Glossary of Terms¶

This glossary defines the 581 concepts in the FanPedia learning graph. Definitions follow the chapter text, which in turn follows the current Twin City Fan catalogs, FanPedia online and the FanPedia PDF.

Access Door¶

A removable or hinged cover over an opening in a fan housing, large enough for inspection, cleaning, or service of the impeller and interior.

Example: A quick-opening access door lets a mechanic clean grease from a kitchen fan wheel without removing ductwork.

Accessory Selection¶

The step of fan selection that adds the optional parts an application needs, each tied to a stated requirement and confirmed against the catalog option list.

Example: An outdoor rooftop fan's accessory selection adds a weather cover, bird screen, and backdraft damper.

Actual Operating Conditions¶

The temperature, altitude, and density at which a fan will actually run, as opposed to the standard air of the catalog table.

Example: 17,000 CFM at 2.5 in. wg, 300°F, and 3,000 ft are actual operating conditions converted before entering the table.

Adapter Sleeve Mount¶

A concentric bearing locking method in which a slit tapered sleeve is drawn into a tapered-bore bearing by a locknut, clamping it to the shaft.

Example: Spherical roller bearings on large fans are commonly installed with an adapter sleeve mount.

Adjustable Pitch Impeller¶

An axial impeller whose separate blades are clamped into the hub by their shanks so the blade angle can be reset to a different pitch.

Example: Resetting the pitch of a B Series impeller moves the fan to a new performance curve without changing diameter or speed.

Air Change Method¶

A ventilation estimate that divides a space's volume by a recommended number of minutes to replace all its air once.

Example: A 60,000 ft³ warehouse at 6 minutes per air change needs 10,000 CFM.

Air Density¶

The mass of air in one cubic foot, stated in pounds per cubic foot (lb/ft³); 0.075 lb/ft³ at standard conditions.

Example: Air at 200°F is thinner than standard air, so a fan at the same speed develops less pressure and draws less power.

Air Handling Impeller¶

The radial bladed impeller for relatively clean air, with heavy-gauge blades welded to both the back plate and the front plate.

Air Kit¶

A forward curved, double-width, double-inlet impeller assembly sold without a housing, for installation in another manufacturer's enclosure.

Example: An oven builder installs an air kit so the fan bearings stay outside the heated airstream.

Air Movement¶

The transfer of a mass of air from one place to another, driven by a difference in pressure.

Example: A lab hood exhaust fan creates air movement that carries fumes from the hood, through the ductwork, and out of the building.

Air Power¶

The rate at which a fan delivers energy to the air, calculated from airflow and fan total pressure before internal losses.

Air Pressure¶

The force that air exerts on each unit of area it touches; a fan creates the small pressure differences that move air.

Example: Sea-level atmospheric pressure is about 14.7 psi, while a fan typically adds only a few inches of water gauge above it.

Air Temperature¶

The temperature of the air at the fan inlet, in °F, which sets air density and determines whether special materials or construction are needed.

Air Velocity¶

The speed of moving air at a point, equal to airflow volume divided by the cross-sectional area the air passes through.

Example: 2,000 CFM through a 1 ft² duct moves at 2,000 fpm; the same flow through a 2 ft² duct moves at 1,000 fpm.

Airflow Continuity Equation¶

The relation stating that airflow equals cross-sectional area times average velocity, constant along a duct without branches or leaks.

Example: By the airflow continuity equation, 4,000 CFM through a 2 ft² duct moves at 2,000 fpm.

Airflow Measurement¶

The determination of the volume of air a fan actually delivers in its installed system, in CFM.

Example: A pitot tube traverse is a traditional method of airflow measurement.

Airflow Volume¶

The volume of air passing a point in a fan or duct system per unit of time, stated in the United States as cubic feet each minute.

Example: A machine shop that needs 2,400 CFM of exhaust has an airflow volume requirement of 2,400 cubic feet every minute.

Airfoil Blade Damper¶

An industrial damper (Type 2) with streamlined, wing-shaped blades that lower open pressure drop and noise.

Airfoil Impeller¶

A centrifugal impeller with backward inclined, hollow blades of airfoil cross section, giving the highest static efficiency of the centrifugal types.

Example: An airfoil impeller suits large clean-air HVAC fans, but not abrasive or sticky airstreams.

Airstream¶

The moving body of air that passes through a fan and its connected ductwork, together with the heat, moisture, dust, fibers, fumes, or vapors it carries.

Example: Arrangement 1 keeps the bearings out of the airstream, while the impeller is always in it.

Airstream Conditions¶

Everything about the gas a fan moves except its quantity: temperature range, elevation, moisture, and contents such as dust or flammable vapors.

Example: Airstream conditions of 400°F and sticky dust set both the construction package and the impeller.

Altitude¶

The elevation of a fan installation above sea level, which lowers barometric pressure and therefore air density.

Example: A fan installed in Denver, about 5,000 feet above sea level, handles noticeably thinner air than the same fan at sea level.

Altitude Correction¶

The density correction factor for a site above sea level, equal to local standard barometric pressure divided by sea-level pressure.

Example: At about 5,000 feet the altitude correction is roughly 0.83.

Aluminum Clad Construction¶

Insulated fan construction in which an outside vendor insulates the housing and covers it with a stucco-embossed aluminum skin.

Example: Aluminum clad construction requires raised access doors that sit above the insulation.

Aluminum Construction¶

Construction using aluminum, a light, corrosion-resistant, non-ferrous metal, for the whole airstream or selected fan parts.

Example: Type B spark-resistant fans typically use an aluminum impeller and rub plate.

AMCA 204 Balance Grade¶

The balance quality and vibration category, BV-1 through BV-5, a fan is built to under ANSI/AMCA Standard 204.

Example: BC impellers are balanced to BV-3, corresponding to balance quality grade G6.3.

AMCA 210 Rating Standard¶

ANSI/AMCA Standard 210 (also ASHRAE 51), the laboratory method for testing fans to rate aerodynamic performance.

Example: Catalog ratings from two manufacturers are comparable only if both tested to the AMCA 210 rating standard.

AMCA Certified Ratings¶

Published fan performance figures that AMCA has licensed the manufacturer to mark with its seal after accredited-laboratory testing and check tests.

Example: A catalog's certification statement names exactly which models and performance types carry AMCA certified ratings.

AMCA Installation Type¶

One of four test configurations defined by AMCA 210 by whether the fan inlet and outlet are free or ducted: Type A, B, C, or D.

Angular Discharge¶

Any of the four centrifugal discharge positions at 45 degrees: top angular up, top angular down, bottom angular down, and bottom angular up.

Example: A TAU discharge meets a duct that rises at an angle without a full elbow.

Annotated Fan Drawing¶

A simple side and drive-side drawing of a fan marked with arrangement, rotation, discharge, motor position, connections, and accessory callouts.

Anti-Backspin Device¶

A one-way clutch on a fan shaft that prevents the rotor from freewheeling in reverse when the fan is not running.

Example: An anti-backspin device stops wind or a parallel fan from spinning an idle fan backward.

Application Requirements¶

The facts about a job that any correct fan must satisfy: duty point, air conditions, airstream contents, space, noise and efficiency limits, power supply, and maintenance preferences.

Example: A missing application requirement, such as minimum air temperature, can cause a bad selection.

Appurtenance Effects¶

Changes in fan performance caused by accessories in or at the airstream, such as inlet boxes, inlet vanes, dampers, and screens.

Example: Adding variable inlet vanes produces appurtenance effects that require more speed and power for the same duty.

Arrangement 1¶

An overhung impeller fan with both bearings on a pedestal and the motor mounted separately on the floor or a fan base, usually belt driven.

Example: Arrangement 1 suits hot or contaminated air because its pedestal bearings are out of the airstream.

Arrangement 10¶

A belt driven, overhung fan with the motor mounted inside the bearing pedestal, more compact than Arrangement 9.

Example: An Arrangement 10 utility set with a weather cover can be installed outdoors.

Arrangement 1P Plug¶

A belt driven pedestal plug fan mounted to grade, with the motor on its own base separate from the fan pedestal.

Arrangement 2¶

An overhung impeller fan, belt or direct driven, whose bearings sit in a bracket supported by the fan housing; not shown among FanPedia's arrangement drawings.

Example: An Arrangement 2 callout may appear on older drawings or other manufacturers' equipment.

Arrangement 3¶

A center hung impeller fan with one bearing on each side of the impeller supported by the housing, and the motor mounted separately on the floor or fan base.

Example: Double width fans are generally supplied in Arrangement 3 for V-belt drive.

Arrangement 3 SI and DI¶

Center hung arrangements that add integral inlet boxes so the bearings stand outside the airstream on independent pedestals: 3SI for single width and 3DI for double width fans.

Example: An Arrangement 3DI fan handles elevated temperatures because neither bearing sits in the air path.

Arrangement 3F¶

An Arrangement 3 fan with an extended frame that carries the motor and slide base as one assembly.

Example: A single width Arrangement 3F fan is typically not suitable for isolators directly under the fan.

Arrangement 4¶

A direct drive fan with the impeller mounted on the motor shaft and the motor on a pedestal, having no fan shaft or fan bearings.

Example: Arrangement 4 needs the least maintenance, but Twin City Fan engineering does not use it for high-temperature applications; its standard ranges are typically up to about 180°F.

Arrangement 4CS and 9CS¶

The clamshell versions of axial and inline centrifugal Arrangements 4 and 9, with a hinged clamshell door giving access to the rotor assembly.

Example: Arrangement 9CS keeps the motor out of the airstream with a V-belt drive.

Arrangement 4P Plug¶

A direct drive pedestal plug fan with the impeller on the motor shaft and the fan mounted to grade rather than to the plenum wall.

Arrangement 4S Swingout¶

A modified Arrangement 4 housed centrifugal fan whose motor and impeller mount on reinforced framework that swings out with a door for access.

Example: Arrangement 4S has no motor position to choose because the motor rides on the door.

Arrangement 4SO and 9SO¶

The swingout versions of axial and inline centrifugal Arrangements 4 and 9, with the rotor assembly built on a swingout door.

Example: Arrangement 4SO keeps the compact Arrangement 4 envelope with the impeller on the motor shaft.

Arrangement 4VI and 4HI¶

Modified Arrangement 4 fans supported entirely by their inlet flange, with vertical (4VI) or horizontal (4HI) inlet airflow and shaft.

Example: A 4VI fan mounted on top of a process vessel draws air vertically into its inlet.

Arrangement 7¶

A direct drive, center hung fan whose motor is coupled to the fan shaft and mounted on a pedestal that also carries one bearing; similar to Arrangement 3 with a motor pedestal.

Example: A large double width fan that must run at motor speed is supplied in Arrangement 7.

Arrangement 7SI and 7DI¶

Direct drive, center hung arrangements with integral inlet boxes that keep the bearings out of the airstream: 7SI single width and 7DI double width.

Example: An Arrangement 7SI fan ships on a unitary base with its motor coupled to the fan shaft.

Arrangement 8¶

A direct drive, overhung fan in which the Arrangement 1 bearing pedestal is extended to carry the motor, joined to the fan shaft by a flexible coupling.

Example: Arrangement 8 keeps the impeller on fan bearings rather than motor bearings, unlike Arrangement 4.

Arrangement 8P and 9P Plug¶

Grade-mounted versions of the Arrangement 8 and 9 plug fans: 8P with the motor coupled to the fan shaft, 9P with a belt-driven motor on the pedestal.

Example: A large oven plug fan uses Arrangement 9P so the foundation, not the wall, carries the load.

Arrangement 9¶

A belt driven, overhung fan with the motor on a slide base on the side of the fan's bearing pedestal, shipped as one assembly.

Example: An Arrangement 9 fan arrives with motor and drive mounted, ready to set on its foundation.

Arrangement 9F¶

An Arrangement 9 whose frame is extended to carry a motor too large for the bearing pedestal.

Example: Arrangement 9F is not suitable for isolators mounted directly under the fan.

Arrangement 9H¶

An Arrangement 9 with the pedestal extended on one side so the motor mounts horizontally beside it, on a slide base, rails, or pivot base.

Example: Arrangement 9H is chosen when horizontal motor adjustment is preferred.

Arrangement 9ST and 9SS¶

Belt driven housed centrifugal swingout fans: 9ST with the motor on top of the bearing pedestal on a slide base, and 9SS with the motor on the side on a pivot base.

Arrangement Selection¶

The choice of the standard arrangement that places impeller, bearings, and motor where the application needs them.

Example: Hot air that bearings cannot tolerate drives arrangement selection toward Arrangement 1 or 9.

ASA Pipe Flange Bolt Pattern¶

The bolt-hole layout of a standard 125-pound-class pipe flange, written "125# ASA," now published in ASME B16.1.

Axial and Inline Arrangements¶

The arrangement numbers applied to tubular fans, both axial and inline centrifugal, including direct drive Arrangement 4 and belt driven Arrangement 9 with their access variants.

Example: An inline centrifugal fan uses the same arrangements as an axial fan but a different impeller.

Axial Blade Count¶

The number of blades on an axial impeller, with more blades generally developing more pressure at a given diameter and speed.

Example: A low-pressure ventilation fan uses a low axial blade count for high volume at low cost.

Axial Blade Tip¶

The outer end of an axial blade, nearest the housing, which moves fastest and whose clearance to the housing controls back-leakage.

Axial Fan¶

A fan in which air enters and leaves the impeller parallel to the shaft, with swirl added by the rotating blades.

Example: Tubeaxial, vaneaxial, panel, and ring fans are axial fans.

Axial Fan Configuration¶

The code stating how an inline, mixed flow, or axial fan is oriented and supported and what brackets, cap, or hood it carries.

Example: An axial fan configuration code replaces a discharge code because the fan always discharges along its axis.

Axial High Temperature Package¶

The construction details for axial fans in hot service, grouped in bands up to 300°F, with temperature limits set by aluminum impellers and in-airstream motors or bearings.

Axial Impeller¶

An impeller whose blades, attached to a central hub, move air parallel to the shaft from the leading edges straight through.

Example: Tubeaxial, vaneaxial, and panel fans all use axial impellers.

Axial Motor Positions¶

The system for locating a belt-drive motor on a tubular fan, used in place of the centrifugal W, X, Y, Z positions because a tubular housing has no scroll; read in the discharge view.

Example: A floor-mounted axial fan is limited to the upper positions G, H, A, B, and C.

Axial Rotation Convention¶

The rule that an axial fan's rotation is viewed from the discharge side and written as left hand or right hand.

Example: An axial impeller's RH designation must not be converted to CW or CCW.

Back Plate Fins¶

Small radial vanes welded to the outside of a centrifugal impeller's back plate that reduce bearing thrust load and draw air inward through the housing shaft hole; also called thrust fins or cooling fins.

Example: Back plate fins help keep a hot airstream from leaking out around the shaft.

Backdraft Damper¶

A damper that opens to let air flow out through a fan in the normal direction and closes to stop reverse flow when the fan stops.

