U.S. patent number 3,758,162 [Application Number 04/881,651] was granted by the patent office on 1973-09-11 for method and apparatus for fluidizing activation of particulate, granular, viscous and similar materials.
Invention is credited to Bengt Lennart Holm, Sven Ivan Arvid Tornmarck.
United States Patent |
3,758,162 |
Holm , et al. |
September 11, 1973 |
METHOD AND APPARATUS FOR FLUIDIZING ACTIVATION OF PARTICULATE,
GRANULAR, VISCOUS AND SIMILAR MATERIALS
Abstract
System for fluidizing and agitating material by introducing
modulated gas into the material to reduce the internal friction
thereof.
Inventors: |
Holm; Bengt Lennart (Malmo,
SW), Tornmarck; Sven Ivan Arvid (Malmo,
SW) |
Family
ID: |
27120744 |
Appl.
No.: |
04/881,651 |
Filed: |
December 3, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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788111 |
Nov 22, 1968 |
3519310 |
Jul 7, 1970 |
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Current U.S.
Class: |
406/85; 222/199;
406/138; 222/195; 406/90 |
Current CPC
Class: |
B01F
13/0266 (20130101); B65D 88/72 (20130101) |
Current International
Class: |
B01F
13/02 (20060101); B01F 13/00 (20060101); B65D
88/00 (20060101); B65D 88/72 (20060101); B65g
053/40 () |
Field of
Search: |
;302/29,52,53,35
;222/195,199 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Blunk; Evon C.
Assistant Examiner: Lane; H. S.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention relates to a method and apparatus for
fluidizing activation of particulate, granular, viscous and similar
materials and is a continuation-in-part of U.S. Letters Pat. No.
3,519,310 granted July 7, 1970 and filed Nov. 22, 1968 as a
continuation of U.S. Pat. application Ser. No. 652,538, filed July
11, 1967, now abandoned.
Claims
We claim:
1. Apparatus for fluidizing particulate, granular, viscous and
similar materials comprising a container for holding said material,
including an outlet, a bottom in said container sloping toward said
outlet, supplying means for introducing gas into said material,
means for modulating the flow velocity of at least a portion of
said gas prior to entrance into said material, said modulating
means being disposed within said container, a lower gas permeable
member in said container spaced from said bottom and defining
together with said bottom a passage for said material from the
interior of the container to said outlet, and an upper gas
permeable member in said container, forming together with said
lower gas permeable member a chamber and supporting material
contained in the interior of said container above said chamber,
said supplying means directing said modulated gas into said
chamber, said gas being discharged from the chamber through said
upper and lower gas permeable members to penetrate into said
material thereby loosening and fluidizing the material.
2. Apparatus for fluidizing particulate, granular, viscous and
similar materials comprising a container for holding said material,
including an outlet, a bottom in said container sloping toward said
outlet, supplying means for introducing gas into said material,
means for modulating the flow velocity of at least a portion of
said gas prior to entrance into said material, said modulating
means being disposed within said container, a lower gas permeable
member in said container spaced from said bottom and defining
together with said bottom a passage for said material from the
interior of the container to said outlet, and an upper gas
permeable member in said container, forming together with said
lower gas permeable member a chamber and supporting material
contained in the interior of said container above said chamber,
said supplying means directing said modulated gas into said
chamber, said gas being discharged from the chamber through said
upper and lower gas permeable members to penetrate into said
material thereby loosening and fluidizing the material, said
modulating means including a sound generator for producing pressure
oscillations and an acoustic resonator horn associated therewith
for modifying the pressure oscillations thus produced, said
resonator horn opening into said chamber.
3. Apparatus according to claim 2 wherein said sound generator is
disposed outside said chamber, and said acoustic resonator horn
extends into said chamber through said upper gas permeable member,
the opening of said horn facing said lower gas permeable
member.
4. Apparatus according to claim 1 wherein said upper gas permeable
member comprises substantially rigid supporting means having
openings therein for the passage of gas therethrough, and a gas
permeable material overlying said openings.
5. Apparatus according to claim 1 wherein said lower gas permeable
member comprises a perforate metal sheet member.
6. Apparatus according to claim 2 wherein said lower gas permeable
member comprises an inverted conical perforate metal sheet member
arranged co-axially with said resonator horn and said outlet and
having the apex thereof directed toward the outlet, and wherein
said upper gas permeable member forms a cover over the inverted
conical member.
