US3645583A

US3645583A - Apparatus and method for handling finely divided solids

Info

Publication number: US3645583A
Application number: US26882A
Authority: US
Inventors: Calvin P Heath
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-04-09
Filing date: 1970-04-09
Publication date: 1972-02-29
Anticipated expiration: 1989-02-28

Abstract

An improved apparatus and method for the discharge, by fluidization and gaseous transport, of finely divided solids from the interior of a storage or transport vessel, such as, for example, a highway bulk transport trailer, or a railway car for similar service. The apparatus includes at least one assembly for providing a fluidizing gas under moderate pressure at the lower boundary of a mass of the finely divided solids in a discharge hopper conventionally forming part of the vessel or tank. The said assembly includes a distributor means and a fabric cover therefor, said cover being characterized in that it has a proportion of looseness, with respect to the gas releasing surface of the distributor means. Upon application of gas pressure to cause flow through said loose cover a vibration is initiated and fluidizing and discharge is accomplished at a greatly accelerated rate.

Description

United States Patent Heath Feb. 29, 1972 [54] APPARATUS AND METHOD FOR HANDLING FINELY DIVIDED SOLIDS [72] Inventor: Calvin P. Heath, 2158 Wisteria Street, Baton Rouge, La. 70806 221 Filed: Apr. 9, 1970 21 Appl.No.: 26,882

FOREIGN PATENTS OR APPLICATIONS 674,707 11/1963 Canada....... .....302/29 765,158 1/1957 GreatBritain ..302/29 Primary Examiner-Richard E. Aegerter Assistant Examiner-Hadd S. Lane Attorney-Lewis R. Krieg ABSTRACT An improved apparatus and method for the discharge, by fluidization and gaseous transport, of finely divided solids from the interior of a storage or transport vessel, such as, for example, a highway bulk transport trailer, or a railway car for similar service. The apparatus includes at least one assembly for providing a fluidizing gas under moderate pressure at the lower boundary of a mass of the finely divided solids in a discharge hopper conventionally forming part of the vessel or tank. The said assembly includes a distributor means and a fabric cover therefor, said cover being characterized in that it has a proportion of looseness, with respect to the gas releasing surface of the distributor means. Upon application of gas pressure to cause flow through said loose cover a vibration is initiated and fluidizing and discharge is accomplished at a greatly accelerated rate.

7 Claims, 9 Drawing Figures Patented Feb. 29, 1972 3,645,583

2 Sheets-Sheet l INVENTOR. ALVIN P. HEATH ATTORNEY;

I Patented Feb. 29, 1972 3,645,583

2 Sheets-Sheet 2 INVENTOR. CALVIN P HEATH APPARATUS AND METHOD FOR HANDLING FINELY DIVIDED SOLIDS BACKGROUND OF THE INVENTION The gas fluidization and gas transport of finely divided .commodities has long been known as an industrial technique. As is generally well known, the term fluidizing means that a system of subdivided solids is formed suspended in a transport gas, said system having the liquid flow characteristics, of a homogeneous gas or vapor. The high utility and common use of fluidization results from this characteristic, as the solid commodity can be stored in bulk, or transported in bulk containers, rather than requiring laborious packaging in bags, fiber drums or the like.

The fluidizing technique is successfully applied for many materials, including dry cement, lime, ground cereal grains, carbon blacks, chemical catalysts, and other products.

In the application of the fluidizing technique in the transport industry, truck trailers or railway cars are provided with tanks having a series of bottom hoppers spaced along the tank body. Such hoppers generally resemble an inverted cone or trapezoid section. At the bottom of each such hopper is usually provided a valve of the slide or flapper type. Connected to this discharge point is a nozzle or adapter in turn connected to a discharge manifold line affixed lengthwise to the vessel or its supporting vehicle. This line is fed by, or can be fed by any or all of thehoppers associated with the tank. Supply means are provided at one end of the manifold discharge line for feeding transport gas. At the other end, a connection is provided to a distribution device, or for connecting to a receiving conduit leading to the storage vessel to which the commodity may be delivered.

Inside each hopper in such above-described apparatus is at least one fluidizing gas assembly. Such an assembly includes a gas inlet conduit, leading through the wall or boundary of the hopper, a distributor device, and a fabric cover for the distributor device. The distributor devices can be in a variety of shapes and certain designs are specifically illustrated hereinafter. Generally, the distributor is an elongated conduit having a number of apertures along its length for the release of the fluidizing gas. Surrounding, or at least covering the portion of the distributor provided with apertures, is a fabric cover element. This serves as the final barrier to gas flow before the gas contacts the commodity in the hopper.

