Inlets, Outlets and Flow Control¶
Summary¶
This chapter covers what happens at a fan's inlet and outlet and how airflow is controlled. It introduces inlet funnels, integral and detached inlet boxes, evasés, transition pieces, companion flanges, and inlet and outlet silencers. It then covers flow control: turndown, parallel and opposed blade dampers, outlet and inlet box dampers, gravity, manual, and motorized dampers, variable inlet vanes, and speed control with a VFD or EC motor controller. After completing it, readers can choose a flow-control method and the inlet and outlet accessories an installation needs.
Concepts Covered¶
This chapter covers the following 27 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Flow Control | 97 |
| Damper | 54 |
| Turndown | 5 |
| Inlet Box | 28 |
| Evasé | 16 |
| Transition Piece | 2 |
| Inlet Silencer | 16 |
| Outlet Silencer | 16 |
| Parallel Blade Damper | 12 |
| Opposed Blade Damper | 9 |
| Gravity Damper | 8 |
| Manual Damper | 1 |
| Motorized Damper | 4 |
| Variable Inlet Vanes | 9 |
| Inlet Funnel | 7 |
| Integral Inlet Box | 2 |
| Detached Inlet Box | 7 |
| Companion Flange | 7 |
| Outlet Damper | 6 |
| Inlet Box Damper | 1 |
| External Inlet Vanes | 1 |
| Speed Control | 28 |
| Damper and Vane Bearings | 1 |
| Butterfly Gravity Damper | 1 |
| Wiper Bar | 1 |
| EC Motor Speed Controller | 23 |
| Relieved Inlet Transition | 1 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 1: Air, Pressure and the Fan
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
- Chapter 5: Roof, Wall and Specialty Exhaust Fans
- Chapter 7: Housing Construction and Exploded Views
- Chapter 8: Centrifugal and Axial Impeller Types
- Chapter 9: Impeller Construction and Material Handling
- Chapter 14: Fan Motors and Enclosures
- Chapter 15: Motor Ratings, Speed Control and Positioners
Welcome to Inlets, Outlets and Flow Control
A fan rarely meets its ductwork bare. Somewhere between the drawing and the job site, someone has to decide what bolts onto the inlet, what bolts onto the outlet, and how the customer will turn the airflow down when the process does not need all of it. By the end of this chapter you will make those calls with reasons you can defend on a quote. Let's follow the airflow.
The Fan Does Not End at Its Flanges¶
A fan's catalog rating describes the fan alone, tested with smooth, straight airflow entering the inlet and leaving the outlet. On a real job, the air arrives through an elbow, a plenum, a silencer or a box, and it leaves through a transition, a damper or a stack. Every one of those parts changes the pressure the fan sees, and some of them change the fan's performance directly. Chapter 10 called the penalty for poor inlet and outlet conditions system effect; this chapter introduces the hardware that either causes that penalty or prevents it.
The chapter follows the air through the fan in three passes:
- The inlet side: the inlet funnel and wiper bar, then the integral and detached inlet boxes.
- The outlet side and the connections: transition pieces, the evasé, companion flanges, and silencers.
- Flow control: dampers, variable inlet vanes, and speed control with a variable frequency drive or an EC motor controller.
Chapter 19 then adds construction detail: blade shapes, actuators, nested vanes, inlet box horns and duct-connection styles.
The Inlet Side¶
Air entering a centrifugal impeller must turn from flowing along the shaft axis to flowing outward between the blades. The smoother that turn, the closer the fan comes to its catalog rating. Two parts inside the housing manage the turn, and an inlet box manages it when the duct cannot approach the fan straight on.
Inlet Funnel¶
An inlet funnel is the aerodynamically shaped piece that guides air into the fan inlet. FanPedia describes it as providing "high efficiency and smooth airflow through the fan" and notes that it is flanged on one end to mount onto the fan housing (FanPedia p. 188). It is also called an inlet cone, a funnel or an inlet bell. The funnel's narrow end sits at the impeller's inlet, so the gap between them is a critical clearance: too large lets air leak back to the inlet, too small risks contact.
FanPedia shows three ways the funnel is made (FanPedia pp. 88-89):
- Solid spun: one spun piece.
- Spun body, spun tip: a spun body with a separately spun tip, welded.
- Fabricated body, ball rolled tip: a fabricated body with a rolled tip and a mounting flange.
FanPedia draws a fourth shape, the inlet venturi, on the same pages and gives it the same aliases, but it is a separate entry piece rather than a fourth way of making the funnel; Chapter 19 covers it.
The funnel material matters for spark-resistant construction. FanPedia's Type C spark-resistant centrifugal fan uses an aluminum inlet funnel as standard, because the aluminum funnel "will be the first point of fan impeller contact if there is a mechanical failure." For high-temperature applications (501°F and above) a steel funnel is required, with a rubbing band and rubbing bars added (FanPedia p. 106).
Worked Example: Reading the Funnel Off an Order¶
An inside sales engineer receives a request for a Type C spark-resistant centrifugal fan handling air at 550°F.
- Type C normally means an aluminum inlet funnel.
- 550°F is above the 501°F threshold FanPedia gives for a steel funnel.
- The order therefore needs a steel funnel with a rubbing band and rubbing bars.
- If the customer also wants variable inlet vanes nested inside the funnel (covered later), replacing those vanes later means replacing the whole funnel assembly (FanPedia p. 188).
The "simple" funnel changed the material, added two components and affected a future maintenance cost.
Wiper Bar¶
A wiper bar is a strip mounted inside the housing in the inlet funnel area. FanPedia lists it with the cutoff as the "Cutoff/Wiper Bar" and describes its job as helping "to direct air through the housing in the inlet funnel area to prevent turbulence and to increase efficiency" (FanPedia p. 59). FanPedia's housing drawings show the wiper bar next to the inlet funnel on single-width (SWSI) and double-width (DWDI) centrifugal housings and on plug fans (FanPedia pp. 60, 65, 66). A wiper bar is not a field accessory that a customer orders; it is part of the housing design. You will meet it when reading an exploded view or a parts list, and if it is damaged or missing during a repair, the fan can lose efficiency even though nothing in the airstream looks broken.
Inlet Box¶
An inlet box is a sheet-metal enclosure on the fan inlet that receives air from the side, at an angle, or from above or below, and turns it into the fan inlet. FanPedia states its purpose directly: an inlet box "is designed to minimize pressure drop and airflow losses," and it is "recommended for applications where uniform flow is difficult to obtain due to limited space or where the air must enter the fan at an angle" (FanPedia p. 141).
The alternative is usually worse. A duct elbow right at the fan inlet sends air into the impeller unevenly and swirling, so the fan delivers less pressure than its curve promises: the inlet system effect from Chapter 10. An inlet box is a designed turn whose losses are known, so they can be included in the selection instead of discovered at start-up.
FanPedia lists four inlet box styles: integral with a horn, integral without a horn, detached bolt-on, and detached free-standing (FanPedia p. 141). The integral and detached types each get their own section below. FanPedia also lists the features and accessories that an inlet box carries (FanPedia p. 145):
- Access door and drain connection, standard on most inlet boxes.
- FGR connection, a special flange on the front of the box for flue gas recirculation.
- Flange at inlet, connecting the box to the customer's ductwork, and a flange or collar at the connection to the fan.
- Insulated inlet box, for high-temperature or sound applications.
- Inlet box damper, a flow-control device covered later in this chapter.
Diagram: Inlet Box Features¶
Run Inlet Box Features Fullscreen
Inlet Box Features
sim-id: ch18-inlet-box-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the inlet box, its damper, its two connections, its access door and drain connection, and the fan housing it feeds on a drawing, given each feature's function.
Content: Seven numbered markers: inlet box, inlet box damper, flange at inlet, access door, flange or collar, drain connection, fan housing.
Provenance: FanPedia page 145, "Inlet Boxes: Features and Accessories, main colour-coded drawing (top left)", with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 145.
