(The Journal of Cotton Science 17:4051 (2013) 40; http://journal.cotton.org, The Cotton Foundation

2013; Engineering and Ginning : Dust Cyclone Technology A Literature Review; Paul A. Funk*

and Kevin D. Baker)

Analyses of the particle size distribution of cyclone exhausts in 1968-1970 showed the

Atomic Energy Commission or 2D2D cyclone to be virtually 100% efficient on particles larger

than 20 microns (Wesley et al., 1972). A 2D2D cyclone has barrel and cone heights each equal

to two barrel diameters, as shown in the Shepherd and Lapple model (1939) in Figure 1.

Figure 1. REPRESENTATIVE DUST CYCLONE DESIGN

Two of the parameters that describe cyclone performance are pressure drop, a measure of

the operating energy cost, and collection efficiency, the operational benefit. Collection efficiency

traditionally has been reported as total mass efficiency the collected mass proportional to what

was conveyed by the incoming gas flow. As collection efficiency is strongly dependent on

particle size, efficiency has also been reported in terms of cut point that particle size for which

the collection (or grade) efficiency is 50%. For particles significantly larger than this, collection

approaches 100%. The cutpoint for many industrial cyclones is typically less than five microns,

normalized for particle density and shape (Koch and Licht, 1977).

Figure 2. Dust cyclone components and vocabulary with top (plan) view above side (elevation) view; original 1D3D

cyclone (left) and fully enhanced 1D3D (right) with shorter and wider inlet, shorter vortex finder, expansion chamber,

and wider trash exit.

However, small details can have just as large an influence on performance. For example,

increasing the diameter of the cone bottom outlet has been shown to decrease pressure drop

(Xiang et al., 2001) and increase mass efficiency (Baker and Hughs, 1999). Other changes to the

dust outlet can have significant impact on collection efficiency, such as including expansion

chambers (Baker et al., 1997; Holt et al., 1999; Obermair and Staudinger, 2001).

Model Limitations. Cyclone performance models have been helpful for designing typical dust

collection systems, and for predicting trends in new design directions. But there will always be a

need for lab and field testing to confirm performance. Models have two major limitations. First,

they can only explain modeled variables. Dust cyclones are complex and their performance is

influenced in statistically significant ways by a large number of parameters, the majority being

omitted for simplicity or overlooked out of ignorance. Case in point: it has long been recognized

that particulate loading attenuates swirl and turbulence, reducing pressure loss and increasing

efficiency at levels up to about 500 g.m-3

, then having the opposite impact at higher levels

(Corts and Gil, 2007), but semi-empirical (and most CFD) models cannot account for this

phenomenon. Other examples of parameters known to influence performance that are not

included in most models include things like dust outlet geometry; (Holt at al., 1999; Obermair

and Staudinger, 2001); or vortex finder shape (Baker and Hughs, 1998; Lim, et al., 2004; Ogawa

and Arakawa, 2006), even though they impact pressure drop and collection efficiency. In

addition, models are only accurate over a limited dimensional range. Most semi-empirical

models predict increasing collection efficiencies with increases in inlet velocity but do not

usually include a term to account for surface imperfections known to have a deleterious effect; at

higher velocities particles near the wall bounce and are re-entrained (Armijo et al., 1992; Baker

and Stedronsky, 1967a; Koch and Licht, 1977).

( Theoretical Study of Cyclone Design. (May 2004); Lingjuan Wang,B. Eng., Anhui Institute

of Finance and Trade, China; M.S., Texas A&M University)

The flow pattern and cyclone dimensions determine the travel distance in a cyclone. The

number of turns was calculated based on this travel distance. The experiment results show that

cyclone pressure drop varies with the inlet velocity, but not with cyclone diameter.

Cyclone cut-points for different dusts were traced from measured cyclone overall

collection efficiencies and the theoretical model for calculating cyclone overall efficiency. The

cut-point correction models (K) for 1D3D and 2D2D cyclones were developed through

regression fit from traced and theoretical cut-points. The regression results indicate that cut-

points are more sensitive to mass median diameter (MMD) than to geometric standard deviation

(GSD) of PSD (Particle Size Distribution). The theoretical overall efficiency model developed in

this research can be used for cyclone total efficiency calculation with the corrected d50

and PSD.

2D2D (Shepherd and Lapple, 1939) and 1D3D (Parnell and Davis, 1979) cyclone designs

are the most commonly used abatement devices for particulate matter control. The Ds in the

2D2D designation refer to the barrel diameter of the cyclone. The numbers preceding the Ds

relate to the length of the barrel and cone sections, respectively. A 2D2D cyclone has barrel and

cone lengths of two times the barrel diameter, whereas the 1D3D cyclone has a barrel length

equal to the barrel diameter and a cone length of three times the barrel diameter. The

configurations of these two cyclone designs are shown in figure 2. Previous research (Wang,

2000) indicated that, compared to other cyclone designs, 1D3D and 2D2D are the most efficient

cyclone collectors for fine dust (particle diameters less than 100 m).

Mihalski et al (1993) reported cycling lint near the trash exit for the 1D3D and 2D2D

cyclone designs when the PM in the inlet air stream contained lint fiber. Mihalski reported a

significant increase in the exit PM concentration for these high efficiency cyclone designs and

attributed this to small balls of lint fiber cycling near the trash exit causing the fine PM that

would normally be collected to be diverted to the clean air exit stream.

Simpson and Parnell (1995) introduced a new low-pressure cyclone, called the 1D2D

cyclone, for the cotton ginning industry to solve the cycling-lint problem. The 1D2D cyclone is a

better design for high-lint content trash compared with 1D3D and 2D2D cyclones (Wang et al.,

1999). Figure 3 illustrates the configuration of 1D2D cyclone design.

