BACKGROUND OF THE INVENTION
The present invention falls into a class of technology
and methods where gasborne charge-carriers are used to neutralize a charge imbalance
on insulating materials and floating conductors. The methods are applied in general
industry for static elimination to reduce hazardous and nuisance static discharges
and improve process operations and cleanliness.
Electrical static eliminators are used in many industries
to control unbalanced charges on insulating materials and floating conductors. Fig.
1 shows one example of a prior art static eliminator system including positive and
negative polarity corona ionizers 1, their environment 10, and a target 11. When
the ionizers 1 are distant from the target 11, gas flow 7 is used to convey the
products of ionization to the target. The corona ionizers 1 can be separate dc or
pulsed-dc emitters, or single emitters with alternating potential to separate the
positive and negative polarity corona in time.
The make-up of ions from a typical ionizer is very complex
and is far from understood. Many species are short-lived, and often highly reactive.
Most ionic species discussed in the literature are found in the interelectrode gap,
after ion molecule reactions have had time to develop. The ions and their distribution
also depend on the corona mode (e.g. glow or pulsed) that is active for the electrode
geometry, the gas, and the potential.
The carriers entrained from a corona by gas flow are only
beginning to be explored. However, it is becoming clear that only about 0.1% of
carriers generated in a corona are entrained, and the control of these carriers
is not achieved by trivial adjustment of positive and negative corona currents.
Conventional charge eliminators produce gasborne charge-carriers
of positive and negative polarity, so that the charge needed for static elimination
is attracted from the gas to charged articles. The equipment includes nozzles, blowers,
and room ionization systems where charged carriers are conveyed from electrical
corona to articles to be neutralized. Other ionizers are simply placed in chambers
where gas circulation conveys the charge-carriers to electrostatically charged articles,
or are static bars fitted with air knives or tubes perforated with an array of orifices.
The corona ionizers can consist of separate positive or negative polarity charge-carrier
generators for direct current (continuous or pulsed) ionization. Alternatively,
the ionizers can be single emitters or arrays of these emitters operated at alternating
A noted deficiency with conventional ionizers is that they
do not perform well in nitrogen, hydrogen, and noble (inert) gases, because control
is difficult where the gases are non-electron attaching. These ionizers also use
corona electrodes with two separate polarities or alternating polarity.
Nitrogen is used to inert processes in many industries,
and can purge areas cooled by the evaporation of liquid nitrogen. In recent years,
static eliminators using nuclear (radioisotope), ultraviolet, soft x-ray, and corona
discharge ionizers have been explored for use in nitrogen environments. Nitrogen,
hydrogen, and the noble gases pose special problems for electrical static eliminators,
since the negative carriers formed in the negative corona discharge are free electrons
and these do not readily attach to atomic or molecular nitrogen species. In industrial
applications, where the impurity is not always well controlled, there will be some
electron attachment, and the effective negative-carrier mobilities and negative
polarity corona current can vary over great ranges without significant effect and
control on carrier entrainment. The mobility effect is also influenced by temperature.
International PCT Publication No. WO 01/09999
entitled "IONIZER FOR STATIC ELIMINATION IN VARIABLE ION MOBILITY ENVIRONMENTS,"
designating the United States, now
U.S. Application No. 09/762,521
, which is incorporated by reference herein, balanced static elimination
is achieved in variable ion mobility environments using positive and negative polarity
corona emitters. The balance, however, is more difficult to control in high purity
nitrogen and at low temperatures where positive carrier generation must occur at
higher electric fields where the ratio of negative to positive polarity emitter
currents can exceed 1000 to 1.
Each of the alternative technologies (nuclear, UV, x-ray)
produces positive ion and free electron pairs in nitrogen. The balance of these
ionizers, however, is not easily controlled in air, let alone nitrogen gas and over
the temperature range of interest (i.e. 200 degrees K to 450 degrees K). Also, the
alternative ionizers can introduce radiation hazards to the work place. X-ray, radioactive
and UV ionizers pose radiation hazards in the environment and typically need to
be licensed or shielded for use in commercial applications. The corona type electrical
ionizer, on the other hand, does not need to be licensed as a source of ionizing
radiation, and operates in the current-limited mode throughout its useful life.
The performance of the corona type electrical ionizer does not decay over time as
will occur for at least the radioactive ionizer. The electrical ionizer is, therefore,
preferred if its balance can be controlled.
Many static eliminators have been proposed for use in industrial
environments. Some have claimed to be useful in nitrogen environments.
U.S. Patent 5,883,934 (Umeda
) describes that imbalance in the entrained carriers from ionizers can
be based on UV ionizer radiation brought into balance by a dc bias. The same is
true for ionizers based on corona ionizer activity and other forms of ionizing radiation,
such as UV and radioactive ionizers, which produce carrier pairs. Umeda, however,
does not recognize the importance of carrier mobility in bringing about balance
in gases such as nitrogen at low temperature. Thus, it is unlikely that balance
of this ionizer can be controlled in a non-electron-attaching environment by the
method proposed in the patent.
