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ImprovingFan SystemPerformance

a sourcebook for industry

U.S. Department of EnergyEnergy Efficiency and Renewable Energy

One of a series ofindustrial energyefficiency sourcebooks

a sourcebook for industry

Bringing you a prosperous future where energy is clean, abundant, reliable, and affordable

ImprovingFan SystemPerformance

Acknowledgments

Improving Fan System Performance: A Sourcebook for Industry has been developed by the U.S. Departmentof Energy’s (DOE) Industrial Technologies Program and the Air Movement and Control AssociationInternational, Inc. (AMCA), a DOE Allied Partner. Industrial Technologies and AMCA International undertook this project as part of a series of sourcebook publications on motor-driven equipment under theBestPractices effort. Other topics in this series include compressed air systems, pumping systems, and motorsand drives. For more information about the Industrial Technologies’ BestPractices effort and AMCAInternational, see Section 3.

AMCA International is a not-for-profit association of the world’s manufacturers of related air system equipment—primarily, but not limited to fans, louvers, dampers, air curtains, airflow measurement stations,acoustic attenuators, and other air system components—for industrial, commercial, and residential markets.The association’s mission is to promote the health and growth of industries covered by its scope and the members of the association consistent with the interests of the public.

DOE, AMCA International, Lawrence Berkeley National Laboratory, and Resource Dynamics Corporation thankthe staff at the many organizations that so generously assisted in the collection of data for this sourcebook.The contributions of the following participants are appreciated for their review and input to this sourcebook:

Gary Benson, The New York Blower CompanyFrank Breining, Airmaster Fan CompanyDon Casada, Diagnostic Solutions, LLCBrad Gustafson, U.S. Department of EnergyTom Gustafson, Hartzell Fan, Inc.Tony Quinn, American Fan Company & Woods USA DivisionPaul Saxon, Air Movement and Control Association International, Inc.Bill Smiley, The Trane CompanySastry Varanasi, ABB Fan Group North AmericaDick Williamson, Twin City Fan Companies, Ltd.Ron Wroblewski, Productive Energy Solutions

Prepared for: The United States Department of EnergyAir Movement and Control Association International, Inc.

Prepared by: Lawrence Berkeley National LaboratoryWashington, DCResource Dynamics CorporationVienna, VA

Cover photo credit: Copyright© CML Northern Blower Inc., 1989. All rights reserved. This image may not be reproduced, stored, or transmitted in any form or means without the prior written consent of the copyright holder.

A Sourcebook for Industry 3

Fans1 are widely used in industrial and commercialapplications. From shop ventilation to materialhandling to boiler applications, fans are critical for process support and human health. In the manufacturing sector, fans use about 78.7 billionkilowatt-hours2 of energy each year. This con-sumption represents 15 percent of the electricityused by motors.3 Similarly, in the commercial sector, electricity needed to operate fan motorscomposes a large portion of the energy costs forspace conditioning.

Performance may range from “free air” to severalpounds per square inch gage (psig)4, with airflow from a few cubic feet per minute (cfm) to more than 1 million cfm. Pressures above 15 psig generally require air compressors, whichare addressed in a separate sourcebook titledImproving Compressed Air System Performance, A Sourcebook for Industry.

In manufacturing, fan reliability is critical to plantoperation. For example, where fans serve materialhandling applications, fan failure will immediatelycreate a process stoppage. In industrial ventilationapplications, fan failure will often force a processto be shut down (although there is often enoughtime to bring the process to an orderly stoppage).Even in heating and cooling applications, fan operation is essential to maintain a productive workenvironment. Fan failure leads to conditions inwhich worker productivity and product qualitydeclines. This is especially true for some productionapplications in which air cleanliness is critical tominimizing production defects (for example, plastics injection molding and electronic componentmanufacturing).

In each case, fan operation has a significant impacton plant production. The importance of fan reliability

often causes system designers to design fan systems conservatively. Concerned about beingresponsible for under-performing systems, designerstend to compensate for uncertainties in the designprocess by adding capacity to fans. Unfortunately,oversizing fan systems creates problems that canincrease system operating costs while decreasingfan reliability.

Fans that are oversized for their service requirementsdo not operate at their best efficiency points. Insevere cases, these fans may operate in an unstablemanner because of the point of operation on thefan airflow-pressure curve. Oversized fans generateexcess flow energy, resulting in high airflow noiseand increased stress on the fan and the system.Consequently, oversized fans not only cost more topurchase and to operate, they create avoidablesystem performance problems. The use of a “systems approach” in the fan selection processwill typically yield a quieter, more efficient, andmore reliable system.