Example: A gravity backdraft damper on a wall exhaust fan keeps cold outside air from blowing in when the fan is off.

Backward Curved Impeller¶

A centrifugal impeller with single-thickness blades curved backward from the direction of rotation.

Example: A backward curved wheel gives smoother air passage than a flat blade while tolerating some dust.

Backward Inclined Impeller¶

A centrifugal impeller with flat, single-thickness blades tilted backward so each blade's outer end trails its inner end as the impeller turns.

Example: A backward inclined impeller is non-overloading, making it a common choice for general ventilation.

Ball Bearing¶

A rolling bearing that uses steel balls as its rolling elements, giving smooth, low-friction motion.

Example: Ball bearings are the usual choice on smaller, lighter-duty fans.

Barometric Pressure¶

The absolute pressure of the atmosphere at a location, stated in inches of mercury and equal to 29.92 in. Hg at sea level under standard conditions.

Bearing¶

A machine element that supports a rotating shaft, lets it turn with little friction, and holds it in radial and axial position.

Example: Most belt-driven fans have two bearings, one fixed to locate the shaft and one floating to allow thermal growth.

Bearing Dowel Pins¶

Rods fixed to the pedestal that pass through holes drilled in the bearing base, so a removed bearing can be reinstalled in exactly the same position.

Example: Doweled bearings return to alignment after a bearing change without re-measuring.

Bearing Life¶

A statistical estimate of how long bearings will run before failing from metal fatigue, based on bearing geometry, load, and speed.

Bearing Lubrication¶

The film of grease or oil that separates a bearing's rolling elements from its raceways.

Example: A fan bearing regreased on the manufacturer's schedule reaches its rated life.

Bearing Positioner¶

A threaded bolt in a bracket on each side of a fan bearing that pushes the bearing housing in small steps to fine-tune its location; also called a bearing alignment jacking screw.

Example: An installer uses bearing positioners to square the shaft before torquing the hold-down bolts.

Bearing RTD and Thermocouple¶

Temperature sensors whose tip sits in the bearing housing and reports bearing temperature to a monitoring system, installed through a grease-fitting T or a tapped hole.

Bearing Seal¶

A seal built into the bearing housing around the shaft that keeps foreign material out and lubricant in, separate from the fan's shaft seal.

Example: The bearing seal type sets the bearing's maximum allowable speed.

Bearing Seal Speed Limit¶

The maximum shaft speed the bearing manufacturer allows for a given bearing and seal combination, which may be lower than the bearing's own speed rating.

Bearing Stop Blocks¶

Brackets or short pieces of keystock welded to a pedestal against each side of a bearing base to hold the bearing's location.

Example: Stop blocks keep a bearing from walking along the pedestal under thrust load.

Bearing Structural Support¶

The welded steel frame that carries a bearing in front of an inlet on a double inlet fan and transfers rotor weight into the housing.

Bearing Vibration Sensor¶

A sensor, usually an accelerometer, mounted on a bearing housing to monitor vibration at the fan bearings.

Example: Twin City Fan drills and taps a 1/4"-28 UNF hole with a spot face to receive a bearing vibration sensor.

Bearings Out of Airstream¶

A design condition in which a fan's bearings are located outside the air path, protecting them from heat and contamination.

Example: Arrangements 1, 3SI/DI, and 7SI/DI keep the bearings out of the airstream for hot or dirty air.

Belt Drive¶

A drive method in which sheaves on the motor and fan shafts, connected by V-belts, let the fan run at a speed different from the motor's.

Example: A belt drive lets an 1,750 RPM motor turn a fan at 1,240 RPM by choosing sheave diameters.

Belt Guard¶

An enclosure over the belts and sheaves of a belt driven fan that prevents anyone from reaching the moving drive.

Example: A spark-resistant axial fan keeps a mild steel belt guard outside the airstream.

Belt Tube¶

An enclosed passage through the side of a belt driven tubular housing that shields the belts and sheaves from the airstream; also called a belt fairing.

Example: On a belt driven vaneaxial fan, the belt tube carries the belts from the outside motor to the inside shaft.

Bi-Directional Positioner¶

A motor positioner that provides horizontal adjustment in both directions of the horizontal plane, along and across the base.

Example: A bi-directional positioner lets a mechanic correct both sheave offset and angle.

Bird and Safety Screens¶

Wire mesh or grilles fastened across fan openings: a bird screen keeps birds and debris out of outdoor-facing openings, and a safety screen keeps hands and objects away from the impeller.

Blade Flutter¶

Rapid, uneven oscillation of partly open damper blades held in position only by the airflow.

Example: Blade flutter at low flow is cured by adding a damper actuator.

Blade Leading Edge¶

The edge of an impeller blade that meets the incoming air first.

Blade Pass Frequency¶

The rate at which impeller blades pass a fixed point such as the cutoff, equal to fan speed in revolutions per second times the number of blades; abbreviated BPF.

Example: A 10-blade impeller at 1,770 RPM has a blade pass frequency of 295 Hz.

Blade Pitch Angle¶

The angle between an axial blade and the plane in which it rotates.

Example: Increasing blade pitch angle at a fixed speed moves more air and draws more power.

Blast Gate¶

A wafer-type butterfly valve that mounts to a blower's outlet flange and controls flow to full shutoff; also called a waffle damper or butterfly damper.

Example: A blast gate lets a customer shut off a pressure blower at its outlet, up to 250°F.

Blower¶

A trade term for a fan built to produce comparatively high pressure, usually a centrifugal design.

Example: A pressure blower supplying combustion air to a burner delivers high pressure at relatively low airflow.

Bolt-On Inlet Box¶

A detached inlet box bolted directly to the fan's inlet flange, with a rectangular inlet flange and a round outlet flange; TCF's preferred design.

Bolt-On Pedestal¶

A fan pedestal that bolts to the housing instead of being welded to it, so it can be removed or rotated.

Example: A bolt-on pedestal comes off so the fan fits through a smaller opening during installation.

Bolted Flange Connection¶

A rigid, airtight joint made by bolting two mating flanges through matching hole patterns, usually with a gasket.

Example: A fan outlet flange and a duct companion flange form a bolted flange connection.

Bolted Housing¶

A fan housing assembled from sections joined with bolts through mating flanges, so it can be taken apart.

Example: A bolted housing can ship in pieces and be opened later to remove the impeller.

Bottom Angular Down Discharge¶

The centrifugal discharge position, coded BAD at 225 degrees, with the outlet at the bottom of the housing pointing downward at 45 degrees; offered on single width, single inlet fans only.

Example: A CW fan with BAD discharge, seen from the drive side, points down and to the left.

Bottom Angular Up Discharge¶

The centrifugal discharge position, coded BAU at 315 degrees, with the outlet near the bottom of the housing and air heading upward at 45 degrees.

Example: A BAU outlet lets a duct leave low and climb over a walkway beside the fan.

Bottom Horizontal Discharge¶

The centrifugal discharge position, coded BHD at 270 degrees, with the outlet at the bottom of the housing pointing sideways.

Example: A fan feeding an under-floor plenum uses bottom horizontal discharge.

Brake Horsepower¶

The power actually delivered to the fan shaft to produce a given airflow and pressure, always larger than the power that ends up in the air; abbreviated BHP.

Example: A fan at 5,000 CFM and 2 in. wg might draw 2.6 BHP to produce about 1.6 hp of air power.

Buffer Strip¶

A strip placed between a corrosion-resistant housing and a mild steel pedestal when the two may not be welded together.

Butterfly Gravity Damper¶

A gravity damper of two hinged halves that fold open with airflow and drop closed when the fan stops, integral to roof-mounted upblast fans.

Bypass Air Plenum¶

A chamber on a lab exhaust fan that admits fresh outside air and mixes it with the contaminated exhaust before it reaches the impeller.

Example: When fume hoods close, the bypass air plenum opens more so the fan flow and plume velocity stay constant.

Bypass Flow¶

Fresh outside air drawn in through a bypass air plenum at a lab exhaust fan inlet, controlled by bypass dampers, which passes through the impeller.

Example: As fume hood sashes close, bypass flow increases to hold stack velocity steady.

C-Face Motor Mount¶

A motor mounting using a machined face with a pilot and tapped bolt holes on the drive end, bolted directly to a matching face on the fan.

Cabinet Ventilator¶

A small centrifugal blower, usually with a forward curved wheel, packaged in a sheet-metal cabinet for ceiling, wall, or inline mounting.

Example: An inline cabinet ventilator exhausts a small office suite through a short duct.

Capstone Fan Specification¶

The final project of this course: a complete written fan specification, with reasoning for each choice, for a realistic application.

Example: The capstone fan specification includes a complete fan description, a CSI-format section, and an annotated drawing.

Carbon Ring Seal¶

A shaft seal that surrounds the shaft with split carbon rings held in a housing at very small clearance, optionally with a purge connection between rings.

Example: A process gas fan uses a carbon ring seal where a contact seal would leak too much.

Cast Housing¶

A fan housing poured as a single metal casting rather than fabricated from sheet or plate.

Example: Pressure blowers are offered with a cast aluminum or cast iron housing.

Catalog Operating Limits¶

The published boundaries a fan selection must stay inside, such as maximum RPM by size and class, temperature limits, and limits stated in notes.

Catalog Structure¶

The standard order of sections inside a Twin City Fan product catalog, close to the same from line to line.

Example: Knowing the catalog structure of Catalog 300 makes Catalog 902 easy to navigate.

Ceiling Hung Isolation¶

Isolation in which a fan is suspended from overhead structure with spring or rubber-in-shear hangers between the structure and the fan.

Ceiling Type Gravity Damper¶

A parallel blade, end-pivoted gravity damper mounted in a horizontal plane for fans and openings in a ceiling.

Example: A ceiling type gravity damper prevents backdraft through an idle ceiling exhaust fan.

Ceiling Ventilator¶

A small exhaust fan mounted in or above a ceiling that draws room air through a grille and discharges it into ductwork.

Center Hung Impeller¶

An impeller mounted on the shaft between the two bearings, one on each side.

Example: A large double width Arrangement 3 fan uses a center hung impeller for a short, stiff shaft.

Centering Plate¶

A plate fitted where a split housing's joint crosses an opening, sealing gaps around the shaft seal and the inlet funnel or plate.

Example: A leak-tight fan with a split housing requires centering plates.

Centrifugal Fan¶

A fan in which air enters the impeller along the shaft and leaves it radially, at a right angle to the shaft.

Example: A utility set is a centrifugal fan whose scroll housing collects the radial discharge into one outlet.

Centrifugal Impeller¶

An impeller that draws air in along the shaft axis and throws it outward at a right angle to the shaft off the blade tips.

Example: Backward inclined, airfoil, forward curved, and radial wheels are all centrifugal impellers.

Centrifugal Roof Exhauster¶

A power roof ventilator with a centrifugal impeller that draws air up from the roof opening and discharges it outward through a shroud, hood, or louvers.

Example: A downblast centrifugal roof exhauster removes restroom exhaust from an office building.

Centrifugal Wall Exhauster¶

A centrifugal exhauster mounted on an exterior wall with a horizontal shaft, discharging air straight out away from the building face.

Example: A building without roof access uses a centrifugal wall exhauster at the end of its exhaust duct.

Certified Drawings¶

Order-specific drawings the manufacturer certifies as accurate for the fan being built, showing dimensions, connections, weights, and orientation as ordered.

Example: Contractors set anchor bolts from certified drawings, not catalog dimensions.

Clad Shaft¶

A mild steel shaft and hub covered with cladding of the airstream material, used on large shafts instead of a solid special-alloy shaft.

Example: A 10-inch stainless-service shaft is clad rather than machined from solid stainless.

Clamshell Fan¶

An axial or inline centrifugal fan whose casing opens on a hinged door to give full interior access without removing the ductwork.

Example: A vertically mounted inline exhaust fan uses clamshell construction so its impeller can be cleaned in place.

Class Shading in Tables¶

A catalog convention of shading, regular type, and italic type within a performance table to show which fan class each duty point requires.

Example: On the RBA 913 page, shaded cells are Class 32 and italic cells to the right are Class 45.

Clockwise Rotation¶

Impeller rotation, abbreviated CW, that an observer on the drive side sees turning the same way as clock hands.

Example: CW 90 designates a clockwise fan with top horizontal discharge.

Coatings¶

Layers such as paint or specialty protective finishes applied to fan part surfaces to protect the base material from corrosion or abrasion or to give a finish.

Example: A coating is usually the lowest-cost protection for mild steel in a mildly corrosive airstream.

Cold Start Horsepower¶

The brake horsepower a fan selected for hot air absorbs when it starts and runs on cold, denser air before the airstream reaches operating temperature.

Example: An oven fan drawing 3.7 BHP at 200°F draws 4.6 BHP on 70°F air at start-up.

Cold Start Motor Sizing¶

Selecting a motor large enough for the brake horsepower a hot-gas fan draws when it starts in colder, denser air.

Example: A fan needing 53 BHP at 500°F draws 96 BHP at a 70°F start, so a 60 hp motor would overload.

Commercial Duty Fan¶

A fan built and rated for building ventilation and HVAC service, typically handling clean air at normal temperatures with quiet operation and standard construction.

Companion Flange¶

A loose flange, round or rectangular, matching the fan's inlet or outlet flange, attached to the customer's ductwork in the field.

Example: Companion flanges ship loose so the contractor can fit them to the duct before bolting to the fan.

Complete Fan Description¶

The compact, ordered statement of every attribute the factory needs to build one fan, together with the duty point and air conditions.

Example: A complete fan description lists quantity, model, size, class, arrangement, rotation and discharge, motor position, drive, motor, construction, and accessories.

Concentric Bearing Mount¶

A bearing locking method in which the inner ring is clamped evenly around the full shaft circumference rather than pushed to one side by set screws.

Conduit Box¶

The motor's power connection point: a small enclosure on the frame where conduit-run supply wiring joins the motor leads.

Example: F2 is standard on IEC frames but a special modification on NEMA motors.

Conduit Box Position¶

The side of the motor on which the conduit box is mounted, coded F1, F2, or F3 as viewed from the drive side looking at the shaft.

Construction Class¶

This book's term for a fan's build designation: the construction choices beyond size, arrangement, rotation, and discharge that fit the housing and impeller to their airstream.

Example: A fan for solvent-laden exhaust is built to the spark resistant Type B construction class rather than standard construction.

Construction Material¶

The metal, alloy, or other material a fan part is made from, which for airstream parts is the first defense against corrosion, heat, abrasion, and sparks.

Example: Mild steel is the default construction material for shafts unless otherwise specified.

Construction Selection¶

The choice of how a fan is built to survive its duty: class, materials, thicknesses, temperature package, spark resistance, abrasion protection, and coatings.

Example: Construction selection is checked against the harshest condition the fan will meet, not the normal one.

Contaminated Airstream¶

An airstream that carries something other than clean air, such as particles, droplets, corrosive gases, flammable vapors or dusts, or hazardous fumes.

Example: Exhaust from a grinding booth is a contaminated airstream because it carries metal dust that erodes blades.

