US3122470A

US3122470A - Method of supplying stock and stock inlet for a papermaking machine

Info

Publication number: US3122470A
Application number: US78092A
Authority: US
Inventors: Charles A Lamb; William F Morgan; Donald G Neill
Original assignee: Kimberly Clark Corp
Current assignee: Kimberly Clark Corp
Priority date: 1960-12-23
Filing date: 1960-12-23
Publication date: 1964-02-25
Anticipated expiration: 1981-02-25

Description

1964 c. A. LAMB ETAL 3,122,470

METHOD OF PPLYING STOCK S TOCK INLET F A PAPERMAKING HINE Filed Dec. 25, 1960 2 Sheets-Sheet 1 ""1 i 15 57 L J 23 E 35 29 27 Z5 5 34 25 \ll-llrgllllll-llm 26 4 105 y 5' i 3 .A; J03 i2 M a i h 1 3 Ffl/PM/NG AREA .94

Feb. 25, 1964 "c. A. LAMB ETAL METHOD OF SUPPLYING STOCK AND STOCK INLET FOR A PAPERMAKING MACHINE 2 Sheets-Sheet 2 Filed Dec. 23, 1960 M m I 2X? NH United States Patent 3,122,470 METHOD OF SUPPLYlNG STOCK AND STOCK INLET FOR A. PAPERMAKWG MACHINE Charles A. Lamb, Neenah, F. Morgan, Oshkosh,

and Donald G. Neill, Neenah, Wis, assignors to Kimberly-Clark Corporation, Neenah, Wis, a corporation of Delaware Filed Dec. 23, 1960, Ser. No. 78,092 8 Claims. (Cl. 1622l4) This invention relates to the mmufacture of sheet or web products from fiuid suspensions of fibrous material and, more particularly, to the manufacture of paper on Fourdrinier papermaking machines. Still, more particularly, the invention has special relationship to paper manufacture on Fcurdrin-ier papermaking machines of the pressure forming type.

In the operation of pressure forming type papermaking machines, aqeuous suspensions of fiber and any other papermaking materials to be used, which is generally designated as stock, is supplied to a web forming region through which a Fourdrinier wire is moved. The stock is applied to a restricted area section of the moving Fourdririer wire as a relatively high energy flowing stream under substantial hydraulic pressure. A large proportion of the water in the stock mixture is forced through the wire in the web forming region at relatively high velocity, which is primarily the result of the substantial pressure ditferential across the forming wire within that region.

in some of the inlets of the pressure forming type, flow passageways within the inlets are so designed that the flowing stream of stock is divided into a plurality of parts. One of these parts is, of course, that part from which the fibrous material is deposited onto the Fourrinier wire with a large par-t of the liquid from this part flowing through the Fourdrinier wire so as to leave the fibers disposed on the wire as a web. Another of the parts is conducted out from the web forming region within the inlet so as to efiiciently control the pressure and velocity relationships of the stock delivered to the web forming region and so as to incidentally allow high speeds of operation of the papermaking machine. Such pressure forming inlets, having the plurality of parts of stock flow, are, for example, disclosed in a patent issued to Charles A. Lee, 2,756,649, on July 31, 1956.

A pressure forming inlet of this type generally has a fiow passage which is restricted or orificed at the upstream end of the pressure forming region on the Fourdrinier wire and which expands toward the downstream end of the pressure forming region, and the downstream end ofthe cforrning region is defined by a slice of some type which is in close proximity with the Fourdrinier wire. Since the flow passage expands from the upstream end of the forming region, the pressure of the stock in the pressure forming region toward the downstream end of this region becomes greater, so that the stock fibers are initially deposited on the wire under a relatively low pressure and, as the felted web builds up on the wire in the pressure forming region, the successive fibers are laid onto the wire under a greater pressure.

Various controls are generally provided for adjusting the parts in such an inlet such as, for example, to change the size of the orifice at the inlet end of the Web forming region or to change the size of the passageway carrying the part of the stock that is drawn out from the web forming region. it has been found that adjustment of the orifice change the strength of the resulting web in the machine direction as compared to the cross direction. It is desirable to have a relatively low ratio of machine direction strength to cross direction strength, and the orifice may be decreased in size to thereby increase the pressure drop across the orifice in order to lower this ratio.

It has been found, however, for very high speeds of paper formation and with relatively small orifice sizes that the resulting web has a tendency to be defective in formation and, in particular, has licks or spots which are of low basis weight or are completely open. The diameters of these spots may vary, such as from /2 inch to 1 /2 inches, and their presence, of course, makes the resulting sheet objectionable from a quality standpoint.

