Greenheck VAD-48F30-27-A150 User Manual
Vane Axial
Application and Design
May
®
2010
Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Vane Axial Fan Arrangements
• Arrangement 4 Direct Drive . . . . . . . . . . . . . . . . . . . . 3
• Arrangement 9 Belt Drive . . . . . . . . . . . . . . . . . . . . 3
Variations of Vane Axial Construction
• Hub-to-Tip Ratio . . . . . . . . . . . . . . . . . . . . . . . 4
• Half-Blade Fans . . . . . . . . . . . . . . . . . . . . . . . 4
• Two-Stage Fans . . . . . . . . . . . . . . . . . . . . . . . 5
• Fans in Parallel . . . . . . . . . . . . . . . . . . . . . . . . 5
Factors Affecting Air Performance
• System Effect . . . . . . . . . . . . . . . . . . . . . . . . 6
• Air Density . . . . . . . . . . . . . . . . . . . . . . . . . 7
Vane Axial Accessories Affecting Performance
• Inlet Bell . . . . . . . . . . . . . . . . . . . . . . . . . . 8
• Inlet Cone . . . . . . . . . . . . . . . . . . . . . . . . . 8
• Outlet Cone . . . . . . . . . . . . . . . . . . . . . . . . . 8
Understanding Direct Drive Performance Charts . . . . . . . . . . . . . 9
• The Total Pressure Concept. . . . . . . . . . . . . . . . . . . . 9
How Outlet Conditions Affect Total, Static and Velocity Pressure . . . . . . . 10-11
• Diagram of Pressure Variations for Various Outlet Conditions . . . . . . . . . 11
Making Fan Selections
• Operating Stability . . . . . . . . . . . . . . . . . . . . . . . 12
• Avoiding Vane Axial Stall. . . . . . . . . . . . . . . . . . . . . 12
• Avoiding Motor Overload . . . . . . . . . . . . . . . . . . . . 13
• Vane Axial Efficiency . . . . . . . . . . . . . . . . . . . . . . 13
Vane Axial Fans in Variable Air Volume Systems . . . . . . . . . . . . . . 13
Methods of Providing Variable Air Volume
• Two-Speed Motors . . . . . . . . . . . . . . . . . . . . . . 14
• Variable Pitch Sheaves . . . . . . . . . . . . . . . . . . . . . 14
• Inlet Vane Dampers . . . . . . . . . . . . . . . . . . . . . . 14
• Outlet Volume Dampers . . . . . . . . . . . . . . . . . . . . . 14
• Variable Frequency Drives . . . . . . . . . . . . . . . . . . . . 14
Vane Axial Sound and Methods of Attenuation
• Greenheck’s Sound Trap Vane Axial . . . . . . . . . . . . . . . . . 15
• Inlet and Outlet Sound Attenuators . . . . . . . . . . . . . . . . . 15
• Acoustical Diffuser Cones . . . . . . . . . . . . . . . . . . . . 15
• Sound Absorbing Materials . . . . . . . . . . . . . . . . . . . . 16
• Fan Speed and Vane Axial Sound . . . . . . . . . . . . . . . . . . 16
• Vibration Isolators . . . . . . . . . . . . . . . . . . . . . . . 16
• Flexible Duct Connections . . . . . . . . . . . . . . . . . . . . 17
• Thrust Restraints . . . . . . . . . . . . . . . . . . . . . . . 17
Economic Considerations of Vane Axial Selection and Application
Maintenance Costs
. . . . . . . . . . . . . . . . . . . . . . . 19
Specifications . . . . . . . . . . . . . . . . . . . . . . Backcover
. . . . . . . 18-19
2
®
INTRODUCTION
This manual provides information on the application of vane axial fans in variable or constant air volume systems.
Many problems encountered with air moving devices such as the vane axial fan, are a result of misapplication
due to lack of easy to read, comprehensive and understandable information. Greenheck makes every effort to
provide the customer with extensive product information. Due to the relatively high volumes, pressures and
velocities generated by vane axial fans and the potential for significant performance variations, this application
manual offers information for proper selection, installation and use.
