Note: Descriptions are shown in the official language in which they were submitted.
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PV 35323
Electrostatic Spraying Process and Apparatus
This invention relates to a process and apparatus
for the electrostatic spraying of liquids. More
particularly the invention is concerned with a process and
apparatus utilising a sprayhead in which liquid is fed along
a surface to an edge of the surface, hereafter termed "an
atomising edge" and a high voltage is imposed between the
atomising edge and a field intensifying electrode, sometimes
referred to/~as a field adjusting electrode spaced from said
atomising edge, whereby an atomising field strength is
created so that the liquid is atomised at least
preponderantly by electrostatic forces to form electrically
charged particles which are projected away from said
atomising edge. Such a process and apparatus are disclosed
iri Canadian Patent No. 1,071,937 issued on February 19, 1980.
Large scale spraying, for example paint spraying in
the car industry, has typically involved the use of
conventional electrostatic sprat' systems. Such systems fall
into two general classes, the first is where liquid to be
sprayed is first atomised and the sprat' so formed is then
charged; the second involves a spinning element for example
a disc or bell that is maintained at a high voltage and
atomises the liquid mainly by g-forces. Currently only such
systems deliver the necessary volumes of liquid with sprat'
characteristics (for example particle size) appropriate for
film formation. The difficulty with such conventional
systems is firstly they produce polydisperse sprat' particles
containing a large population of fines of less than 10~m.
Secondly, charging of the sprat' is less than 100% effective.
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This results in atmospheric pollution because fine particles
of respirable size escape into the air. A further cause of
pollution from such known systems is that their transfer
efficiency tends to be low for example of the order of 60$.
These disadvantages were substantially overcome by
the spray apparatus described in EP 186983 granted March 22, 1989 particular
reference being made to Figures 4 to 7 and the associated
description. Such apparatus produces a near monodisperse
spray (as expressed by the ratio of the volume median
diameter to the number median diameter for the droplets) and
the particle size can be controlled such that risk of
respiration is reduced. In addition complete charging of
the spray and relatively improved transfer efficiency
further reduce the risk of inhalation and atmospheric
pollution. The problem presented by the apparatus described
here is that it can only operate at relatively low flow
rates that are too low for large scale spraying such as
paint spraying.
Various solutions have been proposed for increasing
flow rate whilst maintaining a desired small particle size.
The solution proposed in EP 193348 granted April 4, 1990 is to cause a stream
of
gas to flow through the region of the electrical field, the
direction and velocity of the stream of gas being such as to
remove charged droplets of liquid from the said region,
thereby to reduce any build-up in space charge which affects
the magnitude of the electric field.
Another solution is proposed in EP 186983 granted March 22, 1989. This is
to form the field intensifying electrode as a core of
conducting or semiconducting material sheathed in a material
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of dielectric strength sufficiently high to prevent sparking
between the electrode and the sprayhead and volume
resistivity sufficiently low to allow charge collected on
the surface of the sheathing material to be conducted
through that material to the conducting or semi-conducting
core. It has been found that the use of such an electrode,
inter alia enables a higher potential difference to be
applied between the sprayhead and field intensifying
electrode without disruptive sparking. Hence the flow rate
can be increased when such an electrode is used whilst
maintaining the desired particle size, since the higher the
potential difference, the greater the permitted flow rate
for a given particle size.
An aspect of this invention is to further increase
the flow rate whilst maintaining a desired particle size.
The invention is based upon the very surprising
discovery that two atomising edges and their associated
field intensifying electrodes can be brought close to each
other without substantial uncontrollable electrostatic
interference between them.
The normal expectation by a person skilled in the
art would have been that as the atomising edges were moved
closer so that they effectively formed one spray head this
would lead to a major loss of atomising power because of the
interaction of the electric fields produced by the electric
potentials of the same polarity at the two atomising edges
and the result would have been little different from using
just one atomising edge.
