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Lecture 5 Cyclones EVEN 4386 Air Quality and Pollution Control 1

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Page 1: Lec. 5 Cyclones

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Lecture 5 Cyclones

EVEN 4386 Air Quality and Pollution

Control 

1

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Contents

• Particle removal mechanisms

• Types of particle removal equipment

• Cyclone dimension

• Design and Process Parameters

• Pressure drop• Cyclone collection efficiencies

• Costs

2

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Mechanisms to Remove Particulate

Contaminants from Gas Streams

• The primary mechanisms for removal of particulate

material from gas streams are Brownian motion,

interception, and impaction.

• Enhancement of these mechanisms can occur by using

external forces such as electrostatic, gravitational 

and/or centrifugal forces.

3

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Removal of Particles from a Gas Stream with a

Collector Body via Brownian Motion, Interception,

and Impaction

fluidstreamline

collector body

 impactionBrownianMotion

 interception

4

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Separation and Removal of

Particulate from Gas Streams• Particulate contaminants are typically removed from

industrial gas streams with the use of:

Settling Chambers (gravitational force)

Cyclones (centrifugal force)

Wet Collectors (Brownian motion, interception,

and impaction)

Electrostatic Precipitators (electrostatic force)

Fabric Filters (Brownian motion, interception, and

impaction)

5

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Cyclones (Centrifugal Force)

• Gravitational force is useful to remove coarse particles 

(d p > 10 mm) from gas streams but is not very effective

for smaller particles.

• The centrifugal force can be used to achieve larger

removal efficiencies for smaller particles.• The gas stream is forced to change its direction with the

 particles following a different direction.

• The centrifugal force causes the particle to betransported in a different direction than the gas stream

allowing for separation and collection of particles from

gas streams.6

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Cyclone Schematic Flow

7

Koger Industrial Cyclones Type A-B

Source: Koger Air Corporation website: http://www.kogerair.com

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Advantages

• Low capital cost

• Ability to operate at high

temperatures

• Low maintenance

requirements

• Can handle liquid mists or

dry materials

• Eases re-use or disposal

• Needs relatively small space

for installation

Disadvantages

• Low efficiencies for smallparticles < 1um

• High operating costs due to

pressure drop

• Unable to process “sticky”materials

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Classical Cyclone Dimensions

Cyclone Type

High Efficiency Conventional High Throughput

(1) (2) (3) (4) (5) (6)

Body diameter, D/D 1.0 1.0 1.0 1.0 1.0 1.0

Height of inlet, H/D 0.5 0.44 0.5 0.5 0.75 0.8

Width of inlet, W/D 0.2 0.21 0.25 0.25 0.375 0.35

Diameter of Gas Exit,

De/D

0.5 0.4 0.5 0.5 0.75 0.75

Length of Vortex , S/D 0.5 0.5 0.625 0.6 0.875 0.85

Length of Body, Lb/D 1.5 1.4 2.0 1.75 1.5 1.7

Length of Cone, Lc/D 2.5 2.5 2.0 2.0 2.5 2.0

Diameter of Dust,

Dd/D

0.375 0.4 0.25 0.4 0.375 0.4

Lapple standard Conventional Cyclone

9

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Relationship of collection Efficiency

versus Particle for Cyclones

High throughput

Conventional

High efficiency

d p (μm)10 20

Efficiency,

ɳ (%) 

100

50

10

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Collection Efficiency: Effective Turns

conecycloneof (vertical)length L 

bodycycloneof length L 

duck inlet of height  H  

turnseffectiveof number N  

where2

 L L

 H 

1 N  

c

b

e

cbe

 De

 Do

 Dd

 L1

 L2

 Sc

 H 

Lb

Lc

(4.1)

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Terminal Velocity

12

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Smallest particle that will be

collected

density gas 

 particletheof density

velocityinlet  gasV  

turnseffectiveof number  N  

duck inlet theof widthW  

viscosity gas 

where

V  N 

W 9d  

 g 

 p

i

e

21

 g  pie

 p

  