Every inlet box also has a position, which says which way its opening faces. FanPedia defines the positions in 45° steps, determined from the drive side, the same viewpoint used for rotation and discharge in Chapter 12. Picture a dial drawn on the fan as seen from the drive side, with 360 at the top and the numbers increasing clockwise. The number names where the box opening sits on that dial:
| Position | FanPedia description | Where the opening faces (viewed from drive side) |
|---|---|---|
| 45 | Angular down intake | Upper right |
| 90 | Horizontal right intake | Right |
| 135 | Angular up intake | Lower right |
| 180 | Bottom up intake | Bottom |
| 225 | Angular up intake | Lower left |
| 270 | Horizontal left intake | Left |
| 315 | Angular down intake | Upper left |
| 360 | Top down intake | Top |
Source: FanPedia p. 144.
Worked Example: Elbow or Inlet Box?¶
A plant engineer has a centrifugal fan in a cramped mezzanine. The supply duct drops from the ceiling and must turn 90° to enter the fan inlet, and there is no room for a long straight run before the fan. The engineer asks whether a standard elbow bolted to the inlet will do.
- Identify the condition. Air must enter at an angle and space is limited, exactly the case FanPedia names for an inlet box.
- Compare the choices. The elbow's system effect is hard to predict; the inlet box's loss is known.
- Assign the position. The duct comes down from the ceiling, so the box opening is at the top and air enters going down: position 360, top down intake.
- Pick the accessories. The standard access door allows inspection without dismantling the duct, and a drain connection handles condensation.
The quote line reads "inlet box, position 360, with access door and drain," and the selection includes the inlet box loss.
Check It from the Drive Side
Inlet box positions, like rotation and discharge, are read from the drive side. If you are standing at the inlet side looking at a drawing or a fan, swap left and right before you write the number: 90 seen from the inlet side is 270 from the drive side, and 45 becomes 315. Top and bottom do not change.
Diagram: Inlet Box Position Trainer¶
Run Inlet Box Position Trainer Fullscreen
Inlet Box Position Trainer
Type: microsim
sim-id: ch18-inlet-box-position-trainer
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: apply
Learning Objective: The learner will apply the drive-side viewing convention to assign the correct inlet box position (45, 90, 135, 180, 225, 270, 315 or 360) to eight described or pictured installations, including views drawn from the inlet side.
Prerequisites: inlet box, inlet box position, drive side, inlet side (all defined in the section "Inlet Box" above and in Chapter 12).
Evidence of Mastery: For each of eight installations, the learner commits one position from the eight choices. An answer is correct when it equals the "Correct position" column in Content. Mastery is 7 of 8 correct on the first attempt. Rotating the free-practice dial in step 5 is exploration, not evidence.
Misconceptions: (1) The position can be read from whichever side the viewer happens to stand on; a view from the inlet side mirrors left and right. (2) The two "angular down" positions (45 and 315) are interchangeable, and so are the two "angular up" positions (135 and 225). (3) Position 180 means the box faces up because air moves upward; the number names where the opening is (bottom), and "bottom up intake" describes the air direction.
Instructional Rationale: Assigning a code is an Apply task: the learner must use one rule (drive-side view, clockwise dial from 360 at the top) on new cases. Mixing drive-side and inlet-side views forces the learner to apply the mirror step rather than match pictures from memory.
Content:
The position dial, shown as a reference after each answer is checked: 360 at the top, 45 upper right, 90 right, 135 lower right, 180 bottom, 225 lower left, 270 left, 315 upper left, as viewed from the drive side. Each position carries its FanPedia description: 45 angular down intake; 90 horizontal right intake; 135 angular up intake; 180 bottom up intake; 225 angular up intake; 270 horizontal left intake; 315 angular down intake; 360 top down intake.
Each installation shows a simple end view of a fan housing with its inlet box, a label stating the viewpoint ("Viewed from drive side" or "Viewed from inlet side"), and an arrow showing the direction air enters the box. Item 7 is text only.
| # | Viewpoint | Box opening appears at | Correct position | Why (shown as feedback) |
|---|---|---|---|---|
| 1 | Drive side | Top, air entering downward | 360 | Viewed from the drive side, an opening at the top is position 360, top down intake. |
| 2 | Drive side | Right, air entering horizontally | 90 | Viewed from the drive side, an opening on the right is position 90, horizontal right intake. |
| 3 | Drive side | Upper left, air entering downward at an angle | 315 | Upper left on the drive-side dial is 315; 45 is the upper right. |
| 4 | Drive side | Bottom, air entering upward | 180 | The opening is at the bottom, so the position is 180, bottom up intake; the number names where the opening is, not the air direction. |
| 5 | Inlet side | Right, air entering horizontally | 270 | From the inlet side, left and right are mirrored. An opening that appears on the right from the inlet side is on the left from the drive side: 270. |
| 6 | Inlet side | Lower left, air entering upward at an angle | 135 | Mirror the view: lower left from the inlet side is lower right from the drive side, which is 135. |
| 7 | Text only: "The duct rises from a floor trench and turns straight up into the box." | (none shown) | 180 | Air entering from below means the opening faces down at the bottom of the box: 180, bottom up intake. |
| 8 | Inlet side | Upper left, air entering downward at an angle | 45 | Mirror the view: upper left from the inlet side is upper right from the drive side, which is 45. |
Provenance: Position numbers, descriptions and the drive-side rule from FanPedia p. 144 ("Inlet Box Position Descriptions"; "Inlet Box Positions Determined from Drive Side"). Installations 1-8 are illustrative scenarios written for this chapter; the sim labels them "illustrative."
Rules: Positions are the eight values 45, 90, 135, 180, 225, 270, 315 and 360 only. For a drive-side view, the position is the dial angle of the opening measured clockwise from the top, with the top written as 360, not 0. For an inlet-side view, the drive-side position is (360 − p) where p is the angle read on the inlet-side view; a result of 0 is written 360, and 180 stays 180. A text-only item is judged by where the opening must be for the stated air path.
Learner Activity:
- The learner reads installation 1 and selects one of the eight positions.
- The learner presses Check. The sim marks the answer right or wrong, shows the feedback, and overlays the drive-side dial on the picture. For an inlet-side item, the overlay first shows the mirrored view so the learner sees the opening move to the other side.
- The learner moves to the next installation; items appear in the fixed order 1 to 8.
- After item 8 the sim shows the score.
- Free practice unlocks: the learner drags the box opening around the housing in 45° steps and toggles the viewpoint between drive side and inlet side. The position number and description update at once. The learner should notice that switching viewpoint changes 90 to 270 and 45 to 315, while 360 and 180 never change.
Feedback: Eight items, fixed order, two attempts each. Correct: "Correct: position
Starting State: Item 1 is shown with no position selected and the question "Which inlet box position is this?" The dial overlay is hidden until the first answer is checked.
Chapter Anchors: Eight positions in 45° steps; positions are determined from the drive side; 360 is top down intake, 90 is horizontal right intake, 180 is bottom up intake, 270 is horizontal left intake; from the inlet side 90 reads as 270 and 45 reads as 315; top and bottom do not change. The worked example "Elbow or Inlet Box?" assigns position 360 to a duct dropping from the ceiling.
Integral Inlet Box¶
An integral inlet box, also called an attached inlet box, is built into the inlet side of the fan housing and is supported by the fan. FanPedia describes it as "integral to fan housing" with a "common plate with fan inlet housing sideplate," meaning the box and the housing share one side plate rather than meeting at a bolted flange (FanPedia p. 141). Because it is part of the housing, the box arrives assembled and needs no separate foundation. Integral inlet boxes come with or without a horn, an extension used "when inlet and discharge airflows would intersect" so that the inlet duct can be connected without interfering with the discharge duct (FanPedia p. 141); Chapter 19 covers horns. Several of the arrangements in Chapter 11 that carry the suffix "I", such as 3SI and 7DI, use an integrated inlet box to move a bearing out of the airstream (FanPedia pp. 164-168).
Detached Inlet Box¶
A detached inlet box is a separate assembly that connects to the fan's inlet flange instead of sharing the housing side plate. FanPedia describes two forms (FanPedia p. 142):
- Bolt-on. The box bolts directly to the fan's inlet flange and stands on its own single base angle, with gussets and a reinforcement plate tying the box to the base angle. It has a rectangular inlet flange for the customer's duct and a round outlet flange that matches the fan's inlet flange, and it is available with a straight or flared connection. FanPedia notes that this is "TCF's preferred inlet box design."
