FAN BASIC
Axial Fan - An axial fan discharges air parallel to the axis of the
impeller rotation. As a general rule, axial fans are preferred for
high volume, low pressure, and non-ducted systems.
Axial Fan Types
Propeller, Tube Axial and Vane Axial.
Centrifugal Fan - Centrifugal fans discharge air perpendicular to
the axis of the impeller rotation. As a general rule, centrifugal
fans are preferred for higher pressure ducted systems.
Centrifugal Fan Types
Backward Inclined, Airfoil, Forward Curved, and Radial Tip.
Fan Selection Criteria
Before selecting a fan, the following information is needed.
• Air volume required - CFM
• System resistance - SP
• Air density (Altitude and Temperature)
• Type of service
• Environment type
• Materials/vapors to be exhausted
• Operation temperature
• Space limitations
• Fan type
• Drive type (Direct or Belt)
• Noise criteria
• Number of fans
• Discharge
• Rotation
• Motor position
• Expected fan life in years
Fan Laws
The simplified form of the most commonly used fan laws
include.
• CFM varies directly with RPM
CFM1/CFM2 = RPM1/RPM2
• SP varies with the square of the RPM
SP1/SP2 = (RPM1/RPM2)2
• HP varies with the cube of the RPM
HP1/HP2 = (RPM1/RPM2)3
Fan Performance Tables and Curves
Performance tables provide a simple method of fan selection.
However, it is critical to evaluate fan performance curves in the
fan selection process as the margin for error is very slim when
selecting a fan near the limits of tabular data. The performance
curve also is a valuable tool when evaluating fan performance
in the field.
Fan performance tables and curves are based on standard air
density of 0.075 lb/ft3. When altitude and temperature differ significantly
from standard conditions (sea level and 70
° F) performance
modification factors must be taken into account to ensure
proper performance.
For further information refer to Use of Air Density Factors -
An Example, page 3.
Fan Testing - Laboratory, Field
Fans are tested and performance certified under ideal laboratory
conditions. When fan performance is measured in field conditions,
the difference between the ideal laboratory condition and
the actual field installation must be considered. Consideration
must also be given to fan inlet and discharge connections as they
will dramatically affect fan performance in the field. If possible,
readings must be taken in straight runs of ductwork in order to
ensure validity. If this cannot be accomplished, motor amperage
and fan RPM should be used along with performance curves to
estimate fan performance.
For further information refer to Fan Installation Guidelines,
page 14.
Download Here.
impeller rotation. As a general rule, axial fans are preferred for
high volume, low pressure, and non-ducted systems.
Axial Fan Types
Propeller, Tube Axial and Vane Axial.
Centrifugal Fan - Centrifugal fans discharge air perpendicular to
the axis of the impeller rotation. As a general rule, centrifugal
fans are preferred for higher pressure ducted systems.
Centrifugal Fan Types
Backward Inclined, Airfoil, Forward Curved, and Radial Tip.
Fan Selection Criteria
Before selecting a fan, the following information is needed.
• Air volume required - CFM
• System resistance - SP
• Air density (Altitude and Temperature)
• Type of service
• Environment type
• Materials/vapors to be exhausted
• Operation temperature
• Space limitations
• Fan type
• Drive type (Direct or Belt)
• Noise criteria
• Number of fans
• Discharge
• Rotation
• Motor position
• Expected fan life in years
Fan Laws
The simplified form of the most commonly used fan laws
include.
• CFM varies directly with RPM
CFM1/CFM2 = RPM1/RPM2
• SP varies with the square of the RPM
SP1/SP2 = (RPM1/RPM2)2
• HP varies with the cube of the RPM
HP1/HP2 = (RPM1/RPM2)3
Fan Performance Tables and Curves
Performance tables provide a simple method of fan selection.
However, it is critical to evaluate fan performance curves in the
fan selection process as the margin for error is very slim when
selecting a fan near the limits of tabular data. The performance
curve also is a valuable tool when evaluating fan performance
in the field.
Fan performance tables and curves are based on standard air
density of 0.075 lb/ft3. When altitude and temperature differ significantly
from standard conditions (sea level and 70
° F) performance
modification factors must be taken into account to ensure
proper performance.
For further information refer to Use of Air Density Factors -
An Example, page 3.
Fan Testing - Laboratory, Field
Fans are tested and performance certified under ideal laboratory
conditions. When fan performance is measured in field conditions,
the difference between the ideal laboratory condition and
the actual field installation must be considered. Consideration
must also be given to fan inlet and discharge connections as they
will dramatically affect fan performance in the field. If possible,
readings must be taken in straight runs of ductwork in order to
ensure validity. If this cannot be accomplished, motor amperage
and fan RPM should be used along with performance curves to
estimate fan performance.
For further information refer to Fan Installation Guidelines,
page 14.
Download Here.
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