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A preliminary study of a magnetic precipitator for small airborne magnetic material
Atmospheric Environment Pergamon Press 1970. Vol. 4, pp. 213-217. Printed in Great Britain.
A PRELIMINARY
STUDY OF A MAGNETIC
SMALL AIRBORNE
MAGNETIC
PRECIPITATOR
FOR
MATERIAL
(First received 16 June 1969; and in jinaI form 1 August 1969) Abstract-A type of magnetic filter suitable for gas-borne magnetic particles in the Aitken nucleus sire-range is constructed. The techniques involved in its investigation are those commonly employed in aerosol studies on particles of this size. Its collection efficiency for particles of h 10m5 cm radius at different rates of flow through the tilter is measured. The source of variations in efficiency is discussed. It is shown both theoretically and in practice that the collection efficiency is greater for larger than for smaller particles in the sire-range examined. 1. INTRODUCTION IN RECENTyears several studies have been conducted on the application of the Pollak-Nolan photoelectric nucleus counter (NOLAN and POLLAK,1946; METNIEKSand POLLAK,1959) to the testing of aerosol filters and electrostatic precipitators (MEOAW and WJFFEN,1963; DAMS and CLIFIQN, 1966; SILVERMAN and MCGREEW, 1967; McGrumvv, 1967). These investigations have demonstrated the suitability of this counter for the in situ testing of the efficiencies by particle count of air filter units and it is now being used on a routine basis for such purposes. The investigation described here seeks to show how this approach to the evaluation of air cleaning and air sampling methods may be extended to a simple type of magnetic precipitator suitable for the assessment of an airborne mass of microscopic magnetic particles. Apart from the obvious use of such a precipitator either as a filter of these particles or the separation of iron ores, it may be also adapted for experimental work involving micromagnetics, domains, etc., especially where these investigations involve the formation of an aerosol of disperse magnetic material. The force on a particle in an inhomogeneous magnetic field is of the form
where, if we confine ourselves to magnitudes, Zf is the magnetic field and M is the particle magnetic moment = IV where Z = intensity of magnetization and V = volume of the particle. If Z is the same for all particles in a given aerosol, the force on an individual particle will be proportional to its volume. Assuming that the field gradient is constant, that the particles are spherical and that Stokes’ law applies to them, we may write for the drift velocity Win the magnetic field
w= Z.$rr3 x magnetic field gradient 6qr
2
= jfr_ x magnetic field gradient, tf
where r is the particle radius and r~ is the viscosity of the air. It follows that the larger particles are more mobile in the magnetic field and will be removed preferentially.
2. EXPERIMENTAL
PROCEDURE
The aerosols we have used were either of iron filing dust or of finely powdered ferrite. The test apparatus used is illustrated in FIG. 1. The large radiosonde rubber balloon, A, of diameter 150 cm is first filled with nucleus-free air through a ftne filter. Particles, consisting of either the iron dust or powdered ferrite, were introduced by blowing filtered air into the balloon through a glass vessel which contains small quantities of these substances. Blowing in this fashion transfers the tiner particles into the balloon, leaving the coarser particles behind. Inevitably a certain amount of non-magnetic dust becomes entrained in the aerosol. After the aerosol has had time to reach a fairly uniform particle distribution in the balloon, samples are drawn into the counter for concentration measurement from one point in the balloon with the aid of a vacuum pump. The aerosol flow-rate in the tube 213
214
Tech&al Notes
leading from the balloon to the counter is regulated by a stopcock, & and measured by a rotameter. On the line from the ~~WXI to the counter the aerosol passes between the poies of a magnet. The aerosol is in direct contact with the poles which are enclosed in a tubular container. A conical inlet and outlet (not sketched) are fitted to the container to ensure a smooth uniform flow pattern past the faces of the pole-p&es. Simple bawes surround the poles to ensure that the entire air-flow passes between them. The malplet used in these investigations was a permanent magnet of strength N 3000 oersteds extending over a volume of 5 cm 3. The separation between the pole-pieces was 1.5 cm. The concentration of particles entering the counter, D, at different flow-rates, with and without the magnetic field, is obtained. The Pollak-Nolan photoelectric nucleus counter is used for counting aerosol particles with radii lying approximately between 10m5and 10q7 cm. in the counter, the aerosol sample is saturated with water-vapour by the damp walls of the counter chamber and the particles are converted into visible fog droplets by the adiabatic cooling of the aerosol as a result of increasing the pressure in the chamber and then suddenly releasing it. Each particle forms a droplet so that a
TO
ftor
6
VAC.
