AlllDfc, 4 7 T 3 3

NBSIR 82-2486

/.

The Measurement of Net Space

Charge Density Using Air

Filtration Methods

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Eiectrosystems Division

Washington, DC 20234

April 1 982

I —QC —

100

,U56

82-2486

1982

Prepared for: Office of Electric Energy Systems

Department of Energy

Washington, DC 20535

NATIONAL BURHAL

ON BTANDAHUS

UBBABT

MAY 2 5 1982

NBSIR 82-2486

THE MEASUREMENT OF NET SPACE

CHARGE DENSITY USING AIR no , FILTRATION METHODS /^>

R. H. McKnight

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

April 1982

Prepared for: Office of Electric Energy Systems

Department of Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
i
TABLE OF CONTENTS

Page

LIST OF FIGURES iv

Abstract 1

1. INTRODUCTION 1

2. DESCRIPTION OF AIR FILTRATION 3

3. EXPERIMENTAL APPROACH 4

3.1 Introduction 4

3.2 Measurements Using a Filter and Ion Counter 8

3.3 Measurements Made Using Two Filters 13

3.4 Accuracy of Measurements 13

4. RESULTS 14

4.1 HEPA Filter 14

4.2 Other Filters 17

5. DISCUSSION OF RESULTS 17

6. CONCLUSIONS 22

7. REFERENCES 23

i i i
LIST OF FIGURES

Figure 1

Figure 2

F i gu re 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Page

Experimental configuration for measurements in which the ion

counter is used to measure transmitted charge density 5

Experimental configuration for measurements in which a filter

is used to measure transmitted charge density 6

Low speed air flow facility used to produce a test volume

containing space charge. The filter and ion counter

configuration is shown in place 9

Characteristic I-V curve for the ion counter obtained for

a flow rate of 8.3 x 10-3 n 3/s and unfiltered inlet air.

For this measurement, the positions of the ion counter and

filter shown in figure 3 were interchanged. Also shown is

the space charge detected by the filter following the ion

counter 11

Characteri sti c I-V curve for the ion counter obtained for a flow rate of 8.3 x 10"3 m^/s and filtered inlet air. The

experimental arrangement was the same as for figure 4 12

Summary of HEPA filter transmission measurements. Shown for

comparison are the results of Moore, et a 1 . , Paltridge, and

the factory measured transmission (HOP test). Fi Iter-fi Iter

measurements--*-positi ve ions, o-negative ions. Filter-ion

counter measurements—+-positi ve ions, x-negative ions.

Filter-ion counter measurements (alpha source) —T-positi ve ions, 7-negative ions 15

Transmission through steel wool filter. Flow facility inlet

unfiltered-- ^-positive ions, o-negative ions. Filtered

inlet — A-positive ions. A- negative ions 18

. Transmission through commercial air filter. Flow facility

inlet unfiltered — A-positive ions, o-negative ions. Filtered

i nlet — A-positi ve ions. A- negative ions 19

. Transmission through "furnace" filter. Flow facility inlet

unfi Itered-- A-positi ve ions, A-negative ions. Filtered

inlet —A-positi ve ions, o-negative ions 2C

i v
THE MEASUREMENT OF NET SPACE CHARGE DENSITY

USING AIR FILTRATION METHODS

R. H. McKnight

The efficiency of a high efficiency particulate air

(HEPA) or absolute filter for removing charge from an air stream has been measured for a variety of space charge

and air flow conditions. Ion densities ranged from 10^

to 10 6 /cm3 and were for positive and negative space charge

as well as mixtures. For all conditions studied, the

transmission of the filter was less than 0.1%. For

space charge consisting predominantly of ions of one polarity, space charge density measurements made using HEPA

filters and ion counters may be compared directly. The

filter is well suited for all accurate measurements of net

space charge density. Three other types of fibrous filters

also have been studied.

Key words: corona discharge; HEPA filters; ion counters;

ion density; ions; net space charge.

1. INTRODUCTION

The measurement of electrical quantities which depend on the

ion density in air has been of interest to atmospheric electricity

researchers for many decades. These measurements with others have

provided information about various electrical processes in the

atmosphere [l].l More recently, similar measurements have been made

to characterize the electrical environment in the vicinity of high

voltage dc (HVDC) transmission lines [2], In addition to providing

information about external influences produced by the operation of

such lines, these measurements offer guidance to those involved in

experiments designed to investigate the biological effects of dc

fields and ions.

For the effort described in this report, the electrical

quantities of interest are the positive and negative space charge

densities and the net space charge density. In the following

discussion, the term space charge density will be used to refer to

measurements of either the positive or negative charge per unit

volume (C/m^) while net space charge density refers to the algebraic

sum of the positive and negative space charge densities. It is

possible to have near-zero net space charge density, but very large

positive and negative space charge densities.

