US3055421A

US3055421A - Stock inlet

Info

Publication number: US3055421A
Application number: US719873A
Authority: US
Inventors: Anthony J Cirrito
Original assignee: Rice Barton Corp
Current assignee: Rice Barton Corp
Priority date: 1958-03-07
Filing date: 1958-03-07
Publication date: 1962-09-25
Anticipated expiration: 1979-09-25

Description

p 25, 1962 A. J. ClRRlTO 3,055,421

STOCK INLET Filed March 7, 1958 2 Sheets-Sheet 1 Sept. 25, 1962 A. J. CIRRITO 3,055,421

STOCK INLET Filed March 7, 1958 2 Shets-Sheet 2- Q 4;? A M 1% flfl 0 O o o o o o o o o o i i i i :QI.

iite States Filed Mar. 7, 1958, Ser. No. 719,873 26 Claims. (Cl. 162340) This invention relates to a stock inlet for supplying paper stock to the slice of a paper machine. More particularly, the invention relates to a hydrodynamic type of stock inlet forming in effect a nozzle, the interior configuration of which rigorously controls the flow pattern of paper stock moving therethrough.

A stock inlet, otherwise known as a head box, must not only supply paper stock to a forming wire at a velocity corresponding substantially to that of the Wire, but must also supply such stock in a uniform stream across the entire Width of the inlet, free from gross turbulence and cross flow, but at the same time in a state of agitation or minor turbulence. Otherwise, variations in the solids content per unit area laid down on the wire occurs, with corresponding irregularities in the paper made. In providing flow delivery of the uniform character required, a satisfactory stock inlet must minimize the variations in flow across the width of the machine with naturally results from flow conversion through a flow spreader into a single narrow stream. Additionally, the stock inlet must even out and minimize gross turbulence originating ahead even of the flow spreader, as in the pump, but nevertheless carried on through the flow spreader, to produce therewith variations in flow and gross turbulence which is a composite of the two effects.

In prior art stock inlets, a standard expedient has been to provide therein a long flow path in the course of which to minimize unwanted gross turbulence and cross flow, resulting in inlets of substantial volume and length. Also, prior art inlets have typically included diverging portions following converging portions in providing the extended flow path; the prior art has given a great deal of attention to just what angle of such divergence can be tolerated in a stock inlet without producing eddies, or a tendency of the flow to separate from the wall of the inlet (W. Van der Meer, Hydraulics of Flowbox and Slice Tappi, vol. 37, No. 11, November 1954, pp. 502, 503-504). The prior art has sought to avoid formation of eddies in the stock inlets. The prior art has typically made use in stock inlets of distributing or rectifier rolls, mainly to encourage the desired type of macroturbulence (as opposed to the undesirable gross turbulence which produces diifering flow rates across the stream as Well as agitation Within the stream). Such rectifier rolls, however, have been of very low resistance, with a high percentage of open area and with thin wall thicknesses. Still another prior art characteristic has been the view that when rolls were used in the inlet, the clearances between such rolls and the inlet walls should be very close, to prevent faster flow of the stock along the walls past the roll than elsewhere. Van der Meer (op. cit., p. 508) suggests placing the roll as near as possible to the wall, say A; [inch], which leads him to recommend installing strips along the wall to change flow angles and decrease likelihood of fibers catching. Obviously, all of this requires diflicult or complicated fabrication work.

It is a primary object of the present invention to provide a stock inlet which will minimize cross flow and gross turbulence to deliver to the shoe across the entire width thereof a stream of uniform weight and fiber distribution.

It is a further object to provide such a flow inlet which is simple in construction, is free from need for manufacturing with close fits, and which is small in size, the

3,055,421 Patented Sept. 25, 1962 latter having special advantages as pressure requirements increase.

A further feature of the present invention is to provide for clean operation, but at the same time provide ease of cleaning when such is necessary.

