Abstract:
The flow of granulated materials from a supply hopper to a transport tube is controlled. A material feed friction tube is positioned to receive material from the hopper. The friction tube receives material independent from hopper pressure and the material moves to an orifice. The orifice discharges material to a pickup tube from the orifice and includes a transfer tube for receiving material from the pickup tube. The pickup tube has a source of carrier gas sufficient to move the material into the transfer tube for further use.

Description:
BACKGROUND 
       [0001]    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. 
         [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
       SUMMARY 
       [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of one embodiment of the invention. 
           [0011]      FIG. 2  is a perspective view of a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Feed rate controller  10  controls flow of granulated material from hopper  11  to transport tube  12 , as seen in both  FIG. 1  and  FIG. 2 . The material may be any granulated material, including but not limited to powder, pellets, grit, corn, granulated crystals and the like. Controller  10  is particularly suited for feed rate consistency control of grit for suction blast applications. Hopper  11  may be closed or open, with the latter permitting addition of more material while the device is being used. 
         [0013]    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  FIG. 1  to support friction tube inner wall  13 . Friction tube  13  has a length L from the point where the granulated material enters tube  13  to the point where head pressure reduced by inter material friction, discussed below, ends. Friction tube  13  has a diameter C. Length L is greater than diameter C and must be sufficiently long to provide friction forces from the solid particles within friction tube  13  that is greater than the head pressure caused by material in hopper  11 . Friction tube  13  negates the head pressure from hopper  11  by the friction forces when the particles of granulated materials rub against the side walls of friction tube  13  and against each other. Friction tube  13  length L to diameter C ratio is sufficiently high to eliminate any effect of head pressure on the orifice flow rate. Thus L/C&gt;=4. 
         [0014]    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 .  FIG. 1  shows device  10  in a passive mode, where friction tube  13  is vented to atmospheric pressure above an orifice tube  17  having an orifice  18 . The diameter A of orifice  18  is sized to result in smooth transport of the granulated material caused by suction air flow. Orifice tube  17  is held in place with set screw  21 . Pressure above orifice tube  17  is vented to the atmosphere via vent  23  to provide the passive mode described above. When hopper  11  is itself vented to atmospheric pressure  3 , vent  23  is not required. 
         [0015]    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. 
         [0016]    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 P 1  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. 
         [0017]      FIG. 2  illustrates a second embodiment in which a vibrator  25  is connected to hopper  11  to cause powder to fluidize. Vibrator  25  assists the powder entering the smaller vertical friction tube  13  without clumping or bridging at the entrance to friction tube  13 . Partially down friction tube  13  is a vibration isolator  27  which stops the powder from being fluidized in friction tube  13  in order to allow frictional forces between particles and between particles and friction tube  13  walls. Thus length L is the length of friction tube  13  in which friction forces interact with each other and tube  13  walls to provide the needed head pressure. Length  29  isolates the fluidized powder flow from friction tube  13  and is equal to or greater than the distance the granulated material requires to transition from fluidized by vibration to friction dominated flow through isolator  27 . 
         [0018]    In  FIG. 2 , the powder or granulated material is subjected to a control pressure from pressure tube  31 . The head pressure from the particles in friction tube  13  over length L is greater that the difference between control pressure in tube  31  and pressure in hopper  11 . Orifice  18  has a diameter also sized to result in smooth transport of the granulated material at the control pressure and the desired flow rate. 
         [0019]    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&gt;=4. 
         [0020]    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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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.