lec. 5 cyclones
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Lecture 5 Cyclones
EVEN 4386 Air Quality and Pollution
Control
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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
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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.
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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
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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)
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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
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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
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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
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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
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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
m
m
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
m
m
In practice, the diameter of particle collected with50% efficiency (semi-emperical):
(4.6)
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Control Efficiency jth Particle
range)size particleof diametermediantheis(usually
rangesize particle jtheof diametersticcharacteri
rangesize particle jfor theefficiencycollectionη
1
1
__
th __
th
j
2
__
j
pj
pj
pj
pc
d
d
where
d
d
(4.7)
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Standard Cyclone Efficiency
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Overall Control Efficiency
range size jthein particleof fractionmassm
efficiencycollectionoverall η
where
m
th
j
o
j jo
(4.8)
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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.
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Example-Cyclone Particle Collection
Efficiency –
2/3o j pce d N Strategy:
2
L L H 1 N c
be
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Example-Cyclone Particle Collection
Efficiency –
3/32
__
pj
pc
j
d
d 1
1
j jo m
pj
__
d pj
__
pc d d
2.04
0.58
0.27
0.14
0.07
0.05
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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
V
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
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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
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Fluid Power for Cyclones
s / mrate, flowvolumetricQ
W fluid,theintorateinput work w
where
P Qw
3
f
f
D
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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
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Additional Information
(not required for tests)
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Multiple-tube Cyclones
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Vane-axial cyclone
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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
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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
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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
d
V d F
V d F
C d A
C V A F
V
F
g
g pd rad pV
g
g prad p
m
m
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
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
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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
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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
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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
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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
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• But,
WHQu g
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.
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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
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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
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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
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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
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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
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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 .
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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,
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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
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