^-Numbers in brackets refer to the literature references listed at the

end of thi s report

1
In the following discussion, the term "ion" will be used to

describe molecular-sized ions which have mobilities of approximately

10"4 m

2 /Vs. Other charge carriers, such as charged aerosols,

have much smaller mobilities. The small ions are of concern in this study.

It is common in the literature to find discussion of ion density,

that is, some number of ions per unit volume. This number is obtained

from space charge density measurements by assuming each ion carries

one electron charge. This is a reasonable assumption for molecular

ions, but is not justified for charged particulates which are usually

multiply charged.

Near the surface of the earth, the number of positive and negative

ions varies considerably, but is typically less than 10^ ions per cubic

meter. There is normally an imbalance between positive and negative

ions, with a ratio of positive to negative ion density of about 1.35.

By contrast, near the ground under a HVDC transmission line or in a biological exposure system, the number density may be as large as 1012-1013 ions per cubic meter. Recause of this substantially larger

ion density, the use of traditional measuring instruments and techniques

under these more severe conditions may result in significant errors.

The difficulty of the measurement problem is compounded by the presence

of dc electric fields which may be as large as 50 kV/m.

When using ion counters to measure space charge density, net

space charge density may be obtained by taking the algebraic sum of

the measured positive and negative space charge densities. If these

quantities are nearly equal, there can be a large uncertainty in

this difference. As a result, specialized techniques have been

developed to measure net space charge density directly. The three

most commonly used approaches involve air filtration methods, double

field mills to measure the electric field gradient, and a determi nati on of electrical potential caused by space charge by means of a Faraday

cage. Each of these approaches is thoroughly described in the literature

(the work of Vonnegut and Moore [3] presents an excellent summary of

previous efforts).

Although Obolensky [4] is usually first credited with using a filter technique to measure net space charge, Zeleny [5], in fact, used

filtration methods to study the space charge distribution between two

parallel plates as early as 1898. In the Filtration method, a stream

of air is drawn through a filter medium which is connected to ground

through a sensitive ammeter. Some fraction of the charge in the air

stream is removed by the filter. By measuring the volumetric air flow

6 and the current I, the net space charge density is calculated as

p net = I / 4

> ( 1 )

2
The net space charge density determined by (1) will be in error if a non-negl i gi bl e fraction of the ions in the air stream passes through

the filter.

Various filter materials have been used in previous investigations

including cotton, steel wool, and different types of commercial filters.

The highest efficiency filters available from commercial sources are high efficiency particulate air (HEPA) filters. These filters, sometimes

called "absolute" filters, remove more than 98% of 0.3 ym (micrometer)

particulates from an air stream as measured in a standard test procedure.

The filter material in these devices is a glass cloth made of very fine

fibers. Vonnegut and Moore [3] first described the use of these

filters in net space charge measurements. Other authors including

Bent [6], Paltridge [7], and Moore, et al . [8] describe their work

with HEPA filters and each presents evidence that these filters

exhibit a high efficiency for removing charge from an air stream.

In each case, the conductivity of air was reduced by about 98% in

passing through the filter. Filtration methods have also been used

as space charge detectors in experiments designed to study charge

generation during tank washing in large oil tankers [9] and in work

concerned with downstream charge generation in air conditioning

systems employing electrostatic air filters [10].

2. DESCRIPTION OF AIR FILTRATION

A vast literature exists which describes air filters and- air

filtration processes for many different operating conditions. There

is much more experimental than theoretical work because of the

complexity of the theoretical formalism describing particulate motion

in a complex flow situation. There is little published research

involving charged particles, electrified filters or both. For the

purpose of the present work, a few general references will suffice

and the interested reader may consult these works for further

information. Discussions of filtration mechanisms and some earlier

experimental results may be found in works by Chen [11], Green and

Lane [12], and Dormon [13], An early description of a high efficiency

paper filter is given by Smith and Stafford [14], Gillespie [15]

has considered the role of electrical forces in the filtration process.

When electrical forces are ignored, there are three distinct processes

which are dealt with in discussions of filtration mechanisms. These

processes are diffusion, interception, and inertia. The diffusion

processes are most important for smaller particles. Because of the

Brownian motion of the particles, they can diffuse across flow lines

and impact the filter fiber. Larger particles are more likely to be

filtered by an inertial process in which the particle is unable to

follow the flow lines diverted by the filter fiber and is thereby

captured. Interception results from the geometrical cross section of

the filter fiber itself. Diffusion and inertial effects are functions

of the speed of the particles, while all three depend on filter

construction and particle size.

3
If either or both the particles and the filter are charged,

additional mechanisms arise because of various electrical forces as described by Gillespie [15]. In theoretical and experimental work

reported in the literature, these forces increase the probability

that a charged particle is intercepted by the filter.

It is not clear to what extent the available theory can be used

to predict how effectively a filter will remove small ions from an air stream, since these are molecular-sized particles. In an exposure

system, filtered air will probably be used with the result that there

will be very few larger charged particles in the ion composition.

Outdoors, near a HVDC line, it is quite likely that there will be

substantial numbers of charged particulates. For these, existing

filter data may be applicable.

