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Air Pollution Control – Part A

Cyclone – Basic Principles

Yaacov MamaneVisiting Scientist, CNR

Rome, Italy

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What is a CycloneWhat is a Cyclone?

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Cyclone Performance for various Application

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Standard Standard Cyclone Cyclone Dimensions Dimensions

General guidelines: H < S W < (D-De)/2 Lb+Lc > 3D Cone angle = 7o ~ 8o De/D = 0.4~0.5, (Lb+Lc)/De = 8, S/De = 1

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Conventional Cyclone

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Stokes Law Gravitational forces are balanced by

drag and buoyancy forces. This will lead to stokes law - settling

velocity of particles.

v

m p

d 3 = F

gd 6

= g

d

3p

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18 d g = v p2

t

WhereWhereg – gravity accelerationg – gravity accelerationd – particle diameterd – particle diameterrrpp – particle density – particle density

m - air viscositym - air viscosity

For example VFor example Vtt (1 (1 m) = 0.006 cm/sm) = 0.006 cm/s

When Fd = Fg the When Fd = Fg the settling velocity is settling velocity is given: as follows. given: as follows.

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r

v= w c

For a particle moving at high speed Vc in a circle, the centrifugal

acceleration is given by Vc*Vc/r. The centrifugal force is similar to the

gravity forces

mg = F , r

v m = F g

2c

c

But Fc >> Fg

The equivalent “settling velocity” of the

centrifugal forces is taken from Stokes

Law and is given by the following

Equation:

r

v g

2c

18 d

r

v v p2

2c

t

Vc

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18

kg/m 2000 m 10

m 3.0m/s 18.3

= v326-

2

t

= 1.8 10 kg

m s-5

vt = 0.68 cm/s

While Vt stokes is only 0.006 cm/s!!!

ExampleFor a particle of 1 m moving in a 0.3 diameter circle at 18.3 m/s:

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Settling ChamberSettling Chamber

Particle is entering a chamber at height h an horizontal speed V and settling velocity Vt, may fall inside the chamber. Time (Tl) to cross the chamber is L / V . Time (Th) to fall inside the chamber is h / Vt, thus we could define a collection efficiency = Tl / Th = L Vt / H V

H

L

W

hVt

v

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= L Vt / H V = L Vt / H V oror

= = L g dL g d2 2 pp / H V 18 / H V 18

ButBut V = Q / WH where V = Q / WH where QQ is the flow through the is the flow through the settling chamber, and thussettling chamber, and thus

= L g d= L g d22 pp / H V 18 / H V 18 = = L W g dL W g d22 pp / Q 18 / Q 18

= = L W g d L W g d22 pp / Q 18 / Q 18

This collection efficiency may be applied to a Cyclone This collection efficiency may be applied to a Cyclone wherewhereH and L of the chamberH and L of the chamber are equivalent to are equivalent to W and NW and NDD00 of a cyclone of a cyclone

The Collection Efficiency of a settling chamber used to collect large particles is given by the simple expression:

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18 v W

d g DN = p

2o

c

c

toc v

v

W

DN =

4W = Do = 2r

r 18

d v = v p

22c

t

Since a cyclone is an elongated settling chamber

2r 9 v W

d v2r N =

c

p22

c

= L V= L Vtt / H V for a cyclone may be written as: / H V for a cyclone may be written as:

For centrifugal forces

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W9

d v N = p

2c

lam

turb = 1 - exp - lam

Example:Example:

Calculate efficiency for a cyclone to collect 1 mm particles of density 1. Calculate efficiency for a cyclone to collect 1 mm particles of density 1.

Cyclone width – 15 cm, Vc – 18.3 m/s and N – 5.Cyclone width – 15 cm, Vc – 18.3 m/s and N – 5.

The efficiency is h = 0.023The efficiency is h = 0.023

And for a particle of 10 mm diameter h is larger than 1.And for a particle of 10 mm diameter h is larger than 1.

After rearranging the parameters the equation is now given by:

And for a turbulent flow it is then expressed by the general term:

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pccut Nv 2

W 9 = d

Where

Dcut - cut diameter in mm

- is viscosity

N - number of rotations

Vc - tangential velocity

p - particle density

W - entrance width of the cyclone

Let define a parameter of importance in particulate control, d cut , used to

describe the properties of the cyclone,

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Pressure Drop

Number of gas inlet velocity head

2e

v D

HWKH

Static pressure drop

vig HVP 2

2

1

Power requirement

PQw f K = 16 for normal tangential inlet = 7.5 for one with an inlet vane

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Number of effective turns

2

1 cbe

LL

HN

Gas residence time

ie VDNt /

Terminal velocity

D

VdtWV igpp

t

9

/

22

Smallest collected diameter

gpiep VN

Wd

9

Lapple Theory (laminar flow)

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50% cut size

gpiepc VN

Wd

2

9

The collection efficiency of any size dpj

2/1

1

pjpc

jdd

Overall efficiency

jj f

Penetration

1P Particle size ratio dp/dpc

(%)

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Licht Theory (turbulent flow)

1

1

exp1 npAd

1693.0

n

pc Ad

)1(2

1

318

)1(2

np

D

nKQA

1

1

693.0exp1n

pc

pjj d

d

3.0

14.0

28367.011

TDn (D in )

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Arrangement

Series

Parallel/Battery

Air Pollution Control Equipment, Theodore & Buonicore, CRC Press, 1988

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Discharge

Handbook of Air Pollution Control Engineering & Technology,Mycock, McKenna and Theodore, Lewis Publishers, 1995.

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Advantages & Disadvantages

Advantages: Low capital cost Ability to operate at high

temperatures Low maintenance

requirements because there are no moving parts

Disadvantages: Low efficiencies for fine

particles High operating costs (due to

pressure drop) Cyclones used for removing wood dust