7DI, 4VI and 9SS are starting to blur together, that's normal; most people need a few drawings before the suffixes stick. Anchor each one to its base number first (overhung or center hung, belt or direct), then read the letters as modifications, and keep FanPedia pp. 20–29 open beside you while you practice.

The microsim below lets you practice naming the base arrangement from the features a drawing shows.

Diagram: Arrangement Identifier

Run Arrangement Identifier Fullscreen

Arrangement Identifier

Type: microsim sim-id: ch11-arrangement-identifier
Library: p5.js
Status: Specified
Bloom Level: Analyze
Bloom Verb: distinguish
Learning Objective: The learner will distinguish ten centrifugal fan arrangements (1, 2, 3, 3SI, 3DI, 4, 7, 8, 9 and 10) from schematic side-view drawings by identifying the drive type, the impeller position relative to the bearings, and what carries the bearings and the motor.

Prerequisites: direct drive, belt drive, shaft coupling, sheave, overhung impeller, center hung impeller, bearing pedestal, inlet box, SWSI, DWDI, fan arrangement, Arrangements 1, 2, 3, 3SI, 3DI, 4, 7, 8, 9 and 10 (defined in this chapter).

Evidence of Mastery: For each of 10 drawings, the learner first answers three feature questions (drive: Belt / Direct with coupling / Direct on motor shaft; impeller: Overhung / Center hung; bearings carried by: Pedestal / Fan housing (at least one bearing) / Independent pedestals outside inlet boxes / Motor only), then names the arrangement from a list of the ten. The arrangement answer is correct when it matches the "Arrangement" column. Mastery is 8 of 10 arrangements correct on the first attempt. Feature answers are scaffolding and are scored separately.

Misconceptions: (1) Every direct drive fan is Arrangement 4. (2) Any fan with a coupling is Arrangement 7, whatever the impeller position. (3) Arrangement 1 and Arrangement 9 differ in impeller position (they differ only in motor location). (4) Arrangement 2 and Arrangement 1 are the same (Arrangement 2's bearings are on a bracket carried by the housing).

Instructional Rationale: Distinguishing arrangements is an Analyze task: the learner must break each drawing into its defining features before naming the whole. Asking for the features first, and committing them before the name, makes the learner perform that decomposition rather than pattern-match a picture.

Content:

Each drawing is a simple schematic side view (viewed from the side of the shaft) showing only the parts listed for it. The parts drawn are the content being taught.

#	Parts shown in the drawing	Drive	Impeller	Bearings carried by	Arrangement	Why (feedback)
1	SWSI housing; two bearings together on a pedestal beside the drive side of the housing; impeller at the shaft end; motor on its own base on the floor, belts to the fan sheave	Belt	Overhung	Pedestal	1	Overhung on a pedestal with a separately mounted motor is Arrangement 1 (FanPedia pp. 20, 162).
2	SWSI housing; two bearings in a bracket bolted to the drive side of the housing; impeller at the shaft end; separate motor with belts	Belt	Overhung	Fan housing (at least one bearing)	2	Overhung with bearings in a bracket on the housing is AMCA Arrangement 2.
3	SWSI housing; one bearing on the drive side and one on a bracket across the inlet; impeller between them; motor on the floor beside the fan, belts to the fan sheave	Belt	Center hung	Fan housing (at least one bearing)	3	Center hung with bearings carried by the housing and a separate motor is Arrangement 3 (FanPedia pp. 20, 163).
4	SWSI housing with an attached inlet box; impeller between two bearings, each on its own pedestal outside the housing and inlet box; a coupled motor off the fan assembly	Direct with coupling	Center hung	Independent pedestals outside inlet boxes	3SI	The attached inlet box puts the bearing outside the air on independent pedestals (FanPedia p. 164).
5	DWDI housing with an inlet box on each side; impeller between two bearings on independent pedestals; a coupled motor off the fan assembly	Direct with coupling	Center hung	Independent pedestals outside inlet boxes	3DI	Double width with integrated inlet boxes and independent bearing pedestals is Arrangement 3DI (FanPedia p. 167).
6	SWSI housing; motor on a pedestal; impeller hub on the motor shaft; no separate shaft, no fan bearings	Direct on motor shaft	Overhung	Motor only	4	Impeller mounted to the motor shaft is Arrangement 4 (FanPedia pp. 20, 164).
7	DWDI housing; impeller between two bearings; a pedestal on the drive side carrying one bearing, a flexible coupling and the motor	Direct with coupling	Center hung	Fan housing (at least one bearing)	7	Center hung with a pedestal for motor, coupling and one bearing is Arrangement 7 (FanPedia pp. 21, 168).
8	SWSI housing; two bearings on a long pedestal; impeller at the shaft end; a flexible coupling and the motor on the far end of the same pedestal	Direct with coupling	Overhung	Pedestal	8	An Arrangement 1 pedestal extended for a coupled motor is Arrangement 8 (FanPedia p. 165).
9	SWSI housing; two bearings on a pedestal; impeller at the shaft end; motor on a slide base on the side of the pedestal, belts to the fan sheave	Belt	Overhung	Pedestal	9	Overhung with the motor on the side of the bearing pedestal is Arrangement 9 (FanPedia p. 165).
10	SWSI housing; two bearings on a pedestal; impeller at the shaft end; motor on an adjustable base inside the pedestal, belts to the fan sheave	Belt	Overhung	Pedestal	10	Overhung with the motor inside the bearing pedestal is Arrangement 10 (FanPedia p. 167).