Continuous Seal Weld¶

A weld run without gaps along the full length of a seam so the joint is gas-tight.

Example: Leak-tight and RTO fans use continuous seal welds instead of stitch welds.

Cooler Box¶

An uninsulated open area built around a shaft cooler on an insulated fan so the cooler can dissipate heat.

Cooler Guard¶

A guard enclosing a spinning shaft cooler so hands and tools cannot reach it while air still moves across its fins.

Example: A high-temperature fan with a shaft cooler carries a cooler guard over the cooler.

Corrosive Airstream¶

An airstream containing gases, vapors, or mists that chemically attack fan materials, such as acids, alkalis, chlorine, or solvents.

Example: Plating-tank exhaust is often handled with a fiberglass or stainless steel fan instead of mild steel.

Corten Steel¶

A weathering low-alloy steel that forms a tight, adherent oxide layer slowing further corrosion, listed with mild steel for high-temperature fan packages.

Counterclockwise Rotation¶

Impeller rotation, abbreviated CCW, that an observer on the drive side sees turning opposite to clock hands.

Example: A CCW housing is the mirror image of a CW housing, with every discharge on the opposite side.

CSI Format Specification¶

A specification organized by the Construction Specifications Institute's MasterFormat numbering, with fans in Division 23 and a three-part section structure.

Cubic Feet Per Minute¶

The U.S. unit of airflow volume, abbreviated CFM, equal to one cubic foot of air passing a point each minute.

Example: A large industrial exhaust fan may be rated for several hundred thousand CFM.

Curb Cap¶

The base plate or frame of a roof-mounted fan that sits on and covers the roof curb.

Cutoff¶

The point near a centrifugal fan outlet where the scroll wrapper comes closest to the impeller, separating discharged air from air that would recirculate.

Example: A cutoff set too far from the impeller lets air keep circling the housing, lowering efficiency.

Damper¶

A set of one or more movable blades in a frame placed in an airstream to add resistance, shut off flow, or prevent reverse flow.

Example: Closing a damper moves the fan's operating point to the left on its curve.

Damper Actuator¶

A powered operator, electric or pneumatic, that drives damper blades to a commanded position.

Example: A damper actuator tied to the building control system modulates an outlet damper.

Damper and Vane Bearings¶

Flange bearings that support damper blade shafts and inlet vane control linkage.

Example: A seized damper bearing turns a modulating damper into a fixed one.

Damper Throttling¶

Controlling a fan's airflow by partly closing a damper to add system resistance, moving the operating point left on the fan curve.

Example: Damper throttling to 70 percent airflow saves much less power than slowing the fan.

Decibel¶

A unit equal to ten times the base-10 logarithm of the ratio of a quantity to a fixed reference quantity, used for sound levels; abbreviated dB.

Example: Two identical fans together are 3 dB louder than one, not twice the decibel value.

Density Correction Factor¶

The ratio of actual air density to standard air density, used to convert between catalog and job values of pressure and power.

Example: At a density correction factor of 0.8, a fan develops 80 percent of its catalog static pressure at the same speed.

Detached Inlet Box¶

A separate inlet box assembly that connects to the fan's inlet flange instead of sharing the housing side plate.

Example: A detached inlet box can be removed from the fan for shipping or service.

Dimensional Data Table¶

The catalog table giving approximate outside and mounting dimensions, connection sizes, and weights for each fan size, keyed by letters to a drawing.

Example: Checking dimension "A" in the dimensional data table confirms a fan fits under a beam.

Direct Drive¶

A drive method in which the impeller is mounted on, or coupled to, the motor shaft so the fan runs at motor speed.

Direct Mounted Isolators¶

Isolators placed directly under the foundation holes in a fan's own structure, with no base between, usually limited to impellers 36.5 inches or less.

Discharge Angle¶

The number of degrees from the upblast position to the actual outlet direction, measured in the direction of rotation in 45-degree steps.

Discharge Cone¶

A tapered section at the outlet end of a tubular housing that guides air leaving the impeller and vanes toward the outlet connection.

Example: A belt driven inline centrifugal fan has a discharge cone at its outlet end.

Discharge Position¶

The direction in which air leaves a centrifugal fan's housing outlet, described from the drive side.

Example: A fan feeding an overhead duct is ordered with a top horizontal discharge position.

Double Door Clamshell¶

A clamshell fan with two access doors, one wide enough for impeller removal.

Example: A double door clamshell lets an impeller be removed for rebalancing without disconnecting ductwork.

Double Wall Construction¶

Insulated fan construction in which a second structural steel outer shell surrounds the inner housing with insulation between them.

Double Width Double Inlet¶

A centrifugal configuration, abbreviated DWDI, using an impeller equivalent to two single width impellers back to back on one hub, with an inlet on each side of a wider housing.

Example: A DWDI fan in an air handler draws air into both sides and discharges it through one scroll outlet.

Down Blast Discharge Position¶

The centrifugal discharge position, coded DBD at 180 degrees, with the outlet pointing straight down.

Example: A fan on a mezzanine discharging into a chamber below uses the down blast discharge position.

Downblast Discharge¶

A fan discharge configuration that turns air leaving the impeller back down toward the roof, out and around the unit.

Example: A downblast roof exhauster's mushroom-shaped cover suits clean restroom exhaust.

Drive End Bearing¶

The bearing nearest the sheave or coupling that brings power into a fan shaft; also called the outboard bearing.

Example: On a pedestal, the drive end bearing is set 3/4 in. in from the end of the top plate.

Drive Selection¶

The choice of how the motor turns the impeller at the required speed: V-belts and sheaves, a flexible coupling, or impeller on the motor shaft, with or without a VFD.

Example: Drive selection picks a belt drive when the exact fan speed must be trimmed after startup.

Drive Service Factor¶

A multiplier of at least 1.0 applied to motor power to allow for starting loads, daily hours, and the driven machine, giving the design horsepower for selecting belts and sheaves.

Drive Train¶

The set of parts that carries power from the motor shaft to the impeller, such as a direct mount, a belt drive, or a coupling.

Duct Fan¶

A double-inlet scroll fan mounted inside an enclosure and installed as a packaged unit within a duct system.

Example: A duct fan drops into a supply duct run, drawing air into both inlets and discharging at right angles to the shaft.

Duct System¶

Everything air passes through on its way to and from a fan, including hoods, grilles, filters, coils, dampers, elbows, transitions, straight runs, and the final discharge.

Example: A dust-collection network with dozens of hoods and a baghouse is one long duct system that the fan must overcome.

Dust Collection Fan¶

The fan that moves air through a dust collector, placed on the clean side of a filter or, in cyclone service, in the dust-laden air.

Example: A backward inclined fan serves the clean side of a baghouse, while a radial bladed fan serves a cyclone.

Duty Point¶

The airflow and pressure, at a stated air density, that a specification requires a fan to deliver.

Example: "10,000 CFM at 4.0 in. wg fan static pressure, standard air" is a duty point.

DWDI Discharge Limits¶

The restriction that a double width, double inlet centrifugal fan is offered in seven discharge positions, omitting bottom angular down.

Example: A layout calling for a DWDI fan with BAD discharge must be changed or checked with the factory.

EC Motor¶

An electronically commutated motor that uses an on-board rectifier and controller to run its windings, with speed set by its own controller or a 0-10 V signal.

Example: An EC motor on a ceiling ventilator can be turned down from a building automation signal.

EC Motor Speed Controller¶

A device that sets the speed of an electronically commutated motor, either directly or from an external 0-10 V signal.

Effective Duct Length¶

The length of straight outlet duct, defined in AMCA 201 from outlet velocity and duct equivalent diameter, needed for a fan's outlet velocity profile to become uniform.

Example: An elbow placed beyond the effective duct length adds only its ordinary duct loss.

Efficiency Curve¶

A plot of a fan's efficiency against airflow at one speed, drawn on the same airflow axis as the pressure and power curves.

Example: A static efficiency curve is zero at shutoff, rises to a broad top, and falls to zero at free delivery.

Efficiency Requirements¶

A project's rules on fan energy use, such as a minimum efficiency rating, selection near peak efficiency, a motor efficiency standard, or a BHP cap.

Entrained Flow¶

Outside air pulled into a lab exhaust fan's discharge plume by the high-velocity jet leaving the nozzle or windband, without passing through the impeller.

Example: A lab fan moving 10,000 CFM may entrain several thousand CFM more into its plume.

Evasé¶

A section of ductwork attached to a fan discharge whose outlet is larger than its inlet, reducing outlet velocity and increasing static pressure capability.

Exploded View¶

A drawing of an assembly with its parts pulled apart along their assembly lines so each part is visible and labeled.

Example: FanPedia's exploded view of an upblast roof exhauster labels the motor, bird screen, impeller, shroud, and curb cap.

Explosion Proof Motor¶

A motor with external cooling like a TEFC motor whose internal parts are engineered so an arc or explosion inside cannot ignite a surrounding flammable gas or dust atmosphere.

Example: A fan exhausting a solvent room is supplied with an explosion proof motor rated for the hazardous location.

Explosive Atmosphere¶

Air containing flammable gases, vapors, mists, or combustible dusts in concentrations that a spark or hot surface could ignite.

Example: A solvent-vapor exhaust system is an explosive atmosphere, so its fan is ordered with spark-resistant construction.

External Inlet Vanes¶

Variable inlet vanes built in their own housing and bolted to the fan inlet, suited to contaminated or high-temperature airstreams up to 600°F.

Example: A dusty process fan uses external inlet vanes that can be removed for cleaning.

Fan¶

A machine that moves air by rotating a bladed impeller inside or in front of a housing, raising the air's pressure slightly.

Example: A roof exhauster, a plenum fan, and a vaneaxial fan are all fans, differing in how air passes through the impeller.

Fan Arrangement¶

A standard numbered description of how a fan's drive, impeller, bearings, and motor are put together.

Example: "Arrangement 9" tells a reader the fan is belt driven, overhung, and has its motor on the side of the bearing pedestal.

Fan Base¶

A structural frame that supports a fan, usually with its motor and drive, as one rigid, aligned assembly.

Example: Unitary, isolation, and inertia bases are the three fan base types for centrifugal fans.

Fan Class¶

A grouping of centrifugal fans by the pressure they are built to handle, which sets each size's maximum safe speed. AMCA Standard 99-2408 defines Classes I to IV (outlet velocity and static pressure limits); Twin City Fan's industrial radial catalogs use their own Classes 22, 32, and 45 (about 22, 32, and 45 in. wg), since AMCA has no class system for industrial fans.

Example: A duty beyond Class I limits requires the heavier impeller, shaft, and bearings of a Class II fan.

Fan Comparison¶

A side-by-side table of two or three candidate fans for the same duty point showing performance, construction, arrangement, drive, accessories, and risks.

Example: A fan comparison for a dryer exhaust contrasts radial bladed, radial tip, and backward curved candidates.

Fan Curve¶

A graph of one fan's pressure, power, and often efficiency against airflow at one speed and one air density.

Example: Reading up from 8,000 CFM on a fan curve gives the static pressure and BHP at that airflow.

Fan Efficiency¶

The ratio of air power to brake horsepower, calculated on either a static pressure or a total pressure basis.

Example: A fan using 10 BHP to deliver 7 hp of air power has 70 percent efficiency.

Fan Efficiency Grade¶

An AMCA Standard 205 label classifying a fan's peak total efficiency, adjusted for impeller diameter, into bands such as FEG 67 to FEG 85; abbreviated FEG.

Example: An FEG 85 fan has a more efficient aerodynamic design than an FEG 71 fan.

Fan Energy Index¶

The ratio, defined in AMCA Standard 208, of a reference fan's electrical input power to the actual fan's input power at the same airflow and pressure; abbreviated FEI.

Example: A fan with an FEI of 1.10 uses less power than the reference fan at that duty point.

Fan Family¶

A group of fans that share the same airflow path through the impeller relative to the shaft: centrifugal, axial, or mixed flow.

Fan Family Selection¶

The first step of fan selection: choosing the product line and impeller type before choosing a size.

Example: Fan family selection for a dusty cyclone system points to a radial bladed fan.

Fan Grounding Device¶

A stud, lug, or pad on the fan housing to which a grounding conductor is connected, bonding the fan to plant ground so static charge cannot build up.

Example: A spark-resistant dust fan carries a stainless steel grounding stud with lug.

Fan Housing¶

The stationary shell that encloses the impeller, guides air from inlet to outlet, and supports the inlet, outlet, framing, and structural supports; also called the side, scroll, or casing.

Fan Inlet¶

The opening where air enters a fan, together with the parts that guide air into the impeller.

Example: On a plenum fan the fan inlet is the inlet funnel itself, open to the surrounding cabinet.

Fan Installation¶

Everything that puts a selected fan into service in a particular place: its structure, base, isolation, duct connections, weather protection, and monitoring devices.

Example: A rooftop fan installation includes the curb, isolators, flexible connectors, and weather cover.

Fan Laws¶

Proportionality rules predicting how a fan's airflow, pressure, and power change with speed, size, or air density for geometrically similar fans at the same point of rating.

Fan Leakage¶

Air or other gas leaking into or out of a fan housing, in the direction set by whether the housing is above or below atmospheric pressure.

Example: Fan leakage of toxic process gas through a shaft opening can endanger workers.

Fan Motor¶

The machine that converts electrical energy into the mechanical rotation that turns a fan's impeller.

Example: An induction motor driving a fan through belts and sheaves is the most common fan motor setup.

Fan Outlet¶

The opening where air leaves a fan, together with the parts that connect it to downstream ductwork or open air; also called the discharge.

Example: On a housed centrifugal fan the outlet is the rectangular opening at the end of the scroll, just past the cutoff.

Fan Outlet Area¶

The inside cross-sectional area of a fan's discharge opening, in square feet, as defined by the manufacturer for rating and outlet velocity.

Fan Pedestal¶

The steel structure that supports a fan's bearings, shaft, and often motor, providing the stability needed for balance and vibration limits.

Fan Performance¶

The complete statement of what a fan does at one set of conditions: airflow, pressure, speed, power, and the air density they assume.

Example: "10,000 CFM at 3 in. wg, 1,150 RPM, 7.5 BHP, standard air" is a complete fan performance statement.

Fan Power Equation¶

The relation stating that a fan's shaft power equals airflow times pressure divided by a units constant and by efficiency.

Example: BHP = CFM × in. wg / (6,356 × efficiency).

Fan Selection¶

The process of turning a described application into one fan—family, model and size, impeller, arrangement, drive, construction, motor, and accessories—each chosen for a stated reason.

Example: Fan selection for a dryer exhaust settles the impeller, arrangement, and high-temperature package together.

Fan Selection Checklist¶

A standing list of every value needed to select, describe, and order a fan, filled in for each job before selection starts.

Example: A fan selection checklist records duty point, air temperature, altitude, airstream contents, and space limits.

Fan Selection Range¶

The part of a fan's performance table or curve where it runs stably near its best efficiency, right of the stall region and not so far right that efficiency falls away.