Due to the high speeds of operation of the paper-making machines, it is ditiicult to determine the exact reason for these low basis weight spots in the paper web; however, they apparently ocour because of fluctuations in the absolute pressure from the upstream end of the pressure forming area on the wire to the downstream end of this rea defined by the slice. The pressure of the stock from the upstream end of the forming area to the downstream end generally increases, as has been previously explained; and these fluctuations are imposed on the average pressures and (increase with the speed of web formation and with decreases in size of the orifice. With certain high speeds of web formation at certain small orifice sizes, the fluctuations in pressure are apparently large enough to actually form vacuum at times at the downstream end of the pressure forming area to cause actual lifting of the fibers from the Fourdrinier wire forming the spots of low or no basis weight.

It is an object of the present invention to provide an improved method and apparatus for preventing such low basis weight spots in the formed web at high paperniaking speeds with relatively low orifice sizes to secure a relatively low MD/CD r We have found that the web may be formed without such low basis weight spots by injecting air or other gas upstream of the orifice in the stock passageway. In order to provide very good results, the air is preferably injected very close to the orifice and is supplied by a plurality of jets spaced closely together across the width of the machine. The compressibility of the air apparently has the desired result of cushioning the fluctuations in pressure of the stock between the upstream and downstream ends of the pressure forming area, so that the pressure at the downstream end of this area never becomes a vacuum so as to lift the stock fibers from the web @being formed in this area.

The invention consists of re novel methods, constructions, and devices to be hereinafter described and claimed for carrying out the above stated objects and such other objects as will be apparent from the following description of preferred forms of the invention, illustrated with reference .to the accompanying drawings, wherein:

PEG. 1 :is a side elevational view of a :Fourdrinier papermaking machine embodying the principles of the inven tion;

PEG. 2 is a longitudinal sectional View of a portio of the machine taken on an enlarged scale;

FIG. 3 is a graph showing the variation in head or pressure across the web forming area of the machine; and

FIG. 4 is a fragmentary longitudinal sectional view of a modified form of the machine taken in the vicinity of the web forming area.

Like characters of reference designate like parts in the several views.

Referring now to FIG. 1 of the drawings, the pressure forming Fourdrinier papermaking machine of the general character, to which the present invention has particular application, may be seen to comprise a Fourdrinier E!) wire which passes over a breast roll 16 and a couch roll 17. The wire 15 is in the form of a loop and is held tensioned by means of a guide and tensioning roll 18. The upper pass of the wire 15 is supported by a plurality of table rolls 19, and a plurality of suction boxes 20 may also be provided beneath this pass of the wire.

A tray 21, which is adapted to collect the white water, i.e., the liquid which drains through the wire 15 during the web forming operation, is disposed within the upper and lower passes of the wire 15, as illustrated; and this tray is arranged to discharge into a wire pit or mixing tank 22. A pipe 23 is arranged to discharge into the tank 22 for adding fresh water for makeup purposes, and a'second inlet pipe 24 is provided for adding as much additional stock as is necessary to maintain the proper consistency or fiber concentration of the fluid suspension or stock delivered to the web forming region of the wire 15.

A main flow line 25 connects to the tank at its bottom and is connected with the inflow side of a stock or fan pump 26, which is the principal means relied upon to deliver stock to the web forming region of the machine.

The outlet of the fan pump 26 is connected to the inlet structure of the machine by suitable piping indicated generally at 27. An automatically operable pressure regulating valve 2% is included in this piping, and the piping includes a transition section 29 which converts the round cross sectional stream delivered by the piping 27 to a stream of rectangular outline. A flow spreader 3% is connected to the transition section 29 and accomplishes the function of converting the uniformly dimensioned stream from the section 29 into a relatively shallow stream of rectangular cross section, which desirably has a transverse width substantially equal to the cross machine width of the web forming region of the machine.

The outlet end of the flow spreader 30 is connected to the web forming region of the machine by a conduit 31 which is provided in the lower or stationary portion 32 of the flow control unit 33. The flow passageway 31 is rectangular in cross section and of the sme dimensions as the outlet end of the flow spreader 3G. The piping 27 a so includes a bypass line 34 which connects the outlet side of the fan pump 26 upstream of the pressure regulating valve 28 with the mixing tank 22, as shown. The bypass line 34 includes a shutofi valve 35. Suitable showers or sprays, generally indicated at 36, are provided for effecting continuous cleaning of the rolls and Wire in accordance with usual papermaking practices.

The breast roll 16 constituting a part of the illustrated apparatus is of the open surfaced type and comprises a cylindrical shell 37 which may be of bronze or other corrosion resistant material and which is supported for rotation by means of suitable end shafts 38. At spaced intervals along the shell 37, the roll 16 is provided with a plurality of outwardly projecting rings 39. The rings 39 are integrally attached to the shell and are provided with spaced notches 40 which support a plurality of elongated vanes 41. These vanes 41 extend longitudindly of the roll surface with the axes thereof substan tially parallel of the axis of rotation of the roll.

