Fan and blowers (mech 326)

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