US3485535A - Metering pump for powder materials - Google Patents

Description

Dec. 23, 1969 P, M. A. FABRE 3,485,535

METERING PUMP FOR POWDER MATERIALS Filed April 14, 1967 2 Sheets-Sheet 1 Invefior P. M. A FABR c @MMMAMQ Dec. 23, 1969 P. M. A. FABRE METERING PUMP FOR POWDER MATERIALS 2 Sheets-Sheet 2 Filed April 14, 1967 E s R m M M an W mm m m w M H m .y 7 O A Hm. 5... 5 2 1 Q .r

United States Patent Int. Cl. fiss 53/40 US. Cl. 302-50 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a store of powder contained in a hopper (1) fed by an anger screw (11) into an injector section (5) in which pressure air is injected by a nozzle (9) and through a porous wall (6) to create a powder-in-air suspension that is discharged through a line (10) to a point of use which may be remote. A wall section (3) between the hopper and injection section (5) is made of porous material to permit that portion of the injected air that is forced backwards owing to the flow resistance in the discharge line (10) to drain outwards to atmosphere instead of rising through the body of powder and thereby preventing a stable and controllable feed of powder by the auger (FIGURE 1).

CROSS-REFERENCES TO RELATED APPLICATION This application is filed under international Convention with priority from French patent application PV 58,055, filed Apr. 18, 1966, by the assignees.

This invention relates to the metered feeding of pulverulent solid materials, suspended in a stream of air or other gas. An operation of this sort is required as an essential step in a wide variety of industrial processes. As examples of powder materials that may have to be fed in metered amounts to a point of use, there may be mentioned such inorganic powders as cement, carbon, talc, kaolin, and other minerals, organic powders such as polymerizable resin materials, flour and many others. In many cases the powder material has to be conveyed to a remote point of use over a line that is correspondingly long, narrow and perhaps circuitous, so that the flow resistance therein to the air-suspended powder is high. At the same time it is often essential that the feed of powder be continuous, highly uniform and stable over long periods of time and accurately controllable over a wide range as to the rate of feed, and that the accurate adjustment and readjustment of the feed rate be easily and quickly effected.

One basically satisfactory form of construction for a powder metering pump of this kind, and a form of construction that is very frequently used, comprises a mechanical feed member such as an auger screw rotatably mounted partly in a hopper or other container containing a store of the powder material and partly in a conduit extending from the container. In the conduit is provided an air injection nozzle connectable to a source of compressed air and discharging the air in a generally axial direction in the conduit. Rotation of the auger screw feeds powder material from the store through the conduit and this powder is taken up by the injected air to form an air-suspended powder stream which is conveyed to the point of use over a discharge line.

This conventional construction though basically sound has been found to possess serious practical deficiences. The feed of powder is non-uniform and is diflicult or impossible to control with precision. For a given speed setting of the auger screw drive, the rate of feed of the powder-in-air suspension is not uniquely determined. Further, where the flow resistance of the discharge line 3,485,535 Patented Dec. 23, 1969 is comparatively high, as in the cases mentioned above, the conventional devices are generally inoperative in the absence of elaborate precautions which complicate the construction and operation of the installation, such as pressurizing the powder storage container. This last measure is clearly undesirable not only because of the extra equipment and powder expenditure it entails, but because it requires cutting off the feed of powder every time the storage container has to be refilled.

Another defect of the powder pumps of the specified class is that there is a tendency to clogging in the region of the air injection device. This difficulty is especially marked with certain types of powder material which have a propensity to agglomerate, and is due to the fact that the air discharge from the injection nozzle blows a proportion of the powder particles fed from the auger screw against the inner surface of the conduit surrounding the injection nozzle, where said particles build up gradually into a compact layer. Frequent shut-down is necessary to remove this layer before it has developed to such a thickness that it prevents further operation of the pump.