Example: Catalog 902 underlines maximum static efficiency so the reader can stay within the fan selection range.

Fan Selector Software¶

Twin City Fan's downloadable selection program that computes certified performance and supplies drawings and accessory data the printed catalog omits.

Example: Fan Selector software gives maximum safe speeds derated for elevated temperature.

Fan Shaft¶

The steel bar that carries the impeller and transmits torque from the drive to it, forming the rotor assembly together with the impeller.

Example: A belt-driven centrifugal fan's shaft carries the impeller on one end and the fan sheave on the other.

Fan Size¶

The number Twin City Fan uses to identify one impeller diameter within a model, conforming for industrial fans to AMCA Standard 2402.

Example: In "RBA 913," 913 is the fan size, and the catalog page states the impeller diameter it stands for.

Fan Sound¶

The airborne noise a fan generates, mainly from blades passing the cutoff and from turbulence, and to a lesser degree from bearings, belts, and motors.

Example: A fan selected well off peak efficiency makes more fan sound than one near the peak.

Fan Specification¶

A written document telling bidders what a fan must do and how it must be built, tested, documented, and installed, so any compliant product is acceptable.

Example: A fan specification names a basis of design and may allow comparable products.

Fan Speed¶

The rate at which a fan's impeller rotates, stated in revolutions per minute (RPM).

Example: Raising a fan from 1,000 to 1,100 RPM raises airflow about 10 percent, pressure about 21 percent, and power about 33 percent.

Fan Static Pressure¶

Fan total pressure minus fan outlet velocity pressure, the AMCA pressure most catalog tables use; abbreviated FSP.

Fan Total Pressure¶

The rise in total pressure across a fan: total pressure at the outlet minus total pressure at the inlet; abbreviated FTP.

Example: Fan total pressure counts both the static and velocity energy the fan adds to the air.

FanPedia vs Current Catalog¶

The rule that when FanPedia and a current product catalog disagree, the current catalog governs because it describes what the factory builds and certifies now.

Example: For a maximum speed or a size range, check the current catalog rather than FanPedia.

Fans in Parallel¶

Two or more fans drawing from and discharging into a common system side by side, whose airflows add at any given pressure.

Example: Two identical fans in parallel on a low-resistance system nearly double the airflow.

Fans in Series¶

Two or more fans that move the same air one after another, whose pressures add at any given airflow.

Example: A multi-stage blower is a series arrangement inside one housing.

Feet Per Minute¶

The fan-industry unit of air velocity, abbreviated fpm, giving the distance in feet that air travels in one minute.

Example: Ventilation ductwork typically runs at 1,500 to 3,000 fpm, while dust-conveying ducts often run at 4,000 fpm or more.

Fiberglass Axial Impeller¶

An axial impeller whose blades, and sometimes hub, are molded from fiberglass-reinforced plastic for corrosive airstreams.

Example: An FG4 impeller has fiberglass blades on a fiberglass hub, while an F4 uses an aluminum hub.

Fiberglass Fan¶

A fan whose housing, and often impeller, is molded from fiberglass-reinforced plastic so it can handle moist, corrosive, or chemically contaminated air.

Example: A fiberglass fan exhausts a chemical plating line that would corrode carbon steel.

Fiberglass Hub¶

An axial impeller hub molded from fiberglass instead of cast aluminum, matched to fiberglass blades in corrosive airstreams.

Example: The FG4/FG6 impellers use a fiberglass hub; the F4/F6 use aluminum.

Filtered Supply Fan¶

A roof-mounted centrifugal fan unit that draws outside air through filters and delivers it into a building as make-up air.

First Critical Speed¶

The lowest rotating speed at which a shaft and impeller assembly's natural bending frequency matches its rotation, amplifying unbalance into severe vibration.

Example: Shafts sized for a first critical speed of at least 1.43 times the class maximum speed run safely below it.

Fixed Bearing¶

The bearing that holds a shaft in axial position relative to the housing; also called the non-expansion, thrust, or locating bearing.

Example: On a two-bearing fan shaft, only the fixed bearing resists end-to-end shaft movement.

Fixed Pitch Impeller¶

An axial impeller whose blades are set at one angle that cannot be changed, built cast or fabricated.

Example: The TCTS and TCTA impellers are fixed pitch types.

Fixed-Floating Bearing Rules¶

FanPedia's guidelines for where the fixed bearing goes and whether a floating bearing is needed.

Flange Mount Bearing¶

A bearing in a flanged housing that bolts to a surface perpendicular to the shaft axis.

Example: Some axial fans carry the shaft on flange mount bearings bolted to an internal plate.

Flat Blade Damper¶

An industrial outlet damper (Type 1) with plain flat blades, offered on parallel and opposed blade outlet dampers alongside the airfoil blade (Type 2) version.

Flexible Connector¶

A short section of flexible material joining a fan's inlet or outlet to the ductwork that reduces vibration transmission and allows some misalignment; also called an expansion joint.

Example: Every fan on spring isolators needs flexible connectors at its inlet and outlet.

Floating Bearing¶

The bearing that supports a shaft radially but lets it move axially so the shaft can grow or shrink with temperature; also called the expansion bearing.

Example: As a hot-gas fan's shaft lengthens, the floating bearing's outer ring slides within its housing.

Floor Mounted Isolation¶

Isolation in which isolators sit beneath the fan or its base, bearing on the floor, a housekeeping pad, or a support frame.

Flow Control¶

Any means of changing the airflow a fan delivers while the system runs, such as dampers, variable inlet vanes, or speed control.

Example: A boiler at half load uses flow control to reduce combustion air.

Flow Control Efficiency¶

A measure of how much of the possible power saving a flow-control method delivers when airflow is reduced.

Example: At 70 percent airflow, an outlet damper needs about 86 percent of design power while speed control needs about 34 percent.

Flue Gas Recirculation¶

The practice, abbreviated FGR, of returning part of a combustion system's exhaust gas to the combustion air, commonly to lower flame temperature and emissions.

Example: A burner fan with an FGR connection on its inlet box handles hot recirculated gas.

Foot Mounted Motor¶

A motor supported by feet on the bottom of its frame that bolt to a motor base, slide base, or pedestal.

Example: Nearly every belt-driven fan uses a foot mounted motor on an adjustable base.

Forged Shaft¶

A shaft formed from hot metal by impact or pressure and then machined, used on very large fans with large diameters and several turndowns.

Example: A large Arrangement 3DI fan in a cement plant may use a forged shaft.

Forward Curved Impeller¶

A centrifugal impeller with many short, shallow blades whose tips curve toward the direction of rotation.

Example: A forward curved impeller in a cabinet ventilator moves its airflow at low speed in a compact size.

Free Delivery¶

The airflow a fan moves when discharging against zero static pressure, with nothing connected to resist it.

Example: A propeller fan in an open wall operates near free delivery.

Free-Standing Inlet Box¶

A detached inlet box mounted separately from the fan and fully supported at the floor, connected by a round flange or a collar and rubber sleeve.

Friction Shaft Seal¶

FanPedia's standard shaft seal: a ring of soft material clamped by a cover plate against the housing so it rubs lightly on the shaft; also called a tacky cloth seal.

Example: A friction shaft seal in a hot-air fan may use ceramic felt instead of tacky cloth.

Gas Purge Seal¶

A shaft seal with a connection between its sealing rings through which a clean purge gas is supplied, used when even small leakage is unacceptable.

Example: A fan handling toxic gas uses a gas purge seal fed with clean purge gas so process gas cannot escape along the shaft.

Gasketing¶

Sealing material placed in bolted connections such as flanges, access doors, inlet funnels, and housing splits to prevent leakage.

Example: Nominally leak-tight construction calls for double gasketing on all connections.

Gravity Damper¶

A damper whose blades are pushed open by fan airflow and close by their own weight when the fan stops, preventing backdraft without an actuator.

Example: A wall exhaust fan's gravity damper drops shut when the fan is switched off.

Gravity Relief Ventilator¶

A weather-protected roof opening with no fan that lets air leave or enter a building under natural pressure differences.

Grease Fitting¶

A small threaded nipple with a spring-loaded check ball, screwed into a bearing housing so a grease gun can add grease; also called a zerk.

Example: Lube lines extend the grease fitting of a hard-to-reach bearing to the outside of a guard.

Guide Vanes¶

Fixed, curved blades mounted in a housing downstream of an axial impeller that remove swirl and turn the air to flow straight along the duct.

Example: The guide vanes of a vaneaxial fan recover swirl energy as static pressure.

Hazardous Location Motor¶

A motor built and labeled for a hazardous location classification, in FanPedia's terms a DFG motor with an explosion proof enclosure.

Example: A grain-dust exhaust fan carries a hazardous location motor rated for Group G dust.

Heat Shield¶

A plate mounted between a hot surface and a component it protects so heat from the housing does not reach that component directly.

High Efficiency Centrifugal Fan¶

A housed industrial centrifugal fan with an airfoil or backward curved impeller for high airflow at moderate to high pressure where power cost matters.

Example: A high efficiency centrifugal fan serves the clean side of a large process system.

High Expansion Bearing¶

A bearing and shaft modification, offered for air kits only, that accepts more thermal growth of the shaft than a standard floating bearing.

Example: An air kit in a hot oven uses a high expansion bearing so shaft growth does not load the bearings.

High Moisture Construction¶

A modification for Arrangement 9 axial fans that seals the path between a wet airstream and the drive where steam or condensation may collect.

High Temperature Construction¶

Special construction for fans whose airstream is hot enough to weaken materials, expand parts, and overheat bearings.

Note: Twin City Fan engineering does not use direct drive Arrangement 4 for high-temperature applications; high-temperature construction goes on belt driven or pedestal-mounted arrangements such as 1, 8, 9 and 10.

Example: A kiln recirculation fan at 800°F needs high temperature construction with a shaft cooler.

High Temperature Grease¶

Bearing grease formulated to keep its consistency and lubricating film at elevated bearing temperatures.

Example: High temperature axial fans require high temperature grease over 275°F.

High Temperature Package¶

A set of construction details grouped by airstream temperature band, setting materials, pedestal, shaft cooling, and bearing requirements.

Example: A fan handling 500°F air falls in a different high temperature package than one at 300°F.

High Temperature Pedestal¶

A fan pedestal built to limit heat conducted from the housing into the bearings and motor, using a high temp angle, insulated panel, or separated design.

Hooded Roof Exhauster¶

A centrifugal roof exhauster topped by a weatherhood that turns discharged air down and outward around the base.

Example: The DCLH and BCLH are hooded roof exhausters.

Horizontal Split Housing¶

A split housing cut along its horizontal centerline so the upper half lifts off the lower half; FanPedia's standard split.

Example: Size 807 and larger fans may be horizontally split by the shop for shipping.

Horsepower¶

A unit of power equal to 33,000 foot-pounds of work per minute, or 0.7457 kilowatt, used to rate fan input power and motor output.

Example: A 10 hp motor delivers about 7.5 kW of shaft power.

Hot Airstream¶

An airstream whose temperature is high enough to change fan performance or to call for construction beyond the standard catalog build.

Example: An oven exhaust at 600°F is a hot airstream that may require high temperature construction with a shaft cooler.

Housed Centrifugal Fan¶

A centrifugal fan whose impeller turns inside a scroll housing that collects the air and discharges it through a single outlet at a right angle to the shaft.

Example: An SWSI industrial process fan with a bolted duct connection on its rectangular outlet is a housed centrifugal fan.

Housed Spring Isolator¶

A spring isolator with one or more springs inside an enclosed housing, typically 1 or 2 inches deflection.

Example: A housed spring isolator protects the coil from damage on a plant floor.

Housing Access Panel¶

A whole removable side of a fan housing, larger than an access door, that exposes the impeller, motor, and drive.

Example: A square inline fan has housing access panels removable on three sides.

Housing Drain¶

A pipe connection at the lowest point of a fan housing that lets collected moisture be piped away or closed with a plug or valve.

Housing Framing¶

Structural steel angles or channels fastened to the outside of a fan housing to stiffen the sheet metal and give the fan feet.

Example: A plenum fan's frame holds the inlet panel and motor where a housed fan uses housing framing.

Housing Side Plate¶

One of the two flat sides of a scroll housing, which carries the inlet opening and often the bearing supports.

Example: On a DWDI fan, both housing side plates carry inlets and bearing supports.

Hub on Discharge Side¶

An axial impeller orientation in which the hub faces the leaving air and air passes over the motor before the impeller (MP airflow).

Hub on Intake Side¶

An axial impeller orientation in which the hub faces the incoming air and air passes through the impeller before the motor (PM airflow).

Impeller¶

The rotating, bladed part of a fan that adds energy to the airstream; also called the wheel, and on axial fans the propeller or rotor.

Example: Replacing a forward curved impeller with a backward inclined one changes the fan's curve and power characteristics.

Impeller Airflow Path¶

The route air takes through an impeller's blades: axial in and radial out for centrifugal, along the shaft for axial, and diagonal for mixed flow.

Example: The impeller airflow path is what distinguishes the three fan families.

Impeller Back Plate¶

The solid disc on the closed side of a centrifugal impeller to which the blades are welded and which carries blade loads to the hub.

Example: On an SWSI impeller, air enters through the shroud and is forced outward by the back plate.

Impeller Balancing¶

The process of measuring an impeller's mass unbalance, usually on a balancing machine, and correcting it by adding or removing small weights.

Example: A wheel with grease buildup goes out of balance and must be cleaned and rebalanced to stop vibration.

Impeller Blade¶

One of the surfaces arranged evenly around an impeller's axis that pushes on the air; blade shape, angle, and number set most of the impeller's performance.

Example: A blade that curves back away from the direction of rotation identifies a backward curved impeller.

Impeller End Ring¶

A flat ring joining the blade ends at the face of an impeller; also called an end rim.

Example: End rings hold the many short blades of a forward curved wheel in a drum shape.

Impeller Gusset¶

A triangular or trapezoidal plate welded where a blade meets the hub or back plate to brace the blade against bending.

Impeller Hub¶

The thick central piece that connects an impeller to the shaft and transmits torque to the blades.

Impeller Material¶

The material from which an impeller's blades, hub, and plates are made, controlling corrosion resistance, temperature limit, and spark resistance.

Example: Type B spark resistance requires a non-ferrous impeller material such as aluminum.

Impeller Orientation¶

The position of an impeller on its shaft relative to the bearings or motor, stated for axial fans by whether the hub faces the incoming or the leaving air.

Example: An order must state impeller orientation because it sets which way air passes the motor.

Impeller Selection¶

The choice of impeller type that the airstream and duty allow, trading efficiency against tolerance for dirty air.

Example: Moderately sticky, abrasive dryer dust rules out airfoil and backward curved impellers in impeller selection.

Impeller Shroud¶

The ring-shaped plate on the inlet side of a centrifugal impeller that closes the blade passages; also called the impeller front plate.

Example: The large center hole in the impeller shroud is where air enters the wheel.

Impeller Tie Rod¶

A rod running across the width of an impeller, usually between end rings, that ties them together and resists blade spreading.

