Air flow limiter with closed/open sensing

Abstract

Method and apparatus for air flow regulation comprise a vertically oriented tube, a sail assembly positioned in the tube and moveable therewithin responsively to air flow through the tube to regulate rate of air flow through the tube and to stop air flow through the tube upon air flow rate through the tube exceeding a preselected value with a detector for sensing when the sail assembly has stopped air flow through the tube as a result of air flow rate exceeding said pre-selected value.

Claims

1. An air flow limiter, comprising: a. a vertically oriented tube; b. a pair of open-ended telescoping tubular segments within the tube, one tubular segment being fixed and the other being slidably movable along the fixed segment in the axial direction; c. a plate extending partially across the interior of the vertically oriented tube, positioned for contacting the movable one of the telescoping tubular segments and limiting travel of the moveable telescoping tubular segment, the plate covering an open upper end of the movable telescoping tubular segment upon contact therewith; d. a sail positioned in the vertically oriented tube below the telescoping segments; e. a strut connecting the sail and the moveable telescoping tubular segment; f. a baffle positioned to direct upward air flow within the tube into the telescoping tubular segments; g. an electromagnetic beam emitter mounted on the tube and positioned to direct an electromagnetic beam across the tube interior to intersect the moveable tubular segment when the moveable tubular segment contacts the plate; h. an electromagnetic beam sensor mounted on the tube for sensing the electromagnetic beam when present; the movable telescoping tubular segment moving vertically within the tube unitarily with the sail responsive to air flow upwardly through the tube against the sail.

2. The air flow limiter of claim 1 wherein the tubular segments are cylindrical.

3. The air flow limiter of claim 1 wherein the surface of the plate contacted by the movable tubular segment is planar.

4. The air flow limiter of claim 2 wherein the portion of the moveable tubular segment contacting the plate surface is annular.

5. The air flow limiter of claim 1 wherein a surface of the plate contacted by the movable tubular segment is flat, the tubular segments are cylindrical, and a circular edge of the tubular segment contacting the plate surface is annular and normal to the axis of the tubular segment.

6. An air flow limiter, consisting of: a. a tube; b. a tubular segment within the tube being movable in the axial direction; c. a plate extending at least partially across the interior of the tube, for contacting the movable tubular segment and defining a limit of travel of the movable tubular segment; d. a sail connected to the moveable tubular segment and being movable therewith within the tube; e. a baffle, connected to and within the tube, defining a second limit of travel of the movable tubular segment, the moveable tubular segment being in sliding telescoping engagement with the baffle, the baffle directing air flow within the tube into the tubular segment; and f. a detector for sensing position of the moveable tubular segment; the movable tubular segment being movable unitarily with the sail in response to upward air flow through the tube contacting the sail.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of a resin delivery system with a single air flow regulator in accordance with aspects of the invention.

(2) FIG. 2 is a schematic representation of a resin delivery system with a plurality of air flow regulators in accordance with aspects of this invention.

(3) FIG. 3 is an isometric view of the exterior of an air flow limiter portion of apparatus for pneumatically conveying granular plastic resin as disclosed in co-pending application Ser. Nos. 14/185,016 and 14/574,561 referenced above.

(4) FIG. 4 is a front elevation of the air flow limiter illustrated in FIG. 3.

(5) FIG. 5 is an isometric sectional view of the air flow limiter illustrated in FIGS. 3 and 4, with the section taken at arrows 3-3 in FIG. 4.

(6) FIG. 6 is a sectional view in elevation of the air flow limiter illustrated in FIGS. 3 and 5, with the section taken at lines and arrows 3-3 in FIG. 4, with air flow through the air flow limiter being depicted in FIG. 6 by curved dark arrows.

(7) FIG. 7 is a sectional view in elevation, similar to FIG. 6, of the air flow limiter illustrated in FIGS. 3 through 6, but with the air flow limiter internal parts in position whereby there is no air entering the air flow limiter and hence there is no air flow upwardly through the air flow limiter, in contrast to the condition with such air flow shown in FIG. 6.

(8) FIG. 8 is a sectional view in elevation, similar to FIGS. 6 and 7 of the air flow limiter illustrated in FIGS. 3 through 7, but with the air flow limiter internal parts in position where there is an excessive amount of air attempting to enter the air flow limited but there is no air flow upwardly through the air flow limiter due to the air flow limiter valve having moved to block air flow upwardly through the air flow limiter, in contrast to air flow upwardly through the air flow limiter as shown in FIG. 4.