7. Apparatus for fluidizing particulate, granular, viscous and
similar materials comprising a container for holding said material,
including an outlet, a bottom in said container sloping toward said
outlet, supplying means for introducing gas into said material,
means for modulating the flow velocity of at least a portion of
said gas prior to entrance into said material, said modulating
means being disposed within said container, a lower gas permeable
member in said container spaced from said bottom and defining
together with said bottom a passage for said material from the
interior of the container to said outlet, an upper gas permeable
member in said container, forming together with said lower gas
permeable member a chamber and supporting material contained in the
interior of said container above said chamber, said supplying means
directing said modulated gas into said chamber, said gas being
discharged from the chamber through said upper and lower gas
permeable members to penetrate into said material thereby loosening
and fluidizing the material, and a sound responsive detector
associated with said container and operable to detect said
modulating frequency produced within the container, and indicating
means operatively connected to said detector.
8. Apparatus for fluidizing particulate, granular, viscous and
similar materials comprising a container for holding said material,
including an outlet, the bottom of said container sloping toward
said outlet, means disposed within said container for introducing
gas into said material and for modulating the flow velocity of at
least a portion of said gas prior to entrance into said material,
said means including a sound generator for producing pressure
oscillations and an acoustic resonator horn associated therewith
for modifying the pressure oscillations thus produced, a lower gas
permeable member in said container forming a cup and defining
together with said bottom a passage for said material from the
interior of the container to said outlet, and an upper gas
permeable member in said container forming a cover on said lower
gas permeable member and defining together therewith a chamber,
said upper gas permeable member supporting material contained in
the interior of said container above said chamber, said resonator
horn opening into said chamber to supply said modulated gas to the
interior thereof, such gas being discharged from the chamber
through said upper and lower gas permeable members to penetrate
into said material thereby loosening and fluidizing the material.
Description
BACKGROUND OF THE INVENTION
This invention relates to the activation of particulate, granular,
viscous and similar materials and more particularly to a novel and
improved system for reducing the internal friction of such
materials in order to agitate the material or to facilitate the
flow thereof when removing material by gravity from containers and
other storage spaces and when transporting material by gravity
along slip planes and chutes. The system according to the invention
is useful among other things for efficient removal of cement,
chalk, starch, sand and other different types of particulate and
granular materials as well as mortar, concrete and other different
types of viscous materials from containers, such as tanks, drums,
silos and the like.
Economical aspects impose the necessity of removing and
transporting materials rapidly, e.g. when unloading and loading
vehicles and vessels, and also of exploiting the maximum capacity
of a generally cylindrical, spherical, or parallelpipedonal storage
space to be emptied.
Many systems have heretofore been suggested for a rapid emptying of
containers such as the use of air under pressure to act upon the
material so that there is added to the weight thereof a force
directed toward a discharge opening, which is generally located at
the container bottom, and the use of vacuum which acts in a similar
manner. In these prior art systems it is inevitable that the
material is restrained to a certain extent during emptying because
of the internal friction of the material, i.e., in a particulate or
granular material the friction between the particles or granules
thereof, particularly in a zone in the immediate vicinity of the
discharge opening.
Another difficulty heretofore encountered with known systems
utilizing compressed air for emptying containers is the effect
known as the "rat hole effect." When compressed air is introduced
into the bottom of a container having an outlet, the particulate
material immediately above the outlet will begin to flow
therethrough. This process continues until a vertical hole is
formed above the outlet whereupon the compressed air merely flows
beneath the material and into the outlet without functioning to
remove further material from the container.
SUMMARY OF THE INVENTION
This invention provides an improved system for fluidizing
activation of particulate, granular, viscous and similar materials
in order to loosen up the material and thereby to agitate the
material or to cause it to flow freely.
According to the invention there is provided a method for the
purpose described in the preceding paragraph comprising the steps
of blowing gas into the material to loose up the material and
modulating the gas by a frequency to reduce the internal friction
of the material.
The invention also provides an apparatus for practicing said method
comprising means for introducing compressed gas into the material
and means for modulating such gas to activate the material and
thereby to reduce the internal friction thereof.
The modulation of the gas may be attained by modulating the gas
introduced in said material to be activated at one or more
frequencies, preferably within the audio frequency band.