When the distributor is in the form of an elongated conduit, the said fabric cover resembles a sleeve, and is sometimes so designated herein.

In operation, the valve from the hopper is opened, and transport gas is fed to the manifold discharge line, and fluidizing gas is fed to the above-described assembly, or more commonly to the plurality of identical assemblies in each hopper. The normal loading openings for the tank, at the top of the tank structure, are closed during this operation. The introduction of the fluidizing gas results in the conversion of the at-rest solids to a fluidized system, which flows through the discharge valve into the manifold discharge line and is transported therein to the desired point. The actual discharge is accomplished by the joint effect of gravity and the slight or moderated pressure differential buildup in the tank body as a result of the volume of fluidizing gas.

The above-described apparatus and method are in general operative and effective and tremendous quantities of materials are transported and then delivered in the manner described. It will be immediately realized that the time required for the discharge of a given load is a direct and indirect cost factor. The time required is a direct wage cost with respect to the driver or other transportation company employees. The removal of the tank from active service, in the case of transport vessels, increases overhead costs by lowering the service factor and the revenue-producing capabilities of the equipment, as charges are based on distance of transport rather than on delivery operations. The slowness of discharge time has therefore been a severe limitation to the ultimate efficiencies of the gross storage and transport of finely divided solid commodities. A great need has therefore existed for improved apparatus and procedures in this field.

OBJECTS OF THE INVENTION The general object of the invention is to provide improved apparatus and a method for the discharge of finely divided solids from transport or storage vessels. A more particular object is to provide an improved assembly, and mode of operation thereof, for the purpose of fluidizing solids within the confines of a tank as a step in the discharge of said tanks as above described. Other objects include providing apparatus which in operation or use reduces other time requirements incident to the overall operations. Further objects will appear in the detailed description given herein.

SUMMARY OF THE INVENTION The apparatus of the present invention comprises, in a storage or transport vessel for finely divided solids, having a downwardly directed hopper terminating ina discharge port, and distribution means for supply of fluidizing gas, a fabric cover loosely positioned over a gas-releasing segment of the gas distribution means. In operation, gas is supplied through said distribution means and the loosely fitting fabric cover into the solids load or cargo resulting in vibration of said cover and fluidization of said solids.

In the improved assembly for providing fluidizing gas to the interior of a hopper section, it has been found that a particular level of looseness is required, which I express in terms of a looseness ratio. By this term is meant the ratio of an inside dimension of the fabric cover available for disposal opposite the gas-releasing portion of a distributor element, to the minimum corresponding dimension established by the distributor per se. In the case of an assembly having generally an elongated configuration, this criterion of the completed assembly is determined in a transverse plane, viz., determined with reference to the transverse dimension in a manner hereafter described. In other configurations, for example, in square or circular designs, the looseness ratio applies or is determined in both directions, in the case of a square plan, or to any diameter intersection in the case of a circular configuration.

Explained more fully below is the manner of determination of fabric cover size requirement for assembly with any explicit distributor structure. The looseness proportion or ratio should be below about 1%, preferably from about 1.05 to about 1.20, a preferred ratio being 1.06 to 1.15, and an optimum ratio being 1.10.

FIGURES ILLUSTRATING THE INVENTION The application of the invention and several different embodiments thereof are illustrated by the figures, which include FIG. 1, a side view in partial section of a portion of a typical vessel, in this instance, a tank provided for a highway trailer including a discharge hopper attached thereto,

FIG. 2, a section along line 2-2 of the apparatus or tank and hopper of FIG. 1, showing more specifically the position of the fluidizing gas feed assemblies provided by the present invention,

FIG. 3, a side view of a typical distributor as employed in specific embodiments of the present invention showing in part the fabric cover applied thereto, and illustrating the relation of the wall or boundary of the hopper of the tank hopper section shown in FIGS. 1 and 2,

PEG. 4, a transverse view of the section along line 4-4 of the distributor structure of FIG. 3,

FIG. 5, a view of the section along line 5-5 of the distributor structure of FIGS, showing the intermittent relation of the fabric cover to the structure, during operation of the assembly,