Fans

There are two primary types of fans: centrifugaland axial. These types are characterized by thepath of the airflow through the fan. Centrifugalfans use a rotating impeller to increase the velocityof an airstream. As the air moves from the impellerhub to the blade tips, it gains kinetic energy. Thiskinetic energy is then converted to a static pressureincrease as the air slows before entering the discharge.Centrifugal fans are capable of generating relativelyhigh pressures. They are frequently used in “dirty”airstreams (high moisture and particulate content),in material handling applications, and in systemsat higher temperatures.

Section 1: Introduction to Fan Systems

1 For the purposes of this sourcebook, the term “fan” will be used for all air-moving machines other than compressors.2 United States Industrial Electric Motor Systems Market Opportunities Assessment, U. S. Department of Energy, December 1998.3 Ibid.4 At standard conditions, a column of water 27.68 inches high exerts 1 psig of pressure. Equivalently, 1 inch of water gage =

0.036 psig.

Introduction to Fan Systems

Improving Fan System Performance4

Axial fans, as the name implies, move an airstreamalong the axis of the fan. The air is pressurized bythe aerodynamic lift generated by the fan blades,much like a propeller and an airplane wing.Although they can sometimes be used interchange-ably with centrifugal fans, axial fans are commonlyused in “clean air,” low-pressure, high-volumeapplications. Axial fans have less rotating mass andare more compact than centrifugal fans of compa-rable capacity. Additionally, axial fans tend to havehigher rotational speeds and are somewhat noisierthan in-line centrifugal fans of the same capacity;however, this noise tends to be dominated by highfrequencies, which tend to be easier to attenuate.

◆ Fan SelectionFan selection is a complex process that starts witha basic knowledge of system operating requirementsand conditions such as airflow rates, temperatures,pressures, airstream properties, and system layout.The variability of these factors and other consider-ations, such as cost, efficiency, operating life,maintenance, speed, material type, space con-straints, drive arrangements, temperature, andrange of operating conditions, complicate fanselection. However, knowledge of the importantfactors in the fan selection process can be helpfulfor the purposes of reducing energy consumptionduring system retrofits or expansions. Often, a fantype is chosen for nontechnical reasons, such asprice, delivery, availability, or designer or operatorfamiliarity with a fan model. If noise levels, energycosts, maintenance requirements, system reliability,or fan performance are worse than expected, thenthe issue of whether the appropriate fan type wasinitially selected should be revisited.

Fans are usually selected from a range of modelsand sizes, rather than designed specifically for a particular application. Fan selection is based on calculating the airflow and pressure require-ments of a system, then finding a fan of the rightdesign and materials to meet these requirements.Unfortunately, there is a high level of uncertaintyassociated with predicting system airflow and pressure requirements. This uncertainty, combinedwith fouling effects and anticipated capacityexpansion, encourages the tendency to increasethe specified size of a fan/motor assembly.

Designers tend to protect against being responsiblefor inadequate system performance by “over-specifying.” However, an oversized fan/motorassembly creates a different set of operating problems, including inefficient fan operation,excess airflow noise, poor reliability, and pipe/ductvibrations. By describing some of the problemsand costs associated with poor fan selection, thissourcebook is intended to help designers and oper-ators improve fan system performance through bet-ter fan selection and improved operating andmaintenance practices.

Noise. In industrial ventilation applications, noisecan be a significant concern. High acoustic levelspromote worker fatigue. The noise generated by afan depends on fan type, airflow rate, and pressure.Inefficient fan operation is often indicated by acomparatively high noise level for a particular fantype.

If high fan noise levels are unavoidable, then ways to attenuate the acoustic energy should beconsidered. Noise reduction can be accomplishedby several methods: insulating the duct; mountingthe fan on a soft material, such as rubber or suit-able spring isolator as required to limit the amountof transmitted vibration energy; or installing sounddamping material or baffles to absorb noise energy.

Rotational Speed. Fan rotational speed is typicallymeasured in revolutions per minute (rpm). Fanrotational speed has a significant impact on fanperformance, as shown by the following fan laws:


RPMfinalAirflowfinal = Airflowinitial ( )

RPMinitial

RPMfinalPressurefinal = Pressureinitial ( )

2

RPMinitial

RPMfinalPowerfinal = Powerinitial ( )3

RPMinitial


Rotational speed must be considered concurrentlywith other issues, such as variation in the fan load,airstream temperature, ambient noise, andmechanical strength of the fan.