Figure 2. 1D3D, 2D2D, and 1D2D cyclone configuration

CLASSICAL CYCLONE DESIGN (CCD)

The cyclone design procedure outlined in Cooper and Alley (1994), hereafter referred to as the

classical cyclone design (CCD) process, was developed by Lapple in the early 1950s. The CCD

process (the Lapple model) is perceived as a standard method and has been considered by some

engineers to be acceptable. However, there are several problems associated with this design

procedure. First of all, the CCD process does not consider the cyclone inlet velocity in developing

cyclone dimensions. It was reported (Parnell, 1996) th at there is an ideal inlet velocity for the

different cyclone designs for optimum cyclone performance. Secondly, the CCD does not predict the

correct number of turns for different type cyclones. The overall efficiency predicted by the CCD

process is incorrect because of the inaccurate fractional efficiency curve generated by the CCD

process (Kaspar et al. 1993).

In order to use the CCD process, it is assumed that the design engineer will have knowledge

of (1) flow conditions, (2) particulate matter (PM) concentrations and particle size distribution (PSD)

and (3) the type of cyclone to be designed (high efficiency, conventional, or high throughput). The

PSD must be in the form of mass fraction versus aerodynamic equivalent diameter of the PM. The

cyclone type will provide all principle dimensions as a function of the cyclone barrel diameter (D).

With these given data, the CCD process is as follows:

The Number of Effective Turns (Ne)

The first step of CCD process is to calculate the number of effective turns. The number of

effective turns in a cyclone is the number of revolutions the gas spins while passing through the

cyclone outer vortex. A higher number of turns of the air stream result in a higher collection

efficiency. The Lapple model for Ne calculation is as follows:

...(1)

Based on equation 1, the predicted numbers of turns for 4 cyclone designs were calculated and

listed in the table 1. In table 1, 1D2D, 2D2D, and 1D3D cyclones are the cyclone designs shown

in figures 2 and 3. These three cyclone designs have the same inlet dimensions (Hc and Bc),

referred to as the 2D2D inlet. The 1D3Dt cyclone is a traditional 1D3D cyclone design, which

has the same design dimensions as 1D3D cyclone in figure 2 except the inlet dimensions. The

1D3Dt cyclone has an inlet height equal to the barrel diameter (Hc = Dc) and an inlet width of

one eighth of the barrel diameter (Bc = Dc/8). Table 1 gives the comparison of the predicted Ne

vs. the observed Ne. It has been observed that the Lapple model for Ne produces an excellent

estimation of the number of turns for the 2D2D cyclone designs. However, this model (equation

1) fails to give an accurate estimetion of Ne for the cyclone design uther than 2D2D design. This

observation indicates a limitation for the Lapple model to accurately predict the number of

effective turns. The Ne model is valid only for 2D2D cyclone designs, which was originally

developed by Shepherd and Lapple (1939)

Cut-Point (d50

)

The second step of the CCD process is the calculation of the cut-point diameter. The cut-point of

a cyclone is the Aerodynamic Equivalent Diameter (AED) of the particle collected with 50%

efficiency. As the cut-point diameter increases, the collection efficiency decreases. The Lapple

cut-point model was developed based upon force balance theory. The Lapple model for cut-point

(d50

) is as follows:

... 2

In the process to develop this cut-point model, it was assumed that the particle terminal velocity

was achieved when the opposing drag force equaled the centrifugal force, and the drag force on

every single particle was determined by Stokes law. As a result, the cut-point (dpc

, or d50

)

determined by the Lapple model (equation 2) is an equivalent spherical diameter (ESD), or in

other words, it is a Stokes diameter. The following equation can be used to convert ESD to AED

for the spherical particles:

(Air Pollution88, Ch7)

7.1 COLLECTION EFFICIENCY

We define the collection efficiency (Dp ) of a device for particles of diameter Dp as

The overall efficiency of the device based on particle number is

These efficiencies can be expressed in terms of the particle size distribution functions at the inlet

and outlet sides of the device,

and

The definition of overall efficiency above is based on particle number. We can also define

overall efficiencies based on other particle properties, such as surface area and volume (or mass).

For example, the collection efficiency based on particle mass m is defined as

And the overall efficiency is,

The overall collection efficiency by mass is usually the easiest to measure experimentally.

The inlet and outlet streams may be sampled by a collection device, such as a filter, that collects

virtually all of the particles. A term that is sometimes used to express collection efficiency is the

penetration. The penetration is based on the amount emitted rather than captured; penetration

based on particle mass is just Pm = 1 - m Alternatively, the penetration can be defined on the

basis of particle number, P = 1 - .

We have called the relationship between collection efficiency and particle size simply the

collection efficiency. Other terms that are used for this quantity are the grade efficiency or the

fractional efficiency. An important point on the collection efficiency curve is the size for which

= 0.5. The particle size at this point is called the ellt size or the Cut diameter.

There are a variety of designs of cyclone separators, differing in the manner in which the

rotating motion is imparted to the gas stream. Conventional cyclones can be placed in the

following categories:

1. Reverse-flow cyclones (tangential inlet and axial inlet)

The dirty gas enters at the top of the cyclone and is given a spinning motion because of its

tangential entry. Particles are forced to the wall by centrifugal force and then fall down the wall

due to gravity. At the bottom of the cyclone the gas flow reverses to form an inner core that

leaves at the top of the unit. In a reverse-flow axial-inlet cyclone, the inlet gas is introduced

down the axis of the cyclone, with centrifugal motion being imparted by permanent vanes at the

top.

2. Straight-through-flow cyclones

In straight-through-flow cyclones the inner vortex of air leaves at the bottom (rather than

reversing direction), with initial centrifugal motion being imparted by vanes at the top. This type

is used frequently as a precleaner to remove fly ash and large particles. The chief advantages of

this unit are low pressure drop and high volumetric flow rates.

