Patent Application
20130186920
A wide variety of materials in small particle size are transferred from hoppers to a location where the materials are to be used. Powders as small as dust or chemical powders to as large as pellets or corn, by way of example, are taken from hoppers using devices that are gravity fed to introduce the material into a gas flow for the intended use of the material.
Feed rate control, such as for example, of grit for suction grit blast applications suffers from variability due to changing hopper loading and dust collector vacuum. Variations in these parameters cause the flow of grit through an orifice plate or into other suction pickup devices to fluctuate.
To achieve a constant flow rate, which is highly desirable, the operator must frequently adjust orifice and suction settings. With only loose observational process feedback available to operators, these adjustments are made infrequently and somewhat arbitrarily. This makes locking down process parameters impossible.
It is also a significant problem if the process has to be stopped in order to reload the hopper. It is also a problem if changes in pressures, flow rates, and supply volumes are adjusted arbitrarily based on observations after a change in flow has occurred.
It would be a great advantage in suction grit blast applications if control of grit flow could be achieved to accommodate changes in the process, particularly in compensating for grit material head pressure in the grit supply.
A device, system and method for controlling feed rate consistency in the flow of granulated materials is provided. Granulated materials are transferred from a supply hopper to a material feed friction tube. The tube has a predetermined diameter and length leading to a discharge end. The length of the tube is sufficient to provide a head pressure from the solid particles that is independent of hopper loading. When the hopper is open, which allows additional material to be added to the hopper as needed, the head pressure on the material in the friction tube is dissipated by friction between particles and the walls of the friction tube, making the pressure at the discharge end of the friction tube independent of the head pressure of the material in the hopper. The length to diameter ratio is sufficiently high to eliminate the head pressure. An effective length to diameter ratio, L/C, is equal to or greater than four.
Material is then discharged from the tube through an orifice having a smaller diameter than the diameter of the friction tube. A pickup tube is positioned to receive material from the orifice and move the material into a transfer tube using a source of carrier gas.
The pickup tube has a diameter large enough to accept all the material discharged from the orifice, and the transfer tube has a diameter equal to or less than that of the pickup tube.
In one embodiment, the orifice diameter can be adjusted to insure smooth transport of the solid material. A control pressure source of gas can be provided for adjusting the pressure at the orifice to approximately the pressure of the source of carrier gas in the pickup tube. A vibrator near the bottom of the hopper can be used to help fluidize the powder and assist the powder in powder entry into the friction tube.
Feed rate controller 10 controls flow of granulated material from hopper 11 to transport tube 12, as seen in both
Controller 10 includes material feed friction tube 13 that is positioned to receive material from hopper 11. Friction tube 13 may include an outer wall 15 in
At the bottom of friction tube 13, orifice tube or plate 17 is positioned to receive material from the discharge end 19 of friction tube 13.
Particles exit friction tube 13 into orifice tube 17 and are controlled so that flow of particles is maintained regardless of the quantity of particles in hopper 11. Orifice 18 diameter A is smaller than friction tube 13 diameter C. Particles flow down into pick up tube 33, which has a diameter D that is larger than material height B in order to start airflow without clogging.
Height B is high enough to prevent material from piling up and influencing flow through orifice 18. Carrier gas, such as nitrogen, has a pressure P1 and enters pickup tube 33 to transport the particles into transport tube 12, which has a diameter E that is less than or equal to pickup tube diameter D. The velocity of the transport tube 12 gas is greater than or equal to the pickup tube 33 velocity.
In
Granulated material exiting orifice 18 into pick up tube 33 does not pile up and influence the flow through the orifice because carrier gas flow rate is sufficient to prevent that from occurring. Carrier gas flow in transport tube 12 is greater than or equal to pickup tube 33 velocity. Again, friction tube 13 length L to diameter C ratio is sufficiently high to eliminate any effect of hopper head pressure on the orifice flow rate. Thus L/C>=4.
Hopper 11 can be open, for the addition of more granulated material during operation as long as the head pressure over length L is greater than the delta P between the control pressure in tube 31 and atmospheric pressure.
When carrier gas flow is stopped, granulated material exits orifice 18 into pick up tube 33 and piles up to stop the flow through the orifice. Flow can then be re-established by resuming gas flow. This can be achieved smoothly by maintaining height B at a sufficiently small fraction of the diameter of pickup tube diameter D to allow the initiation of gas flow around the piled up granular material.
In both embodiments, the powder or granulated material exits transfer tube 12 as intended. The present invention has been found to be effective in controlling the flow of particles from a hopper to an end use, such as grit blasting of objects such as metal parts.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
