Patent Application
20140110493
1. Field of the Invention
The present invention relates to electrostatic liquid spray systems, and in particular a nozzle for an electrostatic liquid spray system having a removable and re-settable electrode cap.
2. Background of the Invention
An electrostatic spraying process charges either powder or liquid particulate to improve spray delivery and deposition. Advantages of the electrostatic charging are uniform spray cloud dispersion as well as improved uniformity and mass transfer efficiency in coating of target surfaces. In practice, many types of target surfaces are coated by electrostatic sprays; varying from agricultural crops to automobiles, appliances, furniture and many other manufactured goods. Unique opportunities for electrostatic spraying are still emerging. For example, recently developed applications involve coating of surfaces with sanitizing agents for odor control and the prevention of illness caused by virus and bacteria in areas of high human concentration such as hotels, hospitals, restaurants, schools, day care services, military installations and cruise ships.
In transport from an electrostatic nozzle, unipolar charged particles of relatively low mass maintain separation due to mutual repulsion and are driven along electric field lines to deposit uniformly. Sufficiently charged particulate clouds create strong space charge fields that propel particles near the edge of the spray cloud towards the target. The electrostatic forces due to this space charge are beneficially supplemented by image charge forces that aid the deposition process once individual particulates approach very close to the target substrate. These image charge forces are important to allow very small particles to overcome air boundary layer effects and deposit on the surface. A high ratio of particulate charge-to-mass is important to the process. Very small, highly charged particulates of high numerical density create beneficial space charge and image force fields, maximizing the electrostatic effects and minimizing the influence of gravity.
Choice of the optimal electrostatic charging method to employ for a particular application often depends on the type of spray compound and the target. For example, dry powder sprays to be delivered and deposited onto planar grounded surfaces may be suited for corona or triboelectric types of charging systems. Air assistance can be added to improve charged powder deposition for more complex three dimensional targets. Conductive liquids held in small containers and atomized by hydraulic or gas pressure may be suitably charged by direct contact of the liquid with high voltage probes. Insulating liquids and conductive liquids of relatively high resistivity can be atomized and charged reliably by electrohydrodynamic (EHD) methods as are known in the art. Conductive liquids, such as water based sprays of agricultural or sanitization chemicals, may present leakage current challenges in corona charging systems, EHD nozzles or high voltage contact systems, and may be more suited for charging by non-contact induction methods such as those disclosed in U.S. Pat. No. 3,698,635 to Sickles, U.S. Pat. No. 4,004,733 to Law, and U.S. Pat. No. 5,704,554 to Cooper and Law.
However, use of induction charging methods in the room, equipment and furniture sanitizing applications above typically requires field serviceability and robustness of the design to both servicing a well as providing continuous and extended use of the system. Therefore, it would be desirable to provide an electrostatic sprayer system that has a design robust enough for field servicing and provides the ability to operate continuously for extended periods of time, with low electrical power requirement.
The above objectives, as well as others, are accomplished in a nozzle for an electrostatic sprayer, and electrostatic spray gun and system including the nozzle, as well as a method of operation of the nozzle and system.
The sprayer has a nozzle including an inlet for receiving a liquid and a liquid tip having an outlet at a distal end for ejection of the liquid. An electrode is disposed around the outlet of the liquid tip for generating an electric field between the electrode and the liquid for charging the liquid. The sprayer also has a removable cap that captively secures the electrode and has an aperture for permitting passage of an electrostatically-charged liquid stream. The removable cap provides access to the nozzle components and electrode when the cap is removed from the sprayer. A calibratable stop mechanism is included for controlling a position of the electrode with respect to the outlet of the liquid tip, to ensure that the removable cap is seated at the proper position along the length of the end of the sprayer when the removable cap is installed.
The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.
