CA1161374A - Method and apparatus for separating particulate matter from gases - Google Patents

Abstract

METHOD AND APPARATUS FOR SEPARATING
PARTICULATE MATTER FROM GASES
Inventor: Joseph G. Wilson Abstract of the Disclosure A centrifugal separator has an outer tube and an inner tube concentrically positioned therein, forming an annular gas inlet channel between the outer and inner tubes, and slots are defined in the inner tube to provide communica-tion between this annular channel and the space within the inner tube. A collection chamber for collecting particulate matter and for allowing the flow of bleed gases is provided towards the bottom of the annular channel. Transverse dis-charge means communicate with the collection chamber for removing material therefrom. The separator includes at least one swirling vane for imparting swirling motion to gas flowing in the annular channel. Separation of particulate matter from the swirling gas due to centrifugal force occurs when the gas initially enters the annular channel and as the gas swirls within the annular channel. Further separation of particulate matter occurs when the swirling gas in the channel experiences a sudden change in flow direction during entering the slots in the inner tube. Clean gas discharge means are provided at the bottom of the inner tube so that clean gas exits the separator from below. A plurality of such centrifugal separators are mounted in a pair of upper and lower parallel tube sheets located within a separator vessel. The tube sheets define a main separation chamber therebetween, with a gas inlet chamber thereabove, and a clean gas discharge chamber therebelow. Gas containing particulate matter enters the inlet chamber of the vessel from above, passes directly into the multiple separators in the separation chamber, and is discharged downwardly through the bottom discharge chamber. Means to remove separated particulate matter and bleed gas from the separation chamber are provided.

Description

11613~4 Background of the Invention This invention relates to an improved method and apparatus for separating particles from gases by use of centrifugal separators. It is particularly useful for separating particles of catalyst from hydrocarbon vapors issuing from a catalytic cracking process and can also be advantageo~sly utilized in other applications such as for removing suspended solids from gases fed to boilers, result-ing from coal gasification and liquefa~tion, molecular separation; and for use with supercharged boilers.
In a catalytic cracking process, particle separators embodying the present invention àre especially useful in "third stage" separation, that is, in séparation of relatively smaller particles from gas after the relatively larger particles havé already been removed by cyclone separators. Third stage separation is quite important for air pollution control purposes and for economizing. Some active catalyst can advantageously be recover2d and re-used by this mode OL separation. Furthermore, purified clean gas from this third stage separation can be used to drive a turbine without any significant erosion of the turbine blades which would otherwise be caused if such particles ~tere still present in the gas being fed to the turbine.
Centrifugal separators of the type having an outer tube and a concentric inner tu~e defining an annular ~ass~ge~7ay therehet~Jeen are kno~1n to the art. In United States Patent No. 3,~43,3~8, of ~lhich I am a co-inJentox, 11f~1374 a centrifugal separator of the above type is described. The inner tube, which also acts as a clean gas discharge line, has an open-mouthed end extending into the region defined ~r7ithin the outer tube. The inner surface of the outer tubé is lined with refractory ceramic.
In operation, gas laden with particulate matter enters the annùlar passageway in a swirling motion generated by swirl v~nes associated with the separator. Centrifugal force throws the particulate matter in the gases outwardly against the inner surface of the outer tube. These particles along with some bleed gas, enter a narrow annular trough at the bottom of the separator and are discharged therefrom.
The clean gas, now in a purified state, is sucked into the open mouth lower end of the inner (clean gas discharge) tube and ascends upwardly, exiting the separator at the top~
In my United States Patent No. 3,631,657, an apparatus for cleaning gas comprising an inner vessel enclosed within an outer vessel is disclosed. The inner vessel is partitioned into a clean gas chamber, an inter-mediate inlet chamber and a particle collection chamber.
Gas to be purified is fed into the intermediate inlet chamber, which contains a plurality of centrifugal separa-tors, as for example, those shown in the above-mentioned United States Patent No. 3,443,368 The bottoms of the separators communicate with the particle collection cham~er to deposit the separated particles ther~in. The inner clean ga~ discharge tu~e of the severa l sep arators di roct tl-e clcan cJss opwarclly into _4_ the clean gas discharge chamber. The purified gas flows through holes in the sidewall of this cha~ber, into the space defined bet~een the inner and outer vessel, and is disc'narged from the apparatus through an outlet nozzle associated with the outer vessel.
United States Patent No. 2,941,621 shows another embodiment of a known centrifugal separator including an outer tube and an inner clean gas discharge tube positioned Concentrically therein. Gas to be purified enters an annular channel defined between the inner and outer tubes with a swirling motion imparted by swirl vanes. The separated particles and bleéd gases drop to the bottom of the outeX tube and are discharged therefrom while the purified clean gas ascends through the inner discharge tube and,exits the separator from above. A similar separa-tor is also disclosed n Unite~ States Patent No. 3,066,854.
plurality of such separators are included within a single vessel partitioned into a center inlet chamber which supports the separators ard communicates with the incoming gas to be purified, a lower collecting chamber in co~munication with the bottom of the separators for recei~ing discharged particles and bleed gas, and an upper clean gas outlet chamber in co~munication with the inner discharge tube for recei~ing purified gas~ Duct means are provided for bypassing the upper clean gas dis-charge chamber and introducing the gas to be purified directly into th~ centrcll sepclration ~11z.,~h~,r, ~1 1161374 The prior art centrifugal separators also experienced erosion problems caused by the recycling of the separated particles. That is, even after particles were separated from the gas, these particles ~7ere still trapped Within the separators and subjected to turbulent forces causing them forcefully to impact against and continue ~o churn against the ~alls of the separator, resulting in severe erosion.
Also, because the purified gas exits from the top of the separator while the separated particles and bleed gas exit from the'bottom of thé separator, 'the dis-closed prior art separator véssel, must, by necessi~y, position the clean gas discharge chamber abo~e the separa-~ors, and posi~ion the collecting chamber below the separa-tors. Thus, the chamber in which the separators are housed must be positioned between th'e upper and lower chambers .
thereby requiring the complex arrangement of a separate air inlet duct means to transport the particle-laden gas through the upper clean gas discharge chamber for introduction into the separation chamber of the vessel.
It is an object of the present invention to provide an improved separator, separator vessel and method of separation overcoming the disadvantages o~ the known prior art Specifically, the present invention pro~ides a simplified, economical and durable sepaxator and separator vessel which is highly efficient and easy to maintain More-over, a novel method of scparat~ c~ par~icles, from yas~s is provided.