DEFINITIONS
Adjustable Pitch - Vane axial rotor blades may be manually adjusted to various pitches. Fan must be off,
electrical power locked-out, blade retaining nuts loosened, and blades manually set to desired pitch (within
horsepower limitations).
Hub - The center of the rotor. Hubs contain a provision for attachment to the driven shaft and machined sockets
or holes for attaching the blades. The hub is usually covered by a nose-cone (a spun aluminum cover for
streamlining the hub).
Rotor - A term used to describe the vane axial propeller. The rotor consists of a hub and blades.
Static Regain - Conversion of the energy of motion (kinetic energy) or velocity pressure to potential energy or
usable static pressure. An example is the increase in static pressure as velocity is reduced across an outlet cone.
Swirl (Vortex) - Airflow rotating perpendicular to the intended axis of airflow. It is a swirling movement of air
generated by the vane axial rotor.
System Effect - A pressure loss resulting from fan inlet or outlet restrictions or other condition within the system
affecting fan performance. System effect is difficult to quantify and results in poor efficiency, noise and vibration.
Vane Axial Fan - An air moving device with axial airflow and straightening vanes to reduce swirl created by the
rotor.
Variable Frequency Drive (VFD) - A system for controlling the rotational speed of an AC motor. Traditionally
used on direct drive fans for changing the rotor speed and performance of the fan (may also be used on belt
drive fans).
VANE AXIAL FAN ARRANGEMENTS
Arrangement 4 Direct Drive
Arrangement 4 direct drive vane axial fans have the rotor attached directly to the motor. This arrangement has
several advantages over a belt drive unit in that it is more compact, has no drive losses reducing efficiency, and
requires relatively little maintenance. The disadvantages include fan speeds limited to the motor speed (if used
without a variable frequency drive (VFD)), poor motor accessibility, and maximum airstream temperature of 105°F
using standard motor insulation. Arrangement 4 direct drive fans are available with adjustable pitch rotors and
the sound trap option.
Arrangement 9 Belt Drive
Arrangement 9 belt drive fans are constructed with the motor mounted on the fan housing, out of the airstream.
The rotor is attached to a fan shaft supported by grease lubricated bearings. A belt tube provides passage of the
belts from the motor to the driven pulley. Belt drive advantages include the wide range of fan speeds available,
tolerance of airstream temperatures up to 200°F, and easy motor accessibility. Also, motors for belt drive units
are generally lower cost and more readily available than those in direct drive vane axials. Arrangement 9 belt
drive fans are available with adjustable pitch rotors and the sound trap option.
®
3
VARIATIONS OF VANE AXIAL FAN CONSTRUCTION
Hub-to-Tip Ratio
The hub-to-tip ratio
of a fan is the ratio
of the hub diameter
to the blade tip
diameter. Varying this
ratio will change the
fan's performance
capabilities. Rotors
with higher hub to tip
ratios will generate
higher static pressures.
Rotors with lower
hub-to-tip ratios will
generate less static
pressure. Selecting the
correct hub-to-tip ratio
for a given size fan can
optimize fan efficiency
and reduce chances
of the fan stalling in
the field if the system
resistance increases.
See page 12 for further
details on making fan
selections.
Hub-to-Tip Ratio
Curves
Large H/T High P
Small H/T Low P
s
s
Static Pressure (Ps)
Two-Stage
High Hub-Tip Ratio
Mid Hub-Tip Ratio
Low Hub-Tip Ratio
Half-Blade
CFM
Half-Blade Vane Axial Fans - Direct Drive
Removing every other blade from the rotor has some definite advantages in low pressure selections. Vane axial
fans with a half-blade rotor will require approximately 65 percent of the horsepower required for a full blade rotor,
yet will deliver the same volume (cfm). The downside of a half-blade rotor is that it will generate approximately
65 percent of the pressure of a full blade rotor. Therefore, when the operating point falls low on the vane axial fan
curve and the application is for relatively low static pressures, a half-blade rotor should be considered to reduce
brake horsepower and increase efficiency.
Half-bladed fan selections are available for the smallest hub size for each direct drive fan size in order to extend
the useful pressure range as shown above.