Variations and uncertainties in system requirementsare critical to fan type and fan rotational speedselection. Fans that generate high airflow at relatively low speeds (for example, forward-curvedblade centrifugal fans) require a relatively accurateestimate of the system airflow and pressure demand.If, for some reason, system requirements are uncertain, then an improper guess at fan rotationalspeed can cause under-performance or excessiveairflow and pressure.

Airstream temperature has an important impact onfan-speed limits because of the effect of heat onthe mechanical strength of most materials. At hightemperatures, all materials exhibit lower yieldstrengths. Because the forces on shafts, blades, andbearings are proportional to the square of the rotational speed, high-temperature applications areoften served by fans that operate at relatively lowspeeds.

Airstream Characteristics. Moisture and particulatecontent are important considerations in selectingfan type. Contaminant build-up on fan blades cancause severe performance degradation and fanimbalance. Build-up problems are promoted by ashallow blade angle with surfaces that allow con-taminants to collect. Fans with blade shapes thatpromote low-velocity air across the blades, such asbackward inclined fans, are susceptible to contaminant build-up. In contrast, radial tip fansand radial blade fans operate so that airflow acrossthe blade surfaces minimizes contaminant build-up.These fans are used in “dirty” airstreams and inmaterial handling applications.

Corrosive airstreams present a different set of problems. The fan material, as well as the fan type,must be selected to withstand corrosive attack.Also, leakage into ambient spaces may be a concern, requiring the fan to be equipped with ashaft seal. Shaft seals prevent or limit leakage fromaround the region where the drive shaft penetratesthe fan housing. For example, in corrosive environ-ments fans can be constructed with expensive alloysthat are strong and corrosion resistant, or they can

be less expensively constructed with fiberglass-reinforced plastic or coated with a corrosion-resistant material. Because coatings are often lessexpensive than superalloy metals, fan types thatwork well with coatings (for example, radial fanblades because of their simple shape) are widelyused in corrosive applications; however, wear willreduce the reliability of coatings. Alternately, mate-rials such as reinforced fiberglass plastics havebeen developed for fan applications and function effectively in many corrosive environments.However, there may be size and speed limitationsfor composite materials and plastic materials.

Airstreams with high particulate content levels canalso be problematic for the fan drive train. In directdrive axial fans, the motor is exposed to theairstream. Sealed motors can be used in theseapplications but tend to be more expensive and, in the event of lost seal integrity, they are suscepti-ble to expensive damage. In axial fans, belt drivesoffer an advantage by removing the motor from theairstream. In centrifugal fans, the particulate content is less of a factor because the motor orsheave can be located outside of the fan enclosureand connected to the impeller through a shaft seal.Gear drives are occasionally used in applicationswhere speed reduction is required but the use of belt drives is unfeasible because of access or maintenance requirements.

In flammable environments, fans are usually constructed of nonferrous alloys to minimize therisk of sparks caused by metal-to-metal contact. Insome applications, certain components of the fancan be fabricated out of spark-resistant materials.Fans that operate in flammable environmentsshould be properly grounded, including rotatingcomponents, to minimize sparking because of stat-ic discharge.

Temperature Range. To a large degree, temperaturerange determines fan type and material selection.In high-temperature environments, many materialslose mechanical strength. The stresses on rotatingcomponents increase as the fan’s operating speedincreases. Consequently, for high-temperatureapplications, the fan type that requires the lowestoperating speed for a particular service is oftenrecommended. Radial blade fans can be ruggedlyconstructed and are frequently used in



high-temperature environments. Component materialsalso significantly influence a fan’s ability to servein high-temperature applications, and differentalloys can be selected to provide the necessarymechanical properties at elevated temperatures.

Variations in Operating Conditions. Applications thathave widely fluctuating operating requirementsshould not be served by fans that have unstableoperating regions near any of the expected operating conditions. Because axial, backward-inclined airfoil, and forward-curved fans tend tohave unstable regions, these fans are not recom-mended for this type of service unless there is ameans of avoiding operation in the unstableregion, such as a recirculation line, a bleed fea-ture, or some type of anti-stall device.