Example: A long, narrow-bladed forward curved wheel needs tie rods.

Impeller Width¶

The axial distance across a centrifugal impeller's blades, from back plate to front plate, which governs how much air passes at a given diameter and speed.

Example: Pressure blowers use narrow impeller widths, while plenum fans can vary width to match direct drive speeds.

Inches of Water Gauge¶

The unit used to state fan and duct pressures, abbreviated in. wg, equal to the pressure that supports a column of water one inch high in a manometer.

Example: A fan rated at 2 in. wg static pressure can push air against resistance that would lift a water column two inches.

Induction Motor¶

An AC motor in which rotor current is induced by the magnetic field of the stator winding, needing no brushes or slip rings.

Example: A four-pole induction motor on 60 Hz power runs a little below its 1,800 RPM synchronous speed.

Industrial Duty Fan¶

A fan built for process and plant service where the airstream may be hot, dusty, wet, corrosive, or explosive and continuous operation is required.

Example: A radial bladed fan conveying sawdust in a mill is an industrial duty fan.

Inertia Base¶

An isolation base with reinforcing rods that the customer fills with concrete, adding mass and stiffness to keep long assemblies aligned.

Example: A long direct-drive Arrangement 8 fan sits on an inertia base to limit deflection.

Inlet Box¶

A sheet-metal enclosure on a fan inlet that receives air from the side, at an angle, or from above or below and turns it into the inlet with minimal loss.

Example: An inlet box lets the duct approach a fan from the side where space prevents a straight inlet run.

Inlet Box Damper¶

A parallel blade damper mounted in an inlet box that, when partly closed, pre-spins air in the direction of impeller rotation to save power at reduced loads.

Example: An induced-draft fan with an inlet box uses an inlet box damper for flow control.

Inlet Box Horn¶

An extension on an integral inlet box that carries the inlet connection out past the fan where inlet and discharge airflows would otherwise intersect.

Inlet Box Mounting Planes¶

The relationship between the mounting surfaces of a fan and its detached inlet box: the same plane or different planes.

Inlet Box Position¶

The direction that the opening of a centrifugal fan's inlet box faces.

Example: An inlet box position facing down lets the inlet duct come up from below the fan.

Inlet Collar¶

A short sleeve at a fan inlet that provides a slip-fit duct connection.

Example: A round duct slides over the inlet collar and is fastened with sheet-metal screws.

Inlet Flange¶

A flat ring or frame around a fan inlet, drilled with a bolt-hole pattern, so a duct or accessory can be bolted face to face to the fan.

Example: A companion flange on the duct bolts to the inlet flange for a rigid, sealable joint.

Inlet Funnel¶

The aerodynamically shaped piece, flanged to mount on the housing, that guides air smoothly into the impeller; also called an inlet cone, funnel, or inlet bell.

Example: Too large a gap between the inlet funnel and the impeller lets air leak back to the inlet and lowers performance.

Inlet Silencer¶

A duct section lined with sound-absorbing material that mounts on a fan inlet to reduce noise coming out of it.

Example: A blower drawing from an open room gets an inlet silencer to quiet the inlet roar.

Inlet System Effect¶

Loss of fan performance caused by inlet conditions that keep air from entering the impeller smoothly and evenly.

Example: An elbow bolted directly to the inlet crowds air to one side and causes inlet system effect.

Inlet Venturi¶

A smooth, bell-shaped entry piece mounted on an open fan inlet or on the free end of an inlet silencer so unducted air enters with little turbulence; it shares the inlet funnel's alternate names but is a separate entry piece, not a funnel construction.

Inline Centrifugal Fan¶

A fan that places a centrifugal impeller in a housing that lets air pass straight through in line with the duct, in tubular or square form.

Inline Configuration¶

A code describing an inline, mixed flow, or axial fan's orientation: horizontal or vertical axis, airflow direction, and means of support.

Example: HCH designates a horizontal, ceiling-hung inline fan; VUI-type codes designate vertical-up configurations.

Inline Horizontal Configuration¶

An inline fan configuration with a horizontal axis, coded HOR (no brackets), HCH (ceiling hung), or HBM (base mount).

Inline Lab Exhaust Fan¶

A lab exhaust fan built on a tubular inline centrifugal or mixed flow fan, so air moves straight through the housing.

Inline Motor Positions A to H¶

The eight belt-drive motor locations around the casing of a horizontal Arrangement 9 inline, mixed flow, or axial fan, lettered from A at the top as seen in the discharge view.

Inline Vertical Configuration¶

An inline fan configuration with a vertical axis and air moving up or down, described by ten codes beginning VU, VD, or VR.

Inspection Port¶

A small, closable opening in a fan housing for viewing the interior or inserting a light, gauge, or probe without opening an access door.

Example: A technician checks impeller buildup through an inspection port during a short shutdown.

Insulated Bearing¶

A bearing whose outer race is coated with electrically insulating material so shaft current cannot pass through it to the frame.

Example: An insulated bearing protects a VFD-driven motor from raceway pitting.

Insulated Fan¶

A fan whose housing is covered with insulation and an outer skin to isolate the fan surface from high temperature, condensation, or sound.

Example: A fan handling cold outside air in a warm building is insulated to prevent condensation.

Insulated Inlet Box¶

An inlet box built with insulation for high-temperature or sound applications, matching the fan housing's insulated construction.

Example: An insulated fan with an uninsulated inlet box would leave a hot, noisy surface.

Insulated Jacket¶

A removable insulating blanket wrapped around the fan housing that can be opened to reach access doors, drains, and other features.

Example: A 2-inch insulated jacket ships in labeled pieces for field installation.

Insulation Class¶

A letter (A, B, F, or H) rating how hot a motor's winding insulation may run continuously, with later letters allowing higher temperatures.

Insulation Pins¶

Pins attached to the outside of a fan housing that hold field- or vendor-applied insulation in place.

Example: Insulation pins are optional on high temperature centrifugal fans.

Insulation Plug¶

An insulated section mounted with a plug fan's panel that fills the plenum wall opening around the fan so the wall keeps its insulating value.

Integral Inlet Box¶

An inlet box built into the inlet side of the fan housing, sharing the housing side plate and supported by the fan; also called an attached inlet box.

Example: An integral inlet box arrives assembled with the fan and needs no separate foundation.

Internally Isolated Fan¶

A fan that carries its own isolators inside the unit between the rotating assembly and its outer structure, needing no external isolation.

Example: Many commercial roof ventilators are internally isolated fans.

Interpolation¶

The estimation of RPM and BHP between printed table values by assuming a straight-line change between neighboring rows or columns.

Example: A duty halfway between two airflow rows takes an RPM halfway between the two printed RPMs.

Inverter-Duty Motor¶

An induction motor designed and rated for operation on a variable frequency drive, with insulation and construction that withstand its switched voltage pulses.

Example: A fan to be controlled by a VFD is specified with an inverter-duty motor per NEMA MG-1 Part 31.

Isolation Base¶

A heavy-duty structural channel base supporting fan, motor, and drive, built to sit on spring or rubber-in-shear (RIS) isolators (FanPedia p. 131). Twin City Fan engineering confirms neoprene pads can also be used under an isolation base (FanPedia p. 133), though it is uncommon; RIS mounts are themselves a pad-type isolator.

Example: A fan on an isolation base needs flexible connectors at its inlet and outlet.

Isolation Rails¶

Pairs of flat bars with isolators between them, the top bar bolted to the fan and the bottom bar to the structure.

Example: Isolation rails give springs a clean place to sit under a fan's foundation pattern.

Isolator Deflection¶

The distance a vibration isolator compresses under the weight it carries.

Example: Most isolators Twin City Fan uses have deflections under 1 in., 1 in., or 2 in.

Isolator Mounting Angle¶

A structural angle mounted between a fan and its isolators, used where the fan has no structural support of its own for an isolator to bear on.

Example: An isolator mounting angle supports the inlet side of a fan that lacks framing there.

Isolator Mounting Method¶

The placement of vibration isolators between a fan and its structure, such as directly under the fan, on rails, on mounting angles, on a base, or hung from the ceiling.

K Mount Motor¶

A motor mounted directly on the scroll of a centrifugal fan housing on an adjustable base, located by a clock position; also called a piggyback or scroll mounted motor.

Example: An Arrangement 3 fan with a K mount motor at 12 o'clock saves floor space.

Keystock¶

Bar stock cut into keys that sit half in a shaft keyway and half in a mating keyway, connecting the shaft to a hub, bushing, coupling, or sheave.

Example: A stepped piece of keystock is used only where a square key will not fit.

Kitchen Exhaust Fan¶

A fan that removes hot, moist, smoky, grease-laden air from cooking equipment and discharges it away from building surfaces, listed for grease service.

L-10 Bearing Life¶

The operating hours at which 10 percent of a population of identical bearings at a given speed and load are expected to have failed.

Example: Most TCF fan models offer L-10 life of 40,000 hours.

L-50 Bearing Life¶

The operating hours at which 50 percent of a bearing population is expected to have failed; also called average life, equal to five times L-10 life.

Lab Exhaust Arrangements¶

The arrangements offered on laboratory fume exhaust fans, each tied to particular models.

Example: A lab exhaust arrangement is selected together with the model, since each model is offered only in certain arrangements.

Lab Exhaust Bypass Damper¶

A damper controlling outside air admitted through a lab exhaust fan's bypass air plenum, opening as exhaust demand drops to hold fan flow steady.

Example: When fume hood sashes close, the lab exhaust bypass damper opens.

Lab Exhaust Isolation Damper¶

A damper between a lab exhaust plenum and an individual fan that closes when that fan is off to prevent backflow and allow service.

Lab Exhaust Total Flow¶

The full volume leaving a lab exhaust stack as a plume: contaminated air plus bypass flow plus entrained flow.

Laboratory Fume Exhaust Fan¶

A fan that removes contaminated air from fume hoods and discharges it at high velocity in a tall plume diluted with entrained clean air.

Example: A lab building uses mixed flow laboratory fume exhaust fans with nozzles and windbands on its roof.

Labyrinth Seal¶

A non-contact bearing seal using a winding path of close-clearance grooves between rotating and stationary rings.

Example: A labyrinth seal adds no friction, so it suits higher-speed spherical roller bearings.

Lip Shaft Seal¶

A shaft seal using an elastomer lip, held in a seal housing on the fan housing wall, that presses against the turning shaft and is grease lubricated; also called a grease seal.

Example: A lip shaft seal with a Buna-N lip is standard, with Viton as the higher-performance option.

Louvered Roof Exhauster¶

A centrifugal roof exhauster with a louvered penthouse that deflects air down and outward through fixed weather louvers.

Example: The DCLP and BCLP are louvered roof exhausters.

Low Profile Roof Ventilator¶

A propeller-type roof ventilator built to sit close to the roof deck, offered in upblast and hooded styles.

Example: LUD and LUB low profile upblast units suit roofs where a tall fan would be visible from the street.

Mancooler¶

An axial fan packaged to blow air directly on workers in a hot industrial space, with inlet and outlet screens as standard.

Example: A swivel-mounted mancooler cools workers at a foundry pouring station.

Manual Damper¶

A damper positioned by hand, typically with a handle or locking quadrant, used to set airflow once during system balancing.

Example: A test-and-balance contractor locks a manual damper at the position that gives design airflow.

Material Handling Impeller¶

An open radial impeller with a full back plate, designed to convey long, stringy, or fibrous solids without the material hanging up on or wrapping around the blades and spiders.

Example: The catalog's back plate wool impeller (RBW) handles threads from a buffing system exhaust.

Material Specifications Table¶

The catalog table listing for each size and class the housing gauges, shaft diameter, bearing type by arrangement, and bare fan weight.

Example: In Catalog 300's material specifications table, SR denotes spherical roller bearings in split pillow blocks.

Maximum Operating Temperature¶

The highest airstream temperature at which a fan, or one of its parts, materials, or construction packages, is rated to run.

Example: A fan's overall maximum operating temperature is set by its lowest-rated element, such as its coating.

Maximum Safe Speed¶

The highest RPM at which the manufacturer allows a particular fan size and construction to operate, set by rotating-part strength, critical speed, and bearing limits.

Example: A fan's maximum safe speed is reduced for high-temperature service because hot steel is weaker.

Minimum Conveying Velocity¶

The lowest average duct velocity that keeps a given material suspended in the airstream so it does not settle and clog the duct.

Example: Grinding dust has a minimum conveying velocity of about 3,500 to 4,000 fpm.

Mixed Flow Fan¶

A fan whose impeller discharges air on a diagonal, partly along the shaft and partly outward, combining axial and centrifugal flow.

Example: A mixed flow lab exhaust fan sits in a straight tubular housing yet develops more pressure than an axial fan of similar size.

Mixed Flow Impeller¶

A hybrid axial-centrifugal impeller whose airfoil blades taper from front plate to hub, discharging air partly along and partly away from the shaft.

Example: A mixed flow impeller lets an inline fan develop more pressure than an axial fan of similar size.

Model Code¶

The short letter designation that identifies a Twin City Fan product line and impeller type.

Example: BC-SW, RBA, and TCTA are model codes; adding size, class, and arrangement names one buildable fan.

Motor Base¶

The structure that supports a motor on a fan or its base and, on a belt drive, lets the motor move to set belt tension and alignment.

Example: Slide bases and pivot bases are two kinds of motor base.

Motor Base Saddle¶

A curved support shaped to a cylindrical fan housing on one side that carries a flat motor base on the other.

Example: A belt-driven vaneaxial fan's motor base is bolted to a motor base saddle on the tube.

Motor Cover¶

A hood placed over a fan motor to protect it from rain, snow, and sun; also called a weather cover.

Example: A downblast roof exhauster's motor cover sits on top of the unit above the motor housing.

Motor Efficiency Class¶

A named band of minimum efficiencies a motor must meet, such as Twin City Fan Standard or Premium, or IEC IE1 through IE4.

Motor Enclosure¶

The casing style around a motor's internal parts, which determines how the motor rejects heat and how well its windings are protected.

Example: ODP, TEFC, explosion proof, TEAO, and TENV are the five motor enclosures Twin City Fan uses.

Motor Intended Duty¶

A designation of the operating conditions a motor is built for based on its surrounding environment, separate from its enclosure.

Example: SD in a motor line indicates a severe duty intended duty rating.

Motor Nameplate¶

The metal plate fixed to a motor frame that identifies its manufacturer, serial number, and technical characteristics.

Example: A service call usually starts with a photo of the motor nameplate.

Motor Position¶

The location of a belt-drive motor relative to the fan, read from the drive side for centrifugal fans.

Example: Motor position affects floor space, belt-change access, and clearance from the outlet duct.

Motor Position Restrictions¶

The combinations of rotation, discharge, and motor position that cannot be built or need checking because the motor would block the outlet or not fit.

Example: A motor position that places the motor in front of a bottom horizontal outlet is a restricted combination.

