US2769544A

US2769544A - Material trap

Info

Publication number: US2769544A
Application number: US261874A
Authority: US
Inventors: Joseph H Morrow
Original assignee: Fuller Co
Current assignee: Fuller Co
Priority date: 1951-12-15
Filing date: 1951-12-15
Publication date: 1956-11-06
Anticipated expiration: 1973-11-06
Also published as: ES206675A1; FR1068973A; GB747825A; BE516243A

Description

Nov. 6, 1956 J. H. MORROW MATERIAL TRAP 4 Sheets-Sheet 1 Filed Dec. 15, 1951 INVENTOR. Josasp a 5a. MORROMT ATTORNEY;

Nov. 6, 1956 J. H. MORROW MATERIAL TRAP 4 Sheets-Sheet 2 Filed Dec. 15, 1951 w %5 R o w h m B Nov. 6, 1956 MATERIAL TRAP Filed Dec. 15, 1951 J. H. MORROW.

4 Sheets-$heet 3 EPH f'l N RR BYP IN V EN TOR.

M A My 4747- 1956 J. H. MORROW 2,769,544

MATERIAL TRAP Filed Dec. 15, 1951 4 Sheets-Sheet 4 INVENTOR. JOSEPH 5-1. oRRow Y QTTORNW United States Patent MATERIAL TRAP Joseph H. Morrow, Hokendauqua, Pa., assignor to Fuller Company Application December 15, 1951, Serial No. 261,874

1 Claim. (Cl. 209-474) This invention relates to apparatus for removing undesired substances from fine dry materials, and more particularly, the invention has to do with the separation by gravity of foreign objects from a moving stream of pulverulent material, and their collection in a fixed bed of fluidized material.

In the processing and handling of many pulverulent materials, undesired objects and particles of greater density than the pulverulent material are frequently introduced and must be removed before a subsequent processing or handling operation can be carried out. For example, in grinding a chemical, rock, or mineral, there is usually introduced into the mill stream a variety of metal objects, such as broken pieces of grinding media, mill liners, screen wire, and other metallic items resulting from wear and breakage, all of which are commonly referred to as tramp iron. Undesired oversize particles are also frequently present in the pulverulent material being discharged from a mill and must be separated. In other manufacturing operations, such as conveying and elevating dry materials, tramp iron also enters the material stream due to wear and breakage of equipment.

Magnetic separators associated with a moving stream of material in a spout, or in a conveyor, have long been used successfully to remove tramp iron, but such devices are restricted to the separation of magnetically susceptible materials. Tramp iron and also oversize particles are commonly removed from dry materials by vibrating screens. In the screening of large tonnages of fine dry materials, and particularly abrasive substances, however, operating and maintenance costs are high, and there is always present the chance that a broken or worn screen will pass a substantial quantity of foreign material before the defect is discovered and repaired, the repair in many instances involving costly shutdowns of major plant-operations.

The passage of air through a distribution means into a body of dry material, either with or without mechanical vibration, has previously been employed to cause a classification into components according to particle size or specific gravity. Such air classification methods have been successful in certain industries with a limited number of materials which are in general coarser in size than those to which my invention is applicable. The efliciency of classification decreases rapidly as the average particle size decreases from a crusher product size through'the granular or sand size down to pulverulent materials. In the case of pulverulent materials, the amount of classification obtained is negligible, the result of diffusing relatively small volumes of air therethroughbeing largely the production of a fluidized bed, more fully described below.

The present invention is accordingly directed to a novel and simple apparatus for removing, from a moving stream of pulverulent material, foreign or undesired substances of greater density than the material, by the use of a fluidized bed through which the substantially greater density mateice rial settles. The apparatus of my invention has the further advantage of not in itself being able to introduce contamination due to wear or breakage and of not permitting foreign materials already in the stream to pass because of defects due to wear.

The separation of foreign substances of greater density from a stream of moving material is accomplished in accordance with my invention by maintaining a fixed bed of the material, dilfusing air through the bed to fluidize the material therein, and causing the moving stream to travel over the top of and in contact with the top of the fluidized bed. The undesired foreign material having a density higher than the particles in the moving stream, either due to a higher specific gravity, or to a substantially larger particle size, settles from the moving stream under the influence of gravity through the fixed bed of material of reduced bulk density, so that it can be removed at the bottom of the bed.