Space Constraints. Space and structural constraintscan have a significant impact on fan selection. Inaddition to dimensional constraints on the spaceavailable for the fan itself, issues such as mainte-nance access, foundation and structural supportrequirements, and ductwork must be considered.Maintenance access addresses the need to inspect,repair, or replace fan components. Because down-time is often costly, quick access to a fan can pro-vide future cost savings. Foundation and structuralrequirements depend on the size and weight of afan. Selecting a compact fan can free up valuablefloorspace. Fan weight, speed, and size usuallydetermine the foundation requirements, which, inturn, affect installation cost.

If the available space requires a fan to be locatedin a difficult configuration (for example, with anelbow just upstream or downstream of a fan), thensome version of a flow straightener should be considered to improve the operating efficiency.Because non-uniform airflow can increase the pres-sure drop across a duct fitting and will degrade fan performance, straightening the airflow will loweroperating costs. For more information, see the factsheet titled Configurations to Improve Fan SystemEfficiency on page 39.

An important tradeoff regarding space and fan systems is that the cost of floor space often motivates designers and architects to configure afan system within a tight space envelope. One wayto accomplish this is to use small-radius elbows,

small ducts, and very compact fan assemblies.Although this design practice may free up floorspace, the effect on fan system performance can besevere in terms of maintenance costs. The use ofmultiple elbows close to a fan inlet or outlet cancreate a costly system effect, and the added pressure drops caused by small duct size or acramped duct configuration can significantlyincrease fan operating costs. System designersshould include fan system operating costs as aconsideration in configuring fan assemblies andductwork.

Fan Performance Curves

Fan performance is typically defined by a plot ofdeveloped pressure and power required over arange of fan-generated airflow. Understanding thisrelationship is essential to designing, sourcing, andoperating a fan system and is the key to optimumfan selection.

Best Efficiency Point. Fan efficiency is the ratio ofthe power imparted to the airstream to the powerdelivered by the motor. The power of the airflow isthe product of the pressure and the flow, correctedfor units consistency. The equation for total efficiency is:

An important aspect of a fan performance curve is the best efficiency point (BEP), where a fan operates most cost-effectively in terms of bothenergy efficiency and maintenance considerations.Operating a fan near its BEP improves its performance and reduces wear, allowing longerintervals between repairs. Moving a fan’s operatingpoint away from its BEP increases bearing loadsand noise.

Another term for efficiency that is often used withfans is static efficiency, which uses static pressureinstead of total pressure in the above equation.When evaluating fan performance, it is importantto know which efficiency term is being used.


Total Pressure x AirflowTotal Efficiency =

bhp x 6,362

Where: Total Pressure is in inches of waterAirflow is in cubic feet per minute (cfm)bhp is brake horsepower

A Sourcebook for Industry

Figure 1-1. Region of Instability5

7

Region of Instability. In general, fan curves arcdownward from the zero flow condition—that is,as the backpressure on the fan decreases, the air-flow increases. Most fans have an operating regionin which their fan performance curve slopes in thesame direction as the system resistance curve. A fan operating in this region can have unstableoperation. (See Figure 1-1.) Instability results fromthe fan’s interaction with the system; the fan attemptsto generate more airflow, which causes the systempressure to increase, reducing the generated air-flow. As airflow decreases, the system pressurealso decreases, and the fan responds by generatingmore airflow. This cyclic behavior results in asearching action that creates a sound similar tobreathing. This operating instability promotes poorfan efficiency and increases wear on the fan components.

Fan Start-Up. Start-up refers to two different issues in the fan industry. Initial fan start-up is the commissioning of the fan, the process of ensuringproper installation. This event is important for several reasons. Poor fan installation can causeearly failure, which can be costly both in terms ofthe fan itself and in production losses. Like otherrotating machinery, proper fan operation usuallyrequires correct drive alignment, adequate foundation characteristics, and true fit-up to connecting ductwork.

Fan start-up is also the acceleration of a fan fromrest to normal operating speed. Many fans, particularly centrifugal types, have a large rotation-al inertia (often referred to as WR2), meaning theyrequire significant torque to reach operating speed.


5 Although fan system curves can have a static component, for the purposes of this sourcebook, system curves pass through (0,0).

