®
FAN ENGINEERING
Information and Recommendations for the Engineer FE-2300
Fan Performance
Characteristics of Axial Fans
Introduction Propeller Fans
There are two general classifications of fans: the cen- Propeller fans can be placed in two categories:
trifugal or radial flow fan (see FE-2400) and the propel-
1. Air Circulator or Free Fans A free fan is one that
ler or axial flow fan. In the broadest sense, what sets
rotates in a common unrestricted air space. Examples
them apart is how the air passes through the impeller.
of free fans include ceiling fans, desk fans, pedestal
The propeller or axial flow fan propels the air in an
fans, and wind fans. With the exception of the wind
axial direction (Figure 1a) with a swirling tangential
fans, most of these fans are more decorative than
motion created by the rotating impeller blades.
functional. Low tech, low cost designs function to
In a centrifugal fan the air enters the impeller axially
move and stir the air, but are not necessarily the
and is accelerated by the blades and discharged radi-
most efficient of designs.
ally (Figure 1b).
2. Orifice Panel or Orifice Ring Fans These are the
Figure 1a. Axial Flow Figure 1b. Centrifugal Flow
fans most associated with applications referred to as
ventilating fans. There are many variations of these
arrangements, some with long shaft extensions, direct
connection to a motor, arranged with bearings and
sheaves for belt drive and close coupled belted
arrangements. These fans are designed to transfer air
from one large space to another.
Axial panel and ring fan design must respond to many
variables that affect:
" Materials of construction of the panel or ring
" Materials of construction of the impeller
The axial flow fan increases the air velocity through " Type of impeller blades
rotational or tangential force which produces velocity " Number of impeller blades
pressure (VP), kinetic energy, with a very small increase " Hub configuration
in static pressure (SP), potential energy. For example, typically resi-
The centrifugal fan induces airflow by the centrifugal dential and commercial panel Figure 2. Typical 4-Bladed
Commercial
force generated in a rotating column of air producing and ring fans are constructed
Impeller
potential energy (SP) and also by the rotational (tangen- using shallow drawn light-
tial) velocity imparted to the air as it leaves the tip of weight metal or plastic orific-
the blades producing kinetic energy (VP). es. Impellers for these fans
are also of lightweight con-
struction having from two to
Axial Flow Fans six wide, single thickness,
sometimes overlapping blades
Axial flow fans come in many variations that all have
designed for low cost, low
one thing in common: they rotate about their axis and
speed and low pressure oper-
they move a column of air parallel to that axis.
ation (Figure 2.)
The axial fan is commonly found in residential and
These fans generally oper-
commercial applications where emphasis is on moving
ate against pressures below
large volumes of air against relatively low pressures as
1
D 2" water gauge, are relatively inefficient and have a
economically (low first cost) as possible. The axial fan
steeply rising power curve (Figure 3) which presents the
is also finding greater acceptance in industrial applica-
danger of serious motor overloading in the event the air
tions as alternative equipment to the more expensive
passages in the fan system become accidentally
centrifugal (radial flow) fans.
blocked.
While residential applications are concerned primarily
with creature comfort, commercial and industrial require-
ments are expanded to include ventilation for process
as well as worker comfort.
There are many variations of axial flow fans, all of
which have performance characteristics of the three
basic types: propeller fans, tubeaxial fans and vane-
axial fans.
©2000 Twin City Fan Companies, Ltd.
Figure 3. Characteristic Performance of a Commercial Panel Fan with a Wide Single Thickness 5-Blade Impeller
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PERCENT OF FREE DELIVERY
Like most axial fans, the static pressure curve exhib-
Figure 4. Direct Drive Industrial Panel Fan
its a dip (stall or surge region) where unstable operation
With Deep Draw Venturi
occurs. A fan operating in this region will experience
pulsating behavior and increased noise levels. Extended
operation in this area will result in severe damage to the
structure and the impeller. A fan should be selected to
operate comfortably to the right of this stall region. In
AIRFLOW
the case of our example, the fan should be selected to
operate at 70% to 100% of free delivery. If this is not
possible, a smaller fan should be chosen for the applica-
tion.
On the other hand, a typical industrial orifice panel
or ring fan is constructed of heavier gauge materials
incorporating a deep drawn venturi (Figure 4). These fans
use stronger, more efficiently designed cast aluminum
airfoil or cambered stamped steel impellers (Figures 5
Figure 5. Medium Width Figure 6. Medium Width
and 6). While normally designed for pressures up to 1"
Cast Aluminum Stamped Steel
of water, these fans can be designed to reach 2" to 3"
Airfoil Impeller Impeller
of static pressure.
The designer strives for a fan to have an almost flat
power curve characteristic. Generally speaking, fan
impellers with two to eight narrow-to-medium width
blades have what is called a flat power curve. The
power curve rises only slightly from free air to about
mid-range (Figure 7) and then drops slightly with an
upswing near the condition of no flow. Increasing the
number of blades will usually decrease the free air vol-
ume and increase its ability to work against pressure.
Compare the curves in Figures 3 and 7. Note the
increased operating range (55% to 100%) and higher
Figure 7. Characteristic Performance of an Industrial Panel Fan with a Medium Width 4-Blade Airfoil Impeller
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2 Fan Engineering FE-2300
HORSEPOWER AND EFFICIENCY
PERCENT OF NO FLOW STATIC PRESSURE
HORSEPOWER AND EFFICIENCY
PERCENT OF NO FLOW STATIC PRESSURE
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static pressure capability of the industrial panel fan over Figure 8. 6-Blade Impeller for Medium Low Pressure
the commercial fan. Also note the higher efficiencies Applications
attained by this fan. Now compare the industrial panel
fan performance (Figure 7) against a similar size tube-
axial fan (Figure 10). We can see that there is a negli-
gible performance difference between a well designed
industrial panel fan and a tubeaxial fan.