Motor Positioner¶

A threaded jacking screw mounted beside a motor that pushes it a controlled distance for alignment, providing horizontal adjustment in one direction.

Example: An installer uses a motor positioner to bring the motor sheave into line with the fan sheave.

Motor Positions L and R¶

The two sides, left or right as seen from the drive side, of a bearing pedestal or fan frame on which a fan-carried belt-drive motor can hang.

Motor Positions W X Y Z¶

The four standard AMCA belt-drive motor locations around a centrifugal fan, viewed from the drive side.

Motor Selection¶

The choice of motor horsepower, enclosure, and type from the catalog BHP and job conditions, covering drive losses, cold start, and service conditions.

Motor Service Factor¶

The multiplier above rated horsepower at which a motor may be continuously used.

Example: A 100 hp motor with a 1.15 service factor may run at 115 hp.

Motor Support Base¶

The structure inside a direct drive tubular housing that carries a motor in line with the airflow.

Motor Temperature Rise¶

The change in temperature from the ambient air around a motor to its steady-state operating temperature.

Example: A typical fan motor has Class F insulation but is rated for a Class B motor temperature rise, leaving thermal margin.

Motor Thermal Protection¶

A device built into a motor, such as a thermostat or thermistor, that responds to winding temperature and signals shutdown before insulation damage.

Example: Motor thermal protection trips a fan motor whose cooling is blocked by lint.

Motor Voltage and Phase¶

The electrical supply characteristics in a motor specification: single or three phase, 60 or 50 Hz frequency, and nominal voltage at the motor terminals.

Example: A motor line reading "460 Volt, 3 phase, 60 Hz" states the motor voltage and phase for a typical North American industrial supply.

Motor Winding RTD¶

A resistance temperature detector embedded in a motor's windings that reports continuous winding temperature.

Example: A plant monitoring system alarms at one winding RTD temperature and trips the motor at a higher one.

Motorized Damper¶

A damper with an actuator that powers its blades open and closed rather than relying on airflow pressure.

Example: A motorized damper avoids blade flutter at low flow, where airflow alone might only partly open a gravity damper.

Mounting Panel¶

The flat plate of a plug fan that fastens it to the outside of the customer's plenum wall, with the impeller inside.

NEMA Frame Size¶

A standardized designation of a motor's mounting dimensions, with letters after the number (such as T, C, S, U, TC) describing the frame type.

Example: Two 256T motors from different makers share the same foot and shaft dimensions.

Neoprene Isolation Pad¶

A low-cost isolator of molded, ribbed neoprene pads with a metal plate between them, giving about 1/16 in. deflection and minimal isolation.

Example: Neoprene isolation pads under a fast fan on a solid floor mainly stop noise and fretting at the feet.

Nested Inlet Vanes¶

Variable inlet vanes built into the inlet funnel rather than a separate housing, applied to clean airstreams up to 600°F.

Noise Limits¶

The maximum sound a fan installation may produce, stated as sound power, sound pressure at a location, an A-weighted number, or octave band values.

Nominally Leak-Tight Fan¶

A fan built to reduce leakage to a level agreed with the customer, verified by shop testing and recorded on the inspection form.

Example: A nominally leak-tight fan uses continuous welds, close bolt spacing, and double gaskets.

Non-Drive End Bearing¶

The bearing nearer the impeller on a two-bearing fan shaft; also called the inboard bearing.

Non-Ferrous Material¶

A metal or alloy not based on iron, such as aluminum, bronze, brass, copper, or Monel, far less likely to throw sparks when rubbing steel.

Non-Overloading Power Curve¶

A brake horsepower curve that rises to a maximum and then falls as airflow increases, so a motor sized for that maximum cannot be overloaded at that speed.

Example: Backward inclined, backward curved, and airfoil fans have non-overloading power curves.

Nozzle and Windband¶

The discharge parts at the top of a lab exhaust fan: the nozzle accelerates exhaust into a jet, and the surrounding windband draws in outside air.

Example: The nozzle and windband produce the entrained flow that dilutes lab fumes.

Octave Band¶

A range of frequencies whose upper limit is twice its lower limit, identified by its center frequency.

Example: Fan sound data are published in eight octave bands.

Octave Band Sound Data¶

A fan's sound power level reported separately in each of eight octave bands centered from 63 to 8,000 Hz.

Example: A guide specification requires octave band sound data tested to AMCA Standard 300.

ODP Motor¶

An open drip proof motor whose internal fan blows surrounding air directly across the windings, with openings shielded from dripping water.

Example: An ODP motor suits a clean, dry mechanical room but not a dusty plant.

Oil Mist Lubrication¶

A bearing oil system in which compressed air breaks metered oil into fine droplets carried to the bearings by one pump unit.

Example: Oil mist lubrication keeps a slight overpressure in the bearing housing that keeps dust out.

Open Radial Paddle Impeller¶

A radial impeller with single-thickness paddle blades on the hub, braced by gussets, with no back plate or rings.

Example: The catalog's RBO uses an open radial paddle impeller for coarse, sticky material.

Open Spring Isolator¶

A bare coil spring between two load plates with no restriction of aerodynamically caused motion.

Example: Twin City Fan typically does not use floor-mounted open spring isolators.

Operating Point¶

The single airflow and pressure at which a fan actually runs in a system, where its fan curve intersects the system curve.

Opposed Blade Damper¶

A damper whose adjacent blades rotate in opposite directions, giving airflow that follows control-arm movement closely over the whole range.

Example: An opposed blade damper costs about 10 percent more but suits systems modulated across their entire range.

Outlet Damper¶

A damper mounted on a fan discharge that controls airflow by adding resistance, typically the least expensive flow-control option.

Example: An outlet damper suits a hot, humid, or dusty system that only occasionally runs at reduced capacity.

Outlet Flange¶

A flat frame around a fan outlet, drilled with a bolt-hole pattern, for a bolted connection to downstream ductwork.

Example: The rectangular outlet flange of a housed centrifugal fan bolts to a matching duct flange.

Outlet Silencer¶

A sound-absorbing duct section on a fan discharge that reduces noise carried through the outlet into the duct or atmosphere.

Example: An exhaust stack near a property line gets an outlet silencer to protect neighbors.

Outlet System Effect¶

Loss of fan performance caused by the outlet connection, such as a missing or short outlet duct, a close elbow or damper, or an abrupt expansion.

Outlet Velocity¶

The average speed of air leaving a fan, in feet per minute, equal to airflow volume divided by outlet area.

Example: 6,000 CFM through a 2 ft² outlet gives an outlet velocity of 3,000 fpm.

Outlet Velocity Pressure¶

The velocity pressure corresponding to a fan's outlet velocity, expressing the kinetic energy of the leaving air in in. wg.

Overhung Impeller¶

An impeller mounted on the end of the shaft beyond both bearings.

Example: An Arrangement 1 fan has an overhung impeller, with both bearings on a pedestal outside the housing.

Overloading Power Curve¶

A brake horsepower curve that keeps rising as airflow increases, so a motor sized for the design point can be overloaded at higher airflow.

Example: A forward curved fan whose access door is left open can overload its motor.

Paint Booth Exhaust Fan¶

A fan that removes fumes and overspray from a spray booth at controlled velocity and is easy to clean.

Example: Twin City Fan meets the cleaning need of a paint booth exhaust fan with swingout or clamshell construction.

Panel Fan¶

An axial fan whose impeller is centered in a flat mounting panel, usually installed in a wall opening.

Example: A warehouse wall panel fan carries an inlet safety screen and a weatherhood with a bird screen.

Paper Handling Impeller¶

A material handling impeller made extra rugged for high-impact loading, with a full back plate, trapezoidal gussets, and a paper deflector cone over the hub face.

Example: The catalog's RBP conveys corrugated paper trim, and its deflector cone keeps paper from wrapping around the hub.

Parallel Blade Damper¶

A damper whose blades all rotate in the same direction and stay parallel, recommended for modulating airflow from full open to about 75 percent open.

Example: A parallel blade outlet damper gives low-cost control over the top quarter of a fan's airflow range.

Particulate-Laden Airstream¶

An airstream carrying solid material such as dust, grit, chips, fibers, lint, or granular product that can erode or build up on fan parts.

Example: A woodworking dust collection system moves a particulate-laden airstream, favoring rugged radial or radial tip impellers.

Peak Efficiency¶

The highest efficiency on a fan's efficiency curve, together with the airflow and pressure where it occurs.

Example: Selecting near peak efficiency keeps both power and noise down.

Peak Pressure Point¶

The highest static pressure on a fan curve and the airflow at which it occurs, marking the boundary of the stall region.

Example: A fan whose peak pressure point is 4.9 in. wg at 4,000 CFM is stable only to the right of 4,000 CFM.

Performance Table¶

A grid in a product catalog giving, for one model and size at standard air, the RPM and BHP needed for each combination of airflow and static pressure.

Performance Table Header¶

The block above and around a performance table grid—size block, model line, column heads, and footer—that states what the grid's numbers mean.

Example: On a Catalog 902 page, the header's footer gives the maximum RPM for each class and the legend for shading and italics.

Pie Split Housing¶

A split housing with a wedge cut at 90 degrees or more so the impeller can be removed without disturbing inlet or outlet ductwork.

Example: A pie split housing lets a worn impeller be replaced while the ducts stay connected.

Piezometer Ring¶

Tubing around the throat of a fan's inlet funnel that, with a tap on the funnel face, measures the pressure drop used to calculate airflow.

Pillow Block Bearing¶

A bearing in a housing that bolts to a mounting surface parallel to the shaft axis, such as the top of a fan pedestal, in solid or split styles.

Example: A belt-driven Arrangement 1 fan carries its shaft on two pillow block bearings.

Pivot Motor Base¶

A motor base with the motor on a plate hinged at one edge, so belt tension is set by swinging the motor about the pivot.

Example: Arrangement 9SS swingout fans use an automotive pivot base, which restricts motor location by rotation.

Plenum Arrangement 3SM¶

A horizontal Arrangement 3 plenum fan with the motor on the side of the fan frame on a slide base.

Plenum Arrangement 4V¶

The vertical, direct drive Arrangement 4 plenum fan, available for vertical up or vertical down airflow.

Plenum Arrangements 3HS and 3HA¶

Horizontal Arrangement 3 plenum fans with the motor mounted on top: 3HS on a slide base and 3HA on a heavy-duty adjustable motor base.

Example: An air handler short on floor space uses Plenum Arrangement 3HA.

Plenum Arrangements 3VA and 3VS¶

Vertical Arrangement 3 plenum fans with a side-mounted motor: 3VA on an adjustable motor base and 3VS on a slide base.

Example: An air handler that turns airflow vertically uses Plenum Arrangement 3VA.

Plenum Fan¶

An unhoused centrifugal fan that runs inside an air-filled space within a structure and pressurizes that space, which then distributes air to its outlets.

Example: An air handling unit with a plenum fan can take its discharge ducts off any side of the cabinet.

Plenum Fan Arrangements¶

The set of standard arrangements describing how a plenum fan's motor and drive mount on its frame inside the plenum.

Example: Arrangement 3 is the most common plenum fan arrangement, with variants 3HS, 3HA, 3SM, 3VA, and 3VS.

Plenum Motor Positions¶

The motor locations defined for belt-driven plenum fan arrangements, named differently from housed fan positions because there is no scroll.

Example: On plenum Arrangement 3HS the top-mounted motor sits in the Standard (S) or Optional (O) position.

Plug Fan¶

An unhoused centrifugal fan mounted on a panel fastened to the outside of a plenum wall, with the impeller inside the plenum.

Example: A plug fan on an oven plenum keeps its motor and bearings outside the hot space.

Plug Fan Arrangements¶

The set of standard arrangements describing how a plug fan, with impeller inside the enclosure and drive outside, is supported and driven; seven are listed: 1P, 4, 4P, 8, 8P, 9, and 9P. In Twin City Fan model names, the suffix PL (as in BCPL) means that style of wheel in a plug fan arrangement.

Example: Plug fans come wall-hung, as in Arrangement 9, or grade-mounted, as in Arrangement 9P.

Plume Dilution¶

The mixing of fresh outside air into contaminated exhaust, followed by high-velocity discharge, so the exhaust leaves at lower concentration and rises away from the building.

Example: Bypass flow and entrained flow together provide plume dilution for lab exhaust.

Post Mount Motor Base¶

An adjustable motor base whose motor plate rides on threaded posts so the motor can be moved to tension the belts, used on small motors.

Example: A tubular Arrangement 9 axial fan with a 184T motor uses a standard post mount motor base.

Power Roof Ventilator¶

A motor-driven fan unit, centrifugal or axial, built to mount on a roof opening to exhaust or supply air; abbreviated PRV.

Example: Upblast, downblast, louvered, and hooded roof units are all power roof ventilators.

Power Varies With Speed Cubed¶

The fan law stating that brake horsepower changes in proportion to the cube of the speed ratio.

Example: Cutting speed 20 percent reduces power to 0.8³ = 0.512, about half.

Pre-Spin¶

Swirl given to air entering an impeller, in the same direction the impeller rotates, by inlet vanes or an inlet box damper.

Example: Because pre-spin does part of the impeller's work, variable inlet vanes save power at reduced flow.

Premium Efficiency Motor¶

A motor built with more and better active material, such as copper and electrical steel, so less of its input becomes heat; comparable to NEMA Premium and IEC IE3.

Pressure Blower¶

A housed centrifugal fan with narrow housing and impeller widths, built to produce high pressure at relatively low airflow.

Example: A pressure blower supplying combustion air can be turned down to zero flow with a discharge damper.

Pressure Rise¶

The increase in pressure a fan produces between its inlet and its outlet, stated as fan total pressure or fan static pressure.

Example: A fan moving air from a room into a duct at 2 in. wg static pressure provides that pressure rise.

Pressure Transmitter¶

A device that converts a measured pressure difference into a signal a controls system can use, with or without a local display.

Example: A pressure transmitter on a piezometer ring sends airflow data to the building automation system.

Pressure Varies as Speed Squared¶

The fan law stating that a fan's pressure changes in proportion to the square of the speed ratio.

Example: A 10 percent speed increase raises pressure by 1.1² = 1.21, or 21 percent.

Product Catalog¶

A manufacturer's published document for one product line, giving its models, sizes, classes, arrangements, options, construction, limits, and often rated performance.

Example: Catalog 300 covers the BC-SW and BC-DW backward inclined fans.

Product Options¶

The construction choices and accessories a catalog offers beyond the standard fan, with statements of what each does and when to use it.

Example: Spark-resistant construction, inlet boxes, and variable inlet vanes are product options.

Radial Bladed Fan¶

A housed centrifugal fan with flat blades pointing straight out from the hub, built heavy for dirty, abrasive, sticky, or bulky airstreams.

Example: A radial bladed fan conveys wood chips because material slides off the flat blades instead of building up.

Radial Bladed Impeller¶

A centrifugal impeller with flat blades pointing straight out from the hub along radii, with no backward or forward lean.

Example: A radial bladed impeller handles heavy dust because material flies off its tips rather than building up.