3. EXPERIMENTAL APPROACH

3.1 Introduction

The purpose of this study was to determine the efficiency of

various air filters for removing ions from an air stream passing

through the filter for different air flows and space charge conditions.

In principle, this measurement is made by determining the space charge

density at the inlet and the outlet of the filter. Previous

investigators [6-3] measured the change in conductivity of air

before and after passing through the filter by using a Gerdien tube

to measure conductivity. The present measurements were made using

an experimental arrangement in which the filter under test was followed by a second device which was used to measure the space

charge transmitted through the filter. The experimental arrangements

used are shown in figures 1 and 2, which are geometries using a filter and ion counter and two filters respectively.

The ion counter and the various filters used are expected to have

responses which depend on the mobility spectra of the incident ions.

This aspect of the experiment is discussed in more detail later, but by

using two different techniques to detect the transmitted space charge

it is possible to check for inconsistencies in the experimental procedure.

The transmission of the filter under test is defined as

T(percent) = ( p y/ p i)100 = ( p y/

P y+ P p) ) 100 (2)

where py is the transmitted charge density, p-j is the incident

charge density and p is the charge density determined by the

filter. F

4
•FILTER UNDER TEST

Experinpntal configuration for measurements in which the ion

counter is used to measure transmitted charge density

Figure 1 .
FILTER TO DETECT

TRANSMITTED CHARGE —j j~ FILTER UNOER TEST

Figure 2. Experimental configuration for measurements in which a filter

is used to measure transmitted charge density
The air containing space charge is drawn from a test chamber in

a low-speed air flow facility designed to produce a sizable volume

containing a net space charge. This system, which is described

elsewhere [16], is shown in figure 3. An air stream enters the

inlet and passes through a series of screens to reduce turbulence.

After leaving the screens, the air stream encounters a planar, coronadischarge, ion source. Nine vertical wires, 64 urn in diameter, are

located equidistant between two grounded screens. The wi re-to-screen

spacing is 0.1 m. Ions are carried downstream from the ion source,

through a transition and into the test section. It has been shown

in earlier work that this facility can produce a space charge which

is temporally smooth and spatially uniform to within a few percent

over almost the entire cross sectional area of the test section.

There is a gradual decrease in space charge density downstream from

the source region due to ion losses to the walls. The ion source

can be operated with either positive or negative dc supply and for

the work presented here the operating voltage is 10 kV. Corona

onset for both positive and negative voltages is between 5 and 6 kV.

The space charge density produced in the test volume depends on the

air speed at the ion source. Because the volumetric air flow through

the system is much larger than that extracted by the absolute filter

assembly, the perturbation on the ion density produced by the sampling

process is negligible. The inlet of the air flow facility has been

configured so that a battery of HEPA filters can be inserted in the

air stream. For this operating condition, the air passing through

the corona region should be substantially free of parti cul ates . In the present set of measurements, the use of the inlet filters

resulted in a substantial reduction in air flow through the system.

The fan used to move the air was unable to handle the pressure drop

across the filters (a new fan will be installed for future experiments).

At the reduced air flow, the ion density was approximately 5 x 104/cm3,

and this led to experimental difficulties which are described later.

In a second set of measurements, the corona source was not

used. Instead an alpha particle source was placed in the inlet tube

leading to the filter assembly. By using this source, large positive

and negative ion densities (>106/cm3) were obtained with near zero

net space charge density. Therefore, the filter efficiency could be

determined for a wide range of space charge density conditions.

The filter assembly consists of a commercial 20.3 cm x 20.3 cm HEPA filter, mounted on an insulating support. All air flowing

through the assembly passes through the filter. Current from the

filter to ground is measured using an electrometer, and the volumetric

flow rate was determined by means of a flowmeter. The air

stream containing space charge enters the filter assembly through a 7.6 cm long tube 3.81 cm in diameter. Provision is made to mount a prefilter in front of the HEPA filter to reduce clogging. For measurements of the HEPA filter efficiency, the prefilter is removed.

7
3.2 Measurements Using a Filter and Ion Counter

For the measurements made using the filter and ion counter

(fig. 2), a paral lei -plate ion counter [17] was configured to be in

series with the filter assembly. Polarizing potential for the ion

counter was provided by batteries and the current from the counter

measured with an electrometer. The ion counter used was constructed

in the laboratory and had been used in an extensive set of measurements

so that its operation was well characteri zed . Counters of this type

are extremely microphonic, and this defect, as discussed later, proved

to be a severe limitation in the measurements described here.

The efficiency of the HEPA filter for removing space charge

from an air stream was determined for positive and negative ions as a function of volumetric flow rate through the filter. For these

measurements, the space charge was drawn from the test section (fig. 3),

and the ion counter was used to detect the transmitted space charge.

The initial measurements indicated that substantially less than 1% of

the space charge density passed through the filter, confirming the

results of earlier investigators.