-6- ~' ~ 37~

Summary of the Invention A centrifugal separator embodying the present invention includes a greater diameter metallic outer tube and a smaller diameter metallic inner tube concentrically positioned therein and extending longi~udinally within the outer tube. The innex tube has a closed top and an open bottom. An annular channel is defined between the outer surface of the inner tube and the inner surface of the outer tube. This channel has an open top for receiving gas, and a closed bottom. A p~urality of swirl vanes are positioned proximate to the top of this annular channel for i~parting a swirling movement to gas within the annular channel, The inner tube defines a series of angled slots ~hich are directed inwardly towards the longitudinal center of this tube providing communication between the annular channel and the space within the inner tube~ These slots c~use a sudden change ln the direction of flow of the swirling gas which passes from the annular channel inwardly through the slots and into the inner tube.
Towards the bottom of the annular channel, an annular collection chamber is defined between the inner surface of ~he outer tube, the outer surface of the inner tube, a lo~er annular closure, and an upper annular ridge positioned above the closure. This annular ridge e~.tencls transversely from the outer surface of the inner tube in a direction to~iaxds the irlne~- surface of tlle outer tube. Its radial extel-~ is sliyht:ly smaller than the diference o ~he diameters of the outer and inner tubes. Thus, the outer rim of the ridge and the inner surface of the outer t:ube form a narro~l passageway leading do~mwardly into the collection char~er, The ridge may also include anti-s~"irl vanes (or blades) which extend across the opening of the narrow passageway for converting velocity head into pressure head.
That is, a predetermined percentage of the yas swirling in the annular chaTInel is forcefully directed into the small enclosed bottom collection chamber to increase the pressure therein.
Discharge means, such as a discharge port or a discharge pipe, through thç outer tube communicate with the above-described lower annular collection chamber for pro-viding means for removing material from that chamber.
In op ration, gas iaaen with particles enters the top of the annular channel. The associated swirl vanes produce a swirling flow of gas, Centrifugal force propels the larger particles against the inner surface of the outer tube. These separated particles descend down the inner surface, and together with some bleed gas, enter the lower annular collection cham~er from ~7hich they are dis-charged via the discharge means associated ~7ith the collectio~
chamber. As the gas swirls down the annular char~er and makes further revolutions, additional partic1l1ate matter is centrifugally separated therefrom, After the aforementioned centriugal separation of the larger and intermediate ~ize p<lrticles have occurred in the length of the separating annulus of the annular channel, the partially purified gas swirling downwardly in the annular channel reaches the slots in the inner tube.
The slots are so anyled that the gas in passing from the annular channel int,o the inner tube takes a sudden change in direction of flow as it enters the slots. ~This change in direction and sudden inward flow causes smaller particles (~JhiCh were not initially separated from the gas) to be left within the annular channel while the main flow of gas enters the inner tube and flows downwardly through its open bottom portion. The smaller particles remaining in the annular channel descend'into the lower annular collection cham~er, thus completing the additional stage of separation.
The purified gas leaves each of the separators from the bottom thereof and the separated particles and ~leed gas leaves the annular collection chamber in a transverse direction. Thus, it is possib~e to include a plurality of these separators in an enclosed separator vessel and introduce the gas to be purified directly into tne top of the vessel. This is no, possible in, the pre-viously discussed equipment of the prior art because the known separators disclosed therein discharge purified gas in an up~?ard direction. This prior art structure re~uired that the upper chamber be reserved for clean gas discharged from the separators, and necessitated the use of an i~ter-mediate chamber as an air inlet chamber, thereby requiring tilc-~ vessel ~o ir.clude cluct means for transpo,,t,'Lng the ga,;
to be purilicd throuc3h ~he up~er clean gacs to be ,intrc,duc~d into the intermediate gas inlet chamber, ~ 1161374 In addition to eliminating the need for such duct means, the advantageous arrangement of having the gas inlet chamber at the top of the separator vessel without any such interfering duct allo~s easy access to the separator tube~ for inspection or maintenance purposes, thus render-ing it easy to assemble and disassemble the ~ ssel.
By virtue of the fact that the separated particles and bleed gas leave the separators in a lateral direction, the purified gas can advantageously be discharged from the separator vessel in a do~,mward direction and still avoid xemixing with the separated particles.
Further features, aspects and advantages of the present inv~ntion will become more fully understood from the follo~,Jing detailed description of a preferred embodiment in conjunction with the accompanying drawings.

Brief Description of the jrawings FIGURE 1 shows an elevational sectional view of a centrifugal separator constructed in accordance with the present invention, ~ IGU~E lA shows a portion of the separator of FIG.
1 including a modification, FIGURE lB shows anti-swirl vanes which may be employed, FIGUP2 lC shows the confic3uration of a swirl vane, FIGUR~ 2 shows a top plan v.iew of the c~ntrifu~al e~ rator of ~I5. 1 i ` 11:6 tKa ting ~l .sories c~f s~/irl vanes I

,,,11 . 1, ll 1161374 FIGURE 3 is drawn on a much larger scale than FIGS. 1 and 2, and it shows an elevational sectional vie~.7 and flow diagram of a centrifugal separator vessel including a plurality of centrifugal separators of the type shown in FIG. 1, FIGURES 4A and 4B are plan views whi~! illustrate two typical arrangements of a plurality of centrifugal separa tors mounted in the separating vessel of FIG. 3, FIGURES 5A, 5B, 5C, 5D and 5E are cross sectional vîews taken through the inner tube of a centrifugal separator as dra~n in FIG. 1 showing various advantageous configura-tions of the slots in the innér tube of the separator of FIG. 1~

Description of the Preferred Embodiment Referring to FIG~ 1 of the drawings, a centrifugal separator constructed in accordance with the present inven-tion is indicated generally by the numeral 2 This separator comprises a generally cylindric21 inner tube 4 concen~rically positioned within a generally cylindrical outer-tube 6.
; The inner tube extends in the longitudinal direction in the outer tube. An annular channel 8 for receiving a flowing stream of gas is defined bett7een the inner surface of the outer tube and the outer surface of the inner tu~e.
A pair of supporting tube sheets 10 and 12 are positioned, respectively, to~lards the top and bottom of the centrifugal separator. These tube she~ts rec~ive and hold the upper and lowPr ends of the separ2tor. hn upper .

