RENGA No.1621

ORIENTAL MOTOR

1. IntroductionWith small fans, selecting the right fan (air flow method)

that fits its purpose is extremely important from thestandpoint of energy efficiency, noise reduction, and weightreduction. However, there are a great number of types ofsmall fans. In addition, their purposes range widely fromcooling, air blasting, and ventilation. Therefore, we mustselect a fan that most fits its purpose. Here, we introduce onesmall fan type, the Sirocco fan. We will explain itscharacteristic of high efficiency and low noise in a high staticpressure environment and its appropriateness for coolingrecent high density mounting equipment.

2. Types and Characteristics of Small Fans2.1 Types

Small fans can be divided into three types by the air flowmethod; propeller fan, sirocco fan, and cross flow fan.

The structure and characteristics of these fans are given inthe Table 1.

Characteristics of Centrifugal Blower and Its Effective Use in High Static Pressure Area

Masayuki TAKAHASHI

With small fans, selecting the right fan that most fits the purpose is extremely important from thestandpoint of energy efficiency, noise reduction, and weight reduction. However, the number of small fantypes is large. In addition, their purposes vary widely ranging from cooling and ventilation, to air extraction.Therefore, we must select a fan that most fits its purpose. Here, we introduce one small fan type, theSirocco fan. We will explain how the motor is able to perform with high efficiency and low noise in a highstatic pressure environment and how this fan is most suitable for cooling the recent high density mountingequipment.

Air Flow

Air FlowAir Flow

Figure 1 Propeller Fan Figure 2 Sirocco Fan Figure 3 Cross Flow Fan

Fan Type Structure and Characteristics

PropellerFan

(Axial)

The propeller (blade vanes) in the circular flow path betweencylindrical hub and housing compresses and sends air andgenerates air stream in the direction of rotation axis.Because air flows along the rotation axis, this type hascompact structure. In addition, the fan's capacity to obtaingreat airflow makes it suitable for ventilation/cooling an entiresection inside machine.

Sirocco Fan(Centrifugalwith multiple

blades)

The centrifugal force created by the runner (forward curvedvanes) aligned in cylindrical form creates spiral flow almostperpendicular to the rotation axis. The scroll rectifies the spiralflow in one direction and thus increases pressure.This fan limits the opening where air can come out and unidirectionallyconcentrates airflow. These characteristics lead the fan to be used forlocal cooling. With its high static pressure, this fan is also suitable forcooling in ill-ventilated system or for air flow using ducts.

Cross FlowFan

(Throughflow, Cross

flow)

This fan has a runner similar to that of sirocco fans. However,two sides of the runner are covered with side plates andtherefore, no air flows in from axial direction. This createsthrough flow that passes through the runner. The cross flowfan uses this through flow.Air is sent by the long cylindrical runner, allowing us to obtainwide airflow. The air is sent out horizontally along the runnercircumference, resulting in uniform airflow.This fan is suitable for applications such as uniform cooling ofcircuit boards mounted in systems, or air curtain. Because ofairflow bent at right angle and its rectangular parallelepipedshape, this fan can be installed at a corner of a system,making an effective use of space.

Table 1 Fan Structure and Characteristics

RENGA No.162 2

The best efficiency point (hereinafter referred to asoptimum specific speed) is found at ns = 2000 for propellerfans and ns = 400 for sirocco fans.

2.2.3 Relationship between specific speed and airflow -

static pressure characteristic

In general small fans, the rotational speed N is nearlyconstant at around 3000 r/min.

The equation (4) shows that the known specific speed ns thatgives high efficiency for each air flow method and constantrotational speed N determine the relationship between theoptimum airflow and static pressure for each blast method.

In other words, there is an operation zone that is optimumfor each blast method.

The relationship among the optimum ns, airflow, and staticpressure for sirocco fans and propeller fans is shown in theFigure 6.

For a sirocco fan (optimum specific speed ns = 400), thecurve A represents the relationship between the optimumairflow and static pressure. From this curve, we can see thatthe high efficiency zone for sirocco fans is on high staticpressure side.