Slope Lines

2,000 4,0003,000 13,000 15,000 17,00011,0005,000 7,000 9,000

6,000 8,000 10,000 12,000 14,000 16,000 18,000

Region of Instability

SystemCurvesSt

atic

Pre

ssur

e(in

. wg)

Airflow Rate (cfm)

FanCurve

26

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2

In this region, the slopes of the fan curveand the system curve are near parallel.Instability results when the fan curveintersects the system curve at more than onepoint, causing the fan to hunt.


In addition to the WR2 load, the air mass movedby the fan also adds to the start-up torque require-ments on the fan motor. Although rotational inertiais not typically a problem in heating, ventilation,and air conditioning (HVAC) applications, it maybe a design consideration in large industrial appli-cations. Proper motor selection is essential in ensuring that the fan can be brought to its operating speed and that, once there, the motoroperates efficiently.

Because the start-up current for most motors is 2 to 5 times the running current, the stress on themotor can be significantly reduced by starting afan under its minimum mechanical load andallowing the motor to achieve normal operatingspeed more quickly than when under full load. In many applications, system dampers can be positioned to reduce the load on the fan motorduring start-up. For example, the power requiredby a centrifugal fan tends to increase with increasingflow (although in “non-overloading” fan types, thepower drops off after reaching a peak). In axialfans, the power tends to decrease with increasingflow. Consequently, for most centrifugal fan types,large fan start-ups should be performed withdownstream dampers closed, while for most axialfan types, start-ups should be performed with thesedampers open. However, there are exceptions tothese guidelines, and the actual power curve forthe fan should be evaluated to determine how tosoften the impact of a large fan start-up.

The power surges that accompany the starting oflarge motors can create problems. Among theeffects of a large start-up current are power qualityproblems and increased wear on the electrical sys-tem. In response to increasing demand for equip-ment that minimizes the problems associated withlarge motor starts, electrical equipment manufac-turers are offering many different technologies,including special devices known as soft starters, toallow gradual motor speed acceleration. A keyadvantage of variable frequency drives (VFDs) isthat they are often equipped with soft starting fea-tures that decrease motor starting current to about1.5 to 2 times the operating current. Although VFDsare primarily used to reduce operating costs, theycan significantly reduce the impact of fan starts onan electrical system.

In axial fan applications, controllable pitch fansoffer a similar advantage with respect to reducingstart-up current. Shifting the blades to a low angleof attack reduces the required start-up torque ofthe fan, which allows the motor to reach operatingspeed more quickly. For more information onVFDs and controllable pitch fans, see the factsheet titled Controlling Fans with Variable Loadson page 43.

System Effect. The system effect is the change insystem performance that results from the interactionof system components. Typically, during the designprocess, the system curve is calculated by addingthe losses of each system component (dampers,ducts, baffles, filters, tees, wyes, elbows, grills, louvers, etc.). The governing equation for pressureloss across any particular component is:

The result of this equation is a parabolic line, asshown by the system curve in Figure 1-2. This system curve assumes all components display pressure loss characteristics according to their losscoefficients. However, in reality, non-uniform airflow profiles that are created as the airstreamdevelops swirls and vortices cause system components to exhibit losses that are higher thantheir loss coefficients. The overall effect of theseadded losses is to move the system curve up, asshown by the corrected system curve in Figure 1-2.

The system effect can be minimized by configuringthe system so that the flow profile remains as uniform as possible. However, if space constraintsprevent an ideal system layout, then system effectconsequences should be incorporated into the fanselection process. For more information on how tominimize losses, see the fact sheet titled Configurationsto Improve Fan System Efficiency on page 39.


V∆p = C ( )2ρ

1,097

Where: ∆p = pressure loss in inches of water gage (in. wg)

C = loss coefficient for the componentV = velocity in feet per minuteρ = density of the airstream (0.075 pounds

per cubic foot at standard conditions)

Figure 1-2. System Effect for a Typical Fan and System

9

The system effect can be particularly problematicwhen the airflow into or out of a fan is disruptedinto a highly non-uniform pattern. Poor configurationof ductwork leading to or from a fan can severelyinterfere with a fan’s ability to efficiently impartenergy to an airstream. For example, placing anelbow close to the fan outlet can create a systemeffect that decreases the delivered flow by up to30 percent. This can require an increase in fanspeed, which in turn results in an increase inpower and a decrease in system efficiency.

Although underestimating the system effect causes insufficient air delivery, many designersovercompensate for it and other uncertainties by selecting oversized fans. This practice createsproblems such as high energy costs, high mainte-nance, and reduced system reliability. A more reasonable approach is to combine proper systemlayout practices with an accurate estimate of thesystem effect to determine an appropriate fan size.

