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Performance of the university of Toronto infrasizer MK III as a monosizer and multi-cut classifier
Powder Technology. 0 Elsevier Sequoia
30 (1981) 257 - 261 S.A., Laumnne - Printed
in The
Netherlands
Performance of the University of Toronto as a Monosizer and Multi-Cut Classifier A. H.
VON
FLOTOW
Utzioersily
of
(Received
September
and
Tororzto.
B. ETKIN
Institute 16, 1960;
forrlerospnce in revised
Studies. rorm
March
SUMMARY
This paper presents results of a series of experiments conducted to define the intrinsic performance of an air classifier that is characterized by a uniform low-turbulence flow field. For free-flowing spherical par-titles in the range tested (50 - 230 Dm); the performance, both as a multi-cut device and in monosize production, is excellent_ The sharpness of cut index is only a few percent lower than the ideal value of 1.00 and monosize fractions can be produced with standard deviation about 4% of the average diameter_
INTRODUCTION Air
sively
classifiers both
have
as laboratory
Infk-asker MK III
long
been
used
instruments
estenand
as
production machinery in industry where dry solid powders are processed_ There are numerous patents, going back to at least 1893, for devices that can be called cross-flow classifiers. Noteworthy among these is one by Thomas A_ Edison, 1904 [l] fora wind tunnel designed to separate gold from gravel. A later application of the wind tunnel was by Rouse and Otto, 1939 [ 21 for separating sand into a relatively large number of fractions_ Rumpf, 1939 133 initiated the well-known series of developments at Karlsruhe that led to the subsequent work of Leschonski [4] and others, and TimbreIl [5] developed the Aerosol Spectrometer in the 1950s. All of the above devices have in common that the working medium is air, and that the solid particles cross the streamlines. The separation is effected by the differential balance of aerodynamic, gravitational, and inertial faces, the latter two being the cause of particle trajectories crossing the stieamlinesA different class of device,
4325 Duffcrin
Street.
Dowr~svicw.
Ont..
M31i 51% (C:armdo)
6. 1961)
the vertical column elutriator, is illustrated by the Haultain Infrasizer [6] developed at the University of Toronto in the 1930s. The Infrasizer MK I11 [7, 81 was derived independently of previous work on crossflow classifiers. It was an outcome of research on the interaction of precipitation with wind and jets [ 91 , simultaneously with unrelated work on the Haultain Lnfrasizer. There is little evidence to show that crossflow air classifiers have in the past been designed with particular attention to minimizing those deviations of particles from their intended (ideal) trajectories that are -associated with turbulence and spatial non-uniformity of flow. The lnfrasizer MK III is such a device - i.e. one in which aerodynamic principles have been employed to reduce hot11 non-uniformity and turbulence to less than 1%. Reference [ 71 describes the evolution of the design and presents some of the results obtained in it_ Reference [S] contains an analysis of the factors that contribute to non-ideal performance, and Ref_ [ 101 describes an application to the classification of thin flakes_ It is the purpose of this paper to report the results of a series of experiments conducted to quantify the performance in conventional terms_ The results obtained are thought to represent close to the ultimate in precision of which this class of air classifier is capable. The two performance items emphassized are the production of monosized particles and the sharpness of cut at each of a multiplicity of cut points.
DESCRIPTION
OF
THE
APPARATUS
The principle of the Infrasizer MK III is shown schematically in Fig. 2. The main tunnel in those constructed is typically about
FEED
FINE STREAN
COARSE STREAM Fig. 1. Defbition
of mzsses.
20 - 25 cm in cross-section and about 100 - 125 cm long. (Four have been made, all different_) In addition to the main duct, the device has a variable-speed fan for establishing the flow, an entry section designed to produce a laminar uniform stream, and a system for feeding the particles_ The stream of solid particles entering from above typically arrives in the flow with a vertical velocity nearly eCpd to the settling speed, and forms a stream about 1 - 2 mm thick estending to within about 2 cm of the side walls. The rate of feeding is controllable over a wide range_ The four models have differed from one another in the details of these features [7-S] _ The multiple fractions are of course collected in the set of trays that cover the floor of the due i;_
FACTORS THE
AFFECTING
INFRASIZER
THE
PERFORhlANCE
OF
RIK III
The main factors that can lead to non-ideal performance are the following: spatial non-uniformity of the flow in the y direction, temporal fluctuations in the flow (turbulence). particle interactions, variable particle entry conditions, variable shape and orientation of particles, flocculation of partic!es. These are discussed in some detail in ref. [S] _ We have shown there that particle interactions can effectively be eliminated if the particle feed rate is less than some critical value; and when spherical particles of uniform density are used for testing, the shape/orientation factor is also eliminated. We also found that flocculation is not a significant factor for
Fig. 2_ Schematic
diagram
of Infrasizer.
