blower engineering basics

Blower Application Basics

Blower Application Basics:

Selecting a blower for a particularapplication first requires knowledge oran estimate of the application’s operat-ing point: the flow rate required and theresistance to flow that the system, duct-work, filters, obstructions, etc., will im-pose. This system resistance is oftenreferred to as backpressure or headloss. The performance curves shown inthis catalog represent each model’s typ-ical maximum performance (full speed)at a constant input voltage. The perfor-mance curve describes a blower’s abil-ity to deliver flow rate againstbackpressure - from zero backpressure(“open flow”) to completely blocked flow(“sealed”). Since all Ametek BLDCblowers have adjustable speed control,any operating point that lies underneatha given performance curve can bereached. With knowledge of the de-sired operating point, it is then a simplematter to browse the catalog to identifyblowers having performance curvesthat exceed the desired operating point.Other factors should then be consid-ered, which will be discussed hereafter.

System Resistance Curve: A resis-tance to flow is typically characterizedby a pressure drop for a given flow rate,and often follows a 2nd order relation-ship (see Figure 1) (in some cases suchas certain filters, the relationship ismore linear). If the desired operatingpoint is known, then its associated sys-tem curve can be estimated accordingto the relationship P=CQ2. Plugging thevalues for desired operating point intothe P (pressure drop) and Q (flow rate)terms yields the constant C. The sys-tem curve can then be overlaid graphi-cally onto a given blower performancecurve. The intersection of the twocurves is the actual operating point (seeFigure 2). The pressure rise providedby the blower matches the pressureloss imposed by the system resistance.For quick checks, simply place the op-erating point on a given blower curveand “eyeball” the system curve throughit. There are likely to be several mod-els that can achieve the desired perfor-mance, but not all will operate with thesame efficiency. The blower perfor-mance curves in this catalog include notonly pressure vs. flow rate information,but also current and input power vs.flow rate. An application is normally

best served by selecting the blower thatdelivers the desired performance withthe least amount of input power. Typi-cally, the most efficient blower is theone where the system resistance curveintersects the blower curve at approxi-mately the middle, i.e., half the maxi-mum flow rate. Other questions to consider whenselecting a blower:• Will a blower of this size fit in myapplication?• Would I prefer that the blower havean on-board controller or would I preferto provide an external controller?• Do I need a blower with fast dy-namic response?• Is loudness a major factor?

• Does my application require modu-lating the speed during operation, orsimply a single speed adjustment madeat the time of installation?• What input voltage will I have avail-able for powering the blower?• Are there additional features that Irequire such as a tachometer output,dynamic braking, or speed control func-tions?• Is it a problem for the working fluidto come into contact with electronics?• What are the environmental factorsassociated with my application?

Blower Performance vs. SpeedChanges: Since Ametek’s blowers arespeed controllable, the speed can bereduced to a desired operating point or

Figure 1: Typical System Resistance Curve - Pressure Drop vs. Flow Rate

0

1

2

3

4

5

0 1 2 3Flow Rate

Pres

sure

Dro

p

P = CQ2

P - pressure DropQ - flow rateC - constant for given system

Figure 2: System and Blower Curves - Identifying the Operating Point

0

1

2

3

4

5

0 1 2 3Flow Rate

Pres

sure Actual operating point

Desired operating point

Blower performance curve

System curve

F 1____

This document is for informational purposes only and should not be considered as a binding description of the products or their performance in all applications. The performance data on this page depicts typical performance under controlledlaboratory conditions. AMETEK is not responsible for blowers driven beyond factory specified speed, temperature, pressure, flow or without proper alignment. Actual performance will vary depending on the operating environment and application.AMETEK products are not designed for and should not be used in medical life support applications. AMETEK reserves the right to revise its products without notification. The above characteristics represent standard products. For productdesigned to meet specific applications, contact AMETEK Technical & Industrial Products Sales department.