4
®
Two-Stage Vane Axial Fans - Direct Drive
Where very high static pressures are required, the two-stage vane axial should be considered. Two-stage vane
axial fans have two rotors, one at each end of
the motor. These fans will generate twice the
amount of pressure, require twice the brake
horsepower and will deliver the same volume
as a single stage unit. A second vane section
is used on the exhaust end of the fan to
reduce the swirl from the second stage rotor.
Diagrams showing single- and two-stage
vane axial fans are shown.
Single-Stage
Two-Stage
Fans in Parallel
There are times when one fan may be too large and not fit into a desired space or the required operating range
of a system may necessitate multiple fans instead of one large fan. For these applications it is common to use
multiple fans in parallel. Multiple fans for capacity control may be more economical if cost of operation is critical,
especially at very low flow rates for long time intervals.
For multiple fans in parallel, each
fan will be selected for the same
static or total pressure with the
flow rate being the total flow
divided by the number of fans.
Use care when selecting fans in
parallel to ensure that the system
resistance remains on a stable
portion of the fan curve at all times.
This is particularly true when the
fans have a pronounced surge
area or a dip in the fan curve and
Static Pressure (Ps)
Standard surge
line for single
fan operation.
Single fan surge
line for parallel fan
applications.
some form of control is applied.
The operating point with all fans
running must be lower than
the lowest pressure in the dip.
This minimizes the possibility that the fan will hunt back and forth across the peak of the curve looking for an
operating point. This policy also minimizes the likelihood that the fans will experience unequal loading causing
differences in motor load or creating unequal velocity profiles if used within a plenum, which may result in a
system effect.
For fans in parallel be sure to keep adequate distance
between fans and walls to ensure proper intake conditions.
See diagram for general spacing guidelines.
Single fan
performance curve
Parallel surge line.
1.5 D
CFM
Do not make selections
above this line.
Parallel fan
performance curve
Airflow
2 D
Airflow
1.5 D
5
®
FACTORS AFFECTING AIR PERFORMANCE
System Effect
Imagine a vane axial fan selected with great care to provide exactly the performance required in the
specifications. Once installed, the air balancer reports that air performance is considerably lower than required.
What went wrong?
The answer is probably system effect. The Air Movement and Control Association International Inc. (AMCA)
defines system effect as "a pressure loss which recognized the effect of fan inlet restrictions, fan outlet
restrictions, or other conditions influencing fan performance when installed in the system."
Fan manufacturers go to great lengths to test fans and provide reliable air performance data in their literature.
These fans are tested under very specific conditions as specified on the performance pages. Statements such
as, "Performance shown is for model 'xyz' with inlet and outlet ducts," indicate how the fan was tested. An
installation where elbows, transitions, dampers and other disruptions to airflow are located before or after the fan
can create a condition different from the manufacturer's test methods. Therefore, a performance loss or system
effect is created.
System effect is very difficult to quantify and correct. Frequently, the only means to correct the resulting poor
performance is to
increase fan speed
or increase the
blade pitch. Both of
these situations may
increase horsepower
requirements that
exceed the capability
of the motors. Also, the
system effect may be so
great that the fan is not
capable of generating
enough static pressure
even at maximum fan
speed. This could mean
replacing the fan with
one of greater capacity.
Finally, system effect
will rob an air moving
device of efficiency.
Higher fan speeds and
greater horsepower
used to overcome a
design deficiency result
in wasted energy.
The diagrams show
some of the more
common causes of
system effect. Nonuniform airflow created
by duct elbows,
transitions, dampers
or other obstacles
in the airstream may
dramatically reduce
fan performance. Refer
to AMCA Publication
201 for a quantitative
discussion of system
effects.
Turning
Vanes
Good
Good
One
Fan
Diameter
Good
Good
Good
Three Fan Diameters
Inlet Bell
Length of Straight Duct
Minimum of three fan diameters
D
Ducted Inlet Conditions
Non-Ducted Inlet Conditions
Poor
Ducted Outlet Conditions
Two Fan
Diameter
Outlet Cone
Poor
Non-Ducted Outlet Conditions
Poor
Poor
Poor
6
®
Loading...