glass spheres larger than about 30 pm, and hence confined the test to this range of size_ Our goal was to reduce the remaining non-ideal effects to the least possible. The lnfrasizer MK Ill, Model 4, represents the current stage of this development, and the results presented herein were obtained in that instrument. (Further improvements are being studied.) EXPERIMENTS
The present esperiments were performed with a feed material consisting of solid glass beads of uniform density in the size range 50 - 230 pm. Although the beads were predominantly spherical, some irregular glass fragments, ellipsoids and fused beads contaminated the feed. These non-spherical particles were not counted in the statistical analysis. The experiments were performed at room temperature and humidity; these factors were not monitored. Two feed rates were used, above and below the onset of significant particle interaction, as indicated by the parameter riLl’/V, [ 83 _ The air speed was chosen to use the available collection zone optimally. EXPERIMENTAL
RESULTS
A -Performance as a multi-cut classifier A perfect single-point classifier would be one in which all particles smaller than a certain size would go to the Ike stream while alI particles larger than that size go to the coarse
stream. Similarly, a perfect multi-cut classifier would direct each particle unambiguously to one of several output streams. Real classifiers of course perform imperfectly. Some material will always be found in the wrong output stream. This imperfection can be quantitatively described in a variety of ways. The notation and measures that we have used are those defined in ref_ 1111, with some minor modifications and additions (see Fig_ 1). For convenience, the main items are briefly reviewed below. Recovery The fine recovery is the ratio RDr = w[Jw,~, with a similar definition for the coarse recovery. A perfect classifier would have fine and coarse recoveries of unity_ Contamination The fines contamination is the mass fmction of the fine stream corresponding to particles larger than the cut size, ie. Cr = wrc/wf. Coarse contamination has a similar definition, C, = w,f/w,. A perfect classifier has contaminations of zero. Sharpness of ci~t The probability that a particle from the feed goes to the coarse stream is the ‘size selectivity’ or ‘coarse grade efficiency’, nn and is a function of particle size. The ideal form of no is a step function. In reality, it increases gradually from very small values for very small particles towards unity for the largest particles, indicating the presence of misplaced particles. With this form of vi, the cut size requires further definition, and the recovery and contamination values are no longer ideal. The cut size is normally defined to be the equiprobable size &-, -the size of
particles that have a 50% expectation
of re-
porting to either-stream. The deviation from ideal classification is characterized by the sharpness index - the ratio of the size of the particles with a 25% chance of reporting to the coarse stream to those with a 75% chance. This is denoted P = Dzs/D,s_ A sharpness index of unity would indicate ideal classification, and a value of less than 0.3 is considered poor [12] _ Experimental procedure Two samples of similar composition, each weighing 200 g, were classified_ The feed rates
were chosen to yield values of riD’/v, = 0.062 (Run A) and 0.66 (Run B) (conservatively based on D = 50 pm). These values of the particle-interaction parameter [S] were selected to ensure that Run _A had negligible particle interaction, and Run B had severe particle interactions_ The particles were collected in 24 bounccinhibiting trays distributed on the tunnel floor as shown in Fig. 2. The content of cacti tray was weighed and sampled. hlicrophotographs cf the samples were taken, and diameter measurements of 100 particles yielded values for the mean, standard deviation and a histogram of diameter distributions for each tray. These data were processed t.o yield the curves of Figs. 3 - 5. The average diameter is seen to vary smoothly with downstream distance (Fig. 3), the shape of the curve reflecting approsimately the law for settling velocity at low Reynolds number, i.e. V, - D’, whence D - (s/H)~~“‘. The standard deviation (Fig. 4) is seen to exhibit appreciable scatter, in part from sampling error and in part from measurement error. The latter was determined to be about 1% of D. The precision is best in the range x = II - 11If, and for low feed rate, when u/D - 4%. Figure 5 shows the diameter range associated with 90% of the particles_ This information is a useful supplement to the standard deviation, since the distributions are not necessarily Gaussian_ For esamplc, at 75 ;Im mean diameter, Fig. 5 shows that 90% of the particles lie in a 12 Mm band. To evaluate the performance as a single-cut device, the 24 output streams were mathematically combined into two streams, one including all trays upstream of the division (the coarse stream), the other including all trays downstream of the division (the fine stream)_ This was done for eleven different locations of the dividing line to yield the curves of Figs_ 6 - 10. Figures 6 and 7 are representative of many others and illustrate how the various performance parameters were obtained_ Figure 6 is for a cut after tray No. 9, and shows the computer-generated values of no from which fi was obtained_ Note that the abscissa is a very expanded scale. Figure 7 shows the feed, coarse and fine distributions for this cut, together with the recovery and contamination values. The departure of the dotted line from the solid line on Fig. 7 represents the extent of non-ideal behaviour- It
.
