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

QQ

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

QQ

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