PUMP
Fro. 1. Block diagram of apparatus used to investigate magnetic aerosols. A, Rubber balloon container; B, Magnetic poles in tubular container; C, Cylindrical condenser; D, Polk&-Nolan counter; E, Stopcock. count of the droplets gives the particle concentration. The Polk&--Nolan counter is a relative instrument in the sense that the droplets are not counted directly but by the extinction produced in a lightbeam which passes through the fog formed by the expansion. The intensity of the lightbeam, before and after the fog is formed, is recorded by means of a photocell and microammeter. The extinction produced is a function of the number of droplets. The counter is calibrated by reference to a stereophotomicrographic counter of the Aitken type (P~LLAKand DALY, 1958; POLLAKand METNIEK~, 1959). As a result of various design improvements (~OLLAKand MURPHY,1953; POLLAKand O’CONNOR,1959), two precision counters, model 1957, agree in 72 per cent of all comparisons to a difference of 3 per cent in nucleus concentration. Where one counter is used for the purpose of obtaining relative concentrations as in the experiments described below, self-consistent results reproducible to 1 per cent of the concentration or better at low concentrations, are obtained (MCGREWY,1966). The average size of the particles is usually measured by obtaining the fraction of particles that are charged when the aerosol is brought to electrical charge equilibrium by ionizing radiation (KEEFE, NOLANand RICH, 1959; RICH, POLLAKand METNIEKS,1959). The fraction of charged particles is measured by passing them, on their way to the counter, through the cylindrical condenser, C, across which a saturation ele&rical field can be placed. Alternatively the average particle size may be measured by their rate of diffusion in a diffusion chamber (NOLANand GUBRRINI,1935; POLLAKand Mm, 1957).
Technical Notes 5. DISCUSSION
215
OF METHOD
The analysis of the magnetic material is by a difference method and we do not normally dect it for observation. However, the precipitated material is deposited on the poles of the magnet and it may be inspected if necessary. If the magnetic field is 100 per cent efficient in removing all the magnetic particles present, this technique enables us to find in a simple way the magnetic condition by particle count of an aerosol consisting of a mixture of magnetic and non-magnetic particles. HOWever, the interpretation of the magnetic condition of a particle merely on the basis of 100 per cent efficiency of removal by a magnetic field is subject to a certain amount of restriction. In sticiently strong magnetic fields or at extremely small rates of flow, even paramagnetic particles will be attracted out of the aerosol towards the magnetic poles. As a force acts on a particle only in a non-~0~ magnetic field, it follows that the degree of non-uniformity of the latter should not be too great or the high efficiency of collection leads to a certain ambiguity in the results. In this connection it should be stated that in an auxiliary experiment the magnetic field which we have used failed to remove, even at the lowest flow-rates, any room-air nuclei or amorphous iron oxide particles which we sent through it, so that it would appear to have displaced only the strongly magnetic particles in the main investigation. We have also studied another possibility in a separate series of experiments, namely that charged particles in an aerosol would be removed by the force on moving charges in a magnetic field. The result of these initiations was negative, showing that either the velocity or the magnetic field or both were too small to exert on charged particles in the Aitken nucleus range a force which was sufficient to remove them from the air-stream. It is worth noting that, while we have placed the magnetic pole pieces in contact with the air-stream in our experimental sat-up, this arrangement is not necessary, The magnetic fieId was able to remove magnetic particles from the air-stream when placed outside the flow tube, though less efficiently than when in contact with the air-flow. When the magnet was brought near the counter it was able to reduce the concentration of such particles in the counter itself. 4. DISCUSSION
OF MEASUREMENTS