Fan System Components

A typical fan system consists of a fan, an electricmotor, a drive system, ducts or piping, flow controldevices, and air conditioning equipment (filters,cooling coils, heat exchangers, etc.). An examplesystem is illustrated in a diagram on page 10.

To effectively improve the performance of fan systems, designers and operators must understandhow other system components function as well.The “systems approach” requires knowing theinteraction between fans, the equipment that supports fan operation, and the components thatare served by fans.

Prime Movers. Most industrial fans are driven byalternating current (AC) electric motors. Most areinduction motors supplied with three-phase, 240- or 480-volt power. Because power suppliesare typically rated at slightly higher voltages thanmotors because of anticipated voltage drops in the


System Curve (with system effect)

System Curve(as calculated)

Expected Performance

Actual Performance

Fan Curve

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(in. w

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2,000 4,000

3,000 13,000 15,000 17,00011,0005,000 7,000 9,000

6,000 8,000 10,000 12,000 14,000 16,000 18,000

Airflow Rate (cfm)

A Sourcebook for Industry


distribution system, motors are typically rated at230 or 460 volts. In recent years, because ofefforts by the National Electrical ManufacturersAssociation (NEMA) and motor manufacturers, theefficiency of general-purpose motors has signifi-cantly improved. These improvements are alsoattributable to the Energy Policy Act (EPAct), whichfor most motors went into effect in October 1997.To improve motor efficiency, motor manufacturershave modified motor designs and incorporatedbetter materials, resulting in slight changes inmotor operating characteristics. Although initialcosts of the motors have increased 10 to 20 per-cent, for high run-time applications, improvementsin motor efficiency create very attractive paybacksthrough lower operating costs.

A characteristic of induction motors is that theirtorque is directly related to slip, or the differencebetween the speed of the magnetic field and thespeed of the motor shaft. Consequently, in many

fans, actual operating speeds are usually around 2 percent less than their nominal speeds. Forexample, a theoretical four-pole induction motorwith no slip would rotate at 1,800 rpm with a 60-hertz power supply; however, rated operatingspeeds for this motor are usually around 1,750 rpm,indicating that slip rates are a little over 2.7 percentat rated load. Fans that are driven by older motorsare probably operating at much lower efficienciesand at higher levels of slip than what is availablefrom new motors.

Upgrading to a new motor can reduce operatingcosts, because of improved motor efficiency, whileoffering slightly improved fan performance. EPAct-efficiency motors operate with less slip, whichmeans fans rotate at slightly higher speeds. Forapplications that can effectively use this additionaloutput, this high efficiency can be attractive.However, if the additional output is not useful, theadded power consumption increases operating costs.


Outlet Diffusers

Filter

Inlet Vanes

Centrifugal Fan

Belt DriveMotor

Motor Controller

Heat Exchanger

Turning Vanes(typically used onshort-radius elbows)

Variable Frequency Drive

Baffles

Figure 1-3. Example Fan System Components


Another component of the prime mover is the motorcontroller. The controller is the switch mechanismthat receives a signal from a low power circuit,such as an on/off switch, and energizes or de-ener-gizes the motor by connecting or disconnectingthe motor windings to the power line voltage.Soft starters are electrical devices that are ofteninstalled with a motor controller to reduce theelectrical stresses associated with the start-up oflarge motors. In conventional systems, the high in-rush and starting currents associated with mostAC motors creates power quality problems, such as voltage sag. Soft starters gradually ramp up the voltage applied to the motor, reducingthe magnitude of the start-up current. As industrialfacilities increase the use of computer-basedequipment and control systems, soft starters arebecoming important parts of many motor controlsystems. In fact, a major advantage associated withmost VFDs is that they often have built-in, soft-startcapabilities.

Another common characteristic of motors used infan applications is multiple speed capability.Because ventilation and air-moving requirementsoften vary significantly, the ability to adjust fanspeed is useful. Motors can be built to operate atdifferent speeds in two principal ways: as a singleset of windings equipped with a switch that ener-gizes or de-energizes an additional set of poles, orwith the use of multiple windings, each of whichenergizes a different number of poles. The firsttype of motor is known as a consequent polemotor and usually allows two operating speeds,one twice that of the other. The second type ofmotor can have two, three, or four speeds,depending on application. In general, multiple-speed motors are more costly and less efficient thansingle-speed motors. However, the flow controlbenefit of different motor speeds makes themattractive for many fan applications.