Radial Tip Fan¶

A housed centrifugal fan whose blades curve forward at the entering edge and become radial at the tip, more efficient than a radial bladed fan at high airflow.

Example: A radial tip fan suits a dusty dryer exhaust where airflow is large and particulate is present.

Radial Tip Impeller¶

A centrifugal impeller whose blades curve forward at the entering edge and become radial at the tip of the leaving edge.

Example: A radial tip impeller combines a self-cleaning tip with better efficiency than a radial blade.

Rating Conditions¶

The conditions under which catalog performance numbers apply: air density, installation type, and what BHP includes or excludes.

Reading a Performance Table¶

Finding the row for the required airflow and the column for the required static pressure and taking the RPM and BHP printed where they meet.

Example: Reading a performance table for 10,000 CFM at 4 in. wg gives the cell's RPM and BHP, and its shading shows the fan class.

Recess Cone¶

A cone-shaped pocket formed into the insulated plug or wall of a plug fan or air kit that gives the shaft cooler and shaft seal a recessed space with outside air; also called a drive cone.

Example: "Design #2" for Arrangement 9 plug fans uses a recess cone.

Regenerative Thermal Oxidizer¶

An industrial air pollution control process, abbreviated RTO, that destroys pollutants in process exhaust by oxidizing them at high temperature using heat-storing ceramic beds.

Example: An automotive paint booth sends its exhaust through an RTO driven by a leak-tight fan.

Relieved Inlet Transition¶

A transition shaped to smooth the flow of paper trim and similar material into a fan so it does not snag or pile up.

Restrained Bearing¶

A pillow block bearing whose housing is mechanically prevented from shifting on its pedestal by dowel pins, stop blocks, or spring pins.

Example: Type A and Type B spark-resistant construction includes restrained bearings.

Restrained Spring Isolator¶

A spring isolator with a housing or frame that restrains the spring's vertical or horizontal motion.

Example: A fan that must not lift or rock at startup sits on restrained spring isolators.

Reverse Bore Hub¶

An axial impeller hub bored from the opposite face so the impeller can be mounted facing the other way on the shaft, paired with PM airflow.

Reversible Impeller¶

An axial impeller designed to move air in either direction along the shaft, with standard flow (MP) and reverse flow (PM) modes.

Example: A tunnel ventilation fan uses a reversible impeller so it can supply or exhaust on demand.

Ring Fan¶

An axial fan whose impeller is centered in a short mounting ring rather than a full tube or flat panel.

Example: A ring fan can be mounted in a wall or partition opening to move air between spaces.

Roof Curb¶

A raised frame built into a roof around an opening and flashed into the roofing, on which a rooftop fan sits.

Example: A roof exhauster's curb cap rests on the roof curb, keeping the fan above standing water and snow.

Roof Exhauster Shroud¶

The sheet-metal shell around the impeller of a centrifugal roof exhauster that guides the air the impeller throws off.

Roof Mounted Axial Fan¶

An axial fan mounted on a roof curb to supply or exhaust air, offered with a stack cap for straight-up discharge or a hood that deflects air downward.

Roof Mounted Panel Fan¶

A panel fan used as the base model of a roof unit with roof accessories such as a stack cap or hood.

Roof Ventilator¶

In the current Twin City Fan catalog, an axial, motor-driven rooftop fan unit built on a tubeaxial impeller, offered in upblast, low profile, hooded, filtered supply, and smoke and heat forms.

Example: TUD and TUB upblast units are listed as roof ventilators, not as centrifugal roof exhausters.

Rotation¶

The direction in which a fan's impeller turns, stated for centrifugal fans as clockwise or counterclockwise viewed from the drive side.

Example: A backward inclined wheel turning the wrong way moves only a fraction of its rated air while drawing power.

Rotation and Discharge Code¶

A short designation stating both a centrifugal fan's rotation and its outlet direction, written as a name or as an angle.

Example: "CW THD" and "CW 90" are equivalent rotation and discharge codes.

Rotation Arrow¶

A curved arrow marked on drawings and on the fan housing showing the required direction of impeller rotation.

Example: An electrician checks the motor's bump-start direction against the rotation arrow on the housing.

RTO Service Conditions¶

The operating facts an RTO fan must survive that only the customer or RTO builder knows, which sales must specify.

Example: Temperature swings during the RTO's flow reversals are part of the RTO service conditions.

Rub Plate¶

A non-ferrous plate, typically aluminum, at the housing opening where the shaft passes through, protecting against steel shaft-to-housing contact; also called a striker or spark plate.

Example: A Type B fan with an outside protruding hub does not need a rub plate.

Rub Ring¶

A non-ferrous ring mounted where an impeller could shift into a stationary part, so the ring is touched first instead of steel.

Example: Radial bladed fans and pressure blowers use an inlet rub ring of Monel.

Rubber-in-Shear Isolator¶

An isolator that carries its load through a rubber element loaded in shear, giving moderate isolation with about 0.2 to 0.5 in. deflection; abbreviated RIS.

Example: A floor-mounted RIS pad has two steel load plates embedded in rubber.

Scroll Housing¶

The snail-shaped casing around a housed centrifugal impeller whose clearance from the impeller grows steadily toward the outlet.

Example: The scroll housing collects the air thrown off the impeller tips and converts part of its velocity into static pressure.

Scroll Wrapper¶

The curved band of sheet metal that wraps around the impeller between the two side plates and runs out to the discharge.

Example: The scroll wrapper's growing spiral clearance slows the air and turns velocity into static pressure.

Seismic Restraint¶

A device that holds an isolated fan in place during an earthquake by limiting how far it can move on its soft mounts.

Example: Seismic springs and snubbers are the two seismic restraint forms FanPedia shows.

Seismic Spring Isolator¶

A restrained spring isolator whose housing nearly surrounds the spring so it withstands earthquake loads.

Example: A hospital rooftop fan in a seismic zone sits on seismic spring isolators.

Selection Trade-Offs¶

The conflicts between good properties that no single fan can have at once and that a selection must settle deliberately.

Example: A larger, slower fan trades higher first cost for lower operating cost.

Separated Pedestal¶

A fan pedestal for high-temperature applications that leaves a gap between the fan housing and the pedestal, reducing heat conducted to the bearings and leaving room for insulation.

Set Screw Mount¶

A bearing locking method in which two set screws in the inner ring are tightened onto the shaft.

Setback Outlet Flange¶

The bolted flange frame around a DWDI fan's discharge opening, set back from the end of the outlet.

Example: Duct connections to a setback outlet flange are taken from the certified dimensional drawing.

Severe Duty Motor¶

A motor built to keep running in moisture, chemical fumes, dust, washdown, or outdoor exposure where a general duty motor would fail early.

Example: A paper mill fan uses an IEEE 841 severe duty motor.

Shaft and Coupling Alignment¶

The positioning of motor and fan bearings so the two shaft centerlines form one straight line through the coupling, correcting offset and angular misalignment.

Example: Poor shaft and coupling alignment shows up as vibration and early coupling wear.

Shaft and Coupling Guards¶

Guards that cover a fan's exposed shaft and bearings and enclose the flexible coupling on direct-drive arrangements.

Example: An Arrangement 8 fan needs a coupling guard over the joint between motor and fan shaft.

Shaft Cooler¶

A cast aluminum part mounted on the shaft between the shaft seal and the bearings that dissipates heat conducted along the shaft from the airstream.

Example: Shaft coolers are recommended for applications over 300°F.

Shaft Coupling¶

A device that connects two shafts end to end so one drives the other, such as fan shaft to motor shaft.

Example: Arrangements 7 and 8 use a flexible shaft coupling between the motor and fan shaft.

Shaft Grounding Ring¶

A ring of conductive fibers around a motor shaft that gives VFD-induced shaft voltages a low-resistance path to ground, bypassing the bearings.

Example: A fan run on a VFD may get a shaft grounding ring to keep shaft current from pitting the bearing raceways.

Shaft Keyway¶

A groove cut lengthwise into a shaft to accept a key that transmits torque to a hub, bushing, coupling, or sheave.

Shaft Locking Collar¶

A ring clamped to a shaft that holds it in place axially so it cannot slide and strike a bearing or the impeller.

Example: Vertical fans use a shaft locking collar because gravity pulls the shaft along its axis.

Shaft Material Grades¶

The specific steel or alloy specifications from which fan shafts are made, such as Grade 1045 or 4140 mild steel and Types 304, 316, or 2205 stainless.

Example: A marine axial fan needing AMCA A spark resistance uses an AMPCO-45 shaft.

Shaft Seal¶

A seal mounted on the fan housing around the shaft where it passes through the housing wall, reducing leakage of air or gas along the shaft.

Example: An overhung fan needs one shaft seal on the drive side; a center hung fan with the shaft through both sides needs two.

Shaft Sleeve¶

A tube fitted over a shaft where it would otherwise be exposed to the airstream, shielding a shaft of dissimilar material.

Example: A stainless steel fan with a steel shaft uses a stainless shaft sleeve inside the housing.

Shaft Turndown¶

A section of shaft machined to a smaller diameter than the rest, on one or both ends, to fit a standard bearing, seal, hub, or coupling.

Example: A center hung Arrangement 3DI shaft may carry several turndowns along its length.

Sheave¶

A grooved pulley mounted on the motor shaft or the fan shaft that carries the drive belts, fastened with or without a bushing; the motor-shaft one is the drive pulley and the fan-shaft one the driven pulley.

Example: Swapping a 6-inch motor sheave for a 7-inch one raises fan speed about 17 percent.

Shutoff Pressure¶

The static pressure a fan develops at zero airflow, with the inlet or outlet completely blocked.

Example: Shutoff pressure is the most pressure a fan can impose on its ductwork at a given speed.

Single Width Single Inlet¶

A centrifugal impeller and housing configuration, abbreviated SWSI, that takes air in through one side only, with one back plate, one shroud, and one inlet.

Size Change Calculation¶

An application of the fan laws to a geometrically similar fan of another size, with airflow scaling by the diameter ratio cubed, pressure squared, and power to the fifth power.

Example: At the same RPM, a size 1.12 times larger moves 1.405 times the airflow and absorbs 1.762 times the power.

Slide Base¶

A motor base in which the motor plate slides along a fixed base, moved by an adjusting screw to set belt tension.

Example: Arrangement 9 fans carry the motor on a slide base on the side of the pedestal.

Slide Gate Damper¶

A flat gate that slides across the airway in a frame to throttle flow manually, offered for cast aluminum pressure blowers.

Slip Connection¶

A joint made by sliding one collar or pipe over or into another and sealing it, typically with a rubber sleeve and clamps.

Example: A slip connection between a fan and a free-standing inlet box tolerates small movement.

Smoke and Heat Exhaust Fan¶

A fan built to keep running while moving smoke and hot gas out of a burning building, rated for a minimum temperature and duration.

Example: The current catalog rates its smoke and heat fans for 500°F for four hours or 1,000°F for 15 minutes.

Snubber¶

A pair of brackets with elastomeric material between them, one on the fan or base and one on the floor, that limits excessive movement in any direction.

Example: A snubber engages only when an earthquake or startup jolt pushes the fan beyond its normal float.

Soap Bubble Test¶

A shop leak check in which a pressurized fan housing's seams, joints, and seals are coated with soap solution so bubbles reveal leaks.

Example: An RTO fan passes a soap bubble test before shipment, and the result goes on the inspection form.

Solid Pillow Block Bearing¶

A pillow block bearing with a one-piece cast housing, the common choice on standard fans.

Example: Replacing a solid pillow block bearing usually means sliding it off the shaft end after pulling the sheave.

Sone Rating¶

A measure of perceived loudness on a linear scale, where 1 sone is the loudness of a 1,000 Hz tone at 40 dB sound pressure level.

Example: A 10-sone exhauster sounds about twice as loud as a 5-sone one.

Sound Power Level¶

The total acoustic power a fan radiates, in decibels relative to 10⁻¹² watt, independent of room or distance; abbreviated Lw.

Example: Fan manufacturers publish sound power levels in eight octave bands.

Sound Pressure Level¶

The magnitude of sound pressure fluctuations at one location, in decibels relative to 20 micropascals; abbreviated Lp.

Example: A fan's sound pressure level falls as a listener moves farther away, while its sound power stays the same.

Space Constraints¶

The physical limits on where a fan can go, including footprint, height, duct locations, service clearances, rigging paths, and supporting structure.

Example: A fixed footprint in a mechanical room often decides the arrangement before performance does.

Spark Resistant Construction¶

Fan construction that lowers the chance of the fan striking a spark by using non-ferrous airstream parts and arranging parts so rotating and stationary components are unlikely to touch.

Spark Resistant Type A¶

The highest degree of spark resistance, requiring all fan components in the airstream to be non-ferrous and assembled to reduce stationary-to-rotating contact.

Example: A Type A centrifugal fan has a non-ferrous housing, impeller, inlet funnel, and shaft sleeve.

Spark Resistant Type B¶

Spark-resistant construction requiring a non-ferrous impeller and an assembly that reduces stationary-to-rotating contact, usually with a non-ferrous rub plate.

Example: A Type B fan has an aluminum impeller and rub plate while the housing and shaft stay steel.

Spark Resistant Type C¶

The minimal spark-resistant construction, keeping a steel impeller and housing but adding non-ferrous parts where a shifting rotor would touch first.

Example: A Type C centrifugal fan uses an aluminum inlet funnel and rub plate with a steel impeller.

Specific Speed¶

A single number that characterizes a fan design by combining its speed, airflow, and pressure at the point of rating, usually peak efficiency.

Example: Axial fans have high specific speed, while radial blowers have low specific speed.

Specification Performance Clause¶

The paragraph in Part 2 of a fan specification stating what the fan must deliver, at what air conditions, and how ratings must be proven.

Example: A specification performance clause names the airflow, pressure, temperature, elevation, and AMCA rating standards.

Specification Review¶

The check of a specification, or a submittal against one, for completeness and internal consistency before it is issued or approved.

Example: Specification review catches a duty point stated with no air condition.

Speed Change Calculation¶

An application of the fan laws that finds the new speed, pressure, and power when a fan on the same system must deliver a different airflow.

Example: To raise airflow from 8,000 to 9,000 CFM, speed rises by 9,000/8,000 = 1.125.

Speed Control¶

A flow-control method that reduces airflow by slowing the impeller, using the least power of any method.

Example: Slowing a fan to 80 percent speed cuts its airflow to 80 percent and its power to about half.

Spherical Roller Bearing¶

A rolling bearing with crowned, barrel-shaped rollers in a spherical outer raceway, suited to high and shock loads and tolerant of slight misalignment.

Example: A heavy industrial fan with a large shaft runs on spherical roller bearings.

Split Housing¶

A fan housing built in sections that bolt together along planned joints so it can be opened or shipped in pieces.

Example: Horizontal, pie, and three-way splits are the three centrifugal split housing styles.

Split Pillow Block Bearing¶

A pillow block bearing whose housing splits horizontally through the shaft centerline so the cap lifts off for inspection and replacement.