The average size of the particles employed in these studies was about 2.11Y5 cm radius. For particles of this size there is a certain difficulty in concentration measurement associated with the use of the Polk&-Nolan counter, as spontaneous fogging interferes with the calibration of the counter. This can be avoided by keeping the particle concentration low and using non-hygroscopic particles. At the concentration we employed we did not experience any difficulty on this score. However, there are further diiculties involved with both electrical and diffusion methods of measuring particles above the 10ms cm size range so that our particles size measurements are not very precise. The aerosols which we investigated were all polydisperse but we did not attempt to determine the particle size ~s~bution at any time as it was obvious that it was changing rapidly. The larger particles were quickly lost from the balloon by sedimentation. The sedimentation veiocity for spherical particles of iron of radius 5*1O-5 cm is approximately 2.5 cm min-’ so that all particles with radii greater than this should have disappeared after 1 hr. The loss of smaller particles by coagulation and diffusion to the balloon walls may be neglected, for the size of particle employed and at the concentrations used, compared with sedimentation losses. As a result of these decay mechanisms the concentration of a typical aerosol of ferrite particles dropped from 2062 cm-j to 1092 cmq3 in 2 hr. 5. RESULTS TABLB1 shows how the efficiency of the magnetic field varies with flow-rate for the particles of a sample ferrite aerosol at 2 different times during its decay. The measurements at 1@30were made 1 h after the particles were introduced into the balloon. The variation in efficiency with flow-rate given in TABLE 1 is attributed to the preferential loss of larger particles by sedimentation in the connecting tubing, In order to verify this we performed the following experiment. We differentially removed the larger particles from the aerosol stream by passing them through a sedimentation cell in the form of a diffusion box lying horizontally. We found that at all flow-rates the magnetic efficiencies were greater for the aerosols with a higher percentage of large particles. Thus at 1 l/min flow-rate, when 37 per
216
Technical Notes
cent of the incoming particles is removed in the sedimentation box, the efficiencies of the magnetic precipitator were 46 per cent and 29 per cent respectively for the larger and smaller particle mixtures. This result also implies that the drop in efficiency at all flow-rates between 1630 (1 h old aerosol) and 18.25 (3f h old aerosol) in TASLE 1 is due to sedimentation of the larger particles in the balloon itself. TABLE 1. VARIATIONwrrn FLOW-airs OF EFFICIENCYOF PARTICLERJMOVALBY MAGNE’IKFIELD Time 16.30 1 h after particles introduced into balloon. 18.45 3) h after particles introduced into balloon.
Flow-rate(I/min)
6
4
2
1
Efficiency (per cent)
17
18
29
35
42
(per cent)
12
12
12
31
33
EBciency
05
The difference in efficiency with particle sire can be attributed to either or both of two factors, (i) The magnetic field is more efficient in the removal of large particles as shown in the introduction, (ii) The proportion of magnetic particles was higher among the larger ones. A brief examination of the results of different experiments shows that the second factor is less important. To give one example, when the particle concentration dropped by a factor of SO’%, due mostly to the disappearance of the larger particles, the efficiency only decreased from 35% to 30% of the remaining particles. Thus many of the large particles must be non-magnetic. From a comparison of the variation in flow-rate with efficiency for different aerosols, it appears that the decrease in efficiency with increasing flow-rate is roughly exponential in character. If we take a monodisperse aerosol of magnetic particles and assume an exponential decrease in efficiency with increasing flow-rate, we may write for the efficiency
where W is the drift velocity of the particles to the magnetic poles, V is the velocity of the aerosol past them, L is the length of the inhomogeneous field region parallel to and D its width perpendicular to the direction of flow. This expression, which is analogous to that of DEUKWH (1922) for an electrostatic precipitator, will not be strictly obeyed in our results due to the fact that not all the particles in our aerosols are magnetic and due also to the polydispersity in sire among the magnetic particles present. It should be mentioned that efforts to increase the inhomogeneity of the magnetic field by inserting thin irregular pieces of iron between the pole pieces produced the following result. The efficiency of particle removal was increased by making the magnetic field more non-uniform but