3. Impeller collectors

In straight-through-flow cyclones the inner vortex of air leaves at the bottom (rather than

reversing direction), with initial centrifugal motion being imparted by vanes at the top. This type

is used frequently as a precleaner to remove fly ash and large particles. The chief advantages of

this unit are low pressure drop and high volumetric flow rates. In the impeller collector, gases

enter normal to a many-bladed impeller and are swept out by the impeller around its

circumference while the particles are thrown into an annular slot around the periphery of the

device. The principal advantage of this unit is its compactness; its chief disadvantage is a

tendency toward plugging from solid buildup in the unit.

7.3.3 Cyclone Dimensions

Cyclone collection efficiency increases with increasing (1) particle size, (2) particle

density, (3) inlet gas velocity, (4) cyclone body length, (5) number of gas revolutions, and (6)

smoothness of the cyclone wall. On the other hand, cyclone efficiency decreases with increasing

(1) cyclone diameter, (2) gas outlet duct diameter, and (3) gas inlet area. For any specific cyclone

whose ratio of dimensions is fixed, the collection efficiency increases as the cyclone diameter is

decreased. The design of a cyclone separator represents a compromise among collection

efficiency, pressure drop, and size. Higher efficiencies require higher pressure drops (i.e., inlet

gas velocities) and larger sizes (i.e., body length). The dimensions required to specify a

tangential-entry, reverse-flow cyclone are shown in Figure 7.8. In classic work that still serves as

the basis for cyclone design, Shepherd and Lapple determined "optimum" dimensions for

cyclones. All dimensions were related to the body diameter Dc. A common set of specifications

is given on the right-hand side of Figure 7.8. Other standard cyclone dimensions are given by

Licht (1984) and Cooper and Alley (1986). The number of revolutions that the gas makes in the

outer vortex can be approximated by

where the dimensions are shown in Figure 7.8. Besides collection efficiency the other major

consideration in cyclone specification is pressure drop. While higher efficiencies are obtained by

forcing the gas through the cyclone at higher velocities, to do so results in an increased pressure

drop. Since increased pressure drop requires increased energy input into the gas, there is

ultimately an economic trade-off between collection efficiency and operating cost. A simple

pressure drop equation for cyclones is given by Cooper and Alley (1986). Cyclone pressure

drops range from 250 to 4000 Pa.

7.3.4 Practical Equation for Cyclone Efficiency

We have analyzed the collection efficiency of a cyclone assuming that the particles behave as if

they are in either a laminar or a turbulent flow. Actually, the flow pattern in a cyclone is a

complex one, and the two models that we have presented represent extremes in cyclone

performance. Although a Reynolds number for a cyclone can be de fined as Recv = (p U / JJ-)

(4Ac lJr )1/2, where Ac is the cross-sectional area so that (4 Ac / 7r) I /2 "is an equivalent

diameter, and for the velocity it is sufficient to use u =Q/ W(r2 - r,), a characteristic velocity in

the cyclone, a precise criterion for transition from laminar to turbulent flow in a cyclone does not

exist. The laminar flow theory predicts a well-defined critical value for the smallest particle size

that may be collected completely, whereas the turbulent flow result gives an asymptotic

approach to complete collection as particle size increases. Experimentally determined collection

efficiency curves generally approach 100% efficiency asymptotically and thus appear to conform

more closely to turbulent than to laminar flow conditions. Since operating cyclones do not

conform to either of these limiting cases, one must resort to semiempirical design equations to

predict cyclone performance. There has been a great deal of effort devoted to predicting the

performance of cyclones. Our primary goal in this section has been to present the general

theoretical approaches to the problem so that the various analyses in the literature will be

accessible to the reader. Surveys of design equations are available elsewhere (see, e.g., Bhatia

and Cheremisinoff, 1977; Licht, 1980, 1984). We will present one such semiempirical design

equation that has been applied successfully to cyclone design. If the flow can be considered to be

one of the two limiting cases analyzed above, the collection efficiency may be computed as

shown earlier for a given geometry, flow rate, and number of turns. Practical design equations

are generally derived by considering the particle trajectories under more realistic assumptions

concerning the flow in the cyclone. A theory developed by Leith and Licht (1972) has proved

useful in practical cyclone design. In that theory, account is taken of the fact that the velocity

profile in a cyclone usually does not adhere strictly to the ideal form (7.23). As we noted, a more

general form of the velocity profile is Uo r" = constant [( 7.23) is n = 1], where experimental

observations indicate that in a cyclone n may range between 0.5 and 0.9, depending on the size

of the unit and the temperature. It has been found experimentally that the exponent n may be

estimated from (Licht, 1980, p. 239)

where Dc is the cyclone diameter in meters and T is the gas temperature in kelvin. The collection

efficiency is given by

where N = 1/ (n + 1) and

where Dp is in em, Pp is in g em-3, Qis the gas volumetric flow rate in m3 s -1, it is in g cm- 1

S-l and K is a geometric configuration parameter that depends only on the relative dimensions of

the unit. For the relative dimensions suggested in Figure 7.8, K =402.9; for other dimensions the

values of K are given by Licht (1980, 1984). The calculation of K is explained by Leith and Licht

(1972) and Licht (1980).

(Perry 8th Ed., Ch.17)

Cyclone Separators The most widely used type of dust collection equipment is the cyclone, in

which dust-laden gas enters a cylindrical or conical chamber tangentially at one or more points

and leaves through a central opening. The dust particles, by virtue of their inertia, will tend to

move toward the outside separator wall, from which they are led into a receiver. A cyclone is

essentially a settling chamber in which gravitational acceleration is replaced by centrifugal

acceleration. At operating conditions commonly employed, the centrifugal separating force or

acceleration may range from 5 times gravity in very large diameter, low-resistance cyclones, to

2500 times gravity in very small, high-resistance units. The immediate entrance to a cyclone is

usually rectangular.