Frequencies between 100 and 1,000 cps have proved particularly well
suited to attain a good effect and a thorough penetration of the
material.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be fully understood, it will now be
described with reference to the accompanying drawings in which:
FIG. 1 is a diagrammatical cross sectional view of a container
provided with an emptying system according to the invention;
FIG. 2 is a side elevational view of a practical embodiment of a
container provided with an emptying system according to the
invention;
FIG. 3 is an enlarged fragmentary vertical cross sectional view of
the container in FIG. 2 at a discharge opening thereof and showing
also sound transmitters associated therewith;
FIG. 4 is a fragmentary plan view of the arrangement shown in FIG.
3;
FIG. 5 is an elevational view, partly in vertical cross section, of
a container provided with an emptying system according to the
invention showing another embodiment of the transmitter
arrangement;
FIG. 5A is an elevational view, partly in vertical cross section,
of a container provided with an emptying system according to the
invention showing a modification of the sound transmitter
arrangement of FIG. 5;
FIG. 5B is a fragmentary plan view of the arrangement shown in FIG.
5A;
DESCRIPTION OF PREFERRED EMBODIMENTS
In order to explain the principle of the method according to the
invention reference is made to FIG. 1 in the accompanying drawings
illustrating diagrammatically one example of apparatus for carrying
out the method of the invention. In FIG. 1 there is shown in
vertical cross section a storage container 11 of parallelepipedonal
form filled with material. Arranged in this container is a
perforate slip plane 12 covered with fabric, cloth or like material
12A on the upper side thereof. Slip plane 12 forms the material
supporting surface of the container and slopes at a small angle
.alpha. toward a discharge opening 13 at the bottom of the
container for the removal of material therefrom. A high power sound
transmitter 14 having a pneumatic diaphragm or reed is mounted on
the container and is directed into the interior thereof beneath the
slip plane said sound transmitter being supplied with air under
pressure through a pipe 15. An air supply pipe 16 is also connected
to the interior of the container beneath the slip plane. However,
the provision of air supply pipe 16 is optional.
The air energizing the sound transmitter 14 and exiting from the
resonance horn or other resonator of the sound transmitter together
with air supplied through pipe 16 flows through the air-permeable
slip plane 12 and up into the material in the container in a highly
modulated state created by the sound transmitter producing sound
waves having a frequency which is preferably 100 to 1,000 cps. By
the modulated air thus blown into the material in container 11 such
material is fluidized and activated in a bottom layer 17. It is
believed that this marked fluidizing and activating effect is
produced by the modulation of the air which is caused thereby to
penetrate the material and prevents the bottom layer 17 from
compacting even though the particulate material is of the type that
is hard to cause to flow by gravity on account of tendency of arc
formation as is the case with cement, chalk and starch and other
fine grain particulate material. Thus the internal friction of the
material activated in the manner described is substantially reduced
and the material flows by gravity along the slip plane 12 towards
discharge opening 13 to be removed from the container through such
opening.
It has been established that with the system according to the
invention it is possible to reduce the inclination of slip plane 12
from earlier commonly used values of 20.degree.-45.degree. down to
about 5.degree.. For comparison the slope angle when emptying is
performed by prior art systems has been marked in FIG. 1 by a
dash-and-dot line 18. The angle of inclination thereof is
designated .beta. . The reduction of the slope angle thus achieved
according to this invention affords considerable economical saving
by more efficient utilization of the capacity of the container such
as silos, mobile containers and cargo holds of vessel and
containers of any other type and for different purposes.
It is to be understood that in certain cases sound transmitter 14
or similar modulating means may be used to achieve the desired
result without the supplementary supply of compressed air through
pipe 16. In other cases it may be desirable to use unmodulated air
supply in combination with the air supplied through the sound
transmitter the total air flow through air-permeable slip plane 12
being modulated by the sound waves propagating from sound
transmitter 14.
Several sound transmitters in parallel may be associated with one
and the same container. Generally the gas employed will be
pressurized air but the use of other pressurized gas or gas mixture
is of course conceivable in special cases, for instance when the
material to be emptied from the container is sensitive to the
oxygen of the air.
