Note: Descriptions are shown in the official language in which they were submitted.
` `` ~Z~67~
An air transporting arrangement
The present invention relates to an arrange-
ment for transporting air with the aid of so-called
ion-wind or corona-wind, the arrangement being of the
kind set forth in the preamble of Claim 1.
The arrangement has been developed primari-
ly for use in conjunction with air purif~ving devices,
such as electrostatic precipitators for example, and
air processing systems, such as ventilation systems and
air-conditioning systems, for example, although the in-
vention can also be used to advantage in many other con-
nections where air is required to be transported, such
as when cooling electrical apparatus or electrical
equipment, and inconjunction with heating devices, such
as electric hot-air blaze.
Today,air is transported in the aforesaid
apparatus, systems etc. almost exclusively with the
aid of mechanical fans of mutually different design.
Such mechanical fans and associated drive motors are
relatively expensive, in addition to being heavy and
requiring a considerable amount of space. They also
have a relativeiy high energy requirement, and are con-
sequently expensive to run. In operation the fans also
generate a considerable amount of noise, which is high-
ly troublesome in many areas in which such fans or
blowers are used, for example in dwelling places and
in certain working locations.
It is known that the transportation of air
can be achieved, in princple, with the aid of so-called
ion-wind or corona-wind. An ion-wind is created when a
corona electrode and a target electrode are arranged
at a distance from one another and each connected to
a respective terminal of a direct-current voltage
source, the corona-electrode design and the voltage of
the direct-current voltage source being such as to cause
~.26~76~7
a corona discharge at the corona electrode. This coro-
na discharge results in ionization o the air, with
the ions having the same polarity as the polarity of
the corona element, and possibly also electrically
charged so-called aerosols, i.e. solid particles or
liquid particles present in the air and becoming
electrically charged upon collision with the electric-
ally charged air ions. The air ions move rapidly, under
the influence of the electric field, from the corona
electrode to the target electrode, where they relin-
quish their electric charge and return to electrical-
ly neutral air molecules. During their passage between
the electrodes, the air ions are constantly in colli-
sion with the electrically neutral air molecules,
whereby the electrostatic forces are also transferred
to these latter air molecules, which are thus drawn
with the air ions in a direction from the corona elect-
rode to the target electrode, thereby causing air to
be transported in the form of a so-called ion-wind or
corona-wind~
Arrangements for transporting air with the
aid of ion~windare known to the art, and examples of
such apparatus are described and illustrated, inter
alia, in DE-OS 2 854 716, DE-OS 2 538 959, GB-A
2 112 582, EP-Al-29 421 and US 4 380 720. These prior
art air-transporting arrangements utilizing ion-wind
or corona-wind have been found extremely ineffective
however, and have not achieved any practical signifi-
cance. It would seem that a reason for this is a lack
of understanding of the physical mechanisms decisive
for the total transportation of air through an arrange-
ment of this kind. Consequently, it i9 not possible
with the previously suggested embodiments of ion-wind
operated air transporting arrangements to achieve in
practice the transportation of signiicant quantities
of air without needing to raise the corona current -to
~Z~67~
levels which lie considerably above those levels which
can be considered acceptable when using such an arrange-
ment in populated environments. It is well known, inter
alia, from the electrostatic precipitator field, that
an electric corona discharge generates chemical com-
pounds, primarilyozone and oxides of nitrogen, which
have an irritating effect on human beings" and which
can be harmful to the health when present in the air
in excessively high concentrations. In the event of a
corona discharge these chemical compounds are generated
at a rate which is contingent on the magnitude and po-
larity of the electric corona current. Consequently,
present day electrostatic air ~ilters for use in human,
or populated, environments operate with a positive co-
rona discharge and a corona current having an amperagewhich is substantially proportional to the quantity
of air passing through the filter per unit of time in
normal operating conditions. In this respect the coro-
na current is of the order of 40-80 ~A at an air-
throughput of 100 m3/h, the strength of the currentbeing adapted to the requirement for an acceptable le-
vel of ozone and Nox generation. It will be understood
that the corona current utilized in air-transporting
arrangements which operate with an ion-wind and are
used in the presence of people, i.e. human environments,
must also be restricted to the aforesaid magnitude. This
is not possible to achieve with the prior art air trans-
porting arrangements utilizing ion-wind, due to the
poor efficiency of the arrangements. For example,
according to reports, it is possible to achieve with
the arrangement proposed in EP-Al-29 421 and US 4 380 720
an air throughput of 1 l/s with the aid o~ a corona
power of 1W at a preferred corona voltage of 15 kV.
Thus, when converted to an air throughput of 1~0 m3/h
this arrangement will consume about 1900 ~A, which is
roughly thirty times higher than the corona-current value
12~67~
acceptable in human environments.
Consequently an object of -the present in-
vention is to provide an improved and much more ef-
fective air transporting arrangement of the kind men~
tioned in the introduction, and one which is so effi-
cient as to enable it also to be used in practice in
a human environment.
The arrangement according to the invention
is based on a more profound and improved understand-
ing, previously unachieved, of the mechanisms decisive
for the total transportation of air through an arrange-
ment of this kind, and has the characterizing features
set forth in the following claims.
The invention will now be described in more
detail with reference to the accompanying drawings, in
which
Figure 1 is a schematic illustration of ion
migration between a corona electrode and a target
electrode;
Figures 2-7 and 9-13 illustrate schematical-
ly a number of different embodiments of an arrangement
according to the invention; and
Figure 8 is a diagram of the corona current
as a function of the voltage.
There will first be given a synopsis of the
fundamental conditions determinative for the transpor-
tation of air capable of being obtained with the aid
of an ion-wind or corona-wind generated between a co-
rona electrode and a target electrode arranged axially
downstream of the corona electrode in the desired flow
direction. Figure 1 illustra-tes schematically acorona
electrode K in the form of a thin wire extendin~
across the airflow path, e.g. across an airflow duct,
and a target electrode M which also extends across the
airflow path and which is shown schematically and by
way of example, in the form of a net or grid structure
~26~6t7~7
which is permeable to the airflow. The target elect-
rode M is placed downstream of the corona electrode K
in the deslred direction of airflow, shown by an arrow
_, at an axial distance H from the corona electrode K.
As previously mentioned, the corona discharge
created at the corona electrode gives rise to electric-
ally charged air ions, which migrate in a direction to-
wards the target electrode under the influence of the
electric field present between the corona electrode and
the target electrode.
The mobiliky of the ions varies within a
wide spectrum, altough for the present purpose it can
be assumed that lightweight ions having the mobility
c = 2.5 10-4 m2/Vs
are predominant, and that any electrically charged
aerosols present, which are far less mobile than the
air ions, only constitute a negligible part of the to-
tal charge in the system. It can also beassumed that theair ions constitute a very small fraction of the tota~l
mass of the air within the system, and that the flow
rate of the air is at least one power of ten lower
than the speed of motion of the air ions. Thus, with
respect to the migration velocity of the air ions the
surrounding air can be assumed to be stationary.
The migration velocity v of electrically
charged air ions in relation to the surrounding air is
proportional to the product of their mobility c and
the strength E of the electric field and hence
v = c E (1)
It is also assumed that steady state condi-
tions prevail, so that the charge density in a gi~en
part-volume of the system is constant, i.e. that the
~ ~2~76~7~7
electrical charge per unit time supplied to the system
is equal to that removed from the system. Consequently,
the current density in the air can be expressed as the
product of the migration velocity _ of the charges and
the charge density ~
~ v (2)
where i is the current density.
The specific volumetric force in the air is
the product of the charge density ~ and the electric
field strength E, and hence
f = ~ E (3)
where f is the driving force per unit volume of air.