Low
-
High
Feed Feed
Rate Rate
0.062 0.66
x
Fig. 4. Ratio of standard deviation to diameter,
o/d
be seen that at both feed rates used, the Infrasizer performs exceptionally well as a single-point classifier. The sharpness of cut indes, Fig. 10, is typically greater than 0.8 and can be as large as 0.97 ; the fine and coarse recoveries, Figs. 6, 9, lie between 95 and 100%; and the fine and coarse contaminations are less than 5%. The low recovery and large may
(each-point
represents one tray)_
contamination of the fine fraction at small cut size (II,,, < 80 pm) and of the coarse fraction at 1-e cut size @so > 160 pm) are of course simply refkctions of the difficulty of making a precise cut near the tail of the distribution. The effect of feed rate is seen quite clearly on Figs. 8 - 10. The high rate is about 10
-2 E 2. -0
Q
0
6..
O_
8 i..
-909.
”
l
90% of Particles Lie Eelween Dushed Liws Low Feed Rate. fiD2/Vx = 0.062
2
5
.
Range
b
Y/H
Fig. 5. 90% limits of diameter.
High Feed Rate. Cut Afler Troy 9
#/V_
--_-.
- 066 .
-------------------ae
8
P s
I
-I .
m d.
----
I
‘fD’5
1:
----------------
OI
.+-
4-p”
8 d
. 4_%a.=
-aem-
c3.m
*-
.r3
DIA
Fig_ 6_ Example
l
9.. ?rn.M
I pm
I
-1 1
III.cc
I
, I-
’
! ! _B 0..3 1n.m
In.’
Ilb.C3
1
of T)D for high feed rate.
times the low rate, and is well within the range of strong interaction. Nevertheless, the sharpness of cut has not deteriorated very much. It remains generally above 0.85, and the maximum (on the faired curves) has only decreased from 0.97 to 0.92. The effect of high feed rate on the f%ne &action performance (recovery and contamination) is also modest, whereas the coarse fraction is more influenced, especially the contamination by fines_ This suggests that there are fine particles entrained
in the wakes of coarse ones at the high feed rate, a hypothesis that fits very well the interaction concept reported in ref. [ 8]_ B -Performance in monosize production -4 feature of the Infrasizer MK III is the possibility of producing avariety of monosized samples simultaneously from a single feed stream (Figs. 3, 4). Both the mean size and the diameter range (above a certain minimum) of the monosize sample are controllable. The size range of each sample is controlled by the width of the collection tray. In this study the collection region was subdivided in such a way that the size range of each output sample was approximately proportional to its mean size. In order to study the repeatability of particle trajectories and the value of multi-stage operation, several products of a run identical to Run A in operating conditions, but differing slightly in feed composition were saved and processed further_ Each sample was passed through the Inkrsizer several times and its mean and standard deviation determined after each pass. These results are summarized in
163
COARSE 4
CD1
FEED.
f f
STREAM. \
./-
/
.-‘-FINE
STREAM,
High nD=/Vx
Fig. 7.
Exnmple FINES
of cumulative
Recovey
and
+o (Dl
distributioms;
feed,
Feed
t&(D)
Rate,
= 0.66
fine and coarse.
COARSE
Contamination
Recovery
and
D,,,
Fig.
8_ Fine
recovery
and contamination_
Fig. ll_ As espected, the mean size of ah samples remained within a few pm from run to run, and the standard deviation decreased or remained constant_ The small variations in mean size (bc-j.1 increases and decreases) are probably a coni;- quence of the sampling error
Fig.
9. Co a-se
recovery
Contamination
lw=)
and contamination.
sample consisted of between 60 and 100 particies. -Another expected result is a steady increase in yield, for example from 60% on the second pass to 80% on the seventh. A yield of less than 100% is caused by the loss of particles
-each
4 ,OO~~_~~~~~~~~___ I_
___P_t~----z------_~_
,.__z------.eo--
-_--
Y
0
0
-
Perfecl
--------------n -
-
---LF-----.