The ions which make up the space charge have a range of mobilities.

Since the response of both filters and ion counters to a space charge

is mobility dependent, this results in some uncertainty in the measurements. During the course of the work described here, no means of

determining the mobility spectrum was available in the laboratory.

Ion counters are cha racteri zed by a critical mobility. This

quanti ty i s defi ned as

K c = e<t>/CV (3)

where e is the permittivity of air, the volumetric flow rate, C the

counter capacitance and V the polarizing voltage. All ions with

mobilities equal to or greater than K c will be collected in the

counter. A smaller fraction of less mobile ions will be collected as well. Some information about the ion mobilities can be obtained by

varying the polarizing voltage and measuring the output current from

the counter. In principle, the mobility spectrum can be obtained by

manipulating the I-V data, but this process involves two numerical

derivatives and is very uncertain. As a result the ion counter

measurements were used only to provide some information about the

mobility limits of the ions involved.

Characteri sti c I-V curves were recorded using the ion counter

sampling directly from the air flow facility test chamber. These

measurements were made for different flow rates through the counter.

8
—2.44 m 1.22 m *U 1.22 m

Test Section Transition Inlet

Handling System

inure 3. Low soeed air flow facility used to produce a test volume

containing soace charge. The filter and ion counter

configuration is shown in place.

9
Examples of the space charge density versus polarizing voltage

curves obtained with the ion counter in front of the HEPA filter are

shown in figures 4 and 5. Here the difference is that the air entering

the air flow facility was unfiltered for figure 4, while in figure 5 the inlet air was filtered through HEPA filters. The charge density

produced with filtered inlet air is substantially less than with

no filters as was discussed earlier. The uncertainty in these

measurements is less than ±8%.

The response of the ion counter has saturated at 20 V (volts)

polarizing potential, but there is still a small ion component being

detected by the filter. For the filtered inlet, this component is

2% and for the unfiltered inlet 8% of the total ion density. The

substantial space charge indicated at the lowest polarizing voltage

results from the field due to the space charge itself which causes

ions to be precipitated to both the collecting plates and the

polarizing plates.

Shown on figures 4 and 5 are the critical mobilities for different

polarizing potentials. The small fraction of ions passing through

the counter appear to have mobilities less than 0.005 x 10-4m2/Vs.

A similar series of measurements was made for all the flow conditions

used for the present experiments. In all cases, a polarizing voltage

of 400 V was sufficient to ensure complete collection of any small

ions passing through the filter. The polarizing voltage of 438 V used during the efficiency measurements was battery supplied.

Volumetric flow rates through the system under test were determined

using either a turbine flowmeter or a hot wire mass flow transducer.

Air speeds at the filter inlet were measured with a hot film anemometer

system. During the measurements reported here, which extended over

several months, the laboratory temperature varied from 21-23 °C, and

relative humidity, which was not controlled, ranged from 30 to 75%.

For the measurements described above, the space charge was almost

entirely of one sign. As indicated earlier, an alternative approach

was taken to determine the response of the filter to conditions

where there were large numbers of both positive and negative ions

present, but in nearly equal quantities so the net space charge

density was near zero. This condition was achieved by mounting a small alpha particle source in the inlet tube to the filter. The

space charge density produced by the alpha source was determined by

mounting the source at the ion counter inlet and maintaining the same air speed across the source as would be used in the transmission

measurement. The space charge density determined by this measurement

was used in eq (1) to determine the transmission of the filter for

these conditions. Unlike the case of space charge consisting of

ions of only one sign, the filter itself cannot be used to determine

this information since it responds to the net space charge which for

the alpha source is approxi mately zero. It was observed that the

10
Net

Space

Charge

(C/m

3)

x

10

figure 4. Characteristic I-V curve for the ion counter obtained for

a flow rate of 8.3 x 10 “^ ^3/$ a p r{ unfiltered inlet ai r . For this measurement, the positions of the ion counter and

filter shown in figure 3 were i nterchanged. Also shown is

the space charge detected by the filter followina the ion

counter

11
I I 1 I 1 I

' I

I - : I I - r I

1 5 10 50 100 500

ION COUNTER VOLTAGE (VOLTS)

figure 5. Characteristic I-V curve for the

flow rate of R.3 x 10*^ m^/s and

experimental arrangement was the

ion counter obtained for a filtered inlet ai r . x he same as for figure 4.

12
space charge density depended on the air speed near the source, but

that for all conditions the positive and negative space charge densities

were the same to within experimental uncertainty.

3.3 Measurements Made Using Two Filters

As noted above, the HEPA filter was found to have a transmission

of less than 1% and was therefore suitable for use as a detector

of transmitted space charge in a scheme designed to measure filter

transmission as shown in figure 2. Transmissions of three different

filters were measured using this method and these results are discussed

later. It was found that one of the filters tested transmitted less

than 10% of the incident ions for the test conditions of interest.