When applying the above equations (1), (2)
and (3) there is thus obtained
f = -i / c (4)
i.e. the specific volumetric force can be expressed
as the ratio of the current density to the ion mobili-
ty.
As illustrated in Figure 1, we now consider a
"current duct", which conducts an infinitesimally small
part dI of the total ion flow I between the two elect-
rodes K and M. The centre line of this current duct
is always parallel with the current density vector i
and its cross-sectional area dS has a surface normal
which is parallel with the current-density vector.
We now consider a volume element
dV = dS dl (5)
of this current duct, where dV is an infinitesimal
7~
volume and dl is an infinitesimal length in the direc-
tion of the current duct. The force acting in the di-
rection of the surface normal on each such volume ele-
ment in the current duct becomes
dF = f dV = f dS dl (6)
This volumentric force dF has a component in the direc-
tion w of air transportation and a component at right
angles to said direction. It is assumed that when to-
talled across the whole cross-sectional area of the
airflow path or duct in the arrangement these trans-
verse forces will cancel out each other and can there-
fore be ignored. Consequently, the total transportation
force in a current duct is
M M M
dFT = r w dF = r w ~ f ~ dS ~ dl = r w ~ i/c ~ dS ~ dl=
K K K
M
= dI/c r w dl = H/c dI (7)
K
where H is the distance between the corona electrode K
and the target electrode M in the direction of airflow.
The total transportation force FT in the air-
flow duct can thus be expressed as
FT = f~ dFT = H/c I ~8
where S is the total cross-sectional area of the airflow
duct and I is the ~otal ion or corona currentO
Thus, the average pressure setup can be written
as
6~67~7
~p = FT / S = H/c I/S (9)
The transportation force is thus proportional
to the product of ~he total ion or corona current I and
its migration path H, i.e. proportional 1;o the so-called
"current-distance" H.I.
It can be shown that the total air throughput
as a result of this pressure setup can be written as
~ ~I H S (10~
where Q is the air throughput, k is a dimensionless
aerodynamic resistance coefficient and YA is the densi-
ty of the air.
It will be seen from the equation (10) that
the magnitude of air transportation is directly propor-
tional to the square root of the product between the
total ion or corona current I and its migration distance
H.
Thus, in order to achieve a high air through-
pu~ in the desired direction, i.e. in a direction away
from the corona electrode and towards the target elect-
rode, it should be endeavoured to attain a high productof the ion current and its migration distance in a di-
rection downstream from the corona electrode, i.e. from
the corona electrode toward~ the target electrode. An
increase in the transporting force, and therewith in
the total air throughput, can be achieved either by in-
creasing the strength of the total ion current or by
increasing the distance between the corona electrode
and the target electrode. As beforementioned, when
used in a human environment, however, it is not permiss-
able to increase the strength of the ion or corona cur-
rent to a level which exceeds a given maximum in view
~6~67~
of the ensuing production of harmful ozone and oxides
of nitrogen ~Nox), this production being primarily
proportional to the corona current. Consequently, the
only remaining parameter capable of being influenced
in this regard is the distance migrated by the corona
current, i.e. the axial distance between the corona
electrode and the target electrode. Accordingly, it is
proposed in accordance with the invention that the
distance between the corona electrode and the part of
the target electrode receiving the predominant part
of the ion current is at shortest ~0 mml and preferably
measures at least 80 mm.
It will also be seen that when using an air
transportation arrangement of the aofredescribed kind,
a stream of air ions is also able to migra-te from the
corona electrode in an upstream direction, i.e. in a
direction opposite to the desired direction of air
transportation, if there is located upstream of the
corona electrode an electrically conductive object or
subject having an electrical potential in relation
to the corona electrode which makes such migration of
the air ions possible. It will be understood that this
greatly reduces the total desired transportation of
air through the arrangement. To the extent that this
possibility of a stream of ions passing from the co-
rona electrode in an upstream direction therefrom has
been taken into account when designing known air trans-
porting arrangements of the kind discussed here, it
would appear to have been assumed sufficient to ensure
that electrically conductive objects upstream of the
corona electrode are located at a considerable distance
therefrom and that the flow of ion current directed up-
stream is small. However, since the transportation force
created by the ion flow is proportional to the product
of the strength of said flow and the distance travelled
thereby, as made evident in the above equation ~9), it
~X~76~7~
will be seen, to -the contrary, that even a very small
stream of ions from the corona electrode in a direc-
tion upstream therefrom can give rise to a significant
transportation force in a direction opposite to the de-
sired direction of air transportation, when this up-
stream directed stream of ions has a long path to travel.
It must be observed in the present context
that the term "electrically conductive" must be inter-
preted in relation to the extremely small current
strengths prevailing in an arrangement of the present
kind, these current strengths normally being of the
order of 1 mA/m2. Consequently, in the case of an air
transporting arrangement of the kind to which the pre-
sent invention refers~objects which can be considered
to be electrically conductive or which have a surface
which can be considered as electrically conductive will,
in practice, always be found upstream of the corona
electrode. These objects may, for example, comprise
grids or net structures or other parts of the arrange-
ment itself located at the inlet to the airflow ductof the arrangement. Even in the absence of such
arrangement components, such objects as wall surfaces~
pieces of equipment or furniture and even people, which
are present in the area in which the arrangement is
placed and located in the vicinity of the inIet to the
airflow duct of the arrangement can serve as electric-
ally conductive surfaces to which a stream of ions can
migrate from the corona electrode upstream in the duct.
This sought for improvement in efficiency,
i.e. a high air throughput with the aid of a corona
current limited to an acceptable value, is achieved
in the air transporting arrangement according to the
invention, partly by locating the target electrode at
such a distance from the corona electrode that the dis-
tance from the corona electrode to that part of thetarget electrode receiving the predominant part of the
~L2~76~7
1 1
ion current, i.e. the migration distance o the ion
current downstream from the corona electrode, is at
sho.rtest 50 mm, and preferably not shorter than 80 mm,
and partly by ensuriny that the product of the ion-
current strength and the distance migrated by thecurrent in the upstream direction away from the co-
rona electrode is practically zero, or in all events
much smaller than the corresponding product of ion-
current strength and the migration distance of the
current in the downstream direction, away from the
corona electrode. This latter is effected in accord-
ance with the invention by effectively screening the
corona electrode in the upstream direction, so that
no ion current is able to flow from the corona elect-
rode in the upstream direction, or at least so thatany ion current able to flow in the upstream direc-
tion is only very small and travels through only a
very short distance.
According to one embodiment of the inven-
tion, the aforesaid necessary screening of the coro-
na electrode in the upstream direction can be achieved
by connecting the terminal of the direct current
~source connected to the corona electrode to a poten-
tial which coincides substantially with the potential
of the immediate surroundings of the arrangement, i.e.
in practice is earthed in the same manner as the casing
which houses the arrangement and as the remaining in-
active, electrical components. To the extent that it
has previously been proposed inconjunction with air
transporting arrangements of this kind to locate the
corona electrode at earth potential instead of a
high potential, these two alternatives have previous-
ly been considered to be equivalent to one another
with respect to the mechanism of air transportation,
and connection of the corona electrode to earth po-
tential has not been effected in an endeavour to
... .
126~
12
screen the corona electrode in the upstream directior.