  

     

In theory, the size of the smallest particle that willbe collected:

(4.5)

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Particle collected with 50%

efficiency 

density 

 particletheof density

cityinlet velo 

turnseffectiveof  ductinlettheof widthW

 viscosity 

 N2

W9 

21

e

 gas

 gasV 

number  N 

 gas

where

V d 

 g 

 p

i

e

 g  pi pc

  

  

     

In practice, the diameter of particle collected with50% efficiency (semi-emperical):

(4.6)

14

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Control Efficiency jth Particle

range)size particleof diametermediantheis(usually

rangesize particle jtheof diametersticcharacteri 

rangesize particle jfor theefficiencycollectionη 

1

 __ 

th __ 

th

 j

2

 __ 

 j

 pj

 pj

 pj

 pc

where

 

 

 

 

 

(4.7)

15

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Standard Cyclone Efficiency

16

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Overall Control Efficiency

range size jthein particleof  fractionmassm 

efficiencycollectionoverall η 

where

th

 j

o

 j jo

    (4.8)

17

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Effects of Design and Process

Parameters on Cyclone EfficiencyParameter  If parameter increases, cyclone

efficiency will: 

Particle size  Increase 

Particle density  Increase 

Dust loading  Increase 

Inlet gas velocity  Increase 

Cyclone body diameter   Decrease 

Ratio of cyclone length to diameter   Increase 

Smoothness of cyclone inner wall  Increase 

Gas viscosity  Decrease 

Gas density  Decrease 

Gas inlet duct area  Decrease 

Gas exit pipe diameter   Decrease  18

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Example-Cyclone Particle Collection

Efficiency• For the following particle size distribution, calculate the

particle collection efficiency of a Lapple standard cyclone

with a body diameter of 0.50 meters. The particulate

density ρp = 1200 kg/m3, the gas density ρg = 0.90 g/m3,the gas viscosity μ = 1.67x10-5 kg/m‐s, and the inlet gas

velocity V i  = 25 m/s. 

19

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Example-Cyclone Particle Collection

Efficiency –

 2/3o j pce d  N        Strategy:

2

 L L H 1 N  c

be

20

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Example-Cyclone Particle Collection

Efficiency –

 3/32

 __ 

 pj

 pc

 j

d 1

1

 

 

 

 

 

 j jo m    

 pj

 __ 

d    pj

 __ 

 pc   d d 

2.04

0.58

0.27

0.14

0.07

0.05

21

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• Pressure drop (DP) across air quality control devices is also important

 because operating cost of the device can depend heavily on pressure drop.• An empirical expression describing pressure drop for cyclones is

presented below:

2

e

2

i g 

 D

 HW 

2

 K  P       

D

Pressure Drop for Cyclones

= pressure drop [N/m2

]V i   = inlet gas velocity [m/sec]

= gas density [Kg/m3]

where,

 P D

 g   

22

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K = empirical constant (range from 12 to 18)

H = height of inlet [m]

W = width of inlet [m]

g = gravitational force constant [9.8 m/sec2]

De = diameter of cyclone’s outlet for gas stream [m] 

Pressure Drop for Cyclones – 2/2

2

e

2

i g 

 D

 HW 

2

V  K  P       

D

23

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Fluid Power for Cyclones

 s / mrate, flowvolumetricQ

W  fluid,theintorateinput work w

where

 P Qw

3

 f 

 f 

D

24

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Costs• EPA estimates:

 – Capital costs: $ 2.20/scfm to $3.50/scfm

 – Operating and maintenance costs: $0.7/scfm to $8.5/scfm per

year

• Total purchased cost (1988 dollars) of a cyclone system

= Pc + Pv

Where

Pc is the cost of the cyclone system, Pc=6520 A0.903

Pv is the cost of the rotary air lock valve, Pv=273 A0.0965

A is the cyclone gas inlet area, ft2

25

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Additional Information

(not required for tests)