The spaced vanes 41 vare used to receive and to hold temporarily the liquid which is forced through the wire 15 and the adjacent underlying surface of the roll 16 during the web forming operation. Each of the vanes 41 includes a base or root portion 41a which is rectangular in cross section and which fits within the vane supporting slots 40 in the ring units 39. The slots 40 we so arranged that the root portion 41a of each vane is disposed at an angle of about 15 degrees to the radius which intersects the base of that portion, with the vane portions 41a extending backwardly from the direction of rotation of the roll 16 as indicated by the arrow 42. The remaining outer portion 41b of each of the vanes 41 is inclined forwardly, in the direction of roll rotation indicated by the arrow 42, at an angle of about 120 de- An upper adjustable section or deck 44 is supported on the stationary portion 32 of the flow control unit. The movable portion 44 is adjusted with respect to the stationary portion 32 by means of a plurality of gear units 45. Each of the units 45 comprises a threaded shaft 46 fixed with respect to the movable part 44, and a helical gear 47 enmeshed with a worm 48 which in turn has screw threaded engagement with the shaft 46.

A lobe assembly 49 is disposed within the movable inlet portion 44 and comprises a forward lobe piece 59,

a rearward lobe piece 51 and an intermediate or connect-.

ing lobe piece 52. The lobe pieces 50 and 51 are provided with cylindrical end surfaces 53 and 54, respectively, and the intermediate lobe piece 52 is provided with cylindrical surfaces 55 and 56 which seal with the surfaces 53 and 54, respectively. The lobe parts 50, 51 and 52 are held in assembled relationship, as shown, by means of links 57 having end hooks 58 that pass through and overlap edges of openings 59 provided in wall portions 69 and 61 of the lobe parts 59 and 51.

A lobe insert 62 is connected to and is disposed on the bottom side of the lobe part 59. The lobe insert 62 is held tightly in contact with the lobe part 5! by any suitable means, such as by means of studs 63 having heads 64- received slots provided in wall portions 65 of the lobe insert 62 and an air pressure device 66' having an cxpansible bladder 67 and eifective on the studs 63. An orifice plate 68 having a sharp edge 69 is fixed on the lower surface of the lobe insert 62.

Adjustment between the assembly 49 and the deck 44 is obtained by gear units 45a, which are similar in con- 7 struction to the units 45 and are effective between the assembly 49 and the deck 44.

A thin bottom lip or slice plate 76 carried by a holder 71 is slidably disposed on the stationary portion 32. The

lip 7t) and holder '71 may be adjusted toward or out from the vertical center line 72 of the roll 16 by means of gear units 73. The gear units 73 each comprises a gear 74 enmeshed with a worm 75 drivingly connected withra hand wheel 76. A crank 77 is fixed with respect to the gear 74 and is connected through a link 78 with the slic holder 71.

The lobe part 50 may be tilted or moved up and down by means of

motors

79 and 80 of any suitable type. V The.

motors carry cranks 81 and 82., respectively, and the cranks are respectively connected to shafts 8-3 and 84 on opposite ends of the lobe part 50' by means of

links

85 and 86.

A forward slice 87 having a relatively thin tip 88 is positioned peripherally down stream by a few inches from the center line 72 of the breast roll 16. The slice 87 is carried by the movable deck 44 and may be adjusted toward and outwardly from the roll 16 by means of gear units 455 which are similar to the units 45 and 45a. The units 45b are carried by a support 89 that is fixed to t the deck 44, and the units 4512 are connected to a frame fit that has a slidable connection with respect to the sup- 7 region 94 between the two

lips

70 and 88 on the surface of the Wire 15 defines the web forming region in the papermaking machine.

The conduit 31, as has been previously described, passes through the stationary portion 32 of the machine, and it continues in the movable portion 44, having its upper surface defined by the lobe parts 51, 52 and 50, together with the lobe insert 62. The lower surface of the conduit 31 below the lobe parts is formed by a part of the stationary portion 32 and by the bottom lip 70 and its carrier 71. The orifice plate 68 defines an orifice 95 between its tip 69 and the carrier 71 for the bottom lip 70.

A part of the stock that flows through the conduit 31 is bypassed back into the wire pit 22, and a bypass conduit 96 is provided for this purpose. The bypass conduit 96 just subsequent to the forming area 94 is defined by the lobe insert 62 and the lip 87 and subsequently is defined by other parts of the movable inlet portion 44 and the lobe parts 59 and 52. The conduit 96 is connected, as seen in FIG. 1, to the wire pit 22, and a valve 97 is provided in the conduit 96.