116~374 circular closure piece 17 effectively seals the top of the inner tube while leaving the top of the annular channel 8 unobstructed. A lower annular partition 16 seals the bottom of the annular channel but leaves the lower end of the inner tube unobstructed.
In the preferred embodiment as sho-~ in FIG. 1, t~e top and bottom tube sheets, the upper closure piece, the lower annular partition, and the inner and outer -tubes are formed of metal. Using such metal components renders the separator inexpensive to manufacture as compared to ceramic-lined u~its as often required by the prior art.' The metallic tubes may be surface hardened to reduce erosion.
Advantageously, the upper and lower tube sheets 10 and 12 with the multiple centrifugal separators 2 extending between these two sheets and securea to them provides a rigid truss-like structure which strongly resists downward bending, sagging or distortion. Thé vertically-spaced twin sheets 10 and 12 act like upper and lower webs ~f a truss structure, with the separators 2 acting like struts between these webs. Consequently, the rçspective horizontally-extending tube sheets 10 and 12 can be made relatively thin as comparèd ~7ith prior art structures of comparable diameter, and yet the overall drum-like assembly 10, 12 and 2 is relatively strong and can readily be supported like a basket from its perimeter by means of the sup~ort member 49 to be described further below.
The centri,Eugal ~,e~rator 2 ,includes a scries of associa~ed s~7irl v~ne~; 15 positio:~ed in t31e~ ~nnular cllannel 8 towards the top thereof ~or generating a swirling movement of gas within this annular channel. In the alternative~ a tangential gas entrance to the annular channel 8 can be provided for generating swirling motion of the gas in that channel.
An annular collection cha~ber 14 for collecting separated particles and for allowing the flow~of bleed gas is located in the lower portion of the annular channel 8.
This collection chamber is defined by the lower partition 16, the outer surf~ce of the inner tube 4, the inner surface of the outer tube 6, and a ridge member 18 mounted on the inner tube and having an outer rim 20 closely spaced from the oùter tube. Thé lower partition, ~Ihic'n bridges across horizontally between the inner tube and the outer tube in a dixection perpendicular to the surfaces of these tubes, defines the closed bottom of the collection chamher.
The ridgë 18 is positioned above the partition 16 at the top of the colleetion chamber and extends trans-yersely from the outer surface of the inner tube 4 towards the inner surface of the outer tube 6. The radial extension of this ridge member is slightly less than the difference between the inner diameter (I.D.~ of th~ outer tube and the outex diameter (O.D.~ of the inner tube. Thus, the rim 20 of this ridge 18 and the inner surface o~ thé outer tube 6 define a narrow passageway 22 therebetween ~7hich leads downwardly into the collection chamber 14. The ridge itself can be either transverse or sloped downwardly and outwardly, as sho,iJI in FIC. 1~, r.~ to the ~ raGes o~ thc lnner and outex tubes.

If desired, anti-swirl vanes 19, may be provided e~tending into the area of the passageway 22, and being carried by the ridge 18. These anti-swirl vanes convert velocity head, i.e. momentum of the s~irling gas in annular channel 8, into pressure head to create a pressure differ-ential between the interior of the collection ~amber 14 ~nd the region 44 outside of the outer tube to prevent back-up of separated particulate matter, and to prevent the coupling of adjacent separators.
A discharge port 24. is formed in the wall of the OUter tube 6 communicating with the lower portion of the collection chamber 14. Particulate material which has accumulated ~7ithin the collection chamber 14 is removed th~re~rom via this port 24. The discharge port is of a sufficient size to maintain a 0.1 to 0.15 psi pressure dif~erential between the ~as within the collection chamber 14 and the region 44 outside of the outer tube. The relati~e size of the ports 24 controls the amount of bleed gas discharged from the individual collection c~ambers 14 into the common chamb~r 44 (See FIGS. 1 and 3).
The arrows 27 indicates the outward flow of bleed gas carrying within itself the accumulated particulate matter being removed from the collection chamber 14. By ~irtue of the fact that the pressure within the chamber 44 is purposefully maintained ~t a predetermined differential pressure, e.g. 0.1 to 0.15 psi, b~low the pressure in the upstream chamb~r 14, the flow does not inadvertently re~Prse in the event of sm~ll pressure differences ~.-nong cham~ers 14 n a plurality of separator tubes 2.

;1374 . . ' . .1.

As will be explained further below, a critical-flow no~zle 52 located downstream from the port 24 serves to control the total mass flow of bleed gas from all of the separator tubes 2, each individual port 24 contributing its proportionate share to the total mass flow of the bleed gas. The pressure differential across each port 24 is controlled by the mass of ~leed gas flowing through that port, which is itself controlled by the total mass of bleed gas flowing through critical nozzle 52. This will be a constant value for a given set of operating conditions.
A series of slots 26 are defined in the inner tu~e 4. These slots allow gas swirling within the annular chamber 8 to enter tnto the inner tube, producing a sudden change in direction of flow, causi~g the flow suddenly to turn inwardly into the innPr tube. It is believed that t more efficient results are obtained when the slots are positioned as shown below the approximate longitudinal center of the inner tube 4. -FIGS. 5A through 5E show various alternative configurations of these slots 26. Such slots may,-for example, be normal (FIG. 5A), chamfered (FIG. 5B), opposed (FIG. 5C), congruent (FIG 5D) or combined (FIG. 5E). A
normal slot 26A extends inwardly perpendicular to the tangent to the swirling component of flow 2g of the gases in the annular channel 8 near the 510t. An opposed slot 26C is angled inwardly in a direction which is opposite (or opposed) to the swirliny component of flo~ 29 of the yases in the annular channel near the slo-t. Conversely, a congruent slot 26D is anyled inwardly in a direction which 11~1374 is the same as the swirling component of flow 29. A
ch~mfered slot 26B converges inwardly toward the interior of the inner tube. Slot 26E is a combination of a normal slot on one vertical side and an opposed slot on its other vertica side.
As evident from FI~. 5, and as ~7ill be further discussed below, the slots cause a sudden change 31 in the radial direction of flow of the gas which was previously swirling 29 around the outer periphery of the inner tube 4 and then suddenly enters the slots, thereby suddenly pro-ducing a radial component of flow. The degree of change of direction of flow and the extent of the suddenness of this change depend o~ the specific configuration of the slots in the separator and the positioning of the slots in the inner tube. In FIGS. 5A-5E, the swirling gas is illustrated as circulating in a counterclockwise direction 29, and the gréatesf change in direction of flow 33 is caused by the opposed slot configuration 26C sho~n in FIG. 5 and the combination slot configuration 26E shown in FIG. 5E.
In operation, gas laden with particles is intro-duced i5 into the open top of the annular channel 8 by the action of the s-Jirl vanes 15 which are curved in the axial direction like turbine blades. These swirl vanes impart a swirling ~,novement to the entering gas causing the gas to revolve around the outer periphery of the top o the inner tube 4. The centrifugal force of the s~.~7irling motion causes the larger particles to be thrown outwardly against the inner surface of the outer tube 6. '~ue to thQ ~ction o~ grav.itational forces and the motion of a small amount of bleed gas 27 from the bottom of the annular chamber 8, the separated larger particles descend down along the inner surface of the outer tube and pass through the narrow passageway 22. Thus, the separated particles, together ~lit~ some bleed gas, e.nter t~e collection chamber 14. The discharge port 24 provides means for removing the collected ¦material from t~e chamber 14.
After the initial centrifugal separation of larger particles has occurred, the swirling gas, which is Continually descending i~ the annular channel 8 and causing further separation, reaches the proximuty of,the slots 26 in the inner tube, Because of the pressure differential from the top of the separator to the bottom and because the,bottom of the annular channel is closed while the bottom of the inner ~ube is open, thé swirling gas flows inwardly through the slots and into the inner tube. (In general, the pressure differential between the top and bottom of the separator 2 may be of the order of approximately 1.0 to 1.5 psi. In other words, this is the difference in,pressure of the flow 35 entering the separator 2 and the flow 37 leaving ~s this separator.) As previously noted and shown by FIG, 5, the shape o~ the slots is such that yas entering therein experiences a sudden change in radial direction of flow, This rapid change of direction of the gas strea~ rf~sults in a urther separation of smaller particles ~7hich ~7ere not separated ~7hen the gas entered and sr,~irled throug}l th~ clnn~lax channel.