When positive and negative polarity corona emitters are
used as the corona source, balance can be achieved by adjusting the potentials on
the emitters. The ratio of currents from these emitters is shown in prior art Fig.
6 for gases 213 degrees K and 300 degrees K. The difficulty with the arrangement
of prior art ionizers such as those discussed in
, is that the control point (residual potential = 0) is achieved at large
current ratios or is not achieved at all at lowest temperatures. The ratio of currents
needed to achieve balance in nitrogen is shown in prior art Fig. 7 as a function
of temperature. The method described in
achieves the balance by operating the negative emitter at a high current
(limited) condition and adding positive-polarity corona current as needed to balance
BRIEF SUMMARY OF THE INVENTION
The present invention departs from conventional technology
by relying upon a single polarity corona to generate simultaneously both positive
and negative carriers and to balance this ionization using a corona-free dc bias
electrode to remove unwanted carriers. The invention is best practiced for use with
a negative polarity corona. Negative polarity corona generally contains an extended
corona structure that improves contact between positive and negative ions and gas
flow, and is especially suited for use in nitrogen, hydrogen, and inert gas environments
where there is an intense current-limited discharge. The choice of corona electrode
polarity is driven by the higher mobility of the negative carriers and their relative
abundance in the corona source.
Many balancing and self-balancing circuits have been developed
for electrical ionizers in air, but few have been designed for use in variable ion
mobility environments. The present invention offers improvement over existing balancing
circuits in nitrogen environments, such as described in
International PCT Publication No. WO 00/38484
entitled "GAS-PURGED IONIZERS AND METHODS OF ACHIEVING STATIC NEUTRALIZATION
THEREOF." Unlike conventional balancing circuits based on two polarity corona systems,
a single-polarity (negative) corona is controlled using a passive (corona-free)
control element. The complicated interaction of two corona systems, which could
separately have changing corona modes (morphology) is thereby avoided.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The foregoing summary, as well as the following detailed
description of preferred embodiments of the invention, will be better understood
when read in conjunction with the appended drawings. For the purpose of illustrating
the invention, there is shown in the drawings embodiments which are presently preferred.
It should be understood, however, that the invention is not limited to the precise
arrangements and instrumentalities shown.
In the drawings:
Fig. 1 illustrates the general arrangement of a prior art
Fig. 2 is an ionizer in accordance with the present invention
with point-to-plane electrode geometry;
Fig. 3 is a sectional view of an ionizer in accordance
with the present invention with needle-in-tube electrode geometry;
Fig. 4 is a side elevation view of an ionizer in accordance
with the present invention with needle in tube electrode geometry;
Fig. 5 is a functional schematic, of the power controls
for the electrical ionizer of the present invention;
Fig. 6 is a graph which illustrates the balance control
curves when positive and negative corona emitters are used as the corona source
at 213 degrees K and 300 degrees K;
Fig. 7 is a graph which illustrates the ratio of emitter
currents needed for balanced ionization in nitrogen as a function of temperature
from 200 degrees K to 400 degrees K;
Fig. 8 is a graph which illustrates that a negative corona
has a greater influence on target balance in air at 433 degrees K;
Fig. 9 is a graph which illustrates that a potential on
a sphere does not add carriers to the entrained stream in nitrogen at 300 degrees
K and 433 degrees K; and
Fig. 10 is a graph which illustrates that a potential on
a sphere does not add carriers to the entrained stream in nitrogen at 300 degrees
DETAILED DESCRIPTION OF THE INVENTION
I. OVERVIEW OF PRESENT INVENTION
Fig. 2 shows an ionizer 27 in accordance with one preferred
embodiment of the present invention. The ionizer 27 creates a corona current distribution
having a balanced flow of positive 8 and negative 9 ions in a variable ion mobility
gaseous environment 29. The balanced flow of positive and negative ions is directed
toward a workspace 14 or target 15 located in the gaseous environment 29 and downstream
from the ionizer 27. The ionizer 27 has a corona electrode 20 of negative polarity,
a counterelectrode 26 with an ion collecting surface; and a corona-free dc bias
electrode 23 of positive polarity. The ionizer 27 also has a control circuit 41,
shown in Fig. 5, which controls the output of the corona electrode 20 as a current
limited discharge so as to cause a balanced flow of positive and negative ions to
be emitted from the ionizer 27 and directed towards the workspace 14 or target 15,
thereby creating a static-free environment at the workspace 14 or target 15. The
ionizer 27 may also have a control circuit 41 that controls the potential on the
corona-free electrode 23.
The ionizer 27 may also comprise a corona electrode 20
that is an extended corona structure, thereby improving contact between positive
and negative ions and gas flow. Charge-carriers of positive and negative polarity
are entrained by gas flow through the negative polarity current limited discharge.