Generally speaking, the invention provides for sudden expansion, nearer the upstream end of the stock inlet, of a continuous narrow stream flowing therethrough, the portion of the inlet into which the expansion is done (or expansion chamber) having extending throughout the same, across substantially the entire width and depth of the flow path, a resistance unit. The depth of the stream (thickness normal to mean flow path) at the expansion chamber is at least twice the depth at the narrow opening just theretofore, and an angle of divergence of at least 70 is used therebetween. The resistance unit is very closely adjacent the opening of small depth from which the stream is expanded, the upstream extremity thereof being at no greater distance therefrom than the depth of the expansion chamber at its widest portion.

In the preferred embodiments stable eddies are intentionally propagated upstream of the resistance unit by utilizing linear divergences in excess of 70. However, in other satisfactory embodiments of the invention such eddies are avoided, as by providing for the expansion in an arcuate pattern conforming substantially to the diameter of a resistance roll used as the resistance unit, the divergence angles here being measured as tangents to the arc, directly at the small opening and decreasing steadily to zero at the widest part of the expansion chamber, and placing the roll within a small fraction of an inch of the chamber wall and directly adjacent to the opening. Thus, obviously, a 70 divergence limitation need not characterize the entire divergence portion between the small opening and the maximum depth of expansion chamber.

Another feature is that the transverse walls of the stock inlet converge continuously all the way from the deepest portion of the expansion chamber to the slice, to give maximum flow stability therethrough with minimum length; such continuing convergence is in itself an important and novel feature of the invention.

A further feature of the invention is that precise clearances between resistance units and the walls are not necessary, and increased velocity adjacent the walls is even affirmatively desirable.

Still another feature of the preferred embodiments is that the mean flow path is uphill until the stream is delivered to the slice.

Other objects, features and advantages will appear from the following description of a preferned and several modified embodiments of the invention, taken together with the drawings, in which:

FIG. 1 is a broken away side elevation, pamtially in section, of a paper making machine fitted with the preferred embodiment of stock inlet according to the invention;

FIG. 2 is an end elevation showing a modified fiow spreader for use with inlets according to the invention;

FIG. 3 is a sectional view at 33 of FIG. 2;

FIG. 4 is a vertical sectional view through the upstream end of a slightly modified embodiment of stock inlet according to the invention;

FIG. 5 is a vertical cross section through still another embodiment of stock inlet according to the invention;

FIG. 6 is a ventioal cross section through still another embodiment;

FIG. 7 is a vertical cross section through yet a further embodiment; and

FIG. 8 is a partial cross sectional view illustrating a preferred mounting for a small diameter resistance roll in accordance with the invention.

Referring now in more detail to the drawings, there is shown in FIG. 1 a stock inlet, indicated generally at 10, mounted to accept stock flowing thereinto from pivotally mounted transition pieces 12, and to discharge stock into and through a slice 14 onto a forming wire 16 trained about a breast roll 18.

Each stock inlet 10 is uniform in vertical cross section across the entire width of the machine, which may be for example of the order of 200". Across the width of the machine, the stock inlet is defined by a pair of transversely extending walls 20 and 22, which may if desired be discontinuous to provide for opening for cleaning. in the embodiment shown, the outer transverse wall 20 includes a section 20a which can be separated from sections 20b, 20c and 20d, to facilitate cleanmg.

In the preferred embodiment of FIG. 1, stock is delivered vertically through the transition pieces 12, which in end elevation would have diverging configurations similar to that of the embodiment illustrated in FIG. 2, so that a single continuous thin stream is delivered across the entire 'width of the paper making machine at the entrance 24 to the stock inlet 10. The upstream extremity of the stock inlet 10 comprises a slightly converging length or passage 26, terminating in an opening 28 having a depth -(i.e., provide for a mean stream thickness of) of the order of 2". Immediately upstream of the opening 28 of the walls 22 and 20 diverge (symmetrically about a vertical plane bisecting the opening 28) at an angle of 90, and a resistance roll 30 is mounted directly above said opening, with its axis in the said vertical plane, and with its surface tangent to each transverse wall at portions thereof diverging at 90 from each other.