The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of the invention when read in conjunction with the accompanying Figures, wherein like reference numerals indicate like components, and:
The present invention concerns electrostatic sprayer features, specifically features that permit servicing of the nozzle components by removing a detachable cap at the end of the sprayer. The electrode of the electrostatic sprayer that surrounds a liquid tip of the nozzle is captively secured in the cap. The cap and sprayer include a precision stop mechanism that ensures that the position of the electrode with respect to the liquid tip to be precisely controlled when the cap is removed and re-installed. The detachable cap provides for access to internal components of the nozzle that may become contaminated and require cleaning or replacement. Such contamination is likely to cause electrical leakage currents that diminish spray charging.
Non-contact induction liquid spray charging systems operate by surrounding the spray stream at the atomization zone with an electrode, creating a non-contacting charging field between the electrode and the liquid. Pneumatic energy is often used in induction-charging nozzle systems for atomization and air assisted delivery of spray. High velocity gas, usually compressed air, passes through the gap between electrode and liquid tip. The air generally keeps the liquid from contacting the electrode, which could reduce the charging field or, in the worst case, create a direct electrical short circuit. Distinct advantages of properly implemented induction-charging systems are: the liquid reservoir can be held at or near earth potential, nozzle and tank systems have low electrical capacitance, relatively low voltages in the range of 800 to 1400 V can be used, and relatively low current requirements allow some induction electrode systems to run from portable battery power. The structure of the nozzle systems and spray guns shown below provides such an implementation. However, some or all of the techniques disclosed and claimed herein may provide advantages in other types of liquid spray systems, and the present invention is not limited to induction-charging systems, except as indicated by features recited in particular claims.
During induction charging, if the charging electrode of the sprayer is of positive polarity, a negative charge will be induced onto the liquid surface, and vice-versa. A disadvantage to induction charging is that the surfaces of the nozzle system will become the same polarity as the electrode if wetted or otherwise contaminated by conductive surface films. The spray droplets discharged from the nozzle wrap back to oppositely charged outer nozzle housing surfaces, due to their opposite polarity. The droplets that wrap back to the outer surfaces further contaminate them, wasting spray, causing dripping of large droplets and forming electrical leakage current paths. The problem of charged spray coating the outer housing during prolonged periods of operation was addressed in U.S. Pat. Nos. 4,240,585 and 4,343,433 to Sickles, and partially mitigated using multiple air flow outlets to help prevent charged spray from returning to the nozzle and nozzle mountings. U.S. Pat. No. 5,704,554 addresses the above-described problem by providing structures on the exterior of the nozzle that cause formation of electric force fields to electrically repel spray and keep insulator cavities clean to prevent leakage currents. However, the above-referenced Patents do not address problems caused by contamination and resulting electrical paths on the interior nozzle surfaces.
In addition to reducing the likelihood of direct shorting of the electrode and reduction of spray wrap-back issues onto exterior nozzle surfaces as mentioned above, pneumatic energy provides necessary turbulent aerodynamic forces to assist in improving coating uniformity by reducing Faraday cage effects inside hidden target areas and mitigating fringing effects on prominent target areas. Fringing refers to high deposition on edges and other prominent target geometries where electric field concentrations may be highest. Additionally, air assistance helps overcome environmental factors, such as cross-winds that move spray particulates off target.
The conductive liquid flowing through the nozzle system is earthed to provide the necessary electron flow for the induction process. Liquid resistivity can limit induction charging but only at very high values of liquid resistivity. Generally, induction-charging nozzles begin to show diminished charging capability at liquid resistivity values above approximately 100 Meg ohm cm, a much greater resistivity than would be encountered using solutions mixed with tap water. Induction-charging nozzles are thus suitable for nearly all water-based sprays, but their use is limited for very resistive spray liquids, such as pure oils. Most water-based sprays used in agricultural crop protection, commercial pest control, food safety and sanitizing fall at the opposite end of the resistivity spectrum: generally less than 10,000 ohm·cm. For agricultural applications, water from a local tap source (having resistivity ranges between 1000 to 30000 ohm·cm), is mixed on site with the concentrated chemical bringing resistivity of the final spray solution often down below 1000 ohm cm and often below 100 ohm·cm. The above-mentioned highly conductive liquids help facilitate the induction-charging process. However, highly conductive liquids can also cause various charging issues with art nozzle designs, as mentioned above, as they are operated over long periods of time and interior and exterior surfaces become conductive.