26

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Multiple-tube Cyclones

27

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Vane-axial cyclone

28

vane

outlet forparticles

outlet for

clean gasstream

inlet for particleladen gas

stream

vortex finder

inlet for particleladen gas

stream

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Multiple-tube Cyclones

MultiCyclones

FCC Cyclones

29

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Standard (Involute) Cyclone

dimensions and standard proportions

Ro R*

Ri 

W30

R o  = Cyclone body radius (= Do/2)

R i  = radius to the inner end of gas inlet

W = R o - R i = width of the cyclone's inlet

R* = minimum radius for which a particleof diameter, d p, will just reach the

outer wall of the cyclone and be

removed from the gas stream

De

 Do

 Dd

 L1

 L2

 Sc

 H  L1 = 2Do

  L2 = 2Do

  H = Do/2

  Sc = Do/9

  De= Do/2

  Dd= Do/4

  W = Do/4 (width of 

  cyclone inlet)

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• R o  –  R* represent the particles of diameter dp that will be

collected; and R o  –  R i represent the total particles of size dpentering the device inlet. Then the graded collection efficiency

of an involute cyclone, (d p), can then be described by:

W*RR

RR*RR)d( o

io

op

• A force balance can be used to describe the normal velocity

vector of a particle that is located in a gas stream that is turningwith radius R.

Involute Cyclone: Particle Removal Efficiency

31

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where,

p

2p, tang

ac p

p, tang

a acceleration of particle

VF centrifugal force m

RV

R

tangential velocity vector of particle

radius of curvature

pp ac d

d(V )m F F

d(t)

Particle Force Balance in a Cyclone

32

R

Vp, tang 

Vp, rad 

particle

 

 

p p im a F

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Drag Force, F d , in a Cyclone

33

2

,

2

242

,

2

2

2

,

,

3

4

1

2

1

Reregime),(StokesRe

24 ,

4

1 :ngSubstituti

2

1

 :velocityradial particle

 withforcedrag4Lecture44slidefromrecall

,

,

rad  p g  pd 

rad  p p g d 

d V 

 D p p

 Drad  p p g d 

rad  p

V d  F 

V d  F 

C d  A

C V  A F 

 F 

 g 

 g  pd rad  pV 

 g 

 g  prad  p

m  

   

 

  

  

  

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ac d

2p, tang

p g p p, rad

2p p, tang

p, rad

g p

F F

V

m 3 d VR

m VV

3 d R

m

m

Particle Force Balance in a Cyclone

34

Principle: A particle will deviate from the vortex stream flow when

the centrifugal force equals the drag force, then travels at theresulting terminal velocity in radial direction, and is removed when

it covers the distance to the outer diameter of the cyclone.

Assumptions: transition to terminal velocity is ignored and

 particle mass remains constant, thus,

and 0)t(d

)V(dm p

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R18

Vd 

Rd3

Vd6

1

V

d6

1m

g

2gtan,pp

2p

pg

2gtan,pp

3p

rad,p

p3pp

m

m

• Assuming spherical particles with density ( p)

Particle Force Balance in a Cyclone

35

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• It is also assumed that the tangential velocity of the

 particles is the same as that of the gases, or:

WHQuV g

ggtan,p  

• Radius (R) has some value between the inner radius R i

(R i = R o - W) and the outer radius R o of the cyclone.• The magnitude of is then described by,

radp,V

*o

p,rad

R R

V t

D

  where Dt is some time period for the particle to be

transported from R* to R o

.36

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• Therefore,

2 2*p p go

p,rad

g

2 2p p g*

o g

d uR RV

t 18 R

d u tR R

18 R

D m

D

m

Particle Force Balance in a Cyclone

37

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• It is now necessary to determine Dt.

