Ventilation - Fans - Page 1 2 3 4
Types of Fans
There are several types of fan to choose from in ventilation.
These are:
1. Propeller
2. Axial flow
3. Centrifugal
4. Mixed flow
1. Propeller Fan
Used in situations where there is minimal resistance to air flow.
Typical outputs are; up to 4 m3/s and up to 250 Pa pressure.
Fan efficiency is low at about 40%.
Suitable for wall, window and roof fans where the intake and discharge are free from obstacles.
Can move large volumes of air.
Low installation cost.
2. Axial Flow Fan
High volume flow rate is possible with this type of fan with high efficiency, about 60% to 65%.
Typical outputs are; up to 20 m3/s and up to 700 Pa pressure.
The fan is cased in a simple enclosure with the motor housed internally or externally.
Aerofoil blades can be used to increase efficiency.
Adjustable pitch blades can be used for greater flexibility.
Ductwork can be simply connected to the flange at either end of the fan.
3. Centrifugal Fan
High pressure air flow is possible with this type of fan.
Used in air handling units and other situations to overcome high resistance to air flow.
The impeller is made of thin blades which are either forward or backward curved.
The air changes direction by 90 degrees in a centrifugal fan so more space is required.
Usually the motor is placed external to the casing and a vee belt and pulley drive is commonly used.
Centrifugal Blades
Centrifugal curved fan blades generally have higher efficiencies than if a plain flat blade is used.
The efficiency of a fan with forward curved blades is about 50% to 60%.
The forward curve has a scoop effect on the air thus a higher volume may be handled.
Blade
Direction of
rotation
FORWARD CURVED BLADE
Backward curved blades offer even better efficiency, 70% to 75%.
This improves airflow through the blade and reduces shock and eddy losses.
High pressures can be developed with backward curved blades.
Even further improvements may be made by using an aerofoil section blade in which case the efficiency may be 80%
to 85%.
Another feature of backward curved blades is their non-overloading characteristic.
Flat section non-
overloading
Pressure or
Fan Power
Power Characteristic
Static Pressure
Characteristic
Volume Flow rate
FAN CHARACTERISTIC CURVES
A disadvantage is the high blade tip speed, necessary to obtain a comparable rate of discharge to forward curved
blades, makes the fan noisy.
4. Mixed Flow Fan
Mixed Flow fans can be used for return air, supply, or general ventilation applications where low sound is critical. As
compared to similarly sized axial fans, a mixed flow fan can be 5-20 dB quieter.
Ventilation - Fans - Page 1 2 3 4
Ventilation – Fans - Page 1 2 3 4
Characteristics of Axial Flow and Centrifugal Fans
Axial Flow Fans
1. Axial flow and backward-curved centrifugal fans have similar characteristics as shown below.
Backward-curved
2.0 Centrifugal Fan
Fan Power
Power
kW
1. Efficiency
0 Power
Axial
flow fan
1. 2.0 3.0 Airflow
0
m3/s
TYPICAL FAN CHARACTERISTIC CURVES
2. The axial flow fan is very convenient from an installation point of view, it can be directly duct mounted even in
restrictive areas but they tend to be noisy. This is because they run at a higher speed compared to a centrifugal
fan.
3. Like the Backward-bladed centrifugal fan, the Axial flow fan has a self-limiting power curve as shown above.
Centrifugal Fans
4. The backward curved centrifugal fan runs at a higher speed than the forward curved fan for the same output.
5. A forward-curved centrifugal fan may be liable to overloading because the power rises as the volume increases.
An example of this in practice is if the main dampers are left wide open when the fan is first started up, too
much air will be handled and the excessive power absorbed will overload the driving motor.
6. The backward–curved fan is less liable to over-loading than the forward-curved centrifugal fan and it is also able
to deliver a relatively constant amount of air as the system resistance varies. The power of a backward curved fan
reaches a peak and then begins to fall, this is called the self-limiting characteristic. This is shown below.
Forward-curved
Centrifugal fan
2.0
Fan Power
kW Power
Power
1.
0
Backward-curved
Centrifugal Fan
1. 2.0 3.0 Airflow
0
CENTRIFUGAL FAN CHARACTERISTICS m3/s
7. A backward-curved centrifugal fan must run at higher speed to deliver the same amount of air as a forward-
curved fan because of the shape of the impeller blades and the direction of rotation.