Fields of Application Within the range of their performance capabilities, cyclone collectors offer

one of the least expensive means of dust collection from the standpoint of both investment and

operation. Their major limitation is that their efficiency is low

for collection of particles smaller than 5 to 10 m. Although cyclones may be used to collect

particles larger than 200 m, gravity settling chambers or simple inertial separators (such as

gasreversal chambers) are usually satisfactory for this size of particle and are less subject to

abrasion. In special cases in which the dust is highly agglomerated or in high dust concentrations

(over 230 g/m3, or 100 gr/ft3) are encountered, cyclones will remove dusts having small particle

sizes. In certain instances, efficiencies as high as 98 percent have been attained on dusts having

ultimate particle sizes of 0.1 to 2.0 m because of the predominant effect of particle

agglomeration due to high interparticle forces. Cyclones are used to remove both solids and

liquids from gases and have been operated at temperatures as high as 1200C and pressures as

high as 50,700 kPa (500 atm). Cyclones can be very small or very large. The smallest cyclones

range from approximately 1 to 2 cm in diameter and the largest up to about 10 m in diameter.

The number of cyclones used for a single fluidized bed can vary from 1 to up to 22 sets of first-

stage and secondstage cyclones (44 cyclones total). Hugi and Reh [Chem. Eng. Technol. 21(9):

716719 (1998)] have reported that (at high solids loadings) enhanced cyclone efficiency occurs

when the solids form a coherent, stable strand at the entrance to a cyclone. The formation of such

a strand is dependent upon several factors. They reported a higher cyclone efficiency for smaller

(dp,50 = 40 _m) solids than for larger solids (dp,50 = 125 _m). This is not what theory would

predict. However, they also found that the smaller particles formed coherent, stable strands more

readily than the larger particles, which explained the reason for the apparent discrepancy.

Cyclone Efficiency The methods described below for pressure drop and efficiency calculations

were given by Zenz in Manual on Disposal of Refinery WastesAtmospheric Emissions, chap.

11 (1975), American Petroleum Institute Publ. 931 and improved by Particulate Solid Research

Inc. (PSRI), Chicago. Cyclones work by using centrifugal force to increase the gravity field

experienced by the solids. They then move to the wall under the influence of their effectively

increased weight. Movement to the wall is improved as the path the solids traverse under

centrifugal flow is increased. This path is equated with the number of spirals the solids make in

the cyclone barrel. Figure 17-38 gives the number of spirals Ns as a function of the maximum

velocity in the cyclone. The maximum velocity may be either the inlet or the outlet velocity

depending on the design. The equation for Dpth, the theoretical size particle removed by the

cyclone at 50 percent collection efficiency, is

This equation is a result of the residence time theory of particle collection. In this theory, the

time that it takes for a particle to reach the wall is balanced by the time that a particle spends in

the cyclone. The particle size that makes it to the wall by the time that it exits the cyclone is the

particle size collected at 50 percent collection efficiency, Dpth. When consistent units are used,

the particle size calculated by the above equation will be in either meters or feet.

The equation contains effects of cyclone size, gas velocity, gas viscosity, gas density,

and particle density of the solids. In practice, a design curve such as given in Fig. 17-39 uses

Dpth as the size at which 50 percent of solids of a given size are collected by the cyclone. The

material entering the cyclone is divided into fractional sizes, and the collection efficiency for

each size is determined. The total efficiency of collection is the sum of the collection efficiencies

of the cuts.

The above applies for very dilute systems, usually on the order of 1 gr/ft3, or 2.3 g/m3

where 1 gr = (1/7000) lb. When denser flows of solids are present in the inlet gas, cyclone

efficiency increases dramatically. This is thought to be due to the coarse particles colliding with

fines as they move to the wall, which carry a large percentage of the finer particles along with

them. Other explanations are that the solids have a lower drag coefficient or tend to agglomerate

in multiparticle environments, thus effectively becoming larger particles.

At very high inlet solids loadings, it is believed the gas simply cannot hold that much

solid material in suspension at high centrifugal forces, and the bulk of the solids simply

condense out of the gas stream. The phenomenon of increasing efficiency with increasing

loading is represented by Figs. 17-40 and 17-41 for Geldart group A and B solids, respectively

(see beginning of Sec. 17). The initial efficiency of a particle size cut is found on the chart, and

the parametric line is followed to the proper overall solids loading. The efficiency for that cut

size is then read from the graph.

A single cyclone can sometimes give sufficient gas-solids separation for a particular process or

application. However, solids collection efficiency can usually be enhanced by placing cyclones

in series. Cyclones in series are typically necessary for most processes to minimize particulate

emissions or to minimize the loss of expensive solid reactant or catalyst. Two cyclones in series

are most common, but sometimes three cyclones in series are used. Series cyclones can be very

efficient. In fluidized catalytic cracking regenerators, two stages of cyclones can give

efficiencies of up to and even greater than 99.999 percent. Typically, first-stage cyclones will

have an inlet gas velocity less than that of second-stage cyclones. The lower inlet velocity of

first-stage cyclones results in lower particle attrition rates and lower wall erosion rates. After

most of the solids are collected in the first stage, a higher velocity is generally used in second-

stage cyclones to increase the centrifugal force on the solids and increase collection efficiency.

Inlet erosion rates are generally low in the second stage because of the vastly reduced flux of

solids into the second-stage cyclone. However, cone erosion rates in second-stage cyclones are

much greater than in first stage cyclones.