(9) FIG. 9 is an exploded isometric view of the air flow limiter illustrated in FIGS. 3 through 8.

(10) FIG. 10 is an isometric view of the movable portion of the air flow limiter illustrated in FIGS. 3 through 9.

(11) FIG. 11 is a sectional view of an air flow limiter similar to FIGS. 6, 7 and 8, illustrating an alternate construction of the baffle portion of the air flow limiter.

(12) FIG. 12 is sectional view of the air flow limiter similar to FIGS. 6, 7, and 11, illustrating a second alternate construction of the baffle portion of the air flow limiter.

(13) FIG. 13 is a sectional view in elevation, similar to FIG. 7, of an improved air flow limiter in accordance with the invention, with the air flow limiter internal parts in position whereby there is no air entering the air flow limiter, the sail assembly has not been lifted by any air flow, the electromagnetic beam passes through the air flow limiter without impinging on any internal parts and there is no air flow upwardly through the air flow limiter.

(14) FIG. 14 is a sectional view in elevation of the air flow limiter illustrated in FIG. 13, with the air flow limiter internal parts in position whereby no air can flow through the air flow limiter due to those internal parts having blocked air flow due to instantaneous air flow exceeding the design value and with the electronic beam, which would normally go from an emitter to a detector, being blocked by the air flow limiter internal parts, thereby indicating that air flow has exceeded a maximum design value and has been halted.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND BEST MODE KNOWN FOR PRACTICE OF THE INVENTION

(15) In this application, unless otherwise apparent from the context it is to be understood that the use of the term vacuum means air at slightly below atmospheric pressure. The vacuum (meaning air at slightly below atmospheric pressure) provides a suction effect that is used to draw granular plastic resin material out of a supply and to convey that granular plastic resin material through various conduits to receivers where the granular resin material can be temporarily stored before being molded or extruded. Hence, in this application it is useful for the reader mentally to equate the term vacuum with the term suction.

(16) This invention is based on the flow limiter disclosed and claimed in pending U.S. patent application Ser. No. 14/185,016 referenced above and adds at least a sensor to detect when the limiter has stopped flow, or to detect other flow conditions, according to the location of the sensor.

(17) When air flow is below the design limit, the limiter remains fully open. The moment air flow exceeds the design limit the limiter closes instantly and fully to the point at which it properly stops flow. The movement is sudden and complete. There is no intermediate modulation of the limiter. It does not close gradually nor does any air flow close it partially. For this reason it is easy to detect closing. The preferred way to do this is to place a photo detection device on one side of the limiter, so that it can detect when a beam of light is either blocked, or reflected by the valve closing. A clear window may be used, or the detection device can be mounted into the tubular wall of the limiter.

(18) Referring to the drawings in general and to FIG. 1 in particular, apparatus for conveying granular plastic resin material from the supply to receivers that retain and dispense the resin material when needed by a process machine is illustrated in FIG. 1. The apparatus, which is designated generally 88 in FIG. 1, preferably includes a vacuum pump designated generally 92 and shown schematically in FIG. 1. The vacuum pump preferably includes a vacuum pump suction head 93 also shown schematically in FIG. 1. Connected to the vacuum pump suction head 93 is an airflow limiter 30 shown only in schematic form in FIG. 1, but shown in various details in FIGS. 3 through 14. Airflow limiter 30 receives vacuum drawn by vacuum pump 92 through vacuum drawing conduit 100.

(19) Vacuum drawing conduit 100 is connected to a plurality of receivers 16, each of which receives, retains and dispenses, as needed, granular plastic resin material to a machine, such as a gravimetric blender, or an extruder, or a molding press, as located preferably below a receiver 16. These machines are not illustrated in FIG. 1 to enhance the clarity of the drawing.

(20) Further illustrated in FIG. 1 is a hopper 18 for storage of granular plastic resin material therein and a resin conveying conduit 98, which serves to draw resin from hopper 18 and to deliver the resin through resin conveying conduit 98 to the respective receivers as vacuum is drawn by the vacuum pump, with vacuum propagating through air flow limiter 30, vacuum drawing conduit 100, the various receivers 16, and resin conveying conduit 98, back to hopper 18.