the effect was more pronounced at the larger flow-rates. In one experiment the efficiency of particle removal was increased by factors of 59,28 and 14 per cent respectively at flow-rates of 10 1,41 and 1 I/min when a more non-uniform field was used. This result is to be expected, since it is reasonable to suppose that efforts made to increase the efficiency of removal would be better rewarded at high flow-rates than at low flow-rates when most of the magnetic particles present are in fact already being removed by the inhomogeneity of the unmodified magnetic field. Finally it should be stated that the efficiency of the magnetic field was less for the iron filing particles than for the ferrite particles at corresponding flow-rates. 6. CONCLUSION These investigations, which are of a preliminary nature, illustrate the possibilities and limitations of a novel type of precipitator suitable for microscopic magnetic particles. Improvements in the design
Technical Notes
217
of the magnetic field, preferably by using an electromagnet so that the field strength could be altered as desired, would be the next step in this study. It should be emphasized that the aerosol instrumentation employed here is better suited to particles of slightly smaller sire range than those studied, i.e. it is more easily applied both as regards concentration and particle sire measurement to aerosols of < 10m5 cm radius. Sedimentation losses among large particles are so high that the population and average size changes too rapidly to make accurate quantitative measurements of such items of interest as (i) magnetic efficiency as a function of particle sire, (ii) the intensity of magnetization of the particles, (iii) coagulation coefficients of magnetic particles of different sires-all of which should be obtainable by these methods as the equipment is developed. Acknowledgement-I wish to express my best thanks to Emeritus Professor L. F. BATESof the University of Nottingham for criticism and suggestions. G. MCGREEVY St. Patrick’s College, Maynooth, Co. Kildare, Eire REFERENCES DAVIS R. R. and CLIFTONJ. J. (1966) The use of a Pollak counter for in situ testing of high efficiency filters. Jour. Rech. Atnws. 2,365-374. DEUTSCHW. (1922) AnnlnPhys. 68,335. KEEFeD., NOLANP. J. and RICH T. A. (1959) Charge equilibrium in aerosols according to the Boltzmann law. Proc. R. Ir. Acad. 6OA, 27-45. MCGREEVYG. (1966) Anomalies in measurements made with a Polk&-Nolan photoelectric nucleus counter. Arch. Met. Geophys. Bioklim. 15A, 90-108. McGaaavv G. (1967) The evaluation of the performance of an electrostatic precipitator using a Pollak-Nolan nucleus counter. Atmospheric Environmenf 1,87-95. MEGAW W. J. and WIFFENR. D. (1963) The efficiency of membrane filters. Znt. J. Air Wat. Polka. 7,501-509. METNIEKSA. L. and POLLAKD. W. (1959) Instruction for use of photoelectric condensation nucleus counters. Geophys. Bull. Dubl. No. 16. NOLAN J. J. and Guam V. H. (1935) The diffusion coefficients and velocities of fall in air of atmospheric nuclei. Proc. R. Ir. Acad. 43A2,5-24. NOLAN P. J. and POLLAKL. W. (1946) The calibration of a photoelectric nucleus counter. Proc. R. Ir. Acad. 51A2,9-3 1. POLLS L. W. and MURPHYT. (1953) Comparison of photo-electric nuclei counters. Geofis. pura appl. 25,44-60. POLLAK L. W. and O’CONNOR T. C. (1955) A photoelectric nucleus counter of high precision. Geojis. pura appl. 32,139~146. POLLAKL. W. and METNIEKSA. L. (1957) On the determination of the diffusion coefficient of heterogeneous aerosols by the dynamic method. Geofi. pura appl. 37,183-190. POLLAKL. W. and DALY J. (1958) An improved model of the condensation nucleus counter with stereo-micrographic recording. Geoj7.r.pura appl. 41,211-216. POLLAK L. W. and MEIKI~K~ A. L. (1959) New calibration of photo-electric nucleus counters. Geofis. put-a appl. 43,285-301. RICH T. A., POLLAK L. W. and METNIEKSA. L. (1959) Estimation of average size of submicron particles from the number of all and uncharged particles. Geofi. pura appl. 44,233-241. SILVERMANL. and MCGREEVYG. (1967) Application of the Pollak-Nolan nucleus counter to the routine testing of air filters. Atmospheric Environment 1, l-10
H
A
A PRELIMINARY
STUDY OF A MAGNETIC
SMALL AIRBORNE
MAGNETIC
PRECIPITATOR
FOR
MATERIAL