0
y
Performance
c
EnP .‘a--
d/VI.
=-
Low
Feed
Role
0
High
Feed
Role
0062 0.66
.20--
Fig. 10. Sharpness of cut, fl= D&D-i5_ COMMENTS
40 I
Fig. 11.6 runs_
*
4
5
NUMSEA OF
PLSSES
3
and u Ior succwive
6
passes - monosizing
that land outside the collection tray. These would consist of both particles that do not belong in the collection tray but erroneously arrived there on the previous passes, and of particles that do belong in the collection tray but erroneously arrive in a neighbouring one. The result of these losses of unwanted and borderline particles should be 2 narrowing of the range of particle size in the output stream from one iteration to the next. This is reflected by the values of standard deviation given in Fig. 11.
AND
CONCLUSIONS
The Infrasizer MK III has been shown to be a classifier of considerable merit for freeflowing particles with equivalent aerodynamic diameters in the range 50 - 230 pm. As a multi-cut device, the values of 0 as large as 0.97 speak for themselves_ In monosize production, samples with standard deviations of (5% of mean diameter are obtainable with a single pass. No certain statements can be made about the performance for particles outside the size range tested. However, experience with the apparatus suggests that quite comparable performance can be expected with much larger particles, up to say 3 mm, provided that the collection means is designed to trap the particles and prevent them from bouncing into the wrong tray. It is intrinsic to this classifier, as to others, that particles will only follow the correct trajectories if they do not stick together to form clumps. We have not found this to be a serious problem for particles generally larger than about 40 pm. However, for smaller ones, flocculation may be a serious impediment to accurate classification. IF the feeding means is such as to produce effective dispersion of the particles, then the Infrasizer MK III can be expected to perform well down to about 5 pm. Our current research is directed at the development of a feeder to achieve this objective.
LIST
OF SYIUBOLS
0
contamination
of coarse
w&w, contamination
of
fine
product;
Q,(D)
product;
wiclwc diameter of spherical particle mean diameter of a sample cut size size of a particle that has probability p% of reporting to coarse stream feed rate, particles per set per unit length of feed slot coarse recovery; wcJwoC fine recovery; wfr/wor air velocity in Infrasizing region mass of coarse fraction part of coarse Fraction with D> D5,,.=w, -wcf part of coarse fraction with D < Dso, = wcoc(Dso) mass of fine fraction part of fine fraction with D > Dso, = WI - WLf part of fine fraction with D < Dso,
00(D) Or(D)
REFERENCES
6 7
= wfOrW,o)
8
mass of feed part of feed with D > DsO = wo - IL'"f part of feed with D < Dso, =
9
wooo(D,o) D2,/D,, = sharpness indes size sckctivity, or coarse grade efficiency(probability that a particle of size D reports to coarse stream)
standard deviation of diameter in sample cumulative percent by mass of coarse fraction with particle size less than D cumulative percent by mass of feed with particle size less than D cumulative percent by mass of fine fraction with particle size less than D
10
11 12
Thomas A_ Edison. U.S. Pat. 775.965. 1904. G. H. Otto and H. Rouse. Ciu. Eng.. 9. (‘i) (1939). H. Rumpf, Diss.. T. H. Karlsruhe, 1939_ K_ Leschonski, Chem. Ing. Tech.. 49 (19ii) 708 ‘719. V. Timbrell. in T. T. hlercer (Ed.). Assessnzen~ of Airborne Particles. Charles C. Thomas. 1979. Chap. 15. H. E_ T_ Haultain. Can. Olin. Mefall. Bull. (May 193’7) 301. S. Raimondo, A_ A_ Havz and B_ Etkin, The development of a horizontal elutriatorr the Introsizer MK III. Univ. of Toronto Institute for Aemspace Studies, Rep. 235. 19i9. B. Etkin and A. A. Haan. Proc_ Conf. on Fine Purtictes Processing. Vol_ 1, AWE. New York. 19so. pp_ 209 - 231. B. Etkin and P_ L_ E_ Goering. Phil. Trans. R- Sot. London. 51369 (1971) 5% - 543. B_ Etkin, A. A. Haaa. S. Raimondo and G. D’Eleuterio. inst. Chem. Eng_ SympSer.. (59) (1980) 5:3/l - 5 r3/93_ Equipment Testing Procedure. Particle Size ClassifieTs. AIChE, New York. 1980. T_ _411en. Particle Size Measruement, Chapman and Hall. London, 2nd edn., 1975.