Another important issue in providing optimized charging performance in induction charging systems is control of the positioning of the electrode relative to the atomization zone. A miniature embedded electrode design used in internal air-atomizing nozzles, such as those disclosed in U.S. Pat. Nos. 4,004,733, 5,765,761 and 5,704,554 can be helpful to minimize stray current flows in induction-charging nozzles since the electrode is completely surrounded by an insulator. Only a small interior edge of the conductive cylindrical electrode is exposed along a portion of the length of the walls of the atomization channel. The electrode width needs to be sufficient to provide an adequate field along the length of the entire atomization zone. The atomization zone may shift in position and length with different types of liquid sprays due to changes in liquid flow rate and viscosity, for example. Further, changes in atomizing air energy may change the location or length of the atomization zone. Variations in the manufacturing process of the nozzle cause shifts in nozzle atomization performance. Discontinuities along the interior of the atomization channel, such as may be caused by embedding the electrode between insulators, can cause turbulence and wetting of the electrode. These atomization channel discontinuities cause the spray stream to be less collimated and the diverging turbulent spray pattern is more likely to wrap back and coat the nozzle surfaces. Machining, molding or assembly variations may result in less than optimal positioning of the liquid tip in relation to the electrode. Small variations in dimensions, variations in the linear positioning of the liquid tip along the atomization channel, or concentricity variations will cause spray atomization issues and charging fluctuations. For example, the flexible seal provided between the electrode cap and nozzle body of U.S. Pat. No. 5,704,554 to prevent air leakage, but compression of the flexible seal permits variation of electrode position with respect to the liquid tip, causing up to 0.030″ change in position of the electrode with respect to the liquid tip outlet due to compression of the seal when the cap is removed and replaced.
Another challenge with internal electrode air-atomizing type induction nozzles is in maintaining concentricity of the electrode with the liquid tip outlet. Small side-to-side variations cause atomization issues due to more airflow on one side of the liquid tip outlet. Concentricity variations also change the charging field, increasing the field intensity along one side of the tip and reducing the field intensity on the opposite side. Under such conditions, ionization of the liquid or field breakdown, e.g., arcing, may occur. Nozzles with long liquid tip lengths, such as shown in U.S. Pat. No. 3,698,635 have increased surface distance between the electrode and the liquid outlet. The increased distance may help reduce surface currents, but care must be taken to maintain concentricity between the long liquid tip and the electrode ring. In the nozzle design described in U.S. Pat. No. 6,227,466 to Hartman, the conductive nozzle body in direct electrical contact with the electrode as well as contact at the base of the liquid tip and the liquid hose connection contribute to excessive internal current leakage and resulting charging reductions. In addition, due to small variations in manufacturing, multiple venturi nozzles drawing liquid from a common source compete with each other for liquid flow. The result is a different charge and liquid flow from nozzle to nozzle along the spray boom.
Some induction-charging nozzle designs expose large areas of the electrode to the atomization zone, e.g., by providing longer electrodes. The longer length electrodes may improve charging in systems where the atomization zone is longer or changes with air and liquid flow variations. The larger electrode area is often necessary in air shear, hydraulic or high-volume/low-pressure (HVLP) nozzle types where the atomization zone is partially or fully located on the exterior of the nozzle outlet and may be longer than interior atomizers using higher air pressures and lower gas flows. Examples of air shear or hydraulic atomizers with exterior electrodes are found in U.S. Pat. No. 4,673,132 to Inculet and Castle and U.S. Pat. No. 7,150,412 to Wang, et al.