According to the invention, an electrostatic liquid
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spraying apparatus has a sprayhead comprising a pair of
atomising edges extending side by side, liquid feed means to
each of said atomising edges, a plurality of field
intensifying electrodes extending lengthwise of the
atomising edges, a pair of said electrodes being associated
with each atomising edge with one disposed on either side
thereof and forwardly of the associated atomising edge, and
electric power supply means for imposing a potential
difference between said atomising edges and said electrodes;
and
a) the spacing between said atomising edges is in the range
20-300mm.,
b) the spacing between each of said electrodes and its
associated atomising edge is not less than 3mm., preferably
lcm., and
c) the measurable potential difference between the
conductors through which the potential difference is imposed
between said atomising edges and the electrodes is in the
range 1-3 KV per mm. of the spacing of said electrodes from
the atomising edges.
Thus the sprayhead has,two atomising edges which can
be disposed so close together that they can sprat'
substantially the same area of a target which is moving
relatively to the sprayhead within a period of time which is
so short that in terms of paint spraying it is equivalent to
one sprat'. However for a given particle size the atomising
edges are delivering almost twice the number of particles
per unit area per unit time as would a single atomising
edge.
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The operating parameters of the apparatus may,
be so selected that for a liquid with a viscosity
of 8cP and a resistivity of 2 x 10$ ohm cm a spray can be
achieved with a median particle size diameter not exceeding
10~m. at flow rates up to 20 cc/sec/metre length of
sp~rayhead.
If the spacing of the atomising edges is more than
about 300 mm, the spray coating is not laid down properly
and results in a mottling or striping effect. The
practical space requirements do not allow a spacing between
the atomising edges of less than 20 mm.
It has been found that the longer the atomising
edges the further apart they can be spaced. For example for
atomising edges whose length is of the order of 150 mm. the
spacing between the atomising edges is preferably in the
range of 20 to 100 mm., whereas :Eor atomising edges whose
length is of the order of 600 mm, the spacing between the
atomising edges is preferably in the range 100 to 300 mm.
If the spacing between each of said electrodes and
its associated atomising edge is less than about 3 mm.
liquid is deposited on the electrodes and can form a liquid
bridge seriously affecting the atomisation. If the spacing
is much greater than about 1 cm, then potential gradient is
lost and there can be contamination problems which can only
i
be overcome by working at much higher voltages. Tdeally,
the potential gradient between each said electrode and its
associated atomising edge should be as great as possible
without reaching break down or contamination.
Advantageously the present invention utilises the
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sheathed electrode construction disclosed in EP 186983 granted March 22, 1989
so
that the higher levels of potential gradient within the
above specificed range of 1-3 KV per mm. without breakdown
can advantageously be achieved.
For the larger spacings between the atomising edges
it is necessary to use four field intensifying electrodes.
However at small spacings three field intensifying
electrodes can be used, the central one being common to both
atomising edges. In order to vary the spray footprint or
pattern, the potential on the two adjacent electrodes, where
four are used, or the middle one where three are used may be
varied in value with respect to the potential on the two
outer electrodes.
Advantageously the potential with respect to earth
at the atomising edges is as high as is practical. The
higher the potential the more compressed is the footprint
between the two atomising edges and hence the higher the
density of the spray over this area. This compression
effect also increases with increase in the lengths of the
2p atomising edges. It has been found that this increase in
spray density is very beneficial in achieving a good spray
coating.
To permit flexibility in use of the apparatus the
power supply means preferably is capable of adjustment to
supply adjustable potentials to the atomising edges and the
field intensifying electrodes. Preferably it is also
capable of supplying different potentials to different ones
of the field intensifying electrodes.
The invention will now be further explained by way
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of example with reference to the accompanying diagrammatic
drawings in which:
Figure 1 shows a cross-section of one form of
sprayhead in accordance with the invention,
Figures 2 to 4 show various spray patterns which can
be achieved using various potentials on the sprayheads and
field intensifying electrodes.
Figure 5 shows diagrammatically a modified sprayhead
to that shown in Figure 1 and the spray pattern achieved
with it, and
Figures 6a and 6b illustrate the effect of using
different length atomising heads, Figure 6a showing
the spray pattern for the relatively short atomising
head of Figure 2 and Figure 6b showing the
spray pattern for the longer atomising heads of .