Referring to FIGS. 2-4 in the drawings a practical embodiment of a
container having a system in accordance with the principles
explained with reference to FIG. 1 now will be described in more
detail. A cylindrical container 20 having substantially
hemispherical ends is mounted by shock and vibration absorbing
members 21 fitted to the container by brackets 22, on elements 23
supported by a stationary structure or a mobile structure such as a
vehicle or a vessel. At the top of the container are provided two
conventional fill caps 24. The bottom of the container is provided
with two mutually spaced discharge hoppers 25 each arranged
adjacent one end of the container and terminating in a circular
outwardly projecting flange 26. Slip plates 20A are provided in the
interior of container 20 between hoppers 25 to span the horizontal
bottom surface of the container. Connected with each hopper 25 is a
curved closure member 27 having a circular outwardly projecting
flange 28 at the marginal edge thereof, which is secured to flange
26 by stud bolts having upper and lower nuts 26A and 28A
respectively. As shown in FIGS. 3 and 4 closure member 27 has a
central apertured block 29 forming a central discharge opening of
the closure member said opening communicating with a discharge tube
30 secured to block 29. Between block 29 and flange 28 extend a
plurality of radial cross bars 31 having flat rectangular cross
section and outer ends of said cross bars being received in a
circular recess 32 in the upper surface of flange 28. The cross
bars are covered by a layer 33 of fabric, cloth or other similar
air-permeable material extending over the upper surface of flange
28 to the outer periphery thereof and clamped at its outer marginal
portion between flanges 26 and 28. At their inner ends cross bars
31 and an inner marginal portion of layer 33 define a central
opening in said layer, registering with the central opening in
block 29, and are clamped between the upper surface of block 29,
which forms a conical recess having a bottom sloping toward the
central opening formed by said block, and a conical collar 34
secured to block 29 by means not shown, e.g. countersunk screws
having their heads flush with the upper surface of the collar and
engaging tapped blind holes in block 29. Suitable packing material
may be placed between cross bars 31 and block 29 as indicated at
35. Thus it will be seen that each discharge hopper 25 is closed at
the lower end thereof by an air-permeable bottom forming a conical
slip plane sloping from the peripheral wall of said hopper towards
the discharge opening in block 29.
Communicating with space 27A defined by closure member 27, layer
33, flange 28, and block 29, are two tubes 36 having a
substantially horizontal branch 36A closed at one end by a plate 37
and an upwardly inclined branch 36B extending from the other end of
branch 36A and communicating with space 27A. Plate 37 forms a
mounting member for a sound transmitter 38 of the pneumatic
membrane or reed type which has the operating mechanism outside
said plate and the resonance horn inside the plate and extending
along branch 36A. To each sound transmitter is connected an air
supply pipe 39, and the plurality of pipes 39 are connected to an
air supply manifold 40 having a coupling 41 at one end for
connection to an air compressor. Thus it is seen that sound
frequency modulated air is supplied by the sound transmitters 38 to
the interior of container 20 beneath the air-permeable bottom 31,
33 and passes through said bottom to penetrate into particulate or
similar material supported thereby in the container as is indicated
by arrows in FIG. 3. The fluidizing activation of the material as
described in connection with FIG. 1 thereby is obtained, and the
flow of material by gravity towards the central discharge opening
thus is facilitated. However, it may be necessary to supply
additional air through the material beyond that supplied by the
sound transmitters, and for this purpose there is connected to
space 27A a pipe 42 which is branched from a manifold 43 supported
by container 20 on brackets 44. The manifold is closed at one end
by a shut-off valve 45A and is provided with a coupling 45B at the
other end to be connected to the air compressor. The communication
between manifold 43 and each pipe 42 is regulated individually by a
valve 46. As explained in connection with FIG. 1 the additional air
supplied through pipes 42 is modulated by the sound waves
propagating from the sound transmitters and contributes to the
fluidizing of the material. The material entering the discharge
opening in block 29 is carried by air flow from container 20
through pipe 30 communicating with said opening. Pipes 30 join to
have a common outlet 30A provided with coupling means for the
connection of pipe, hose or similar conduit means.
According to the invention there is provided for the system
described with reference to FIGS. 2-4 an electrical indicating
circuitry which is schematically shown in FIG. 2 and now will be
described with reference to said FIGURE.