In many cases, however, it is not desirable
to connect the corona electrode to earth potential,
since for various practical reasons it may be desired
to connect the target electrode to earth potential, or
to connect the corona electrode and the target electrode
to opposite polarities relative to earth, and therewith
reduce the need for high-voltage insulation. In cases
such as these the desired screening of the corona
electrode in the upstream direction can be achieved,
in accordance with another embodiment of the invention,
with the aid of a method known from other areas of the
electrotechnical field, by arranging an eletrically
conductive screening element upstream of the corona
electrode and giving to said element a potential which
coincides substantially with the potential of the co-
rona electrode, so that they form upstream of the co-
rona electrode an equipotential barrier which is sub-
stantially impenetrable to ions flowing in the up-
stream direction. To the extent that the provision of
a screen electrode upstream of the corona electrode
and connected to the same potential as said electrode
has been previously proposed inconjunction with air
transporting arrangement of the kind in ~uestion, such
proposals have been made in conjunction with an air
transporting arrangement of cascade construction, com-
prising a plurality of corona-electrode arrays and
target-electrode arrays arranged in axial sequential
relationship in an airflow duct. It has not earlier
been understood or perceived that effective screening
of the corona electrode against an ion current in the
upstream direction is, under all circumstances, essen-
tial to the efficiency of the air transporting arrange-
ment.
A third and extremely surprising possibili-
ty of effecting the necessary screening of the corona
~Z6~t7
13
electrode against an undesirable flow of ions in the
upstream direction resides in extending an airflow
duct encompassing the electrodes of the arrangement
through a substantial distance upstream of the corona
electrode, i.e. at the inlet end of the airflow duct,
the walls of said duct expediently consisting of a
dielectric material, for example a suitahle plastics
material, in a known and obvious manner. Tests have
shown that when operating an air transporting arrange-
ment of the kind in question, there appears on thedielectric walls of the airflow duct an excess of
electric surface charges which remain all the time
the material is subject to the prevailing electric
field. By "excess charges" is meant here electrical
charges on the surface of the dielectric material
additional to the surface charges assumed by the
classical understanding of dielectric material of
weak electrical conductivity.Ithasnot been clearly
established why these excess charges occur on the
dielectric walls of the airflow duct, although the
phenomenon itself has been established experimentally.
The phenomenon would seem to be related to the pheno-
mena utilized when manufacturing dielectric electrets.
In this latter case, special dielectric material is
subjected to a combination of highly electric field
and ion currents. Electrical excess charges are
therewith bound permanently in the structure of the
material, and are not conducted away despite the fact
that the material is electrically conductive to a cer-
tain degree. Conseyuently, in conjunction with afore-
stated phenomenon encountered in air transporting
arrangements of the kind in question r it is an ob-
vious assumption to one skilled in this art that the
electrical excess charges on the dielectric walls of
the airflow duct are also bound to the structure of
the dielectric material, but only provided that the
, ,,
.....
~,2~7~7t7
14
material is exposed to -the influence of an electric
field. This phenomenon can be used beneficially to
achieve necessary screening of the corona electrode
in the upstream direction, by extending the airflow
duct and its dielectric walls upstream, away from the
corona electrode, i.e. at the inlet end of the duct,
through a distance such that the excess charges
appearing on the duct walls under the influence of
an ion cuxrent from the corona e~ectrode immediately
after switching on the arrangement, effectively screen
the ion cloud present around the corona electrode
against the possible occurrence of an electric field
upstream of the corona electrode, so as to obtain
thereby an effective shield against an upstream-
directed ion current from the corona electrode. Itwill be seen that the further the airflow duct is ex-
tended upstream of the corona electrode, the greater
the efficiency of the screen provided. Tests have
shown that a satisfactory screening effect can be
obtained when the distance through which the airflow
duct is extended upstream of the corona electrode is
at least 1.5 times the distance between the corona
electrode and the target electrode. It will also be
seen that the screening effect becomes more efficient
with decreasing widths of the airflow duct, i.e. the
smaller the distance between mutually opposing di-
electric walls, the greater the efficiency of the
screening effect produced. In the case of an airflow
duct of relatively large cross-sectional area, the
screening effect can be increased substantially, by
dividing the duct into a plurality of mutually pa-
rallel part-ducts upstream of the corona electrode,
with the aid of elongated partition walls extending
parallel with the walls of the duct, for example par-
tition walls in the form of strips or the like of di-
electric material. An arrangement such as this will
-` 1267~
enable the corona electrode to be screened effectively
against an ion current in the upstrearn direction even
though the distance to which the airflow duct is ex-
tended upstream of the corona electrode is only rough-
ly equal to the distance between the corona electrodeand the target electrode.
Another serious problem encountered with air
transporting arrangements of this kind intended for
use in a human environment, is that they must be sa~e
to touch in spite of the high voltayes used. A touch
guard can, of course, be provided with the aid of me-
chanical means, by providing the airflow duct surround-
ing the electrodes of the arrangement with fully imper-
vious walls and fitting the duct with a protective
grid at both its inlet and its outlet end, so that it
is impossible to touch the voltage carrying electrodes
of the arrangement, either unintentionally or inten-
tionally. Suchguards, however, present a significant
resistance to flow and therewith seriously impair the
transport of air through the arrangement, and there-
with its efficiency. It has been found possible in
an arrangement according to the invention, however,
to provide perfectly satisfactory safety precautions
against contact with the arrangement in a much simpler
and more advantageous manner. As described in the
aforegoing, an arrangement constructed in accordance
with the present invention operates with an extremely
low corona current, in the order of 20-50 ~A per
100 m3/h transported air. This extremely low specific
value of the corona current is made possible due to
the large axial distance between corona electrode and
target electrode, and the effective screening of the
corona electrode in the upstream direction. As a re-
sult of this low current consumption, the voltage
carrying electrodes of the arrangement, irrespective
of whether it is the corona electrode or the target
16
electrode, can be connected to its associated terminal
of the voltage source through an extremely high re-
sis-tance, without needing to increase the voltage of
the voltage source to an unacceptable extent. It has
been found that this series resistance can be readily
given, with no difficu]ty whatsoever, a resistance
value of such high magnitude that in the event of the
voltage carrying electrode being short-circuited di-
rectly, the short circuiting current is so low as to
be totally harmless. A limit value of 2 n~ is normally
set with regard to a harmless short circuiting current
from the aspect of bodily contact with such electrical
appliances. If the short circuiting current is made as
low as about 100-300 ~A, no unpleasant sensations at
all are experienced when touching the voltage carrying
electrode. This can readily be achieved with an
arrangement according to the invention. If it is assumed,
for example, that the voltage carrying electrode of an
arrangement shall have an operating voltage of 20 kV
and the corona current is 50 ~A, the voltage carrying
electrode can be connected to the corresponding ter-
minal of the voltage source through a resistance of,
for example, 150 MQ, wherewith the voltage source it-
self must thus have a terminal voltage of 27.5 kV.
When the voltage carrying electrode is directly short-
circuited, the short circuiting current will therewith
be solely about 185 ~A, which is of such low magnitude
as to cause no discomfort, should the shortcircuit be
caused by direct contact with the electrode. This li-
mitation of the short circuiting current to a valuewhich causes no discomfort when coming into direct
personal contact with the voltage carrying electrode
has been totally unattainable in practice, however,
with the large corona currents, in the order of 2000
~A, which must, of necessity, be used inprior art air
transporting arrangements operating with an electric
1.26~6~
\
17
ion-wind. ~nother significant factor of the contact
safety-precaution, additional to the low level of
the short circuiting current, is the capacitive dis-
charge current which can occur when an electrode of
a given capacitance is touched. In the case of elect-
rodes of such design as to have significant capacitance,
however, the capacitive discharge current: can be re-
duced to fully acceptable levels, by forming these
electrodes from a material of high resistivity, in
accordance with the invention. This creates no other
drawbacks, since the electrodes do not need to be
highly conductive, in view of the low current strengths
which can be used in accordance with the invention
while still providing an efficient air transporting
arrangement.