FIG. 6, a side view of an alternative distributor structure, showing for part of its length the fabric cover applied thereto,

FIG. 7, a view of the assembly of FIG. 6, along line 77,

FIG. 8, being a top view of an embodiment having a square configuration. and

FIG. 9, a sectional view along the line 9--9 of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 and FIG. 2 show the general orientation and positioning of the apparatus of the present invention in a representative installation. Referring to these figures, in this instance the container for the commercial solids is a tank comprising or mounted on a highway trailer chassis. A section of the tank 1 1 is shown, the tank having a generally circularcross section, although in some instances elliptical cross sectioned tanks are used, with the major dimension of the ellipse parallel to the ground.

Included in the tank section is a loading port 12 and a discharge hopper section 21. Inthis instance, the hopper section 21 is an inverted conic section, decreasing downwardly in diameter to a discharge port having a valve 23 attached. The valve 23 connects, by an adapter or nipple 25 to a discharge line 26. A supply of transport gas is fed through the discharge line 26 as shown for movement of fluidized solids along the line to a receiving vessel or a distributor device, not shown.

Spaced uniformly, and generally abutting the wall 22 of the hopper 21 is a group, usually three or four, of fluidizing gas feed assemblies according to the present invention, 41a, 41b, 41c, 41d. Gas is supplied to the assemblies from a compressed gas line 18, feeding a ring manifold 24 having coupling outlets uniformly spaced apart to connect gas inlet nozzles associated with or part of the individual fluidizing

gas feed assemblies

41a, 41b, 41c, 41d. It is seen that each of the said assemblies is a generally elongated shape, the details of construction and assembly being described below for a particular class of embodiments as illustrated by FIGS. 3, 4, and 5.

Referring to FIGS. 3 and 4, the distributor assembly for fluidizing gas includes an elongated base member 43 having a slight downward curvature, as is shown in the sections views FIGS. 4 and 5.

Supported on the concave side of the troughlike base member 43 is a generally U-shaped channel member 44, having a series of apertures 45a, 45b in each

sidewall

46a, 46b. The top of the inverted U-shaped channel 44 continues beyond the ends of the sidewalls 46a, 56b and is bent downward at each end toward the base member 43. End openings for gas flow are thus provided at each end as at 47.

In this embodiment a nozzle 42 is centrally attached to the base member 43, at a hole therein, and is adapted to pass snugly through a hole in the hopper wall 22 to connect to the ring manifold 24.

The distributor structure above-described, as well as other distributor structures which can be employed, is preferably made of light metal sheet of about one-sixteenth to one-eighth inch in thickness. The particular metal chosen can be dictated by the expected service. Thus, mild steel or galvanized steel, aluminum, cuprous metal, or nickel alloy are all satisfactory. The channel member 44 is desirably attached to the base member 43 by tack welding or other fastening means. The nozzle 42 is desirably threaded in to the hole in the base member 43.

Referring to FIGS. 3 and 5, a part of the distributor is shown covered by a fabric sleeve cover SI. In this instance, the sleeve 51 is made of two

rectangular cloth pieces

55, 56 fastened together by side seams 52a, 52b. The length of the pieces is in excess of the length of the distributor structure to permit end closure.

Usually, one end is presewn shut as at 53. In assembling the complete fluidizing gas feed assembly, the thus-formed openended tube or sleeve is slid over the metal distributor structure, with the nozzle 22 removed, and the open end is then sewn shut to resemble the presewn end, the metal distributor structure then being completely surrounded by the fabric cover, except for a precut hole, not shown, in the bottom piece 56 for admitting the nozzle into its connection.

The vital feature of the assembly is therelative looseness of the fabric cover as already mentioned. This attribute or feature is illustrated by FIG. showing the section 55 of the final assembly. In this illustration, the fabric sleeve cover 51 is shown in a position fully distended away from the gas-releasing side of the gas distributor, viz., away from'the side occupied by the U-shaped channel 44 and snugly fitting the bottom of the curved bottom member 43. It is to be understood that the method of operation does not require the fabric cover 51 to be in this position continually during an unloading cycle, as in fact, it cannot be, because the characteristic of the method is the vibration effect. While it is not intended to be limited to any theory of function, it is postulated that the fabric cover,

under the joint influence of the gas pressure and the weight of the solids in contact therewith, inside the container, is alternately fully distended and collapsed against the contained or underlying distributor support structure. This alternate distension and collapsing cycle, apparently occurs at relative high frequency, so that an obvious vibration, accompanied by sound, is evidenced, It is therefore concluded that a mechanical shaking or vibrating action is occasioned in addition to the conventional action of flowing gas on the solids cargo.