On the outer surface of container 20 there is fixed a detector 47
which is sensitive to sound within the frequency range used for the
modulation of the activating air and produced by sound transmitters
37, and this detector is adapted to generate an electrical signal
in response to the pick-up of sound within said range. Detector 47
is connected by a line 48 to a device 49 controlled by the electric
signal generated by the detector e.g. an amplifier or a relay which
is supplied with electric power by a line 50 from suitable power
source not shown in the drawing. Device 49 is connected by a line
51 with suitable indicating apparatus 52 which may include an audio
indicator such as a bell or a horn, a visual indicator such as a
lamp, or a coil controlling an indicator of any other type as is
shown diagrammatically in FIG. 2. Detector 47 and/or device 49
provide a threshold function, i.e. a signal is transmitted to
indicating apparatus 52 only when the sound picked up by detector
47 is of an intensity greater than a predetermined value. This
threshold value may be determined by the characteristics of the
detector which may be an electro-mechanical or electronic
transducer, or by the characteristics of device 49. When container
20 is more or less filled with material and is being emptied, the
sound produced by sound transmitters 38 as picked up by detector 47
is attenuated by the bulk of material supported by bottom 31, 33.
The threshold value of the indicating circuitry may be adjusted in
such manner that no indication is given when the detector picks up
sound that has been attenuated by the bulk of material. However,
when the container is empty or substantially empty there is little
if any material stored in the container, between the sound
transmitters and the detector which means that the sound picked up
by the detector will have a relatively greater intensity when the
container is empty or is about to become empty. Thus, it is
possible to adjust empirically the threshold value of the
indicating system so as to produce an indication only when the
container is empty or is about to become empty.
As will be seen, the indicating circuitry described above and
disclosed in FIG. 2 provides simple means to indicate a
substantially empty condition of the container. This circuitry may
be applied to the container without it being necessary to intrude
upon the space in the container or to pierce the wall thereof. If
desirable the signal supplied by detector 47 may be transmitted
over a telephone line for remote control purposes.
The embodiment according to FIG. 5 comprises a substantially
spherical container 53 of a common type having a fill cap 54
providing a central discharge opening 55 at the bottom thereof, a
discharge tube 56 being connected to said opening. Means for
mounting the container movable or stationary may be provided e.g.
as shown in FIG. 2. In the interior of the container there is
provided a supporting structure consisting of an annular set of
inclined legs 57 connected to the container at their lower ends and
converging towards their upper ends, and a horizontal platform 58
connected to said legs at their upper ends. Supported by the
supporting structure is a sound transmitter 59 of the pneumatic
membrane type having an air supply pipe 60 extending to the
exterior of the container to be connected to an air compressor. A
set of several sound transmitters may be provided if desired. As
will be seen in the drawing the sound transmitter has the operating
mechanism disposed on the upper side of the platform, the resonance
horn being disposed below the platform and extending axially on the
vertical central axis of the container towards the discharge
opening 55. On brackets 61 secured one on each leg 57 at the lower
end thereof is mounted an inverted conical metal sheet member 62
which is arranged coaxially with the discharge opening of the
container the tip or apex thereof being directed towards the
discharge opening. The lower surface of member 62 defines together
with the inner surface of the bottom of the container a conical
space 63 surrounding discharge opening 55 and having a bottom
sloping down towards such opening. The upper side of member 62
forms a cup which is covered by a perforate metal cover plate 64
joining the marginal edge of conical member 62 and defining a space
65 together with said member. Cover plate 64 is provided with a
layer 66 of cloth, fabric or similar material on the upper side
thereof. As is realized the perforated cover plate may be
substituted by a cross bar structure as that disclosed in FIGS. 3
and 4 extending between and supported by the resonance horn of
sound transmitter 59 and the marginal edge of member 62. Preferably
there is arranged in the cover plate and the layer thereon a
central opening registering with the outlet opening of the
resonance horn of sound transmitter 59 in order that the perforate
plate and the layer thereon will not attenuate the sound produced
by the sound transmitter. A tube 67 extending through the wall of
the container is connected to space 65 and provides for the supply
of unmodulated air to said space.