For a propeller fan (optimum specific speed ns = 2000), thecurve B represents the relationship between the optimumairflow and static pressure. From this curve, we can see thatthe high efficiency zone for propeller fans is on high airflowside.

ORIENTAL MOTOR

2.2 Specific Speed and Efficiency2.2.1 Specific speed

Two fans with structural similarity shown in the Figure 4demonstrate similar performance.

While the airflow of a fan is proportional to the third powerof the runner diameter and to the rotational speed, pressure isproportional to the square of the runner diameter and to thesquare of the rotational speed. This can be expressed in thefollowing equations.

....................................................(1)

............................... (2)

Q : Airflow D : Runner diameter : DensityP : Static pressure N : Rotational speedg : Gravitational acceleration

By eliminating D1/D2 from equations (1) and (2), we canobtain an index that does not depend on fan size.

.......... (3)

The equation (3) expresses specific rate and generallyrepresented by ns.

........................................ (4)

Using past design examples, we can acquire important datain designing fans such as relationship between specific rateand efficiency.

2.2.2 Relationship between specific speed and efficiency

The relationship between specific speed and efficiency bythe air flow method is shown in the Figure 5.

NQ11/2

[P1/( g)]3/4=ns

Q11/2

[P1/( 1 g)]3/4=N1

Q11/2

[P2/( 2 g)]3/4N2

P1/( 1 g)P2/( 2 g)

D1D2

= 2 N1

N2

2

( ) ( )

Q1Q2

D1D2

N1N2

= 3

( )

w1c1

u1w2

c2u2

Figure 4 Fans with Structure Similarity

90

80

70

60

50

40

30

2040 5 6 8 100 2 3 4 5 6 8 1000 2 3 4000

Specific Speed nsMax

imum

Tota

l Pre

ssur

e Ef

ficien

cy

m

ax [%

]

Centrifugal Fan withNarrow Straight Vanes

Turbo BlowerRadial Fan

Turbo Fan

Propeller Fan

Fan with Multiple Vanes

Axial Fanwith StreamlinedStator Vanes

Tubelike Axial Fan

Axial Fan with Stator Vanes

Figure 5 Relationship between Specific Speed and Efficiency(By Air Flow Method)

00

100

200

300

400

2 4 6 8 10 12Air Flow [m3/mn]

Stat

ic Pr

essu

re [P

a]

Specific Speed ns, = 400Maximum Efficiency for Sirocco Fan

MB1255-B 100(V)50(Hz)

MB1040-B 100(V)50(Hz)

MRS16-BUL 100(V)50(Hz)

MRS18-BUL 100(V)50(Hz)

B

Specific Speed ns, = 2000Maximum Efficiency for Propeller Fan

A

Figure 6 Relationship between Specific Speed ns and Airflow-StaticPressure Characteristic (By Air Flow Method)

To prevent such issues, we need to know the noise level atthe operating point in advance. The noise level at theoperating point is measured with loaded noise measuringsystem (Figure 8).

This structure allows us to control the static pressure in thechamber with the aperture mounted to the outlet in order tomeasure the noise level of the fan with a load. The system isalso soundproof, absorbs sound, and is vibration-proof toavoid measuring a noise level caused by other factors than thefan (such as sound reflected inside enclosure and soundemitted from enclosure due to vibration transmitted from thefan).

2.4.2 Result of measuring loaded noise level

The result of measuring loaded noise level of the siroccofan MB1255-B and propeller fan MRS16-BUL is given inFigure 9.

While the propeller fan shows lower noise level with higherairflow which means low static pressure, the sirocco fanshows lower noise level with higher static pressure. In termsof noise level, small fans have optimum operation zone.Propeller fans will function better at low static pressure whilethe sirocco fans at high static pressure.

The noise level of the sirocco fans arises greatly frompressure noise*1 and this results in lower noise level at highstatic pressure area where airflow is lower. On the other hand,the noise level of the propeller fans arises greatly fromturbulent noise*2 and the turbulent noise is greater at highstatic pressure.