Pressure Drop Pressure drop is first determined by summing five pressure drop components

associated with the cyclone.

1. Inlet contraction

where K is taken from Table 17-3. Using SI units gives the pressure drop in Pa. In English units,

the factor of 32.2 for g must be included. This loss is primarily associated with cyclones located

in the freeboard of a fluidized bed. If the cyclone is located external to a vessel and the high-

pressure tap used to measure the cyclone pressure drop is in the inlet pipe before the cyclone, the

measured pressure drop will not include this pressure loss, and this term should not be used to

calculate total cyclone pressure drop. However, if the high-pressure tap to measure the cyclone

pressure drop is located in the freeboard of the bed, this component will be included in the

measured pressure drop, and it should be included in the calculation of the total cyclone pressure

drop.

2. Particle acceleration

For small particles, the velocity is taken as equal to the gas velocity and L is the solids loading,

kg/m3.

3.Barrel friction

The inlet diameter din is taken as 4 (inlet area)/inlet perimeter. Then

where the Reynolds number for determining the friction factor f is based on the inlet area.

4. Gas flow reversal

5. Exit contraction

where K is determined from Table 17-3 based on the area ratio of barrel and exit tube of the

cyclone.

The total pressure drop is the sum of the five individual pressure drops. However, the actual

pressure drop observed turns out to be a function of the solids loading. The pressure drop is high

when the gas is free of solids and then decreases as the solids loading increases up to about 3

kg/m3 (0.2 lb/ft3). The cyclone P then begins to increase with loading. The cause of the initial

decline is that the presence of solids decreases the tangential velocity of the gas [Yuu, Chem.

Eng. Sci., 33, 1573 (1978)]. Figure 17-42 gives the actual pressure drop based on the loading.

When solids are absent, the observed pressure drop can be 2.5 times the calculated pressure drop

with solids present.

Cyclone Design Factors Cyclones are sometimes designed to meet specified pressure drop

limitations. For ordinary installations, operating at approximately atmospheric pressure, fan

limitations generally dictate a maximum allowable pressure drop corresponding to a cyclone

inlet velocity in the range of 8 to 30 m/s (25 to 100 ft/s). Consequently, cyclones are usually

designed for an inlet velocity of 15 to 20 m/s (50 to 65 ft/s), although this need not be strictly

adhered to. Because of the relatively high gas velocities at the inlet of cyclones, particle attrition

in fluidized-bed systems is generally dominated by the attrition produced in the cyclone. In some

catalytic systems with very expensive catalysts, the economics of the process can be dependent

on low attrition losses. In such cases, reducing the inlet velocity of the cyclone will significantly

reduce the attrition losses in the process. To compensate for the reduction in inlet velocity, the

exit gas velocity will generally be increased (by reducing the diameter of the outlet tube) in order

to maintain high cyclone efficiencies. Reducing the outlet tube diameter increases the outlet gas

velocity and increases the velocity in the vortex of the cycloneincreasing collectionefficiency.

However, as the vortex velocity is increased, its length is also increased. Therefore, care must be

taken to ensure that the cyclone is long enough to contain the increased vortex length. If it is

not, the vortex can extend far into the cone and can entrain solids flowing on the sides of the

cone as it comes near them.

Cyclone Roughness Large weld beads, etc., can also reduce cyclone efficiency. If the solids flow

along the wall of a cyclone encounters a large protuberance such as a weld bead, the weld bead

acts as a type of ski jump and causes the solids to be deflected farther into the center of the

cyclone, where they can be thrown into the vortex and carried out of the cyclone. In small pilot

or research cyclones, this is especially common, because the distance between the wall of the

cyclone and the vortex tube is very small. Because of theirdetrimental effect on cyclone

efficiency, weld beads should be ground off to make the cyclone inner wall smooth.

In high-temperature processes, cyclones are often lined with refractory to both minimize heat

loss and protect the metal surfaces from abrasion. These refractory surfaces are not as smooth as

metal, but after a few days of operation, the refractory becomes smoother because of the abrasive

action of the solids. With very small laboratory or pilot cyclones, some solids (large polymer

beads, spherical particles, etc.) can sometimes bounce off the cyclone wall immediately across

from the cyclone inlet and be deflected into the vortex. Very large particles can be found in the

gas outlet stream of the cyclone with these very small cyclones and with particles that bounce.

To increase cyclone efficiency with these types of solids, the cyclone barrel diameter can be

increased. This increases the distance between the cyclone vortex and the wall and prevents

most of the solids from bouncing back into the vortex. Theoretically, a primary design factor that

can be utilized to control collection efficiency is the cyclone diameter. A smaller-diameter unit

operating at a fixed pressure drop has a higher efficiency than a larger diameter cyclone

[Anderson, Chem. Metall. 40: 525 (1933); Drijver, Warme 60: 333 (1937); and Whiton, Power

75: 344 (1932); Chem.Metall. 39: 150 (1932)]. In addition, smaller-diameter cyclones have a

much smaller overall length. Small-diameter cyclones, however, will require multiple units in

parallel to give the same capacity as a large cyclone. In such cases, the smaller cyclones

generally discharge the dust into a common receiving hopper [Whiton, Trans. Am. Soc. Mech.

Eng. 63: 213 (1941)].

However, when cyclones discharge into a common hopper, there is a tendency of the gas to

produce cross-talk. This occurs when the gas exiting from one small cyclone passes up the exit

of an adjoining cyclone, thus reducing efficiency. Various types of mechanical devices are

generally added to the bottom of these smallcyclones in parallel to reduce the cross-talk. The

final cyclone design involves a compromise between collection efficiency and the complexity of

equipment. It is customary to design systems for a single cyclone for a given capacity, resorting

to multiple parallel units only if the predicted collection efficiency is inadequate for a single unit

or single units in series.