In this embodiment of FIGS. 3, 4, 5 the portion of the cover 55 in contact with the outside surface of the bottom member 43 is not available for gas flow, the portion serving generally to secure the top portion 56.- In an example of this embodiment the complete peripheral distance around the distributor structure comprising the bottom member 43 and the U-shaped channel 44 together is 10 inches. This measurement is conveniently determined by a tape measure. Of this full peripheral dimension, the outside arc of the bottom member 43 is 5 inches. The sleeve or fabric cover 51 is made with the seams 52a, 52b spaced 5% inches apart, measured, of course, when the fabric. pieces are flat. This spacing provides an internal peripheral dimension of the cover of 10% inches. It is thus seen that the portion of the cover above the edges of the base member 43 is 5% inches, which provides the optimum looseness factor or ratio of l .10, a highly effective value IO.5-5 )/5 =l.10.

Further illustrations showing an additional embodiment are FIGS. 6 and 7. Referring to FIG. 6, in this design the distributor element is an elongated tubular element 61. having a transverse section of an ellipse flattened on one side. A series of elongated slots are provided on the top are.

The critical proportions needed for a fabric cover 69 for this design of distributor structure, are readily determined. The full peripheral dimension of the tube is determined by tape measuring. This is equal to the sum of the segments A and B as illustrated in FIG. 7. The length of the periphery to the extremities of the gas-releasing slots 62 is then determined, in this instance, the segment B.

As heretofore defined, the length of fabric opposite the zone of gas-releasing apertures should have a looseness ratio of about 1.05 to 1.20, preferably 1.06 to 1.15. To illustrate the required internal periphery of the fabric sleeve cover, it is seen that it will be selected from a value equal to B plus 1.05A up to B plus 1.2OA. In an illustration of this embodiment, A is 6 inches, and B is 4.5 inches. The total internal transverse dimensions of the sleeve or cover 69 will then be from 4.5-H l. 05X6)=10.8 inches up to 4.5+( l.20 6)=l 1.7 inches. The preferred range, determined by the same simple calculation, being from about 10.9 inches to l 1.4 inches.

A further fluidizing gas feed assembly of the invention is illustrated by FIGS. 8 and 9. In this example, the assembly is square in plan as shown by the top view of FIG. 8. Referring to FIG. 9, a section of the assembly at line -9 of FIG. 8 is shown. The distributor portion in this example includes a square pan-type base 71, having a gas feed nozzle 73 attached at a hole in the bottom of the base 71. The base is surrounded by a rim flange 72 on which is mounted the margin of the fabric cover 80. The cover is secured in place by a clamping frame 77, which is fastened down by a series of fasteners such as bolt 78.

nun: n-iru A deflector-distributor device 75'is provided in the base 71 directly over the open end of the feed gas line or nozzle 73.1t is made of light-gage sheet metal. As shown, it is in the form of a square with side extensions bent downwardly, the ends of these being secured preferably by welding to the base 71. The deflector 75 thus diverts incoming gas flow laterally through the four corner openings established by the above-described construction. In this manner incoming gas pressure is distributed to essentially the full internal area of the assembly instead of being directed as a jet at the central position.

Alternate deflector arrangements, differing in details from that shown, to perform the desired internal gas distribution, will be readily thought of.

As in the other illustrative embodiments alreadydescribed, the required looseness factor must be provided for the fabric cover element 80. This is readily determined by merely multiplying the actual distance across the recessed portion of the base 71 by the desired ratio. For example, if the base portion provides a recessed portion inches square, the cloth cover within the confines of the clamping frame, will have a square portion, when distended flat, of from 10.5 to l2 inches, preferably 10.6 to l 1.5 inches. To this, of course is added the edge widths secured by the clamping frame 77.

The fabric covers employed in the fluidizing gas distributor assemblies of this invention can be made of a variety of materials. Ordinary canvas is effective, but has a tendency to rot or deteriorate rapidly when chemical products are being handled. It is preferred that the fabric be made of a synthetic fiber, such as nylon or polyester materials.