A plurality of small diameter orifices 98 are provided in the intermediate lobe part 54. Each of these orifices is connected by means of a tube 99 with a source of air pressure 169. A pressure gauge 101 and a flow meter 102 of any suitable constructions may be provided for measuring the pressure of the air in the tubes 99 and the rate of air flow through the tubes, and the rate of air flow may be controlled by any suitable means such as a valve 192a. The orifices 93 are preferably located a short distance upstream in the conduit 31 with respect to the orifice 95-, such as 6 inches upstream, for example. The orifices 98, however, if desired, may be located immediately upstream of the tip 69 of the orifice plate 63, and, in fact, they may actually pass through the plate 68. The latter construction is illustrated in FIG. 4. In the FIG. 4 construction, an orifice plate 68a is provided which has orifices 98a formed in it and connected to the tubes 99. The orifices 98 may also, if desired, be located farther upstream in the conduit 31; however, preferably the orifices 98 are so located within the conduit 31 that there are no sharp corners or crevices in the conduit 31 between the orifices 98 and the stock orifice 95- that may collect air and discharge it unevenly or spasmodically. The provision of the orifices 98 connected with a source of air have been found to materially upgrade the quality of the web that is formed on the Fourdrinier wire 15, and the advantageous functioning of the air orifices 98 will become apparent from the operation of the web forming mechanism Which will at this point be briefly set forth.

In operation, the pressure forming apparatus functions to produce a paper web A on the Fourdrinier wire 15. The progressive drainage through the web and the forming wire within the web forming region 94 causes the stock consistency to increase at the downstream side of the Web forming region 94 to such an extent that the web and the thin overlying layer of stock in the region where the web passes out of the web forming region constitute essentially a plastic body. Thus, it is required that the web A which has been formed on the wire 15 in its passage through the web forming region shall be extruded from the region 94 under conditions approximating the conditions of plastic flow and having the formed web proceed from the web forming region out into the atmosphere without disturbance or scuffing is facilitated by careful control of the flow passageways in this region of the machine.

The stock is pumped from the wire pit 22 through the piping 27, the pressure regulating valve 28, the transition section 29 and the spreader 30 to the conduit 31 within the flow control unit 33. It is to be understood, of course, that additional stock fibers are supplied to the wire pit 22 to maintain the stock at uniform consistency and to make up for the fibers that are deposited on to the wire 15 as the web A. The stock flows through the restricted orifice 95 provided by the orifice plate 68 and the holder 71 for the bottom lip 70. The restricted orifice 95 has the function of increasing the velocity of the stock flowing through the orifice compared to the stock velocity prior to the orifice in the conduit 31, and since the velocity is increased at the orifice 95, its pressure is correspondingly decreased at this point. It will be noted that the lower surface of the lobe insert 62 located just above the web forming region 94 slants upwardly so as to increase the cross sectional area of the conduit 31 toward the lip 87, and therefore the velocity of the stock as it flows through the wire 15 in the web forming region 94 and as it flows through the conduit 31 toward the bypass conduit 96 decreases, and thus the pressure of the stock increases. Therefore, stock fibers are initially deposited on the wire 15 at the beginning of the web forming region 94 adjacent to the bottom lip 7 0 at a relatively low pressure, and subsequent stock fibers are deposited on the wire 15 in the downstream portions of the region 94 closer to the slice tip 88 with greater pressure, so as to cause substantial water drainage through the wire 15, not only at the initial parts of the :web forming region 94 where the resistance to water drainage is relatively slight but also in later portions of the region 94 where the resistance to water drainage is greater.

The stock flowing to the web forming region 94 divides into two main portions of flow and a third minor portion. The two main portions are the flow through the wire 15 in the web forming region 94 to form the web A on the wire and a top layer of flow, above the web forming region 94 through the bypass conduit 96. The stock flowing through the bypass conduit 96 returns to the wire pit 22, and the flow is regulated by the valve 97 within the conduit 96. A minor flow of stock also takes place upstream underneath the bottom lip 70, since the lip 70 is not tightly sealed with respect to the wire 15 and breast roll 16.

The second main division of stock involving the continuous removal above the web forming region 94 of the upper boundary layer of the flowing stream of stock by means of the bypass conduit 96 facilitates the control of flow within the flow control unit 33 and the attaining of the desired stock pressures within the web forming region 94. The size of the orifice95, the direction in which the lower surface of the lobe insert 62 extends with respect to the wire 15 above the web forming region 94 and the amount the valve 97 is opened, together with the size of the bypass conduit 96 are the main factors determining the manner in which the pressure of stock in the web forming region 94 increases from the bottom lip 70 to the slice tip 88. The curve 103 shown in FIG. 3 indicates the increase of head that may be obtained from the bottom lip '70 to the slice tip 88 under conditions in which no air is being supplied to the orifices 98.