As will be seen the system disclosed in FIG. 5 will operate in the
same manner as those of FIGS. 1-4 and in accordance with the
principles of the invention. When the container is to be emptied
the material therein is activated and fluidized by air supplied
through sound transmitter 59 and tube 67 into space 65 and
modulated by sound frequency waves propagating from the sound
transmitter. The modulated air passes through perforate plate 64
and layer 66 and penetrates into the material supported thereon, as
is indicated by arrows in FIG. 5. Thus the internal friction of the
material in the container is reduced in the manner earlier
described herein and the material will flow easily by gravity
through space 63 towards the discharge opening 55 into discharge
tube 56 as is also indicated by arrows in FIG. 5.
The embodiment disclosed in FIG. 5 and described above has the
advantage that the sound transmitter or a plurality of sound
transmitters arranged inside the container are surrounded by the
material in the container when filled. This means that the sound
produced by the sound transmitter or transmitters is attenuated by
the surrounding material, the sound level in the immediate
surroundings of the container thereby being reduced. Further, the
sound activating system is out of the way when disposed inside the
container which is substantially free of external apparatus for
emptying the container.
In the embodiment of FIG. 5 an indicating circuitry as that
disclosed in FIG. 2 may be provided.
An improved form of the embodiment of FIG. 5 is disclosed in FIGS.
5A and 5B. In this form there is provided a pneumatic sound
transmitter 159 having an air supply pipe 160 extending to the
exterior of the container 153 to be connected to an air compressor
in the manner described above. The supporting structure provided in
the interior of the container to support the sound transmitter and
elements connected therewith in the manner to be described,
comprises two channel beams 158 disposed on opposite sides of the
sound transmitter and connected at their ends to the inside of
container 153 as by welding. Beams 158 are each connected with the
sound transmitter by threaded studs 170 and nuts 171, said studs
being formed by the end portions of bars 172 connected to an
outwardly projecting circumferential flange 173 on sound
transmitter 159 by nuts 174 screwed on said studs 170. Bars 172
extend vertically and are substantially parallel to the axis of a
downwardly directed horn 175 forming part of sound transmitter 159,
to an annular ring 176 surrounding said horn at the mouth thereof.
Having passed through ring 176 bars 172 are angled outwardly and
terminate in lugs 177. Two annular rings 178 and 179 are clamped
together by bolts 180 having nuts 181 and passing through lugs 177
as well as both rings 178 and 179.
Between rings 178 and 179 there is clamped an inverted conical
perforate metal sheet member 162 which is arranged coaxially with
horn 175 and the discharge opening 155 of the container, the top or
apex thereof being directed towards the discharge opening. In the
manner described above the lower surface of member 162 defines
together with the inner surface of the bottom of the container a
conical space 163 surrounding discharge opening 155 and having a
bottom sloping down towards each opening. A conical perforate metal
cover plate 164 covering the cup formed by member 162 is also
clamped between rings 178 and 179 at the large diameter end
thereof, the small diameter end of said cover plate being connected
to the mouth of horn 175 by a ring 182 having pressure fit or being
otherwise secured on the outside of the horn. Cover plate 164 is
provided with an air permeable synthetic fiber cloth layer 166 on
the upper side thereof such layer being secured between rings 178
and 179 and between horn 175 and ring 182 together with said cover
plate. Bars 172 pass through tunnels on the upper surface of layer
166 formed by securing to said layer rectangular pieces 183,
preferably of the same material as layer 166.
The embodiment according to FIGS. 5A and 5B operates in the same
manner as that described with reference to FIG. 5, and the same
advantages are achieved thereby. However, there is achieved in
conical space 163 additional force downwards on the material
passing through said space to discharge opening 155 by means of the
sound modulated air streams passing through the holes in the
perforated metal sheet member 162.
Although the invention has been described with regard to the
fluidizing of material in order to facilitate the flow of the
material in connection with the removal or transport thereof it is
contemplated that the system according to the invention may be
applied in cases wherein flow of material is not required but the
material has to be agitated to prevent compaction on the supporting
surface.
The modulation of the air may be achieved by modulating the air
before penetrating into the material to be activated as in the
examples shown and described herein. However, it is possible to
modulate the air or other gas penetrating into the material by
enclosing the surface in which pressure modulations are produced,
the air or other gas penetrating into the material is thus
modulated by the modulated back pressure encountered thereby.
It is readily understood by those skilled in the art that the
invention is not limited to practicing the method with the aid of
the apparatus illustrated herein and that the invention can be
carried into effect by other means within the scope of the appended
claims.
* * * * *