(First received 16 June 1969; and in jinaI form 1 August 1969) Abstract-A type of magnetic filter suitable for gas-borne magnetic particles in the Aitken nucleus sire-range is constructed. The techniques involved in its investigation are those commonly employed in aerosol studies on particles of this size. Its collection efficiency for particles of h 10m5 cm radius at different rates of flow through the tilter is measured. The source of variations in efficiency is discussed. It is shown both theoretically and in practice that the collection efficiency is greater for larger than for smaller particles in the sire-range examined. 1. INTRODUCTION IN RECENTyears several studies have been conducted on the application of the Pollak-Nolan photoelectric nucleus counter (NOLAN and POLLAK,1946; METNIEKSand POLLAK,1959) to the testing of aerosol filters and electrostatic precipitators (MEOAW and WJFFEN,1963; DAMS and CLIFIQN, 1966; SILVERMAN and MCGREEW, 1967; McGrumvv, 1967). These investigations have demonstrated the suitability of this counter for the in situ testing of the efficiencies by particle count of air filter units and it is now being used on a routine basis for such purposes. The investigation described here seeks to show how this approach to the evaluation of air cleaning and air sampling methods may be extended to a simple type of magnetic precipitator suitable for the assessment of an airborne mass of microscopic magnetic particles. Apart from the obvious use of such a precipitator either as a filter of these particles or the separation of iron ores, it may be also adapted for experimental work involving micromagnetics, domains, etc., especially where these investigations involve the formation of an aerosol of disperse magnetic material. The force on a particle in an inhomogeneous magnetic field is of the form
where, if we confine ourselves to magnitudes, Zf is the magnetic field and M is the particle magnetic moment = IV where Z = intensity of magnetization and V = volume of the particle. If Z is the same for all particles in a given aerosol, the force on an individual particle will be proportional to its volume. Assuming that the field gradient is constant, that the particles are spherical and that Stokes’ law applies to them, we may write for the drift velocity Win the magnetic field
w= Z.$rr3 x magnetic field gradient 6qr
2
= jfr_ x magnetic field gradient, tf
where r is the particle radius and r~ is the viscosity of the air. It follows that the larger particles are more mobile in the magnetic field and will be removed preferentially.
2. EXPERIMENTAL
PROCEDURE
The aerosols we have used were either of iron filing dust or of finely powdered ferrite. The test apparatus used is illustrated in FIG. 1. The large radiosonde rubber balloon, A, of diameter 150 cm is first filled with nucleus-free air through a ftne filter. Particles, consisting of either the iron dust or powdered ferrite, were introduced by blowing filtered air into the balloon through a glass vessel which contains small quantities of these substances. Blowing in this fashion transfers the tiner particles into the balloon, leaving the coarser particles behind. Inevitably a certain amount of non-magnetic dust becomes entrained in the aerosol. After the aerosol has had time to reach a fairly uniform particle distribution in the balloon, samples are drawn into the counter for concentration measurement from one point in the balloon with the aid of a vacuum pump. The aerosol flow-rate in the tube 213
214
Tech&al Notes
leading from the balloon to the counter is regulated by a stopcock, & and measured by a rotameter. On the line from the ~~WXI to the counter the aerosol passes between the poies of a magnet. The aerosol is in direct contact with the poles which are enclosed in a tubular container. A conical inlet and outlet (not sketched) are fitted to the container to ensure a smooth uniform flow pattern past the faces of the pole-p&es. Simple bawes surround the poles to ensure that the entire air-flow passes between them. The malplet used in these investigations was a permanent magnet of strength N 3000 oersteds extending over a volume of 5 cm 3. The separation between the pole-pieces was 1.5 cm. The concentration of particles entering the counter, D, at different flow-rates, with and without the magnetic field, is obtained. The Pollak-Nolan photoelectric nucleus counter is used for counting aerosol particles with radii lying approximately between 10m5and 10q7 cm. in the counter, the aerosol sample is saturated with water-vapour by the damp walls of the counter chamber and the particles are converted into visible fog droplets by the adiabatic cooling of the aerosol as a result of increasing the pressure in the chamber and then suddenly releasing it. Each particle forms a droplet so that a
TO
ftor
6
VAC.