Reducing the gas outlet diameter should increase both collection efficiency and pressure drop.

To exit the cyclone, gas must enter the cyclonic flow associated with the outlet tube. If the outlet

diameter is reduced, the outlet vortex increases in length to compensate. Therefore, when the

outlet area is less than the inlet area, the length of the cyclone must increase. Too short a cyclone

is associated with erosion of the cone and reentrainment of solids into the exit flow. Table 17-4

gives the required increase in cyclone length as a function of outlet-toinlet area.

The cyclone length is measured centrally along a cylinder 10 cm larger than the inner diameter

of the outlet tube to prevent interference with the cone. If the cone interferes with this extended

vortex, the barrel must be lengthened.

As discussed above, theoretically a smaller-diameter cyclone should be able to collect smaller

particles because it can develop a higher centrifugal force. However, using smaller cyclones

generally means that many have to be used in parallel to accommodate large gas flows. The

problem with parallel cyclones (as indicated above) is that it is difficult to get even distribution

of solids into all the cyclones. If maldistribution occurs, this can cause inefficiencies that can

negate the natural advantage of the smaller cyclones.

Cyclone diameters can be very large. Perhaps the largest cyclones are those used in circulating

fluidized-bed combustors, where cyclone diameters approach 10 m. Large-diameter cyclones

also result in very long cyclones, and so these large-diameter, long-length cyclones arereally not

feasible as internal cyclones in fluidized beds (they make the vessel too tall). The minimum cone

angle of the cyclone should be 60. It is generally greater, with steeper cone angles appropriate

to materials that are more cohesive.

The cyclone inlet is usually rectangular (more efficient at getting material to the wall), but in

some cases has been circular. In either case, projection of the inlet flow path should never cause

interference with the outlet tube. This generally means that the inlet width of a cyclone should

always be less than the distance between the wall and the outside diameter of the outlet tube. If a

very heavy solids loading is anticipated, the barrel diameter should be increased slightly to

minimize interference with the outlet gas tube.

Collection efficiency is normally increased by increasing the gas throughput (Drijver, op. cit.).

However, if the entering dust is agglomerated, high gas velocities may cause breakup of the

agglomerated solids in the cyclone, so that efficiency remains the same or actually decreases.

Also, variations in design proportions that result in increased collection efficiency with dispersed

dusts may be detrimental with agglomerated dusts. Kalen and Zenz [Am. Inst. Chem. Eng.Symp.

Ser. 70(137): 388 (1974)] report that collection efficiency increases with increasing gas inlet

velocity up to a minimum tangential velocity at which dust is either reentrained or not deposited

because of saltation. Koch and Licht [Chem. Eng. 84(24): 80 (1977)] estimate that for typical

cyclones the saltation velocity is consistent with cyclone inlet velocities in the range of 15 to 27

m/s (50 to 90 ft/s).

Lapple (private communication) reports that in cyclone tests with talc dust, collection efficiency

increased steadily as the inlet velocity wasincreased up to a maximum of 52 m/s (170 ft/s). With

ilmenite dust, which was much more strongly flocculated, efficiency decreased over the same

inlet velocity range. In later experiments with well-dispersed

talc dust, collection efficiency continued to increase at inlet velocities up to the maximum used,

82 m/s (270 ft/s). Another effect of increasing the cyclone inlet gas velocity is that friable

materials may disintegrate (or attrit) as they hit the cyclone wall at high velocity. Thus, the

increase in efficiency associated with increased velocity may be more than lost due to generation

of fine attrited material that the cyclone cannot contain.

Cyclones can be either placed in the freeboard above the fluidized bed or located outside of the

fluidized-bed vessel. There are advantages and disadvantages to each type of placement, and the

optimum type of placement depends on what is best for a particular process. Internal cyclones

have the advantages that they require no inlet piping (their inlets can be open to the freeboard)

and no high-pressure shell, and they have straight cyclone diplegs. Internal cyclones are

generally smaller in diameter than external cyclones because their size is limited by the

headspace available in the freeboard above the fluidized bed. These size limitations result in

using several smaller cyclones in parallel instead of one large cyclone. In addition, it is difficult

to aerate second-stage cyclone diplegs (generally an advantageous technique) when internal

cyclones are used. Aerating secondary cyclone diplegs can improve the operation of the diplegs

significantly.

The advantages of external cyclones are that (1) they can be much larger than internal cyclones,

(2) they are more accessible than internal cyclones, and (3) their diplegs can be aerated more

easily. The disadvantages of external cyclones are that (1) they require a pressure

shell and (2) external cyclone diplegs generally require a section with an angled or a horizontal

pipe to return the solids to the bed. The angled or horizontal dipleg sections can result in poor

dipleg operation, if not designed correctly. Cyclones in series may be justified under the

following circumstances:

1. The dust has a broad size distribution, including particles under 10 to 15 m as well as larger

and possibly abrasive particles. A large low-velocity cyclone may be used to remove the coarse

particles ahead of a unit with small-diameter multiple tubes.

2. The dust is composed of fine particles but is highly flocculated or tends to flocculate in

preceding equipment and in the cyclones themselves. Efficiencies predicted on the basis of

ultimate particle size will be highly conservative.

3. The dust is relatively uniform, and the efficiency of the secondstage cyclone is not greatly

lower than that of the first stage.

4. Dependable operation is critical. Second-stage or even thirdstage cyclones may be used as

backup.