Figure 2 of the accompanying drawings illust-
rates schematically and by way of example the principle
construction of a first embodiment of an air transport-
ing arrangement according to the invention. This
arrangement includes an airflow duct 1 which is made
of an electrically insulating material and through
which a flow of air is to be produced in the direction
identified by an arrow 2. Arranged in the airflow duct
is a corona electrode K which is permeable to the air-
flow, while arranged axially downstream of the coronaelectrode is a target electrode M, which is also per-
meable to the airflow. The corona electrode K comprises
an electrically conductive material, which is prefer-
` ably ozone and ultraviolet resistant, and may be con-
structed in a number of different known ways, to pro-
duce an electric corona discharge under the influence
of an electric field. The corona electrode K of the Fi-
gure 2 embodiment is shown, by way of example, to com-
prise a thin wire or filament which extends across
the airflow duct 1. The corona electrode may have many
other different forms however. For example, it may com-
~,67~7
18
prise a plurality of thin wires or filaments arrangedeither parallel with one another or in the form of an
open mesh grid or net. Instead oE using straight, thin
wires or filaments, the wires may be wound spirally,
or thin strips exhibiting straight, serrated or un-
dulating edge surfaces may be arranged in a similar
manner. The corona electrode may also comprise one or
more needle-like electrode elements directed substan-
tially axially in the airflow duct 1. The target
electrode M comprises an electrically conductive or
semi-conductive material, or a material coated with an
electrically conductive or semi-conductive surface,
and is provided with~surfaces which will not give rise
to a powerful concentration of electric fields. The
target electrode may also be constructed in a number
of different, known ways, partly in dependence on the
construction of the corona electrode. In the Figure 2
embodiment the target electrode M is shown to comprise,
by way of example, two mutually parallel plates located
in the direction of the airflow duct. In the case of
needle-shapedcorona electrode the target electrode ad-
vantageously has the form of a cylinder arranged co-
axially with the airflow duct. An electrically conduct-
ive surface coating on the inside of the airflow duct 1
may aIso serve as the target electrode. The target
electrode may also comprise a plurality of planar or
cylindrical electrode elements arranged in side-by-
side relationship, with their sidesurfaces substantial-
ly parallel with the longitudinal axis of the air~low
duct 1. The target electrode may also comprise straight
or helically wound wires, or straight rods which may be
arranged mutually parallel with one another or to cross
one another to form a grid structure, or may have the
form of a perforated disc. A particular advantage is
afforded, however, when the target electrode has the
form of an electrically conductive or semi-conductive
~767~
1 9
surface which embraces the airflow duct in the form of
a frame and which has an extension parallel with the
airflow direction corresponding to at least one ~ifth
of the distance between corona electrode and target
electrode.
The aforedescribed exemplifying embodiments
of the corona electrode and the target electrode can,
in principle, be used in all of the embodiments or
arrangements according to the invention described here-
inafter.
In the arrangement illustrated in Figure 2 thecorona electrode K and the target electrode M are each
connected in a conventional manner to a respective pole
or terminal of a direct-current voltage source 3. In
the illustrated example the corona electrode K is con-
nected to the positive terminal of the voltage source 3,
so as to obtain a positive corona discharge. In prin-
ciple, however, the polarity of the voltage source 3
may also be the opposite, so as to obtain a negative
corona discharge. A positive corona discharge is ge-
nerally to be preferred, however, since less ozone,
which is a poisonous gas, is produced with a positive
corona discharge than with a negative discharge.
In the arrangement illustrated in Figure 2 the
terminaI of the voltage source 3 connected to the co-
rona electrode K is earthed, in accordance with the
invention, so that the potential of the corona elect-
rode K coincides substantially with the potential of
all other electrically inactive parts of the actual
arrangement similarly earthed, and also with the po-
tential of the immediate surroundings of the arrange-
ment. The potential of the corona electrode K will,
in this way, be the same as the potential of the en-
vironmental conditions located upstream of the corona
electrode K, with any electrically conductive objects
or surfaces located in said environment, and hence no
~2676'~7
undesirable ~low of ions will be obtained from the co-
rona electrode K in a direction upstream therefrom.
As mentioned in the aforegoing, the axial
distance between the corona electrode X and that part
of the target electrode M which receives the predominant
part of the ioncurrent is at least 50 mm, and preferab-
ly at least 80 mm, whereby air can be transported
through the airflow duct at a throughput of, for example,
100 m3/h with the aid of a low corona current in the
order of 20-50 ~A, which is an acceptable valvue with
respect to the production of ozone and oxides of nit-
rogen. Further, as previously mentioned, an advatage
is gained when the target electrode M is connected to
the d.c. voltage source 3 through a large limiting re-
sistance 8, which in the event of a short circuit causedby touching the target electrode M limits the short cir-
cuiting current to a value of at most about 300 ~A.
Since, as a result of its construction, the target
electrode M has a not insignificant capacitance, it
can suitably be made from a material of high resistivi-
ty. A suitable material in this respect, having a high
resistivity and, at the same time, the requisite abi-
lity to conduct electricity, is a plastics material
which incorporates a finely divided electrically con-
ductive material, such as carbon black for example.Known materials of this kind from which target elect-
rodes can be produced have a surface resistivity in
the order of 100 kQ and more.
It will be understood from the aforegoing
that an arrangement constructed in accordance with the
invention, for example in the manner illustrated in
Figure 2, is quite safe to touch, and hence it is not
necessary to take any other safety measures or to pro-
vide any form of safety device in order to prevent
intentioanl or unintentional contact with either the
corona electrode K or the karget electrode M. Further-
i7~67~7
21
more, since the corona electrode K is earthed, thereis no risk of ion current flowing through any other
location than the target electrode. When seen as a
whole, this surprisingly enables, in reality, an air
transporting arrangement according to the invention
to be constructed without including any Eorm of air-
flow duct 1 whatsoever/ at least when the primary
purpose of the arrangement is to cause air to move
in the space or area in which the arrangement is in-
stalled. For example, an arrangement constructed inaccordance with the invention may have the extremely
simple form illustrated in Figure 3. This embodiment
of the arrangement according to the invention includes
a corona electrode K in the form of a wire stretched
between holder means (shown solely schematically)
carried by suitable frame means (not shown in detail),
and a target electrode M which is spaced from the
corona electrode K and also carried by the aforesaid
frame means. The target electrode M may comprise two
mutually parallel, electrically conductive surfaces,
which also lie parallel to the corona electrode K.
Alternativel~, the target electrode M may comprise a
rectangular or circularframe-like electrode surface
whose axial extension coincides with the desired air-
flow direction 2, as illustrated in the figure, thisembodiment of the target electrode being the one pre-
ferred. It will be seen that in this embodiment there
is no airflow duct whatsoever surrounding the two
electrodes K and M. As with the Figure 2 embodiment,
the corona electrode K is connected to earth and to
one terminal of the d.c. voltage source 3, whereas the
target electrode M is connected to the other terminal
of the source 3 through a large ohmic resistance ef-
fective to limit a short circuiting current to an
acceptable value, in the event of a short circuit
created by contact with the target electrode M. The
~.2~;767~
22
target electrode M is also formed from a material of
high resistivity, so as to limit the capacitive dis-
charge current when contact is made with the target
electrode. Tests carried out with an arranyement con-
structed in the manner illustrated in Figure 3 showedthat the arrangement is able to transport air very
effectively in the direction indicated by the arrow
2, within the area embraced by the target electrode M.
The tested arrangement incorporated a rectangular,
frame-like target electrode M having a cross-sectional
area of 600 x 60 mm and an axial length of 25 mm. The
distance of the target electrode from the coxona elect-
rode K was 100 mm. A voltage of 25 kV was applied to
the target electrode M, and the corona current was 30
~A. The d.c. voltage source 3 had a terminal voltage
of 29 kV, and the series resistance 8 had a resistance
of 132 MQ. This extremely simple arrangement resulted
in an airflow of 60 m3/h through the area enclosed by
the target electrode M. When short circuiting the tar-
get electrode M of this arrangement, the short cir-
cuiting current was found to be only ~ 220 ~A, i.e.
a current strength which can hardly be felt should
personal contact be made with the target electrode M.