The apparatus of my invention, referred to as a material trap, includes a chamber for containing a column of fluidized pulverulent material and adapted to be placed below an opening through the bottom of a trough or like structure, along which the pulverulent material is being advanced. A gas permeable medium forms the bottom of the chamber and serves as a means for diffusing air or other suitable gas into the pulverulent material to maintain it in a fluidized condition, and a plenum chamber is disposed below the porous medium.

It is well known that the introduction of a gas, in appropriate quantities, into a pulverulent material by diffusion will result in fiuidization of the mass, and the gassolids mixture will acquire many of the properties of a liquid. Individual particles will be separated from each other, the bulk volume considerably expanded and thus the bulk density reduced, and the internal particle to particle friction reduced. To maintain the fluidized condition in a column the gas must have suflicient velocity to support the particles, but not sufficient to entrain any substantial amount of solid as the gas emerges from the upper surface of the bed.

My material trap includes the use of such a fluidized bed of reduced bulk density, and the heavier tramp iron or oversize particles sink readily through the bed, a movement which would not be possible in an unaerated bed due to the internal friction and close packing of the fine particles.

As previously stated, the gas emerging from the upper surface of the fixed bed is directed through the moving stream of material producing a fluent effect similar to that in the fixed bed. As a result, the tramp iron and oversize particles settle out of the stream and into the fluidized bed.

The apparatus of my invention may be applied to the separation of foreign material from a variety of pulverulent substances among which may be mentioned for purposes of illustration: cement and cement raw materials, gypsum, soda ash, fly ash, barytes, bentonite, flour, hydrated lime, ground ores, alumina, catalysts, silica, phosphates, talc, resins, and detergents. Generally speaking, any of the above materials, to be effectively fluidized in my trap, should be finer than 20 mesh and preferably finer than 65 mesh, while at the same time containing a substantial proportion of minus mesh, and with some materials a substantial amount of minus 200 mesh.

The moving stream of pulverulent material from which foreign material is separated, in accordance with my invention, may be present in a variety of conveyors. Typical examples are: the air activated gravity conveyor referred to below, screw conveyors, flight conveyors, drag chains and Redlers, and vibrating conveyors.

A clearer understanding of my invention may be had by reference to the drawings in which:

3 Fig. l. is a 'schematicelevation of an air activated gravity-conveyor --utilizing my --material trap; I n

Fig. 2 is a sectional view of the material trap disclosing a portion of .the air activated gravity conveyor;

Fig; is a sectional view of the material' -trap"disclosing a portion of-ascrewconveyor;

"FigJ-S is atransverse sectionalong'li-ne -50f Fig. 4; t

Fig. 6' is a side elevation, partiallyin-sectiomof a modified form of" the material trap disclosing -amortion of -a serevr-conveyor;

Fig-7&5 a transverse-section along line"7- 7"of-Fig."6;

' FigrS is -an* enlarged vertical section ofthe material '7 trap-disclosed in Fig. -6;

-Figf9is a-side elevation; partially in section; disclosingan air'aetivated; gravity' conveyor;

V PigL-l0 is-avertical section of-a secondmodified form 7 0f 'myunaterial trap;

Fig. '11 is' a transverse sectionialong line ill-11 of Fig. 10. V a

"Referringto thedrawingsyit will beiseen thatimy material trap in'itssimplest form comprises a housing,.

or container, 1 inthei form of a. rectangular. box-like structure. A gas permeable. medium, or wall, 2 extends across-the lower portion of. the'housing 1 and'provides' a materialsupporting. surface which at the same time divides 'the'housing intoytwo' portions, a plenum chamber 3-and a material confining. chamber 4. The plenum chamberfiiis' formed' by a rectangularsections, attached to'thehousing It by means. offlan'ges andbo'lts 6, the gas permeable medium being held between the respective flanges ofthe' housing and plenum chamber inasecure manner. .Air, or other desired operating 'gas, enters the plcnnmchamber 3 through conduit 7 and is-furnished from any convenientsource, notdisclosed. Aciean-out portfi is covered by a hingedclosure 9 secured in a closed position bynvin'g nuts 10.

Fig. .1 discloses a typical schematic arrangement of air activated. gravity material conveyor, a detailed description oflwhi'ch is found in U. S. Patent Nos.