A particular advantage of induction-charging systems with miniature internal electrodes positioned in very close proximity to the liquid stream and a well-defined atomization zone are much lower power supply voltage requirements for the same or higher intensity charging field strength compared to induction systems with wide electrode gaps. The required current supplied to the electrode can be held very low if electrical leakage is prevented. Theoretically, the amount of current required from the induction electrode's power source should be extremely low—equal only to that required for maintaining the electrode at the proper voltage level. However, prior art nozzle electrode input currents 10 to 100 times higher than the emitted spray cloud currents are sometimes observed, especially after nozzles are operated for extended time periods and have become contaminated. A fraction of the excess current may be due to ionization at electrode discontinuities, but much of the excess is a result of electrical leakage along interior and exterior nozzle surfaces. The power loss due to leakage currents is usually below 0.2 to 2 Watts per nozzle for very conductive spray mixes, which is not a concern when operating from a large power supply such as a vehicle or line-operated power system. However, reducing the leakage current becomes critical when the electrostatic nozzle system is battery-powered. Leakage currents may also cause physical damage to the nozzle interior surface, reducing spray-charging reliability and reducing the life of nozzle components.
Incorporating a liquid outlet tip molded or machined to be an integral part of a plastic insulating nozzle body, as shown for example in U.S. Pat. No. 5,704,554, is an effective construction method to assist in preventing electrode leakage currents from contacting the grounded liquid through seams within the nozzle body. Unfortunately, an integral design requires that the entire nozzle body must be replaced in the event of a damaged liquid outlet tip. Such structures make repair significantly more expensive and difficult to perform since air and liquid hoses as well as electrical connections often must be disconnected and then reconnected to the replacement nozzle body.
As mentioned above, leakage currents over exterior surfaces of induction nozzles and mountings can be significant. Leakage currents on the interior of the nozzles described therein may be much smaller than those over the exterior; however, the impact on charging levels can be more significant. Not only do interior leakage paths cause excess current draw from the power supply, they also cause diminished charging as the stray voltages from the electrode eventually touch the liquid in the nozzle channels, fittings or at the tip outlet. Interior nozzle surfaces surrounding the electrode and liquid inlets and outlet areas become contaminated with moisture or other conductive residues causing the potential of the liquid to be elevated towards that of the electrode, greatly diminishing the charging field and level of the charge on the spray droplets. The liquid in the nozzle tip, being earthed some distance downstream in the nozzle channel or the tubing to the nozzle, will achieve elevated voltage as a function of the current and the resistance of the liquid in the channel. The resulting reduced level of charge is often seen happening gradually over long periods of time while the nozzles are operated continuously. It also may be seen as intermittent charge reduction when moisture from atomizing air or spray liquid builds inside the nozzle, causing momentary shorts between the electrode and the liquid, which are then generally cleared by the moving air. Embedded electrode designs having an insulated electrode are not immune to the above-described problem; contaminants render insulating surfaces of the nozzle interior conductive, and those contaminated surfaces then are in contact with surfaces of the liquid tip or fittings connected to the liquid channels. The structure of the nozzle systems and spray guns described in detail below reduce or eliminate the above-described problems.
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Flexible seal 18 provides additional protection against leakage currents contacting the liquid within liquid channel 19 of nozzle body 2 or within the liquid channel extending through liquid tip 3. Electrode shroud 7 may be integral to cap 5 or be a separate piece of the same or a different material removably attached by threads, press-fit, molded into place, or otherwise attached to cap 5. The material of electrode shroud 7 is generally non-conductive plastic, ideally a plastic providing good electrical insulation and low surface wettability characteristics, such as PTFE, UHMW, Glass, or other suitable dielectric material. Nozzle electrode wire 72 passes into nozzle body 2 and terminates at socket 34 that receives contactor 15. In the example, the insulation of nozzle electrode wire 72 is sealed into the nozzle body 2. An o-ring can be provided within the channel that receives nozzle electrode wire 72 or a non-conductive adhesive or both can be used to ensure an air and liquid seal as well as an electrically tight seal between the wire insulation and nozzle body 2. In the depicted example, the base of the contactor 15 is shown within dielectric shroud 4 and a contactor pin 16 that contacts the electrode 6 extends above dielectric shroud 4. Alternate placement of contactor pin 16 could be outside of dielectric shroud 4, to the side of dielectric shroud 4 and away from base 17 of liquid tip 3. The placement of dielectric shroud 4 blocks stray currents between high voltage portions of the nozzle system, the liquid channels, and liquid tip 3. Nozzle electrode wire 72 is connected to a suitable power supply (not shown) preferably providing 400 to 2500 volts DC. Contactor pin 16 may be spring loaded and extend and retract into contactor base 15, as cap 5 is removed and attached or adjusted. Socket 34 simplifies removal and replacement of contactor 15 from the distal face of nozzle body 2 when cap 5 is removed. However, in accordance with alternative implementations, nozzle electrode wire 72 may be connected to a solid contactor without requiring socket 34 or contactor pin 16.