30 (1981) 257 - 261 S.A., Laumnne - Printed
in The
Netherlands
Performance of the University of Toronto as a Monosizer and Multi-Cut Classifier A. H.
VON
FLOTOW
Utzioersily
of
(Received
September
and
Tororzto.
B. ETKIN
Institute 16, 1960;
forrlerospnce in revised
Studies. rorm
March
SUMMARY
This paper presents results of a series of experiments conducted to define the intrinsic performance of an air classifier that is characterized by a uniform low-turbulence flow field. For free-flowing spherical par-titles in the range tested (50 - 230 Dm); the performance, both as a multi-cut device and in monosize production, is excellent_ The sharpness of cut index is only a few percent lower than the ideal value of 1.00 and monosize fractions can be produced with standard deviation about 4% of the average diameter_
INTRODUCTION Air
sively
classifiers both
have
as laboratory
Infk-asker MK III
long
been
used
instruments
estenand
as
production machinery in industry where dry solid powders are processed_ There are numerous patents, going back to at least 1893, for devices that can be called cross-flow classifiers. Noteworthy among these is one by Thomas A_ Edison, 1904 [l] fora wind tunnel designed to separate gold from gravel. A later application of the wind tunnel was by Rouse and Otto, 1939 [ 21 for separating sand into a relatively large number of fractions_ Rumpf, 1939 133 initiated the well-known series of developments at Karlsruhe that led to the subsequent work of Leschonski [4] and others, and TimbreIl [5] developed the Aerosol Spectrometer in the 1950s. All of the above devices have in common that the working medium is air, and that the solid particles cross the streamlines. The separation is effected by the differential balance of aerodynamic, gravitational, and inertial faces, the latter two being the cause of particle trajectories crossing the stieamlinesA different class of device,
4325 Duffcrin
Street.
Dowr~svicw.
Ont..
M31i 51% (C:armdo)
6. 1961)
the vertical column elutriator, is illustrated by the Haultain Infrasizer [6] developed at the University of Toronto in the 1930s. The Infrasizer MK I11 [7, 81 was derived independently of previous work on crossflow classifiers. It was an outcome of research on the interaction of precipitation with wind and jets [ 91 , simultaneously with unrelated work on the Haultain Lnfrasizer. There is little evidence to show that crossflow air classifiers have in the past been designed with particular attention to minimizing those deviations of particles from their intended (ideal) trajectories that are -associated with turbulence and spatial non-uniformity of flow. The lnfrasizer MK III is such a device - i.e. one in which aerodynamic principles have been employed to reduce hot11 non-uniformity and turbulence to less than 1%. Reference [ 71 describes the evolution of the design and presents some of the results obtained in it_ Reference [S] contains an analysis of the factors that contribute to non-ideal performance, and Ref_ [ 101 describes an application to the classification of thin flakes_ It is the purpose of this paper to report the results of a series of experiments conducted to quantify the performance in conventional terms_ The results obtained are thought to represent close to the ultimate in precision of which this class of air classifier is capable. The two performance items emphassized are the production of monosized particles and the sharpness of cut at each of a multiplicity of cut points.
DESCRIPTION
OF
THE
APPARATUS
The principle of the Infrasizer MK III is shown schematically in Fig. 2. The main tunnel in those constructed is typically about
FEED
FINE STREAN
COARSE STREAM Fig. 1. Defbition
of mzsses.
20 - 25 cm in cross-section and about 100 - 125 cm long. (Four have been made, all different_) In addition to the main duct, the device has a variable-speed fan for establishing the flow, an entry section designed to produce a laminar uniform stream, and a system for feeding the particles_ The stream of solid particles entering from above typically arrives in the flow with a vertical velocity nearly eCpd to the settling speed, and forms a stream about 1 - 2 mm thick estending to within about 2 cm of the side walls. The rate of feeding is controllable over a wide range_ The four models have differed from one another in the details of these features [7-S] _ The multiple fractions are of course collected in the set of trays that cover the floor of the due i;_
FACTORS THE
AFFECTING
INFRASIZER
THE
PERFORhlANCE
OF
RIK III
The main factors that can lead to non-ideal performance are the following: spatial non-uniformity of the flow in the y direction, temporal fluctuations in the flow (turbulence). particle interactions, variable particle entry conditions, variable shape and orientation of particles, flocculation of partic!es. These are discussed in some detail in ref. [S] _ We have shown there that particle interactions can effectively be eliminated if the particle feed rate is less than some critical value; and when spherical particles of uniform density are used for testing, the shape/orientation factor is also eliminated. We also found that flocculation is not a significant factor for
Fig. 2_ Schematic
diagram
of Infrasizer.