As a result, the roles of the HEPA filter and the test filter were

exchanged. The test filter then acted as the detecting filter in a measure of the HEPA filter transmission. This testing methodology,

which appears at first to be circular, was used to check for possible

inconsistencies in the experimental procedure. The ion counter and

filter are expected to have mobility dependent response, and some evidence for this is seen in the results described below. The filterfilter method cannot be used where the incident space charge is not

predominantly of one polarity, since a filter responds to the net

space charge and is not polarity selective.

3.4 Accuracy of Measurements

As noted earlier, the parallel plate ion counter is microphonic

and as a result of vibration of the counter caused by both air flow

through the system and mechanical coupling to the air flow facility,

a noise signal was produced which was for some measurements a substantial fraction of the desired signal. Because of this, data

were acquired by recording the electrometer output on a strip chart

recorder and subtracting the background signal from the "ion-on"

signal. For the lowest flow rates, where the current from the counter

was smallest, this subtraction resulted in considerable uncertainty.

When the inlet was filtered, the available space charge density

dropped by about a factor of 10. This reduction resulted in a transmitted space charge which was too small for meaningful measurements

to be made when the HEPA filter was the device under test. As a consequence, there are no transmission results presented for a filtered inlet for the HEPA filter. For the other filters, which

transmitted a larger fraction of the incident space charge, signal

levels were high enough to obtain useful data.

Volumetric flow measurements have an estimated uncertainty of 5% and

were made with a turbine flowmeter. Air speed measurements were made with

a hot film anemometer operated near the lower end of the calibrated range.

As a result, air speed values are subject to considerable uncertainty.

Since the results displayed depend on volumetric flow rates and not

directly on the air speed, there was no effort made to obtain an improved

calibration. The el ectrometers used to measure currents are accurate

13
to 3%. For the filter-filter arrangement, there were no microphonic

problems, but the detecting filter itself has a transmission which is

uncertain to approximately 10%. As a result of these various errors,

the transmissions presented for the HEPA filter have relatively

large uncertainty, which can be as large as 0.005% in the value for

the transmission. This does not affect the usefulness of the HEPA

filter, since the transmission of charge is so small that other

sources of error dominate, i.e., measurement of volumetric air flow

and current.

4. RESULTS

4.1 HEPA Fi 1 ter

Three different types of measurements were made during the

investigation reported here. They are (1) transmission through the

HEPA filter using the ion counter as a transmitted space charge

detector, (2) transmission through other filter materials using the

HEPA filter as a detector, and (3) transmission through the HEPA

filter using a second filter as a detector. The results from (1) and

(3) will be presented together to facilitate discussion and maximize

clarity. The initial set of measurements made using the filtercounter arrangement indicated that the HEPA filter transmitted

substantially less than 1% of the incident ions for all conditions

examined. Because of this, it was decided to use the HEPA filter in

determining the transmission of three other filter materials. These

transmissions were measured and one of the three filters (the commercial

filter) was selected for use as a detector in a remeasurement of the

HEPA filter transmission. During the course of the third set of

measurements, the transmission of the filter was observed to increase

substantially. This increase occurred after the filter system unit

had been disassembled for maintenance. After this observation, the

system was again opened and the filter carefully inspected for damage.

There was no visual indication of change in the physical state of

the filter. As a result of this, all earlier data were discarded and

repeats were made of previous runs. Only the more recent data are presented here. It would be necessary to return the filter to the

manufacturer for recerti fi cati on to confirm that a small leak in the

filter assembly had occurred. This information would not affect the

present results but might offer an explanation for the change in

transmission of charge.

All of the transmission numbers obtained using both filtercounter and filter-filter configurations are summarized on figure 6.

The data points displayed represent the average of several measurements

and the width of the error bars are indicative of the spread in the

data

As noted earlier, the results presented here were determined in

terms of volumetric flow rate and not directly as a function of the

air speed at the face. The air speed values shown were measured using

14
i gu re

10p

1 : Moore et al (1962)

c

O)a

o

Q.

C

,o

£

0)c

CO

<u

0.1

0.01

0.001

Paltridge (1967)

Ii X

Filter Transmission

(DOP Test)

2 4

Air Speed, m/s

h. Summary of HEPA filter transmission measurements . Shown c or comDarison are the results of Moore et al , D altridge and the

factory measured transmission (DOP test). Fi 1 ter-f i 1 ter

neasurements--«-positi ve ions, o-negative ions. Filter-ion

counter neasu rements --+-pos i ti ve ions, x-negative ions.

Filter-ion counter measurements (alpha source )--T-posi ti ve ions, 7-negative ions
a hot film anemometer near the lower end of the calibrated range. As

indicated previously, these numbers are subject to considerable uncertainty.

This uncertainty does not affect the results presented. The choice of

face velocity rather than volumetric flow rate for use in presenting the

data was made to facilitate comparison with previous data. Volumetric

flow rates ranged from (3.3-18.3) x 10‘3m 3/ Si

Transmission factors measured with the filter-filter arrangement

were consistently higher than those obtained using the filter-counter

configuration. There are a number of reasons for this observation.