As mentioned previously, specialty panel fans can be
designed to work against pressures of 2" to 3" of water.
In addition to additional blades these impellers also have
higher hub-to-tip ratios (the outside hub diameter
divided by propeller diameter) than typical panel fan
impellers. A low pressure commercial impeller (Figure 2)
might have a hub-to-tip ratio in the range of 0.15, while
a well designed industrial impeller (Figures 5 and 6) is
in the range of 0.25. A typical higher pressure impeller
(Figure 8) will have a hub-to-tip ratio of 0.4 or greater.
Another popular speciality fan utilizes a reversible
propeller, in a double orifice panel. Designed with a
hub-to-tip ratio of 0.25, this S shaped blade is capa-
Figure 9. Direct Drive Tubeaxial Fan
ble of moving the same airflow at the same horse-
power, in either direction, with the flip of a switch. This
IMPELLER
propeller exhibits a static pressure curve similar to
Figure 7, combined with a horsepower curve similar to
Figure 3.
AIRFLOW
Tubeaxial Fans
The tubeaxial fan (Figure 9) is a propeller fan mounted INLET
OUTLET
BELL
in a cylindrical tube or duct and is often called a duct
CONE
fan. Fans of this type employ a variety of impeller
designs similar to those already described under the
MOTOR WITH
industrial panel fan. The tubeaxial fan can operate in
COOLING FAN
pressure ranges up to 4" water gauge primarily because
its strong construction allows for higher speeds and
horsepower.
The performance characteristics of the tubeaxial fan
are very similar to those previously shown for the indus-
trial panel fan. The performance curve (Figure 10) is for
peratures up to 600°F, or air heavily contaminated with
a tubeaxial fan using the same impeller that was used
corrosive chemicals or explosive vapors. They can be
in the industrial panel fan (Figure 7). Generally speaking,
mounted in parallel for higher airflows or they can be
the tubeaxial fan will develop slightly better pressure
staged in series to increase their pressure capabilities.
characteristics than a similar well designed panel fan.
Tubeaxial fans are designed for use in ducted appli- Also, as mentioned under the panel fans, using larger
hub-to-tip ratio impellers increases the tubeaxial fan s
cations. Much more versatile than the panel fan by
ability to work against pressure for a given speed or
virtue of their construction, they are most adaptable to
conversely enables the fan to work against the same
ventilation of industrial processes. They can be built of
materials which will stand up under light abrasion, tem- pressure at a lower speed.
Figure 10. Characteristic Performance of a Tubeaxial Fan with a Medium Width 4-Blade Airfoil Impeller
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PERCENT OF FREE DELIVERY
3 Fan Engineering FE-2300
HORSEPOWER AND EFFICIENCY
PERCENT OF NO FLOW STATIC PRESSURE
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Figure 11. Belt Driven Vaneaxial Fan
Vaneaxial Fans
The vaneaxial fan (Figure 11) is a variation of the duct BELT
TUBE
fan design which operates in the medium-to-high pres-
IMPELLER
sure ranges. Two to 10 inches water gauge is the
expected pressure range for a single stage fan.
The performance of the vaneaxial fan (Figure 12)
GUIDE
shows the pressure curve to rise steeply from free deliv-
VANES
ery to a maximum point and then dip sharply into stall.
From the bottom of the stall range the pressure rises
again to a higher pressure value at the point of no flow.
The increased operating pressure characteristic of the
AIRFLOW
vaneaxial fan is the combined result of impeller design
and the guide vanes.
The guide vanes are usually located at the discharge
of the impeller. The function of the vanes is to recover
BEARING
the energy of rotation and convert this into useful work.
CASING
The efficiency of the vaneaxial fan rises to a maximum
near the midrange peak pressure point. Its efficiency is
higher than the efficiency of other types of axial fans,
but the horsepower characteristic is not as flat as that
sions. The adjustable pitch versions are limited to clean
of the industrial panel or tubeaxial fans. The power rises
air applications; however, fans with cast solid impellers
from free delivery to the mid-range peak pressure, dips
can be designed to handle high temperatures and
similarly as does the static pressure curve, and then rises
chemical contaminated air. Vaneaxial fans are not recom-
again toward the point of no flow.
mended for applications containing abrasives, dust,
In designing a system for the vaneaxial fan, it is nec-
stringy materials or overspray since buildup on the guide
essary to be sure that the point of operation is to the
vanes will decrease fan performance.
right of the dip in the performance curve, but not too
far from the peak pressure point to take advantage of
Conclusion
maximum efficiency. When operating vaneaxial fans in
parallel, care should be taken to ensure that the flow is Propeller fans have many advantages over other forms
divided equally. Vaneaxial fans work well in series, either of air moving devices and the recognition of these has
as two stages in a common housing or as two separate brought about rapid progress in their development and
fans installed end to end. use. Among the main advantages of propeller fans are
One valued feature of the vaneaxial fan is its ability their high capacity-to-weight ratio, the inline flow design
to allow pitch changes for controlling air volumes, either making installation in ducts simple, and the broad range
through in-flight adjustable or manually adjustable ver- of high efficiency performance.
Figure 12. Characteristic Performance of a Vaneaxial Fan with a Medium Width 7-Blade Airfoil Impeller
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PERCENT OF FREE DELIVERY
®
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5959 trenton Lane N | minneapolis, mN 55442 | Phone: 763-551-7500 | Fax: 763-551-7501
HORSEPOWER AND EFFICIENCY
PERCENT OF NO FLOW STATIC PRESSURE
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