Figure 5.
The sprayhead shown in Figure 1 comprises two linear
and substantially parallel nozzle assemblies 10 and 11 the
edges 12 and 13 of which form respective atomising edges.
Each nozzle assembly 10,11 is formed of two plate members 14
and 15 arranged face to face. The facing surfaces are so
configured that they butt over their portions 16 but are
spaced apart over the remainder to form a liquid flow slot
17 extending over the whole length of the plates 14 and 15
and leading from a gallery 13 to the atomising edge 12,13.
One of the facing surfaces extends beyond the other to form
a projecting lip 20 leading from the outlet of the slot 17
to the atomising edge 12,13. The edges of the plates 14 and
15 are bevelled as shown. The edges 12 and 13 may be
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toothed or straight. The gallery 18 comprises a
longitudinally extending channel formed in the surface of
the plate member 15. Each gallery 18 is connected with a
liquid supply source (not shown) through tubes 19. As an
alternative the edges of the plates 14 and 15 could be
aligned as shown in Figures 2 to 4,
The plates 14 and 15 are of insulating material and
to enable an electrical potential to~ be applied to the
liquid feed an electrode 22 is provided in each of the
nozzles 10 and 11. Each electrode 22 is positioned adjacent
the atomising edge 12, 13 and .is located in the facing
surface of the plate 15. Each of the electrodes 22 is
connected to a high voltage generator through insulated
leads 23. The actual potential on the atomising edge 12,13
will depend upon the potential drop between the electrode 22
and the atomising edge 12,13 through the liquid being
sprayed. The resistivity of liqui<~ paint is of the order of
107 ohm cm. and at an applied potential of between 40-80Kv
a potential drop of about 10~ can be tolerated. The
essential requirement is that the electric field at the
atomising edge is sufficient to produce the required
ligamentary spray. As an alternative to the electrodes 22
one or both of the plates 14 and 15 of each nozzle may be
made of electrically conducting or semi-conducting material
and connected to the high voltage generator,
The nozzles 10 and 11 are supported in a suitable
frame 25 which also supports three linear electrodes 26,27
and 28, termed herein field intensifying electrodes, but
also sometimes referred to as field adjusting electrodes.
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These electrodes extend substantially parallel to each other
and to the atomising edges 12 and 13 with the electrodes 26
and 27 on the outer sides of the atomising edges 12 and 13
and the electrode 28 extending midway between the atomising
edges 12 and 13. The electrodes 26 to 28 are disposed
forwardly in the spraying direction of the atomising edges
12 and 13. Thus a pair of electrodes 26 and 28 are
associated with atomising edge 12 and a pair of electrodes
27 and 28 are associated with atomising edge 13.
Each of the electrodes 26 to 28 comprises a core 30
of electrically conducting material such as carbon and a
sheath 31 of semi-insulating material such as soda-glass.
The resistivity of the sheath is of the order of 1011 ohms
cm. These electrodes are of the form described in detail in
EP 186983. The cores 30 of the electrodes may be held at
earth potential, Alternatively a voltage with respect to
earth may be imposed on them, which voltage creates the
desired potential difference between the atomising edges 12
and 13 and the electrodes 26 to 28.
Tn Figure 1 the atomising edges are shown spaced
apart a distance a. The nearest point on the nozzles 10 and
ll to the surfaces of the associated electrodes 26 to 28 are
the atomising edges 12 and 13. The distance between the
atomising edges and the surfaces of electrodes 26 to 28 is
labelled b in Figure 1.
In one experimental example of paint spraying
utilising the apparatus of Figure 1 the value of a was 36
mm. The spacing b was about 1 cm. It is to be noted that
the drawings are not to scale. The atomising edges 12 and
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13 can be of any desired length and in this example were
150 mm. The effect of varying the length will be
demonstrated hereafter. The width of each slot 17 was such
as to allow the desired flow rate from each nozzle with a
normal reservoir pressure head taking into account the
viscosity of the liquid to be sprayed. In this example it
was set to deliver at least 10 cc per second per metre
length of sprayhead per nozzle of a liquid having a
viscosity of 8 cP so that the total liquid delivered per
second per metre length of the sprayhead was at least 20 cc.