AMETEK TECHNICAL & INDUSTRIAL PRODUCTS627 Lake Street, Kent OH 44240USA: +1 215-256-6601 - Europe: +44 (0) 845 366 9664 - Asia: +86 21 5763 1258www.ametektip.com


Figure 3: Speed Change Calculation Example

0

1

2

3

4

5

6

7

8

9

10

11

12

0 2 4 6 8 10 12 14 16 18

Flow Rate (cfm)

Pres

sure

(in

H2O

)

0

2

4

6

8

10

12

14

16

18

20

22

24

Inpu

t Pow

er (W

)Sp

eed

(krp

m)

increased if a different operating point isneeded in a dynamic system. Blowersconveniently have very predictable be-havior when it comes to changes inspeed. Table 1 summarizes these rela-tionships.

Figure 3 with Example 1 demonstrateshow to calculate the power requirementfor an operating point that occurs at areduced speed.Example 1:An application calls for air performanceof 8 cfm (ft3/min) at a pressure of 6 inch

H2O. The blower whose performance isshown in Figure 3 has been chosen.What will be the power consumption at8 cfm, 6 in H2O?

Answer: At full speed, the blower willdeliver about 8.8 cfm in this system,whose backpressure is about 7.2 inH2O at that flow rate (see Figure 3).Since this is delivering too much, thespeed must be reduced to achieve thedesired operating point.At full speed, Figure 3 shows the blowerspeed to be about 19.8 krpm and thepower consumption is 18.3 W. Usingthe fan laws shown in Table 1 yields:

So, the power consumption at the re-duced speed is:

blower pressure vs. flowblower input power vs. flowblower speed vs. flowsystem resistance

Table 1: Fan Laws Related toChanges in Blower Speed

N2

N1)(2 = 1

3

N2

N1( )Q2 = Q1

N2

N1( )P2 = P1

2

m2 = m1N2

N1)(

Where, N fan rotational speedm mass flow rate

Q volume flow rateP static pressure (gage)

blower power demandAll other variables constant.

•

N2 = 18.0 krpm

19.8 krpm(8 cfm = 8.8 cfm )N2

18.0 krpm19.8 krpm)(2 = 18.3 W

3

2 = 13.7 W

Pabsolute = Pgage + Patm

Gage Pressure vs. Absolute Pressure:Before proceeding, it’s important at thispoint to note that this tutorial makes refer-ence to both “absolute pressure” and“gage pressure.” Gage pressure is simplythe difference above or below atmo-spheric (or barometric) pressure. Abso-lute pressure is the sum of atmosphericand gage pressures:

Pressure values stated hereafter aregage values unless otherwise stated.

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The speed at the desired operatingpoint could also have been found byusing the ratio of pressures:

Note: There is some minor error in thiscalculation because there will likely besome small change in motor efficiencyat the different operating point. Thiscan usually be neglected for estimatingpower requirements.

Static and Velocity Pressure: Theenergy of a fluid can be viewed as hav-ing two types of pressure: static pres-sure and velocity pressure. Static pressure is the force per unitarea exerted by the fluid on its sur-roundings, and is independent of thefluid’s motion. This is the pressure thatwould be measured by a pressure sen-sor ported normal to the direction offlow. Dividing this pressure by the fluiddensity yields an energy term (per unitmass) often referred to as “pressurehead.” Velocity pressure is essentially thekinetic energy content, or velocity head,of the fluid. It is the pressure that wouldresult from slowing down the streamlinevelocity to zero, converting the kineticenergy into pressure head. Mathemati-cally, it takes the form P=½ 2, where

velocity (including the density valuemaintains force per area, i.e., pressureunits). The sum of the static pressure andvelocity pressure is the total pressure. Within a flow system, there are alsolikely to be changes in elevation(potential energy or elevation head) thatcontribute to the energy content of thestream at a given point. But thechanges in elevation head for gas floware usually small enough to be safelyignored. The blower performance curves

herein are plotted as static pressure (orvacuum) rise vs. flow rate. The velocitypressure is typically not a useful energyquantity for overcoming resistance toflow. In other words, in most cases theresistance to flow results in a loss ofstatic pressure of the streamline and notto a loss of velocity pressure. Technical-ly, a diverging nozzle could be used toconvert velocity pressure to static pres-

sure, thereby increasing the blower’sability to provide flow against backpres-sure, but this is not a common practice.