Answer list for the arrangement question, in this order: 1, 2, 3, 3SI, 3DI, 4, 7, 8, 9, 10.

A note on screen reads: "Schematics are simplified. Twin City Fan arrangement drawings: FanPedia pp. 20–23. Arrangement 2 is from the AMCA standard and is not shown in FanPedia."

Provenance: FanPedia pp. 20–23 (arrangement drawings and captions), p. 30 (impeller orientation), pp. 162–168 (technical descriptions cited per row). Arrangement 2: AMCA arrangement standard definition, as stated in this chapter's "Arrangement 2" section.

Rules: Drawings are shown in shuffled order. The arrangement question unlocks only after all three feature questions are answered. Feature answers are scored but do not block the arrangement answer. If the three committed features match the drawing's columns, the arrangement list highlights the arrangements consistent with those features (for drawing 1, features Belt / Overhung / Pedestal highlight 1, 9 and 10). If any feature is wrong, no highlighting is shown.

Learner Activity:

1. A drawing appears. The learner answers the drive question, the impeller question and the bearings question.
2. The learner presses Check Features. Each feature is marked correct or incorrect, with the correct answer shown.
3. The learner selects an arrangement from the list and presses Check Arrangement. The drawing's parts are labeled and the "Why" text appears.
4. After 10 drawings the learner sees separate scores for features and arrangements and may start a new shuffled round.

Feedback: 10 drawings, shuffled, one attempt per question per round. Correct arrangement: "Correct: Arrangement ." Incorrect: "This is Arrangement . ." Added messages for likely confusions: choosing 4 for drawings 4, 5, 7 or 8: "There is a coupling and a separate fan shaft, so this is not Arrangement 4." Choosing 7 for drawing 8: "The impeller is overhung, not between the bearings; a coupled overhung fan is Arrangement 8." Choosing 1 for drawing 9 or 10, or 9 for 10: "Look at where the motor is mounted; Arrangements 1, 9 and 10 differ only in motor location." Choosing 1 for drawing 2: "The bearings are on a bracket carried by the housing, not on a pedestal." The answer is revealed after each check. Running counts "Features correct: n of 30" and "Arrangements correct: n of 10" are shown.

Starting State: The first drawing of a shuffled set is shown with all questions unanswered and the prompt "How is this fan driven, where is the impeller, and what carries the bearings?"

Chapter Anchors: The chapter's worked example names an Arrangement 9 from a drawing with a belt guard, an overhung impeller with both bearings on a pedestal, and the motor on a slide base on the side of the pedestal, and states that Arrangements 1, 9 and 10 differ in motor location. The chapter states that Arrangement 2 is not shown in FanPedia's arrangement drawings (pp. 20–29).

Read a Drawing in Three Glances

Want to name an arrangement in ten seconds? Glance once for a belt guard or a coupling, once to see whether anything stands on the inlet side of the impeller, and once to find the motor; check it from the drive side, and the number follows.

Arrangement Families Beyond the Housed Centrifugal

The numbers above were defined for housed centrifugal fans. The other fan families from Chapters 4 through 6 reuse many of the same base numbers, with letters that describe how each family is mounted. This section introduces the families; Chapter 24 covers each lettered variant in detail.