(21) FIG. 2 shows an alternate embodiment of a resin conveying system where this alternate embodiment of the conveying system has been designated 88A. FIG. 2, as in FIG. 1, depicts a vacuum pump 92 shown in schematic form having a vacuum pump suction head 93 also depicted in schematic form. In the alternate embodiment illustrated in FIG. 2, vacuum drawing conduit 100 leads directly into and communicates with vacuum pump suction head 93. In the embodiment illustrated in FIG. 2, an air flow limiter 30 is provided for each receiver 16, with the air flow limiter 30 for a respective receiver 16 being located in a portion of a connection conduit 102 that connects a respective receiver to vacuum drawing conduit 100. In FIG. 2, each air flow limiter 30 is depicted in a vertical orientation, just as is airflow limiter 30 depicted in a vertical orientation in FIG. 1. Each receiver is connected by connection conduit 102 to vacuum drawing conduit 100 with air flow limiter 30 forming a portion of connection conduit 102.

(22) In FIG. 2, as in FIG. 1, a first conduit 98 serves to convey granular plastic resin from hopper 18 to the respective receivers in response to vacuum drawn by vacuum pump 92 as that vacuum propagates from vacuum pump 92 through second conduit 100, connection conduits 102, receivers 16, and resin conveying conduit 98 to hopper 18.

(23) During operation of the resin conveying systems shown schematically in FIGS. 1 and 2, upon actuation of vacuum pump 92, a vacuum is drawn at vacuum pump suction head 93. This vacuum, as it propagates back to hopper 18, serves to draw resin out of hopper 18 and into the respective receivers 16. In the embodiment illustrated in FIG. 2, individual air flow limiters 30 limit the suction or vacuum drawn by vacuum pump 92 through a given associated receiver 16. In the embodiment illustrated in FIG. 1, a single air flow limiter 30 limits the vacuum drawn through all of receivers 16 forming a portion of the granular resin conveying system illustrated in FIG. 1.

(24) Referring to FIGS. 1 and 2, the air flow limiter 30 portion of the resin delivery systems is preferably in the general form of a vertically oriented tube, preferably having inlet and outlet ends 54, 56 respectively. The tubular character of air flow limiter 30 is apparent from FIGS. 3 through 14, where air flow limiter 30 preferably includes a vertically oriented exterior tube 32, with open-end caps 58, 60 defining and providing open inlet and outlet ends 54, 56 respectively. End caps 58, 60 are open, of generally cylindrical configuration, and are configured to fit closely about vertically oriented tube 32 so as to provide a substantially air tight fit between end caps 54, 56 and tube 32.

(25) As illustrated in FIG. 5, air flow limiter 30 preferably includes, within vertically oriented exterior tube 32, a horizontally positioned plate 46, which is oriented perpendicularly to the axis of tube 32. Plate 46 is preferably configured as a circular disk of lesser diameter than the inner diameter of vertically oriented tube 32, with plate 46 further preferably including three legs extending outwardly from the circular interior disk portion of plate 46. Legs of plate 46 are designated 62 in FIG. 9, while the circular interior portion of plate 46 is designated 64 in FIG. 9. Plate 46 is secured to the interior of vertically oriented outer tube 32 by attachment of legs 62 to the interior surface of tube 32. Any suitable means of attachment, such as by welding, adhesive, mechanical screws, or end portion of legs 62 defining tabs fitting into slots within tube 32 as shown in FIG. 5, may be used.

(26) As shown in FIGS. 5, 6, and 7, a baffle 52 is positioned within vertically oriented outer tube 32, below plate 46. Baffle 52 has a lower conical portion 66 and an upper cylindrical portion 44, with cylindrical portion 44 defining a fixed internal tubular segment of air flow limiter 30. Baffle 52 is preferably retained in position by a pair of screws designated 68, 70 respectively. Baffle 52 preferably rests on screw 68. Screw 70 preferably fits against the fixed internal tubular segment 44 portion of baffle 52 to secure baffle 52 in position within vertically oriented external tube 32. Lateral force applied by screw 70 in a direction perpendicular to the axis of vertically oriented external tube 32, with screw 70 in contact with fixed internal tubular segment 44, serves to effectively retain baffle 52 against movement within vertically oriented external tube 32.