- Free-standing. The box is mounted apart from the fan and fully supported at the floor on a base plate with gussets. Its connection to the fan is either bolted (a round outlet flange) or a slip connection (an outlet collar joined to the fan's inlet collar with a rubber sleeve and clamps).
The deciding question for a detached box is whether the fan and box sit on the same plane. FanPedia shows both cases (FanPedia p. 143). When the fan and box share the same support, either both on grade or both on one unitary or isolation base, they move together and can be bolted together. When they sit on different planes, such as a fan on an isolation base and a box on legs to grade, they move differently, and FanPedia's note is explicit: a flex connector (slip connection) is required between the fan and the inlet box.
Worked Example: Fan on Isolators, Box on Grade¶
A specifying engineer's drawing shows a large centrifugal fan on a spring isolation base, with a detached inlet box standing on legs to grade next to it. The engineer has drawn a bolted flange between the two.
- Find the planes. The fan is on an isolation base and the box is on grade: different planes.
- Predict the motion. The fan moves on its springs; a rigid bolted joint to a box fixed to the floor would carry that motion into the box, the joint and the duct.
- Apply the rule. FanPedia requires a flex connector (slip connection) between fan and box in this configuration, so the free-standing box with an outlet collar, rubber sleeve and clamps is the correct choice.
- Alternative. If the engineer wants a bolted connection, both fan and box must sit on the same base, for example a single isolation base carrying both.
The response to the engineer is a one-line drawing comment: "Fan and inlet box on different planes; provide slip connection with flex connector per manufacturer."
The Outlet Side and Duct Connections¶
The outlet of a centrifugal fan is a rectangle sized for the fan, and the outlet of an axial fan is a circle sized for the fan. The customer's duct is sized for the building. The parts in this section make the two fit, recover energy at the outlet, or quiet the fan.
Transition Piece¶
A transition piece (usually just "transition") is a section of ductwork that connects a fan opening to a duct of a different shape or size. FanPedia lists two jobs: changing the opening "from one shape to another (i.e. square to round, rectangular to round, etc.)" or keeping the same shape while enlarging or reducing the opening (FanPedia p. 59). Transitions are used "when corresponding ductwork connecting to the fan does not directly match the shape or size of the opening and/or flange bolt pattern," and they can be welded to the fan or supplied as a separate bolt-on piece (FanPedia p. 67). FanPedia's examples include a welded rectangular-to-round transition with an outlet flange, a welded square-to-round transition with a slip connection on the outlet, a non-welded rectangular-to-round transition, and a round-to-round transition (FanPedia p. 67). One special case has its own name: when a transition is used on a standard fan to enlarge the housing opening for proper fan performance, it "would then be called an evasé."
Relieved Inlet Transition¶
A relieved inlet transition is a transition shaped for material handling. FanPedia describes it as "used to smooth the flow of paper trim and similar material through the fan" (FanPedia p. 67). Where an ordinary transition is designed mainly for air, a relieved inlet transition is shaped so that strips of trim carried in the airstream do not snag or pile up as they enter the fan. You will meet it on paper-converting and trim-conveying jobs, which use the material-handling impellers described in Chapter 9; if a customer describes trim catching at the fan inlet, the inlet transition is one of the first parts to check.
Evasé¶
An evasé (pronounced "ev-ah-SAY") is a section of ductwork, usually bolted, attached to the fan discharge, with an outlet larger than its inlet. FanPedia gives its purpose: it "ultimately expands the outlet area of the fan" in order "to reduce the outlet velocity of the fan and to increase the static pressure capability" (FanPedia p. 146). FanPedia shows rectangular and round evasés.
The physics comes from Chapter 2. Air leaving a fan carries velocity pressure, energy the fan has already paid for, and air discharged at high velocity into a plenum or the atmosphere loses it. A gradually expanding section slows the air and converts part of that velocity pressure into static pressure, called static regain. How much is regained depends on the evasé's angle and length; it is never the full drop in velocity pressure.
Velocity pressure at standard air density follows from the outlet velocity:
Velocity Pressure at Standard Air¶
where:
- \( VP \) is the velocity pressure in inches of water gauge (in. wg)
- \( V \) is the air velocity in feet per minute (fpm), equal to airflow in CFM divided by area in ft²
Worked Example: What an Evasé Buys¶
An illustrative fan delivers 12,000 CFM through a fan outlet area of 3.0 ft² and discharges to atmosphere. An evasé expands the outlet to 4.5 ft². All numbers here are illustrative, including the fraction regained.
| Step | Without evasé | With evasé |
|---|---|---|
| Outlet area | 3.0 ft² | 4.5 ft² |
| Velocity, \( V = Q/A \) | 12,000 / 3.0 = 4,000 fpm | 12,000 / 4.5 = 2,667 fpm |
| Velocity pressure, \( (V/4005)^2 \) | 1.00 in. wg | 0.44 in. wg |
| Velocity pressure lost at discharge | 1.00 in. wg | 0.44 in. wg |
The evasé lowers the velocity pressure leaving the system by 0.56 in. wg. If it recovers 60% of that drop as static pressure, the fan gains about 0.33 in. wg of usable static pressure, a meaningful margin on a system that needs 3.0 in. wg.
FanPedia adds two practical notes. For some fans, the published performance ratings already include an evasé, so leaving it off the order means the fan will not meet its catalog numbers. And the customer may use "a well-designed transition piece in place of the evasé as long as the outlet area matches" (FanPedia p. 146).
Read the Rating Note Before Deleting the Evasé
A common trap is treating the evasé as an optional extra and removing it to save cost or space. When the catalog rating includes the evasé, the bare fan is not the fan that was rated. Check the catalog's rating notes first, and if the customer supplies their own transition instead, confirm that its outlet area matches the evasé's.
Companion Flange¶
A companion flange is a loose flange, round or rectangular, that matches the fan's inlet or outlet flange and is attached to the customer's ductwork in the field. FanPedia's description is short: companion flanges "are connected to the connecting ductwork in the field and ensure a matching connection to the fan. They are shipped loose for field mounting" (FanPedia p. 180). FanPedia's accessory page shows them for both the inlet and the outlet, in round and rectangular shapes (FanPedia p. 125).
The value of a companion flange is the bolt pattern. A fan flange has a specific size, hole count and hole spacing. If the contractor fabricates a duct flange from the drawing, any error in hole location shows up on installation day, when the duct is hanging and the fan is set. A companion flange is made to match the fan, so the contractor attaches it to the duct and the bolt holes line up.
Worked Example: Adding Companion Flanges to a Quote¶
A mechanical contractor orders a housed centrifugal fan with a round inlet and a rectangular outlet, ducted on both sides. The contractor's sheet-metal shop will build the duct.
- Inlet side. The inlet duct is round, so the order adds a round inlet companion flange matched to the fan's inlet flange.
- Outlet side. The outlet duct is rectangular, so the order adds a rectangular outlet companion flange matched to the fan's outlet flange.
- Shipping. Both flanges ship loose. The quote states that they are for field attachment to the contractor's duct, so nobody expects them welded to the fan.
- Coordination. The flanges go to the sheet-metal shop before the duct is finished, so the shop builds the duct to the flange instead of to a drawing dimension.
If the duct is a different shape or size from the fan opening, a companion flange alone is not enough; the job needs a transition piece first, with the companion flange on the transition's far end.
Inlet Silencer¶
An inlet silencer is a duct section lined with sound-absorbing material that mounts on the fan inlet to reduce noise coming out of it. FanPedia describes Twin City Fan's inlet silencer as "constructed of welded steel with acoustical absorption material to reduce noise emanating from fan inlet." A flanged connection is suggested for mounting to the fan, and the opposite end can be furnished with an inlet venturi, an inlet flange or an inlet pipe assembly; unless otherwise specified, the silencer has punched flanges at both ends (FanPedia p. 188). FanPedia shows it with a support leg, so the silencer's weight does not hang on the fan inlet (FanPedia p. 127).
