Fan selection is based on the calculated total pressure increase
required (sum of pressure losses) and desired air flow rate per
Normally, the desired operating point can be achieved with several
fan sizes. If a small fan is selected, the operating point will
lie far to the right of the optimal point. The result will be
low efficiency. If a larger fan is selected, the operating point
will lie further to the left of the diagram and greater efficiency
will be achieved. The initial cost of a larger fan is therefore
often offset by lower operating costs.
CENTRIFUGAL FANS TYPE IMPELLER DESIGN HOUSING DESIGN
Highest efficiency of all centrifugal fan designs. Ten to 16
blades of airfoil contour curved away from direction of rotation.
Deep blades allow for efficient expansion within blade passages.
Air leaves impeller at velocity less than tip speed. For given
duty, has highest speed of centrifugal fan designs. Scroll-type
design for efficient conversion of velocity pressure to static
pressure. Maximum efficiency requires close clearance and alignment
between wheel and inlet.
BACKWARD INCLINED BACKWARD CURVED
Efficiency only slightly less than airfoil fan. Ten to 16 single-thickness
blades curved or inclined away from direction of rotation. Efficient
for same reasons as airfoil fan. Uses same housing configuration
as airfoil design.
Higher pressure characteristics than airfoil, backward-curved,
and backward-inclined fans. Curve may have a break to left of
peak pressure and fan should not be operated in this area. Power
rises continually to free delivery. Scroll. Usually narrowest
of all centrifugal designs. Because wheel design is less efficient,
housing dimensions are not as critical as for airfoil and backward-inclined
FORWARD CURVED FANS
Flatter pressure curve and lower efficiency than the airfoil,
backward-curved, and backward-inclined. Do not rate fan in the
pressure curve dip to the left of peak pressure. Power rises continually
toward free delivery. Motor selection must take this into account.
Scroll similar to and often identical to other centrifugal fan
designs. Fit between wheel and inlet not as critical as for airfoil
and backward-inclined fans.
Low efficiency. Limited to low-pressure applications. Usually
low cost impellers have two or more blades of single thickness
attached to relatively small hub. Primary energy transfer by velocity
pressure. Simple circular ring, orifice plate, or venturi. Optimum
design is close to blade tips and forms smooth airfoil into wheel.
Somewhat more efficient and capable of developing more useful
static pressure than propeller fan. Usually has 4 to 8 blades
with airfoil or single thickness cross section. Hub is usually
less than half the fan tip diameter. VANEAXIALGood blade design
gives medium- to high-pressure capability at good efficiency.
Most efficient of these fans have airfoil blades. Blades may have
fixed, adjustable, or controllable pitch.
Performance similar to backward-curved fan except capacity and
pressure are lower. Lower efficiency than backward-curved fan.
Performance curve may have a dip to the left of peak pressure.
Cylindrical tube similar to vaneaxial fan, except clearance to
wheel is not as close. Air discharges radially from wheel and
turns 90° to flow through guide vanes.
POWER ROOF VENTILATORSCENTRIFUGAL
Low-pressure exhaust systems such as general factory, kitchen,
warehouse, and some commercial installations.
Provides positive exhaust ventilation, which is an advantage over
gravity-type exhaust units. Centrifugal units are slightly quieter
than axial units. Normal housing not used, since air discharges
from impeller in full circle. Usually does not include configuration
to recover velocity pressure component.
PERFORMANCE CURVESa PERFORMANCE CHARACTERISTICS APPLICATIONS
Highest efficiencies occur at 50 to 60% of wide open volume. This
volume also has good pressure characteristics. Power reaches maximum
near peak efficiency and becomes lower, or self-limiting, toward
free delivery. applications. Usually only applied to large systems,
which may be low-, medium-, or high-pressure applications. Applied
to large, clean-air industrial operations for significant energy
savings. Similar to airfoil fan, except peak efficiency slightly
lower. Same heating, ventilating, and air-conditioning applications
as airfoil fan. Used in some industrial applications where airfoil
blade may corrode or erode due to environment. Higher pressure
characteristics than airfoil and backward-curved fans. Pressure
may drop suddenly at left of peak pressure, but this usually causes
no problems. Power rises continually to free delivery. Primarily
for materials handling in industrial plants. Also for some high-pressure
industrial requirements. Rugged wheel is simple to repair in the
field. Wheel sometimes coated with special material. Not common
for HVAC applications. Pressure curve less steep than that of
backward-curved fans. Curve dips to left of peak pressure. Highest
efficiency to right of peak pressure at 40 to 50% of wide open
volume. Rate fan to right of peak pressure. Account for power
curve, which rises continually toward free delivery, when selecting
motor. Primarily for low-pressure HVAC applications, such as residential
furnaces, central station units, and packaged air conditioners.
High flow rate, but very low-pressure capabilities. Maximum efficiency
reached near free delivery. Discharge pattern circular and airstream
swirls. For low-pressure, high-volume air moving applications,
such as air circulation in a space or ventilation through a wall
without ductwork. Used for makeup air applications. High flow
rate, medium-pressure capabilities. Performance curve dips to
left of peak pressure. Avoid operating fan in this region. Discharge
pattern circular and airstream rotates or swirls. Low-and medium-pressure
ducted HVAC applications where air distribution downstream is
not critical. Used in some industrial applications, such as drying
ovens, paint spray booths, and fume exhausts. High-pressure characteristics
with medium-volume flow capabilities. Performance curve dips to
left of peak pressure due to aerodynamic stall. Avoid operating
fan in this region. Guide vanes correct circular motion imparted
by wheel and improve pressure characteristics and efficiency of
fan. General HVAC systems in low-, medium-, and high pressure
applications where straight-through flow and compact installation
are required. Has good downstream air distribution. Used in industrial
applications in place of tubeaxial fans. More compact than centrifugal
fans for same duty. Performance similar to backward-curved fan,
except capacity and pressure is lower. Lower efficiency than backward-curved
fan because air turns 90°. Performance curve of some designs
is similar to axial flow fan and dips to left of peak pressure.
Primarily for low-pressure, return air systems in HVAC applications.
Has straight-through flow. Usually operated without ductwork;
therefore, operates at very low pressure and high volume. Only
static pressure and static efficiency are shown for this fan.
Low-pressure exhaust systems, such as general factory, kitchen,
warehouse, and some commercial installations. Low first cost and
low operating cost give an advantage over gravity flow exhaust
systems. Centrifugal units are somewhat quieter than axial flow
units. Usually operated without ductwork; therefore, operates
at very low pressure and high volume. Only static pressure and
static efficiency are shown for this fan. Low-pressure exhaust
systems, such as general factory, kitchen, warehouse, and some
commercial installations. Low first cost and low operating cost
give an advantage over gravity flow exhaust systems.
Fan Noise & Isolation