The open breast roll 16 functions to allow relatively unrestricted flow of water through the wire 15 (the water draining from the web may be termed white water) beneath the web forming region 94 and to throw ofi the water that is received within the roll 16 below the upper pass of the wire 15, so that the water may drain into the wire pit 22. The forwardly extending vane portions 41b function to hold the white water for a time subsequent to the discharge of the water into the open roll 16 through the web forming region 94 and to throw off the water below the upper pass of the wire 15.

The slice 87 preferably has the thin tip 83 in contact with the wire 15. The relatively thin tip has been found to allow the web A to pass beneath the slice 88 with substantially no plugging of stock fibers at the slice, and this is considered to be due principally to the fact that the tip 88 of the slice is preferably relatively thin, to /1 inch, for example, and to the sharp leading edge 83a of the slice 87. The slice 87 in its illustrated form also has been found to overcome and disrupt small vortices within the stock adjacent the slice which would otherwise cause longitudinal streaks in the Web A.

Preferably the machine is used with the slice 87 disposed on a portion of the wire 15 that is unsupported by the breast roll 16, as shown in FIG. 2, so that the white a water draining from the web A adjacent the slice 87 does not flow directly into the roll 16. In this position, the slice tip 88 is downstream of the vertical center line 72 through the breast roll 16. The deck 44 is adjusted with respect to the stationary portion 32 by means of the gear mechanism 45 in order to so position the slice 87. The gear mechanism 45a may be adjusted for properly positioning the lobe part 59 and associated lobe parts 51 and 52 so as to provide a proper sized bypass conduit 96. The lobe part 50 may be tilted and moved vertically as desired by actuating the motors 70 and 84 which are effective through the cranks 81 and 82 and the

links

85 and 86 on the lobe part 50. The bottom lip 70 may be adjusted by means of the gear mechanism 73 so as to provide a suitable size of forming area 94. The size of the orifice 95 is, of course, adjusted by raising or lowering the lobe part 59.

It has been found that changing the size of the orifice 95 functions to change the machine direction strength with respect to the cross direction strength (MD/CD) of the web A. As the size of the orifice 95 is decreased, lower MD/ CD ratios are obtained, and the sheet is more square. It is desirable, for certain products, that this ratio be relatively low. As a result, however, of radically reducing the size of the orifice 95 to obtain very low MD/CD ratios, without using air supplied through the orifices 98; licks, which are low or no basis weight spots, have been formed in the web A. These spots apparently are due to high velocity, low pressure fluctuations, associated with a submerged jet, near the end of the forming area 94 adjacent the slice 88. These spots have been found to vary in size, and they may be as large as two inches in diameter.

The high velocity low pressure fluctuations of the stock in the vicinity of the slice have been found by actual measurement to occur with a high frequency, such as of 50 to 100 per second. The pressure fluctuations are apparently due to an oscillating condition of the stock as it flows over the forming area. They are of such intensity that the pressure at times in the vicinity of the slice 87 is negative, that is, there is a vacuum; and the result is that fiber is actually drawn from the wire 15 so as to form these spots of little or no basis weight. Obviously, these defects in the sheet render the sheet worthless for most purposes.

We have found that these sheet defects may be cured by injecting air under a pressure into the stream of stock prior to the orifice 95. The air flows from the source of air supply 100* through the tubes 99 and through the orifices 98 into the stock stream. The orifices 98 extend across the inlet and may, for example, be spaced about three inches apart. They may be spaced closer together or farther apart with satisfactory results, although unduly large spacing will not assure that the spotting will be absent across the full width of the web A.

The advantageous functioning of the injected air is apparently due to its compressibility. Other gases, which are compressible relative to the fluid stock, may instead be used if desired; however, air is relatively economical to use and, therefore, is preferred. It has been found that when air under pressure is injected into the flowing stream of stock from the orifices 98, high frequency pressure fluctuations still exist, but the average pressure at the slice 88 is raised, so that the absolute pressure of the stock at the slice tip 87 does not decrease below atmospheric. Consequently, the spots, above referred to, do not exist. With the average pressure at the slice 8% being higher, the fluctuations do not represent as large a portion of the local head as they did without air injection. For example, if there were an average pressure fluctuation amounting to about .28 of the local head at the slice with no air injection, the average air pressure may be about .22 of the local head at the slice with air injection. Obviously, therefore, the local pressure on the wire adjacent the slice 88 is not as likely to decrease to a condition of low or negative drainage through the wire when air injection is used as compared to the cases in which air is not injected.