In one preferred embodiment illustrated in Fig. 2, the
corona-free electrode 23 is spherically shaped. However, other shapes are within
the scope of the invention, such as a wire or cylinder of sufficient diameter to
prevent corona (where the curvature of the surface is sufficiently large to prevent
Fig. 2 shows one embodiment of the ionizer 27 wherein the
corona electrode 20 is arranged in a point geometry, the counterelectrode 26 is
arranged in a plane geometry, and the corona-free electrode 23 is arranged in a
point geometry on the opposing side of the counterelectrode 26 from the corona electrode
Fig. 3 shows another embodiment of the ionizer 27 wherein
the corona electrode 30 is a needle electrode, the counterelectrode 36 is arranged
in a ring or tube geometry about the corona electrode 30, and the corona-free electrode
33 is arranged in a ring or tube geometry about the counterelectrode 36.
Referring to Fig. 2, in operation, the ionizer 27 creates
a balanced flow of positive and negative ions directed toward a workspace 14 or
target 15 located in a variable ion mobility gaseous environment 29. The corona
electrode 20 may be controlled with a fixed voltage potential, current limiting
power supply 45 of negative polarity; and the corona-free electrode 23 may be controlled
with a voltage controlled power supply 42 of positive polarity based on the output
signal 17 of a balance sensor 16 located near the workspace 14 or target 15.
The ionizer 27 may be operated in the gaseous environment
29 when the variable ion mobility gaseous environment is substantially nitrogen,
hydrogen, or a noble gas such as helium, neon, argon, krypton, xenon, or radon.
The ionizer 27 may also be operated in the gaseous environment 29 when the variable
ion mobility gaseous environment is between about 200 degrees Kelvin to about 450
II. DETAILED DESCRIPTION
Referring again to Fig. 2, the present invention employs
a single polarity corona to generate simultaneously both positive and negative carriers
and to balance this ionization using a corona-free dc bias electrode to remove unwanted
carriers. Fig. 5 shows a self-balancing circuit 41, for use with the present invention.
The circuit 41 avoids the complications associated with the interaction of two corona
The present invention is best practiced with a negative
polarity corona, since negative polarity corona generally contains an extended structure.
Extended discharge structures introduce both positive and negative polarity carriers
to the gas stream. These extended structures include streamers, Trichel pulses,
burst pulses, and sparks. Conversely, glow corona, such as Hermstein glow of positive
corona, introduce positive carriers with few negative carriers. The difficulty with
positive corona is that the glow corona can transition to a pre-breakdown streamer
mode with a somewhat random onset condition. When this transition occurs, the positive
corona will change from introducing positive carriers to introducing both positive
and negative polarity carriers to the entrained flow. This transition will upset
use of a conventional design, but is partially overcome in the method described
The corona is produced by application of potential differences
between electrodes. The resulting electric fields not only produce the corona, but
also electric forces which remove charge-carriers from the gas stream. The small
fraction of carriers (typically 0.1%) that are entrained with the gas flow is determined
against this removing action. The difference in carrier mobility is also important,
since more mobile carriers move faster in a given electric field and are more easily
removed from the gas stream. This is especially true in nitrogen, where the negative
carriers (free electrons) have mobilities from 100-1000 times greater than the positive
carriers. At lower temperatures, higher electric fields are needed to initiate corona,
and thus, stronger forces act to remove carriers from the gas stream. The large
difference in carrier mobility in nitrogen and noble gases is used to their best
advantage in the present invention.
The research has shown that negative polarity corona in
nitrogen produces extended corona structures and the generation of positive and
negative polarity carriers in the entrained gas stream. The negative polarity carriers
in air, and especially in nitrogen, generally have higher mobility than the positive
polarity carriers. For this reason, positive carriers are more likely to be entrained
from the corona. In negative polarity corona, the positive carriers that are generated
are typically closer to the high voltage electrode and in a higher field. The bias
of the entrained carriers is negative for the negative polarity dc corona.
For conventional ionizers, including ionizers described
, to be used in variable ion mobility environments, a positive polarity
corona is used to inject positive carriers into the gas stream and provide an electric
field to remove excess carriers and balance targets placed in the entrained carrier
stream. The positive corona may inject some negative carriers, making balance more
difficult. Fig. 8 shows that in air at 433 degrees K, a negative corona has a greater
influence on target balance than a positive polarity corona, when the other polarity
is operating at normal voltages.
When one emitter is replaced with the spherical target
and a positive potential is placed on this electrode, the balance condition is not
significantly affected in air. The charge concentrations are reduced by the bias
field, and the charge extraction rates decrease. This is expected, because the positive
and negative carriers have a similar mobility.
In nitrogen, a potential on the corona-free electrode,
in this case a sphere, does not add carriers to the entrained stream, but preferentially
removes mobile free electrons over positive carriers. This leads to a more easily
established balance condition. This is shown in Fig. 9 for data at 300 degrees K
and 433 degrees K. Similarly, Fig. 10 shows the balance control at 213 degrees K.
Since the negative corona is generally an extended corona structure, the underlying
negative corona process generates positive and negative polarity carriers that can
be balanced by the corona free electrode at positive potential. This is an important
feature of the present invention and has not previously been demonstrated in the
known prior art.
It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without departing from
the broad inventive concept thereof. It is understood, therefore, that this invention
is not limited to the particular embodiments disclosed, but it is intended to cover
modifications within the scope of the present invention as defined by the appended