As mentioned above, locking ring 9 provides adjustable stop edge 26 to limit the movement of electrode cap 5 along adjustment threads 27 which mate to threads on nozzle body 2. The above-described arrangement provides an adjustable positioning and setting mechanism to allow fine adjustment of the position of the outlet end of liquid tip 3 within the electrode channel 11 relative to electrode outlet 12. Locking ring 9 enables precise and repeatable adjustment of the positioning of liquid tip 3 with respect to electrode 6 by rotating cap 5, and the adjustment can be performed while the nozzle is operating. The position of stop edge 26 may be re-settable, which enables calibration of the location of liquid tip outlet 35 relative to electrode 6 to allow for variations in the length and location of the atomization zone due to nozzle component manufacturing variations, as well as for variations in the flow rates or pressure of gas or liquid, or for variation in liquid properties such as viscosity and solids content. Locking ring 9 is generally constructed of a non-conductive plastic. Locking ring 9 may incorporate an air gap 22 to provide increased electrical tracking distance between the high voltage components of the nozzle assembly and the mount for the nozzle. In the depicted embodiment, a non-conductive mounting tube 51 is shown attached to the nozzle body 2. Air gap 22, which extends between locking ring 9 and the outer surface of mounting tube 51 provides a cavity on the outer surface that is less susceptible to contamination from drifting spray around the nozzle. The depicted structure of the spray nozzle provides for access to gap 22 for periodic cleaning if necessary, especially with very conductive sprays used for long periods of time. In one exemplary design, seal 24 may be used to provide additional protection against electrical leakage currents on contaminated nozzle surfaces. An additional seal 25 may be used to prevent electrical current flows along interior surfaces to the outside of the nozzle system which may become conductive due to surface films. Seal 25 may be positioned to allow a sealing surface between the nozzle body 2 and cap 5 even as electrode 6 is adjusted by rotation of cap 5 along mating threads 8. Seal 25 allows a seal maintaining air pressure and against stray electrical currents during operation as cap 5 (and thus the position of electrode) is adjusted by rotation.