The arrangement is thus perfectly safe to touch, pro-
vided that the actual voltage source 3 itself is elect-
rically safe to touch.
As before mentioned, many cases are to be
found in which it is not desirable for the corona
electrode to be connected to earth potential. In cases
such as these, the requisite screening of the corona
electrode in accordance with the invention can be
achieved with an arrangement of the kind illustrated
schematically and by way of example in Figure 4. In
this arrangement, the negative terminal of the d.c.
voltage source 3, and therewith also thé target elect-
rode M, is connected to earth, whereas the corona
677
electrode K is connected to the positive terminal
through a large resistance effective to limit the
short cirauiting current to an acceptable value in
the event of a short circuit due to contact with the
corona electrode K. In order to prevent :ions from
migrating upstream from the corona electrode K, a
screen electrode S is arranged upstream of the coro-
na electrode and connected thereto, so that the
screen electrode S and the corona electrode K both
have mutually the same potential. The screen elect-
rode S may have one ofa number of different forms,
depending upon the construction or form of the coro-
na electrode used. When the corona electrode K com-
prises a thin, straight wire, the screen electrode
may, for example, have the form of a rod or a helic-
ally formed wire. The screen electrode may also com-
prise a plurality of rods or wires arranged in mutually
parallel relationship or in a diamond configuration.
The screen electrode S may aIso be in the form of a
net or grid-like structure. Alternatively, the screen
electrode may comprise electrically conductive sur-
faces placed in the close proximity of the wall of
an airflow duct 1 or on the inner surfaces of said wall.
In principle, the screen electrode S is ~iven a geomet-
ric configuration and position relative to the coronaelectrode K such that the screen electrode S forms an
equipotential barrier or surface which is impermeable
to ions emanating from the corona electrode K.
The screen electrode S need not necessarily
be electrically connected directly to the corona
electrode K, but may also be connected to the one ter-
minal of a further d.c. voltage source 4, as schematic-
ally illustrated in Figure 5, in a manner such that the
screen electrode S has the same polarity as the corona
electrode K in relation -to the target electrode M, and
preferably a potential which coincides substantially
767~7
24
with the potential of the corona electrode K. The screen
electrode S is, herewith, connected to the voltage
source ~ through a large resistance 9 efective to li-
mit the short circuiting current in the event of con-
tact with the screen electrode 5.
It will be seen that in the case of an arrange-
ment according to Figure 5 when the screen electrode S
has a higher positive potential in relat:Lon to the tar-
get electrode M than the corona electrode K, the flow
of ions in a direction upstream from the corona elect-
rode K is also effectively prevented hereby. Even though
the screen electrode S might have a somewhat lower posi-
tive potential than the corona electrode ~, so that a
small ion current is able to flow from the corona elect-
rode to the screen electrode S upstream thereof, thiscan be accepted provided that there is only a short dis-
tance between the corona electrode K and the screen
electrode S, so that the distance through which the ion
current migrates in the upstream direction is very short,
and therewith also the so-called current distance.
It will be understood that when the screen
electrode S of the~embodiment of Figure 4 or Figure 5
has a form, or construction, such as to present a sig-
ni~icant capacitance, the electrode is preEerably made
of a material of high resistivity, so as to limit the
capacitive discharge current to an acceptable level in
the event of contact being made with the electrode. This
applies generally to all voltage carrying electrodes
incorporated in an arrangement constructed in accordance
with the invention, when these electrodes have a not
insignificant capacitance. The corona electrode, how-
ever, is normally always designed to have a very small
capacitance, such as to be incapable of giving rise to
significant capacitive discharge currents. Another ge-
nerally applicable feature is that all electrodes ofan arrangement according to the invention connected to
~ 267~
a non-earthed terminal of a d.c. voltage source are
preferably connected to said source through a resist-
ance of such high magnitude that in the event of a
short circuit creaked by contact with the electrode,
the short circuiting current is limited to at most
30Q ~A.
As mentioned in the aforegoing, requisita
screening of the corona electrode against an undesirable
flow of ions in the upstream direction can also be
achieved electrostatically, for example in the manner
illustrated in Figure 6. In this embodiment, the air-
flow duct 1, the walls of which consist of a dielectric
material, such as a suitable plastics material, is ex-
tended through some considerable distance from the co-
rona electrode K in the upstream direction. When thearrangement is in its operational mode there is produced
on the walls of the duct 1 an excess of surface charges
which generate an effective shield against the ion
cloud in the vicinity of the corona electrode K, pro-
vided that the duct 1 extends through a sufficientdistance from the corona electrode in said upstream di-
rection. This effectively prevents the migration of an
ion current in a direction upstream of the corona
electrode K. The efficiency of the screen can be further
improved, by dividing the airflow duct upstream of the
corona electrode K into a plurality of part-ducts, with
the aid of elongated partition walls, plates or strips
7 made ofa dielectric material, as schematically illust-
rated in Figure 6. In order to provide an effective
screen, the length of duct 1 located upstream of the
corona electrode K should be at least equal to the
distance of the corona electrode from the target elect-
rode M, and preferably at least 1.5 times this distance.
The length of duct required to provide an effective and
efficient screen depends on the geometry of the airflow
duct 1, and then primarily on its cross-sectional confi-
~2~;76~7~
26
guration, and on whether or not dielectric partitionwalls 7 have been provided in the duct 1,upstream of
the corona electrode 7. When seen generally, it will
also be understood that the demands placed on this
screening of the corona electrode will depend upon the
difference in potential between the corona electrode
and the earthed surroundings; a smaller difference in
these potentials will thus lessen the demands which
need be placed on the screen.
When the corona electrode of an air trans-
porting arrangement according to the present invention
is effectively screened in one of the ways aforede-
scribed, such that substantially no ions will flow in
the upstream direction from the corona electrode, the
effective transportation of air through the arrangment
is determined primarily by the transport force generated
by the ion current flowing from the corona electrode K
to the target electrode M, and is proportional to the
product of said ion current and the distance between
the corona electrode and the target electrode.
An increase in the distance between the corona
electrode K and the target electrode M, while simulta-
neously maintaining an unchanged ion current between
the electrodes, can be achieved by increasing the volt-
age connected between the two electrodes, from the volt-
age source 3. Consequently, in accordance with the in-
vention, there is advantageously applied between the
corona electrode and the target electrode a difference
in potential of higher magnitude than has hitherto been
usual in, for example, electrostatic filters or preci-
pitators of the kind used in domestic dwellings. It will
be understood that when the potential of the corona
electrode is increased relative to the surroundings,
there is a still greater need to screen the corona
electrode in the manner aforementioned. An incrase in
voltage, however, is also encumbered with an increase in
12~ 7~7
27
the costs entailed, inter alia, by the high-voltage in-
sulation in both the actual voltage source itself and
in the ion-wind arrangement as such, and because of
this there is naturally an upper limit to which the
voltage can be increased in practice. One advantageous
method of reducing these difficulties is to connect the
corona and target electrodes to potentials of opposite
polarities in relation to earth.