ltl37~ l These smaller particles are left behind when this sudden change in direction occurs. In other words, due to their momentum, they continue traveling in a swirling motion 29 ~FIGS. S~-SE) in the annular channel 8 when the gas flow suddenly turns inwardly 31 or 33 through the slots. Also, some particles strike the downstream sides 39 of the slots and are bounced back into the annular channel. These ¦additionally separated particles remain in the annular ch~annel and move towards the inner surface of the outer tube, descending into the collection chamber 14 at the bo~tom thereof, while the main stream of gas enters the inner tube through the slots 26. The later separated particles leave the collectio~ chamber together with the previously separated particles through the discharge port 24, as was described above.
After all the separation stages have occurred, the main swirling clean g~s stream flows through the slots and into the inner tube and descends therein, exiting as shown at 37 from the bottom of the separator through the lower open end of the inner tube.
During the above described separation, a small amount of the gas to be purified remains in the annular channel and does not pass through the slots in the inner tube. This bleed gas swirls downwardly towards the lower ridge 18, which may carry at least one anti-swirl vane 19 (FIG. lB) for forcefully directing the flo~ of this gas through the passageway 22. In cffect, the anti-~wixl v~nes 19 convert velocity head into pressure h~1d with:in the small 37 ~ ' collection chamber 14, causing a pressure differential of about .l to .15 psi between the interior of the collection chal~er and the region 44 outside of the outer tube. This pressure differential prevents the coupling of the centri-fugal separator with any other similar separators in the vicinity, and thus prevents the back-up of particles collected in the collection chamber through the annular channel of the separator. This pressure difference between the collection chamber 14 and the downstxeam region 44 also facilitates the discharge of material from the chamber.
Although the anti-swirl vanes are advantageous, they are not absolutely essential. This results from the fact that some bleed gas will enter the passageway 22 into the collection chamber 14 due to the pressure differential caused by the cxitical flow nozzle and the ports 24 even if no anti-swirl vanes are presént.
Because the collection ch~ber is substantially isolated from the swirling forces of the annular channel above it by the ridge 18, the effect of these swirling forces on the particles collected in the collection chamber is greatly diminished. (These now diminished forces may facilitate the removal of particulate matter from the collection chamber through the ports). Also, unlike some of the noted prior art separators, the separat d particles are not required to negotiate any sharp turns or to "jump"
across a gap, because the entrance 22 to the collection charnber l~ is positioned dir~ctly ~ 0',7 the inn~r sur~ace of the outer tube and form~ a contin~latiorl the~r~o~ ~itho~t interruption or change in direction.

Thus, the present embodiment of the invention advantageously eliminates t~70 major causes of erosion due to recycling of the separated particles that occurred in many of the prior art structures. Therefore, th~ presently described separators may, depending on the concentration of particles in the gas to be treated, be manu~actured without lining the inner and outer surfaces of the outer and inner tubes, respectively, with an erosion reducing material (such as the expensive ceramic lining used in c~rtain prior art units), thus markedly reducing the cost of the separators. Use of unlined tubes also provides a lishter weight vessel.
In the preferred embodiment, the clearance between the rim 20 of the ridge 18 and the inner surface of the outer t~be 6 is sufficiently small so that larger particles of debris, which would obstruct the discharge port 24 if allowed entry into the collection chamber, are prevented from entering the collection chamber.
An example of the p~efer~ed dimensions of the -above described new centrifugal separator is as follows.
The inner diameter of the outer tube should be less than six inches and the outer diameter of the inner tube should be less than four inches. The length of both the inner and outer tubes is preferably at least three times the length of the inner diameter of the outer tube. The slots in the inner tube are approximately four inches long and approxi-mately one-quarter inch ~7ide, a.nd in the pr~lerre(~ einboc1imerlt, there are six ~o t~7elve such Sl.0~,5, ¦ ~ The inner and outer tubes should preferably be long. In fact, FIG. 1 shows these tube lengths as being about six times as great as the diameter of the inner tube~
The advantage of utilizing longer tubes is that they provide I ~ longer annular channel 8, which can result in more effi-cient separation of particulate matter from ~s as the gas swirls down the channel, prior to its entry into the slots.
The longer annular channel provides a large spiral path for particulate matter and gas swirling within the annular channel, This èmbodiment includes six to eight swirl vanes 15 associated with each separator. As shown on larger scale in FIG, lC, the angle A of the downstream or discharge lip 43 of each swirl vane with the horizontal should not be greater than 30, q'he spouting velocity of the swirling g~s can be within the range of 100 to 250 ft,/sec. The spouting velocity for larger particles should be in the ~o~eX portion of the range, while the spouting velocity fo~ s~aller particles should be in the higher portion of the range, This spouting velocity is the velocity at 41 ~,n FI~. lC as the gas shoots away from the downstream lip 43 of each swirl vane and into the annular channel 8 below the swirl vanes.
The width of the passayeway 22 should be between 1/8" and 1/16" in order to prevent laryer sized particles or debris from entering the collection cham~er and obstruct-iny the dischar~e port 2~. There are three or io~r of these discharge ports 2~.

Referring to FIG. 3 of the drawings, a novel separator vessel, advantageously utilizing a pl~rality of thP above-described new centrifugal separators, is indicated genexally by the numeral 28. The vessel is defined by an outer shell 30. A layer of heat resistant insulation 32 is a~fixed to the interior of this outer shell.
A reduced diameter inlet passage 34 is formed a~
the top of the vessel and a similar necked outlet passage 36 ~s formed at the bottom o~ the vessel. A main body, ~ene~ally indicated at 40, is formed between the inlet and outlet passageways, The main body 40 of the separator vessel is shown as being gener~lly cylindrically shaped when high internal pressure ~s involved, and is divided into three sections: an upper gas inlet chamber 42 positioned below the ~nlet passageway 34, an intermediate annular particle separation chamber 44, and a lower clean gas discharge cham~er 46 which leads ~nto the outlet passageway 36 there-~elow.
The annular separation charnber 44 is defined by the uppex and lower tu~e sheets 10 and 12 (shown in FI5. 1) and an annular sid~wall 48. The separation cham~er accom-modates a plurality of the separators 2. Th~ upper ends of the inner and outer separator tubes extend through openings in the upper tube sheet 10 into the gas inlet cham~er 42 thereabove. The lower ends of the inner and outer separator tubes e~tend through openinc3s Gn th~ 10,1er tube sheet and into the clean gas discharge charn',~er ~,6 thrrcbelo~,l, Holes for accor~odatiny the separators on the up~er and lower tube sheets are in axial alignment so that the cylindrical separators stand ~ertically ~7hen inserted therethrough.