8. The backward-bladed fan is used in high velocity systems where high pressures are required and is often made
with aerofoil blades to increase efficiency.
9. Up to about 750 N/m2 fan pressure, the forward-curved centrifugal fan tends to be quieter and cheaper. Above
this value of pressure backward-curved fans take over.
Ventilation – Fans - Page 1 2 3 4
Ventilation – Fans - Page 1 2 3 4
Choosing a Fan
To choose a suitable fan one must look at the performance curves.
Performance curves are found in fan catalogues.
These curves show the pressure developed by a fan at a given flow rate.
The pressure to be developed by the fan is found from duct sizing data (See DUCT SIZING section) and the flow rate
is found from design data (See VENTILATION DESIGN section).
The operating point of the system is marked as a point on the curve.
Example 1
The example below shows a system operating point of 250 Pascals (Pa) pressure and 0.48 (m3/s) flow rate.
Go to the curve above the operating point, this is the fan curve for the appropriate fan.
The fan size is chosen as a 250mm-diameter fan (1350 r.p.m. speed).
Example 2
The example below shows a system operating point of 320 Pascals (Pa) pressure and 1.25 (m3/s) flow rate.
The fan performance curve for a 400mm-diameter fan will be suitable for the requirements for this example since
the curve is above the operating point.
The fan size is chosen as a 400mm-diameter fan (1350 r.p.m. speed).
Example 3
An axial flow fan is required for a ventilation system for a Workshop.
Four fans are represented below in the four curves – 2 green and 2 red curves.
The left-hand diagram shows fans with 4-pole electric motors, and the right hand diagram shows fans with 2-pole
electric motors.
Four pole electric motors are slower than two pole motors, in this example 4-pole is at 1420 r.p.m. and 2-pole is at
2840 r.p.m.
The system operating point requirements are 100 Pascals (Pa) pressure
and 0.60 (m3/s) flow rate.
The fan size is chosen as a 350mm-diameter fan (1420 r.p.m. speed).
The electric motor for this fan has a 4-pole winding and will run at 1420 r.p.m. which will be slower than a 2-pole
motor and therefore quieter.
Ventilation – Fans - Page 1 2 3 4
Ventilation – Fans - Page 1 2 3 4
Fan Laws and Running Costs
The Fan Laws are as follows:
No.1 Speed / Volume
N1 Q1
=
N2 Q2
Where;
N = Fan speed (rev. per minute or r.p.m.)
Q = Volume flow rate of air (m3/s)
This means that fan speed and volume flow rate of air are directly proportional.
No.2 Speed / Pressure
2
N1 p1
=
N2 p2
Where;
N = Fan speed (rev. per minute or r.p.m.)
p = Fan pressure (N/m2)
This means that as the fan speed is doubled, for example, the pressure developed is raised by a factor of 4.
No.3 Speed / Power
3
N1 P1
=
N2 P2
Where;
N = Fan speed (rev. per minute or r.p.m.)
P = Fan power (Watts)
This means that as the fan speed is doubled, for example, the power required to drive the fan is raised by a factor of
8.
The above three laws may be written differently to aid calculations, as follows;
No.1 Speed / Volume
N2 Q2
Q2 = Q1 or N2 = N1
N1 Q1
No.2 Speed / Pressure
2 1/2
N2 p2
p2 p1 or N2 N1
= N1 =
p1
No.3 Speed / Power
3 1/
N2 P2 3
P2 P1 or N2 N1
= =
N1 P1
Fan Running Costs
Running costs can be examined as follows;
Fan power (W) = Fan pressure (N/m2 or Pa) x Air Volume flow rate (m3/s) / Efficiency
Annual running cost (£/annum) = Fan power (kW) x Hours run per annum x Electrical price £ per kWh
For example; if a fan runs for 2496 hours per year and delivers 0.625 m3/s against a pressure of 200 Pa, then the
annual running costs can be calculated as follows;
Take electricity cost at 11.2 p/kWh = 0.112 £/kWh.
Fan power (W) = Fan pressure (N/m2 or Pa) x Air Volume flow rate (m3/s) / Efficiency
Fan power (W) = 200 x 0.625 / (say) 0.6
Fan power (W) = 208 Watts.
Fan power (kW) = 0.208 kW.
Annual running cost (£/annum) = 0.208 x 2496 x 0.112
Annual running cost (£) = £58.15 per annum
Ventilation – Fans - Page 1 2 3 4