Cyclone Inlets The design of the cyclone inlet can greatly affect cyclone performance. It is

generally desired to have the width of the inlet Bc as narrow as possible so that the entering

solids will be as close as possible to the cyclone wall where they can be collected. However,

narrow inlet widths require that the height of the inlet H be very long in order to give an inlet

area required for the desired inlet gas velocities. Therefore, a balance between narrow inlet

widths and the length of the inlet height has to be struck. Typically, low-loading cyclones

(cyclones with inlet loadings less than approximately 2 to 5 kg/m3) have height/width ratios

H/Bc of between 2.5 and 3.0. For high-loading cyclones, this inlet aspect ratio can be increased

to as high as 7 or so with the correct design. Such high inlet aspect ratios require that the

cyclone barrel length increase.

A common cyclone inlet is a rectangular tangential inlet with a constant area along its length.

This type of inlet is satisfactory for many cyclones, especially those operating at low solids

loadings. However, a better type of inlet is one in which the inner wall of the inlet is angled

toward the outer cyclone wall at the cyclone inlet. This induces solids momentum toward the

outer wall of the cyclone. The bottom wall of the inlet is angled downward so that the area

decrease along the inlet flow path is not too rapid and acceleration is controlled. In addition,

the entire inlet can be angled slightly downward to give enhanced efficiencies.

This type of inlet is superior to the constant-area tangential inlet, especially for higher solids

loadings (greater than 2 to 5 kg/m3). Hugi and Reh [Chem. Eng. Technol. 21(9):716719 (1998)]

report that continuous acceleration of the solids throughout the inlet is desired for improved

efficiency and that the angled inlet described above achieves this. If the momentum of the solids

is sufficient and the solids are continuously accelerating along the length of the inlet, the stable,

coherent strand important for high collection efficiencies is produced.

The best inlet for high solids loadings is the volute cyclone inlet. At high inlet loadings (above

approximately 2 to 3 kg/m3) in a tangential cyclone inlet, the gas-solids stream expands rapidly

from its minimum width at the point of contact. This rapid expansion disturbs the laminar gas

flow around the gas outlet tube and causes flow separation around the tube. At some loadings,

the inlet stream can expand to such an extent that the solids can impact the gas outlet tube. Both

effects result in lowered cyclone efficiency.

However, when a volute inlet is used, the expanding solids stream is farther from the gas outlet

tube and enters at an angle so that the solids do not induce as much flow separation or

asymmetric flow around the gas outlet tube. Therefore, cyclone efficiency is not affected to as

great a degree. If a tangential cyclone is used at high solids loadings, an extra distance between

the gas outlet tube and the cyclone wall should be designed into the cyclone to prevent the solids

from impacting on the gas outlet tube.

At low solid loadings, the impacting on the gas outlet tube does not occur. Because tangential

cyclone inlets are less expensive than volute inlets, the tangential cyclone is typically utilized for

low loadings and the volute inlet cyclone is used for high loadings.

The nature of the gas solids flow in the inlet ducting to the cyclone can affect cyclone efficiency

significantly. If the solids in the inlet salt out on the bottom and result in dune formation and the

resulting unsteady or pulsing flow, cyclone efficiency is adversely affected. To minimize the

possibility of this occurring, it is recommended that the inlet line to the cyclone operate above

the saltation velocity [Gauthier et al., in Circulating Fluidized Bed Technology III, Basu, Horio,

and Hasatani (eds.), 1990, pp. 639644], which will prevent the solids from operating in the

dune or pulsing flow regime. If this is not possible, then the inlet line can be angled downward

(approximately 15 to 20) tto let gravity assist in the flow of the solids. Keeping the inlet line as

short as possible can also minimize any pulsing of the solids flow.

A cyclone will operate equally well on the suction or pressure side of a fan if the dust receiver is

airtight. Probably the greatest single cause of poor cyclone performance, however, is the leakage

of air into the dust outlet of the cyclone. A slight air leak at this point can result in a tremendous

drop in collection efficiency, particularly with fine dusts. For a cyclone operating under pressure,

air leakage at this point is objectionable primarily because of the local dust nuisance created. For

batch operation, an airtight hopper or receiver may be used. For continuous withdrawal of

collected dust, a rotary star valve, a doublelock valve, or a screw conveyor may be used, the

latter only with fine dusts. A collapsible open-ended rubber tube can be used for cyclones

operating under slight negative pressure. Mechanical trickle and flapper valves at the end of

cyclone diplegs can also be used for continuous withdrawal into fluidized beds or into the

freeboard of fluidized beds. Open diplegs simply immersed in a fluidized bed can be used in

cases where start-up losses are not excessive and are the simplest type of discharge system

returning solids to a fluidized bed (see Fluidized-Bed Systems: Solids Discharge). Special

pneumatic unloading devices can also be used with dusts. In any case it is essential that sufficient

unloading and receiving capacity be provided to prevent collected material from accumulating in

the cyclone.

Solids Loading Cyclones can collect solids over a wide range of loadings. Traditionally, solids

loadings have been reported as either kilograms of solids per cubic meter of gas (kg/m3), or as

kilograms of solids per kilogram of gas (kgs/kgg). However, loading based on mass is probably

not the best way to report solids loadings for cyclones. This is so because the volume of solids

processed by a cyclone at the same mass loading can vary greatly, depending on the density of

the solids. For example, many polymers have a bulk density of approximately 400 kg/m3, and

iron ore has a bulk density of approximately 2400 kg/m3. This is a factor of 6. Therefore, a

cyclone operating with polymer would have to process 6 times the volume of solids that a

cyclone operating with iron ore would process at the same mass loading.

If the cyclone operating with the polymer were designed to operate at high loadings on a mass

basis, it would probably plug. In addition, the diplegs below the cyclone operating with the

polymer may experience operational problems because of the high volumetric loading. At

ambient conditions, cyclones have been operated at solids loadings as low as 0.02 kg/m3 (0.0125

kg/kg) and as high as 64 kg/m3 (50 kgs/kgg) or more. This is a factor of 3200. In general,

cyclone efficiency increases with increasing solids loading. This is so because at higher loadings,

very fine particles are trapped in the interstices of the larger particles, and this entrapment

increases the collection efficiency of the small particles. Even though collection efficiencies are

increased with increased loading, cyclone loss rates are also increased as loading is increased.