Experiments have shown that even small quantities of air supplied through the orifices 93 gave an improved sheet A, overcoming local high frequency pressure fluctuations at the slice 87. A preferred minimum amount of air which should be added for alleviating all of the low basis weight spots has been found in one particular embodiment of machine to be .5 to 1 percent by volume, that is, the air injected through the orifices 98 has between .5 to 1 percent of the volume (the air being meas' ured at atmospheric pressure) of the amount of stock that is flowing at the time through the conduit 31. Air in the amount of 2 percent by volume may be desirable in certain cases. It has been found, however, thatrany amount of air injected even substantially below this figure makes an improvement in the sheet. Referring to FIG. 3, the curve 104 indicates the head across the forming area 94 of a certain machine when air of only 1 percent by volume of the stock flow is supplied through the orifices 98, and the curve 1G5 shows the head that exists across the forming area when a substantially larger amount of air, such as one percent by volume, is supplied through the orifices 98. In each case, the pressure of the air in the tubes 99 is simply maintained at a value which is sufiicient to cause the air to flow through the orifices 93 against the pressure of the stock in the conduit 31 in the air volumes mentioned. Referring to FIG. 3, it will be obse ed that only percent of air by volume raises the pressure substantially at the slice 88 (about 15 percent), and the pressure with this amount of air is nearly as high as the pressure at the slice that exists with 1 percent of air being added. Since the pressure at the slice is substantially raised by air injection, the high frequency pressure fluctuations which are effective particularly at the slice do not cause the low basis weight spots that would otherwise exist.

In view of the fact that the air has no substantial weight as compared to the stock that is flowing through the conduit 31, the air flows in bubbles along the lower surface of the lobe insert 62 and orifice plate 68, and the air passes into the bypass conduit 96 and passes through the valve 97, along with the bypassed stock, into the wire pit 22. The air thus increases the volume of flow through the orifice 95 without increasing the mass of substance that fiows through the orifice. It thus causes the in creased pressure at the slice 83. Its compressibility is also effective at the orifice 95 to take up some of the oscillatory forces in the stock adjacent the slice 88, and

these are thought to be reasons for the advantageous effect of the air injected in the stream of stock.

In order to obtain this advantageous effect of air with: in the stock, the air should be injected into the stream of stock upstream of the orifice plate 68. Preferably the air should be injected as close as possible to the orifice 95 upstream of it, such as within six inches, as is illustrated in FIG. 2 of the drawings or right at the orifice 95 as shown in FIG. 4. The orifices 98 should be distributed crosswise of the machine closely enough so that the air is distributed well as it passes through the orifice 95. A spacing of three inches between orifices 98 has been found satisfactory.

There should be at least some stock bypassing through the conduit 96, so that the air is not forced through the wire 15 to disturb the resulting sheet A. The stock bypassing through the conduit 96 in conjunction with the slice 87 having the relatively sharp edge 88a bearing on the wire 15 has the advantageous function of preventing small vortices in the stock in the forming area 94 and resultant streaking of the sheet. There is, however, no particular minimum of bypass flow that is necessary in order to obtain the benefits of air injection; but good results have been obtained when 40 percent to 60 percent (preferably about 40 percent) of the stock is bypassed. However, 10 percent to 60 percent of stock bypass is effective for obtaining the advantages of the invention. If the volume of flow is greater through the orifice 95, the pressure drop at the orifice may be greater, and the stock may increase in pressure to a greater extent across the forming area. The greater the pressure drop across the orifice 95 is, the lower the ratio of MD/ CD becomes, which is desirable for some purposes, and therefore, a substantial bypass flow of stock in conduit 96 is desirable for increasing the velocity at the orifice 95. Since the amount of flow through the bypass conduit 96 is adjustable by means of the valve 97, there is a great flexibility of control insofar as the MD/CD ratio is concerned.

Referring to FIG. 3, it will be observed that when air is injected into the stock stream, the initial pressure right at the end of the bottom lip 70 is less. This also is considered desirable, since it may be expected under these conditions that the stock fibers are initially put onto the wire with less force. When the pressure is lower above the forming area, such as slightly downstream of the lip '76, the lay on force of stock fibers is decreased. It may thus be expected that the sheet A will not become as firmly attached to the wire 15 as otherwise, and the sheet may be more easily removed from the wire 15 after it has been dewatered by the usual table rolls 19 and suction boxes 29.