According to a further development of the in-
vention it has pro~en possible, however, to increase
the distance between the corona electrode K and the tar-
get electrode M substantially, and therewith the migra-
tion distance of the ion current, without any decisive
reduction in the strength of the ion current between
these two electrodes and without needing to increase
the voltage level, by arranging a so-called excitation
electrode E in the proximity of the corona electrode K,
as illustrated by way of example in Figure 7. In the
exemplary embodiment of Figure 7, this excitation elect-
rode E has the form of a rotational symmetrical ring Ecomprising an electrically conductive material, or at
least presenting a partially electrically conducting
inner surface, which is arranged coaxially around the
corona electrode K, which in this embodiment has the
form of a needle e]ectrode. In view of the particular
configuration of the corona electrode K of the illust-
rated embodiment, the target electrode M has the form
of a cylinder arranged coaxially in the duct, whereas
the screen electrode S has the form of a ring arranged
coaxially in relation to the corona electrode K and up-
stream thereoE. Thus, the excitation electrode E is lo-
cated at a shorter axial distance from the corona
electrode K than the target electrode M and, in the
illustrated embodiment, is connected to the same termin-
al of the d.c. voltage source 3 as the target electrodeM, through a high ohmic resistance 6. The excitation
~2~6'7~
28
electrode E thus adopts a potential having the same
polarity as the potential of the target electrode M
in relation to the corona electrode K. The potential
difference between the excitation electrode E and the
corona electrode K, however, becomes smaller than the
potential difference between the target electrode M
and the corona electrode K. The excitation electrode
E contributes towards generating a corona discharge
and maintaining the same at the corona electrode K,
even when the distance between the corona electrode K
and the target electrode M is increased without in-
creasing the voltage of the voltage source 3 at the
same time. Only a minor part of the corona ion-flow
eminating from the corona electrode K will pass to the
excitation electrode E, while the major part of this
corona flow or current will still pass to the target
electrode M and contribute in transporting air through
the arrangement.
The effec-t produced by the excitation elect-
rode E can be illustrated by the diagram shown in Fi-
gure 8, in which the curve A illustrates the corona
current I as a function of the voltage U between the
corona electrode and the target electrode in the ab-
sence of an excitation electrode. As will be seen, no
corona discharge, and therewith corona ion-current,
will take place at all until a given threshold voltage
UT is exceeded. On the other hand, when an excitation
electrode is arranged adjacent the corona electrode,
the circumstances illustrated by the curve B prevail,
namely that a corona ion-current is initiated at a much
lower voltage with the axial distance between corona
electrode and target electrode unchanged. Only a part
of this corona ion-current will flow to the excitation
electrode, whereas the remainder passes to the target
electrode.
6~7~
-
29
The excitation electrode together with the
target electrode can also be considered as a two-part
target electrode, whose one part is located close to
the corona electrode, when seen in the axial direction,
and serves as an excitation electrode, while the other
part is located at a substantial axial distance from
said corona electrode and serves as a target electrode
for that part of the corona ion-current providing the
motive force Eor the air flow.
Consequently, an "excitation electrode" can
be obtained, for example, in the manner illustrated in
Figure 9, by extending a part of the target electrode
M axially towards the corona electrode K, up to the
proximity of said electrode or even beyond the same;
the target electrode ~ in this embodiment comprising
a number of mutually parallel plates extending axially
in the duct 1. In this case those parts of the target
electrode M located axially nearest the corona electrode
K function.as an excitation electrode, although the ma-
jor part of the corona ion-current will flow to that
part of the target electrode located further away from
the corona electrode in the axial direction, to generate
the desired ion-wind. When the excitation electrode E
is combined with the target electrode M in this manner,
by extending the target electrode M axially to a loca-
tion in the vicinity of the corona electrode, the tar-
get electrode may advantageously comprise a highly re-
sistive material or a highly resistive surface coating
applied to the inner surface of a tube of insulating
material, the distal end of the target electrode ~ in
relation to the corona electrode K being connected to
one terminal of the d.c. voltage source 3. That part
of the target electrode located nearest the corona
electrode K in the axial direction will therewith serve
as an excitation electrode E, which receives only a
minor part of the corona ion-flow. Alternatively, a
~2~6'7~7
combined target and excitation electrode can be obtain-
ed by providing the target electrode M with parts which
extend axially towards the corona electrode K and up to
the vicinity thereof, and which exhibit a much smaller
electrically conductive area than the major part of the
target electrode M located further away from the corona
electrode K and connected to one terminal of the d.c.
source. Those parts of the target electrode of small
conducting area located axially in the proximity of the
corona electrode K will thus serve as an excitation
electrode, to which only a minor part of the total co-
rona ion-flow deriving from the corona electrode K will
pass.
The excitation electrode can be formed and
arranged in many different ways. Any form of electrode
which is located in the axial proximity of the corona
electrode K and which does not in itself produce a
corona discharge and which is connected to one terminal
of a direct-current voltage source, the other terminal
of which is connected to the corona electrode, is able
to serve as an excitation electrode, if only a minor
part of the total corona ion-current flows to this
excitation electrode while the larger part of the co-
rona ion-currentflowsto the target electrode. Thus, a
screen electrode located upstream of the corona elect-
rode and arranged to receive a given, small ion-current,
for example in accordance with the embodiment of Figure
5, is able to function as an excitation electrode.
The geometric form of the excitation electrode
E may also vary in dependence on the configuration of
the corona electrode K. For example, when the corona
electrode comprises a plurality of geometrically sepa-
rated but electrically connected electrode elements, for
example straight thin wires arranged side-by-side, the
excitation electrode may advantageously also comprise a
plurality of geometrically separated but elec~rically
7~77
31
connected electrode elements, which are then arranged
between the electrode elements oE the corona electrode
so as to be screened Erom each other, which in respect
oE such a corona electrode is advantageous to the
creation of the corona ion-current.
Figure 9 illustrates schematically and by
way of example an arrangement according to the inven-
tion which incorporates a corona electrode K, a target
electrode M, a screen electrode S and an excitation
electrode E. In this embodiment each electrode comprises
a plurality of geometrically separated but electrically
connected electrode elements, which in the case of the
corona electrode K comprise straight, thin wires made
of tungsten for example, whereas the other electrodes
comprise helically formed wires of, for example, stain-
less steel.
Since, as evident from the aforegoing, an
arrangement according to the invention can be readily
constructed so that all electrodes are safe to touch,
it will be understood that the embodiments illustrated,
for example, in Figures 4, 5, 7, 9 and 10, in which the
target electrode M is earthed and the corona electrode K
and the screen electrode and also optionally the exci-
tation electrode E are connected to a higher potential,
can also be constructed to exclude an airflow duct
which surrounds the electrodes, provided that the
screen electrode is constructed in a manner which en-
sures that it will effectively prevent the ion current
eminating from the corona electrode from flowing in any
other direction than towards the target electrode.
Although an arrangement according to the in-
vention is able to function quite satisfactorily in
the absence of any form of airflow duct around the
electrodes of the arrangements, the provision of such
a duct may be desirable in some instances, however, for
example for psychological reasons or because such a
7~
duct will conduct the air through the arrangement in
a more orderly fashion. The provision of such a duct
may also be unavoidable in some instances~ or example
when the arrangement is to be placed within a ventila-
tion duct in a ventilation system, or in other instanceswhere the airstream generated by the arrangement is to
be conducted from and/to specific locations. The pre-
sence of such an airflow duct which encloses the
electrodes of the arrangement and the walls of which,
quite naturally, consist of an electrically insulating
material, gives rise to troublesome problems however.