PUMP
Fro. 1. Block diagram of apparatus used to investigate magnetic aerosols. A, Rubber balloon container; B, Magnetic poles in tubular container; C, Cylindrical condenser; D, Polk&-Nolan counter; E, Stopcock. count of the droplets gives the particle concentration. The Polk&--Nolan counter is a relative instrument in the sense that the droplets are not counted directly but by the extinction produced in a lightbeam which passes through the fog formed by the expansion. The intensity of the lightbeam, before and after the fog is formed, is recorded by means of a photocell and microammeter. The extinction produced is a function of the number of droplets. The counter is calibrated by reference to a stereophotomicrographic counter of the Aitken type (P~LLAKand DALY, 1958; POLLAKand METNIEK~, 1959). As a result of various design improvements (~OLLAKand MURPHY,1953; POLLAKand O’CONNOR,1959), two precision counters, model 1957, agree in 72 per cent of all comparisons to a difference of 3 per cent in nucleus concentration. Where one counter is used for the purpose of obtaining relative concentrations as in the experiments described below, self-consistent results reproducible to 1 per cent of the concentration or better at low concentrations, are obtained (MCGREWY,1966). The average size of the particles is usually measured by obtaining the fraction of particles that are charged when the aerosol is brought to electrical charge equilibrium by ionizing radiation (KEEFE, NOLANand RICH, 1959; RICH, POLLAKand METNIEKS,1959). The fraction of charged particles is measured by passing them, on their way to the counter, through the cylindrical condenser, C, across which a saturation ele&rical field can be placed. Alternatively the average particle size may be measured by their rate of diffusion in a diffusion chamber (NOLANand GUBRRINI,1935; POLLAKand Mm, 1957).
Technical Notes 5. DISCUSSION
215
OF METHOD
The analysis of the magnetic material is by a difference method and we do not normally dect it for observation. However, the precipitated material is deposited on the poles of the magnet and it may be inspected if necessary. If the magnetic field is 100 per cent efficient in removing all the magnetic particles present, this technique enables us to find in a simple way the magnetic condition by particle count of an aerosol consisting of a mixture of magnetic and non-magnetic particles. HOWever, the interpretation of the magnetic condition of a particle merely on the basis of 100 per cent efficiency of removal by a magnetic field is subject to a certain amount of restriction. In sticiently strong magnetic fields or at extremely small rates of flow, even paramagnetic particles will be attracted out of the aerosol towards the magnetic poles. As a force acts on a particle only in a non-~0~ magnetic field, it follows that the degree of non-uniformity of the latter should not be too great or the high efficiency of collection leads to a certain ambiguity in the results. In this connection it should be stated that in an auxiliary experiment the magnetic field which we have used failed to remove, even at the lowest flow-rates, any room-air nuclei or amorphous iron oxide particles which we sent through it, so that it would appear to have displaced only the strongly magnetic particles in the main investigation. We have also studied another possibility in a separate series of experiments, namely that charged particles in an aerosol would be removed by the force on moving charges in a magnetic field. The result of these initiations was negative, showing that either the velocity or the magnetic field or both were too small to exert on charged particles in the Aitken nucleus range a force which was sufficient to remove them from the air-stream. It is worth noting that, while we have placed the magnetic pole pieces in contact with the air-stream in our experimental sat-up, this arrangement is not necessary, The magnetic fieId was able to remove magnetic particles from the air-stream when placed outside the flow tube, though less efficiently than when in contact with the air-flow. When the magnet was brought near the counter it was able to reduce the concentration of such particles in the counter itself. 4. DISCUSSION
OF MEASUREMENTS