3. Fan Selection3.1 Fan Selection Procedure

There are several methods to select a fan. A fan shall beselected with the following procedure.

1. Find required ventilation flow rate and airflow fromthe upper limit of the temperature rise in the system,generated calorie, and required passing air speed.

2. Find pressure loss (resistance curve) of the system.3. Calculate required airflow of the fan from the step 1

and 2, and select the best fan.

This is because while centrifugal-flow sirocco fans aresuitable for increasing pressure, axial-flow propeller fans aresuitable for increasing airflow.

The characteristics of sirocco fans MB1040-B andMB1255-B, and that of propeller fans MRS16-BUL andMRS18-BUL are also shown in the Figure 6.

Sirocco fans show high static pressure characteristics andpropeller fans shows high airflow characteristics. In otherwords, both fans show characteristic that agrees with what wesaw with the optimum specific speed ns.

The intersection of the airflow-static pressure characteristiccurve with curve A and B is the optimum operating point. Thearea shown with oval around the point is the recommendedoperation zone.

2.3 InputAirflow-Input characteristic of sirocco fan MB1255-B

and propeller fan MRS16-BUL is shown in the Figure 7.

While the propeller fan shows smaller input with higherairflow, the sirocco fan gives smaller input with lowerairflow, which means higher static pressure.

This is because the sirocco fan is more efficient at higherstatic pressure while the propeller fan is more efficient withhigher airflow.

2.4 Noise Level2.4.1 How to measure loaded noise level

The noise level given in catalogues is the one at maximumairflow and is not the noise level of the fan actually installedin a system, which means not of the fan with a load (atoperating point).

For this reason, the fan actually installed to a system doesnot yield the expected noise level.

RENGA No.1623

ORIENTAL MOTOR

0 2 4 6 8 10 12 1440

60

80

100

120

Air Flow [m3/mn]

Inpu

t [W]

Sirocco Fan MB1255-B 100(V)60(Hz)Propeller Fan MRS18-BUL 100(V)60(Hz)

Figure 7 Airflow-Input Relationship (By Air Flow Method)

1m

Frame Chamber

Aperture

Sound AbsorbingMaterial

Outlet

Microphone

Figure 8 Loaded Noise Measuring System

0 2 4 6 8 10 12 1450

60

70

80

Air Flow [m3/mn]

Noi

se L

evel [d

B(A)

] Sirocco Fan MB1255-B 100(V)60(Hz)

Propeller Fan MRS18-BUL 100(V)60(Hz)

Figure 9 Loaded Noise Level of Propeller Fan and Sirocco Fan

Defining pressure loss (resistance curve) should be a keypoint in selecting a fan.

3.3 How to Calculate Pressure Loss (Resistance Curve)Here we describe two major calculation methods.1. Measurement with multi-nozzle airflow measuring

system (double chamber, *3)2. Calculation with CFD (Computational Fluid

Dynamics)

3.3.1 Calculation with actual measurement

Pressure loss of the device under test (DUT) is measuredwith multi-nozzle airflow measuring system. An auxiliary fanflows air into the DUT and the resulting pressure loss ismeasured.

With this, data with highest accuracy can be obtained.

3.3.2 Calculation with Computational Fluid Dynamics

This method uses Computational Fluid Dynamics (CFD) toanalyze pressure distribution in the system. We usedPHOENICS (5), major CFD software, for this analysis.

CFD divides the model into meshes and calculates heatequation and fluid equation for each mesh.

This method is very convenient for relative comparisonsuch as seeing change in pressure loss upon change on shapeor internal structure.

This method is also used when simplified equations cannotbe applied due to complicated shape. However, such absoluteevaluation requires verification experiment on actual model.

Besides pressure loss, CFD also allows us to find airflowdistribution and temperature distribution in the system.

4. Fan Selection ExamplesSince its shape and characteristics provide limited

application, selecting a cross flow fan is relatively easy.By contrast, we need to select the right fan type, referring

to airflow-static pressure characteristics and it may often bedifficult to choose either a propeller fan or sirocco fan forright application.