A feature of the above-described nozzle system is that each principle component of the nozzle system is easily removable and replaceable from the front (distal end) of the nozzle. The primary components are accessible by removing cap 5. Some nozzle components may become damaged in use or cleaning, may wear out over time, or may need to be changed to provide a different spray characteristic, flow rate, or spray pattern. Cap 5 may be removed from the nozzle body 2 by un-threading of mating threads 8. During re-assembly, the positioning of the stop is preserved by the position of locking ring 9. Locking ring 9 may be cemented into place at the threads or otherwise anchored, e.g., by a set screw, to prevent movement and keep the position of stop edge 26 fixed during repeated removal and re-installation of cap 5. In one implementation, electrode 6 may be press fit into a recess in cap 5. Electrode 6 may be removable from cap 5 for replacement, or alternatively the entire cap 5 and electrode 6 assembly may be integral and replaced together. In instances where various flow characteristics or spray patterns are desired, such as may be achieved with a larger or smaller electrode opening or channel length, the entire cap 5 may be replaced with an alternate cap. In some configurations, the length of a skirt 28 on cap 5 contacting stop edge 26 may be longer or shorter depending on the spray characteristics desired and depending on how a larger or smaller electrode opening may change the necessary electrode-to-liquid-tip placement dimension to achieve optimum atomization and spray droplet charging. For example, the adjustment of the distance between electrode outlet 12 and outlet 35 of liquid tip 3 using adjustment provided by threads 8 and locking ring 9 and/or various electrode cap skirt lengths 28 may be useful to provide optimum adjustment to obtain a very narrow, collimated spray stream 30 at an outlet 29 of cap 5. Collimation of spray stream 30 helps to carry droplets in charged spray cloud 31 further from the nozzle at increased velocity, helping to prevent them from being electrically attracted back onto outer nozzle surfaces and mountings. Electrode 6 in the depicted embodiment is constructed as one piece with a smooth interior channel with the central channel and outlet 12 of electrode 6 is smaller or equal in diameter compared to outlet 29 of cap 5.
An implementation of the spray nozzle of
As described above, dielectric shroud 4 is included to provide a barrier to electrical leakage currents between high voltage parts, such as electrode 6, contactor 15, contactor pin 16, socket 34, and liquid tip outlet 35. Dielectric shroud 4 may surround liquid tip 3 and base portion 17 and may have solid walls or may have walls with openings to allow gas to pass through and around liquid tip 3. Dielectric shroud 4 may be cylindrical (annular) in shape or have a hexagonal, square or other cross-section. Dielectric shroud 4 is formed from an electrically-insulating material and may be integral to the nozzle body or constructed as a separate piece and press-fit or threaded into a channel in nozzle body 2, as shown, or over a flange (not shown) raised above the mating surface of nozzle body 2 and liquid tip 3. It may be advantageous to fabricate dielectric shroud 4, electrode shroud 7 or other mating parts from a different type dielectric material than that to which dielectric shroud 4 and electrode shroud 7 are adjacently joined, since disruption of the paths of surface leakage currents appear to be improved at the seams of the dissimilar insulating materials. One or both of dielectric shroud 4 and electrode shroud 7 may be removable for cleaning or replacement. It may also be desirable to fabricate any or all of dielectric shroud 4, electrode shroud 7 and liquid tip 3 from a material that has low surface wettability and characteristics that prevent formation of continuous electrical leakage paths. Some examples of such materials are ultra-high-molecular-weight polyethylene (UHMW), polytetrafluoroethylene (PTFE), or other materials such as glass or materials with surface coatings, such as non-stick treatments, that render them desirable as insulators in wet and contaminated environments encountered in spraying. Electrical tracking may be interrupted more effectively if dissimilar materials are used for nozzle body 2 and dielectric shroud 4 and/or electrode shroud 7.
While the presence of either of electrode shroud 7 or dielectric shroud 4 will generally reduce leakage currents, the presence of both is desirable, especially if very conductive liquid sprays are used, for instance below 500 ohm cm resistivity, or for spray liquids that tend to leave surface residues. As shown in the embodiment depicted in
In one exemplary nozzle system, the compressed gas moving past liquid tip outlet 35 may create a negative pressure in liquid channel 19. The resulting venturi-pumping action may be adjusted by the position of electrode outlet 12 relative to liquid tip outlet 35 of liquid tip 3. The adjustment may be facilitated by the adjustment threads 8 provided between mating parts of nozzle body 2 and cap 5. Setting of the adjustment can be controlled by the locking ring 9 and stop position 26. Alternate embodiments may include electrode caps with a skirt edge 28 of different lengths to allow for different spray characteristics, such as the aforementioned venturi setting. The interchangeable electrode caps may be switched out as needed.
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While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.
This U.S. Patent Application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application Ser. No. 61/716,884 filed on Oct. 22, 2012.
| Number | Date | Country | |
|---|---|---|---|
| 61716884 | Oct 2012 | US |