527,455 and 12,527,488, utilizing my material trap. in any of the specific forms. I have. shown a conveying .duct

13 delivering-material from a' hopperiZto. a receiver 13. HA gas permeable .medium'14,provides.a. material conveying surface which at the. same time .fdividesthe conveyor into two portions, a. conveying...air ..plenum is supplied'fby a fan: i7 drivenby a motor.18.;through an appropriate connection 19 to the plenum chamberifi'.

in the normal operation of. .the..conveying-..duct.11,

material capable 'of'being, fluidized isdischargedfrem the hopper'iz ontothe gas permeable medium.14,.wherein it is fluidized by air passing. through the. gaspermeable medium. from the plenum .chamber .15 and .fiows .by

gravity to the receiver 13*through the materialconveyingduct 16.

Theymaterial trap .is incorporated into. the..conveying system as generally disclosed-in. Fig. l, by. inserting an expansion joint 2% into theconveying duct.11 .atrthe .desired location of the trap. Theexpansion joint. 20 providesan unobstructedv opening for..the material. convey- 1 .Inthe. operation ofthe material strap-disclosed inzF-ig conveyed. enters and...fills :..the :chamber. .Themnaterial within chamber .4, referred toast-the V fixed; ,of mate:

7 material supported by thegas permeable medium 2 in chamber i-San'da material conve'yingflduct 16. .Th'ecain material sinks into the fluidized column of material in '-thez materialconfining chamber of the trap and isfdeposited on'th'e' surface -'of the gas-permeable mediu'mu2,

' l, land 3.the. chamber 4,.is;filled .the.-materialt.-to" be conveyed ina preliminary. operation for, .during1 the initial parrot" the conveying operation, thetmamrial being ceiver 13;

rial, rests upon the gas permeable element 2 and air from the-plenum-chamberB is 'difiused through-themement 2 into the fixed bed in suflicient quantity. and at the proper pressure to fiuidize'the bed. Thebed fills the chamber 4 and the top of the bed forms the support over which the'materi-al' being conveyed travels across the trap.

The material being conveyed,' containing foreign ma terial, moves downi the conveyor "through the material conveying duct 16. The foreignmaterialbeing carried in the moving stream is usually moving along'the-su'rface of the conveyor gas permeable medium 14 due to its densityibeing greater than. that. of t'thecparticlestofrmate' rial being conveyed. Whenthe foreign. material moving along the conveyor 11 reacheslthefiuidized fixed bed of material in the trap; the foreign material, being heavier, will fall by gravity through the fluidized column 7 to the surface of the gas permeable medium 2, while the moving bed of material being conveyed will continue through the material conveying duct 16 toward thelre- The foreign material collectedin' the trap is later removed through the clean-outport' 8 at any convenient time when the conveyor is not in operation. The frequency of clean-out is dictated by the proportion of foreign material in. the :material stream. A bed ofv severai inches of foreign material" is not detrimental to maintenance of. a fiuidized'b'e'd above it.

Fig. 4 discloses my material trap incorporated into.

a conveying systemutilizing a conventional screw'conveyor. The structure of thematerial trap is thesameas: that shown-in Figs. 2 and 3. A transition section 23zis welded,. or connected; in. any conventional manner, :to thetrough 24. of. the screwconveyor 25,.surroundingaan opening .26lin..the trough.24. .The material trap is attached to.the.transition sectionv 23. by means .of flan'ges and .bolts 1 .27. ..thereby forming. -.a material. tight structure integralwiththe. conveying system wherein; the material confining chamber .4 of thematerialtrap is :in'direct communicationwith the interior of the conveying trough .Tnsthe operation of the material trap disclosed in Figs.

4. and 5, airris. 'supp'liedto.the plenum chamber 3 in sufficientgquantity. and at suflicient pressureto fiuidize the thermaterial1confiningchamber-4. This material containedflinfthe trap'iseither placed therein prior -to;the V operation'of the-conveyor, oris collected from the ma.-

.terial being conveyedafter the start of operation a of the conveyor. The -air diflused through the. gas permeable me'diumii creates a. fluidizedfcolumn of material,

in thematerial confining chamber-4 thatiextends upwardly into-the stream-:ofmaterial inthescrew conveyor 25.

. The: material: being conveyed, containingf foreign material, moves through the conveyor trough 24 by means'of the'rOtatiQn-of.:the:screw fiight28. 'Theforeignmaterial'; containedrinthisfmoving' stream. of material is" distributed throughout 'thefmoving. stream of material; "Asathet'ma- :terial; is pushed z'through thesconveyor trough -24 .by.