An inlet silencer matters most on fans with an open inlet, where the fan draws from the room rather than from a duct. A pressure blower drawing room air, for instance, radiates its inlet noise straight into the space where people work. Every silencer also adds airflow resistance, and that resistance must be added to the system before the fan is selected.
Worked Example: Open-Inlet Blower in a Work Area¶
A pressure blower draws room air through an open inlet in a production area, and operators complain about the noise at the blower. The proposed fix is an inlet silencer. All numbers are illustrative.
- Locate the noise path. The inlet is open to the room, so noise leaves through the inlet directly toward the operators. An inlet silencer treats that path.
- Choose the inlet end. The silencer will draw from the room, so the open end is furnished with an inlet venturi rather than a flange, giving the air a smooth entry.
- Add the resistance. Suppose the silencer's published loss at the design airflow is 0.4 in. wg (illustrative). If the system previously needed 20.0 in. wg, the blower must now be checked at 20.4 in. wg.
- Recheck power. Higher pressure at the same airflow means more brake horsepower. The engineer confirms the existing motor still has margin at the new point before quoting the silencer as a drop-in fix.
Outlet Silencer¶
An outlet silencer does the same job on the discharge side: it reduces noise carried out of the fan through the outlet and into the duct or the atmosphere. FanPedia notes that silencers "are available for both the inlet and outlet of fans" (FanPedia p. 127). The choice between them depends on where the noise goes. Noise radiating from an open inlet calls for an inlet silencer; noise carried down the discharge duct to occupied rooms, or out of an exhaust stack toward a neighbor, calls for an outlet silencer.
The outlet side brings one extra concern. Air leaving a fan outlet does not have a uniform velocity profile for some distance downstream; it is concentrated toward the outside of the scroll. A silencer bolted tight to the outlet sees that uneven flow, and its pressure loss rises above its published rating, which is another form of system effect. Allowing a short straight duct between the outlet and the silencer, or following the silencer manufacturer's spacing guidance, keeps the loss close to its rating.
Worked Example: Stack Noise at a Property Line¶
A process exhaust fan in a mechanical room discharges through a stack on the roof, and a neighboring property has complained about noise at night. Numbers are illustrative.
- Locate the noise path. The fan's inlet is ducted inside a closed room; the noise reaches the neighbor through the stack, so an outlet silencer is the right tool.
- Place the silencer. It goes after a straight run of duct from the fan outlet, not bolted to the flange, so the airflow entering it is closer to uniform.
- Add the resistance. The illustrative rated loss of 0.5 in. wg raises the system requirement from 3.5 to 4.0 in. wg.
- Recheck the fan. The engineer checks whether the fan can be sped up within its maximum safe speed and motor capacity, or whether a larger, lower-loss silencer is the better trade.
Every Accessory Lives on the System Curve
Notice the pattern in the last four examples: the inlet box, the evasé and the silencers all changed the pressure the fan had to supply. Think of each accessory as a point added to, or taken off, the system curve from Chapter 10. If you can say which way each part moves the curve, you can predict what it does to airflow and horsepower before anyone opens a catalog.
Flow Control¶
What Flow Control Means¶
Flow control is any means of changing the airflow a fan delivers while the system is running. A fan is sized for the worst case, but most systems spend much of their life below it: a boiler at half load needs less combustion air, and a dust collection system with branches closed needs less exhaust. Flow control is how the fan follows those changes.
Chapter 10 showed that a fan runs where its fan curve crosses the system curve. That gives exactly two ways to change the airflow:
- Change the system curve. Add resistance with a damper so the operating point slides left along the fan's curve. The fan still turns at full speed.
- Change the fan curve. Either change how the air enters the impeller (inlet vanes or an inlet box damper, which pre-spin the air) or change the impeller's speed (a variable frequency drive, an EC motor controller, or a sheave change on a belt drive). The fan produces a smaller curve, and the operating point moves down along the system curve.
The two approaches use very different amounts of power at reduced flow. A damper throws away pressure that the fan has already produced. Speed control never produces it. Pre-spin from inlet vanes sits between them.
Worked Example: The Same 70% Flow Three Ways¶
A process fan needs to run at 70% of design airflow for long periods. Using the illustrative comparison values in the sim below, the fan's shaft power at 70% flow would be:
| Method | How it reduces flow | Shaft power at 70% flow (illustrative, % of design) |
|---|---|---|
| Outlet damper | Adds resistance; operating point slides left on the fan curve | 86% |
| Variable inlet vanes | Pre-spin the air, producing a lower fan curve | 67% |
| Speed control | Lowers speed, producing a smaller fan curve | 34% |
The damper saves 14% of the power, the vanes save about a third, and speed control saves about two-thirds. For a fan that runs at 70% flow most of the year, that gap is the main input to the payback calculation. For a fan that runs at 70% flow for one hour a month, the cheaper damper may be the right answer. The speed-control figure is the ideal fan-law value for a system with no fixed static pressure; a real drive has its own small losses, and a system with a fixed pressure requirement saves less.
Turndown¶
Turndown is the range over which a flow-control method can reduce airflow and still control it well, usually stated as a ratio of maximum to minimum controllable airflow. A method with 4:1 turndown can control a 10,000 CFM fan down to 2,500 CFM; below that, control becomes unstable, the fan nears its stall region, or the motor cannot run properly at the low speed. FanPedia makes one turndown comparison directly: EC motors "have a larger usable turndown range than a traditional speed controllable motor" (FanPedia p. 68). When a customer says a process "runs from 100% down to 30%," the first check is whether the proposed method's turndown covers that range.
Two Different Turndowns
FanPedia also uses "turndown" for a shaft machined to a smaller diameter at a bearing, seal or hub (see Chapter 16). If a customer note says "turndown," ask whether it is about airflow range or shaft diameter before you answer; the context of the sentence usually tells you, but a one-line question saves a wrong quote.
Before comparing the methods in detail, use the sim below to see how much power each method uses across the whole flow range. It uses the same illustrative values as the worked example.
Diagram: Flow Control Power Comparator¶
Run Flow Control Power Comparator Fullscreen
Flow Control Power Comparator
Type: chart
sim-id: ch18-flow-control-power-comparator
Library: Chart.js
Status: Specified
Bloom Level: Analyze
Bloom Verb: differentiate
Learning Objective: The learner will differentiate outlet dampers, variable inlet vanes and speed control by the shaft power each needs at reduced airflow, correctly ranking the three methods and estimating the savings gap at a stated airflow between 50% and 100% of design.
Prerequisites: flow control, operating point, fan curve, system curve, outlet damper, variable inlet vanes, speed control, brake horsepower (defined in this chapter's "What Flow Control Means" section and in Chapter 10).
Evidence of Mastery: Three prediction challenges, each answered before the curves are revealed. In each, the learner ranks the three methods from lowest to highest power at the stated airflow, then types the power saved by the best method compared with the outlet damper. A challenge is correct when the ranking matches Content and the typed saving is within ±3 percentage points of the value in Content. Mastery is 3 of 3. Hovering points and moving the airflow selector are exploration, not evidence.
Misconceptions: (1) Closing a damper to half flow cuts power roughly in half. (2) All flow-control methods save about the same energy. (3) Inlet vanes save as much as speed control. (4) Power falls in proportion to airflow when speed is reduced, rather than with roughly the cube of the airflow.
Instructional Rationale: Analyze-level differentiation needs the learner to compare the three methods' power side by side at specific airflows. Committing a ranking and a saving before the reveal forces a prediction that exposes misconceptions 1 to 3, and the hover readout supplies the concrete values for the comparison.
Content:
A line chart with airflow (% of design) on the horizontal axis from 50 to 100 and shaft power (% of design) on the vertical axis from 0 to 100. Three series, each labeled "illustrative":
| Airflow (% of design) | Outlet damper (% power) | Variable inlet vanes (% power) | Speed control (% power) |
|---|---|---|---|
| 100 | 100 | 100 | 100.0 |
| 90 | 96 | 88 | 72.9 |
| 80 | 91 | 77 | 51.2 |
| 70 | 86 | 67 | 34.3 |
| 60 | 80 | 59 | 21.6 |
| 50 | 74 | 52 | 12.5 |
Hovering any point shows its airflow, method and power. An airflow selector shows the three powers and the two savings (inlet vanes vs. damper; speed control vs. damper) at the selected airflow.