As discussed above with reference to Figure 6, there
appears on the inner surfaces of the wall of such a
duct an excess of electrical surface charges. A similar
excess of surface charges will naturally also appear on
that part of the duct wall located between the corona
electrode and the target electrode, and will influence
the desired ion-current flowing from the corona electrode
downstream towards the target electrode, in a manner
such as to tend to restrict the ion-current to the
central region of the cross-sectional area of the air-
flow duct, which results in an uneven distribution of
the airflow across the width of the duct, therewith
impairing transportation of air therethrough. This prob-
lem is greatly exacerbated by variations in the voltageapplied to the corona electrode and the target electrode
through the aforesaid voltage source. A tempory increase
in the voltage will namely result in an increase in the
aforesaid surface charges, these charges persisting
even when the voltage is subsequently lowered, and there-
with cause a strong reduction in the corona current and
therewith in the transporation of air through the arrange-
ment. The drawbacks created by this phenomenon can be
overcome, or at least greatly alleviated, by stabilizing
the voltage delivered by the voltage source, -this expe-
dient being of no particular interest from other aspects
6'7~7
33
in an arrangement of the kind in question, or by brief-
ly cutting-off the voltage to the electrodes at uniform-
ly spaced time intervals. The excess surface charges
present on the inner surfaces of the duct; wall namely
S disappear relatively quickly when the voltage supply
is interrupted and the electric field thereby removed.
The presence of excess electrical charges on the inner
surfaces of the electrically insulating duct wall give
rise, however, to an additional, highly surprising and
serious problem. It has namely been found that when
the inner surface of the insulating duct-wall is touch-
ed, even briefly, the flow of corona current will cease
totally, and is not automatically stored, not even after
the lapse of a very long period of time from when the
surface was touched. Obviously, a solution to this prob-
lem must be found.
One possible solution to this problem is to
apply an electrically conductive layer to the outer
surface of the insulating wall of the duct and to earth
said layer. However, this would give a high capacitance
to a target electrode located in the close proximity of
the duct wall, or located directly on the inner surface
of said wall, which as mentioned in the aforegoing is
undesirable with respect to the safe-to-touch aspect of
the target electrode. It has been found possible to avoid
this, however, by increasing the cross-sectional dimen-
sions of the airflow duct to a size substantially greater
than the corresponding dimensions of the area enclosed
by the target electrode, so that the target electrode
is located at a substantial distance from the inner sur-
face of the airflow duct. One such embodiment is illust-
rated schematically in Figure 11. In this embodiment,
the outer surface of the insulating wall of the duct 1
is provided with an electrically conductive layer 10,
which is earthed. The duct 1 of th~s embodiment is also
significantly wider than the target electrode M, so that
~Z~
34
the duct walls are further away from the target elect-
rode, which thereby obtains a much lower capacitance.
The duct walls have, in this way, also been placed
further away from the corona electrode K, and hence
the excess charges occurring on the inner surface of
the insulating duct-wall have a much less disturbing
effect on the corona current flowing from the corona
electrode K to the target electrode M. This increase
in the cross-sectional dimensions of the airflow duct
1 in relation to the cross-sectional dimensions of the
target electrode M has not been found to have any de-
leterious effect on the transporation of air through
the arrangement, but that in fact such transportation
is increased at an unchanged corona current. In the
embodiment illustrated in Figure 11, the centre point
of the d.c. voltage source 3 is earthed, so that the
target electrode M and the corona electrode K have op-
posite polarities in relation to earth, which restricts
the total high-voltage level required and therewith the
necessity to insulate the arrangement against high volt-
ages r and also reduces the demands on the screening of
the corona electrode K, as mentioned in the aforegoing.
Since, in this case, a high voltage is applied to the
screen electrode, the corona electrode and the target
electrode, all of the said electrodes are connected to
the d.c. voltage source through a large resistance 8
effective to limit the short circuiting current in the
event of contact with the electrodes. Moreover, both
the target electrode M and the screen electrode 7 are
suitably manufactured from a material of high resistivity,
in order to limit the capacitive discharge current in
the event of contact.
In an embodiment of this kind, an advantage
is gained when the cross-sectional dimensions of the
airflow duct 1 are adapted so that the distance between
the duct wall and corona electrode K is equal to approxi-
. :,., :
67t~ ~
mately half the distance be-tween the corona electrode
and target electrode, and so that the distance between
duct wall and the surface of the target electrode is
approximately 50% of the cross-sectional dimension of
the target-electrode aperture.
The aforedescribed unavourable effects caused
by the presence of excess charges on the inner surface
of the duct wall can also be reduced with the aid of
an excitation electrode having the function described
in the aforegoiny, this excitation electrode comprising
an electrically conductive layer applied to the inner
surface of the duct wall. As will be understood, no
excess charges are able to appear on the inner surEace
of the duct wall in the presence of such an excitation
electrode. If, in this respect, the cross-sectional di-
mensions of the airflow duct are increased to an extent
such that the target electrode is located at a signifi-
cant distance from the wall of the duct, as illustrated
in Figure 11 and described above, the excitation elect-
rode mounted on the inner surface of the duct wall canbe very surprisingly extended in the downstream direc-
tion, to a location beyond the target electrode. In
actual fact, in this particular case an electrically
conductive layer can be provided on the inner surface
of the duct wall throughout the whole length of the
duct, i.e. even in the upstream direction to a location
beyond the corona electrode. One such embodiment is
illustrated schematically in Figure 12.
Thus, the embodiment illustrated in Figure 12
includes an airflow duct 1, the wall of which is assumed
to consist of an electrically insulating material and
the inner surface of which is provided with an electric-
ally conductive coating E, which is earthed and which
functions as an excitation electrode in the vicinity of
the corona electrode K. The cross-secti.onal dimensions
of the duc-t 1 are such that a target electrode M, o~
36
frame-like configuration and extending parallel with
the walls of the duct 1l is located at a significant
distance from the inner surface of the duct wall, and
is thus well insulatedfrom the electrically conductive
coating E on th~ inner surface of the duct wall. Located
upstream of the corona electrode K is a number of screen
electrodes S~ for example in the form of coarse rods. The
d.c. voltage source is earthed at its central point, so
that the corona electrode K and the target electrode M
have opposite polarities in relation to earth, which
affords the aforedescribed advantages. The electrodes
are also connected to the d.c. voltage source through
large resistances 8, to limit the short circuiting cur-
rent. It will be seen that no excess surface charges
whatsoever can appear on the inner surface of the duct
wall in an embodiment of the arrangement such as this,
and hence the arrangement is not encumbered with those
problems arising from the presence of such excess sur-
face charges. This embodiment of an arrangement accord-
ing to the invention has also been found to transport
air in an exceedingly satisfactory manner. The condi-
tions mentioned above with reference to Figure 11 also
apply with regard to the dimensioning of the airflow
duct 1 of the Figure 12 embodiment.
It will be understood that since it is possible
with an arrangement such as that illustrated in Figure
12 to provide the inner surface of the duct wall with an
electrically conductive, earthed coating along the whole
length of the duct, there is nothing to prevent the duct
wall from consisting entirely of an electrically conduct-
ive material, which would naturally facilitate manufact-
ure considerably, and also afford other valuable advan-
tages. Thus, it is possible that the inner surface of
the duct can lined, at least along a given part of its
length, with a chemically adsorbing or absorbing mate-
rial, for example a carbon filter, eEfective to remove
6767~
gaseous contaminents from the air, such as odouxs and
the oxides of nitrogen generated by the corona dis-
charge, by absorption or adsorption. It is also possible,
for the same purpose, to pass a thin liquid Eilm, for
example water or a chemically active liquid, along the
inner surface of the airflow duct. The wall of the air-
flow duct can also be cooled or heated, with the aid
of suitable means, for example circulating water, in
order to cool or heat the transported air. All this is
made possible by the fact that the wall of the airflow
duct is electrically conductive and earthed.