' itpasses over. the: opening 26.1nthe conveyor. trough :above' the screw :flightf28;iit is 1 at:least. partiallyf fluidized as the: material'trap. As-gpreviously described; :the. foreign while the moving bedof-propermaterial-size will continue through i the screw conveyor towards thedis-' charge'endh Qi. V V

In.F. igs.;6,5 7,' 8 and. 9,'I show.amodification. of my. invention. f Referring' to Fig. 8; it w'ill'be .seenjthatlth'e modified form. of.-my-.materialltrap. comprises .a housing 40.. provided .wtih. two. aeration. units A1 ..and l.42 positionedttojform generally -a V-like,: or-wedge shaped,- sup portingstructure the .housing 40: The-aeration units, 141 and 42, .-arey,. similar; in;- structure, .neachv-heing:

provided with a plenum chamber 43 formed by a rectangular section 44 having one side closed with a gas permeable medium 45. The plenum chambers 43 are supplied with air from any convenient source through connections 46.

The aeration unit 42 is longer than unit 41 and extends below the end of unit 41, so that aeration is provided throughout substantially the entire cross-sectional area of housing 40. The lower end of aeration unit 41 lies above and spaced from the lower end of unit 42, and a discharge opening 47 is formed between the adjacent ends of the units. The opening is surrounded by a valve seat 48 and a gate 49 attached to a valve shaft 50 cooperates with the seat to control flow through opening 47. A valve handle 51 is attached to the valve shaft 50 is used as a means for opening and closing the valve gate 49. A butterfly valve 52 is positioned beneath an opening 53 in the lower portion of the material trap and is fastened to the housing 40 in any conventional manner. The butterfly valve 52 is provided with a valve gate 54 operated by a valve handle 55.

An expansion joint 56 is positioned in the air activated gravity conveyor 11 at the desired location of the trap and provides an unobstructed opening for the material conveying duct 16, but forms a closure 57 for the conveying air plenum chamber 15. The housing 40 of the material trap is connected to the expansion joint 56 by means of flanges and bolts 58 thereby forming a material tight structure integral with the conveying system wherein the material confining chamber 59 of the material trap is in direct communication with the conveying duct 16.

In the operation of the modified form of the material trap disclosed in Figs. 8 and 9, air is supplied to the plenum chamber 43 through the connections 46 in sufficient quantity and at suflicient pressure to fluidize the material supported by the gas permeable mediums 45 in the material confining chamber 59. The material contained in the trap is either placed therein prior to the operation of the conveyor, or is collected from the material being conveyed after the start of operation of the conveyor. The air difiused through the gas permeable mediums 45 creates a substantially vertical fluidized column in the material confining chamber 59 that extends upwardly into the stream of material in the air activated gravity conveyor.

The material being conveyed, containing foreign material, moves down the conveyor through the material conveying duct 16. The foreign material being carried in the moving stream is usually moving along the surface of the conveyor gas permeable medium 14 due to its density being greater than the particles of material being conveyed. When the foreign material moving along the conveyor 11 reaches the fluidized fixed bed of material in the trap the foreign material, being heavier, will fall by gravity through the fluidized column to the opening 47 at the bottom of the V-like supporting structure formed by the two aeration units 44, while the moving bed of proper material size will continue through the material conveying duct to the discharge end. The foreign material will accumulate at the bottom of the V-like supporting structure until a sufiicient amount is collected for discharge of the material trap.

This modified form of my invention permits the discharging of foreign materials from the trap without interrupting the flow of material through the conveyor 11. The foreign material is discharged from the trap by opening the gate 49 by the rotation of valve handle 51 thereby permitting the foreign material to discharge from the V-like supporting structure on to the top of valve gate 54 of the butterfly valve 52. After the material has been discharged from the supporting structure, the valve gate 49 is closed by rotating the valve handle 51 in the opposite direction thereby closing the discharge opening 47 and permitting the further accumulation of foreign materials from the conveyor 11. The foreign material is finally discharged from the trap structure by opening valve 54 by rotating valve handle 55 to position the valve 54 in a vertical position. The foreign material will then fall by gravity from the material trap structure whereupon the valve 54 is returned to its horizontal position for the next cycle of foreign material discharge.

