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6 Fans and Flow Control Devices
Comparison between Centrifugal and Axial Fans
The main difference is that centrifugal fans deliver low quantities at high pressures,
while axial fans provide higher flow rates at lower pressures.
In addition to the above, all things being equal, axial fans as compared with
centrifugal:
• Are more compact–smaller, lighter–than centrifugal.
• Tend to be more economical.
• Have fewer breakdowns if they do not use complex motor to fan drive systems.
• Have less inertia, making them easier to start.
• Do not require deep foundations for installation.
• Do not store water–which could worsen their performance–in their interior.
• Are reversible, although if they are reversed they provide less flow and their
efficiency is much lower.
• Tend to be more flexible to operating requirements. This is due to the possibility
of controlling them by varying both the angle of the blades
5 and the rotation speed
of the shaft.
However, all things being equal, axial fans, as compared with centrifugal fans:
• Are usually less energy efficient.
• Are more likely to overload.
• Are less resistant to harsh environments.
• Are noisier.
Mixed Flow Fans
A mixed flow fan
6 is similar to an axial fan. However, in the first, the impeller is modified incorporating blades with axial and radial elements that deflect the airflow from
its straight path onto a diagonal angle of about 45° relative to the shaft. This effect
is obtained by employing a conical impeller that draws the air through expanding
channels. Then, the spiral flow created by the impeller is usually transformed into
straight flow using straightening vanes located in the casing, thus increasing the static
pressure. Mixed flow fans (Fig. 6.4) combine some advantages of axial (high airflow
rates) and centrifugal fans (high pressure).
5 This is not usual in centrifugal fans.
6 Sometimes referred to as diagonal fans.
6 Fans and Flow Control Devices
Comparison between Centrifugal and Axial Fans
The main difference is that centrifugal fans deliver low quantities at high pressures,
while axial fans provide higher flow rates at lower pressures.
In addition to the above, all things being equal, axial fans as compared with
centrifugal:
• Are more compact–smaller, lighter–than centrifugal.
• Tend to be more economical.
• Have fewer breakdowns if they do not use complex motor to fan drive systems.
• Have less inertia, making them easier to start.
• Do not require deep foundations for installation.
• Do not store water–which could worsen their performance–in their interior.
• Are reversible, although if they are reversed they provide less flow and their
efficiency is much lower.
• Tend to be more flexible to operating requirements. This is due to the possibility
of controlling them by varying both the angle of the blades
5 and the rotation speed
of the shaft.
However, all things being equal, axial fans, as compared with centrifugal fans:
• Are usually less energy efficient.
• Are more likely to overload.
• Are less resistant to harsh environments.
• Are noisier.
Mixed Flow Fans
A mixed flow fan
6 is similar to an axial fan. However, in the first, the impeller is modified incorporating blades with axial and radial elements that deflect the airflow from
its straight path onto a diagonal angle of about 45° relative to the shaft. This effect
is obtained by employing a conical impeller that draws the air through expanding
channels. Then, the spiral flow created by the impeller is usually transformed into
straight flow using straightening vanes located in the casing, thus increasing the static
pressure. Mixed flow fans (Fig. 6.4) combine some advantages of axial (high airflow
rates) and centrifugal fans (high pressure).
5 This is not usual in centrifugal fans.
6 Sometimes referred to as diagonal fans.