Prediction challenges, in this order:
| # | Airflow | Correct ranking, lowest to highest power | Saving of best method vs. damper | Feedback when wrong |
|---|---|---|---|---|
| 1 | 80% | Speed control, inlet vanes, outlet damper | 39.8 percentage points (91 − 51.2) | The damper still drives the fan at full speed and burns most of the power; speed control at 80% flow needs about 0.8³ = 51% power. |
| 2 | 70% | Speed control, inlet vanes, outlet damper | 51.7 percentage points (86 − 34.3) | At 70% flow the damper saves only 14% of power; speed control needs about 0.7³ = 34%. |
| 3 | 50% | Speed control, inlet vanes, outlet damper | 61.5 percentage points (74 − 12.5) | Halving the flow with a damper still leaves about three-quarters of the power; halving the speed cuts power to one-eighth. |
Provenance: Speed-control values are the ideal fan-law relation (power proportional to the cube of airflow for a system curve through zero), computed from the Rules. Outlet damper and inlet vane values are illustrative values written for this chapter to show the typical ordering; they are not Twin City Fan test data. FanPedia supports the ordering: outlet damper horsepower savings are "usually much less than other methods" (FanPedia p. 182), and inlet box dampers and inlet vanes save horsepower at reduced loads by pre-spinning the air (FanPedia pp. 145, 183, 188). The sim labels all three series "illustrative."
Rules: Speed control power % = 100 × (airflow % / 100)³, rounded to 0.1. Damper and inlet vane values between table rows are found by straight-line interpolation between the neighboring rows. The airflow selector ranges from 50 to 100% with a step of 5% and a default of 70%. Saving = damper power − method power, in percentage points. A footnote states that real speed control has small drive losses and that a system with fixed static pressure saves less than the ideal cube value.
Learner Activity:
- The learner reads challenge 1 (80% airflow) with the three curves hidden, orders the three methods from lowest to highest power, and types the saving of the best method compared with the damper.
- The learner presses Check. The three curves are drawn, the 80% points are marked with their values, and the answer is judged.
- The learner completes challenges 2 and 3 the same way; the curves are hidden again at the start of each challenge.
- After challenge 3 the chart stays visible and the airflow selector unlocks. The learner moves it from 100% down to 50% and watches the gap between the damper and speed-control curves grow. The learner should notice that the damper curve is nearly flat while the speed-control curve drops steeply.
Feedback: Three challenges, fixed order, two attempts each. Correct: "Correct:
Starting State: Axes are drawn with no curves; challenge 1 asks, "At 80% airflow, which method uses the least power, and how much does it save compared with an outlet damper?"
Chapter Anchors: At 70% airflow the worked example "The Same 70% Flow Three Ways" gives outlet damper 86%, variable inlet vanes 67%, speed control 34%; the damper saves 14% of power, the vanes about a third, speed control about two-thirds. The Speed Control section's worked example uses 80% airflow and 51.2% power.
Damper¶
A damper is a set of one or more movable blades in a frame, placed in the airstream to add resistance, shut off flow, or prevent reverse flow. Open blades offer little resistance; closing them adds resistance until the passage is blocked. A flow-control damper makes the system more restrictive, so the fan runs farther left on its own curve.
FanPedia divides dampers into commercial grade and industrial grade (FanPedia pp. 136-137). Commercial-grade dampers are the gravity dampers used on roof and wall exhaust and the motorized dampers with center or end pivots. Industrial-grade dampers are the parallel blade and opposed blade outlet dampers, each available as Type 1 with flat blades and Type 2 with airfoil blades, the parallel blade inlet box damper (Type 2 only), the manual outlet damper, and the outlet damper with actuator. Separately, the damper's operator (gravity, a hand quadrant or an actuator) decides how its blades move.
Worked Example: Which Kind of Damper Is the Customer Asking For?¶
Three requests arrive in one week. Sorting them by purpose shows that "damper" means three different products:
| Request | Purpose | Damper family |
|---|---|---|
| "Keep cold air from blowing back in through our wall fan at night." | Prevent backdraft when the fan is off | Commercial grade, gravity damper |
| "We need to throttle the dust collector fan when two branches are shut." | Reduce airflow while running | Industrial grade, outlet damper |
| "Lock the supply fan at the airflow the balancer measured." | Set airflow once | Manual damper |
The first question to ask when a customer says "damper" is what the damper must do: stop backflow, control flow, or set a balance point. The answer picks the family before any size or blade type is discussed.
Outlet Damper¶
An outlet damper is a damper mounted on the fan discharge to control airflow. FanPedia describes how it works and when to use it: outlet dampers "add resistance to the fan by shifting the operating point to the left of the rating point. The horsepower savings depends on the relative position on the fan curve and is usually much less than other methods." They "are typically the least expensive option and should be considered when infrequent operation at lesser capacity is desired or when handling hot, humid or particulate-laden air," and both blade styles are available to 750°F construction (FanPedia pp. 182-183).
Outlet dampers suit dirty and hot air because their blades sit downstream of the impeller and are simple, robust parts, while inlet vanes put small moving vanes and linkages right in the inlet stream.
Worked Example: Following the Operating Point¶
An illustrative fan runs at 10,000 CFM and 4.0 in. wg. The process now needs about 8,000 CFM for a short shift each week, and the customer has a constant-speed motor.
- Close the damper partway. The damper adds resistance, so the system curve becomes steeper.
- Find the new point. The new system curve crosses the unchanged fan curve farther left, at about 8,000 CFM and a higher pressure, say 4.6 in. wg on this illustrative curve. The fan is now producing more pressure, and the damper throws the extra pressure away.
- Check the power. Following the fan's brake horsepower curve back to 8,000 CFM shows only a small drop, roughly 91% of design power from the illustrative comparison.
- Check the stall region. The new point must stay to the right of the fan's stall region on the curve; a damper closed too far can push a fan into unstable operation.
- Decide. Because reduced operation is infrequent, the small power saving does not matter, and the outlet damper is the least expensive solution.
Parallel Blade Damper¶
A parallel blade damper has all its blades linked so they rotate in the same direction and stay parallel to each other at every position. As it closes, the blades act like a set of tilted louvers that turn the airflow to one side. FanPedia recommends parallel blade outlet dampers "for systems where air volume is modulated between full-open to about 75% of open" (FanPedia p. 183). Over that upper range a parallel blade damper gives useful control at the lowest cost; below it, small blade movements cause large and uneven airflow changes, which makes the damper harder to control.
Parallel blade construction also appears in two other places in this chapter: the ceiling and wall gravity dampers are "parallel blade, end-pivoted" dampers (FanPedia p. 181), and the inlet box damper is a parallel blade damper that uses its tilted blades on purpose to pre-spin the air (FanPedia p. 183).
Worked Example: Trim Duty on a Dust Collector Fan¶
A dust collector fan is sized for all branches open. When one branch is closed for maintenance a few times a month, the operator wants to trim the fan back modestly so the remaining hoods do not over-draw. The trimmed position is estimated at about 80% to 90% of full-open blade position.
- Duty. Reduction is infrequent and modest.
- Airstream. The air is particulate-laden, which FanPedia lists as a reason to consider an outlet damper.
- Range. The required modulation stays between full open and about 75% open, inside the parallel blade recommendation.
- Choice. A parallel blade outlet damper, the lowest-cost industrial-grade outlet damper, is the fit.
Opposed Blade Damper¶
An opposed blade damper links alternate blades so that neighboring blades rotate in opposite directions. As it closes, pairs of blades come together like hands closing, and the air passes through straight rather than being turned to one side. FanPedia states that opposed blade dampers "cost about 10% more and are recommended for systems where volume is modulated over the entire range," because "opposed blades reduce air volume in a closer relationship to the control arm movement" (FanPedia p. 182). Airflow follows the control arm more evenly, so a controller can position it accurately across the range.