In those embodiments of the arrangement accord-
ing to the invention in which the electrodes are enclosed
in an airflow duct it has been found to advantage to
use one single corona electrode K arranged centrally
therein, since the greatest possible distance between
the duct wall and the corona electrode is obtained in
this way, and therewith the least possible disturbance
in the function of the corona electrode as a result of
the duct wall. Alternatively, there can be used, however,
two corona electrodes placed symmetrically on a respect-
ive side of the symmetry plane of the duct. In this
arrangement each electrode will be affected solely by
one wall or side of the duct and both electrodes will
operate under mutually similar conditions. This does
not apply, however, when more than two electrodes are
installed in the duct. In those embodiments where two
corona electrodes are placed symmetrically in the air-
flow duct, it can be to advantage to also install two
target electrodes side-by-side in a similar symmetric-
al relationship, the target electrodes in this respect
suitably having a common electrically conducting wall.
In the case of an embodiment such as that
illustrated in Figure 12, it will be understood that
the electrically conductive and earthed coating or
lining E on the inside of the insulating airflow duct 1
~l2~767~
38
need not be extended upstream of the corona electrode
K, in which case the excess charges consequently
appearing on the inner surface of the electrically
conductive duct wall upstream of the corona electrode
K will co-operate in establishing the necessary screen-
ing of the corona electrode K.
A further problem, affecting the total trans-
portation of air through an arrangement of this kind,
occurs when the corona electrode has the form of a
wire extending across the path of the airflow and
attached at both ends to electrically insulated attach-
ment means. The same problem can also occur with other
types of electrode which extend across the path of the
airflow. In this respect it has been found that the
corona electrode gives much more corona current per
unit of length within the central region of the air-
flow path than at the end parts of the electrode. This
would appear to be due to a screening effect created
through the electrode attachment means and through the
wall of the duct at both ends of the electrode, when
an airflow duct is included in the arrangement. In the
case of a low corona current, a considerable part of
both ends of the corona electrode can even be "exting-
uished" or cut-out. This results in uneven distribution
of the ion current and therewith uneven distribution of
the airflow across the cross-sectional area of the path
taken by the airflow. When the arrangement incorporates
an airflow duct which surrounds the electrodes, it has
been found that when seen in cross-section, those parts
of the airflow duct located opposite respective ends of
the corona electrode exhibit an airflow which moves
in a direction opposite to that intended. This pheno-
menon can greatly impair, and even totally eliminate
effective transportation of air through the arrangement
This problem can be overcome, however, in accordance
with a further development of the invention, by giving
i7~7
39
the target electrode and/or the excitation electrode
a particular form. An embodimenk of a target electrode
suitably ~ormed in this latter respect is illustrated
schematically and by way of example in Figure 3, which
shows an arrangement according to the invention, in-
corporating an airflow duct 1, shown in broken lines,
of narrow, elongated rectangular cross-section. Extend-
ing across the duct 1, between the two short walls
thereof, is a wire-like corona electrode K. The target
electrode M has the form of a conductive layer or
coating on the inner surfaces of the duct wall and, in
this embodiment, is so formed that when seen in the
axial direction of the duct it lies closer to the end
portions of the corona electrode K than to the central
region of said corona electrode in the transverse di-
rection of the duct. For example, the axial distance
between the target electrode M and the corona electrode
K at the centre region thereof may be 60 mm, while the
corresponding axial distance from the target electrode
to the opposite located end portions of the corona
electrode is only 40 mm. A target electrode M of this
configuration will eliminate the problem discussed
above, so as to obtain substantially uniform distribu-
tion of the corona current along the whole length of
the corona electrode.
The same result can be achieved when an ex-
citation electrode arranged between the corona electrode
K and the target electrode M is formed in the manner
described above with reference to Figure 13 in respect
of the target electrode. In this case the target elect-
rode can either be formed in the manner illustrated in
Figure 13 or in a normal manner, i.e. so that its axial
distance from the corona electrode is the same at all
points thereon. A corresponding result can also be ob-
tained with the aid of excitation electrodes which arelocated solely in the vicinity of both end portions of
676~t7
the corona electrode. A most essential feature, however,
is that the target electrode and/or the excitation
electrodes is, or are, so formed that the corona elect-
rode IC extending across the airflow path provides sub-
stantially the same amount of corona current per unitlength over the whole of its length, i.e even at the
end portions of the corona electrode.
A target electrode and excitation electrode
having the form described with reference to Figure 12
may also be used to advantage in an arrangement in which
the electrodes are not enclosed in an airflow duct,
since a target electrode and excitation electrode thus
formed will enable the corona current to be distributed
more uniformly over the whole length of the electrode.
An arrangement according to the invention and
constructed in accordance with the embodiment illustrated
in Eigure 10 was used in practice for experimental pur-
poses. In this experimental arrangement, the distance
between the plane of the screen electrode S and the plane
of the corona electrode K was 12 mm, whereas the distance
between the plane of the corona electrode K and the tar-
get electrode M was 85 mm. The mutual distance between
the wire-like electrode elements in the corona elect-
rode K was 50 mm, and the electrode element oE the
excitation electrode E was arranged in the same plane
as the electrode elements of the corona electrode K
centrally therebetween. The various electrodes were
connected to the voltages given in the drawings. The
airflow duct 1 measured 35 x 22 cm in cross-section,
and an earthed protective grid G was arranged at the
inlet to the duct. When this apparatus was placed free-
ly on a table, an airflow velocity in excess of 0.5 m/s
was obtained. The total corona current from the corona
electrode K was about 50 ~A, of which about 40 ~A
passed to the target electrode M. An airflow velocity
of about 0.5 m/s was obtained at a power consumption
7677
41
of 5-6 W/m2 of the area of the flow duct. The power re-
quired to obtain a corresponding airflow velocity in a
similar apparatus lacking the screen electrode S and
the excitation electrode E but with the same voltage
on the corona electrode was about 100 W/m2. In this
case, the distance between the corona electrode K and
the target electrode M was about 50 mm, and the distance
between the corona electrode K and the protective grid
G at the duct inlet was 100 mm. In this embodiment of
the apparatus according to the invention, the distance
of the protective grid G from the corona electrode K
had no noticable influence on the efficiency of the
apparatus.
The transportation of air through an arrange-
ment, or apparatus, constructed in accordance with theinvention can be further increased by arranging a plu-
rality of electrode arrays, each array comprising a
corona electrode, target electrode, screen electrode and
optionally an excitation electrode, sequentially in one
and the same airflow duct. The arrangement of a screen
electrode upstream of each corona electrode, in the
aforedescribed manner, will effectively prevent the
undesirable and harmful flow of ions in the upstream
direction, such flow being unavoidable in such a cas-
cade arrangement in the absence of a screen electrode.
The arrangement provides an extremely effect-
ive air transporting arrangement of relatively simple
cons-truction. In addition, an arrangement constructed
in accordance with the invention is relatively inex-
pensive, and has small dimensions and a low weight.Such an arrangement also has a low energy consumption
and is absolutely silent in operation.
When an air transporting arrangement accord-
ing to the invention is used in conjunction with an
electrostatic filter device, the target electrode M
in the air transporting arrangement can be arranged to
~L2~ 7~7
42
form simultaneously parts of the precipitation surfaces
incorporated in the eleckrostatic filter arrangement for
receiving the impurities charged upon collision with the
air ions, for example in a capacitor separator of a kind
known per se. ~hen the target electrode M functions as
a precipitation surface for impurities carried by the
air transported through the arrangement, the target
electrode is suitably constructed in a manner which en-
ables it to be readily dismantled for replacement or
cleaning purposes when the electrode becomes excessively
coated with precipitated contaminents. It will be seen
that this can be readily achieved when the arrangement
does not incorporate an airflow duct surrounding the
electrodes. In contexts such as these the target elect-
rode can conceivably have the form of strip materialfed from a storage reel or fed through a cleansing de-
vice when the part of the strip material used as a tar-
get electrode has been dirtied by precipitated conta-
minents.
