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ORIENTAL MOTOR

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 the
standpoint of energy efficiency, noise reduction, and weight reduction. However, the number of small fan
types 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, the
Sirocco fan. We will explain how the motor is able to perform with high efficiency and low noise in a high
static pressure environment and how this fan is most suitable for cooling the recent high density mounting
equipment.

1. Introduction

Table 1 Fan Structure and Characteristics

With small fans, selecting the right fan (air flow method)
that fits its purpose is extremely important from the
standpoint of energy efficiency, noise reduction, and weight
reduction. However, there are a great number of types of
small fans. In addition, their purposes range widely from
cooling, air blasting, and ventilation. Therefore, we must
select a fan that most fits its purpose. Here, we introduce one
small fan type, the Sirocco fan. We will explain its
characteristic of high efficiency and low noise in a high static
pressure environment and its appropriateness for cooling
recent high density mounting equipment.

2. Types and Characteristics of Small Fans
2.1 Types
Small fans can be divided into three types by the air flow
method; propeller fan, sirocco fan, and cross flow fan.
The structure and characteristics of these fans are given in
the Table 1.

Fan Type

Structure and Characteristics

Propeller
Fan
(Axial)

The propeller (blade vanes) in the circular flow path between
cylindrical hub and housing compresses and sends air and
generates air stream in the direction of rotation axis.
Because air flows along the rotation axis, this type has
compact structure. In addition, the fan's capacity to obtain
great airflow makes it suitable for ventilation/cooling an entire
section inside machine.

Sirocco Fan
(Centrifugal
with multiple
blades)

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

Cross Flow
Fan
(Through
flow, Cross
flow)

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

Air Flow
Air Flow
Air Flow

Figure 1 Propeller Fan

1

RENGA No.162

Figure 2 Sirocco Fan

Figure 3 Cross Flow Fan

ORIENTAL MOTOR
Maximum Total Pressure Efficiency  max [%]

2.2 Specific Speed and Efficiency
2.2.1 Specific speed

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

w1

c1

w2

u1

c2
u2

90
80
70

Turbo Blower

Radial Fan

Centrifugal Fan with
Narrow Straight Vanes

Axial Fan
Turbo Fan with Streamlined
Stator Vanes

60
Fan with Multiple Vanes

50

Axial Fan with Stator Vanes
40
Tubelike Axial Fan
30
Propeller Fan
20
40 5 6

8 100

2

3

4

5 6

8 1000

2

3 4000

Specific Speed ns

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

Figure 4 Fans with Structure Similarity

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

Q1
=
Q2

3

( )
D1
D2

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

·

N1 ....................................................(1)
N2

2

2

( )( )
D1
D2

·

N1
N2

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

Q : Airflow
D: Runner diameter
 : Density
P : Static pressure
N : Rotational speed
g : Gravitational acceleration

The best efficiency point (hereinafter referred to as
optimum specific speed) is found at ns = 2000 for propeller
fans 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 nearly
constant at around 3000 r/min.
The equation (4) shows that the known specific speed ns that
gives high efficiency for each air flow method and constant
rotational speed N determine the relationship between the
optimum airflow and static pressure for each blast method.
In other words, there is an operation zone that is optimum
for each blast method.
The relationship among the optimum ns, airflow, and static
pressure for sirocco fans and propeller fans is shown in the
Figure 6.