The resistance roll, which is preferably rotatably mounted by any conventional technique, may suitably have a diameter of 10 and a wall thickness of and be drilled in /2 diameter perforations to provide an open area of 36%. Preferably there are provided at the innor and outer extremities of the perforations radii (not shown) to minimize sticking of fibers.

The stock inlet .10 has its greatest depth, or area in a plane normal to the mean flow direction, substantially where said plane passes through the axis of the resistance roll 30. Downstream of this plane the depth of the inlet or stream thickness normal to mean flow direction continuously diminishes, the walls 20 and 22 continuously converging until the stream is delivered at the slice 14.

Downstream of the resistance roll 30 are smaller resistance rolls 32 and 34; center line distances between each pair of resistance roll-s may suitably be of the order of 16", the entire length of the inlet 10 being suitably of the order of feet.

The resistance roll 34 may suitably be 6" in diameter, with a /8" wall thickness, and drilled with /2 diameter holes arranged on a 60 staggered pattern to provide 43% open area.

The intermediate resistance roll 32 of diameter intermediate between the

rolls

30 and 34 may suitably be of wall thickness and hole spacing in size conrespondin g substantially to that of roll 30.

Downstream of the smallest resistance roll 34 is a slot 36, extending across the entire width of the machine, through which an upper layer of stock flowing through the inlet may be continuously drawn off, such layer having been found to be particularly rich in entrapped air, this bleed-off thus protecting in large measure against delivery of air against the slice.

This stock inlet, with its hydrodynamic characteristics and short, compact size, may accurately be designated as a stock nozzle.

There is shown in FIGS. 2 and 3 a modified form of flow spreader. In this embodiment the transition pieces (or diifusors) 112 discharge directly through the small opening 128 in the stock inlet into the expansion chamber thereof. (The expansion chamber is, in each inlet, that portion thereof downstream of the small opening Where divergence begins, and upstream of a plane drawn through the upstream resistance element normal to the mean flow path.) The

transition pieces

12 and 112 in both embodiments described diverge at an included angle of 12, the maximum permissible without production of separation from the walls as discussed by Van der Meer, supra.

In the embodiment of FIGS. 2 and 3 the stock is transmitted to the transition pieces 112 through branch pipes 142 leading vertically from a tapered, inclined, single ended header 144.

As is well known in the art, where the stock first comes together in a continuous narrow stream (at the opening 128, in the embodiment of FIGS. 2 and 3; and at the entrance 24 in the embodiment of FIG. 1) the flow velocity is inherently variable across the width of the stream, being :at maximums adjacent the center lines of the individual transition elements 12 and 1:12, and at minimums intermediate thereof, where the different transition pieces come together.

In operation, stock is pumped in (preferably with a fan pump designed to give as smooth, continuous flow as possible, although other pumps will serve us is known to the art) through the header 144, branch pipes 42, and

transition pieces

12 or 112. Gross turbulence and cross flow components result both from irregularities in flow occurring prior to and in header 144, and in and through the branch pipes and transition pieces. The continuous wide narrow stream then passes through the openings 28 or 128, which in the preferred embodiments is 2" deep, and is expanded substantially five times into the

expansion chambers

40 or 140. The angle of divergence just downstream of the openings 28 and 128, being 90, produces stable eddies just downstream of the opening, with substantial consequent head loss. Also the resistance roll 30, because of its wall thickness and the small diameter of holes and percentage of open area creates substantial head loss about 1 2" (inches of head loss refers to inches of water column throughout). The total head loss resulting from these two factors from the openings 28 and 128 through the resistance roll 39 is of the order of 18" to 24" when the stock flow is at the rate of 50 gal-Ions per inch of width across the inlet.