Thus, the injection of air into the stock prior to the orifice 95 has a number of advantages. It cures low basis weight spots that may be occurring in the sheet A due to a relatively high pressure drop across the orifice 95 which has been provided for a relatively low MD/CD ratio. Air injection clears up the sheet defects and does not change the original MD/ CD ratio. It is thus possible to obtain a good sheet A with a lower MD/CD ratio by using air injection than without air injection. Any amount of air added upstream of the orifice 95 gives advantageous results; however, one to two percent by voiume has been found to completely overcome any tendency for the low basis weight spots to occur. The injection should preferably be as close as possible to the orifice 95, and the air orifices should be sufficiently close together so as to give a good distribution of air to the stock as it flows through the orifice 95. The stock bypass conduit 96 functions to carry substantially all of the air out from the forming area 94, so that air passing through the forming area does not disturb the sheet formation. The air injection gives a steeper pressure gradient across the forming area 94, with a lower pressure at the beginning and a higher pressure at the end of the forming area than when no air is added, and these are desirable conditions as has been explained. Air injection allows successful operation with a large pressure drop across the orifice 95 for which low pressure spots would ordinarily occur in the sheet and with which there is a desirably low MD/CD strength ratio.

We wish it to be understood that our invention is not to be limited to the specific methods, constructions and devices shown and described, except only insofar as the claims may be so limited, as it will be understood to those skilled in the art that changes may be made without departing from the principles of the invention.

What is claimed is:

1. In the manufacture of continuous webs from a fluid suspension of fibrous material, conducting a stream of fluid suspension of fibrous material from a tank for the suspension through a passageway having a restricted orifice therein onto a moving, fluid permeable, forming member which substantially closes the end of the passageway so as to provide a web forming region on the forming member on which the fluid suspension may drain to form a web on the forming member, injecting a gas into the flowing stream upstream of said orifice so as to change the pressure distribution of the suspension across said web forming region, and withdrawing an upper boundary layer of said stream above said forming region and including the injected gas in the stream and returning the upper stream boundary layer to said tank.

2. In the manufacture of continuous webs from a fluid suspension of fibrous material, pumping a stream of fluid suspension of fibrous material from a tank for the fluid suspension through a passageway having a restricted orifice therein onto a moving, fluid permeable, forming member, said passageway having an outlet which is adjacent said orifice and is substantially closed by the forming member so as to provide a web forming region on the forming member on which the suspension may drain to form a web on the forming member, injecting a gas into the flowing stream within said passageway upstream and adjacent to said orifice at a volume of percent to 2 percent measured at atmospheric pressure with respect to the volume of the stream of fluid suspension flowing through said passageway so as to change the pressure distribution of the suspension across the web forming region, and withdrawing an upper boundary layer of said stream at the downstream end of said forming region and including said gas and returning the stream layer to said tank.

3. In the manufacture of continuous webs from a fluid suspension of fibrous material, pumping a stream of fluid suspension of fibrous material from a tank for the fluid suspension through a passageway having a restricted orifice therein onto a moving, fluid permeable, forming member, said passageway having an outlet which is adjacent said orifice and is substantially closed by said forming member so as to provide a web forming region on the forming member on which the suspension may drain to form a web on the forming member, injecting a gas into said passageway from a top surface of the passageway and adjacent said orifice at a volume of percent to 2 percent measured at atmospheric pressure with respect to the volume of the stream of fluid suspension flowing through said passageway so as to change the pressure distribution of the suspension across said web forming region, and withdrawing an upper boundary layer including said gas from said flowing stream at the end of said web forming region remote from said orifice in an amount equal to 10 percent to 60 percent of the volume of 'flow of the suspension through said orifice and returning the stream layer to said tank.

4. In the manufacture of continuous fibrous webs by the use of a confined flowing stream of a fluid suspension of fibrous material which is pumped from a tank for the fiuid suspension through a passageway having a restricted orifice therein and having an outlet adjacent to the orifice which is closed by a moving, fluid permeable, forming member so as to provide a web forming region on the forming member on which the suspension may drain to form a web on the forming member, an upper boundary layer of the flowing stream of fluid suspension being conducted away from the web forming region at its downstream end to said tank so as to cause a predetermined distribution of pressure of the fluid suspension to exist across the web forming region, the improvement which consists in changing the predetermined distribution of fluid pressure within said web forming region by injecting air into said passageway from the top of the passageway at a point adjacent to and upstream of said orifice and thereby into the flowing stream of fluid suspension in said pas sageway upstream of said orifice with the air being conducted away from said forming region with said upper boundary layer.