A
400

Q11/2
Q11/2
= N2
N1
3/4
[P1/( 1 g)]
[P2/( 2 g)]3/4

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

Specific Speed ns, = 400
Maximum Efficiency for Sirocco Fan
300
Static Pressure [Pa]

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

MB1255-B
100(V)50(Hz)
MB1040-B
100(V)50(Hz)

200

MRS16-BUL
100(V)50(Hz)
MRS18-BUL
100(V)50(Hz)
Specific Speed ns, = 2000
Maximum Efficiency for Propeller Fan

100

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

ns = N

Q11/2
[P1/(  g)]3/4

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

Using past design examples, we can acquire important data
in designing fans such as relationship between specific rate
and efficiency.
2.2.2 Relationship between specific speed and efficiency

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

B
0
0

2

4

6
Air Flow [m3/mn]

8

10

12

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

For a sirocco fan (optimum specific speed ns = 400), the
curve A represents the relationship between the optimum
airflow and static pressure. From this curve, we can see that
the high efficiency zone for sirocco fans is on high static
pressure side.
For a propeller fan (optimum specific speed ns = 2000), the
curve B represents the relationship between the optimum
airflow and static pressure. From this curve, we can see that
the high efficiency zone for propeller fans is on high airflow
side.
RENGA No.162

2

ORIENTAL MOTOR
This is because while centrifugal-flow sirocco fans are
suitable for increasing pressure, axial-flow propeller fans are
suitable for increasing airflow.
The characteristics of sirocco fans MB1040-B and
MB1255-B, and that of propeller fans MRS16-BUL and
MRS18-BUL are also shown in the Figure 6.
Sirocco fans show high static pressure characteristics and
propeller fans shows high airflow characteristics. In other
words, both fans show characteristic that agrees with what we
saw with the optimum specific speed ns.
The intersection of the airflow-static pressure characteristic
curve with curve A and B is the optimum operating point. The
area shown with oval around the point is the recommended
operation zone.
2.3 Input
Airflow-Input characteristic of sirocco fan MB1255-B
and propeller fan MRS16-BUL is shown in the Figure 7.
120
Sirocco Fan
MB1255-B
100(V)60(Hz)

Input [W]

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

To prevent such issues, we need to know the noise level at
the operating point in advance. The noise level at the
operating point is measured with loaded noise measuring
system (Figure 8).
This structure allows us to control the static pressure in the
chamber with the aperture mounted to the outlet in order to
measure the noise level of the fan with a load. The system is
also soundproof, absorbs sound, and is vibration-proof to
avoid measuring a noise level caused by other factors than the
fan (such as sound reflected inside enclosure and sound
emitted from enclosure due to vibration transmitted from the
fan).
2.4.2 Result of measuring loaded noise level

The result of measuring loaded noise level of the sirocco
fan MB1255-B and propeller fan MRS16-BUL is given in
Figure 9.
While the propeller fan shows lower noise level with higher
airflow which means low static pressure, the sirocco fan
shows lower noise level with higher static pressure. In terms
of noise level, small fans have optimum operation zone.
Propeller fans will function better at low static pressure while
the sirocco fans at high static pressure.

80
80

40
0

2

4

6

8

10

12

14

3

Air Flow [m /mn]

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

While the propeller fan shows smaller input with higher
airflow, the sirocco fan gives smaller input with lower
airflow, which means higher static pressure.
This is because the sirocco fan is more efficient at higher
static pressure while the propeller fan is more efficient with
higher airflow.
2.4 Noise Level
2.4.1 How to measure loaded noise level

The noise level given in catalogues is the one at maximum
airflow and is not the noise level of the fan actually installed
in a system, which means not of the fan with a load (at
operating point).
For this reason, the fan actually installed to a system does
not yield the expected noise level.
Frame

Chamber
Aperture

1m

Microphone

Sound Absorbing
Material
Outlet

Figure 8 Loaded Noise Measuring System

3

RENGA No.162

Noise Level [dB(A)]

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

70

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

50
0

2

4

6
8
3
Air Flow [m /mn]

10

12

14

Figure 9 Loaded Noise Level of Propeller Fan and Sirocco Fan

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

3. Fan Selection
3.1 Fan Selection Procedure
There are several methods to select a fan. A fan shall be
selected with the following procedure.
1. Find required ventilation flow rate and airflow from
the 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.