(27) The upper portion of baffle 52, defining fixed internal tubular segment 44, is adapted for sliding telescopic engagement with movable tubular segment 42. Fixed to movable tubular segment 42 is a first strut 48 preferably extending transversely across the upper portion of movable tubular segment 42 and preferably secured on either end to movable tubular segment 42, as illustrated in FIG. 10. Preferably extending downwardly from first strut 48 is a second strut 50, preferably secured to first strut 48 and preferably also to a sail 34, as illustrated in FIG. 10 and in FIGS. 5, 6, 7, 8, 9, 13 and 14.

(28) Movable sail 34 is preferably planar and positioned fixedly on second strut 50 to remain perpendicular with respect to the axis of vertically oriented outer tube 32. Movable sail 34 is preferably of generally triangular configuration, as illustrated in FIGS. 9 and 10, with the sides of the triangle curving slightly inwardly. The curved edges 72 of movable sail 34 converge and terminate to form small rectangularly shaped extremities of sail 34, which are designated 76 in FIG. 9.

(29) Movable sail 34 is positioned within generally vertically oriented outer tube 32 so that rectangular extremities 76 are closely adjacent to but do not contact the inner surface of vertically oriented outer tube 32, so long as sail 34 moves vertically up and down within vertically oriented external tube 32. The rectangular shape of extremities 76 with their outwardly facing planar surface assures minimal friction and consequent minimal resistance to movement of movable sail 34 in the event one of rectangular extremities 76 contacts the interior surface of vertically oriented tube 32, should sail 34 for some reason move laterally or otherwise and become skew to the vertical axis of tube 32.

(30) Movable internal tubular segment 42 is telescopically movable, unitarily with sail 34, relative to and along fixed internal tubular segment 44. A lower limit of movement of movable tubular segment 42 is illustrated in FIG. 7, where the first strut portion 48 of movable tubular segment 42 (shown in FIG. 10) rests on the upper circular edge of fixed internal tubular segment 44. This is the condition when no air is flowing or drawn through the air flow limiter and gravity causes sail 34 together with movable internal tubular segment 42 to drop, with first strut 48 coming to rest on the upper circular edge of fixed tubular segment 44.

(31) When air is flowing through air flow limiter 30, as illustrated generally in FIG. 6, the moving air pushes against movable sail 34, moving it upwardly. Movable internal tubular segment 42 moves upwardly unitarily with sail 34 due to the fixed connection of movable tubular segment 42 and movable sail 34 made via first and second struts 48, 50, as illustrated in FIGS. 5, 6, 7, 9, and 10.

(32) If air flow upwardly through air flow limiter 30 reaches an extreme value, above an acceptable level of operation of the resin delivery system of which air flow limiter 30 is a part, the excessive force (resulting from the high volume of air flow contacting sail 34) pushes sail 34 upwardly to the point that upper annular edge 78 of movable internal tubular segment 42 contacts plate 46. In this condition, which is illustrated in FIG. 8, no air can pass between the upper annular edge 78 of movable tubular segment 42 and flow limiting horizontal plate 46, and air flow stops.

(33) Once air flow stops through vertically oriented outer tube 32, gravity pulling downwardly on sail 34, connected movable internal tubular segment 42, and first and second struts 48, 50, causes these parts, which may be connected together and fabricated as a single integral assembly as shown in FIG. 8, to move downwardly, thereby again permitting air flow upwardly through air flow limiter 30 as depicted generally in FIG. 6. Consequently, air flow limiter 30 is self-regulating in that when air flow is too high, the force of air moving or impinging on sail 34 pushes movable internal tubular segment 42 upwardly until upper annular edge 78 of movable tubular segment 42 contacts plate 46 and no air can then escape upwardly between the upper annular edge 78 of movable tubular segment 42 and plate 46. This stops air flow through flow limiter 30 until downward movement of sail 34 together with movable internal tubular segment 42 moves upper annular edge 78 of movable tubular segment 42 away from plate 46, again permitting air to flow through the upper extremity of movable tubular segment 42, with air passing between upper annular edge 78 of movable internal tubular segment 42 and flow limiting horizontal plate 46, and then escaping through upper outlet end 56 of air flow limiter 30.

