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Fans and Blowers Principles
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Fan is a machine used to add energy to the gaseous fluid to increase its
pressure. Fans are used where low pressures (from a few mm of water to
50 mm Hg) and comparatively large volume are required. They run at rela-
tively low speed, the casing and impeller usually built of sheet iron.
FAN TYPES
1) AXIAL FLOW FANS - the flow of the gases is parallel to the fan shaft.
a. tube axial
b. vane axial
c. Propeller
2) RADIAL OR CENTRIFUGAL FLOW FANS- the flow of gases depends
upon the centrifugal action of the impeller or rotor.
a. Straight blades
b. Forward curved blades
c. Backward curved blades
d. Double curved blades
Propeller Fan Tubeaxial Fan Vaneaxial Fan
Air in
Air out
Motor
Rotor
Housing
Centrifugal Fan
COMMON USES OF FANS
1. Ventilation and air conditioning
2. Forced and induced draft service for boilers
3. Dust collection
4. Drying and cooling of materials
5. Cooling towers
6. Mine and tunnel ventilation
7. Pneumatic conveying and other industrial process work
Head Calculations
1
2
suction
discharge
For a fan Z = 0 ; PE = 0 and Q = 0, because fans are designed to
overcome fluid friction. No cooling system is needed due to small temperature
differential between suction and discharge.
3. For fans installed with only discharge duct; P1 = 0 gage and v1 = 0
1. For fans installed with both suction and discharge duct
gas of m 2g
vvPPh
2
1
2
212t
γ
gas of m 2g
vvP0h
2
1
2
21t
γ
2. For fans installed with only a suction duct; P2 = 0 gage
gas of m 2g
vPh
2
22t
γ
From Bernoulli’s energy theorem
gas of m PP
h 12s
γ
gas of m 2g
vvh
2
1
2
2v
let
ht = hs + hv m of gas
Where:
hs - static head at which a fan operates, m of gas
hv - velocity head at which a fan operates, m of gas
ht - total head added to the fluid, m of gas
Head Conversion: From m of gas to m of water
waterof m h
h
hw
gg
w
gg
wρ
ρ
γ
γ
htw = hsw + hvw
Where:
h - stands for ,total head, static head or velocity head
w - refers to water; g - refers to gas
FAN POWER FP = Qwhtw KW
STATIC POWER SP = Qwhsw KW
where Q - capacity in m3/sec
w - specific weight of water (gage fluid) in KN/m3
htw - total head in m of WG
hsw - static head in m of WG
FP - total fan power in KW
SP - Static power in KW
Static Power - is that part of the total air power, that is
used to produced the change in static head.
FAN EFFICIENCY
STATIC EFICIENCY
% 100 xBP
FPη
F
100% xBP
SPη
S
BP - Brake or shaft power in KW
FAN LAWS
A. Variation in speed and impeller diameter
Q ND3
H N2D2
B. Variation in impeller Speed
Q N ; H N2 ; Power N3
C. Variation in impeller size; Tip speed = C ; = C and
same proportions; H = C
Q D2 ; Power N2 ; N 1/D
D. Variation in impeller size; N = C; = C ; Same proportions
Q D3 ; Power D5 ; H D2 ; Tip Speed D
E. Variation in density; Q = C; N =C; D = C; system = C
H ; Power
F. Variation in Density; D = C; H = C
ρ
1N ;
ρ
1 Power ;
ρ
1Q
G. Variation in density; m = C;D = C; system = C
2
1 Power
; ρ
1 N ;
ρ
1 H ;
ρ
1 Q
A certain fan delivers 340 m3/min of air at a static
pressure of 25.4 mm WG when operating at a
speed of 400 RPM and requires an input of 3 KW.
If in the same installation 425 m3/min of air are
desired, what will be the new Q, hsw and Fan power
required? (40 mm WG;500 RPM;6 KW )
KW 6
400
500
3
BP
WGmm 1.39
400
500
25
h
RPM 500
400340
425
;; Q
BP
N
/minm 425Q
KW 3 BP
RPM 400N
OH of 025.0
min/340
2
3
2
2
2
s2
2
2
1
2
1
2
32
2
2
3
2
1
1
21
3
1
BP
h
N
N
N
N
Q
Q
NPNhN
LawsFanFrom
mhs
mQ
s
BLOWERS
Blower is a machine used to compressed air or gas by centrifugal force to a
final pressure not exceeding 241 KPa gage. Usually blower has no cooling
system or it is not water cooled.
COMPRESSION OF GASES
The design of blower is usually based upon either an adiabatic or isothermal
compression.
A. For Adiabatic or Isentropic Compression:
P
V
P1
P2
1
2 PVk = C
meters in head adiabatic - H
/secm incapacity - Q
V Q where
HQW
1P
P
1k
QkPW
P
P
T
T
3
1
k1k
1
21
k1k
1
2
1
2
γ
gas of m 1
P
P
1kg
1000kRTH
k1k
1
21
B. For Isothermal Compression:
P
V
P1
P2
1
2 PV = C
meters P
Pln
g
1000RTH
KW HQW
KW P
PlnmRT
P
Pln QPW
CVPVP
1
21
1
21
1
21
2211
γwhere
H - isothermal head in meters
Q - capacity in m3/sec
g - gravitational acceleration in m\sec2
Efficiency:
A. Adiabatic or Isentropic Efficiency
100% xWork Actual
Work Isentropick η
B. Isothermal Efficiency
100% xWork Actual
Work IsothermalI η
RATIO OF THE ADIABATIC TEMPERATURE RISE TO THE
ACTUAL TEMPERATURE RISE
1
'
2
k1k
1
21
TT
1P
PT
Y
RELATIONSHIP FOR CORRECTING PERFORMANCE CURVES
1. Volume Flow
A
B
A
B
N
N
Q
Q
1B
1A
1A
1B
A
B
A
B
T
T
P
P
N
N
m
m
2. Weight Flow
3. Pressure Ratio
ratio) (pressure r P
P
T
T
N
N
1P
P
1P
P
p
1
2
1B
1A
2
A
B
A
k1k
1
2
B
k1k
1
2
2
A
2
B
A
B
N
N
H
H
4. Head
5. Brake Power
A
k1k
1
2
B
k1k
1
2
A
B
1A
1B
A
B
1B
1A
1A
1B
3
A
B
A
B
1P
P
1P
P
Q
Q
P
P
BP
BP
T
T
P
P
N
N
BP
BP
Where:
1 - suction
2 - discharge
A - 1st condition
B - 2nd condition
R - gas constant, KJ/kg-K
P - absolute pressure in KPa
- density, kg/m3
T - absolute temperature, K
H - head, m - specific weight, KN/m3
Q - capacity, m3/sec
BP - brake power, KW
N - speed, RPM
W - work, KW
m - mass flow rate, kg/sec
Copyright: YURI G. MELLIZA
324619CYE