The substantial loss of head resulting from this combined effect goes a long way toward dampening gross turbulence and evening out cross flowing components. Further head loss having additional such effect takes place through the resistance rolls 32 and 34, which also serve the function of encouraging the desired macroturbulence. The substantially continuously converging configuration of the transverse walls of the inlet from the expansion chamber to the slice provide stability, while the very substantial head loss created as just described makes possible the very short and compact stock nozzle character.

The

rolls

30, 32 and 34 need not fit closely against the walls 20 and 22. Clearances may be of the order A", and a higher flow rate on each side of the rolls than through the rolls, which can thus be achieved, is often afiirmatively desirable to produce scrubbing along the walls to promote cleanness.

There is shown in FIG. 4 a slightly modified embodiment of stock inlet according to the invention. In this case the upstream portion of the inlet 226 of small diameter is at an angle of 26 from the vertical. As in the embodiments heretofore described, the center line of the small inlet portion extends also through the center of the resistance roll 230, but in this embodiment the walls 220 and 222 are symmetrically positioned (relative to the plane bisecting the opening 226 and passing through the axis of the resistance roll 230), as far as the downstream extremity of the roll 230. In this embodiment the angle of divergence between the walls 220 and 222 just downstream of the opening 228 is 85, and the distance between the opening 228 and the upstream extremity of the roll 230 is substantially less than the depth of the inlet at its "Widest portion (defined by a plane normal to the mean flow path and passing through the axis of roll 230).

Still another embodiment is shown in FIG. 5. In this embodiment the

walls

320 and 322 are symmetrical over most of the inlet length about a horizontal axis, and accordingly are much more likely to coat up in operation, particularly the outer wall 320. However, this embodiment is satisfactory where flow rates are sufiiciently high and where stock relatively free from additives and binders used, as for example in the manufacture of tissue paper and newsprint at speeds of the order of 2000 feet per minute. In this embodiment a bleed-off slot 336 for an air-rich layer of stock is introduced at the deepest portion of the inlet, at the downstream extremity of the expansion chamber 340, rather than nearer the slice. Another respect in which this embodiment is different from those heretofore described is that the resistance rolls 330 and 332 are fabricated of round bars 337 extending across the width of the stock inlet throughout the expansion chamber, and welded to metal discs 333 spaced 1 apart, the bars being 1" in diameter and spaced to form A" by 1" openings, with a total of 20% open area. In this embodiment clearance at the walls 32% and 322 is A and flow rate is greater around the rolls 3'30 and 332 than through the same, with beneficial cleaning action as above described. The smallest roll 334- is perforated and of the same character as the roll 34 previously described. In this embodiment as in the others described, there is substantial continuous convergence between the

rolls

320 and 322 from the expansion chamber to the slice, to provide flow patterns of maximum stability.

Still another embodiment is disclosed in FIG. 6. This embodiment corresponds closely to the embodiment of FIG. 1, except that provision is made for an air pressure dome 450, defined by the outer wall 420. The

rolls

430, 432 and 434 are resistance rolls similar in character to the rolls 3d, 32 and 34. Use of a high resistance roll beneath the pressure dome would, however, upset the airliquid interface thereabove. Accordingly, a typical distribution roll of the low resistance characteristic of the prior art is preferably used beneath the pressure dome 450 for agitation in that zone of the inlet, though such a roll could be omitted altogether. Such a roll may suitably be of A wall thickness, and with holes 1% in diameter, to provide 45% open area. (The head loss across such a roll, under a flow rate of 50 gallons per minute, per inch of stock inlet width, would be considerably less than 1" (of water).) Compare this with the 18" to 24" across the roll 30 and expansion chamber 40.

Although the

walls

420 and 422 do not continuously converge in this embodiment because of the pressure dome 450, the air pressure in the latter acts in such a Way that the flowing stock does in effect continue to converge thereacross, the air acting to bound or confine the stream along a horizontal in that zone, and the inner wall 422 continuing to converge.