glass spheres larger than about 30 pm, and hence confined the test to this range of size_ Our goal was to reduce the remaining non-ideal effects to the least possible. The lnfrasizer MK Ill, Model 4, represents the current stage of this development, and the results presented herein were obtained in that instrument. (Further improvements are being studied.) EXPERIMENTS
The present esperiments were performed with a feed material consisting of solid glass beads of uniform density in the size range 50 - 230 pm. Although the beads were predominantly spherical, some irregular glass fragments, ellipsoids and fused beads contaminated the feed. These non-spherical particles were not counted in the statistical analysis. The experiments were performed at room temperature and humidity; these factors were not monitored. Two feed rates were used, above and below the onset of significant particle interaction, as indicated by the parameter riLl’/V, [ 83 _ The air speed was chosen to use the available collection zone optimally. EXPERIMENTAL
RESULTS
A -Performance as a multi-cut classifier A perfect single-point classifier would be one in which all particles smaller than a certain size would go to the Ike stream while alI particles larger than that size go to the coarse
stream. Similarly, a perfect multi-cut classifier would direct each particle unambiguously to one of several output streams. Real classifiers of course perform imperfectly. Some material will always be found in the wrong output stream. This imperfection can be quantitatively described in a variety of ways. The notation and measures that we have used are those defined in ref_ 1111, with some minor modifications and additions (see Fig_ 1). For convenience, the main items are briefly reviewed below. Recovery The fine recovery is the ratio RDr = w[Jw,~, with a similar definition for the coarse recovery. A perfect classifier would have fine and coarse recoveries of unity_ Contamination The fines contamination is the mass fmction of the fine stream corresponding to particles larger than the cut size, ie. Cr = wrc/wf. Coarse contamination has a similar definition, C, = w,f/w,. A perfect classifier has contaminations of zero. Sharpness of ci~t The probability that a particle from the feed goes to the coarse stream is the ‘size selectivity’ or ‘coarse grade efficiency’, nn and is a function of particle size. The ideal form of no is a step function. In reality, it increases gradually from very small values for very small particles towards unity for the largest particles, indicating the presence of misplaced particles. With this form of vi, the cut size requires further definition, and the recovery and contamination values are no longer ideal. The cut size is normally defined to be the equiprobable size &-, -the size of
particles that have a 50% expectation
of re-
porting to either-stream. The deviation from ideal classification is characterized by the sharpness index - the ratio of the size of the particles with a 25% chance of reporting to the coarse stream to those with a 75% chance. This is denoted P = Dzs/D,s_ A sharpness index of unity would indicate ideal classification, and a value of less than 0.3 is considered poor [12] _ Experimental procedure Two samples of similar composition, each weighing 200 g, were classified_ The feed rates
were chosen to yield values of riD’/v, = 0.062 (Run A) and 0.66 (Run B) (conservatively based on D = 50 pm). These values of the particle-interaction parameter [S] were selected to ensure that Run _A had negligible particle interaction, and Run B had severe particle interactions_ The particles were collected in 24 bounccinhibiting trays distributed on the tunnel floor as shown in Fig. 2. The content of cacti tray was weighed and sampled. hlicrophotographs cf the samples were taken, and diameter measurements of 100 particles yielded values for the mean, standard deviation and a histogram of diameter distributions for each tray. These data were processed t.o yield the curves of Figs. 3 - 5. The average diameter is seen to vary smoothly with downstream distance (Fig. 3), the shape of the curve reflecting approsimately the law for settling velocity at low Reynolds number, i.e. V, - D’, whence D - (s/H)~~“‘. The standard deviation (Fig. 4) is seen to exhibit appreciable scatter, in part from sampling error and in part from measurement error. The latter was determined to be about 1% of D. The precision is best in the range x = II - 11If, and for low feed rate, when u/D - 4%. Figure 5 shows the diameter range associated with 90% of the particles_ This information is a useful supplement to the standard deviation, since the distributions are not necessarily Gaussian_ For esamplc, at 75 ;Im mean diameter, Fig. 5 shows that 90% of the particles lie in a 12 Mm band. To evaluate the performance as a single-cut device, the 24 output streams were mathematically combined into two streams, one including all trays upstream of the division (the coarse stream), the other including all trays downstream of the division (the fine stream)_ This was done for eleven different locations of the dividing line to yield the curves of Figs_ 6 - 10. Figures 6 and 7 are representative of many others and illustrate how the various performance parameters were obtained_ Figure 6 is for a cut after tray No. 9, and shows the computer-generated values of no from which fi was obtained_ Note that the abscissa is a very expanded scale. Figure 7 shows the feed, coarse and fine distributions for this cut, together with the recovery and contamination values. The departure of the dotted line from the solid line on Fig. 7 represents the extent of non-ideal behaviour- It
.
