Chapter 4. Cyclones

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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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(4.6)

R

VdV

igpp

t

18

)( 22


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(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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Chapter 4. Cyclones