(34) With the self-regulating characteristic of air flow limiter 30, the assembly consisting of movable internal tubular segment 42, first and second struts 48, 50 and sail 34 may oscillate somewhat about the position at which air flow drawn by suction is at the desired level, as the vacuum pump drawing air through flow limiter 30 varies in cubic feet per minute of air drawn.

(35) Desirably, ends of first strut 48, which is depicted as being horizontally disposed in the drawings, are mounted in movable tubular segment 42 in movable fashion such that first strut 48 can move slightly, rotationally, relative to movable internal segment 42. This is to provide a small amount of play in the event movable sail 34 and second strut 50, which is vertically oriented and connected to movable sail 34, become skew with respect to the vertical axis of vertically oriented exterior tube 32. Should this occur, the movable characteristic of first strut 48, being slightly rotatable relative to movable internal tubular segment 42, effectively precludes movable internal tubular segment 42 from binding with respect to fixed internal tubular segment 44 and thereby being restricted from what would otherwise be freely telescoping movement of movable internal tubular segment 42 relative to fixed internal tubular segment 44.

(36) Desirably first strut 48 is rotatable relative to movable internal tubular segment 42, to provide maximum freedom of vertical motion of movable internal tubular segment 42 in the event movable sail 34 becomes skew to the axis of vertically oriented exterior tube 32, with consequent frictional force restricting vertical movement of movable sail 34.

(37) Baffle 52 preferably includes two portions, the upper portion preferably being defined by fixed internal tubular segment 44 and a lower portion preferably being defined by conical portion 66 of baffle 52. A lower edge of baffle 52 is circular and is designated 84 in the drawings. Circular edge 84 fits closely against the annular interior wall of vertically oriented exterior tube 32 so that all air passing upwardly through air flow limiter 30, namely through vertically oriented exterior tube 32, is constrained to flow through the interior of baffle 52. The tight fitting of the circular lower edge of baffle 52 against the interior wall of vertically oriented exterior tube 32 forces all air entering flow limiter 30 from the bottom to flow through the interior of baffle 52, flowing upwardly through lower conical portion 66 of baffle 52.

(38) The air then flows further upwardly through the interior of fixed internal tubular segment 44. Thereafter, if movable internal tubular segment 42 is spaced away from flow limiting horizontal plate 46, air flows along the surface of movable internal tubular segment 42, passing the upper annular edge 78 of movable internal tubular segment 42; air then flows around the space between edge 82 of flow limiting horizontal plate 46 and the interior annular wall of vertically oriented exterior tube 32. The air then flows out of air flow limiter 30 via open outlet end 56 formed in end cap 60.

(39) In an alternate embodiment of the air flow limiter, baffle 52 may be constructed from two pieces that fit closely together, with the two pieces being in facing contact in the area where they define fixed internal tubular segment 44, but diverging one from another in the area where they define conical portion 66 of baffle 52. As illustrated in FIG. 12, the two portions of baffle 52 are designated 66A and 66B where they diverge, with baffle portion 66A serving to channel air flow upwardly through vertically oriented exterior tube 32 into fixed internal tubular segment portion 44 of baffle 52. The space between the lower parts of baffle portions 66A and 66B is filled with a filler material 86 to provide additional assurance that all air entering vertically oriented exterior tube 32 from the bottom flows through fixed internal tubular segment 44 and on through movable internal tubular segment 42, and does not pass around the edge of baffle 52, namely between baffle 52 and the interior surface of vertically oriented exterior tube 32. Filler material 86 provides additional structural rigidity for flow limiter 30.

(40) In another alternative environment of the air flow limiter, baffle 52 is one piece, preferably molded plastic, as illustrated in FIG. 11, where baffle 52 is designated 52B to distinguish it from the baffle construction illustrated in FIG. 12 and the baffle construction illustrated in the other drawing figures. In the baffle construction illustrated in FIG. 11, the one piece construction means that there is no need or space for any filler material.

(41) The assembly illustrated in FIG. 10 comprising the moveable internal tubular segment 42, first strut 48, second strut 50 and moveable sail 34 may be constructed as a single piece or several pieces as required. The assembly of moveable internal segment 42, first and second struts, 48, 50 and moveable sail 34 is referred to as a sail assembly. It is not required that first and second struts 48, 50 be separate pieces; they may preferably be fabricated as a single piece. Additionally, second strut 50, which has been illustrated as a machine screw in FIGS. 9 and 10, need not be a machine screw. Any suitable structure can be used for second strut 50 and it is particularly desirable to fabricate first and second struts 48 and 50 from a single piece of plastic or metal, by molding, by machining, by welding, or by otherwise fastening two pieces together. Similarly with the hex nut, which is unnumbered in FIG. 10 and illustrated there, any other suitable means for attachment of the second strut or a vertical portion of a strut assembly to moveable sail 34 may be used.