Example: Larger heavy-duty fans often use split pillow block bearings.

Spring Isolator¶

A steel coil spring with load plates placed between a fan or its base and the structure, giving high deflection and high isolation.

Example: A 2-inch-deflection spring isolator lets a fan run far above its natural frequency.

Square Inline Centrifugal Fan¶

An inline centrifugal fan with a square housing and removable side panels that can pass air straight through or turn it 90 degrees.

Example: A square inline fan fits a rectangular duct run above a ceiling, and its side panels come off for service.

Stack Cap¶

A short, open-topped discharge stack on the outlet of a roof-mounted or vertically mounted fan that sends air straight up in a vertical jet.

Stainless Steel Construction¶

Construction in which airstream parts, and where specified the shaft, are made of stainless steel, an iron-based alloy made corrosion resistant by chromium.

Example: A food-plant washdown exhaust fan uses Type 304 stainless steel construction.

Stall Region¶

The part of a fan curve to the left of the pressure peak, where airflow separates from the blades and performance becomes unsteady.

Standard Air¶

The reference air condition used to rate fans: air with a density of 0.075 lb/ft³, very nearly that of dry air at 70°F and 29.92 in. Hg barometric pressure.

Example: Unless a catalog table says otherwise, its static pressures and brake horsepowers are standard-air values.

Standard Motor Position Rule¶

FanPedia's default pairing of motor side with rotation on pedestal-mounted motors: clockwise fans take position L and counterclockwise fans take position R.

Example: A CCW Arrangement 9 fan normally has its motor in position R.

Static Efficiency¶

Fan efficiency computed from air power based on fan static pressure, divided by brake horsepower.

Static Oil Lubrication¶

A bearing oil system that keeps a constant oil level in a sump in each bearing housing, topped up by a constant-level oiler.

Static Pressure¶

The pressure that air exerts equally in all directions on the walls of a duct or housing, independent of air motion, measured perpendicular to the flow.

Example: A manometer tube connected flush to the side of a duct reads static pressure.

Straight Bore Hub¶

An impeller hub whose bore runs straight through, fastened to the shaft with a key and set screws.

Straightening Vanes¶

Fixed vanes inside a tubular housing downstream of the impeller that remove swirl and return the air to straight-through flow.

Example: Straightening vanes in a tubular inline centrifugal fan turn the radially thrown air back along the duct.

Stuffing Box Seal¶

A shaft seal in which rings of packing are stacked in a housing around the shaft and compressed by an outer plate, like valve-stem packing; also called a packing gland or Graphoil seal.

Submittal Data¶

The package of documents sent for the engineer's review before a fan is released for manufacture, confirming it meets the specification.

Example: Submittal data include performance curves, octave band sound data, motor ratings, and dimensioned drawings.

Surge¶

An unstable, cycling condition left of the pressure peak in which airflow and pressure oscillate between the fan and the air volume in the ductwork.

Example: Surge shows up as rhythmic "breathing" of the ductwork and fluctuating motor amps.

Swingout Door Assembly¶

The hinged door and reinforced framework that carry a swingout fan's rotating parts out of the housing.

Example: On Arrangement 9ST, the motor, shaft, bearings, and impeller ride on the swingout door assembly.

Swingout Fan¶

A fan whose impeller, shaft, and bearing assembly is mounted on a large hinged door so it swings out of the housing for cleaning and inspection.

Example: A food plant uses a swingout fan where the impeller must be washed often.

Swingout Motor Position Rule¶

The rule tying motor position to the swing side of an Arrangement 9ST and to both rotation and swing side of an Arrangement 9SS.

Example: A 9SS left-swing fan must be CCW with the motor in position R and an F2 conduit box.

Synchronous Motor Speed¶

The speed of the rotating magnetic field in an induction motor, set by supply frequency and number of poles: 120 times frequency divided by poles.

Example: A four-pole motor on 60 Hz has a synchronous motor speed of 1,800 RPM.

System Curve¶

A parabola-shaped graph of one duct system's resistance against airflow, starting at zero pressure at zero airflow.

Example: A system needing 2 in. wg at 5,000 CFM has a system curve passing 8 in. wg at 10,000 CFM.

System Effect¶

The loss of fan performance caused by the way a fan is connected to its system, expressed as an added pressure loss the selection must account for.

Example: An elbow bolted directly to a fan inlet creates system effect that a catalog rating does not include.

System Effect Curve¶

A lettered chart in AMCA 201 giving system effect in in. wg against velocity at the fan inlet or outlet for a given connection.

Example: A designer looks up the system effect curve assigned to an inlet elbow, then reads the loss at the inlet velocity.

System Resistance¶

The pressure needed to push a given airflow through everything connected to a fan, rising with the square of airflow.

Example: Doubling airflow through a duct system raises its system resistance to four times the original.

Taconite Seal¶

A bearing seal for dirty or abrasive environments that combines a labyrinth with a grease-purged cavity that pushes contaminants out.

Example: A mining-plant fan on split pillow blocks is ordered with taconite seals.

Taper Lock Hub¶

An impeller hub with a tapered bore that is clamped to the shaft by a tapered bushing drawn in by screws.

TEAO Motor¶

A totally enclosed air over motor with no cooling fan of its own, relying on the fan's airstream for cooling.

Example: A direct-drive vaneaxial fan with the motor inside the tube uses a TEAO motor.

Technical Description¶

A short written paragraph in FanPedia or a catalog that defines one component, arrangement, or option and states its restrictions.

Example: The technical description of Arrangement 1 SWSI states that the motor can go in any of the four AMCA motor positions.

TEFC Motor¶

A totally enclosed fan cooled motor with sealed windings and an external fan that blows air over the ribbed outer frame.

Example: A TEFC motor is the usual choice for a fan in a dusty or damp industrial space.

Temperature Correction¶

The density correction factor for an air temperature other than 70°F, computed from absolute temperatures in degrees Rankine.

TENV Motor¶

A totally enclosed non-ventilated motor with no cooling fan, dissipating heat through its frame alone.

Example: A TENV motor is chosen where lint would clog the fan cover of a TEFC motor.

Thermal Expansion¶

The increase in a part's dimensions as its temperature rises, proportional for a straight part to its length and temperature rise.

Example: Thermal expansion lengthens a hot fan's shaft, which the floating bearing must accommodate.

Three-Way Split Housing¶

A split housing cut into three sections, up to 180 degrees, used for shipping or to pass the fan through a restricted opening.

Thrust Restraint¶

A device that prevents excessive fan motion on isolators caused by the aerodynamic reaction force of the moving air.

Example: A fan on soft springs pushing 4 in. wg through a 4 ft² outlet needs a thrust restraint against an 83 lb reaction.

Tip Speed¶

The linear speed of the outermost point of the impeller blades, in feet per minute, set by impeller diameter and rotating speed.

Example: A 3-foot-diameter impeller at 1,000 RPM has a tip speed of about 9,425 fpm.

Top Angular Down Discharge¶

The centrifugal discharge position, coded TAD at 135 degrees, with the outlet near the top of the housing and air heading downward at 45 degrees.

Example: A TAD outlet can collide with a motor on that side, restricting motor position.

Top Angular Up Discharge¶

The centrifugal discharge position, coded TAU at 45 degrees, with the outlet near the top of the housing and air heading upward at 45 degrees.

Example: A TAU outlet meets an exhaust duct climbing toward an offset roof penetration.

Top Horizontal Discharge¶

The centrifugal discharge position, coded THD at 90 degrees, with the outlet at the top of the housing pointing sideways.

Example: A fan feeding a duct that runs near the ceiling uses top horizontal discharge.

Total Pressure¶

The sum of static pressure and velocity pressure at a point in an airstream, representing all of the energy the air carries there.

Totally Split Roller Bearing¶

A specialty bearing whose housing and every internal part are split in halves so it can be replaced without removing the shaft.

Example: A large fan where pulling the shaft means a long outage may justify a totally split roller bearing.

Transition Piece¶

A section of ductwork that connects a fan opening to a duct of different shape or size; usually called a transition.

Transmission Losses¶

The power lost between the motor shaft and the fan shaft in the drive, such as belt flexing and slip and drive bearing friction.

Example: A belt-driven fan's motor must supply the catalog BHP plus transmission losses.

Trantorque Hub¶

An axial impeller hub that uses a keyless compression bushing which grips the shaft when tightened.

Example: A Trantorque hub needs no shaft key, unlike a taper lock hub.

Tri-Directional Positioner¶

A motor positioner combining horizontal positioners with vertical jack screws, giving adjustment horizontally and vertically.

Tube Adapter and Sleeve¶

A flexible outlet connection for pressure blowers: a metal collar and flange bolted to the discharge plus a molded rubber slip sleeve with hose clamps.

Example: A tube adapter and sleeve connects a pressure blower to a pipe line rated up to 5 psi and 180°F.

Tubeaxial Fan¶

An axial fan in a plain cylindrical housing with no guide vanes, so air leaves with some rotational swirl.

Example: A tubeaxial fan provides general ventilation through a wall or short duct run.

Tubular Housing¶

A round, straight-through cylindrical housing in which air enters one end and leaves the other in line with the duct.

Example: Tubeaxial, vaneaxial, tubular inline centrifugal, and mixed flow fans all use tubular housings.

Turndown¶

The range over which a flow-control method can reduce airflow and still control it well, stated as a ratio of maximum to minimum controllable airflow.

Example: A method with 4:1 turndown can control a 10,000 CFM fan down to 2,500 CFM.

Two and Four Hole Mount¶

The two pillow block base styles: a two hole mount with one hold-down bolt on each side of the shaft, and a four hole mount with two on each side.

Example: A replacement bearing from another maker may not match the pedestal holes if it offers only a two hole mount.

Two-Bearing Housing¶

A specialty housing, also called a monoblock bearing, holding both shaft bearings in one casting to preserve their alignment.

Unhoused Centrifugal Fan¶

A centrifugal fan whose impeller runs without a scroll housing, discharging radially in all directions into a surrounding plenum or cabinet.

Unitary Base¶

A fan base of structural channel, also called a channel base, designed to bolt directly to the floor without isolators.

Example: A fan on a solid industrial slab with no vibration-sensitive neighbors sits on a unitary base.

Up Blast Discharge Position¶

The centrifugal discharge position, coded UBD at 360 degrees, with the outlet pointing straight up.

Example: A fan exhausting into a vertical stack uses the up blast discharge position so no outlet elbow is needed.

Upblast Discharge¶

A fan discharge configuration that sends exhaust straight up, away from the roof surface and nearby air intakes.

Example: Kitchen exhaust fans use upblast discharge so the grease plume clears the roof.

Utility Set¶

A compact single-inlet centrifugal fan package, belt or direct driven, for general exhaust and supply in commercial and light industrial buildings.

Example: A utility set with a weather cover exhausts a paint storage room from an outdoor pad.

V-Belt¶

A drive belt with a trapezoidal cross section that wedges into the matching V-shaped groove of a sheave.

Example: A 25 hp fan drive uses three V-belts side by side on multi-groove sheaves.

Vaneaxial Fan¶

An axial fan whose tubular housing includes a section of stationary guide vanes that straighten the airflow as it passes through.

Example: A vaneaxial fan reaches higher pressure and efficiency than a tubeaxial fan of the same diameter.

Variable Frequency Drive¶

An electronic controller, abbreviated VFD, that converts fixed-frequency AC power to adjustable-frequency power, changing induction motor speed in proportion.

Example: Running a VFD at 45 Hz slows a 60 Hz fan to about 75 percent speed.

Variable Inlet Vanes¶

A ring of adjustable radial vanes at a fan inlet that, when partly closed, pre-spin entering air to control flow; also called inlet guide vanes or a vortex damper.

Example: Variable inlet vanes with an actuator let a building automation system modulate a large centrifugal fan.

Velocity Pressure¶

The pressure that a moving airstream would produce if brought to rest, representing the kinetic energy of the air.

Example: At standard air, air moving at 4,005 fpm has a velocity pressure of 1 in. wg.

Velocity Pressure Equation¶

The equation converting between air velocity and velocity pressure; for standard air, velocity in fpm equals 4,005 times the square root of velocity pressure in in. wg.

Ventilating Set¶

The trade name for a compact Arrangement 10 centrifugal fan in Class I or II, sizes 122 through 365.

Vertical Arrangement¶

A fan arrangement in which the shaft is vertical, so air enters or leaves the impeller vertically.

Example: Plenum Arrangement 4V can be built for vertical up or vertical down airflow.

Vertical Inline Motor Location¶

The place a motor occupies on a belt-driven Arrangement 9 vertical inline fan, centered on the curb cap or between the support brackets, with no motor position letter.

Vertical Jack Screws¶

Screws threaded through drilled and tapped motor feet that lift the motor so shims can be set, removed after shimming.

Vibration¶

The repeated back-and-forth motion of a machine or structure about its resting position, mainly caused in fans by rotating unbalance.

Example: A fan whose impeller collects dirt develops vibration once per revolution.

Vibration Isolation¶

The prevention or limitation of the force an operating fan transmits to its supporting structure, using resilient isolators.

Viewed From Drive Side¶

The convention of describing a centrifugal fan's rotation, discharge, and motor position as seen from the side where the sheave, coupling, or motor is located.

Example: A fan that is clockwise viewed from the drive side appears counterclockwise from the inlet side.

Wall Mounted Fan¶

A fan installed in or on a wall to supply or exhaust air directly through it, usually without ductwork.

Example: A belt-driven wall mounted fan provides cost-effective general ventilation for a warehouse.

Wall Type Gravity Damper¶

A parallel blade, end-pivoted gravity damper mounted in a vertical plane for fans and openings in a wall.

Example: A wall exhaust fan uses a wall type gravity damper whose blades hang closed.

Weather Cover¶

A cover that protects a fan's motor from rain, snow, and sun when installed outdoors.

Example: An Arrangement 10 fan with an optional weather cover is suitable for roof installation.

Weatherhood¶

A hood mounted over the outdoor side of a wall fan that keeps rain and snow out while letting air pass.

Example: A panel fan's weatherhood carries a bird screen across its open side.

Weep Hole¶

A small open hole at the lowest point of a fan housing that drains wash water before painting and moisture after installation.

Example: When water must be piped away rather than dripped, a housing drain replaces the weep hole.

Welded Housing¶

A fan housing whose scroll, sides, and framing are joined by welds into one permanent piece.

Example: Nominally leak-tight construction calls for a welded housing with continuous welds.

Wet Airstream¶

An airstream carrying enough water vapor, mist, or steam that liquid water can condense and collect inside the fan.

Example: Commercial dishwasher exhaust is a wet airstream, so the fan may need a housing drain and corrosion-resistant coating.

Wiper Bar¶

A strip mounted inside a centrifugal housing in the inlet funnel area that directs air to prevent turbulence and increase efficiency.

Zero Speed Switch¶

A sensor that detects slowing or stopping of the fan shaft; also called a speed sensor.

Example: A zero speed switch alarms when a belt breaks even though the motor starter is still on.