SUMMARY OF THE INVENTION The applicants experiments have shown that the chief above-mentioned deficiencies of conventional powder pumps of the auger-feed, air-injection type are ascribable to a precise cause. This lies in the fact that a small but appreciable proportion of the air injected by the injector nozzle is forced back in a direction counter to the direction of powder feed, and this backflowing air traverses the body of powder surrounding the auger screw, partly fluidizing the powder adjacent the screw threads and impeding or preventing the correct action of the auger screw in feeding the powder in the forward direction. Apparently the powder particles when thus fluidized fail to adhere positively to, and hence are not gripped by the screw thread surfaces, and as a consequence the rate of forward feed of the powder by the screw cannot be positively and accurately controlled by the speed of rotation imparted to the screw.

Further, since the proportion of injected air forced back as just described obviously increases as the flow resistance of the discharge line beyond the air injector is increased, the degree of fluidization of the powder mass by said backflowing air can become so great, in cases where the discharge line is relatively long and narrow, that the auger screw may then completely fail to drive the surrounding powder particles owing to lack of adherence between the fluidized particles and the screw thread surfaces; this explains the inoperability of the conventional devices in the circumstances just mentioned, unless the storage container is placed under substantial air pressure high enough to oppose the back flow of the injected air.

Objects of the invention are to provide an improved pump of the mechanical-feed air-injection type for powder material, which include the following features:

Full effectiveness even in, cases where the discharge line has high flow resistance, as where the powder is to be conveyed in an airstream to a remote point;

Achievement of such effectiveness without having to pressurize the powder store, thereby permitting easy replenishment of the store without shutting down the feed of powder;

Stable and uniform feed of powder at a rate that is a precisely determined function of the speed of rotation of the auger screw (or other mechanical feeder), whereby the rate of feed can be simply and reliably controlled and adjusted by adjusting the speed setting of the drive.

The above objects are achieved by providing, in the conduit means extending from the powder store and ahead of the air injection nozzle, venting means arranged for allowing any air flowing back from the air injection nozzle to escape laterally outwards and thereby preventing such backflowing air from interfering with the feed of powder by the mechanical feeder surfaces.

Preferably the venting means takes the form of an air-permeable wall section of a part of the conduit means between the powder source and the air injection nozzle.

Another object of the invention is to eliminate any tendency to clogging in the region of the conduit means surrounding the air injection nozzle.

For this purpose, according to an aspect of the invention the wall section of the conduit means in the region surrounding the air injection nozzle is made of air-permeable material, means are provided to define a pressure chamber around said permeable wall section, and means for creating in said chamber an air pressure exceeding the air pressure cerated in said conduit section by the air injection nozzle. Due to this provision, powder particles that might tend to cling to the inner surface of the conduit means around the air injection nozzle are prevented from building up into a compact layer thereat due to the positive pressure differential between the inside and outside of the permeable conduit wall.

While in the preferred embodiments of the invention to be described hereinafter, an auger screw is used as the feeder member, it is to be understood that the principles of the invention are equally applicable to other types of mechanical feeder devices, such as vaned rotors and the like, since it will be apparent that the correct feed of powder by the moving surfaces of any such mechanical feeder devices are equally prone to interference from the backflow of air from the air injection means, and would therefore be susceptible to improvement through use of the venting means of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified view in axial cross section of the main parts of a powder metering pump according to a first form of embodiment of the invention; and

FIG. 2 is a similar view of another embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS The powder pumping apparatus shown in FIG. 1 comprises a powder storage hopper or container 1 which may be conventionally made of sheet metal to a cylindroconical shape, including a cylindrical upper section not shown and a frusto-conical lower section 1a. Secured by way of flanges 15 to the lower end of the hopper 1 is a further frusto-conical wall 3 which is porous. The porous wall section 3 is shown surrounded by a cylindrical casing 16 made of non-porous material, e.g. sheet metal, and provided with at least one vent opening such as 17 whereby the annular chamber defined between walls 3 and 16 is normally at atmospheric pressure. The lower end of porous frusto-conical wall 3 has secured to it the upper end of a cylindrical tube section 4.

An auger screw 11 is rotatably supported axially of the structure so far described as by being suspended at its upper end by means of a swivel or cardan joint 13 power driven from a shaft 12. Shaft 12 may be connected to any suitable variable speed driving means, not shown, such as to the output of a conventional speed variator driven by a suitable electric motor for example. The auger screw 11 extends downwardly to a point about flush with the lower end of tube 4.