Plug Fan Arrangements

A plug fan is an unhoused centrifugal fan whose impeller and inlet funnel sit inside the customer's enclosure, while the shaft, bearings and motor stay outside it, mounted through a wall panel; "the system plenum acts as the fan housing" (Chapter 7; FanPedia p. 172). Plug fan arrangements describe how that fan is supported and driven. FanPedia shows six (FanPedia pp. 26–27):

• Arrangement 9: belt driven, "fully supported by the customer's wall"; FanPedia calls it "the most common plug fan arrangement" (FanPedia p. 172).
• Arrangement 4: direct drive, impeller on the motor shaft, wall mounted; compact and low maintenance, though "high airstream temperatures may limit the use of this arrangement" (FanPedia p. 171).
• Arrangement 8: direct drive, motor coupled to the fan shaft, wall mounted; "the least common plug fan arrangement due to the length of the assembly" (FanPedia p. 171).
• Arrangements 1P, 4P, 8P and 9P: the "P" (pedestal plug) versions are floor mounted. FanPedia explains why: they are "typically used on larger fans and/or larger HP motors where the customer's wall may not be sufficient by itself," and "mounting to the foundation also makes it better for meeting lower vibration requirements" (FanPedia pp. 170–172).

Worked example: wall or floor? An oven builder wants a large belt driven plug fan to recirculate hot air, and the oven wall is light-gauge insulated panel. The default plug fan choice is Arrangement 9, which hangs the whole fan on the wall. Here the wall may not carry a large fan and motor, and an oven line usually cares about vibration. Arrangement 9P mounts the same belt driven fan on its own pedestal at grade, so the wall only seals around the fan, with the mounting panel optional (FanPedia p. 172). Because the bearings and motor are outside the oven in both versions, the hot air never reaches them.

Plenum Fan Arrangements

A plenum fan is an unhoused centrifugal fan that sits inside an air plenum and pressurizes it, with its motor and drive inside the plenum too, unlike a plug fan (FanPedia pp. 5, 172). Plenum fan arrangements describe how the motor and drive are mounted on the fan frame. FanPedia shows belt driven Arrangements 1 and 3, the top-mounted motor versions 3HS and 3HA, the side-mounted 3SM, the vertical 3VA and 3VS, and direct drive Arrangements 4 and 4V (FanPedia pp. 24–25). Arrangement 3 is "the most common plenum fan arrangement" in OEM and site-built air handlers, and direct drive Arrangement 4 plenum fans serve cleanroom and pharmaceutical work (FanPedia pp. 168, 170).

Vertical Arrangement

A vertical arrangement is one in which the fan shaft is vertical, so air enters or leaves the impeller vertically. FanPedia's vertical arrangements include Arrangement 4VI, a centrifugal fan "designed to mount directly on the inlet of the fan" with "a vertical airflow into the fan (vertical motor shaft)" (FanPedia p. 164); the vertical plenum arrangements 3VA and 3VS (FanPedia pp. 169–170); and plenum Arrangement 4V, available "with either vertical up airflow (inlet under the motor) or vertical down airflow (inlet above the motor)" (FanPedia p. 170). Vertical fans need bearing and shaft details suited to a vertical shaft, covered in Chapter 16.

Axial and Inline Arrangements

Axial and inline arrangements apply the same numbering to tubular fans: axial fans and inline centrifugal fans. FanPedia shows two base arrangements with access variants, and notes that "inline centrifugal fans are available with the same arrangements but use different impellers" (FanPedia p. 28):

• Arrangement 4 (direct drive): "the impeller is mounted directly on the fan motor shaft," making it "the logical choice when space is at a premium or a simple, dependable fan with minimum maintenance is required." Exact performance is reached by adjusting the blade angle (FanPedia p. 172). The motor sits inside the tube on its support.
• Arrangement 9 (belt driven): "the perfect choice for applications that require the motor to be out of the airstream," with "either a fixed or adjustable V-belt drive system," so a rating change is "a simple sheave change or blade angle adjustment" (FanPedia p. 173). The motor sits outside the tube and the belts run through a belt tube.
• CS and SO variants (4CS, 4SO, 9CS, 9SO) add a clamshell door or a swingout rotor for maintenance access (FanPedia pp. 172–173).