Fluid Density: The density of the work-ing fluid has a significant influence onblower performance, as can be seen inthe relationships listed in Table 2. Be-cause fluid density is a variable that isindependent of the blower, the densitymust be stated as part of any blowerperformance specification. In the previ-ous example, the density was ignoredfor simplicity. But the performancecurves for all blowers in this cataloghave the statement “normalized to airdensity = .075 lb/ft3,” or equivalentstatement. So, in the previous exam-ple, it was assumed that the desiredperformance of (8 cfm, 6 in H2O) hadalso been normalized to .075 lb/ft3. The density value of .075 lb/ft3 is asomewhat arbitrary selection, but it isclose to typical air density at sea level.It is commonly referred to as “StandardDensity” in the fan and blower industry.Any target operating point must be nor-malized to .075 lb/ft3 when evaluatingblower performance curves in this cata-log. Examples hereafter demonstratehow to do this. The vast majority of blower applica-tions are designed to move air, butAmetek blowers can be used with anynon-explosive, non-corrosive gas mix-ture (exception - see Ametek’s Nautilairseries for blowers designed for combus-tion pre-mix applications). Caution:most of the blowers in this catalog arenot designed to be leak-proof, andmany vent some working fluid to coolthe motor. At the temperature and pres-sure ranges for most blower applica-tions, the gas mixture can beconsidered an ideal gas, and it will be-have according to the relationship:

For gas mixtures, the molar mass of themixture can be determined from theweighted average of the componentgases. Refer to texts on thermodynam-ics or contact Ametek engineering forhelp in calculating density for gas mix-

tures. Because of the difference inviscosity between air and other gases atthe same conditions, the blower perfor-mance curves herein will have someerror when evaluating non-air perfor-mance, even if the operating point isnormalized to standard density. Con-tact Ametek engineering for help in se-lecting a blower for non-air applications.

Determining air density requiresknowing the temperature, barometricpressure, and humidity of the working environment. Measuring temperatureis relatively easy, but barometric pres-sure requires a barometer, and humidityrequires either a wet bulb thermometer,a hygrometer, or a dew point detector. (Note: be careful if using barometric values obtained from weather services - they are normalized for altitude andare not the actual barometric pressure measurements. Once these values are known, theeasiest way to determine density is touse a psychrometric chart, which givesthe properties of air for large ranges oftemperature and humidity. (There arealso on-line calculators available).Some notes on using a psychrometricchart:1) Instead of density, the specific vol-ume, which is the reciprocal of density,usually is plotted.2) The specific volume is typically givenin terms of volume per unit mass of dryair. But the working fluid is a mixture ofair and water vapor (except at 0% hu-midity, of course). See the example

PM

Where, P is the absolute pressureis the density

Ru is the universal gas constant T is the absolute temperature M is the molar mass

RuT

19.8 krpm(6 in H2O = 7.2 in H2O )N22

Table 2: Fan Laws Related toChanges in Fluid Density:

2

1 )(2 = 1

P2 = P12

1 )(

m2 = m12

1 )(

Note: volume flow rate, Q, is independentof density for gases.

•

Where, fluid densityP static pressure (gage)

power demand m mass flow rate

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hereafter for calculating the actual spe-cific volume, i.e., volume per unit massof the air-water vapor mixture.3) Dry bulb temperature is simply thetemperature measured using a regularthermometer.4) For humidity, the wet bulb tempera-ture can be used directly without havingto convert to another type of humidityquantity. As an alternative, relative hu-midity or humidity ratio can be used.5) Psychrometric charts are plotted ata single barometric pressure which isnoted in the heading of the chart. Acorrection is needed to adjust densityvalues to the actual barometric pres-sure.