Connected with the lower end of tube 4 is a frustoconical tubular section 6 made of porous material. This porous section 6 is encased in an airtight casing 7 having a union 18 connetced with a side of it for connection to a source of compressed air. Moreover, an air injector is positioned within the porous tube section 6, the injector including as shown a nozzle 9 which is bent to extend along the axis of the tube section 6 in the direction of powder discharge (here in the downward direction) and extends in sealing relation through the wall of porous tube 6 and the wall of casing 7 for connection through a threaded union with an air pipe 8 supplied with compressed air at a pressure somewhat less than the air pressure, which is in operation connected to the pipe union 18.

A discharge pipe 10 is connected to the lower end of porous pipe section 6 e.g. through the threaded union shown. The pipe 10 leads to any desired receiving station at which the powder stream is to be utilized, and may be long and circuitous. It may in part or in whole consist of flexible hose if occasion demands.

In operation, a store of powder material is contained in the hopper 1. This may, for example, be a polymerizable resin powder which is to be conveyed at an accurately metered flow rate by means of the metering pump of the invention to a powder coating station, not shown. The auger screw 11 is set in rotati n by way of drive shaft 12 at a predeterminable angular speed proportionate with the desired metered rate of flow for the powder. Rotation of the screw feeds the powder downwards from hopper section 1a through porous conical section 3, solid tubular section 4 and porous conical tubular section 6. Compressed air at a suitable pressure such as 1 kilogram per sq. cm. (gauge) is injected through pipe 8. This air discharged from nozzle 9 into the powder filling the discharge tube section 7 discharges the steam 0f powder-in-air suspension into the discharge pipe 10 for conveyance to the remote point of use.

In the type of operation just described, it has been found in past practice that a smooth uniform flow of powder has been extremely difficult or impossible to obtain. Specifically, it has been found impossible to prevent an appreciable fraction of the air flow from injector nozzle 9 from flowing back in a direction reverse from the direction of powder discharge, i.e. in the upward direction as here shown. The proportion of the injected air flow that is forced back in this manner is increasingly great as the flow resistance of the discharge line 10 is great, that is, as said line is long and circuitous and narrow; but some of this backflow is invariably found to occur, and as a consequence of the backflowing air rising through the bed of powder, the powder is to some extent fluidized in the hopper and boils around the auger screw, so that the feeding efiiciency of the auger screw is gravely impaired. It is found impossible to maintain a uniform, controllable downward feed of powder.

This defect of prior powder metering pumps has been completely eliminated in the improved apparatus described above, owing to the provision of the p rous venting section 3. Apparently the backflowing air from injector nozzle 9 after rising through the cylindrical tube section 4 chooses the path of least resistance presented to it in the form of the porous wall 3, and seeps out through the porous wall rather than continuing upward through the mass of powder. As a consequence it is found that there is no boiling or fluidization of the powder mass, and a perfectly smooth and controllable feed of powder is effected by the auger.

The discharge arrangement including the lower porous wall section 6 and surrounding pressure air box 7 serves a different and also a useful function. In the operation of pneumatic powder pumps of the general class considered it has been found that clogging frequently occurs in the conduit section, usually of tapered frusto-conical shape as here shown, that surrounds the air injection nozzle. The injected airflow tends to press a proportion of the powder particles against the inner surface of the tapered connecting conduit, gradually building up a compact layer thereat wh1ch ultimately impedes the flow of the powder- 1n-a1r suspension. This defect is eliminated with the arrangement disclosed herein. The air injected through pipe 18 at a pressure somewhat exceeding the pressure of the injected air from nozzle 9 creates an overpressure in the annular space within casing 7 around the frustoconical discharge tube 6, as compared to the air pressure inside this tube. Due to this over-pressure, air tends to seep continually inward through the pores of the porous wall of said tube 6, continually preventing the formation of a compact powder layer coating its inner surface.

Preferably, a vibrator 14 may be associated as sh wn with the hopper section It: in order to promote the downward feed of powder especially in the case of compact powder materials of the kind that tend to build into bridges.