Here we will give a selection example for a sirocco fan, aswell as the result of choosing a propeller fan for anapplication in which a sirocco fan was to be used.

Here we skip step 1, supposing that these values are knownand describe from step 2. See (1), (2), and (3) in thebibliography for the step 1 and other selection examples andmethods.

3.2 Pressure Loss (Resistance Curve)If an object is placed in the air current, the object acts as

resistance that blocks the air current.For example, when ventilating inside the enclosure, internal

components, flow path, and the shape of outlet act asresistance. If a fan is to be used for cooling, the object to becooled and ducts connected act as resistance.

The loss caused by such resistance is called pressure lossand represented by static pressure [Pa].

Pressure loss is almost proportional to the square of airspeed or airflow and therefore, it can be expressed withquadratic curve. If airflow doubles, the pressure lossquadruples.

The curve that plots the relationship between airflow andpressure loss is called resistance curve. This resistance isinherent in each system.

The pressure loss (resistance curve) of 35 mm and 60mm square straight pipe (1000 mm in length) is shown in theFigure 10.

Compared to 60 mm pipe, 35 mm pipe has narrowerflow path and thus greater resistance, resulting in greaterpressure loss.

The pressure loss produced by flowing air at airflow of2.5m3/min is 30 Pa for 60 mm pipe while it is 380 Pa for35 mm pipe, which is more than 10 times greater.

A fan shall be selected according to data on pressure loss(Figure 10).

Let's think about flowing air into each pipe at airflow of2.5m3/min.

As for 60 mm straight pipe, we shall look at airflow-static pressure characteristic of fans and select a fan thatgenerates static pressure of 30 Pa or higher at airflow of2.5m3/min.

With 35 mm pipe, we need a fan that generates staticpressure of 380 Pa or higher at airflow of 2.5m3/min.

ORIENTAL MOTOR

RENGA No.162 4

0 1 2 3 4 5 60

100

200

300

400

500

600

Air Flow [m3/mn]

Stat

ic Pr

essu

re [P

a]

Resistance Curve for 35 mm 1000 mm Square Straight Pipe

Resistance Curve for 60 mm 1000 mm Square Straight Pipe

Figure 10 Pressure Loss

Auxiliary BlowerFan under Test

Manometers

Nozzle

Chamber BChamber A

Ps P

Figure 11 Multi-Nozzle Airflow Measuring System

2. Calculation with CFDThe result of analyzing the pressure loss generated at

airflow of 2.5 m3/min is shown in the Figure 15.

The Figure 15 is a contour map. The color in the modelshows the distribution of the pressure loss and the value of thecolor bar (on the left of the Figure 15) corresponding to thecolor in the model represents the scale of the pressure loss.

The maximum value of the color bar indicates themaximum pressure loss. We can see that the maximumpressure loss of 353 Pa is taking place near the air inlet.

Varying airflow for more pressure loss analysis gives us aresistance curve. This resistance curve is also given in theFigure 14.

The result of actual measurement with multi-nozzle airflowmeasuring system and that of calculation with CFD is almostidentical, and thus, we can see that simulation is highlyreliable.

4.3 Fan SelectionNow we are to select a fan according to the resistance curve

obtained by actual measurement with multi-nozzle airflowmeasuring system.

From measurement result given in the Figure 14, theresistance curve is sharp, increasing more as static pressureincreases. For this reason, we can expect that selecting asirocco fan will be advantageous since it is highly efficientwith lower noise level at higher static pressure.

1. Selecting a sirocco fanTo flow required airflow of 2.5m3/min, static pressure of

320 Pa or higher is required.

4.1 Selection ConditionsHere is an example where a fan is to be selected for the

model that supplies air via square duct (Figure 12). The ductis equipped with a square gate valve to adjust outlet air speed.

Required airflow shall be known. Here we calculatepressure loss (resistance curve) and select a fan.