5. In web forming apparatus, the combination of an endless, fluid permeable, forming member, means for movably mounting said forming member, a tank for a fluid suspension of fibrous material, means forming a passageway connected With said tank, means for pumping the fluid suspension from said tank through said passageway, said passageway having its end substantially closed by said forming member so as to provide a forming area on the forming member at the end of the passageway on which the fluid suspension is deposited and drains to form a web on the forming member, a restricted orifice in said passageway adjacent and upstream of said forming area, means forming a second passageway for conducting away an upper boundary layer of said suspension from above said forming area and back to said tank, a source of gas, means for injecting gas from said source into said first named passageway adjacent and upstream of said orifice to change the distribution of pressure of the suspension across the forming area with the gas being conducted away from the forming area through said second passageway, and means for controlling the rate at which the gas is so injected.

6. In web forming apparatus, the combination of an endless, fluid permeable, forming member, means for movably mounting said member, a tank for a fluid suspension of fibrous material, means forming a passageway connected with said tank, means for pumping the fluid suspension from said tank through said passageway, said passageway having its end closed by said forming member so as to provide a forming area on the forming member on which fluid suspension drains to form a Web, a restricted orifice in said passageway adjacent and upstream of said forming area, means forming a second passageway for drawing off an upper boundary layer of fluid suspension at the downstream end of said forming area and returning it to said tank, a source of air under pres sure, means connecting said source of air with said first named passageway adjacent and upstream of said orifice so as to inject air into said first named passageway to change the distribution of pressure of the suspension across the forming area with the air being conducted away from the forming area through said second passageway, and means for regulating the amount of air supplied from said air source to percent to 2 percent by volume at atmospheric pressure of the fluid suspension flowing through said first named passageway.

7. In web forming apparatus, the combination of an endless, fluid permeable, forming member, means for movably mounting said member, a tank for a fluid suspension of fibrous material, means forming a passageway connected with said tank, means for pumping the fluid suspension from said tank through said passageway, said passageway having its end closed by said forming member so as to provide a forming area on the forming member on which fluid suspension drains to form a web, a restricted orifice in said passageway adjacent and upstream of said forming area, means forming a second passageway for drawing oil an upper boundary layer of fluid suspension at the downstream end of said forming area and returning it to said tank, means for regulating 1 2 the amount of fluid suspension flowing through said second passageway to 10 percent to 60 percent of the flow through said first named passageway, a source of air pressure,

conduit means connecting said source of air with an upper surface of said first named passageway adjacent and upstream of said orifice so as to inject air into said first named passageway to change the distribution of pressure of the suspension across the forming area with the air being conducted away from the forming area through said second passageway, and means for regulating the air supplied from said air pressure source to /1 percent to 2 percent by volume at atmospheric pressure of the fluid suspension flowing through said first named passageway.

8. In web forming apparatus having a fluid permeable forming member in the form of a movably mounted endadjacent to the forming area, said flow control unit hav ing a second passageway therein for drawing off an upper boundary layer of fluid suspension adjacent the downstream end of the forming area and connected with said tank for discharging therein, the improvement which consists in providing a source of air, means for injecting air from said source of air under pressure into said first named passageway upstream and adjacent to said restricted orifice for changing the distribution of pressure of the fluid suspension across the forming area with the injected air flowing with said upper boundary layer away from said forming area and back to said tank, and means for controlling the rate at which the air is so injected.

References Cited in the file of this patent UNITED STATES PATENTS 1,5 63,095 Lewthwaite Nov. 24, 1925 1,662,226 Witham Mar. 13, 1928 2,756,649 Lee July 31, 1956 FOREIGN PATENTS 356,294 Great Britain Sept. 10, 1931 627,520 Great Britain Aug. 10, 1949.

Claims

1. IN THE MANUFACTURE OF CONTINUOUS WEBS FROM A FLUID SUSPENSION OF FIBROUS MATERIAL, CONDUCTING A STREAM OF FLUID SUSPENSION OF FIBROUS MATERIAL FROM A TANK FOR THE SUSPENSION THROUGH A PASSAGEWAY HAVING A RESTRICTED ORIFICE THEREIN ONTO A MOVING, FLUID PERMEABLE, FORMING MEMBER WHICH SUBSTANTIALLY CLOSES THE END OF THE PASSAGEWAY SO AS TO PROVIDE A WEB FORMING REGION ON THE FORMING MEMBER OF WHICH THE FLUID SUSPENSION MAY DRAIN TO FORM A WEB ON THE FORMING MEMBER, INJECTING A GAS INTO THE FLOWING STREAM UPSTREAM OF SAID ORIFICE SO AS TO CHANGE THE PRESSURE DISTRIBUTION OF THE SUSPENSION ACROSS SAID WEB FORMING REGION, AND WITHDRAWING AN UPPER BOUNDARY LAYER OF SAID STREAM ABOVE SAID FORMING REGION AND INCLUDING THE INJECTED GAS IN THE STREAM AND RETURNING THE UPPER STREAM BOUNDARY LAYER TO SAID TANK.