In the embodiment of FIG. 2, parts similar to parts in FIG. 1 are designated with the same reference numerals and only the differences between the two embodiments will be described. As shown, between the bottom of the hopper 1 and the porous feed section 3 earlier disclosed as permitting the discharge of backflowing air, there is interposed additional structure as follows. Connected to the lower end of hopper 1 by means of the flanges 15 is an additional porous frustconical wall section 19 which is encased in a sealed casing 20 having a side connection with a compressed-air pipe 21. Below this additional porous section is a cylindrical wall section 22 which has a diameter larger than that of the lower end of the section 19, and which is shown connected therewith by means of a centrally apertured sealing plate 23 which separates the interior of cylindrical section 22 from the annular chamber defined between the p rous section 19 and its surrounding casing 20. The lower end of cylindrical wall section 22 connects with the upper end of the porous section 3 earlier described, by means of flanges 24.

It will be'noted that the threaded or vaned portion of the auger screw 11 is shown as extending upwards only to a level corresponding to the top of the cylindrical section 22. A vibrator 25 may, if desired and as here shown, be associated with the outer side surface of said cylindrical section 22.

In the operation of this embodiment, compressed air at a relatively low superatmospheric pressure of eg 50 grams/sq. cm. is applied through pipe 21 into the annular chamber 20A and seeps inward through the porous wall 10. This fluidizes the powder particles in the region above the active, threaded or vaned section of auger 11, and ensures that the powder is fed smoothly and regularly by gravity into the cylindrical chamber 22. In this chamher the powder is taken up by the threads of the auger screw 11 and is fed downward thereby in a positive manner and at a rate that is strictly controlled by the speed of rotation of the auger, owing to the fact that there is good adhesion of the p wder particles to the screw thread surfaces because of the absence of any air traversing the body of powder in said chamber 22, since any air backflowing from the air injector section 5 of the apparatus has been vented to atmosphere in the venting section 3 as in the first embodiment.

Another difference between the two embodiments is that, whereas in FIG. 1 the auger screw 11 is freely suspended from its upper end and centered in the surrounding structure mainly by the action of gravity, in FIG. 2, the auger 11 is shown positively centered both at its upper and lower ends, by means of an upper centering ring 26 supported through spider arms from the surrounding hopper structure, and a lower centering ring 27 similarly supported in the lower end of tube 4. Except for the noted differences, the general operation of this embodiment is the same as that earlier described.

While the various porous wall sections utilized in the apparatus of the invention may be made from any of a number of suitable porous or gas-permeable materials, it is indicated that good results have been obtained with the porous sintered bronze material sold as Poral by Socit d Electrochimie et dElectromtallurgie des Aciries dUgine, France, used in thicknesses in a range of from 0.5 to 2 mm., and more especially the materials identified as Poral BK42005.

In one practical construction of the embodiment of the invention shown in FIG. 1, the metering pump as designed for pumping epoxy resin powder having an average granulometry of from 10 to microns and a true density of 1.2, to an electrostatic powder coating station. 'It was specified that the pumping rate was to be continuously adjustable over a range from zero to kilograms powder per hour. The discharge line 10 was a Rilsan pipe 11 mm. in internal diameter and 15 meters in length.

The pump device of the type ShOWn in FIG. 1 constructed to meet the above specifications was 23 centimeters high as measured from the base of discharge casing 7 up to the top of the upper casing 16, and the outer diameter of said latter was 9.5 cm. This was surmounted with a hopper 1 having a maximum diameter of 50 cm. and a total height of 75 cm, which was open at the top for easy replenishment with the powder material during operation. The structure was made generally from mild steel sheet"1 to 2 mm. gauge, except for the porous wall sections 3 and 6 which were made of the above-identified Poral material, 1 mm. thick. The auger screw 11 was 50 cm; long, 35 mm. outer diameter and 35 mm. pitch, made of steel. The auger was freely suspended through a simple cardan joint 16 mm. in diameter from a drive shaft 12, 10 mm. in diameter, driven through an infinitely variable speed drive of the friction cone type from a three-phase synchronous motor of 0.200 kilowatt rating. The air injector 9 was made of tubing 2 mm. inner and 4 mm. outer diameter and was connected to a pressure air source through a restrictor valve providing a pressure of 1 kg./sq. cm. at the injection nozzle outlet. The air pipe 18 was connected to deliver a pressure of 1.4 kg./ sq. cm. in the outer casing 7.