The flow rate was advantageously adjustable to different
values.
In use of the apparatus a high voltage with respect
to earth as specified hereafter was imposed on each of the
nozzle electrodes 22 and a lower voltage with respect to
earth as also specified was imposed on each of the cores of
electrodes 26 to 28 so that a potential difference existed
between the nozzle electrodes 22 and the electrodes 26 to
28. The flow rate of liquid paint supplied from a reservoir
was also adjusted to a desired value. The liquid reaching
the atomising edges was subjected to a high intensity
electric field created by the imposed potential difference
and this caused the liquid to form a series of ligaments
extending from each of the edges 12 and 13 and which at a
distance from the edges 12 and 14 broke up into atomised
particles as is fully described in our aforementioned Canadian Patent No.
1,071,937 granted February 19, 1980 and EP Patent 186983 granted March 22,
1989
Referring to Figures 2 to 4 these show various spray
patterns which were achieved within this example by imposing
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~~1~~~:~
various voltages on the electrode 26 to 28.
In all of Figures 2 to 4 the voltage on the nozzle
electrodes 22 was 40 KV. In Figure 2 the voltage on all the
field intensifying electrodes 26 to 28 was 15 KV. This
produced a sprat' pattern which had a slight overlap between
the two sprays as shown,
In Figure 3 the voltage on the outer electrodes 26
and 27 was again l5 KV but that on the central electrode 28
was 11 KV. This produced a large overlap between the two
sprays as shown.
In Figure 4 the voltage on the outer electrodes 26
and 27 was again 15 KV and that on the inner electrode 28
was 19 KV. This caused a spacing of the two sprays as
shown. It can thus be seen that there is no significant
interference between the sprays and that in fact the sprat'
pattern is controllable simply by varying the voltages. It
could similarly be varied by varying the distances b so that
it was different for the central e:l.ectrode 28.
In another example shown diagrammatically in Figure
5, the spacing a between the two atomising edges 12 and 13
was 20 cms. This was so large, that it was necessary to use
two central electrodes which have been referenced for
consistency with the other Figures as 28a and 28b. The
distance b was 180 mm. The length of each atomising edge
was 60 cm.
The voltage on the electrode 22 was 80 KV and the
voltage on the field intensifying electrodes 26 to 28 was 40
KV. The target surface which was at earth potential was at
a distance of 30 cms. from the atomising edges 12 and 13 and
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moved at a speed equivalent to 1 meter in 19 sees. The
liquid paint which was a red grey surfacer used in the
automotive industry had a resistivity of 4 x 10~ ohms/cm.
and a viscosity of about 3 poise. Its flow rate was about
100 cc, per min, through each nozzle.
It was found that the resulting coating had a good
flat appearance with high gloss, low mottle and no striping.
The coat thickness was about 34-6~m.
Referring now to Figures 6a and 6b, these show a
comparison of the spray patterns achieved using the
apparatus of Figure 2 and Figure 5. The lengths of the
atomising heads and spacings a referred to in Figures 2 and
5 are shown proportionally in Figures 6a and 6b. Also the
spray patterns are shown in contour form so that the closer
the contours to each other, the denser the spray pattern. It
can be seen that with the longer atomising edges 12 and 13
o.f Figure 5 the end effect i,e, the end spread is not so
pronounced and there is more compression of the spray
pattern between the atomising edcJes, This produces a
denser spray which is very advantageous for paint spraying.
A similar compression effect can be achieved by increasing
the voltage with respect to earth on the electrodes 22.
The twin nozzle arrangement of the present invention
cannot only be arranged to produce a relatively high
delivery rate for a given median particle size as described,
but can also be used in two component systems which require
substantially simultaneous application of the components on
to a target, one component being supplied through one nozzle
and the other component through the other nozzle.
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