ORIENTAL MOTOR
Here we skip step 1, supposing that these values are known
and describe from step 2. See (1), (2), and (3) in the
bibliography for the step 1 and other selection examples and
methods.
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 as
resistance. If a fan is to be used for cooling, the object to be
cooled and ducts connected act as resistance.
The loss caused by such resistance is called pressure loss
and represented by static pressure [Pa].
Pressure loss is almost proportional to the square of air
speed or airflow and therefore, it can be expressed with
quadratic curve. If airflow doubles, the pressure loss
quadruples.
The curve that plots the relationship between airflow and
pressure loss is called resistance curve. This resistance is
inherent in each system.
The pressure loss (resistance curve) of □35 mm and □60
mm square straight pipe (1000 mm in length) is shown in the
Figure 10.

Defining pressure loss (resistance curve) should be a key
point 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 measured
with multi-nozzle airflow measuring system. An auxiliary fan
flows air into the DUT and the resulting pressure loss is
measured.
With this, data with highest accuracy can be obtained.

Fan under Test

Auxiliary Blower
Nozzle

Chamber A

Ps

Chamber B

P

Manometers
600

Figure 11 Multi-Nozzle Airflow Measuring System
500
Static Pressure [Pa]

Resistance Curve for □35 mm  1000 mm Square Straight Pipe

3.3.2 Calculation with Computational Fluid Dynamics

400

300

200

Resistance Curve for □60 mm  1000 mm Square Straight Pipe

100

0
0

1

2

3
3
Air Flow [m /mn]

4

5

6

Figure 10 Pressure Loss

Compared to □60 mm pipe, □35 mm pipe has narrower
flow path and thus greater resistance, resulting in greater
pressure loss.
The pressure loss produced by flowing air at airflow of
2.5m3/min is 30 Pa for □60 mm pipe while it is 380 Pa for
□35 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 of
2.5m3/min.
As for □60 mm straight pipe, we shall look at airflowstatic pressure characteristic of fans and select a fan that
generates static pressure of 30 Pa or higher at airflow of
2.5m3/min.
With □35 mm pipe, we need a fan that generates static
pressure of 380 Pa or higher at airflow of 2.5m3/min.

This method uses Computational Fluid Dynamics (CFD) to
analyze pressure distribution in the system. We used
PHOENICS (5), major CFD software, for this analysis.
CFD divides the model into meshes and calculates heat
equation and fluid equation for each mesh.
This method is very convenient for relative comparison
such as seeing change in pressure loss upon change on shape
or internal structure.
This method is also used when simplified equations cannot
be applied due to complicated shape. However, such absolute
evaluation requires verification experiment on actual model.
Besides pressure loss, CFD also allows us to find airflow
distribution and temperature distribution in the system.

4. Fan Selection Examples
Since 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 be
difficult to choose either a propeller fan or sirocco fan for
right application.
Here we will give a selection example for a sirocco fan, as
well as the result of choosing a propeller fan for an
application in which a sirocco fan was to be used.
RENGA No.162

4

ORIENTAL MOTOR
4.1 Selection Conditions
Here is an example where a fan is to be selected for the
model that supplies air via square duct (Figure 12). The duct
is equipped with a square gate valve to adjust outlet air speed.

2. Calculation with CFD
The result of analyzing the pressure loss generated at
airflow of 2.5 m3/min is shown in the Figure 15.

Gate Valve

Air Flow

Figure 12 Selection Model
Figure 15 Result of Pressure Loss Analysis with CFD

Required airflow shall be known. Here we calculate
pressure 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 system
We measured pressure loss in the model with multinozzle airflow measuring system. A photo of mounted
system is shown in the Figure 13, and the measurement
result of the pressure loss (resistance curve) in the Figure
14.