When the auger screw drive was adjusted to any one of each of a plurality of speed settings from 20 r.p.m. to r.p.m., it was found that the rate of delivery ofthe powder material was proportionally varied from 12.6 kg./hr. to 106 kg./hr. At each speed setting the feed of powder was smooth and stable and could be continued indefinitely without any tendency to fluctuate around or depart from the precise rate of powder feed corresponding to the selected speed setting of the auger.

As a simple control of the utility of the invention, the air venting means constituted by the porous wall section 3 was rendered inoperative by stopping the vent hole 17 with a thumb. It was observed that the free surface of the store of powder in hopper 1 immediately started to heave and boil, demonstrating that air from the injector nozzle 9 (and from the additional air inlet 18) was rising through the body of powder. Within a very few seconds, the discharge of powder from output line 10 ceased completely.

After many hours of continuous operation the appara tus was dismantled. The converging tube section 6 surrounding the air injector was examined and its interior surface was seen to be virtually free of adhering powder particles.

Various modifications may be introduced into the exemplary constructions illustrated and described without departing from the scope of the invention. Thus, as earlier noted, the auger screw 11 may be replaced with some other form of mechanical feeder, e.g. one or more vaned rotor members. This would simply entail a suitable modi fication in the geometry of the structure between the storage container 1 and the injection section 5, but would not otherwise affect the teaching of the invention. The venting means, here shown in the form of a rigid wall section of porous material, may assume other forms, as for example an array of small perforations in an otherwise solid and compact wall material, the perforations being made small enough to prevent outward leakage of, or obstruction with, the powder material with which the apparatus is used. As another alternative, the venting means may comprise one or more sections of fabric or other permeable sheet material. It is noted further that while a permeable wall section 3 of the frustoco'nical shape shown herein has given very good results, this particular shape does not appear essential and it would be feasible, in a construction otherwise similar to that shown in FIG. 1, to provide the wall section 3 of solid compact material, and form the cylindrical tube section 4 of porous material instead.

The apparatus disclosed may be used in any orientation of its center axis relative to the vertical, provided the auger screw is suitably supported e.g. as described with reference to FIG. 2.

What is claimed is:

1. A metering pump device for pulverulent material comprising: a container (1) for a store of said material; conduit means (19, 22, 3, 4, 6, 10) extending from the container; an auger screw (11) positioned and extending axially along a part (22, 3, 4) of said conduit means, said auger screw being rotatable at a controllable speed for feeding material from the container through said conduit means; injector means (5) comprising a discharge nozzle (9) externally connectable to a source of pressure gas and positioned in said conduit; means associated with a section of said conduit upstream of said discharge nozzle for venting any air discharged by said discharge nozzle which has flowed upstream from said nozzle toward the container, said conduit means comprising a gas-permeable wall section (6) surrounding said discharge nozzle means; means (7) defining a pressure chamber around said gas-permeable wall section; and means (18) creating a gas pressure in said chamber exceeding the gas pressure created in said wall section by said injection nozzle means.

2. A pump according to claim 1 wherein said conduit means further includes a gas-permeable wall section (20) positioned upstream of said venting means and upstream of said last-mentioned conduit portion, and means (21) connectable with a gas pressure source for discharging gas through said gas-permeable wall section into the conduit means for fluidizing the pulverulent material.

3. A pump device as claimed in claim 1 in which the average cross-section of the conduit portion in which said nozzle is located is greater than the maximum diameter of the screw in that conduit portion, and the conduit portion with which the venting means is associated and the conduit portion upstream of said venting means are successively larger.

References Cited UNITED STATES PATENTS 2,448,745 9/ 1948 Struckmann 302 3,099,496 7/1963 Kayser 302--50 FOREIGN PATENTS 1,055,653 10/1953 France. 1,115,327 12/1955 France. 1,129,406 9/ 1956 France.

ANDRES H. NIELSEN, Primary Examiner U.S. Cl. X.R. 302-52

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