1. Required airflow 2.5 [m3/min]2. Duct Square straight pipe 100 X L420 [mm]3. Gate valve Clearance: 25 [mm]

Position: 80 [mm] from outlet

4.2 Calculating Pressure Loss (Resistance Curve)1. Actual measurement using multi-nozzle airflow

measuring systemWe measured pressure loss in the model with multi-

nozzle airflow measuring system. A photo of mountedsystem is shown in the Figure 13, and the measurementresult of the pressure loss (resistance curve) in the Figure14.

ORIENTAL MOTOR

RENGA No.1625

Gate Valve

Air Flow

Figure 12 Selection Model

Figure 13 Nozzle

0 1 2 3 4 5 60

100

200

300

400

500

600

Air Flow [m3/mn]

Stat

ic Pr

essu

re [P

a]

CFD

Actual Measurement

Figure 14 Pressure Loss (Resistance Curve)

Figure 15 Result of Pressure Loss Analysis with CFD

2 4 6 8 10 12 14 160

100

200

300

400

500

600

Air Flow [m3/mn]

Stat

ic Pr

essu

re [P

a] MB1255-B

100(V)60(Hz)MRS18-BUL 2 Units in Series 100(V)60(Hz)

Resistance Curve

Required Characteristic

Figure 16 Selection

5. ConclusionAlong with miniaturization and technical advantages of

recent systems, the pressure loss of the system where a fan isto be used tends to be higher and higher.

Sirocco fans have characteristics of higher efficiency andlower noise level at higher static pressure. And we found thatsirocco fans are more advantageous than propeller fans interms of efficiency, noise level, and weight if the pressure lossis great and operating point is in high static pressure area.

When selecting a fan, finding pressure loss (resistancecurve) of the system and defining the operating point isessential.

*1: Noise caused by impact with turbulent flow vanes*2: Noise caused by stream turbulence and swirl of turbulent

flows*3: Compliant with AMCA (The Air Moving and

Conditioning Association) STANDARD 210

Reference documents

(1) "Denshi kiki sekkei no tameno fan motor to souon netsu taisaku (2001)", Kogyo Chosakai PublishingCo., Ltd.

(2) "Koumitsudo jissou kiki ni okeru netsu taisaku" byTakahiro Ito of Oriental Motor, Internepcon LecturePaper, (2001)

(3) "Koumitsudo jissou souchi ni taiou suru cooling fan nokashikoi sentaku", extract from Oriental Motorbrochure

(4) Concentration, Heat & Momentum Limited Websitehttp://www.phoenics.co.jp./

(5)"Soufuuki to asshukuki" by Takefumi Ikui andMasahiro Inoue (1986), 122, Corona Publishing, Co.,Ltd.

The characteristic of MB1255-B, a sirocco fan, is givenin the Figure 16.

We can see that this fan gives the required characteristic.

2. Selecting a propeller fanInstalling two units of MRS18-BUL in serial meets the

required characteristic (Figure 16).

4.4 ComparisonTable 2 shows the comparison of input, noise level, size,

and weight of the selected sirocco fan and propeller fans.

Although the value of airflow-static pressure at operatingpoint is around the same in both fans, the sirocco fan giveslower input and noise level.

As described before, sirocco fans have lower input andnoise level at higher static pressure while propeller fans havegreater input and noise level at higher static pressure.

Especially, while the propeller fans have lower noise levelin catalogue (at maximum airflow), the value is lower withsirocco fan at operating point. The result is reversed.

As is the case with this example, if the operating point is athigh static pressure, selection of a sirocco fan is advantageousin terms of input, noise level, and weight.

Conversely, if the operating point is at higher airflow,propeller fans will be advantageous.

ORIENTAL MOTOR

RENGA No.162 6

Fan

Sirocco Fan

Propeller Fan

Selected Fan Masskg

MB1255-B

MRS18-BUL2 units in serial

Input W Noise Level dB(A)Operating

PointMaximum

AirflowOperating

PointMaximum

Airflow68

152

105

146

62

76

72 3.2

59 4.6

Writer

Masayuki TAKAHASHI

TMS Company

Table 2 Difference in Input and Noise Level between the Fans Selected

RENGA No.162 (Japanese Original Edition): Published in October 2002Copyright 2004 ORIENTAL MOTOR CO., LTD.