(42) Referring to FIGS. 13 and 14, an air flow limiter in accordance with the invention has been designated generally 30 and includes a vertically oriented exterior tube 32 and a moveable sail 34; these components are preferably the same as those described with respect to air flow limiter 30 above.

(43) Air flow limiter 30 further includes a pair of concentric telescoping tubular segments 40, a moveable internal tubular segment 42, a fixed internal tubular segment 44, a flow limiting horizontal plate 46, first and second struts 48, 50, a baffle 52, and an inlet end 54 and an outlet end 56, all preferably the same as the air flow limiter illustrated in the other drawing figures and as described above. Other parts that are shown in FIGS. 13 and 14, and that appear to be identical to corresponding parts shown in FIGS. 3 through 12, are identical and are not further delineated herein for the sake of some brevity.

(44) Air flow limiter 30 in accordance with the invention includes a pair of housings 110, 112 mounted on either side of vertically oriented exterior tube 32, where housing 110 is provided generally for an emitter 114, and housing 112 is provided generally for a detector 116. Any suitable emitter-detector combination can be used. Preferably emitter 114 emits an electromagnetic beam 118 and detector 116 detects beam 118 when beam 118 impinges on detector 116. Emitter 114 and detector 116 are mounted within housings 110, 112 at the positions illustrated in FIGS. 13 and 14. At this position, when emitter 114 emits an electromagnetic beam 118 and moveable internal tubular segment 42 is in contact with flow limiting horizontal plate 46, electromagnetic beam 118 is blocked by the cylindrical wall of tubular segment 42. Hence, detector 116 does not detect any electromagnetic beam and detector 116 sends a signal to an appropriate monitoring station or computer or other device indicating that the air flow limiter has blocked flow due to air flow through the air flow limiter exceeding the pre-selected design value.

(45) When air is permitted to flow through the air flow limiter, due to moveable internal tubular segment being below and not contacting flow limiting horizontal plate 46, electromagnetic beam 118 from emitter 114 is detected by detector 116. (The beam is indicated as 118 in the drawing.) Detection of electromagnetic beam 118 by detector 116 indicates that either there is some permissible flow through the air flow limiter 30 or the entire resin delivery system has been shut down, meaning there is no air flow entering air flow limiter 30 at inlet end 54. However, the no air flow operation, or no air flow entering inlet end 54 is essentially of no interest, in that the air flow limiter of the invention, with the emitter-detector combination, is a self-limiting device as described above.

(46) Air flow limiter 30 preferably contains no springs. Air flow limiter 30 preferably contains no sensors to provide operating feedback to a control device for regulation of air flow limiter 30. No feedback control sensors are needed since because flow limiter 30 is self-regulating and once in place, an air flow limiter is not subject to outside intervention or control. Air flow limiter 30 preferably includes a tubular valve, closing against a flat surface, where the tubular valve is defined by movable internal tubular segment 42 closing against flow limiting horizontal plate 46. Movable internal tubular segment 42 is in the form of an open-ended cylinder and is connected to a plate in the form of movable sail 34 to move movable tubular segment 42 against flow limiting horizontal plate 46. Air flow limiter 30 uses gravity alone to open the valve defined by the assembly of movable internal tubular segment 42, movable sail 34, and the connecting structure therebetween.

(47) In the air flow limiter illustrated in FIGS. 3 through 14, the movable internal tubular segment 42 is preferably made with a very thin wall, preferably from metal tubing, where the wall is preferably less than 1/32 inch in thickness.

(48) The air flow limiter of the invention functions equally well with a vacuum pump drawing air through air flow limiter 30 from bottom to top by application of vacuum to outlet end 56 as depicted generally in FIGS. 1 and 2, or by air being supplied under positive pressure at inlet end 54 for passage upwardly through air flow limiter 30.

(49) In the claims appended hereto, the term comprising is to be understood as meaning including, but not limited to while the phrase consisting of should be understood to mean having only and no more.