The second filter is tightly coupled to the HEPA filter so there are

no ion losses between the filter elements. When the ion counter is

used as a detector, there is a travel path of approximately 25 cm between filter and counter so ions can be lost by transport to the walls.

The ion counter has a definite critical mobility for each operating

condition so some lower mobility ions pass through the counter. The

detecting filter may be more efficient at removing these ions from the

air stream. As the volumetric flow rate decreases, the critical

mobility becomes smaller, and there is some indication of convergence

between ion counter and filter results as this occurs.

The lowest transmission factors were observed for the situation

where the net space charge density was near zero, but the positive

and negative space charge densities were high. A number of interesting

features enter into explaining this result. Perhaps the most important

is that for this space charge condition there is a very small

contribution of slow ions to the total space charge. The air speed near the source is high (5 to 24 m/s) so little diffusional charging

is expected since the particles spend little time in the space charge.

There are no applied electric fields so there is no field charging.

This situation is contrasted to that where the ions are produced in

the corona source. Here both field and diffusion charging mechanisms

are important [18].

If the surface of the filter becomes charged during exposure to

a unipolar ion environment, el ect rostati c forces between the surface

charge and an ion near the surface would tend to decrease the

probability of collecting the ion. When the space charge is neutral,

this mechanism would not come into play and an increased ion removal

probability would be expected. There was indication of some charging

of the filter during these measurements. During data acquisition, the

current to the filter remained constant over periods of tens of

minutes, which suggests that the charging was not large enough to cause a repulsion of ions at the front of the filter. However, when

the ion source was turned off, the output current from the filter

did not drop completely to zero. There was a small (less than 5%)

residual current which decayed to zero in a period of several minutes.

No extensive investigation was made of this phenomena, since part of

the current was presumably due to space charge in the laboratory

reci rculating through the air flow facility.

16
4.2 Other Filters

Three other filter materials were examined to determine their

usefulness in measuring space charge. These filters were (1) a densely

packed steel wool filter ~ 4 cm thick, (2) an inexpensive 2.5 cm "furnace" filter, and (3) a commercial filter of pleated design with

a manufacturers' efficiency rating of 30-35% based on a standard test.

For these measurements, the HEPA filter was used as a detector of

transmitted charge. Figures 7, 8, and 9 show the transmission of

these three filter materials determined for ion spectra produced with

and without inlet filters on the air flow facility. The data plotted

in figures 7 and 8 are the average transmission values determined for

these filters for all conditions studied. The error bars show a typical spread in the data, which were acquired over a several month

period. In figure 9, the data points represent the maximum transmission

values observed, while the error bar illustrates the spread in data.

There is a strong dependence on the nature of the ions being measured

(i.e., air flow facility inlet filtered or unfiltered), but except

for the furnace filter little effect due to flow rate through the

medium. These filters transmit a significant percentage of the

incident ions, and would not be useful for accurate measurements.

The furnace filter shows a large variation with flow rate

and also exhibited the greatest variability from run to run. This

filter was operated substantially above its rated flow rates. The

furnace filter was mechanically weak and very nonuniform. The

results for this filter are present for comparison purposes only.

It is quite surprising that the transmission of ions through the

steel wool and the commercial filter is as high as measured. The steel

wool filter, in particular, was optically dense. However, the steel

fibers making up the wool were not fine when compared to the fibers

in other filters.

5. DISCUSSION OF RESULTS

The results presented here show clearly that for a wide range of

space charge conditions and flow rates the HEPA filter may be used to

accurately determine net space charge. For the particular filter

studied here, the transmission of the filter for all conditions

observed was less than 0.1%.

A number of experimental difficulties reduced the overall accuracy

of the measurements. As noted earlier, the parall el -pi ate ion counter

used was microphonic. At the higher flow rates this resulted in a significant noise level with the large polarizing voltage used.

The noise level combined with the low transmission of the filter to

limit the quality of measurements obtained with the ion counter. As

a result, it was not possible to determine an I-V curve for the ions

transmitted through the filter. This measurement would have been

extremely useful in estimating the mobility of the ions passing through

the filter.

17
o

f0)

20

c

0

U

0

Q.

C

o

<7> . - 0 10

c/) c

0

0

j i i i l 1 2 3 4 5 6

Air Speed, m/s

Figure 7. Transmission through steel wool filter. Flow facility inlet

unfiltered-- •-positive ions, o-negative ions. Filtered

ini et -- i-posi ti ve ions, A- negative ions

13
20 -

c

0

U

6

0

a

c

o

'</) co£

<nc

0

0

10

o

A ¥

A

I

i

A

A

0 1 2 3 4 5

AirSpeed, m/s

Figure 8. Transmission through commercial air filter. Flow facility

inlet unfiltered-- •-positive ions, o-regative ions. -ilte r ed

i nlet-- 4-posit i ve ions, a- negative ions

19
Figure 9. Transmission through "furnace 11 filter. Flow facility inlet

unfi ltered--A-positi ve ions, A-neqative ions. Filtered

i nl et --•-pos i ti ve ions, o-negative ions

20
Considerable day-to-day variation was observed in the measured

transmission. It is felt that this was a real effect and was a result

of being unable to carefully control the composition of air entering

the air flow facility. Although it was possible to place filters at

the inlet of the facility to remove particulates, the resultant drop

in net space charge produced in the test volume meant that reliable

measurements could not be obtained for this operating condition.