The average size of the particles employed in these studies was about 2.11Y5 cm radius. For particles of this size there is a certain difficulty in concentration measurement associated with the use of the Polk&-Nolan counter, as spontaneous fogging interferes with the calibration of the counter. This can be avoided by keeping the particle concentration low and using non-hygroscopic particles. At the concentration we employed we did not experience any difficulty on this score. However, there are further diiculties involved with both electrical and diffusion methods of measuring particles above the 10ms cm size range so that our particles size measurements are not very precise. The aerosols which we investigated were all polydisperse but we did not attempt to determine the particle size ~s~bution at any time as it was obvious that it was changing rapidly. The larger particles were quickly lost from the balloon by sedimentation. The sedimentation veiocity for spherical particles of iron of radius 5*1O-5 cm is approximately 2.5 cm min-’ so that all particles with radii greater than this should have disappeared after 1 hr. The loss of smaller particles by coagulation and diffusion to the balloon walls may be neglected, for the size of particle employed and at the concentrations used, compared with sedimentation losses. As a result of these decay mechanisms the concentration of a typical aerosol of ferrite particles dropped from 2062 cm-j to 1092 cmq3 in 2 hr. 5. RESULTS TABLB1 shows how the efficiency of the magnetic field varies with flow-rate for the particles of a sample ferrite aerosol at 2 different times during its decay. The measurements at 1@30were made 1 h after the particles were introduced into the balloon. The variation in efficiency with flow-rate given in TABLE 1 is attributed to the preferential loss of larger particles by sedimentation in the connecting tubing, In order to verify this we performed the following experiment. We differentially removed the larger particles from the aerosol stream by passing them through a sedimentation cell in the form of a diffusion box lying horizontally. We found that at all flow-rates the magnetic efficiencies were greater for the aerosols with a higher percentage of large particles. Thus at 1 l/min flow-rate, when 37 per
216
Technical Notes
cent of the incoming particles is removed in the sedimentation box, the efficiencies of the magnetic precipitator were 46 per cent and 29 per cent respectively for the larger and smaller particle mixtures. This result also implies that the drop in efficiency at all flow-rates between 1630 (1 h old aerosol) and 18.25 (3f h old aerosol) in TASLE 1 is due to sedimentation of the larger particles in the balloon itself. TABLE 1. VARIATIONwrrn FLOW-airs OF EFFICIENCYOF PARTICLERJMOVALBY MAGNE’IKFIELD Time 16.30 1 h after particles introduced into balloon. 18.45 3) h after particles introduced into balloon.
Flow-rate(I/min)
6
4
2
1
Efficiency (per cent)
17
18
29
35
42
(per cent)
12
12
12
31
33
EBciency
05
The difference in efficiency with particle sire can be attributed to either or both of two factors, (i) The magnetic field is more efficient in the removal of large particles as shown in the introduction, (ii) The proportion of magnetic particles was higher among the larger ones. A brief examination of the results of different experiments shows that the second factor is less important. To give one example, when the particle concentration dropped by a factor of SO’%, due mostly to the disappearance of the larger particles, the efficiency only decreased from 35% to 30% of the remaining particles. Thus many of the large particles must be non-magnetic. From a comparison of the variation in flow-rate with efficiency for different aerosols, it appears that the decrease in efficiency with increasing flow-rate is roughly exponential in character. If we take a monodisperse aerosol of magnetic particles and assume an exponential decrease in efficiency with increasing flow-rate, we may write for the efficiency