Still another embodiment is disclosed in FIG. 7. Here, as in the embodiment of FIG. 4, the axis of the small opening 528, which passes also through the center of the resistance roll 530, is at an angle of 26 to the vertical. In both embodiments this angular orientation sometimes has advantages, despite the fact that there is a greater tendency to coat up, particularly on the walls 220 and 520 (no doubt because of the rising of entrapped air thereagainst). In this embodiment, however, the small hole 526, and opening 528 in which it terminates, are of much greater depth than in any embodiment heretofore described. In fact, the depth of thi sopening 528 may suitably be as great as half the depth of the expansion chamber at its deepest portion (along a plane normal to the mean flow path and passing through the axis or the roll 530). In this embodiment, an arcuate divergency rather than a linear divergency in the

walls

520 and 522 extends from the opening 5228 marking the beginning of the expansion chamber to the plane just referred to (which defines the downstream extremity of the expansion chamber), the angle of divergency between the walls varying from a maximum of (measured as tangents) at the upstream extremity of the expansion chamber to a minimum of zero at the downstream extremity thereof. Because not only is linear divergency avoided, but an arcuate divergency corresponding in radius substantially with the outer radius of the roll 530 is used and the roll 530 is spaced directly adjacent the opening 528, eddies are avoided in this embodiment. Thus, the head loss to be achieved in the expansion chamber must in this embodiment be accomplished solely by means of the resistance of the roll 530, and accordingly it is desirable in this embodiment to use such a roll of even higher resistance than heretofore described. In the embodiment shown the wall thickness is approximately 2", and the holes approximately /2 in diameter, to provide an open area of approximately 25%.

By making the arc in the

walls

520 and 522 substantially the same as or slightly greater than the radius of the roll 530, the latter can be brought as close as desired to the opening 528, minimizing flow around the roll, while at the same time avoiding any need for close fit between the roll 530 and other portions of the

walls

520 and 522 around the arcuate portions thereof. Thus, when desired, substantially the entire flow can be made to pass directly through the roll 530, and any deflection, for example from load, caused by flow is in the direction of safety.

In all the embodiments above described, the resistance unit disposed in the expansion chamber has been a resistance roll, preferably rotatably mounted. As noted, such rolls may be either perforated rolls or bar rolls, for example. However, the resistance unit need not even be a roll; it might suitably for example be some other grid-like resistance unit extending through the expansion chamber, and kept clean for example by oscillation rather than rotation. When, as preferred, a rotating round resistance roll is used, it is preferred to rotate the same slowly and with the upper surface moving downstream, so that any entrapped air is swept forward rather than counteracted to encourage build-up to stagnation and periodic release.

Preferably the largest resistance roll used is 10" to 14" in diameter. By resistance rolls there is meant a roll having much higher resistance and producing much great er head loss than the distributor rolls of the prior art. As above described, in connection with FIG. 6, the roll 452, which was such a roll of perhaps 8" in diameter, would have a head loss of the order of considerably less than 1" at a flow rate of 50 gallons per minute per inch of width. For the purposes of this invention, unless a head loss of at least 5" is produced by a roll under such a flow rate, the same is not regarded as a resistance roll; rather such rolls are referred to herein as distribution or distributor rolls.

The absolute amount of resistance to be given each roll will depend in each case on the amount of gross turbulence and cross flow which must be damped by production of head loss. The greater the head loss required of each roll, obviously the greater must be the resistance given the same by use of thicker walls, smaller holes, lower percentage of free area, or all three.

As above noted, preferably the inlet is given internal configuration such that there is a substantial continuing convergence from the deepest portion of the inlet (at the downstream end of the expansion chamber), all the way to the slice. However, as will appear, certain advantages of the invention may be achieved even though the walls of the inlet are parallel for certain distances, for example. See also the discussion of the embodiment of FIG. 6.