The differences in observed transmissions as measured by the

filter-ion counter and filter-filter technique appear to be due to

differences in the response of the detector instrument to the

transmitted space charge. Resolution of this question would require an enhanced measurement capability which would result from substantial

equipment modification and the development of a method for measuring

the mobility spectrum of the ions in the test volume. From a practical

standpoint, it is a moot point since net space charge measurements

can clearly be made with less than one percent error being due to ion

transmission through the filter for ions with the mobility spectra used

here.

The mechanisms by which small ions are precipitated out by the

filter are of interest. It is unlikely that inertia plays a significant role for these molecular-sized ions. Diffusion would be expected

to dominate with direct interception having only a small role. Roth

processes would be affected by electrical forces. If electrification

of the filter fiber is ignored, then the most likely electrical

force involves the charge and its image force. If a particle of

charge q is located a distance r from the center of a fiber of radius

R and dielectric constant E, the radial force on the particles due

to an image charge is [14]

q2 ( £o - £ ) Ffl = — ~

7

( 4

) 16ire

0 (r-R) 2

(

0 + e )

where e 0 is the dielectric constant of air. Paltridge [7] proposed

that a distance d 0 be defined such that at this distance the average

transl ati onal energy KT/2 be equal to the potential energy (here, K is Boltzmann's constant.)

q2 ( e0

- e ) do = (r-o-R) . (5)

4tT£qKT ( £ 0

- £ )

It is highly probable that an ion located (r 0 -R) < d 0 from a fiber

will move to the fiber and be neutralized. As a result, the effective

radius of the ion will be much greater than its physical radius. (It

should be noted that similar considerations are encountered when dealing

with charge transfer collisions between slowly moving ions and neutral

atoms or molecules.

21
If the surface of the filter medium becomes charged (say on the

surface) with the same polarity as the incident ion, then the situation

becomes more complicated and the net force may in fact tend to inhibit

the precipitation of the ion to the fiber. This may suggest a reason

for the difference in transmission between a unipolar space charge

and one in which the net space charge is effectively neutral. For the

zero net space charge, it would be expected that the fibers would not

on the average become charged. Because of this, there would be no repulsive force between ion and fiber which may be the case for a space charge made up of ions of one polarity.

One force that has not been described is that due to the space

charge itself. At the high ion densities studied here, there is a significant electric field generated due to the space charge which

will in effect cause the ions to separate and be forced to the filter

material. Again for the zero net space charge condition this field

is absent. In this case, however, the argument would say that the zero net space charge effect would be less than that for a unipolar

charge density, which is opposite to what is observed.

It is clear that it is unlikely that a satisfactory description

of the filter process will be forthcoming. The physical state of the

surface of the filter material itself during the time that there is

space charge near the filter is unknown. This lack of adequate theory,

however, does not detract from the attractiveness of using the filter

for net space charge measurements.

6. CONCLUSIONS

The efficiency of a HEPA filter for removing charge from an air

stream has been measured for a variety of space charge and air flow

conditions. For all conditions studied the transmission of the

filter was less than 0.1%. The mobility spectrum of the ions was

not measured but the ions were predominantly small ions. There was

some indication that there was a small amount of filter charging.

No adequate theoretical development is available to explain the

results of these measurements. Diffusion is expected to be the

dominant loss mechanism enhanced by electrical forces of which image

forces should be the most important.

Other filter materials studied have transmissions of 5-15% and

are not suitable candidates for accurate measurements.

Because of the low transmission of the HEPA filter, accurate

measurements can be made of space charge densities where the ions are predominately of one sign. Under these conditions, measurements made

using the filter and an ion counter may be directly comparable,

depending on the mobility spectrum of the ions. Although much more work is needed, the HEPA filter appears to form the basis for a means

of calibrating ion counters.

22
7. REFERENCES

[1] Chalmers, J. A., Atmospheric Electricity, 2nd ed . (Pergaman Press,

Oxford, 1967).

[2] Bracken, T. D., Capon, A. S., and Montgomery, D. V., Ground Level

Electric Fields and Ion Currents on the Cel i 1 o-Sylmar ±400 KVDC

Intertie During Fair Weather, IEEE Trans. PAS97, 1 978, p. 370.

[3] Vonnegut, B. and Moore, C. B., A Study of Techniques for Measuring

the Concentration of Space Charge in the Lower Atmosphere, (Final

Report, Contract AF 191604), 1920, 31 Jan. 1958.

