Surface static reduction device

Details Surface static reduction device Surface static reduction device

1999-10-18

2004-05-25

Kim, Christopher (Department: 3752)

Fluid sprinkling, spraying, and diffusing

Rigid fluid confining distributor

Having interior filter or guide

C239S590500, C239S690000

Reexamination Certificate

active

06739530

ABSTRACT:

The present invention relates to a surface static reduction device for use, for example, in reducing and preferably eliminating static electricity from surfaces to be sprayed with paint. In particular, the present invention provides an improved radioactive gun for the reduction of static.
There are many ways of eliminating static electricity. These may involve the use of high voltage devices which use a corona discharge to generate ionized air, there are so-called passive devices which consist of unpowered arrays of sharp points, there are electrically powered X-ray tube devices which ionize air by emitting low energy X-rays and there are radioactive devices which are generally described as bars, guns or cartridges which use radioactive sources to ionize, air. Devices can be used in combination with each other and in conjunction with, blowers, fans, compressed air lines and the like which guide the ionized air to where it is needed. All the methods seek to produce and direct as many ions as possible to the charged work surface. There they can neutralize unwanted electrostatic charge which may have built up.
The basic design concept and operating principle for radioactive guns and ionizing cartridges is described below by reference to FIG.
1
. Devices work by passing air from a high pressure feed line
3
at a high velocity into an input nozzle end
2
of a hollow cylindrical cartridge
1
which is open at the other end with an outlet nozzle
4
. Inside the cartridge
1
is placed a source of ionizing radiation
5
which is commonly a metal foil containing the radioisotope polonium-210 which emits alpha particles. This causes the air flowing through the cartridge to become ionized. The air exits the cartridge through the outlet nozzle
4
and is then directed towards a charged surface by the operator of the device. Ions in the air stream are blown onto the surface and/or are drawn towards it by an electrostatic field associated with the charge on the surface and they cause the charge on the surface to be neutralized. Static radioactive guns and ionizing cartridges such as the one represented in
FIG. 1
are well known in the industry and such devices using this operating principle have been available for many years.
The main field of application is in manufacturing industry where it is important for certain articles to be kept clean and free from dust and charge during their assembly. An important application is in the paint spraying industry. In this application it is well known that both dust and charge on a surface give rise to a poor quality paint finish and there can be a significant cost associated with rework. The radioactive ionizing gun provides a means of improving the quality of the surface finish by eliminating both dust and charge simultaneously prior to painting. High voltage corona discharge devices are potential fire hazards in this application and they are generally not used by industries which perform paint spraying on safety grounds.
The efficiency of existing radioactive guns and cartridges can be adversely affected where local conditions vary and also due to poor design. Factors which can affect performance include such parameters as the air input pressure, the air flow volume, air turbulence, air cleanliness and particulate content, temperature, humidity, work surface material, geometry and distance from the gun, local electrostatic fields, individual operator training and product age. In poorly designed devices, performance may also be adversely affected due to inefficient ion production, inefficient transport to the work surface and ion losses due to recombination and dispersion in the outside air. The present invention seeks to address the problems encountered with existing radioactive guns and in particular the present invention seeks to provide a radioactive gun which is substantially insensitive to changes in local conditions.
The present invention provides a surface static reduction device for generating a stream of ionized air comprising a cartridge having a chamber with an inlet for communication with a pressurised air supply and an outlet for the stream of ionized air, the chamber containing at least one radioactive source for ionizing air within chamber characterized in that the cartridge is arranged to produce an external stream of ionized air having a core region and a perimeter region with the average ion concentration in the core region being greater than the average ion concentration in the perimeter region.
A cylindrical static reduction device may be provided with a cylindrical radioactive source, the internal diameter of the cylindrical source being greater than 11 mm and less than 23 mm diameter and in which the input air pressure, the inlet diameter and the outlet diameter are matched to produce an internal air density and air velocity contour which causes the production rate of ionisation to be greater in the centre of the chamber than adjacent the walls of the chamber so that the stream of ionized air has an ion concentration in the core region of the air stream which is maximized and an ion concentration in the perimeter region of the air stream which is minimized. In addition the inlet diameter and the outlet diameter may be matched so that the internal air density inside the ionizing cartridge is substantially independent of pressure variation in the compressed air supply line.
Preferably, but not exclusively, the device is designed and operated so that the internal air density inside the cartridge is such that the maximum path length of alpha particles from the source is between about 0.55-0.85 (preferably 0.65-0.8, more preferably about 0.75) of the internal diameter of the source, (or if the source is planar, that fraction of the average height of the air space above the source). This produces an ion distribution in which the ion cloud from opposite sides of the cartridge overlap in the middle to produce a core region of higher (i.e. about double) ion concentration. The larger the cartridge diameter, the longer the alpha path length needs to be before there is overlap in the middle. The optimum air density is lower for large cartridge diameters.
Because of the need to balance ion concentration with air pressure in dependence on the diameter of the cartridge a practical limit arises for the useful range of internal diameters for such devices of between about 12-22 mm. The 12 mm devices need to be designed to operate at high internal pressure for optimum performance whereas the 22 mm diameter devices need to be designed to operate at low internal pressure for optimum performance.
In order to ensure the optimum internal operating pressure (i.e. air density) is achieved for any given cartridge diameter more usually in the range 12-22 mm the ratio of the internal diameter of the radioactive source to the output nozzle diameter is important. In a preferred embodiment, optimum performance is achieved when this ratio is in the range 2.5-4.5, preferably 3-4, more preferably 3.5.
In a second preferred embodiment the inlet nozzle diameter and the outlet nozzle diameter are matched so that the inlet nozzle has an air flow resistance which is greater than the air flow resistance of the output nozzle. The air inlet nozzle acts as the primary barrier to air flow through the device. When the velocity at the inlet is close to supersonic (as it usually is in practical conditions of use) the air input is said to be “choked”. This causes the internal air density of the cartridge to be substantially independent of the air input pressure of the high pressure feed line. In other words, changing the input pressure does not substantially alter the air flow through the input nozzle. This enables the device to operate at optimum efficiency over a wide range of possible input pressures. This is achieved and optimized with the current invention when the ratio of the diameters of the output nozzle to the input nozzle is in the range 1.2-1.4, preferably 1.3. A representative set of workable design parameters for practical devices is summarized in the table

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