~ ` 11~1374 This discharge ports 24 of the separators 2 connect the collection chambers 14 of each of the separators with the separation chamber 44. One end of a duct 50 for carryin~
a,7ay the particle-laden bleed gas 27 co~municates t7ith the separation chamber 44, while its other end passes through the lower clean gas discharge outle, to join ~th a critical-flow nozzle 52 shown as being positioned outside of the vessel. Thus, a line of communication is formed between the interior of the individual collection chambers 14 of the separators and the exterior of the vessel for expelling particle-laden bleed gas.
The critical-flow nozzle 52 regulates the amount of bleed gas 55 discharged from the separation chamber throug the duct 50,~ Pre~erablyJ 1/4 to 2% of the gas 25 intro-duced into the vessel is discharged through this duct. In any event, the ~mount of ~leéd gas 55 must be sufficient to carry the load of collected particulate matter which has been deposited in the separation cham~er from the collection chambers of the individual separators.
Furthermore, by regulating the flow of gas from the separation chamber, the critical-~low nozzle controls the pressure differential between the collection chambers of the individual separators and the region e~ternal to them, as discussed above~ This differential prev~nts coupling between the individual separators anc avoids any back-up of material into tnese collection ch~nbers or into the annular channel thereabove. Also, the pressuxe differential het~Jeen the collection chc!~!bexs 1~ arl~l the sfparation chamber 44 of the vessel facilitates the discharge ~f material from the collection ch2mbers into the sèparation chamber, as discussed above i1374 ¦ An expansion-contraction accommodation bellows 51 ¦is provided connecting the duct 50 to the neck ~6. Because the operating temperature within the vessel can attain a s:ignificantly high magnitude (ap~roximately 1300F), the bellows avoids undue stress on the duct by expanding and contracting. Ways for preventing accumulation~of separated ¦particles in the folds of the bellows, such as by continuous steam flushing, are known to those s~illed in the art. As ¦ evident from FIG. 3, the bellows is positioned in the vessel in a location which provides ready access thereto for inspection, maintenance or replacement purposes through a removable access door 53 in the structural support wall 63.
The separation char.~ber is suspende~ and supported from the interior of the vessel by a support 54. The support can be affixed to or integral with cylindrical sidewall 48, thus, in effect, providing a basket-like structure fol-supporting the separation chamber 44. The support S4 is cylindrically shaped, the top thereof being affixed to the interior of the vessel 28. In FIG. 3, the top of the support 54 is affixed to the inner surface of the outer shell 30. As also shown in FIG. 3, the support 54 isolates inlet chamber 42 from outlet charr~er 46, thus preventing any inlet gas and particles con~ained therein from by-passing the separatiorl char~er 44.
In the alternative, support members (not sho~,m~
can be provided underneath the lower tube sheet 12 of the separation chamber 44 to support it from. below. In such an ernbodim2nt, the support rr,.er,~ers wo.lld be afi~ed to the inside of the vessel shell 30 and would project beneath the `~ 1 1~i1374 ~ 11 lower tube shee~ 12 for accommodating relative expansion and contraction of the various members. Also, as noted above, means to isolate the inlet chamber 42 from the outlet chamber 46 ~lould be provided be~ween the top of the separa-tion chamber 44 an2 the vessel 28 to prevent inlet gas and particles from by-passing the separation chamber by going around its outer peripnery.
The support 54 is shown as being partially insulated ~y the insulation 61 to reduce thermal stress therein. The insulation 61 of the support 54 is desira~le because, in operation, t~e interior of the vessel can achieve temperatures of approximately 1300F ~7hile the insulated vessel shell 30 is maintained significantly colder in the , range of 300~F to 40~F.
FIG, 4 illustrates two possible arrangements of a tube sheet in accordance with the present invention. The large circular area repres~nts the lower tube sheet 12, and the smaller circular openings 56 répresent the holes in the tube sheet through which the individual separators 2 are inserted.
In operation, gas 25 laden with particles enters the gas inlet 34 ancl is introduced into the gas inlet chamber 42. The gas flows through the swirl vanes associated with the individual separators 2 as shown hy the arror,7s 35 i.mparting a po~exful s~irling motion to the yas as shown by arrow 41. As previously describea, particulate matter is separatecl from the gas, and the separatea part.icle~ and some bleed gas enter .into the lo~.Jer coll~ction chamhers 1 of the individual separators 2. Separated particles and i bleed gas contained within the collection chambers 14 of the individual separators pass out laterally through the d}scnarge ports 2a and into the separation area 44, and are expelled from the vessel through the duct 50 and nozzle 52, as indicated by the arrow 55. ~
The clean gas 37 exits the separators downwardly through the open bottom portion of the inner separator tube 4, enters the clean gas discharge chamber 46 directly from the bottoms of the individual separators, and passes through the discharge outlet 36 into an outflow conduit 58, as sho~n by the arrow 59.
As is now evident, the use of the separators shown in FIG. 1 (in which clean gas is discharged downwardly from the bottom of the separator and the separated particles are discharged laterally ~rom the separators) in the vessel of FI5. 3 advantageously enables the uppermost region of the vessel to act as a particle laden gas inlet chamber.
This was not possib~e in the prior art separa~ing vessels because those separators discharged the clean gas upwardly, thus requiring the uppermost chamber to be used as a clean gas discharge area. This necessitated the use of duct means to transport incoming gas through this upper chamber to the center separation chamber of the vessel and complicate~
the arrangement of connecting piping.
In the ~IG. 3 embodiment of the present invention, the above r,lentione~ ~luct m~ns of th~ p~-ior art have advantageously been eliminatea ~ecause use of ~he ne~.7 -`ll 11f~1374 separators allows the upper chamber of the vessel to be us~d as the gas inlet chamber. Elimination of the prior art inlet ducts reduces the cost of cons~ruction and manu-facture of the vessel.
Also, the vessel is easier to repair and maintain because access to the separator tubes is uno~itructed by the inlet ducts and partitions of the prior art.
Because the separated particles are discharged laterally from the individual separators, the clean gas can advantageously exit from the bottom of the separators in a downward direction, ~et avoid remixing with the sep-arated particles. This assures that the gas in the vessel flows in a downward dixection from the time it enters the ve~el until it exits therefrom~ In contrast, the known prior art vessels admitted incoming gas in a downward direction, discharged ~he purified gas in an upward d;`rection, and discharged the separated particles in a do~nward direc-tion.
Thus, the disclosed embodiment of ~he present invention provides a less complicated vessel bécause the main stream of gas is always ~lowing in a downward direction, unlike the prior art in which the gzs enters the vessel in a downwardly direction but exits therefrom in an upwardly direction. Increased efficiency resul~s from the use of a plurality of separators comprising long, narro~ tubes.
Furthermore, as previously ex,plain~d, efficienc~ of operation is enhanced b~ the novel s~paxators ;J'nic'n pro,~id~ more distinct stages of s~paration ~han is providea by the prior art embodiments.