The two industrial outlet dampers compare as follows:
| Property | Parallel blade outlet damper | Opposed blade outlet damper |
|---|---|---|
| Blade motion | All blades rotate the same way | Neighboring blades rotate in opposite directions |
| Recommended modulation range | Full open to about 75% open | The entire range |
| Airflow vs. control arm movement | Less proportional | Closer relationship to arm movement |
| Relative cost | Base | About 10% more |
| Blade types | Type 1 flat, Type 2 airfoil | Type 1 flat, Type 2 airfoil |
| Temperature | Available to 750°F construction | Available to 750°F construction |
Source: FanPedia pp. 137, 182-183.
Worked Example: Continuous Modulation from a Control Loop¶
A drying process uses a constant-speed fan, and a temperature controller must vary the exhaust airflow continuously between full flow and a small fraction of it, all day. A variable frequency drive has been ruled out by the owner for this machine.
- Duty. Modulation is continuous and covers nearly the whole range, and the controller needs airflow to follow the actuator closely.
- Choice. An opposed blade outlet damper with an actuator, at about 10% more than the parallel blade damper.
- Note for the quote. Because the process runs reduced most of the time, the engineer notes the damper's small power savings so the owner sees the cost of ruling out speed control.
Ask How Far and How Often
Two questions pick most outlet dampers in under a minute: "How far down does the airflow go?" and "How often?" Modest and occasional points to a parallel blade damper; anywhere across the full range under control points to opposed blades; reduced most of the time is your cue to raise inlet vanes or speed control.
Inlet Box Damper¶
An inlet box damper is a parallel blade damper mounted in an inlet box. FanPedia's description: "When partially closed, the inlet box damper pre-spins the air in the direction of impeller rotation, resulting in a savings in horsepower at reduced loads" (FanPedia p. 183). Its tilted parallel blades turn the air before it reaches the impeller, so the impeller does less work on it. That makes the inlet box damper a fan-curve method, not just added resistance, which is why it saves more power than an outlet damper. FanPedia lists it as "Type 2 only," meaning airfoil blades, and also calls it a pre-spin parallel blade inlet box damper (FanPedia p. 137). It requires an inlet box, so it is a natural choice when the fan already needs one. Its blades must turn the air the same way the impeller rotates, so the fan's rotation (Chapter 12) is part of the damper order.
Manual Damper¶
A manual damper is a damper positioned by hand, typically with a handle or quadrant that locks the blades at a set angle. FanPedia notes that manually operated outlet dampers are available in parallel or opposed blade configurations and that "manual dampers are typically used to balance a system" (FanPedia p. 182). Balancing means setting the airflow once, during commissioning, to match the design; after that, the damper stays put. A manual damper is the right choice when the airflow does not need to change during operation and the wrong choice whenever someone would have to walk up and move it every shift.
Motorized Damper¶
A motorized damper has an actuator that drives its blades open and closed. FanPedia explains why that matters: motorized dampers are "usually furnished with an actuator that powers the blades open rather than relying on the velocity pressure of the airflow," which is "of particular importance in low flow conditions that might otherwise only partially open the automatic damper blades creating blade flutter and potentially more noise" (FanPedia p. 182). FanPedia's commercial-grade examples have an actuator with either a center pivot or an end pivot (FanPedia p. 136). Chapter 19 covers actuators and blade flutter in more detail.
Diagram: Industrial Grade Dampers¶
Run Industrial Grade Dampers Fullscreen
Industrial Grade Dampers
sim-id: ch18-industrial-damper-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the parallel blade and opposed blade outlet dampers, the parallel blade inlet box damper, the manual outlet damper and the outlet damper with actuator on FanPedia's drawings, given a description of each.
Content: Five numbered zones: parallel blade outlet damper; opposed blade outlet damper; parallel blade inlet box damper; manual outlet damper; outlet damper with actuator.
Provenance: FanPedia page 137, "Dampers: Industrial Grade (parallel and opposed blade outlet dampers with Type 1/Type 2 blade insets, inlet box damper, manual outlet damper, outlet damper with actuator)", with the captions and blade-type tags removed. Drawing: FanPedia, Twin City Fan, p. 137.
Gravity Damper¶
A gravity damper opens when the fan's airflow pushes its blades open and closes by its own weight when the fan stops. It uses no actuator and no power. Its job is not to control airflow during operation; it prevents backdraft, the reverse flow of outside air or building air through an idle fan. FanPedia describes the ceiling and wall types as "designed to open automatically when the fan is energized and to close by gravity when power is turned off," with "parallel blade, end-pivoted" construction that prevents "the air from back flowing through the system while also serving to exclude outside elements such as rain and snow" (FanPedia p. 181).
Because the airflow itself holds the blades open, a gravity damper adds some pressure loss when the fan runs, and it needs enough airflow to open fully. At low airflow the blades open only partway and may flutter, which is the condition that leads to a motorized damper instead.
Worked Example: Winter Backdraft at a Warehouse Wall Fan¶
A warehouse uses wall exhaust fans in summer. In winter they are off, and staff report cold air and snow blowing in through them.
- Diagnose. With the fan off, nothing closes the opening, and wind pressure drives outside air in. This is backdraft.
- Choose. A wall type gravity damper closes by gravity when the fan is off and excludes rain and snow, with no wiring.
- Check the running condition. When the fan runs, the damper's pressure loss is added to the system, so the engineer confirms the fan still meets its airflow with the damper installed.
- Watch the low-flow case. If the fans will run at reduced speed, the damper may open only partway and flutter; in that case a motorized damper is the better recommendation.
Butterfly Gravity Damper¶
A butterfly gravity damper is a gravity damper made of two hinged halves that fold up like a butterfly's wings when air flows through and drop closed when it stops. FanPedia describes the butterfly type as "integral to roof mounted upblast fans," where it "prevent[s] a backdraft of outside air and provide[s] weather protection when the fan is not in operation" (FanPedia p. 181). Because it sits in the discharge of an upblast fan, closing it also keeps rain from falling straight down into the fan and the duct below. You will meet it on the roof and wall exhaust fans of Chapter 5; when a customer asks for an upblast fan that "doesn't let the rain in when it's off," this is the part they are describing.
Damper and Vane Bearings¶
Damper and vane bearings are the bearings that support damper blade shafts and the control linkage of inlet vanes. FanPedia identifies them as flange bearings, "mounted within a flanged housing," used "when the bearing mounting surface is perpendicular to a shaft axis," and lists their uses on Twin City Fan products: dampers with bearing bridges, bearings mounted directly to a damper without bearing bridges, and control linkage rod support for inlet vanes (FanPedia p. 177). These are small parts, but a seized damper bearing turns a modulating damper into a fixed one. On leak-tight RTO construction, FanPedia adds that dampers, if required, "must have stuffing boxes" to seal where the shafts pass through the frame (FanPedia p. 121).
Variable Inlet Vanes¶
Variable inlet vanes are a ring of adjustable radial vanes at the fan inlet. When partly closed they make the entering air spin in the same direction as the impeller before it reaches the blades. FanPedia describes them simply: "radial vanes at the fan inlet pre-spin the air entering the fan to control the flow," and they "come standard with a manual handle operator, but can be provided with an actuator" (FanPedia p. 188). Other names for the same device include vortex damper, inlet damper, inlet guide vanes and radial inlet damper (FanPedia p. 139).
Pre-spin is why vanes save power. An impeller adds energy by spinning the air; air that arrives already spinning its way needs less added spin, so the fan produces a lower curve and draws less power, though not as little as with speed control. FanPedia's inlet vanes are available in two forms for airstreams up to 600°F: external vanes, for contaminated or high-temperature air, and nested vanes inside the inlet funnel, for clean air (FanPedia p. 139). Chapter 19 covers nested vanes and pre-spin in more detail.
Worked Example: Field Adjustment on a Direct-Drive Plenum Fan¶
An Arrangement 4 plenum fan in a pharmaceutical air handler has its impeller mounted directly on the motor shaft. FanPedia notes that for these fans, "performance changes in the field are usually achieved by means of variable inlet vanes or VFD" (FanPedia p. 170), because a direct-drive fan has no sheaves to change.
