Ventilation
5 Fans
Vladimír Zmrhal (room no. 814)
Dpt.
Dpt. Of
http://users.fs.cvut.cz/~zmrhavla/index.htm Environmental
Engineering
1
Introduction
Fans
air pump that creates a pressure difference and causes airflow
generally for gas transport
rotary blade machine not only for HVAC applications
Fan parameters
total pressure difference ∆p
volume airflow rate V
power input P
1
Types of fans
According to the direction of airflow
centrifugal fan
axial fan
mixed-flow fan
cross-flow fan
Drive arrangements
direct drive
coupling drive
belt drive
Types of fans
According to pressure rise across the fan
Centrifugal fans
low pressure ∆p < 1000 Pa
medium pressure ∆p = 1000 to 3000 Pa
high pressure ∆p > 3000 Pa
Axial fans
propeller
tubeaxial
vaneaxial
2
Types of fans
According to using
residential fans
ducted fans
jet fans
roof fans
smoke-ventilating fans
hot-gas fans
corrosion-resistant fans
etc.
Centrifugal fans
impeller wheel (1), inlet (2), discharge (3), scroll (4), motor (5),
frames (6)
scroll (diffuser) - kinetic energy → pressure energy
Centrifugal fans blow air at
right angles to the intake of
the fan; the impeller 3
rotates, causing air to enter 1
the fan near the shaft and
move perpendicularly from
the shaft to the opening in 2
the scroll-shaped fan
casing
4 1 5 4 6
3
Centrifugal fans
impeller design
a) forward-curved blades
b) backward-curved blades
c) radial blades
d) aerofoil blades
low pressure fans with forward-curved blades – most widely used
in ventilation and air-conditioning – lots of blades (40 to 50),
ηm = 0,55 to 0,65
medium pressure and high pressure fans with backward-curved
blades – small number of blades (6 to 15), ηm= 0,8 to 0,85
Centrifugal fans
a) b) c) d)
4
Axial fans
the axial-flow fans have blades that force air to move parallel to
the shaft about which the blades rotate
because the discharge opening is inline with its entrance, and
axial flow fan offers the advantage of simplified duct
arrangement
wide variety of applications, ranging from small cooling fans for
electronics to the giant fans used in wind tunnels
standard axial flow fans have diameters from 300–400 mm or
1800 to 2000 mm and work under pressures up to 800 Pa
Axial fans
Propeller fans
low pressure fans
Tubeaxial fans
generally considered to be heavy-duty
propeller fans
Vaneaxial fans
basically tube axial fans plus vanes.
behind the fan blades are vanes which
straighten the spiral flow of air, thus
increasing the static efficiency
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5
Axial fans
Jet fans
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Mixed-flow fans
in a mixed flow fan the air flows in both axial and radial direction
relative to the shaft.
mixed flow fans develops higher pressures than axial fans
the angle of leaving air < 90°
example: roof fans
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6
Cross-flow fans
also tangential fan or tubular fan
main flow moves transversely across the impeller, passing the
blading twice
forward-curved blades, the fan is usually long in relation to the
diameter L = (1 to 5)D
example: fan-coil units, ηm = 0,45 to 0,55
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Fan parameters
Volume airflow rate V [m3/s]
volume of air, which is transferred by fan
performance data are based on dry air at standard conditions
101,325 kPa and 20 °C → ρ = 1,2 kg/m3
Total pressure difference ∆p [Pa]
the fan have to pass the system pressure losses (static
pressure)
Electrical power P [W]
V ∆p
P=
ηtot
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7
Fan parameters
Specific fan power SFP [W/(m3/s)]
P ∆p
SFP = =
V ηtot
Energy consumption [MWh/a]
Volume airflow rate V [m3/h]
15
Energy consumption
τ n
Etot = ∫ Pdτ =∑ P [kWh/year]
0 0
V ∆p
Etot = τ = SFP .V .τ
1000 ⋅ηtot
V1∆p1 V ∆p 1 n Vi ∆pi
Etot = τ 1 + 2 2 τ 2 + ... = ∑ τi
1000 ⋅ηtot ,1 1000 ⋅ηtot ,2 1000 1 ηtot ,i
τ … working time of the fan [hours]
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8
Fan performance curves
∆p = f (V)
P = f (V)
ηtot = f (V)
n = const.
17
Fan laws
n = var.; ρ = const. ρ = var.; n = const
n2
V2 = V1 V2 = V1
n1
2
n ρ2
∆p2 = ∆p1 2 ∆p2 = ∆p1
n1 ρ1
3
n ρ2
P2 = P1 2 P2 = P1
n1 ρ1
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9
Temperature rise across fans
P = M +Q
P ∆pV
Q= −M = − ∆pV = V ρ c ∆t
ηtot ηtot
∆p
∆tfan = (1 − ηc )
ρ cηc
∆p SFP
∆ttot = =
ρ cηc ρ c
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Temperature rise across fans
Temparature rise across the fan ∆ tfan
Total temparature rise ∆ ttot
Total pressure difference ∆p [Pa] Total pressure difference ∆p [Pa]
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10
Fan and system pressure relationship
21
Fan and system pressure relationship
dynamic pressure
w2
pd = ρ
2
total pressure
pt = ps + pd
total pressure difference of the fan
∆p = pt 2 − pt 1 = ∆pt 1 + ∆pt 2 = ∆pl 1 + ∆pl 2 + pd 2
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11
Fan and system pressure relationship
∆ pl 2 = 0
23
Duct system charakteristics
Pressure losses of ductwork
friction
local losses
l w2 w2 l w
2
∆ploss =λ ρ + ∑ζ ρ = λ + ∑ ζ ρ = KV 2
14d24
2 3 1424 23 d 2
friction local losses
λ ⋅l 8⋅ ρ
K = + ∑ζ 2 4
d π ⋅d
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12
System and fan charakteristics
System
curve
Fan
charakteristics
Working
point
25
Parallel fan operation
System curve
Fan charakteristics
2 fan operation
26
13
Serial fan operation
System curve
Fan charakteristics
2 fan operation
27
Control of the fans
System characteristic
change
dampers
reducing of airflow by
increasing the system
pressure required
incresing power
consumption
∆P = V2 ( ∆p2 − ∆p2' )
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14
Control of the fans
Fan characteristic change
economical
speed control n
29
Control of the fans
a) variable speed motors
variable speed motors (twin or three speed motors)
number of motor poles p (s …slide of the motor)
120f
n= (1 − s )
p
b) frequency control
optimal control
control in wide range od 0 do 100 %
for bigger systems
c) voltage control
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15
Control of the fans
d) EC motors
electronically commutated
(EC) motors
brushless DC motors where
the direction of the electric
current is switched using
electronic controllers
variable speed control,
without the drawbacks of
brushes
DCV (demand control
ventilation systems)
31
Control of the fans
Adjustable pitch-blades
a) Controllable-pitch in suction of centrifugal fan
for backward-curve impellers only
b) Controllable-pitch of impeller wheel
for axial fans
high operation efficiency
complicated mechanism a expensive
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Thank you for your
attention
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