If desired, the expansion joint 56 may be provided with a material inlet duct 60 to permit the location of the material trap to be directly beneath a substantially vertical supply of material. In this event the material trap will serve not only as a means for removing foreign material from the moving stream, but will also serve as a means for cushioning the fall of the vertical body of material which becomes particularly important in an embodiment incorporating an air activated gravity conveyor wherein the impingement of material on the woven fabric may produce excessive wear.

Figs. 6 and 7 disclose a modified form of my material trap incorporated into a conveying system utilizing a conventional screw conveyor. The structure of the material trap is the same as that described for Figs. 8 and 9. A transition section 70 is welded, or connected in any conventional manner, to the trough 24 of the screw conveyor 25, surrounding an opening 26 in the trough 24. The material trap is attached to the transition section 70 by means of flanges and bolts 71 thereby forming a material tight structure integral with the conveying system wherein the material confining chamber 59 of the material trap is in direct communication with the interior of the conveying trough 24.

The operation of this modified form of my material trap is the same as that described for the operation of the material trap disclosed in Figs. 8 and 9, and the characteristics of the screw conveyor 25 are the same as those described for Figs. 4 and 5.

A second modified form of my material trap is shown in Figs. 10 and 11 wherein the material trap housing is in the form of a valve body and the material confining chamber 81 is in the form of a corresponding rotating valve plug 82. The plug 82 is provided at each end with shafts 83 and 84 positioned in bearing housings 85 and 86 of the valve body 80. The shaft 83 is provided with an externally operated handle, not disclosed, for rotating the valve plug 82. An aeration unit 87, comprising a gas,

permeable medium 88 and a plenum chamber 89 is positioned in the lower portion of the valve plug 82 and serves as a material supporting surface for the material confining chamber 81. Air is supplied to the plenum chamber 89 to a conduit 90 passing into the valve body 80 through a hollow section of the valve plug shaft 84 in an airtight arrangement. The valve plug structure 82 forms an opening 91 in the upper portion of the plug opposite the aeration unit 88 for the admission of material into the material confining chamber 81. An opening 93 is provided in the lower portion of the valve body 80 to permit removal of the foreign material from the trap.

The second modified form of my invention is incorporated into the previous mentioned types of conveying systems by attaching the material trap to the expansion joint of the air activated gravity conveyor, or to the transition piece of the screw conveyor, by means of flanges 92. This arrangement will, as described before for the other conveyors, provide a material tight structure that permits a direct communication between the material being conveyed and the material confining chamber 81.

In the operation of this second modified form of my material trap, a fluidized column of material is maintained in the material confining chamber 81 and extends upwardly into the moving stream of material being conveyed, such a column being produced by the fluidizing effect created by the diffusion of air from the plenum chamber 89 through the gas permeable medium 88 into the material contained in the chamber 81. The material being conveyedwill deposit the:.foreigrnmaterial.otmthea'supporting a surface :of' the;aerationsunit,=:.asepreviously described. r The foreigmmaterial. accumulatedrmay 'befrernoved from the. material trap,-without interruption in the flow'of the materialtbeing conveyed, :by the rotation'of the-valve 'plug 182* one-halfzturn; by meansrofthe shaft83uand the turningdiandle. .iThisone-half turn willpositionzthe opening 91 of the valve plug intcomrnunication'with the valve body opening=93gand vpe'rmit the'.foreign materialiaccumulated in the trap -.to:be1discharged therefrom by gravity. This rotation ofthe valveiplugis onlyamomentary movement and the valve plugz'isi immediately returnedto-its normal; operating position; for. further accumulation of. the

foreign material: being.- separated Y from the material: being conveyed. V e 7 The efiiciency of collection of'myitrap: depends principally'on first, the density oftthe'foreign'material; second, the degree ofifiuidizationmaintained in the.fixed bed, which is measured by the bulk density of the'fiuidized material compared to its unae rated bulk density; third,

the degree to which the material-in the. moving stream is'aeratedas it passes overithe fluidized bed; and'fourth,

the lengthof the-fluidized :bed over which the moving stream passes. The. properties of eachmaterial handled and: partieularlythe particle-size range-will, as explained above, determine in: each case the most desirable vertical gas velocity to maintain the .fixed'lbedsat aminimumbulk density. The moving stream as it passes over the fluidized bed should be aerated t0 the maximum possible extent Without causing entrainment of the finesin the;gas emergingfrom the top of the stream. The most-'desirable length for the trap is dependent on the first'three items listed above, andon the efiieiency-of. separationdesired. 'I have found 'that in general, sa'tisfactoryv separation .is.