[4] Obolensky, W. N., Uber elektrische Ladungen in der Atmosphare, Ann.

Phys. Lpz 77_,

1 925, p. 644.

[5] Zeleny, J., On the Ratio of the Velocities of the Two Ions

Produced in Gases by Rontgen Radiation; and on Some Related

Phenomena, Phil. Mag. _46, 1898, p. 120.

[6] Bent, R. 8., The Testing of Apparatus for Ground Fair Weather

Space Charge Measurements, J. of Atm. and Terr. Phy. _26 , 1 964,

p. 313.

[7] Paltridge, G. W. , The Retention of Atmospheric Radioactivity by

Fibrous Filters, J. of Geo. Res. 72_,

1 967 , p. 1 269.

[8] Moore, C. B., Vonnegut, B. and Mallabon, F. J., Airborne Filters

for the Measurement of Atmospheric Space Charge, J. Geo. Res., 66_,

1961 , p. 3219.

[9] Owen, T. E. and Spiegel, R. J., Bulk Carrier Operations Safety

Enhancement Project-Phase II, Volume I I-Shi ptank Electrostatic

Model Studies, Final Report for Contract S-38044, U.S. Dept, of

Commerce, 1978.

[10] Penney, G. W. and Hewett, G. W. , Electrically Charged Dust in

Rooms, Trans. AIEE 68_, 1 949, p. 278.

[11] Chen, C. Y., Filtration of Aerosols by Fibrous Media, Chem. Rev.

55_, 1 955, p. 595.

[12] Green, H. L. and Lane, W. R., Particulate Clouds: Dusts, Smokes,

and Mists, 2nd ed. E. & F. N. Spon, Ltd. (London), 1 964.

[13] Dormon, R. G. , Theory of Fibrous Filtration in High Efficiency Air

Filtration, White, P. A. F. and Smith, S. E., editors, Butterworths

(London), 1964, p. 67.

23
[14] Smith, W. J. and Stafford, E., Development of a Dry Fibrous Filter,

in Air Pollution, McCabe, L. C., editor, McGraw Hill, 1952, p. 264.

[15] Gillespie, T., The Role of Electric Forces in the Filtration of

Aerosols by Fiber Filters, J. Colloid, Sci. J_D, 1 955, p. 299.

[16] McKnight, R. H., Kotter, F. R., Misakian, M., and Ortiz, P., 1980

Annual Report: Electric and Magnetic Field Measurements, Nat. Bur.

Stand. (U.S.), NBSIR 81 -2267 (1 981 ).

[17] Knoll, M., Eichmerer, J. and Schon, R. W. , Properties, Measurement

and Bioclimatic Action of "Small" Multimolecular Atmospheric Ions,

in Advances in Elec, and Electron. Phy., Marton, L. ed, T9, 1964,

p. 178.

r 18] White, H. J., Industrial Electrostatic Precipitation, Addi son-Wesley

1963.

24
NBS-1 14A 'rev. 2 -ac) U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2486

2. Performing Organ. Report No. 3. Publication Oate

April 1982

4. TITLE AND SUBTITLE

The Measurement of Net Space Charge Density Using Air Filtration Methods

5. AUTHOR(S)

R. H. McKnight

S. PERFORMING ORGANIZATION (If joint or other than N 8 S, see in struczion s) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

EA-77-01 -601

..Tas k Nn . A018 -.EF_S

8. Type of Report 3t Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADORESS ( Street , City. State, ZIP ) Department of Energy

Office of Electric Energy Systems

Washington, DC 20585

10. SUPPLEMENTARY NOTES

_

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT i A 200-word or less factual summary of most si gnificant information, if document incluaes a significant

Oi bl i o graohy or literature survey, mention it here)

The efficiency of a high efficiency particulate air (HEPA) or absolute filter for

removing charge from an airstream has been measured fojc a var^ety^of space charge

and air flow conditions. Ion densities ranged from 10 to 10 /cm and were for

positive and negative space charge as well as mixtures. For all conditions studied,

the transmission of the filter was less than 0.1%. For space charge consisting

predominantly of ions of one polarity, space charge density measurements made using

HEPA filters and ion counters may be compared directly. The filter is well suited

for all accurate measurements of net space charge density. Three other types of

fibrous filters also have been studied.

12. KEY WORDS (Six to twelve entries; alohaoetical order; capitalize only proper names; and separate <ey words :y s,emicoions)

corona discharge; HEPA filters; ion counters; ion density; ions; net space c.narge .

13. AVAILABILITY

~X~ Unlimited For Official Distribution. Do Mot Release to NTIS _ Order From Suoenntenaenc of Documents, U.S. Government Printing Office, <Vasn in gton , D.C. 20402.

14. NO. OF

PRINTED PAGES

28

1 15. Price

_j£_ Order From National Technical Information Service (NTIS), Soringfieia, VA. 22161 i i .30

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