The Figure 15 is a contour map. The color in the model
shows the distribution of the pressure loss and the value of the
color bar (on the left of the Figure 15) corresponding to the
color in the model represents the scale of the pressure loss.
The maximum value of the color bar indicates the
maximum pressure loss. We can see that the maximum
pressure loss of 353 Pa is taking place near the air inlet.
Varying airflow for more pressure loss analysis gives us a
resistance curve. This resistance curve is also given in the
Figure 14.
The result of actual measurement with multi-nozzle airflow
measuring system and that of calculation with CFD is almost
identical, and thus, we can see that simulation is highly
reliable.
4.3 Fan Selection
Now we are to select a fan according to the resistance curve
obtained by actual measurement with multi-nozzle airflow
measuring system.
From measurement result given in the Figure 14, the
resistance curve is sharp, increasing more as static pressure
increases. For this reason, we can expect that selecting a
sirocco fan will be advantageous since it is highly efficient
with lower noise level at higher static pressure.
1. Selecting a sirocco fan
To flow required airflow of 2.5m3/min, static pressure of
320 Pa or higher is required.

Figure 13 Nozzle

600

600
Resistance Curve

CFD
500

500
Static Pressure [Pa]

Static Pressure [Pa]

Actual Measurement
400

300

200

MB1255-B
100(V)60(Hz)

400

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

300

200

100

100

0

0

Required Characteristic

0

1

2

3
Air Flow [m3/mn]

4

5

Figure 14 Pressure Loss (Resistance Curve)

5

RENGA No.162

6

2

4

6

8
10
Air Flow [m3/mn]

Figure 16 Selection

12

14

16

ORIENTAL MOTOR
The characteristic of MB1255-B, a sirocco fan, is given
in the Figure 16.
We can see that this fan gives the required characteristic.
2. Selecting a propeller fan
Installing two units of MRS18-BUL in serial meets the
required characteristic (Figure 16).
4.4 Comparison
Table 2 shows the comparison of input, noise level, size,
and weight of the selected sirocco fan and propeller fans.

5. Conclusion
Along with miniaturization and technical advantages of
recent systems, the pressure loss of the system where a fan is
to be used tends to be higher and higher.
Sirocco fans have characteristics of higher efficiency and
lower noise level at higher static pressure. And we found that
sirocco fans are more advantageous than propeller fans in
terms of efficiency, noise level, and weight if the pressure loss
is great and operating point is in high static pressure area.
When selecting a fan, finding pressure loss (resistance
curve) of the system and defining the operating point is
essential.

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

Selected Fan

Sirocco Fan

MB1255-B

MRS18-BUL
Propeller Fan
2 units in serial

Noise Level dB(A)

Operating Maximum Operating Maximum
Point
Airflow
Point
Airflow

Mass
kg

68

105

62

72

3.2

152

146

76

59

4.6

Although the value of airflow-static pressure at operating
point is around the same in both fans, the sirocco fan gives
lower input and noise level.
As described before, sirocco fans have lower input and
noise level at higher static pressure while propeller fans have
greater input and noise level at higher static pressure.
Especially, while the propeller fans have lower noise level
in catalogue (at maximum airflow), the value is lower with
sirocco fan at operating point. The result is reversed.
As is the case with this example, if the operating point is at
high static pressure, selection of a sirocco fan is advantageous
in terms of input, noise level, and weight.
Conversely, if the operating point is at higher airflow,
propeller fans will be advantageous.

*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 Publishing
Co., Ltd.
(2) "Koumitsudo jissou kiki ni okeru netsu taisaku" by
Takahiro Ito of Oriental Motor, Internepcon Lecture
Paper, (2001)
(3) "Koumitsudo jissou souchi ni taiou suru cooling fan no
kashikoi sentaku", extract from Oriental Motor
brochure
(4) Concentration, Heat & Momentum Limited Website
http://www.phoenics.co.jp./
(5)"Soufuuki to asshukuki" by Takefumi Ikui and
Masahiro Inoue (1986), 122, Corona Publishing, Co.,
Ltd.

Writer

Masayuki TAKAHASHI

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

RENGA No.162

6

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