In the description of the embodiments of FIGS. 35, no reference was made to the overall length of the distributor assembly. As is apparent from the preceding description, the critical proportions are those determined sectionally or transversely with respect to the long dimension of an elongated type of fluidizing gas distributor assembly. The length is merely selected to correspond to, preferably, about three-fourths to about nine-tenths of the length of a hopper wall.

It should also be noted that, in the sleeve cover asdescribed with reference to FIGS. 3-5, inclusive, the seams can be of overlapped edges, or of edges of strips merely laid together. In this latter instance, the cover is readily made by sewing together along the edges of two superimposed strips. The tubular element thus established is desirably reversed, to place the raw edges internally, before sewing one end closed. After installation on the distributor structure, as already mentioned, the opposite end can be sewn shut, sufficient length of course being provided, preferably sufficient to allow folding over and then sewing. This last end can also be closed by any convenient mechanical clamping means. It will be apparent that a sleeve cover can be also readily made by merely folding over a single piece of fabric and sewing one seam joining its edges.

The novel method of fluidizing solids by the present invention comprises passing fluidizing gas through a fabric septum supported at vibratory intervals apart from a solid support structure by the fluidizing gas, said vibratory support being opposed by the weight of the solids being fluidized. In carrying out the process, it is clearly evident that a vibrating action is being achieved which has not heretofore been encountered because of the uniform practice of tightly fastening a fabric layer onto a perforated distributor structure. When a fabric cover is provided having a high degree of looseness-Le, from above 1.20 and higher, the rate of fluidization and removal from a tank is decreased from the results attainable in the preferred range of up to 1.15, although results will still be superior to prior art performance.

In carrying out the method, as in prior practice, fluidizing gas is supplied under pressure to the fluidizing gas feed assembly, concurrently with a gas supply through the final discharge line, and a supply of gas to the tank vessel proper, supplied above the main body of cargo. In most commercial trailers a small gas compresser and drive engine is provided with the unit to provide the necessary gas. In most instances the operating gas is air. In some instances, air would create an explosion hazard, and in these cases an inert gas such as nitrogen or carbon dioxide would desirably be used. In such instances,-a convenient supply would be by compressed gas cylinders, fitted, for uniform delivery pressures, with conventional gas pressure regulators on the output line.

With respect to the gas pressure used in carrying out the new method, the pressure needed for effecting efficient unloading for a given situation is very readily determined. The total weight of load, and the density and particle size distribution of the cargo, are factors having some influence on the desired pressure range. Thus, slightly higher feed pressures are required for a material having a high bulk density, such as cement, then are needed for a low-density material such as flaxseed meal. To determine the pressure, the operator increases, by appropriate adjustment, the supply of gas to the fluidizing gas feed units until a perceptible and audible vibration occurs. This pressure will almost always be above an 8 pound per square inch supply pressure, and mostly from 10 to 20 pounds per square inch, gage, 12 to 18 pounds pressure being preferred,.and 15 pounds per square inch being optimum for many commodities.

During operation, gas is customarily supplied to the body of the tank in the usual manner. This supply prevents a time lag which would otherwise occur if an increase in pressure above the solids cargo had to be supplied by the fluidizing gas as such. Such gas is normally supplied by a branch line from the same compressor discharge line which supplies the fluidizing gas feed assemblies. Similarly, the transport gas, to the discharge manifold conduit fed by the hoppers containing the improved fluidizing feed gas assembly, is supplied in the usual manner.

To illustrate the high effectiveness of the apparatus and method of the present invention, data are given below on performance of typical commercial tank truck trailer before and after the installation of the present invention. The tank used was a highway trailer type, bulk transport type, manufactured by the Butler Manufacturing Company, and having a nominal or approximate total volume of 1,500 cubic feet. In this unit, five conic section discharge hoppers were provided, spaced uniformly and adjacently along the bottom of the body of the tank, which had a circular cross section. Each hopper was about 38 inches in diameter at the top and decreased, in a vertical depth of 44 inches, to a discharge port size of about 4 inches in diameter, at which point a plate or butterfly valve was provided, connecting to a discharge manifold line of 3 inches size. The side dimension of the hopper, from the said valve position to the top of the conic section averaged about 4 feet, being longer or shorter than this according to the point of junction of the hopper wall with the curving sides of the tank proper.