- Options. The two field methods are a VFD on the motor or variable inlet vanes at the fan inlet.
- Constraint. In this illustrative case the owner's existing motor is not suitable for drive operation (see inverter-duty motors in Chapter 15) and the owner will not replace it.
- Airstream. The air is clean and well below 600°F.
- Choice. Variable inlet vanes with an actuator tied to the building controls. Because the air is clean, nested vanes inside the inlet funnel are an option; the engineer notes that replacing nested vanes later means replacing the inlet funnel assembly.
- Expectation. From the illustrative comparison, the vanes save about a third of the power at 70% flow, more than a damper and less than a VFD.
External Inlet Vanes¶
External inlet vanes are variable inlet vanes built in their own housing and bolted to the fan inlet, rather than nested inside the inlet funnel. FanPedia gives their application: "used for contaminated airstreams or for high temperature airstreams up to 600°F" (FanPedia p. 139). Being outside the funnel, the vane section can be removed, cleaned or replaced without replacing the funnel, which is why external vanes are the choice when the air carries dust or sticky material. They come standard with a manual handle and can be provided with an actuator, and the actuator's linkage is carried on the damper and vane bearings described above.
Speed Control¶
Speed control reduces airflow by slowing the impeller. A slower impeller produces a smaller fan curve, and the operating point moves down along the unchanged system curve. Nothing is added to the airstream and no pressure is thrown away, so speed control uses the least power of any method. The relationships between speed and performance are the fan laws, developed fully in Chapter 25: at constant air density and for the same fan, airflow changes in proportion to speed, pressure with the square of speed, and power with the cube of speed.
Fan Law for Power¶
where:
- \( BHP_1 \) and \( BHP_2 \) are brake horsepower at the original and new speeds
- \( N_1 \) and \( N_2 \) are the original and new fan speeds in RPM
There are three ways to change a fan's speed. A variable frequency drive (VFD) changes the frequency of the power supplied to an AC induction motor, so the motor and fan run at any speed within the drive's range; Chapter 15 covers VFDs and the inverter-duty motors they need. An EC motor speed controller sets the speed of an electronically commutated motor, covered in the next section. And on a belt-driven fan, changing the sheaves (pulleys) gives a fixed new speed; that is a one-time adjustment, not continuous control.
Worked Example: Slowing a Fan to 80% Airflow¶
An illustrative belt-driven fan delivers 20,000 CFM at 1,000 RPM and 4.0 in. wg, using 40 BHP, on a system with no fixed static pressure. The owner wants 80% airflow most of the time.
- Speed. Airflow is proportional to speed, so 80% airflow needs 80% speed: 0.8 × 1,000 = 800 RPM.
- Pressure. Pressure goes with speed squared: 0.8² = 0.64, so 0.64 × 4.0 = 2.56 in. wg.
- Power. Power goes with speed cubed: 0.8³ = 0.512, so 0.512 × 40 = 20.5 BHP.
- Method. Because the reduction is most of the time and may vary, a VFD is the fit; if the owner wanted a permanent 800 RPM, a sheave change would do it without a drive.
Cutting airflow by 20% cut power by almost half, which is why speed control dominates wherever reduced operation is common.
The Cube Law Feels Wrong at First
If "20% less air, half the power" seems too good to be true, you are in good company; most people need to work one example by hand before they believe it. Redo the 80% example at 70% and 50% airflow, then compare your numbers to the comparator sim above. Chapter 25 explains where real systems fall short of the ideal cube.
EC Motor Speed Controller¶
An EC motor speed controller is the device that sets the running speed of an electronically commutated (EC) motor. An EC motor uses on-board electronics, a rectifier that converts AC to DC and a controller that directs current through each winding at the right time, to drive a permanent-magnet rotor. FanPedia notes that "the speed can be controlled through a speed controller or the motor can receive a 0-10V signal from an outside source," that this allows "highly efficient operation, even at reduced speeds," and that EC motors have "a larger usable turndown range than a traditional speed controllable motor" (FanPedia p. 68). Chapter 14 introduced EC motors; this section is about setting their speed.
FanPedia describes Twin City Fan's controller for its second-generation EC motors, the GridPoint EC Motor Controller (FanPedia p. 70). Its functions are:
- Switching the motor between constant speed mode and 0-10V mode, where the motor follows an external control signal.
- Letting a user view and set the operating speed and the maximum speed of the motor.
- Plug and play connection, so one controller can be used on multiple fan and EC motor packages.
FanPedia refers readers to Twin City Fan installation manual IM-4055 for details (FanPedia p. 70).
Worked Example: Commissioning an EC Exhaust Fan¶
A building has several small direct-drive exhaust fans with second-generation EC motors. At start-up, a test-and-balance technician must set each fan to its design airflow, and later the building automation system (BAS) will vary the fans.
- Set the balance speed. The technician plugs the controller into the first fan in constant speed mode, adjusts the operating speed until the measured airflow matches the design, and records the speed.
- Cap the speed. The technician sets the maximum speed so the BAS can never drive the fan beyond the design condition, protecting against over-exhaust and noise.
- Hand over control. The technician switches the motor to 0-10V mode, so the BAS signal now sets the speed between its limits.
- Move on. Because the controller is plug and play, the technician unplugs it and repeats the steps on the next fan.
The same flow-control job on an induction-motor fan would need a VFD for each fan, while the EC motor carries its speed electronics on board and needs only the controller during setup.
Choosing a Flow-Control Method¶
Every method in this chapter answers one of three needs: stopping reverse flow when the fan is off, setting airflow once, or changing airflow while the fan runs. For the third need, the choice depends on how often and how far the airflow drops, what the airstream carries, and what the motor and budget allow. The table below gathers the sources' guidance from the sections above.
| Method | Main use | Strengths | Limits |
|---|---|---|---|
| Gravity damper (wall, ceiling, butterfly) | Prevent backdraft when off | No power or wiring; excludes weather | Needs airflow to open; may flutter at low flow |
| Manual damper | Balance a system once | Simple, low cost | Not for changing airflow in operation |
| Motorized damper | Open and close on command | Opens fully at low flow; can isolate | Needs power and controls |
| Parallel blade outlet damper | Infrequent, modest reduction | Lowest cost; hot, humid or dirty air; to 750°F | Recommended only from full open to about 75% open; small power savings |
| Opposed blade outlet damper | Modulation across the full range | Airflow follows arm movement closely; to 750°F | About 10% more cost; small power savings |
| Inlet box damper | Reduced loads on a fan with an inlet box | Pre-spin saves horsepower | Needs an inlet box; Type 2 airfoil blades |
| Variable inlet vanes | Frequent reduction on a constant-speed fan | Pre-spin saves horsepower; to 600°F | Less saving than speed control |
| VFD speed control | Frequent or continuous reduction | Lowest power at reduced flow | Needs a suitable motor and a drive |
| EC motor speed controller | Speed setting and control of EC motors | Efficient at reduced speed; wide turndown | Applies to EC motor packages |
Diagram: Flow Control Method Selector¶
Run Flow Control Method Selector Fullscreen
Flow Control Method Selector
Type: microsim
sim-id: ch18-flow-control-method-selector
Library: p5.js
Status: Specified
Bloom Level: Evaluate
Bloom Verb: recommend
Learning Objective: The learner will recommend the flow-control or backdraft device that best fits each of ten job descriptions and justify the choice by naming the deciding job condition, scoring at least 8 of 10.
Prerequisites: gravity damper, butterfly gravity damper, manual damper, motorized damper, parallel blade outlet damper, opposed blade outlet damper, inlet box damper, external inlet vanes, variable inlet vanes, speed control, variable frequency drive, EC motor speed controller, turndown, blade flutter (all defined in this chapter above).
Evidence of Mastery: For each of ten job cards, the learner selects one device from ten choices and then selects the deciding condition from three offered conditions. A card is correct when both the device and the deciding condition match Content. Mastery is 8 of 10 cards correct on the first attempt.
Misconceptions: (1) Any damper can control airflow during operation; gravity and manual dampers cannot. (2) Speed control is always the right answer; for infrequent reduction an outlet damper is the least expensive fit. (3) Parallel and opposed blade dampers are interchangeable. (4) An outlet damper saves about as much power as inlet vanes. (5) A gravity damper works at any airflow.