Low
-
High
Feed Feed
Rate Rate
0.062 0.66
x
Fig. 4. Ratio of standard deviation to diameter,
o/d
be seen that at both feed rates used, the Infrasizer performs exceptionally well as a single-point classifier. The sharpness of cut indes, Fig. 10, is typically greater than 0.8 and can be as large as 0.97 ; the fine and coarse recoveries, Figs. 6, 9, lie between 95 and 100%; and the fine and coarse contaminations are less than 5%. The low recovery and large may
(each-point
represents one tray)_
contamination of the fine fraction at small cut size (II,,, < 80 pm) and of the coarse fraction at 1-e cut size @so > 160 pm) are of course simply refkctions of the difficulty of making a precise cut near the tail of the distribution. The effect of feed rate is seen quite clearly on Figs. 8 - 10. The high rate is about 10
-2 E 2. -0
Q
0
6..
O_
8 i..
-909.
”
l
90% of Particles Lie Eelween Dushed Liws Low Feed Rate. fiD2/Vx = 0.062
2
5
.
Range
b
Y/H
Fig. 5. 90% limits of diameter.
High Feed Rate. Cut Afler Troy 9
#/V_
--_-.
- 066 .
-------------------ae
8
P s
I
-I .
m d.
----
I
‘fD’5
1:
----------------
OI
.+-
4-p”
8 d
. 4_%a.=
-aem-
c3.m
*-
.r3
DIA
Fig_ 6_ Example
l
9.. ?rn.M
I pm
I
-1 1
III.cc
I
, I-
’
! ! _B 0..3 1n.m
In.’
Ilb.C3
1
of T)D for high feed rate.
times the low rate, and is well within the range of strong interaction. Nevertheless, the sharpness of cut has not deteriorated very much. It remains generally above 0.85, and the maximum (on the faired curves) has only decreased from 0.97 to 0.92. The effect of high feed rate on the f%ne &action performance (recovery and contamination) is also modest, whereas the coarse fraction is more influenced, especially the contamination by fines_ This suggests that there are fine particles entrained
in the wakes of coarse ones at the high feed rate, a hypothesis that fits very well the interaction concept reported in ref. [ 8]_ B -Performance in monosize production -4 feature of the Infrasizer MK III is the possibility of producing avariety of monosized samples simultaneously from a single feed stream (Figs. 3, 4). Both the mean size and the diameter range (above a certain minimum) of the monosize sample are controllable. The size range of each sample is controlled by the width of the collection tray. In this study the collection region was subdivided in such a way that the size range of each output sample was approximately proportional to its mean size. In order to study the repeatability of particle trajectories and the value of multi-stage operation, several products of a run identical to Run A in operating conditions, but differing slightly in feed composition were saved and processed further_ Each sample was passed through the Inkrsizer several times and its mean and standard deviation determined after each pass. These results are summarized in
163
COARSE 4
CD1
FEED.
f f
STREAM. \
./-
/
.-‘-FINE
STREAM,
High nD=/Vx
Fig. 7.
Exnmple FINES
of cumulative
Recovey
and
+o (Dl
distributioms;
feed,
Feed
t&(D)
Rate,
= 0.66
fine and coarse.
COARSE
Contamination
Recovery
and
D,,,
Fig.
8_ Fine
recovery
and contamination_
Fig. ll_ As espected, the mean size of ah samples remained within a few pm from run to run, and the standard deviation decreased or remained constant_ The small variations in mean size (bc-j.1 increases and decreases) are probably a coni;- quence of the sampling error
Fig.
9. Co a-se
recovery
Contamination
lw=)
and contamination.
sample consisted of between 60 and 100 particies. -Another expected result is a steady increase in yield, for example from 60% on the second pass to 80% on the seventh. A yield of less than 100% is caused by the loss of particles
-each
4 ,OO~~_~~~~~~~~___ I_
___P_t~----z------_~_
,.__z------.eo--
-_--
Y
0
0
-
Perfecl
--------------n -
-
---LF-----.