Drive System. The drive system often offers substantial opportunities to improve energy efficiency and to lower overall system operatingcosts. There are two principal types of drive systems:direct drive and belt drive. Gear drives are alsoused but are less common. In direct drive systems,the fan is attached to the motor shaft. This is a simple, efficient system but has less flexibility withrespect to speed adjustments.

Because most fans are operated with inductionmotors, the operating rotational speeds of direct-drive fans are limited to within a few percent ofthe synchronous motor speeds (most commonly1,200, 1,800, and 3,600 rpm). The sensitivity offan output to its operating rotational speed meansthat errors in estimating the performance require-ments can make a direct-drive system operate inef-ficiently (unlike belt drives, which allow fan rota-tional speed adjustments by altering pulley diame-ters). One way to add rotational speed flexibility toa direct-drive system is to use an adjustable speeddrive (ASD). ASDs allow a range of shaft speedsand are quite practical for systems that have varyingdemand. Although ASDs are generally not a prac-tical option for fans that are only required to oper-ate at one speed, ASDs can provide a highly effi-cient system for fans that operate over a range ofconditions.

In axial fans, direct drives have some importantadvantages. Applications with low temperaturesand clean system air are well-suited for directdrives because the motor mounts directly behindthe fan and can be cooled by the airstream. Thisspace-saving configuration allows the motor tooperate at higher-than-rated loads because ofadded cooling. However, accessibility to the motoris somewhat restricted.

Belt drives offer a key advantage to fan systems by providing flexibility in fan speed selection. Ifthe initial estimates are incorrect or if the systemrequirements change, belt drives allow flexibilityin changing fan speed. In axial fans, belt driveskeep the motor out of the airstream, which can bean advantage in high temperature applications, orin dirty or corrosive environments.

There are several different types of belt drives,including standard belts, V-belts, cogged V-belts,and synchronous belts. There are different cost andoperating advantages to each type. In general, synchronous belts are the most efficient, while V-belts are the most commonly used. Synchronousbelts are highly efficient because they use a mesh-type contact that limits slippage and can loweroperating costs. However, switching to synchronousbelts must be done with caution. Synchronousbelts usually generate much more noise than other belts. They also transfer shock loads through the



drivetrain without allowing slip. These suddenload changes can be problematic for both motorsand fans. Another problem with synchronous beltsis the limitedthe pulleys halters the pitengagementin discrete sadvantage ofrotational spBecause of tas widely us

In contrast, Vtheir efficienV-belts have VVwhich meanabout them.their protectload changesudden drivewear or suddtend to transshafts and mprotection. Asynchronoussuch as low flexibility. Inupgrades to

Although theoffer some atend to be malternatives; frequent inspand are preflimited acceconfiguratiooffset drivesmay provide an attractive advantage in someapplications. Gear-system efficiency depends largelyon speed ratio. In general, gear efficiencies rangefrom 70 to 98 percent. In large horsepower (hp)applications (greater than 100 hp), gear systemstend to be designed for greater efficiency becauseof the costs, heat, and noise problems that resultfrom efficiency losses. Because gears require lubri-cation, gearbox lubricant must be periodicallyinspected and changed. Also, because gears—likesynchronous belts—do not allow slip, shock loadsare transferred directly across the drivetrain.

Ductwork or Piping. For most fan systems, air isdirected through ducts or pipes. In general, ductsare made of sheet metal and used in low-pressure

and used inause ducts areions, “duct” will

sourcebook; how-can be applied to

h a fan pullsn one side andspace (like at losses are not aost applications,es of a fan and

ormance. Frictionct surface is usu-rall load on a fan.

er airflowough larger ductsof material andnergy because of

e costs and shouldign process ands. For moret titled Fan

Other considera-nd leakage class.a per unit cross

ucts and, as ar cool airstreams,he amount ofnt.

tified by the/linear foot) is anes that determine

CL include the type of joints used in construction,the number of joints per unit length of duct, andthe shape of the duct. Depending on the lengthof the duct system, leakage can account for asignificant portion of a fan’s capacity. This isespecially applicable to systems with rectangularducts that have unsealed joints. In many cases, thesystem designer can improve the performance ofthe ventilation system by specifying ducts thathave low CLs. For more information see the factsheet titled System Leaks on page 37.


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