Instructional Rationale: Evaluate-level recommendation requires judging options against job criteria and defending the choice. Pairing each device choice with the deciding condition makes the learner state the criterion, not just match keywords, and the ten cards cover every device in the chapter so each criterion is tested once.
Content:
Device choices (the same ten for every card): A. Butterfly gravity damper; B. Wall or ceiling gravity damper; C. Manual damper; D. Motorized damper; E. Parallel blade outlet damper; F. Opposed blade outlet damper; G. Parallel blade inlet box damper; H. External variable inlet vanes; I. Speed control with a VFD; J. EC motor speed controller.
| # | Job card shown to the learner | Correct device | Deciding condition (correct) | Two wrong conditions offered | Why (shown as feedback) |
|---|---|---|---|---|---|
| 1 | A roof-mounted upblast exhaust fan lets rain and outside air in when it is off. | A | Upblast fan needs backdraft and weather protection when off | Needs airflow reduced while running; Needs a set balance point | FanPedia describes the butterfly gravity damper as integral to roof-mounted upblast fans, preventing backdraft and providing weather protection when off. |
| 2 | Wall exhaust fans in a warehouse let cold wind and snow blow in during winter, when they are off. Full airflow whenever they run. | B | Backdraft and weather exclusion on a wall fan, no wiring wanted | Continuous modulation; Low-flow flutter | Wall type gravity dampers open when the fan runs, close by gravity when it stops, and exclude rain and snow. |
| 3 | A supply fan must be set to the airflow the balancer measured at commissioning and left there. | C | Airflow set once and not changed in operation | Frequent reduction; Hot, dirty air | Manual dampers are typically used to balance a system. |
| 4 | An exhaust outlet damper opens only partway at low airflow, flutters and is noisy. | D | Blades must be powered open at low flow | Infrequent modest reduction; Fan has an inlet box | Motorized dampers power the blades open instead of relying on airflow, avoiding partial opening, flutter and noise at low flow. |
| 5 | A hot, particulate-laden process fan on a constant-speed motor needs modest trimming, between full open and about 75% open, a few times a month. | E | Infrequent, modest reduction within full open to about 75% open, dirty hot air | Modulation across the full range; Reduced operation most hours | FanPedia recommends outlet dampers for infrequent reduced operation and dirty or hot air, and parallel blades for modulation from full open to about 75% open, at the lowest cost. |
| 6 | A drying process on a constant-speed fan needs a controller to vary airflow continuously from full flow to a small fraction. The owner has ruled out a drive. | F | Modulation over the entire range with airflow following the actuator | Infrequent reduction only; Airflow set once | Opposed blade dampers are recommended for modulation over the entire range and reduce volume in closer relationship to control arm movement, for about 10% more cost. |
| 7 | A large centrifugal fan that already has an inlet box runs at reduced load often. The owner wants horsepower savings without changing the constant-speed motor. | G | Fan already has an inlet box; pre-spin saves horsepower | Backdraft prevention; Airflow set once | The inlet box damper pre-spins air in the direction of rotation, saving horsepower at reduced loads, and it fits the existing inlet box. |
| 8 | An open-inlet fan handling a contaminated airstream at 400°F runs at reduced flow most of the day; the constant-speed motor must stay. | H | Frequent reduction, contaminated air under 600°F, constant-speed motor | Airflow set once; Backdraft prevention | External inlet vanes are used for contaminated or high-temperature airstreams up to 600°F and save horsepower by pre-spin. |
| 9 | A large induction-motor fan with an inverter-duty motor runs at 60% to 80% flow for most of the year, and energy cost is the owner's top concern. | I | Frequent large reductions; lowest power wanted; motor suits drive operation | Infrequent modest reduction; Backdraft prevention | Speed control uses the least power at reduced flow, and power falls with roughly the cube of airflow. |
| 10 | Small direct-drive exhaust fans with second-generation EC motors must be set to design airflow at start-up, then follow a 0-10V signal from the building controls. | J | EC motor; set speed and maximum speed, then switch to 0-10V mode | Hot dirty air; Fan has an inlet box | The GridPoint EC motor controller switches between constant speed and 0-10V modes and sets operating and maximum speed. |
Provenance: Device descriptions and application criteria from FanPedia: butterfly, ceiling and wall gravity dampers p. 181; manual and motorized dampers and opposed blade outlet damper p. 182; parallel blade outlet damper and inlet box damper p. 183; external inlet vanes p. 139 and p. 188; EC motors p. 68; GridPoint EC motor controller p. 70. The cube relation for speed control is the standard fan law (Chapter 25). Job cards 1-10 are illustrative scenarios written for this chapter; the sim labels them "illustrative."
Rules: Each card has exactly one correct device and one correct deciding condition, as listed. The three deciding conditions on a card are the correct one and the two wrong ones listed, shown in a shuffled order. A card scores 1 only when both selections are correct on the first attempt. The device must be chosen before the condition choices appear.
Learner Activity:
- The learner reads job card 1 and selects one of the ten devices.
- The three deciding conditions appear; the learner selects the one that decided the choice.
- The learner presses Check. The sim marks the device and the condition separately and shows the "Why" text.
- The learner continues through the remaining nine cards, which appear in a shuffled order chosen at the start.
- After the tenth card, a summary lists every card, the learner's device, the correct device and the "Why" text, and highlights cards where the device was right but the condition was wrong.
Feedback: Ten cards, shuffled order, two attempts per card. Correct: "Recommended:
Starting State: A randomly chosen first job card is shown with the ten devices and the prompt "Which device would you recommend for this job?" Condition choices are hidden until a device is chosen.
Chapter Anchors: Parallel blade outlet dampers are recommended from full open to about 75% open; opposed blade dampers over the entire range at about 10% more cost; outlet dampers are available to 750°F construction; inlet vanes to 600°F; manual dampers balance a system; motorized dampers power blades open at low flow; the butterfly gravity damper is integral to roof-mounted upblast fans; the GridPoint controller switches between constant speed and 0-10V modes and sets operating and maximum speed.
Key Takeaways¶
- Every part bolted to the fan changes the pressure the fan must supply, so it belongs on the system curve before the fan is selected.
- The inlet funnel guides air into the impeller and the wiper bar reduces turbulence beside it. Type C spark resistance uses an aluminum funnel, or steel with a rubbing band and bars at 501°F and above.
- An inlet box turns air into the fan where space is tight or air enters at an angle. Integral boxes share the housing side plate; detached boxes are bolt-on (TCF's preferred design) or free-standing, and need a flex connector when fan and box sit on different planes. Positions run from 45 to 360, read from the drive side.
- Transition pieces match fan and duct shapes and sizes; a relieved inlet transition passes paper trim. An evasé lowers outlet velocity and raises static pressure capability, and some ratings include it. Companion flanges ship loose to match the fan's bolt pattern.
- Inlet and outlet silencers treat noise on the path it actually travels, and both add pressure loss.
- Flow control changes the system curve (dampers) or the fan curve (pre-spin or speed). Turndown is the controllable range.
- Outlet dampers are the cheapest and suit infrequent reduction and hot or dirty air but save little power: parallel blade from full open to about 75% open, opposed blade across the entire range for about 10% more.
- Gravity and butterfly gravity dampers stop backdraft, manual dampers balance, motorized dampers open fully at low flow, and flange-type damper and vane bearings keep them moving.
- Inlet box dampers and variable inlet vanes, including external inlet vanes for dirty air to 600°F, pre-spin the air and save more power than outlet dampers.
- Speed control saves the most, because power falls with roughly the cube of speed; a VFD drives induction motors, and an EC motor speed controller sets EC motor speed, maximum speed and 0-10V mode.
You Can Now Equip the Fan's Ends
You can now pick the inlet box and its position, decide whether an evasé or a silencer belongs on the order, and recommend a flow-control method with a reason the customer will accept, whether that is a parallel blade damper for occasional trimming or a VFD for a fan that runs at part load all year. That covers most of the conversations that happen around a fan's inlet and outlet.