Example 2: A laboratory makes thefollowing measurements of the ambientair conditions: Barometric pressure = 28.60 in Hg Dry bulb temperature = 72 oF Wet bulb temperature = 65 oFWhat is the ambient density?Answer: Using a psychrometric chartplotted at 29.921 in Hg, the intersec-tions of the curves corresponding to thedry bulb and wet bulb values aboveyields a specific volume of 13.64 ft3/lbdry

air and a humidity ratio of .0116 lbwater-

vapor per lbair. Correct the specific vol-ume per lb air-water vapor mixture:

Finally, correcting this value for the ac-tual barometric pressure yields:

The density of air varies substan-tially with weather and altitude. TheStandard Atmosphere Table (not in-cluded herein, but widely available in

reference texts and internet sites) givesa good estimate for typical outdoor am-bient conditions as a function of altitude.The following example demonstrateshow to account for the lower density athigh altitude:

Example 3: An application calls for ablower to deliver 50 cfm of air at 30 inchH2O at an altitude of 7000 ft above sealevel where the average air density is0.062 lb/ft3, according to the StandardAtmosphere Table. For the purpose ofselecting a blower from this catalog,what operating point should be used?Answer: Refer to Table 2. Since vol-ume flow rate does not change withdensity, the flow rate remains at 50 cfm.The pressure does change in proportionto density:

So, the operating point normalized tostandard density is 50 cfm, 36.3 inchH2O.

Example 3 continued: A blower is se-lected from the catalog that can deliverthe above performance with a powerdemand of 600 W at standard density,as shown on the published performancecurves. What will be the power demandat the intended location at 7000 ft alti-tude?

Answer: Refer to Table 2 for correctingpower demand for a change in density.

Example 3 continued - Redefining theproblem in terms of mass flow rate: Theapplication requires a mass flow rate of3.1 lb/min of air regardless of altitude.A) How much must the speed be re-duced if the application is moved from alocation at 7000 ft (.062 lb/ft3) to sealevel (.075 lb/ft3)?B) If the speed is not adjusted to com-pensate for the change is density, whatwill be the change in volume flow ratebetween the two locations?C) How much will the power demandchange?

Answer:A) Table 2 shows that the change inmass flow rate is a simple ratio of thedensities. If the blower speed is main-tained between the two locations, themass flow rate delivered at sea level willbe:

Mass flow rate is directly proportional toa change in speed (Table 1), so tomaintain 3.1 lb/ft3, the speed must bereduced by:

B) Since volume flow rate is indepen-dent of density, the volume flow rate willnot change between the two locations ifthe speed remains constant. This illus-trates the difference between the quan-tities of volume flow and mass flow - afactor that must be kept in mind whenevaluating an application’s air perfor-mance needs.

C) Tables 1 and 2 show that the powerdemand is directly proportional to bothdensity and speed, although a speedchange has a much larger influence.Combining both relationships yields:

Incompressible Flow: Although thedensity of the working fluid is a variablethat must be accounted for, in a givenapplication the density is assumed to beconstant for applications involving theblowers herein. In other words, for agiven blower in operation the fluid enter-ing the blower has the same density asthe fluid leaving the blower. Anotherterm for this is “incompressible flow.”Density variations are less than one

m2 = 3.10 lb/min )(.075 lb/ft3

.062 lb/ft3

m2 = 3.75 lb/min

mmixture = mwv + mair

= .0116mair+ mair

= 1.0116mair

= .0116mwv

mair

= .989mair

mmix

.989(13.64 ft3/lbair) = 13.49 ft3/lbmix

(= 14.12 ft3/lbmix

13.49 ft3/lbmix )28.60 in Hg29.921 in Hg

= .0708 lbmix/ft3

P2 = 30 inch H2O )(.075 lb/ft3

.062 lb/ft3

P2 = 36.3 inch H2O

2 = 600 W )( .075 lb/ft3

.062 lb/ft3

2 = 496 W

3.10 lb/min = 3.75 lb/min )(N2

N1

= .827 = 17.3% reduction)(N2

N1

2 = 1 )( 2

2 = .684 1 = 31.6% reduction

1 )(N2

N1

3

2 = 1 )( )(3

.062 lb/ft3

.075 lb/ft3.827

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percent for flow speeds less than Mach= 0.2, which is greater than typical ap-plications for these blowers.