where W is the drift velocity of the particles to the magnetic poles, V is the velocity of the aerosol past them, L is the length of the inhomogeneous field region parallel to and D its width perpendicular to the direction of flow. This expression, which is analogous to that of DEUKWH (1922) for an electrostatic precipitator, will not be strictly obeyed in our results due to the fact that not all the particles in our aerosols are magnetic and due also to the polydispersity in sire among the magnetic particles present. It should be mentioned that efforts to increase the inhomogeneity of the magnetic field by inserting thin irregular pieces of iron between the pole pieces produced the following result. The efficiency of particle removal was increased by making the magnetic field more non-uniform but the effect was more pronounced at the larger flow-rates. In one experiment the efficiency of particle removal was increased by factors of 59,28 and 14 per cent respectively at flow-rates of 10 1,41 and 1 I/min when a more non-uniform field was used. This result is to be expected, since it is reasonable to suppose that efforts made to increase the efficiency of removal would be better rewarded at high flow-rates than at low flow-rates when most of the magnetic particles present are in fact already being removed by the inhomogeneity of the unmodified magnetic field. Finally it should be stated that the efficiency of the magnetic field was less for the iron filing particles than for the ferrite particles at corresponding flow-rates. 6. CONCLUSION These investigations, which are of a preliminary nature, illustrate the possibilities and limitations of a novel type of precipitator suitable for microscopic magnetic particles. Improvements in the design
Technical Notes
217
of the magnetic field, preferably by using an electromagnet so that the field strength could be altered as desired, would be the next step in this study. It should be emphasized that the aerosol instrumentation employed here is better suited to particles of slightly smaller sire range than those studied, i.e. it is more easily applied both as regards concentration and particle sire measurement to aerosols of < 10m5 cm radius. Sedimentation losses among large particles are so high that the population and average size changes too rapidly to make accurate quantitative measurements of such items of interest as (i) magnetic efficiency as a function of particle sire, (ii) the intensity of magnetization of the particles, (iii) coagulation coefficients of magnetic particles of different sires-all of which should be obtainable by these methods as the equipment is developed. Acknowledgement-I wish to express my best thanks to Emeritus Professor L. F. BATESof the University of Nottingham for criticism and suggestions. G. MCGREEVY St. Patrick’s College, Maynooth, Co. Kildare, Eire REFERENCES DAVIS R. R. and CLIFTONJ. J. (1966) The use of a Pollak counter for in situ testing of high efficiency filters. Jour. Rech. Atnws. 2,365-374. DEUTSCHW. (1922) AnnlnPhys. 68,335. KEEFeD., NOLANP. J. and RICH T. A. (1959) Charge equilibrium in aerosols according to the Boltzmann law. Proc. R. Ir. Acad. 6OA, 27-45. MCGREEVYG. (1966) Anomalies in measurements made with a Polk&-Nolan photoelectric nucleus counter. Arch. Met. Geophys. Bioklim. 15A, 90-108. McGaaavv G. (1967) The evaluation of the performance of an electrostatic precipitator using a Pollak-Nolan nucleus counter. Atmospheric Environmenf 1,87-95. MEGAW W. J. and WIFFENR. D. (1963) The efficiency of membrane filters. Znt. J. Air Wat. Polka. 7,501-509. METNIEKSA. L. and POLLAKD. W. (1959) Instruction for use of photoelectric condensation nucleus counters. Geophys. Bull. Dubl. No. 16. NOLAN J. J. and Guam V. H. (1935) The diffusion coefficients and velocities of fall in air of atmospheric nuclei. Proc. R. Ir. Acad. 43A2,5-24. NOLAN P. J. and POLLAKL. W. (1946) The calibration of a photoelectric nucleus counter. Proc. R. Ir. Acad. 51A2,9-3 1. POLLS L. W. and MURPHYT. (1953) Comparison of photo-electric nuclei counters. Geofis. pura appl. 25,44-60. POLLAK L. W. and O’CONNOR T. C. (1955) A photoelectric nucleus counter of high precision. Geojis. pura appl. 32,139~146. POLLAKL. W. and METNIEKSA. L. (1957) On the determination of the diffusion coefficient of heterogeneous aerosols by the dynamic method. Geofi. pura appl. 37,183-190. POLLAKL. W. and DALY J. (1958) An improved model of the condensation nucleus counter with stereo-micrographic recording. Geoj7.r.pura appl. 41,211-216. POLLAK L. W. and MEIKI~K~ A. L. (1959) New calibration of photo-electric nucleus counters. Geofis. put-a appl. 43,285-301. RICH T. A., POLLAK L. W. and METNIEKSA. L. (1959) Estimation of average size of submicron particles from the number of all and uncharged particles. Geofi. pura appl. 44,233-241. SILVERMANL. and MCGREEVYG. (1967) Application of the Pollak-Nolan nucleus counter to the routine testing of air filters. Atmospheric Environment 1, l-10
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