### I - SELECTING A FAN

```Technical guide
I - SELECTING A FAN
Forced convection cooling is an effective solution to dissipate heat in electronic equipment. This
BASIC REQUIREMENTS
VOLTAGE: The first step is to determine the
nominal voltage; this can be AC or DC. ETRI
fans cover all voltage ranges from 5 V to 500 V.
DIMENSIONS: Optimising performance within
the available space envelope is the most
important criteria. ETRI offers a complete range
of fans and blowers from 25 x 25 x 10 mm up
to Ø 350 mm.
DETERMINING NECESSARY AIRFLOW
Dissipated power has to be determined first. If
this value is unknown, the estimation can be
done by taking the power consumed by the equipment and the efficiency (which is approximately
75 % for electronics equipment).
Example :
Consumed power = 500 W
Power to be dissipated : 500 x 25 % = 125 W
The graph below (fig1) shows how to calculate
the airflow according to the dissipated power,
using the formula:
Airflow (l/s) = P(W)
1,2 x ∆t
This calculation does not take into consideration
pressure drop, which has a direct impact on the
airflow. Pressure drop is defined in the next
paragraph.
CALCULATING NECESSARY
STATIC PRESSURE
Each component mounted in the equipment
opposes a resistance to air circulation. These
accumulated resistances are called “pressure
drop”. The pressure drop is balanced by the fan
static pressure which is expressed in mmH2O
or in Pa.
The necessary airflow of the fan must be specified
at a certain static pressure.
Pressure drop is not easily calculable, especially
in complex equipment. In cooling applications,
pressure drop can be calculated according to duct
diameter, length, bends or other deviations.
Here is one basic principle to calculate pressure
drop:
A specified fan, which air performance is known,
is mounted on the equipment. The air speed
can be measured at the outlet of the equipment
with an anemometer. The airflow is calculated
as follows:
A = Airflow (l/s)
O = Outlet Section (cm2)
S = Air Speed (m/s)
The static pressure, corresponding to the
measured airflow, can therefore be read on
the fan performance curve.
The value of this static pressure is the second
parameter to consider when selecting a fan.
Fig.1 - Determining necessary airflow
P: represents the power
to be dissipated (in watts)
∆t: represents the temperature difference
between internal temperature
of the equipment and ambient temperature
Q: represents
P(W)
the airflow of the fan =
L/s
1,2 x ∆t
THE AIRFLOW IS DETERMINED BY THE
PROJECTION ON THE GRAPH BELOW
FROM THE INTERSECTION POINT OF THE
LINES W AND ∆T ON THE AIRFLOW SCALE.
Example: Power to be dissipated: 200 W
∆t: 20°C
Necessary airflow to cool is: 8,5 l/s
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•
A= SxO
10
E-mail: [email protected]
5
Technical guide
CHOOSING THE APPROPRIATE FAN OR
BLOWER WITH THE WORKING POINT
Example:
Working point: 22 l/s at 1,8 mm H2O
Fig.2
The combination of necessary airflow and static
pressure gives a value which is called working
point. It is now very easy to choose the appropriate fan or blower, by selecting a model in
the catalogue, which curve meets the working
point (see fig 2).
Static Pressure (mm H2O)
6,0
OPTIMISING FAN SELECTION
If the air requirement is high, and available space
limited, it is possible to double the airflow at free
delivery by putting two fans in parallel or to
increase the pressure by putting two fans in series.
50 Hz
60 Hz
5,0
4,0
3,0
2,0
Working-point
1,0
0,0
0
5
10
15
20
25
30
35
40
Airflow (I/s)
The noise level will then increase by approximately 3dBA depending on the fan model.
rtant
Impo e !
notic
The next chapter shows the benefits of the different fan designs.
Values indicated in this catalogue are based on
free delivery (no pressure value) and shutoff
(no flow condition).
II - AERODYNAMIC FAN TYPES
These drawings describe the air direction for each aerodynamic fan type.
AXIAL FANS
BACKWARD CURVED FANS
Benefits:
Benefits:
- High airflow
- Low static
pressure
- Motor cooled
by the flow
- Low noise
- Best design for
combination of high
airflow and high
static pressure
Fig.3
Fig.5
CENTRIFUGAL BLOWERS
CROSS FLOW FANS
Benefits:
Benefits:
- Low airflow
- High static pressure
- Best design to
create over or
under pressure
- High static
pressure
- Best design