Worked example: an inline fan for oily, moist exhaust. A designer specifies a tubeaxial exhaust fan for air that carries oil mist and moisture. A direct drive Arrangement 4 would put the motor inside the tube, in that air. Arrangement 9 keeps the motor outside the tube with the belts shielded by the belt tube, so the motor stays clean and cool, and if the system turns out to need a little more air after balancing, a sheave change handles it without replacing the motor. If cleaning access matters, the 9CS (clamshell) or 9SO (swingout) variant adds a door to reach the rotor (FanPedia p. 173).

Lab Exhaust Arrangements

Lab exhaust arrangements are the arrangements offered on laboratory fume exhaust fans, the high-plume fans that dilute and discharge hazardous fumes above a building (Chapter 5 and Chapter 6). FanPedia's fume exhaust arrangement page (FanPedia p. 29) and descriptions (FanPedia pp. 174–175) show five, each tied to particular models. The table below gathers them; the models were introduced in Chapter 4.

Arrangement	Drive	Models shown (FanPedia p. 29)	Notes from FanPedia pp. 174–175
1	Belt	BAIFE, BCIFE	Overhung; two bearings on the pedestal, out of the airstream
4	Direct	BAIFE, BCIFE, TVIFE	Impeller on motor shaft; motor on a pedestal (BAIFE, BCIFE) or motor drive plate (TVIFE)
8	Direct (coupled)	BAIFE, BCIFE	Arrangement 1 pedestal extended for the motor; flexible coupling
9	Belt	BAIFE, BCIFE, TFE, QFE, QIFE	Motor slide base on the pedestal (BAIFE, BCIFE) or adjustable motor base on the housing exterior (inline models, excluding TVIFE)
10	Belt	BAIFE, BCIFE	Scroll-type centrifugal only; motor base inside the pedestal; weather cover option for roof or outdoor use

Worked example: matching a lab exhaust model to an arrangement. A lab project calls for an induced-flow mixed flow fan and asks for "belt drive with the motor out of the exhaust." Of the two induced-flow mixed flow models, FanPedia shows TVIFE only in direct drive Arrangement 4 (on a motor drive plate) and QIFE in belt driven Arrangement 9 (FanPedia pp. 29, 174–175). So the request points to a QIFE in Arrangement 9, with its adjustable motor base on the outside of the housing. If the project had specified a scroll housed BCIFE with a compact rooftop footprint, Arrangement 10, with the motor inside the pedestal and the optional weather cover, would be worth offering.

Key Takeaways

• Direct drive turns the impeller at motor speed, either with the impeller on the motor shaft (Arrangement 4) or through a shaft coupling to a fan shaft (Arrangements 7 and 8). It is simple and low maintenance, but fan speed is fixed unless a VFD is added. A direct drive tubular fan carries its motor on a motor support base inside the tube.
• Belt drive sets fan speed through the sheave ratio, \( N_{fan} = N_{motor} \times D_{motor} / D_{fan} \). A larger fan sheave slows the fan. V-belts wedge into sheave grooves; belt count is design horsepower divided by per-belt rating, rounded up.
• The drive service factor multiplies motor horsepower to give design horsepower for belts and sheaves. It is not the motor nameplate's service factor.
• An overhung impeller sits at the end of the shaft with both bearings on one side (Arrangements 1, 8, 9, 10 and 4); a center hung impeller sits between the bearings (Arrangements 3 and 7).
• A fan arrangement number answers three questions: how the fan is driven, where the impeller sits relative to the bearings, and what carries the bearings and motor. Arrangement 1 is overhung with a separate motor; Arrangement 2 (AMCA, not in FanPedia) carries its bearings on a housing bracket; Arrangement 3 is center hung with one bearing (SWSI) or both (DWDI) in the airstream; Arrangement 4 puts the impeller on the motor shaft; Arrangement 7 is a coupled, center hung fan with a motor pedestal.
• Arrangements 3SI and 3DI (and 7SI, 7DI) use inlet boxes to move center hung bearings out of the air.
• Keeping the bearings out of the airstream matters for hot, dirty, corrosive and spark-resistant duty; overhung arrangements, inlet-box arrangements and plug fans all achieve it.
• The same numbers reappear in the plug, plenum, vertical, axial and inline, and lab exhaust families, with letters that describe the mounting.

You Can Name the Arrangement

You can now size a belt drive from its sheaves, tell an overhung rotor from a center hung one, and turn a drawing into an arrangement number by asking three questions. That's the vocabulary every order, outline drawing and field service call in the rest of this book will use.

See Annotated References