Blower Life: Brushless DC blowershave the advantage of long life com-pared to blowers driven by brush mo-tors. Since the motor is electronicallycommutated there are no brushes towear out, so the end of life scenario forbrushless blowers is normally bearingfailure. Since long life is one of the mainattributes of brushless technology, thequestion “How long will it last?” is com-mon. Unfortunately, there is no simpleanswer to this question. There are anumber of factors that influence life, andwe currently have no way to account forevery possible application. The primaryfactors that influence life are:

• temperature• bearing contamination• mechanical shock• operating point

Temperature: The deterioration ofbearing grease is directly proportionalto its temperature. So, the higher thetemperature, the shorter the bearinglife. There is no hard limit on bearingtemperature, but just a general rule thatcooler is better. There is a point ofdiminishing returns that is applicationdependent - reducing temperature isunnecessary if the blower life is ade-quate for the application. A rule ofthumb for Ametek blowers is 45 oCmaximum ambient temperature, but thiscan be exceeded under certain circum-stances. Please consult Ametek engi-neering for applications a hightemperatures. Ultimately, it is up to thecustomer to determine whether or not ablower can provide adequate life. Thor-ough testing is recommended. Alsokeep in mind that extreme temperaturecan cause premature failure of electron-ics or the motor winding.

Bearing Contamination: The introduc-tion of particulate matter increases therolling friction of the bearing, which in-creases temperature. Particulates alsoscar the surface of the bearing race-ways and balls, which increases pittingand flaking, further exacerbating thecondition. Corrosive gases can reactnegatively with grease or cause corro-sion of the steel in the bearings. Liquidsof any kind should be avoided. Water

vapor is not a problem as long as it doesnot condense into liquid water inside theblower.

Mechanical Shock: Mechanicalshock can cause denting of thebearing’s rolling elements. This resultsin unwanted noise and can lead to flak-ing and pitting inside the bearing. Appli-cations that subject the blower tomechanical shock should be thoroughlytested to ensure adequate life.

Operating point: The amount of back-pressure a blower works against is an-other influencing factor in some blowermodels. The backpressure results in apressurized blower housing, putting apressure gradient across the adjacentbearing. The pressure gradient tends topush the grease out and contaminationin. Ametek has implemented designfeatures to minimize this effect, and thecustomer should not be hesitant to op-erate at high pressure if the applicationcalls for it.

Again, blower life is highly applica-tion dependent. Customer requests forMTTF (Mean Time To Failure) informa-tion are difficult to answer. Moreover,what constitutes a failure is often opento interpretation. For example, therecan be situations where a blower isdelivering air performance as required,but it has deteriorated bearings thatproduce unacceptable noise for sound-critical applications. Ametek does conduct an ongoinglife test program to gather life informa-tion and to improve the durability of ourblowers. Tests are conducted on blow-ers that represent all model families.History has shown that Ametek blowerssurvive beyond 10,000 hours of contin-uous running in typical applications. Itis common for blowers to endure morethan 20,000 hours, and some blowershave accumulated 30,000+ hours. Butagain, it must be stressed that blowerlife is application dependent, and thecustomer is encouraged to conduct alife test in-situ under the application’smost rigorous conditions. The life val-ues mentioned above are not to beused as a basis for warranty claims.

Safety Bulletin: In the applica-tion of Ametek, Inc. motors and/or blow-ers as a component in your product, youmust exercise the following minimumprecautions:

THE FAILURE TO OBSERVE THEFOLLOWING SAFETY PRECAU-TIONS COULD RESULT IN SERIOUSBODILY INJURY, INCLUDING DEATHIN EXTREME CASES. We recommendthat adequate instructions and warningsby the original equipment manufactur-ers (OEM) include labels setting forththe precautions listed below to the enduser. The motors and/or blowers must beconnected to a proper and effectiveground or mounted in a manner that willguarantee electrical isolation and insu-late the user and others from electricshock. End product design should notrely solely on the primary insulation ofthe motor. Standard blowers and/or motorsare not designed to handle volatile orflammable materials through the fansystem unless specifically designed.Passing combustible gases or otherflammable materials through the fansystem could result in leakage whichcould cause a fire or explosion. These products must not be used inan area contaminated by volatile orflammable materials since sparking ispredictable in the normal operation ofthe motor and may ignite the volatilecausing a dangerous explosion. The Technical and Industrial Prod-ucts Division of Ametek, Inc. can supplyspecifically designed motors and blow-ers for use in handling combustiblegases or for use in hazardous dutylocations. These specially designedunits should only be used in conjunctionwith combustible gases, which theywere specifically designed to handle.The type of gases must be so noted onthe product label and in the instructions. The rotation of the motor shaft, oranything mounted on the shaft, is apotential source of injury and must betaken into account in the design of yourend product. You must provide thenecessary guarding or housing as re-quired by the finished product and youmust indicate to the user the direction ofrotation. Do not remove guard as se-vere bodily injury may occur to fingersor appendages. Products incorporating vacuummotors/blowers must be designed so asto prevent the vacuum or air pressurefrom being concentrated in a mannerthat can expose the user to injury bycoming into contact with any body area,such as eyes, ears, mouth, etc.

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The motors and/or blowers mustnot be exposed to moisture or liquid orused outdoors, except in equipmentwhich is specifically designed for out-door use and meets the appropriateregulatory agency requirements for out-door use. Moisture or liquid can dam-age the motor/blower and defeat theelectrical insulation resulting in a severeelectrical shock to the user. Ametek motors/blowers must notbe operated above the design voltage.Over voltage conditions can cause ex-cessive speed of the motor and canresult in severe electrical shock and/orother traumatic injury to the operator. Precautions must be exercised toensure motor leads are properly routedand connected in your equipment.Lead wires must be routed and retainedto ensure that they do not becomepinched or come in contact with rotatingparts during assembly or subsequentoperations. Connections must be de-signed so that proper electrical contactis established and the connections mustbe properly insulated. Disassembly or repairs of Ametekproducts should not be attempted. Ifaccomplished incorrectly, repairs cancreate an electrical shock and/or opera-tional hazard. It is recommended thatrepairs be made only by Ametek andnot by others. In the event that the motor orblower ceases to operate, power must

be disconnected before examinationand/or removal from the system. Contact Ametek, Inc. to discuss anyquestionable application before select-ing a standard motor or blower. In setting forth the above listedrecommendations with regards to pre-cautionary steps that must be consid-ered, we in no way intend to imply thatif these steps are taken a product willmeet the applicable safety standards.We, at Ametek, are not sufficiently con-versant with the specific safety hazardswhich may be associated with particularproducts. We can only advise precau-tions to be employed generally for thesafe use of Ametek products as compo-nents. For testing specifically related tothe safety of the product, we recom-mend that you contact the appropriateregulatory agency as indicated by thetype of product being manufactured.

Unit Conversions

Length: 1 ft = 12 inch = .3048 m = 30.48 cm = .0001894 mile

Volume: 1 ft3 = .02832 m3

= 28.32 L = 7.481 gal

Mass: 1 lbm = .4536 kg

Force: 1 lbf = 32.174 lbm•ft/s2

= 4.448 N

Volume Flow Rate: 1 cfm (ft3/min) = 28.32 L/minm 996.1 = 3/hr

Pressure: 1 inch H2O = .249 kParabm 94.2 =

bl( isp 1630. = f/in2)gH hcni 5370. =

Density: 1 lbm/ft3 = 16.03 kg/m3

= .01602 g/cm3

Energy: 1 ft•lbf = 1.356 J = 1.356 N•m = .001285 Btu

Power: 1 W = .00134 hp = 3.413 Btu/hr = 550 (ft-lbf)/s

Temperature: T (oR) = T(oF) + 459.7 = 1.8[T(oC + 273.15] = 1.8T(K)

Universal Gas Constant, Ru: = 10.73 psia•ft3/(lbmol•oR)bl•tf 5451 = f/(lbmol•oR)

)K•lomgk(/Jk 413.8 = = 8.314 kPa•m3/(kgmol•K)

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blower engineering basics

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