g

e

u

)R)(N(2 

velocity)gasal(superfici

turns)of number (R radiushvortex wit

theof ncecircumfere

 

velocity)gasal(superfici

directiontangentialtheinstreamgastheof ntdisplacemesome

t

 

  

 

  

  

D

38

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where,

 Ne   number of equivalent turns of

the gas stream in the cyclone

  

  

2LL

H1  2

1

H = height of inlet to involute cyclone

L1 = height of cylindrical portion of cyclone

L2 = height of conical portion of cyclone

De

 Do

 Dd

 L1

 L2

 Sc

 H 

39

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• Therefore,

g

egp2p

gg

e2gp

2p*

o

9

Nud 

uR18

)N)(R2(udRR

m

m

and,

W9

Nud

W

RR)d(

g

egp

2

p

*

op

m

Particle Force Balance in a Cyclone

40

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• But,

WHQu g

• Then,

HW9NQd)d(

2g

egp2pp

m

Involute Cyclone: Particle Removal Efficiency

• This expression for (d p) exhibits some problems

 because (d p) can be calculated to be 100% for all

 particles > d p and the other assumptions in its derivation.

41

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• Therefore, define d p,50 as the particle diameter that is

collected at (d p) = 0.5.

HW9NQd5.0)d(

2g

egp

2

50,pp

m

Involute Cyclone: Particle Removal Efficiency

2/1

egp

2g

50,pNQ2

HW9d

 

 

 

 

m

42

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• This expression for d p,50 is similar as the equation 4.6

shown on page 141 of the textbook.

• Now the problem is how to apply the equations to

calculate collection efficiency for the cyclone.

• A calibration curve can be used for cyclones of

standardized proportion (see Figure 4.3 in the text).

43

Calibration Curve for Cyclones of Standardized

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Calibration Curve for Cyclones of Standardized

Proportions

B

         (   d

  p   )

 C

 A

 dp / dp,50

A = High throughput

B= Conventional

C = High efficiency

44

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• d p,50 can be readily calculated given the geometry of

the cyclone, density of the particles, viscosity of the

gas and volume flow rate of the gas stream.

• d p/d p,50 values can then be determined for the particlesize distribution of interest.

• Values of (d p) can then be read from the calibration

curve for calculated values of d p/d p,50.

Involute Cyclone: Particle Removal Efficiency

45

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• The overall collection efficiency (T) can then be

calculated:

)()(

1 ,i

i

 pi inT 

 pi

T d 

m

d m  

   

 

 

 

Involute Cyclone: Overall Removal Efficiency

46

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Typical Values for Involute Cyclones

m/sec2010u

m3to5.0D

m101dfor %50)d(

g

o

pp

m

 De

 Do

 Dd

 L1

 L2

 Sc

 H 

47

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Example (Cyclone)

An involute cyclone of standard proportions with a 2 m

diameter is operated at a gas flow rate of 10 m3/sec.

The gas stream consists of air at 500 K and 1 atm.

What is the collection efficiency for a 10 m m diameter particle? 

Use the calibration curve for the cyclone is presented

in Figure 4.5 (p.142) of textbook . 

48

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• An empirical expression describing pressure drop

(DP) for cyclones is presented below:

)PP(D

HW

Kg2

u

P 1T2T2e

g

2

g

L

D

Pressure Drop (DP) for Cyclones

DP = pressure drop [N/m

2

]ūg = inlet superficial gas velocity [m/sec]

g = gas density [Kg/m3]

where,

49

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K = empirical constant

= 16 for standard tangential inlet

= 7.5 for vane axial entry

H = height of cyclone’s inlet [m] W = width of cyclone’s inlet [m] 

g = gravitational force constant [9.8 m/sec2]

L = density of liquid water [1,000 Kg/m3]

De = diameter of cyclone’s outlet for gas stream

[m]

Pressure Drop (DP) for Cyclones

50