This is so because the cyclone efficiency increase isalmost always less than the increase in the

solids loading.

Generally cone-and-disk baffles, helical guide vanes, etc., placed inside a cyclone, will have a

detrimental effect on performance. However, a few of these devices do have some merit under

special circumstances. Although an inlet vane will reduce pressure drop (and may result in

significant erosion), it generally causes a correspondingly greater reduction in collection

efficiency. Its use is recommended only when collection efficiency is normally so high as to be a

secondary consideration, and when it is desired to decrease the resistance of an existing cyclone

system for purposes of increased air handling capacity or when floorspace or headroom

requirements are controlling factors. If an inlet vane is used, it is advantageous to increase the

gas exit duct length inside the cyclone chamber.

A disk or cone baffle located beneath the gas outlet duct may be beneficial if air in-leakage at the

dust outlet cannot be avoided. A heavy chain suspended from the gas outlet duct has been found

beneficial to minimize dust buildup on the cyclone walls in certain circumstances.

Such a chain should be suspended from a swivel so that it is free to rotate without twisting.

Substantially all devices that have been reported to reduce pressure drop do so by reducing spiral

velocities in the cyclone chamber and consequently result in reduced collection efficiency. At

low dust loadings, the pressure in the dust receiver of a single cyclone will generally be lower

than in the gas outlet duct. Increased dust loadings will increase the pressure in the dust receiver.

Such devices as cones, disks, and inlet vanes will generally cause the pressure in the dust

receiver to exceed that in the gas outlet duct. A cyclone will operate as well in a horizontal

position as in a vertical position. However, departure from the normal vertical position results

in an increasing tendency to plug the dust outlet. If the dust outlet becomes plugged, collection

efficiency will, of course, be low. If thecyclone exit duct must be reduced to tie in with proposed

duct sizes, the transition should be made at least five diameters downstream from the cyclone

and preferably after a bend. In the event that the transition must be made closer to the cyclone, a

Greek cross should be installed in the transition piece to avoid excessive pressure drop.

Cyclone Length As described above, the cyclone length should be great enough to contain the

vortex below the gas outlet tube. It is generally advisable to have the cyclone somewhat longer

than required so that modifications to the gas outlet tube can be made if required. Either the

barrel or the cone can be increased in length to contain the vortex. However, cyclone barrels can

be made too long. If the barrel is too long, the rotating spiral of solids along the wall can lose its

momentum. When this happens, the solids along the wall can be reentrained into the rotating gas

in the barrel, and cyclone efficiency will be reduced.

Hoffman et al. [AIChE J. 47(11): 24522460 (2001)] studied the effect of cyclone length on

cyclone efficiency and showed that the efficiency of a cyclone increases with length. However,

they also found that after a certain length, cyclone efficiency decreased. They reported that

cyclone efficiency suddenly decreased after a certain cyclone length, which in their cyclone was

at a length/diameter ratio of 5.65. (Although many researchers employ this length/diameter ratio

as a correlating parameter to make the length parameter dimensionless, it is likely that it is the

actual length of the cyclone that is important.) Hoffman et al. stated that the probable reason for

the sudden decrease in cyclone efficiency was the central vortex touching and turning on the

cyclone cone. When this occurred, the efficiency collapsed, causing increased solids

reentrainment. Hoffman et al. also reported that cyclone pressure drop decreased with increasing

cyclone length. This probably occurs for the same reason that cyclone pressure drop decreases

with increasing cyclone loading. For long cyclones, the increased length of the cyclone wall

results in a longer path for the gas to travel. This creates greater resistance to the flow of the gas

in the cyclone (much as a longer pipe produces greater resistance to gas flow than a shorter pipe)

that results in reducing the tangential velocity in the cyclone and, therefore, the cyclone pressure

drop.

Most processes operate at high temperatures and/or high pressures. Therefore, it is important to

know how cyclones operate at these conditions. Efficient cyclones are able to collect very small

particles sizes. Therefore, cyclone efficiency is proportional to 1/Dpth.

The effects of temperature and pressure manifest themselves in how they affect the gas density

and gas viscosity. From the equation above, it can be seen that cyclone efficiency is theoretically

related to gas density and gas viscosity as

As pressure is increased, gas density will increase. However, the term p - g does not change

with increases in gas density because particle density is so much greater than the gas density

(typically about 2000 kg/m3 versus approximately 20 kg/m3 at high pressure) that it dominates

this term. Therefore, it is expected that gas density would have little or no effect on cyclone

efficiency. Conversely, cyclone efficiency would be expected to decrease with system

temperature because gas viscosity increases with increasing pressure. Knowlton and Bachovchin

[Coal Processing Technol. 4:122127 (1978)] studied the effect of pressure on cyclone

performance andfound little change in overall cyclone efficiency with pressure over a pressure

range from 0 to 55 barg. However, fractional efficiency curves from the same study showed that

cyclone efficiency decreased with pressure for particle sizes less than about 20 to 25 m. For

particle sizes greater than about 25 m there was no effect of pressure on cyclone efficiency. The

effect of temperature on cyclone efficiency was studied by both Parker et al. [J. Environ. Sci.

and Technol. 15 (4):451 (1981)] and Patterson and Munz [Canadian J. Chem. Eng. 67:321

(1989)]. Both studies showed that cyclone efficiency decreased with increasing gas viscosity. As

with the studies at high pressure, Patterson and Munz (1989) reported that only the collection

efficiency of particles less than about 10 m was reduced because of operation at high

temperature.