Each hopper was provided with four distributor units of the type previously described with reference to FIGS. 1 to 4, inelusive. As normally equipped, the distributor units were covered with canvas covers tightly fitting the metal assemblies. A change was made to fabric covers made of Dacron polyester fabric, and performance on unloading was satisfactory, but required the normal time. A new set of fabric covers according to the present invention were installed, having a looseness ratio of 1.10. A dramatic reduction in unloading time was experienced. A typical unloading schedule previously required was in the range of 75 to minutes to unload about 50,000 pounds of cement.- A series of comparable unloadings with the new method and apparatus reduced the unloading time requirements to periods of about 10 minutes, in some instances even less.

On the basis of a substantial number of unloadings with dry cement, slaked lime, quick lime, and soda ash, it is found that a loading time reduction is obtained of from 70 to 90 percent of the time formerly needed. These improvement factors were obtained using a fabric cover of Dacron polyester fabric, having the optimum looseness ratio of 1.10, on a distributor structure of the type illustrated by FIGS. 3-5, inclusive, having a gas releasing dimension of 5 inches. Thus, the available fabric transverse dimension adjacent this space was 5.5 inches.

The apparatus and method, and the best mode of performing the invention having been described fully, what is claimed 1. An improvement in a method of discharging finely divided solids from a storage or transport vessel, said method comprising fluidizing by supplying fluidizing gas to the solids in the vessel in a discharge zone downwardly diminishing in cross section and terminating in a discharge port from said discharge zone, the improvement consisting of supplying the fluidizing gas through openings in a distribution means positioned adjacent the wall of the discharge zone and thence through a fabric cover loosely covering said openings and in contact with the solids to be discharged, the fabric cover having a looseness ratio as defined in the specification of about 1.05 to about 1.20 and the gas being supplied at a constant pressure of from 8 to 20 pounds per square inch gage and sufficient to induce vibration in the audible frequency range during discharge.

2. The improved method of claim 1 further defined in that said gas pressure is from about 10 to 20 pounds per square inch, gage.

3. The method of claim 2 further defined in that said gas pressure is from about 12 to 18 pounds per square inch, gage.

4. The method of claim?) further defined in that said gas pressure is about 15 pounds per square inch, gage.

5. Apparatus for discharge of finely divided solids from a storage or transport vessel by gaseous fluidization and downward transport including a bottom hopper portion from said vessel diminishing downwardly in transverse area to a discharge opening for the solids, at least one fluidizing gas assembly therein adjacent the hopper wall, and gas supply means to supply gas to said assembly at a constant pressure of from at least about 8 to 20 pounds per square inch gage, each fluidizing gas assembly comprising a gas distribution means having openings for gas release in a finite portion, and a fabric cover loosely secured over said finite portion with a looseness ratio of about 1.05 to 1.20 which is the ratio of the fully distended fabric to the corresponding dimension of the finite portion, whereby said fabric cover vibrates in the audible frequency range upon application of sufficient gas pressure.

6. The apparatus of claim S'further defined in that the gas distribution means thereof is relatively long and narrow and has a finite portion for gas release along one side thereof, and the looseness ratio of the fabric cover is from about 1.06 to 1.15 with respect to the short dimension of the finite portion.

7. An apparatus according to claim 6 wherein the looseness ratio is about 1.10.

Claims

1. An improvement in a method of discharging finely divided solids from a storage or transport vessel, said method comprising fluidizing by supplying fluidizing gas to the solids in the vessel in a discharge zone downwardly diminishing in cross section and terminating in a discharge port from said discharge zone, the improvement consisting of supplying the fluidizing gas through openings in a distribution means positioned adjacent the wall of the discharge zone and thence through a fabric cover loosely covering said openings and in contact with the solids to be discharged, the fabric cover having a looseness ratio as defined in the specification of about 1.05 to about 1.20 and the gas being supplied at a constAnt pressure of from 8 to 20 pounds per square inch gage and sufficient to induce vibration in the audible frequency range during discharge.

4. The method of claim 3 further defined in that said gas pressure is about 15 pounds per square inch, gage.

6. The apparatus of claim 5 further defined in that the gas distribution means thereof is relatively long and narrow and has a finite portion for gas release along one side thereof, and the looseness ratio of the fabric cover is from about 1.06 to 1.15 with respect to the short dimension of the finite portion.

7. An apparatus according to claim 6 wherein the looseness ratio is about 1.10.