0
y
Performance
c
EnP .‘a--
d/VI.
=-
Low
Feed
Role
0
High
Feed
Role
0062 0.66
.20--
Fig. 10. Sharpness of cut, fl= D&D-i5_ COMMENTS
40 I
Fig. 11.6 runs_
*
4
5
NUMSEA OF
PLSSES
3
and u Ior succwive
6
passes - monosizing
that land outside the collection tray. These would consist of both particles that do not belong in the collection tray but erroneously arrived there on the previous passes, and of particles that do belong in the collection tray but erroneously arrive in a neighbouring one. The result of these losses of unwanted and borderline particles should be 2 narrowing of the range of particle size in the output stream from one iteration to the next. This is reflected by the values of standard deviation given in Fig. 11.
AND
CONCLUSIONS
The Infrasizer MK III has been shown to be a classifier of considerable merit for freeflowing particles with equivalent aerodynamic diameters in the range 50 - 230 pm. As a multi-cut device, the values of 0 as large as 0.97 speak for themselves_ In monosize production, samples with standard deviations of (5% of mean diameter are obtainable with a single pass. No certain statements can be made about the performance for particles outside the size range tested. However, experience with the apparatus suggests that quite comparable performance can be expected with much larger particles, up to say 3 mm, provided that the collection means is designed to trap the particles and prevent them from bouncing into the wrong tray. It is intrinsic to this classifier, as to others, that particles will only follow the correct trajectories if they do not stick together to form clumps. We have not found this to be a serious problem for particles generally larger than about 40 pm. However, for smaller ones, flocculation may be a serious impediment to accurate classification. IF the feeding means is such as to produce effective dispersion of the particles, then the Infrasizer MK III can be expected to perform well down to about 5 pm. Our current research is directed at the development of a feeder to achieve this objective.
LIST
OF SYIUBOLS
0
contamination
of coarse
w&w, contamination
of
fine
product;
Q,(D)
product;
wiclwc diameter of spherical particle mean diameter of a sample cut size size of a particle that has probability p% of reporting to coarse stream feed rate, particles per set per unit length of feed slot coarse recovery; wcJwoC fine recovery; wfr/wor air velocity in Infrasizing region mass of coarse fraction part of coarse Fraction with D> D5,,.=w, -wcf part of coarse fraction with D < Dso, = wcoc(Dso) mass of fine fraction part of fine fraction with D > Dso, = WI - WLf part of fine fraction with D < Dso,
00(D) Or(D)
REFERENCES
6 7
= wfOrW,o)
8
mass of feed part of feed with D > DsO = wo - IL'"f part of feed with D < Dso, =
9
wooo(D,o) D2,/D,, = sharpness indes size sckctivity, or coarse grade efficiency(probability that a particle of size D reports to coarse stream)
standard deviation of diameter in sample cumulative percent by mass of coarse fraction with particle size less than D cumulative percent by mass of feed with particle size less than D cumulative percent by mass of fine fraction with particle size less than D
10
11 12
Thomas A_ Edison. U.S. Pat. 775.965. 1904. G. H. Otto and H. Rouse. Ciu. Eng.. 9. (‘i) (1939). H. Rumpf, Diss.. T. H. Karlsruhe, 1939_ K_ Leschonski, Chem. Ing. Tech.. 49 (19ii) 708 ‘719. V. Timbrell. in T. T. hlercer (Ed.). Assessnzen~ of Airborne Particles. Charles C. Thomas. 1979. Chap. 15. H. E_ T_ Haultain. Can. Olin. Mefall. Bull. (May 193’7) 301. S. Raimondo, A_ A_ Havz and B_ Etkin, The development of a horizontal elutriatorr the Introsizer MK III. Univ. of Toronto Institute for Aemspace Studies, Rep. 235. 19i9. B. Etkin and A. A. Haan. Proc_ Conf. on Fine Purtictes Processing. Vol_ 1, AWE. New York. 19so. pp_ 209 - 231. B. Etkin and P_ L_ E_ Goering. Phil. Trans. R- Sot. London. 51369 (1971) 5% - 543. B_ Etkin, A. A. Haaa. S. Raimondo and G. D’Eleuterio. inst. Chem. Eng_ SympSer.. (59) (1980) 5:3/l - 5 r3/93_ Equipment Testing Procedure. Particle Size ClassifieTs. AIChE, New York. 1980. T_ _411en. Particle Size Measruement, Chapman and Hall. London, 2nd edn., 1975.