- ``I 1~ ~i1374 ` The separator tubes themselves form struts between the tube sheets increasing the strength of the structure markedly and permitting lighter weight and low cost construc-tion, These tu~ sheets can withstand the large forcesassociated with the incoming and outgoing flowing gas. At the same time, the arrangement of the tube she~'s and individual Separators defines the separation chamber 44 of the Separating vessel 28. .
It is believed that the many advantages of this .
invention will now be apparent to those skilled in the art.
It is also apparent that a number of variations and modifica-tionS may be made in this invention without departing from its SCope and spirit, Accordingly, the foregoing description iS to be construed aS illustrative only, rather than limit-ing, The inVention iS limited only by the following claims and all equivalents thereto,

Claims (31)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A centrifugal separator for separating particulate matter from downflowing gases, said separator comprising:
a vertical outer tube having an inner and outer surface, a vertical inner tube having an inner and outer surface, said inner tube being of a diameter of less than that of said outer tube, said inner tube being positioned within said outer tube and extending in the longitudinal direction within said outer tube with said outer surface of said inner tube and said inner surface of said outer tube defining an annular channel therebetween, said inner tube having a plurality of longitudinally extending slots therein in the lower portion of said inner tube for providing communication between said annular channel and the space within said inner tube, said slots being dimensioned and positioned so that gas swirling in circumferential direct-ion around said inner tube in said annular channel and then turning inwardly and entering through said slots into said inner tube experiences a sudden change from a circumferential direction of flow to a radially inward direction of flow, first closure means for sealing the top of said inner tube, and second closure means positioned proximate to the bottom of said annular channel for sealing the bottom of said annular channel, said inner tube being closed at the top and open at the bottom and said annular channel being open at the top and closed towards the bottom, means associated with said annular channel for swirl-ing a stream of gas containing particulate matter flowing down in said annular channel above said slots, said first and second closure means defining a path of flow of said stream of swirling gas in said annular channel, said gas swirling down through said annular channel above said slots, and then suddenly flowing inwardly through said slots into said inner tube, and down through the open bottom of said inner tube, said particulate matter in said swirling gas in said annular channel being separated from said gas by the force of centrifugal force thrusting said particulate matter outwardly against said inner wall of said outer tube (i) when said gas begins swirling in said annular channel and (ii) as said gas swirls down through said annular channel above said slots, and (iii) a further separation of particulate matter occurring as a result of said sudden change in direct-ion of flow of said swirling gas upon entering said slots, said separated particulate matter remaining in said annular channel for subsequent removal along with bleed gas from the lower portion of said channel, and said swirling gas stream exiting from said separator by flowing downwardly and out through the bottom of said inner tube.

2. A centrifugal separator as claimed in Claim 1 further including:
an annular ridge positioned within said annular channel and above said second closure means, the inside of said annular ridge being affixed to said outer surface of said inner tube, the outside of said annular ridge being closely spaced from said inner surface of said outer tube for defining a narrow passageway between said annular ridge and said outer tube, said second closure means, said annular ridge, said inner surface of said outer tube, and said outer surface of said inner tube defining an annular collection chamber towards the bottom of said annular channel, and said separator having at least one discharge port communicating with said collection chamber for dis-charging bleed gas and particulate matter from said collection chamber.

3. A separator as claimed in Claim 1 wherein said means for swirling gas in said annular channel includes a plurality of swirl vanes positioned in the upper portion of said annular channel towards the top of said separator.

4. A centrifugal separator as claimed in Claim 1, 2 or 3 wherein said slots in said inner tube are in the lower half of said inner tube.

5. A centrifugal separator as claimed in Claim 2 further including means for controlling the amount of said bleed gas discharged from said collection chamber through said discharge port, said controlling means including a critical flow nozzle positioned downstream from said dis-charge port and communicating therewith.

6. A separator as claimed in Claim 1 wherein said inner tube is concentrically positioned within said outer tube and is co-axial therewith.

7. A centrifugal separator as claimed in Claim 1, 2 or 3 wherein the axial length of said annular channel above said slots in said inner tube is at least three times greater than the diameter of said inner tube.

8. A centrifugal separator as claimed in Claim 2 wherein said annular ridge is sloped downwardly in a radially outward direction relative to the surfaces of said inner and outer tubes.

9. A centrifugal separator as claimed in Claim 1 wherein said inner tube has six to twelve slots in its wall, said slots extending longitudinally in the wall of said inner tube and having a length greater than the diameter of said inner tube and a width of approximately one-quarter of an inch.

10. A centrifugal separator as claimed in Claim 9 wherein each of said slots has one edge normal to a tangent to the swirling direction of the gas in said annular channel.

11. A centrifugal separator as claimed in Claim 9 or 10 wherein each of said slots has a downstream edge which is chamfered on the outside.

12. In a centrifugal separator of the type having an outer tube, an inner tube positioned within said outer tube and extending in the longitudinal direction within said outer tube, said inner and outer tubes extending vertically and defining an annular channel extending downwardly therebetween, and swirling means at the top of said annular channel for swirling a downwardly flowing stream of gas having parti-culate matter in said annular channel, said gas swirling in a direction around said inner tube while flowing downwardly in said annular channel, the improvement comprising:
first closure means sealing the top of said inner tube, second closure means sealing the bottom of said annular channel, a plurality of slots in said inner tube providing communication between said annular channel and the space defined within said inner tube, said slots being positioned in a lower portion of said inner tube and extending longitudinally in said tube for causing swirling gas in said annular channel to experience a sudden change in direction of flow during entry of said gas into said slots during flow of said gas into said inner tube, particulate matter in said stream of gas being initially separated from said gas and propelled against the inner surface of said outer tube by centrifugal force generated by said swirling motion of said gas when said gas is first swirled in said annular channel and as said gas swirls downwardly through said annular channel above said slots, further particulate matter being separated from said gas by said sudden change in direction of flow of said gas when said gas enters said slots, wall means defining a collection chamber communi-cating with said annular channel below said slots, said wall means defining a small passageway from said annular channel into said collection chamber, said centrifugal separator having a discharge port from said collection chamber to the exterior of said separator for discharging bleed gas plus the separated particulate matter from said collection chamber, and said inner tube being open at the lower end for allowing the cleaned gas which has entered through said slots into said inner tube to exit in a downward direction from the lower end of said inner tube.

13. A centrifugal separator as claimed in Claim 12 in which said wall means defining said collection chamber include an annular ridge secured to the outside surface of said inner tube below said slots, said annular ridge extending radially outwardly toward the inside surface of said outer tube, said annular ridge having a periphery closely spaced from the inside surface of said outer tube forming a narrow passageway between the periphery of said annular ridge and said outer tube.

