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 TYPES1) AXIAL FLOW FANS - the flow of the gases is parallel to the fan shaft. a. tube axial b. vane axial c. Propeller2) 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 FANS1. Ventilation and air conditioning2. Forced and induced draft service for boilers3. Dust collection4. Drying and cooling of materials5. Cooling towers6. Mine and tunnel ventilation7. Pneumatic conveying and other industrial process work
Head Calculations
1
2
suctiondischarge
For a fan Z = 0 ; PE = 0 and Q = 0, because fans are designed toovercome fluid friction. No cooling system is needed due to small temperaturedifferential 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
21
2212
t
γ
gas of m 2g
vvP0h
21
221
t
γ
2. For fans installed with only a suction duct; P2 = 0 gage
gas of m 2gvP
h2
22t
γ
From Bernoulli’s energy theorem
gas of m PP
h 12s γ
gas of m 2g
vvh
21
22
v
let
ht = hs + hv m of gas
Where:hs - static head at which a fan operates, m of gashv - velocity head at which a fan operates, m of gasht - 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
ggw ρ
ρ
γ
γ
htw = hsw + hvw
Where:h - stands for ,total head, static head or velocity headw - 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 KWSP - 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 xBPFP
ηF
100% xBPSP
ηS
BP - Brake or shaft power in KW
FAN LAWSA. 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/DD. Variation in impeller size; N = C; = C ; Same proportions Q D3 ; Power D5 ; H D2 ; Tip Speed DE. 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 500400340
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
32
1
1
21
31
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 GASESThe design of blower is usually based upon either an adiabatic or isothermalcompression.
A. For Adiabatic or Isentropic Compression:
P
VP1
P2
1
2PVk = C
meters in head adiabatic - H /secm incapacity - Q
V Q whereHQW
1PP
1kQkP
W
PP
TT
3
1
k1k
1
21
k1k
1
2
1
2
γ
gas of m 1PP
1kg1000kRT
Hk
1k
1
21
B. For Isothermal Compression:
P
VP1
P2
1
2PV = C
meters PP
ln g
1000RTH
KW HQW
KW PP
lnmRT PP
ln QPW
CVPVP
1
21
1
21
1
21
2211
γwhere H - isothermal head in metersQ - capacity in m3/secg - gravitational acceleration in m\sec2
Efficiency:
A. Adiabatic or Isentropic Efficiency
100% xWork Actual
Work Isentrop ick η
B. Isothermal Efficiency
100% xWork Actual
Work IsothermalI η
RATIO OF THE ADIABATIC TEMPERATURE RISE TO THEACTUAL TEMPERATURE RISE
1'2
k1k
1
21
TT
1PP
T
Y
RELATIONSHIP FOR CORRECTING PERFORMANCE CURVES 1. Volume Flow
A
B
A
B
NN
1B
1A
1A
1B
A
B
A
B
TT
PP
NN
mm
2. Weight Flow
3. Pressure Ratio
ratio) (pressure r PP
TT
NN
1PP
1PP
p1
2
1B
1A
2
A
B
A
k1k
1
2
B
k1k
1
2
2A
2B
A
B
NN
HH
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
1PP
1PP
PP
BPBP
TT
PP
NN
BPBP
Where:1 - suction2 - dischargeA - 1st conditionB - 2nd conditionR - gas constant, KJ/kg-KP - absolute pressure in KPa - density, kg/m3
T - absolute temperature, K H - head, m - specific weight, KN/m3
Q - capacity, m3/secBP - brake power, KWN - speed, RPMW - work, KWm - mass flow rate, kg/sec
Copyright: YURI G. MELLIZA324619CYE