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 relatively 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
Centrifugal Fan
Air in
Housing
Rotor
Motor
Air out
Vaneaxial 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
2
1
discharge
suction
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.
From Bernoullis energy theorem
1. For fans installed with both suction and discharge duct
2
P2 P1 v 2 v 1
ht
2g
m of gas
2. For fans installed with only a suction duct; P2 = 0 gage
2
0 P1 v 2 v 1
ht
2g
m of gas
3. For fans installed with only discharge duct; P1 = 0 gage and v1 = 0
2
P2 v 2
ht
2g
m of gas
let
P2 P1
hs
m of gas
v 2 v1
hv
2g
m of gas
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
hw
gh g
w
gh g
w
m of water
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
FP
x 100 %
F BP
STATIC EFICIENCY
SP
x 100%
S
BP
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
1
Q
; Power
1
1
; N
G. Variation in density; m = C;D = C; system = C
1
Q ; H
1
1
1
; N ; Power 2
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 )
Q1 340m 3 / min
hs1 0.025m of H 2 O
N1 400 RPM
BP1 3 KW
Q 2 425 m 3 /min
N2
BP2
From Fan Laws
Q N; h N 2; P N 3
Q2 N 2
Q1 N1
425 N 2
340 400
N 2 500 RPM
2
h s2 500
25 400
hs 2 39.1 mm WG
BP2 500
3
400
BP2 6 KW
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
P2
P1
PVk = C
k 1
T2 P2 k
T1 P1
k 1
kP1Q P2 k
W
1
k 1 P1
W QH
where Q V1
Q - capacity in m3 /sec
H - adiabatic head in meters
k 1
1000kRT1 P2 k
H
1 m of gas
gk 1 P1
B. For Isothermal Compression:
P
P2
PV = C
P1
P1V1 P2 V2 C
P2
P2
W P1Q ln
mRT1ln
KW
P1
P1
W QH KW
H
1000RT1 P2
ln
g
P1
meters
where
H - isothermal head in meters
Q - capacity in m3/sec
g - gravitational acceleration in m\sec2
Efficiency:
A. Adiabatic or Isentropic Efficiency
Isentropic Work
k
x 100%
Actual Work
B. Isothermal Efficiency
Isothermal Work
I
x 100%
Actual Work
RATIO OF THE ADIABATIC TEMPERATURE RISE TO THE
ACTUAL TEMPERATURE RISE
P k 1k
T1 2
1
P1
Y '
T2 T1
RELATIONSHIP FOR CORRECTING PERFORMANCE CURVES
1. Volume Flow
Q B NB
Q A NA
2. Weight Flow
mB NB P1B
m A N A P1A
T1A
T1B
3. Pressure Ratio
P k 1k
2
1
2
P1
B NB T1A
N T
P k 1k
A 1B
2
1
P1
A
P2
rp (pressure ratio)
P1
4. Head
HB NB2
2
HA NA
5. Brake Power
BPB NB
BPA N A
P1B
P1A
T1A
T1B
P k 1k
2
1
BPB P1B Q B 1
B
BPA P1A Q A k 1k
P2
1
P1
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