14. A centrifugal separator as claimed in Claim 13 in which:
a plurality of anti-swirl vanes are mounted on the periphery of said annular-ridge and project into said narrow passageway for converting the velocity "head" in the swirling gas in said annular channel into pressure "head" in said collection chamber for causing the interior of said collection chamber to have a higher pressure than the lower portion of said annular channel.

15. A centrifugal separator as claimed in Claim 12 wherein the length of each of said inner and outer tubes extending along said annular channel above said slots is at least three times greater than the diameter of said inner tube.

16. A centrifugal separating vessel for separating particulate matter from gases to be purified, said vessel including:
a main housing including a gas inlet passageway positioned at the top thereof and a gas outlet passageway positioned at the bottom thereof, partition means within said housing defining an upper gas inlet chamber, an intermediate separation chamber, and a lower clean gas discharge chamber, said upper gas inlet chamber being adjacent to and communicating directly with said gas inlet passageway thereabove, said lower clean gas discharge chamber being adjacent to and communicating directly with said gas outlet passageway therebelow, said partition means including upper and lower tube sheets defining said separation chamber between them, a plurality of centrifugal separators extending vertically through both of said tube sheets and through the separation chamber between said sheets, each of said separators including an inner and an outer tube, said inner tube having an O.D. less than the I.D. of said outer tube and extending vertically coaxially within said outer tube for providing an annular channel between said tubes, said inner tube being closed at the top and open at the bottom, said annular channel being open at the top and closed at the bottom, said inner tube having a plurality of longitudinally extending slots therein providing communication between the lower portion of said annular channel and the interior of said inner tube, said open tops of said annular channels in said separators communicating directly with said inlet chamber for admitting particle-laden downflowing gas from said inlet chamber into each of said annular channels, a plurality of swirl vanes in the open top of each separator for causing the particle-laden gas in said channel to swirl around the inner tube as the particle-laden gas flows downwardly through said channel for separating particles from the swirling gas by centrifugal effect, further separation of particles from the swirling gas occurring as the swirling gas suddenly turns inwardly flowing through said slots into the interior of the inner tube, said open bottoms of said inner tubes in said separators communicating directly with said lower clean gas chamber, means for expelling separated particles and bleed gas from the bottom of the annular channels of said separators communicating with said intermediate separation chamber of said separator vessel to discharge particles and bleed gas therein, and duct means communicating with the bottom of said separation chamber for carrying away the bleed gas and separated particles, wherein said gas to be purified is introduced into the vessel at the upper gas inlet chamber, flows downwardly through the plurality of centrifugal separators extending vertically through said intermediate separation chamber where it is purified, and exits downwardly from said separators passing through said lower clean gas discharge chamber in a downwardly direction

17. A centrifugal separating vessel as claimed in Claim 16 in which:
said means for expelling separated particles and bleed gas from the bottom of the annular chambers of said separators includes a discharge port in each of said separators communicating with said intermediate separation chamber, and a critical flow nozzle in said duct means for controlling the total mass flow of bleed gas and separated particles for controlling the flow of bleed gas and separated particles through each of said discharge ports from the res-pective centrifugal separators.

18. A centrifugal separating vessel as claimed in Claim 16 wherein said intermediate separation chamber which is defined between said upper and lower tube sheets is strength-ened by said plurality of centrifugal separators being secured to said upper and lower tube sheets adding structural support ?

to said intermediate separation chamber.

19. A centrifugal separating vessel as claimed in Claim 18 further including means solely connected to the perimeter of said upper and lower tube sheets for supporting said upper and lower tube sheets defining the intermediate separation chamber solely from the perimeter as a basket-like structure, and means for heat insulating said supporting means for said intermediate separation chamber, whereby said downflow separators enable a simplified structure and said downflow separators connected to said upper and lower tube sheets act as stiffening struts for enabling the entire basket-like structure to be relatively lightweight but strong.

20. A centrifugal separating vessel as claimed in Claim 17 in which:
said duct extends vertically down through said clean gas discharge chamber, and said duct is connected to said vessel through an expansion and contraction bellows.

21. A centrifugal separating vessel as claimed in Claim 16 wherein each of said plurality of centrifugal separators further includes:
an annular ridge positioned within said annular channel below said slots and spaced above the bottom of said annular channel, said annular ridge being affixed to the outer surface of said inner tube, said annular ridge extending in a direction towards the inner surface of said outer tube, the periphery of said annular ridge being closely spaced from the inside surface of said outer tube for providing a narrow passageway between said outer tube and the periphery of said annular ridge, ?

said annular ridge, the inner surface of said outer tube, and the outer surface of said inner tube defining a collection chamber towards the bottom of said annular channel, said narrow passageway leading down from said annular channel into said collection chamber such that particles separated from said swirling gas descend down said annular channel, through said narrow passageway, and into said collection chamber, and said means for expelling separated particles and bleed gas from the respective separators providing communica-tion between the individual collection chambers of the res-pective separators and said separation chamber between said tube sheets.

22. A method of separating particulate matter from gases, said method using a vertical outer tube and a vertical inner tube having a plurality of axially extending slots therein, said inner tube being positioned concentrically within said outer tube such that a vertical annular channel is defined between the outer surface of said inner tube and the inner surface of said outer tube, said method including the steps of:
introducing a stream of gas containing particulate matter into said annular channel, causing said stream of gas to swirl in a circum-ferential direction around said inner tube as the stream flows downwardly in said annular channel such that larger particles of particulate matter are thrust against the inner surface of said outer tube by centrifugal force of said swirling gas, allowing the swirling gas to flow downwardly in said annular channel for causing additional particles to be flung against the inner surface of the outer tube such that said additional particles are separated from the stream of gas, thereafter directing the flow of said swirling gas inwardly through said slots in said inner tube passing into the interior of said inner tube such that said swirling gas experiences a sudden change in direction of flow in entering said slots so that further particles are separated and temporarily remain in said annular channel while said swirl-ing gas enters the interior of said inner tube, discharging said stream of gas downwardly from the interior of said inner tube and removing the separated particulates together with bleed gas from the lower end of said annular channel.

