Fans.pdf

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MECH7350 Rotating Machinery
11. Fans
11. FANS
(This section is taken mainly from Black and Veatch)
11.1 Types of Fans
Fans are used in electricity generation to supply or exhaust air or flue gas. They increase the
pressure of a flow stream to offset the pressure losses that result from system resistance.
Centrifugal fans move air or gas perpendicular to the impeller shaft. Axial fans move air or
gas parallel to the impeller shaft.
11.1.2 Centrifugal Fans
Figures 11.1 and 11.2 show the construction and components of centrifugal fans. Centrifugal
fan blades are mounted in an impeller that rotates within a spiral housing. They are designed
with either one or two inlets, and normally run at constant speed with flow and pressure
controlled with inlet dampers.
Fig. 11.1 Double-inlet centrifugal fan (from Black and Veatch).
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11. Fans
The performance of centrifugal fans is highly dependent on the type of blades used; backward
curved, straight or radial tip. This is shown in Fig. 11.3.
Fig. 11.2 Centrifugal fan components (from Black and Veatch).
Fig. 11.3 Fan blade types and performance curves (from Black and Veatch).
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11. Fans
Backward curved blades have an aerofoil shape and are most widely used in power plant
operations. Their advantages are:
•
•
•
•
Highest efficiency, over 90 percent
Very stable operation
Low noise
Capacity for high speed operation
They should be avoided where large or adhesive particles are present.
11.1.3 Axial Fans
Single-stage (one rotor and one set of blades) axial fans are typically used in a forced draft
situation on a balanced draft steam generator. When axial fans are designed for induced draft
service, the higher pressure requirements normally dictate use of a two-stage fan such as in
Fig. 11.4. Axial fans are driven by single-speed or two-speed motors, with flow and output
pressure controlled by varying blade pitch with a hydraulically actuated mechanism. Axial
fans can maintain higher efficiencies at various steam generator loads than can constant-
speed centrifugal fans controlled with inlet dampers.
Fig. 11.4 Two-stage axial fan (from Black and Veatch).
Use of lightweight fan blade materials such as aluminium or magnesium reduces the strength
requirements of the fan hub and the fan blade thrust bearing loads. This reduces equipment
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11. Fans
costs but in erosive situations blade protection might be needed, such as a hardened steel strip
on the leading edge.
11.2 System Resistance
A loss of pressure occurs when gas is forced through a duct system. This loss of pressure is
called system resistance, and is composed of two parts; friction losses and dynamic losses.
Friction losses mainly occur at the walls of the duct system and can be quantified by the
following empirical equation:
∆
P
friction
where
f
ρ
V
2
L
=
Dg
(11.1)
∆
P
friction
= frictional pressure loss
f
= a dimensionless friction factor
ρ
=
gas density
V
= gas velocity
L
= duct length
D
= duct diameter
g =
gravitational constant
Dynamic losses occur at changes of direction in gas flow and at sudden duct enlargements
and contractions. They are sometimes called velocity pressure losses. Provided that the gas
flow is turbulent, as is the case for nearly all combustion air and flue gas handling systems
and equipment, dynamic losses can be empirically quantified by the following equation:
∆
P
dynamic
where
1
ρ
KV
2
=
2
g
(11.2)
K
= system constant based on geometry of duct and determined experimentally.
Then
∆
P
= ∆
P
friction
+ ∆
P
dynamic
(11.3)
When the system geometry is not modified (i.e. when
f, L, D, g
and
K
are held constant) only
changes in gas density and flow rate cause the system resistance to change. Once the
pressure test has been made at a known gas flow rate and density, the system resistance can
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11. Fans
be predicted for any flow condition. If the gas density is not changed, system resistance can
be plotted against flow
Q
which is proportional to
V.
11.3 Fan Performance
Fans are used to provide the pressure necessary to overcome system resistance.
Fan
performance characteristics are developed from test data. A typical fan performance curve
(in American units; in. wg = inches water gauge, acfm = actual cubic feet per minute) for a
centrifugal fan operating at a given speed and gas density is shown in Fig. 11.5.
Fig. 11.5 Typical centrifugal fan performance curve (from Black and Veatch).
Once the fan is installed into a ductwork system, the intersection of the system resistance
curve and the fan characteristic curve defines the system operating point, as shown in Fig.
11.6.
Fig. 11.6 Typical installed centrifugal fan performance curve (from Black and Veatch).
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