achieved by. using a width approximatelyzthe widthiof the conveyor trough, and :.-a length 'of approximately two feet. For example, traps varyin'g in,widthfrorn- 610.16

7 inches and two feet in len th have been eifective'when 6 used vvithairgravity conveyors. In .the case of screw conveyors, the length of the' atrap iswith advantage made equalto about one to two screw :fiights. This length normally gives the. gas sufiicientltime to penetratethe 7 moving stream, and theioreignmaterial suflicient time to drop into the trap. The depth of the vfluidized bed employed 'Will' besdetermined by the properties 1 of v the r pulverulent material,

Woven fabric .of the type referred 'to' below, ;a;t rap :and

conveyor of the type illustrated in Figs. 8 -and 9.-was:'em-

' ployed in handling'a cement raw material mixture having area to maintain maximum fluidization.

abolit- 41C. F. -M'./Sq.' ft. were used '-2U1d--Wr 1 passedthrough :the bed' EVlih-fl-Pf6SSUf6 of 528"':W. gm (1 lbl p.1-st ii):in= lthesplenurn: chambers of the trap. The' fluidized bed varied in depth from about 7" a t -theendsto' 1-6" in; the center. The,raw"material:passedgthrough theifi'fwide. conveyor.;at.-a rate of ZS 'R/hr. :in .a-bed about; 1. /2 deep. A: variety. of foreign materials includg ingnuts and bolts,-screen'.-wire. pieces, shattered :grinding balls, ametal: eutti'ngs,. .and rock oversize 1 0f different; dimensions, :werefzadded .Ito. the flowing-.zstrearnabout 10.

feet a'hcadof the:;t1;ap. wThei trap was emptiedwhiletthe stream. continuedntozflow .-Substantially.l00% of the metal: :objectsvivere: thus. recovered; 9.0 .of. the 4 inch oversize-sand :ahout S0%.-:of;the inchoversize.

. In the; einhodimentofstny'inventionillustrated ir LFigs; 6-9,. .a: woven; fabric. of flowtpermeability is, preferredin order to. promote .uniforrn iair distribution. through; the entire bedtin. spite: ofnthev substantial. difierencein mate rialthead hetweentthe, center. andends o f'the trap. I have found that a fabric; havingla permeability belowabout. S

is ebest suited for I :this. purpose. This ist equivalent to theiporous mediumflescribedin 'S chemmaU. 8.. Patent No..2,527,455 for use in the air activated gravity. conveyor,- which; is :the. conveyorsused intheabove described test. i n x this installation, :more .highly -peianeahle membersivould. eauaeexcessively.aerated; portions of thesbed near theends, with-.a-relatively dead area. in the. center,

thuszdecreasing the efiiciency :of separation .of foreign materials, .In-the otheramodifications of: myv trap itl-is. frequently -preferredgto'use porous mediums of similarilow permeability to promote uniform distribution of In [an apparatus; for. separating foreign material "from:

. a streamq-of material moving through a conveyor. trough, V acontainerdisp'osed beneaththe .troughandin commuiii= cation .with the. rinteriorlthereof; a .pain of. aeration units a fineness ofabout90%:minusi200mesh. .Pulverulent materials require. vertical.gas velocitiesof 0:05 10.03 feet per second i. e.. 3'10 18 C. F. M./sq.ift. ofab'ed' Within .thecontainer, each unit including .a .plenurn chamber. closed :at. its-.1011 by; ajflat gas-permeable Wall .andthe' units being ad-isposed-at opposite sides offl-the' containeiji with theirtopwialls' lying at. opposite angles toi'the-vertical' and .dcfining .a wedge-shaped bottom".for*the container;

oneunitbeing longer than'the second and having its lower endfbeneatiiarid spacedfrom the lowererid of'the second unit,. a.discharge port' between 'thejlower ends .of the units, a' gate operabie to open and 'closefthe port, .and. means for supplying a gas under pressuretoj the plenu m. chambers for diifusionfihrough the: -gas -permeable walls int'o'the contents 'of the container References 'Cited injthelfile of 'thi spatent I UNITEDZ STATES. PATENTS V