23. A centrifugal separator for separating particulate matter from gas, said separator comprising:
an elongated outer cylindrical tube;
an elongated inner cylindrical tube having an outside diameter (O.D.) less than the inside diameter (I.D.) of said outer tube;
said inner tube extending longitudinally within said outer tube and being positioned concentrically within said outer tube;
said inner tube having an O.D. approximately 2/3rds of the I.D. of said outer tube;
said concentric outer and inner tubes extending vertically and defining an elongated vertical annular channel therebetween;
said concentric outer and inner tubes being adapted to be mounted with their axes extending vertically;
the upper end of said inner tube being closed;
the lower end of said annular channel being closed;
said inner tube having a plurality of circumferen-tially spaced slots therein;

said slots being positioned in the lower part of the inner tube for defining a long annular channel above the upper ends of said slots;
the lower ends of said slots being positioned above the closed lower end of said annular channel;
swirl means associated with the upper end of said annular channel for causing the particle-laden gas entering the upper end of said annular channel to swirl vigorously around the axes of said tubes as the gas flows down through said long annular channel above said slots;
thereby providing a first stage of separation as the particle-laden gas is initially swirled by said swirl means as the gas enters the upper end of said annular channel and thereby providing second stage of separation as the gas swirls down through the long annular channel and makes further revolutions therein before reaching the upper ends of said slots for causing the gas to become partially purified before reaching the upper ends of said slots, with the separated particles descending down along the inner surface of the outer tube;
the partially purified swirling gas then taking a sudden change in direction of flow as the gas enters and passes through said slots into the interior of said inner tube;
thereby providing a third stage of separation as said partially purified gas takes said sudden change in direction causing smaller particles which were not separated from the gas in said first two stages of separation to remain within the annular channel for removal therefrom while the main flow of gas enters through said slots into the inner tube;
the clean gas flowing down through said inner tube and being discharged from the lower end of said inner tube; and said annular channel having an outlet therefrom at the lower end of said annular channel for removing the collected particles from said channel.

24. A centrifugal separator for separating particulate matter from gas as claimed in Claim 23, in which:
a ridge is provided on the inner tube positioned below the lower ends of said slots and above the closed lower end of said annular channel;
said ridge extending circumferentially around the inner tube and projecting out closely adjacent to the outer tube defining a narrow annular passageway between the inner surface of the outer tube and the perimeter of said ridge for allowing the collected particles to be removed from the lower end of said annular channel through said narrow passage-way;
a chamber being provided below said ridge, said chamber being defined by said ridge, the closed lower end of said annular channel and the portions of said inner and outer tubes extending between said ridge and said closed lower end;
and said chamber having at least one exit port therefrom for discharging the collected particles through said port together with a minor proportion of the gas.

25. A centrifugal separator for separating particulate matter from gas as claimed in Claim 24, in which:
anti-swirl vanes are positioned in said narrow annular passageway for converting the velocity head of the swirling gas in the lower end of said annular channel into pressure head in the chamber below said ridge.

26. A centrifugal separator for separating particulate matter from gas as claimed in Claim 24 or 25, in which:

said exit port(s) from said chamber are sufficiently small for providing a pressure drop of between 0.1 and 0.15 pounds per square inch for isolating pressure variations in said annular channel from the region downstream from said exit port(s) for preventing any flow reversal through said exit port(s);
thereby preventing any back-up of particles and facilitating the discharge of collected particles from the chamber near the closed lower end of said annular chamber through said exit port(s) regardless of pressure variations in said annular channel.

27. A centrifugal separator for separating particulate matter from gas as claimed in Claim 23, 24 or 25, in which:
said swirl means are a plurality of circumfer-entially spaced swirl vanes positioned in the upper end of said annular channel;
each of said swirl vanes having a discharge lip at its downstream end; and the angle "A" between said discharge lip and the horizontal is no greater than 30° at the portion of said discharge lip adjacent to the wall of the outer tube.

28. A separator vessel for containing a plurality of centrifugal separator tube units for separating particulate matter from a gas to be purified comprising:
a main housing having a gas inlet passageway positioned at the top and a clean gas outlet passageway positioned at the bottom;
a pair of vertically spaced tube sheets;
support means in said main housing for supporting said pair of tube sheets extending across the interior of said hous-ing for separating the interior of said housing into an upper chamber communicating with said inlet passageway for receiving the particle-laden gas and a lower chamber communicating with said outlet passageway;
said tube sheets being vertically spaced apart forming an intermediate chamber between said tube sheets, said intermediate chamber being located between said upper and lower chambers;
a plurality of centrifugal separator tube units extending vertically through both of said tube sheets, said centrifugal separator tube units being of the type in which the particle-laden gas enters the upper end thereof and clean gas is discharged from the lower end thereof;
the upper ends of said centrifugal separator tube units being positioned in said upper chamber and the lower ends of said tube units being positioned in said lower chamber;
said tube units each being secured to both of said tube sheets for forming a rigid truss-like structure which strongly resists downward bending, sagging or distortion;
whereby said tube sheets can be formed of relatively thin metal;
said centrifugal separator tube units being of the type in which the collected particles together with bleed gas exit therefrom through at least one exit port intermediate the upper and lower end of the tube unit;
said exit ports of the centrifugal separator tube units opening into said intermediate chamber; and a duct connected to the bottom of said intermediate chamber for carrying away the collected particles together with the bleed gas.

29. A separator vessel as claimed in Claim 28, in which:
said duct includes a critical flow nozzle for controlling the total mass flow of bleed gas flowing from said intermediate chamber through said duct; and said total mass flow being sufficient for carrying away the particulate matter from said intermediate chamber.

30. A separator vessel as claimed in Claim 29, in which:
said critical flow nozzle controls said total mass flow of bleed gas to be between 1/4% and 2% of the total amount of gas entering the vessel through said inlet passageway.

31. A downflow centrifugal separator for separating particulate matter from gas, said downflow separator compris-ing:
an elongated outer cylindrical tube, an elongated inner cylindrical tube having an outside diameter (O.D.) less than the inside diameter (I.D.) of said outer tube;
said inner tube extending longitudinally within said outer tube and being positioned concentrically within said outer tube;
said concentric outer and inner tubes being vertical and defining an elongated vertical annular channel therebetween having an axial length at least six times said I.D. dimension of said outer tube;
said concentric outer and inner tubes being adapted to be mounted with their common axis extending vertically;
the upper end of said inner tube being closed and the lower end of said inner tube being open;
the lower end of said annular channel being closed;
a plurality of swirl vanes near the upper end of said annular channel for rapidly swirling the particle-laden gas entering the upper end of said annular channel around said common axis of said downflow separator for providing a first stage of separation of particles;
said inner tube having a plurality of circumferential-ly spaced axially extending slots therein;

the upper ends of said slots being positioned below the longitudinal center of the inner tube for defin-ing a long annular channel above the upper ends of said slots for providing a second stage of separation of particles as the swirling gas flows down through said long annular channel above said slots;
the lower ends of said slots being positioned above the closed lower end of said annular channel;
said axially extending slots defining an abrupt change in direction for the gas swirling in said annular channel for causing the partially purified swirling gas to take a sudden change in direction of flow as the gas enters through said slots into the interior of said inner tube for providing a third stage of separation as said partially purified gas takes said sudden change in direction for causing smaller particles which were not separated from the gas in said first two stages of separation to be left within the annular channel while the main flow of gas enters through said slots into the inner tube;
the purified gas flowing down through said inner tube and being discharged downwardly from the open lower end of said inner tube;
the closed lower end of said annular channel having a chamber for collecting the particles which were separated in said three stages and which have traveled down through said annular channel; and at least one outlet port from said chamber for removal of the collected particles.