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Chapter 3. Cyclones
Ph.D25th September 2012
An Introduction to Air
Pollution
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Cyclones
The particulate-laden gas stream is forced to spin
in a cyclonic manner.
The mass of the particles causes them to move
toward the outside of the vortex.
Most of the large-diameter particles enter a hopper
below the cyclonic tubes while the gas streamturns and exits the tube.
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There are two main types of mechanical collectors: (1)
large-diameter cyclones, and (2) small-diameter multi-
cyclones.
Large-diameter cyclones are usually one to six feet indiameter; while small-diameter multi-cyclones usually
have diameters between 3 and 12 inches.
A typical large-diameter cyclone system is shown inFigure 1.
The gas stream enters the cyclone tangentially and creates
a weak vortex of spinning gas in the cyclone body.
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Large-diameter particles move toward the cyclone
body wall and then settle into the hopper of the
cyclone.
The cleaned gas turns and exits the cyclone.
Large-diameter cyclones are used to collect
particles down to 1/16 inch (1.5 mm) diameterand above.
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Standard Cyclone
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In systems where the large-diameter cyclone is
located after the fan (positive pressure), the treatedgas is usually discharged directly from the cyclone.
In systems where the cyclone is located before the fan
(negative pressure), the gas stream is either exhaustedfrom a separate stack or from the discharge of the fan
itself.
In negative pressure systems, a solids discharge valve
is used to prevent air infiltration up through the
hopper area.
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A small-diameter cyclone tube is shown in Figure 2.
Vanes located on the inlet of each of the tubes create
the spinning movement of the gas stream.
Most of the commercial tubes are six, nine, or twelve
inches in diameter.
Due to the limited gas handling capacity of each tube,
large numbers of tubes are mounted in parallel in a
single collector.
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Small-diameter multi-cyclones, such as the one shownin Figure 2 are capable of removing particles having
diameters down to 5 micrometers.
Small-diameter multi-cyclones are not generally used
for very large diameter material, such as 3 mm and
above, because large particles may plug the spinner
vanes in the multi-cyclone tubes.
Some mechanical collectors are specially designed toprovide high-efficiency PM collection down to a
particle size of one micrometer.
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The small-diameter of the cyclone tube creates morerapid spinning of the gas stream than in large-diameter
cyclones.
The particles moving outward in the spinning gasstream have a relatively shorter distance to travel in a
small-diameter multi-cyclone tube before they reach
the cyclone body wall.
These features allow small-diameter multi-cyclones to
collect considerably smaller particles than large-
diameter cyclones can.
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Mechanical collectors are used whenever the particle size
relatively large (> 5 micrometers) and/or the control
efficiency requirements are in the low-to-moderate range
of 50 to 90%.
They are also used as the pre-collector of large-diameter
embers generated in some combustion systems.
Removal of the embers is necessary to protect high-efficiency particulate control systems downstream from
the mechanical collectors.
Most mechanical collectors are not applicable to industrialsources that generate sticky and/or wet particulate matter.
These materials can accumulate on the cyclone body wall
or the inlet spinner vanes of conventional multi-cyclone
collectors.
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Advantages and Disadvantages of cyclones
Advantages of cyclones are:
1. Low capital cost
2. Ability to operate at high temperatures
3. Low maintenance requirements because there are
no moving parts.
Disadvantages of cyclones are:
1. Low efficiencies (especially for very smallparticles)
2. High operating costs (owing to power required to
overcome pressure drop).
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Generalized efficiency curves for
three types of cyclones
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Collection Efficiency
(4.1)
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Collection Efficiency
(4.6)
R
VdV
igpp
t
18
)( 22
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Collection Efficiency
(4.7)
(4.8)
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Design Considerations
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Design Considerations
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Applications
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Applications
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Problem
A cyclone with a flow rate of 150 m3/min has an efficiency of
80%. Estimate the efficiency if the flow rate is doubled.
Step 1
Q1 = 150 m
3
/minQ2 = 300 m3/min
Pt1 = 100% - 80% = 20%
Pt2/Pt1 = (Q1/Q2)0.5
Step 2Final Efficiency = 1- Pt2
= 86%
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Pressure Drop
(4.12)
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Pressure Drop and Power
(4.13)
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Power Requirement
(4.14)
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Detailed Design Process
Given - particle size distribution and densities and gas
flow rate, temp, pressure, viscosity
Specify - desired removal efficiency and pressure
drop Select desired design geometry
Select body diameter
Calculate other dimensions from geometryuseTable 4.1 p.127
From inlet area, calculate inlet velocity, Vi = Q/(WH)
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Detailed Designpart II
Calculate number of turns, Ne from eqn 4.1 From inlet
velocity, 50% particle removal diametereqn 4.6
Using particle diameters - calculate particle collection
efficiency for each sizeeqn 4.7
Using mass fraction mj in size class dj, calculate mass
removedright side eqn 4.8
Calculate overall removal efficiency by summingeqn 4.8 If set terms in spreadsheet, can rapidly calculate overall
removal efficiency from several body diameter diameters and
plot a curve!
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Designpart III: Pressure Drop
Calculate number of velocity headseqn 4.12
Calculate pressure dropeqn 4.13
Calculate power requirement, eqn 4.14
See example 4.4 for pressure drop using
English units (k = 0.0001575)
If have in spreadsheetcan calculate pressure
drop vs body diameter
PkQwf
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Cyclone inner vortex core dimensions
Assumed that Vt equals the average air
stream inlet velocity Vin when r equals the
radius of the cyclone wall (R)
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Stokes Equation
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Stokes Law
Newtons drag (Re > 1000) applies to particle motion for
which the viscous effects of the gas can be neglected
compared with the inertial effects.
In 1851, George Gabriel Stokes (1819-1903) derived an
expression for the drag force on spherical objects with
very small Reynold's numbers (e.g., very small particles)
by solving the generally unsolvable Navier-Stokesequations. Stokes' law for drag force is expressed as:
pD VdF 3
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Stokes Law
Rep < 1
The net force acting on the particle is obtained by
integrating the normal and tangential forces over the
surface of the particle.
Normal Force
Frictional Force
Total resisting force
pn VdF
pVdF 2
pD VdF 3
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Stokes Law
By equating Newton's resistance law and Stokes'
law, the drag coefficient can be solved for
(applicable when Rep
< 1):
22
83 VdCVdF pgDpD
ppg
DVd
CRe
2424
Th ti l A l i f C l
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Theoretical Analysis of Cyclone
Collection EfficiencyThe particle motion in the cyclone outer vortex can be modeled by Newtons law
For flows in which Stokes law applies, the drag force on a spherical particle may be
determined by the Stokes law and the centrifugal force is determined
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In the cyclone outer vortex fluid field, there are only two forces (centrifugal
force Fc & drag force FD) acting on the particle in the radial direction. When Fc> FD, the particle moves towards the cyclone wall to be collected. Whereas,
when Fc < FD, the particle will move to the inner vortex and then to penetrate
the cyclone. The force balance (Fc = FD) gives the particle a 50% chance to
penetrate and a 50% chance to be collected. The force balance differential
equation can be setup by letting equation 4 equal to equation 5, i.e. Fc = - FD, it
yields equation 6.
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