Research Category: Bulk Material Handling
Researchers: Kleineidam, Andreas|
Location: Elmshorn, DE, United States
The invention concerns a process and device for conveying powdered material. The process and device are used especially in powder coating installations to convey powder coating material by compressed air in dense phase from a reservoir into a delivery line and through this to a spray gun or other type of spray application device.
Until now, in powder coating installations, the powder coating material was usually pneumatically conveyed by the dilute phase method from a reservoir to a spray gun through a hose-like delivery line. However, this causes problems, because, first, relatively large amounts of compressed air are needed, second, the diameter of the hose-like delivery line must be relatively large, and third, wear occurs at the bends in the delivery lines. For this reason, over the past several years, tests have been conducted at a number of powder coating installations with so-called plug flow conveying or dense phase conveying, in which the powder coating material is cyclically conveyed either by gravity or by negative pressure into a chamber and the discharged from the chamber with compressed air and conveyed in the form of successive “plugs” through the delivery line to the spray application device.
A process and device of the type cited at the beginning for the plug flow or dense phase conveyance of powdered materials with suction by negative pressure is already known from DE 196 43 523, DE 196 54 648, and EP 0 937 004 B1. The previously known device has a cylindrical pump chamber, which is equipped with a discharge opening for the conveyed material at its lower end and a plate-like filter element that is impenetrable by the conveyed material at its upper end, by which the pump chamber can be alternately connected with a vacuum pump and with a source of compressed gas, to suck the conveyed material from a reservoir through a connection that opens into the pump chamber from the side for the purpose of filling the pump chamber, or to push the conveyed material through the discharge opening into a discharge line for the purpose of emptying the pump chamber. To allow exact metering of the conveyed material and at the same time a high discharge volume, the pump chamber should have a filling volume that is as small as possible, and it should be possible to fill it and empty it in an operating cycle that is as short as possible. To achieve the latter objective, however, the gas must be sucked from the pump chamber and fed into the pump chamber relatively quickly, which requires a pressure difference that is as large as possible between the inside of the pump chamber and the source of negative pressure or the source of compressed gas. However, a high pressure difference at the filter element causes larger bending loads and pressure loads on the filter element and thus a reduction of its service life, which is why the filter element must be supported with support lattices or the like. However, this in turn results in a reduction of its passage cross section, which makes it necessary to choose between a greater load and thus a shorter service life of the filter element and a higher gas throughput and thus a shorter operating cycle. In the case of the pneumatic conveyance of powder coating material, there is the additional consideration that this material has a particle size of <80 .μm, and about 10-15% of this material is in the particle size range of <5 μm. Since this is of the same order of magnitude as the pore diameter of the filter materials that are used, small particles can penetrate deeply into the filter element or even pass through it. Some of the particles that were mentioned first are retained in the filter element during the subsequent admission of compressed gas and may not become detached from the filter element again until after a prolonged period of time, which can lead to contamination of the coated surface after a color change. The particles mentioned last can cause pump damage, at least when diaphragm pumps are used to produce the negative pressure. However, the use of a smaller pore diameter to avoid these problems would in turn result in a lower gas throughput and thus longer operating cycles. Furthermore, in the previously known device, when the chamber is emptied, residual pigmented powder coating material can also remain inside the suction intake connection, which can also result in contamination of the powder coating material during a color change.
Proceeding on the basis of this prior art, the object of the invention is to improve a process and device of the type specified at the beginning in such a way that the service life of the filter element is prolonged and contamination of the filter element can be more easily avoided.
In accordance with the invention, this object is achieved by designing the gas-permeable filter element as a hollow cylinder in such a way that it forms a part of the cylindrical surface of the chamber, unlike the filter element of the state of the art, in which it is designed as an end wall of the chamber. The invention is based on the idea that this measure provides a simple means of increasing the filter surface of the filter element and thus reducing the pressure difference between the outer and inner surface of the filter element at the same gas throughput, without increasing the volume of the chamber and thus adversely affecting the metering precision. Furthermore, as a filter element constructed as a section of the cylindrical wall, it allows axial conveyance of the powdered material through the chamber in a straight line, which ensures frictionless conveyance, unlike the situation with a filter element installed on an end face. Moreover, no moving parts are required other than at the chamber inlet and outlet.
It is advantageous for the hollow-cylindrical filter element to be made of a sintered material, preferably a sintered plastic powder, since, when rigid filter materials of this type are used, the use of a support material becomes unnecessary. In addition, a cylindrical filter element made of a rigid filter material has greater stability than a flat filter element of the same size and therefore can be produced with a smaller wall thickness at equivalent permeability. It is advantageous for the pore diameter of the sintered material to be smaller than the smallest particle diameter of the powdered material being conveyed, i.e., in the case of the conveyance of powder coating material, preferably smaller than 5 μm.
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