Device for insulating the spray liquid source from the high tension voltage of an electrostatic spray system when using an electrically conductive spray liquid

A device for insulating the spray liquid source from the high tension voltage of an electrostatic spray system when using an electrically conductive spray liquid. The device comprises a spray gun (13), a supply line (12) which connects the spray gun (13) to a spray liquid reservoir (10) and which includes a vessel (16) containing a barrier forming liquid (17) of low electrical conductivity and a density other than that of the spray liquid. The vessel (16), which is substantially non-conductive is connected at its inlet end (24) to ground potential and at its outlet end (19) to the same high potential as the spray gun (13). An electrode means (22; 25) is provided in the vessel (16) to expose the spray liquid passing through the vessel (16) to at least one intermediate potential for controlling the tension gradient in the barrier forming liquid by limiting and controlling the charging of the spray liquid.

BACKGROUND OF THE INVENTION 
This invention relates to a device for insulating parts of the spray liquid 
supply line, the spray liquid reservoir and the feed pump from the high 
tension voltage of an electrostatic spray systeem when using an 
electrically conductive liquid spray material such as a water based paint 
or a paint containing metallic particles. 
In particular, the invention concerns a device of the above type in which a 
vessel containing a barrier forming liquid is incorporated in the spray 
liquid supply line as described in European patent application No. 
87850083.4. 
A problem inherent in the above described device is identified as an 
uncontrolled tension gradient within the barrier forming liquid such that 
electrical fields of very high magnitudes are formed, and the spray 
liquid, which is transported through the barrier forming liquid in drop 
form, is torn up into a cloud of very small droplets. When disintegrated 
to such an extent, the spray liquid does not respond to gravitation 
forces, whereby it is difficult to have the spray liquid transported 
through the barrier forming liquid by the influence of gravitation. 
The above problem is solved by the invention as it is defined in the claims 
.

DETAILED DESCRIPTION 
The spray liquid supply shown in FIG. 1 comprises a spray liquid reservoir 
10, a feed pump 11, a supply line 12 connecting the feed pump 11 to an 
electrostatic spray gun 13, and an insulating device 14 incorporated in 
the supply line 12. The insulating device 14 comprises a pressure vessel 
16 made of a substantially nonconductive material such as plastics, and 
contains a substantially nonconductive liquid 17 which has the physical 
properties of not being mixable with the spray liquid and which has a 
density that is different from that of the spray liquid. In the drawing 
figures there are shown alternative vessel designs all of which contain a 
barrier forming liquid which has a lower density than the spray liquid. 
As a barrier forming liquid having a density lower than that of a water 
based paint any suitable fraction of petroleum may be used, for example 
fuel oil which has a density of about 0.8 g/cm.sup.3. 
In the spray system shown in FIG. 1, a water based liquid paint is supplied 
from the reservoir 10 to the electrostatic spray gun 13 via the vessel 16 
and the supply line 12. Instead of connecting the high tension source to 
the spray gun as is common practice in previously known electrostatic 
spray systems, the high tension source 15 in this improved system is 
connected via a conduit 20 to the outlet end 19 of the vessel 16. Through 
the conductive water soluble paint the high tension potential propagates 
downstream to the spray gun 13. This means that the spray gun 13 as well 
as the supply line 12 downstream of the vessel 16 are exposed to the high 
tension voltage and form a high tension section 18 of the system. The 
electrical lead upstream through the paint is interrupted by the 
nonconductive barrier forming liquid in the vessel 16. 
At the top of the vessel 16 there is located an inlet nozzle 21 by which 
the paint is disintegrated into small quantities like drops which fall 
through the insulating liquid 17 by gravity. At the bottom end 19 of the 
vessel 16, the drops regather to form a continuous paint flow when leaving 
the vessel 16. Since the paint is transported through the insulating 
liquid 17 in the form of separate drops, the high tension voltage is not 
able to propagate further upstream through the paint. Thereby, the paint 
supply system upstream of the vessel 16, including the feed pump 11 and 
the paint reservoir 19, is effectively protected from the high tension 
voltage. 
In the embodiment of the invention illustrated in FIG. 1, the barrier 
liquid containing vessel 16 is provided with four electrode elements 22 
located after each other in the flow direction of the spray liquid. These 
electrode elements 22 are connected to intermediate potential outputs 23 
of the high tension source 15. These outputs 23 have different potentials 
between ground potential and the high potential to which the outlet end 19 
of the vessel 16 is charged. The intermediate potentials of the electrode 
elements 22 are arranged in such a way that a successively increasing 
potential is obtained toward the outlet end 19 of the vessel 16. The 
uppermost electrode element may, however, be connected to ground 
potential. Such an arrangement is advantageous in that the spray liquid 
would not be exposed to any electrical charging at all during its drop 
forming sequence when leaving the nozzle 21 at the top of the vessel 16. 
By arranging a number of electrode elements with successively increasing 
potentials, there is obtained a controlled charging of the spray liquid. 
This is accomplished in that the tension gradient within the vessel 16 is 
limited to substantially the steps in potential represented by the 
different electrodes. In a practical application, the high tension 
potential supplied at the bottom end 19 of the vessel 16 is about 100 kV, 
and the difference in potential between each of the four electrodes 22 is 
20-30 kV. 
According to the embodiment of the invention shown in FIG. 3, the electrode 
means for accomplishing a controlled charging of the spray liquid 
comprises a vertical tube 25 of a low-conductive material. The tube 25 
extends upstream in the vessel 16 from the high tension supplied outlet 
end 19 of the latter. This means that the lower end of the tube 25 is 
connected to the high potential supplied through the high tension lead 20 
and, due to the limited conductivity of the tube 25, a successively 
decreasing potential is exposed to the liquid over the length of the tube 
25. A suitable material to be used in the tube 25 is a conductive plastic, 
such as carbon impregnated polytetrafluoroethylene. 
In the embodiment of the invention illustrated in FIG. 4, a successively 
increased potential is obtained by a lining 26 of a low-conductive 
material attached to the vessel walls. A suitable material for this lining 
26 is the same as mentioned above in connection with the tube shaped 
electrode. 
In a further embodiment of the invention, the walls of the vessel 16 itself 
are made of a low-conductive material as described above so as to 
accomplish a successive charging of the spray liquid.