Any flow spreader capable of delivering stock in a wide, thin stream may be used. A cross spreader, as illustrated in FIGS. 9 and 10 (p. 506) of Van der Meer, op. cit., may be used if desired.

In the preferred embodiments, a linear divergency is used from the small opening into the expansion chambers, to produce stable eddies which have the desirable effect of themselves producing head loss to dampen turbulence and cross flow, reducing the resistance requirements of the rolls. In order to achieve these stable eddies, divergencies of 70 or more are used, the preferred divergency as above described being a linear divergency of 90. If an arcuate divergency is used (angles of divergency being measured as tangents), a resistance roll may if desired be moved so close tothe neck or opening (528 in the embodiment of FIG. 7) that the stream passes directly from the opening 528 into the roll without diverging, so that eddies are avoided, in which case a roll of higher resistance will normally be desired.

In the preferred embodiments the ratio between the depth of re small opening (where the expansion chamber begins) and the maximum (downstream extremity) depth of the expansion chamber may suitably be of the order of five or ten times. However, this ratio should be at least 2:1.

The distance from the small openings at which the expansion chamber begins to the upstream extremity of the resistance unit is preferably very small, and preferably should be not more than "a distance corresponding to the distance across the expansion chamber at its deepest area.

In FIG. 8 is shown a preferred means of mounting the 6" diameter resistance roll 34, only one extremity of the same being shown. A solid bar 50 of outside diameter corresponding with the inside diameter of the roll 34 extends a short distance thereinto and is welded thereto at 52. Packing 54 assures a seal. The shaft 50 is supported in controflecture by a pair of

nntifriction bearings

56 and 58, the former being supported from below and the latter being supported from the top as indicated by the arrows, it being possible to exert a variable force above the bearings 58 in order to counteract warping, which occurs more easily in a roll of diameter as small as 6".

Other embodiments within the spirit and scope of the invention will occur to those skilled in the art, and accordingly I do not wish to be limited to the particular embodiments above described.

I claim:

1. A stock inlet for a paper making machine comprising a nozzle generally uniform in vertical cross section across a width corresponding to the width of the paper to be formed and defined across the width thereof by a pair of transversely extending opposed walls, at least one mechanical unit mounted on a line in the direction of stock flow and having sufiicient resistance to cause a head loss of at least inches of water at a flow rate of 50 gallons per minute per inch of width in said nozzle, said unit being mounted in said nozzle between said walls, said walls being spaced farthest apart at said unit and being spaced much more closely together at an inlet opening upstream immediately adjacent said unit to provide immediately downstream of said inlet opening an expansion chamber of increased cross sectional area at least twice the depth of said inlet opening, the downstream extremity of said expansion chamber being defined by a plane extending across said nozzle normal to the mean flow path and passing through the center of said mechanical unit, and said mechanical unit having openings therethrough and filling substantially the entire depth between said walls across the entire width of said nozzle in said expansion chamber.

2. The stock inlet of claim 1 in which, between said inlet opening and said mechanical unit, said opposed walls diverge downstream at an angle therebetween of at least 70 to provide said expansion chamber.

3. The stock inlet of claim 2 in which said walls diverge at an angle therebetween of 4. The stock inlet of claim 1 in which said inlet opening is spaced upstream from the upstream extremity of said mechanical unit a distance less than the maximum depth of said nozzle in said expansion chamber.

5. The stock inlet of claim 2 in which said walls diverge symmetrically about a plane passing through the centerlines of said inlet opening and said mechanical unit.

6. The inlet of claim 2. in which said walls diverge along arcuate paths of radius substantially equal to the radius of said mechanical unit, said unit being of generally cylindrical outer form.

7. A stock inlet for a paper making machine comprising a nOzzle generally uniform in vertical cross section across a width corresponding to the width of the paper to be formed and defined across the width thereof by a pair of transversely extending opposed walls, at least one mechanical unit having sufiicient resistance to cause a head loss of at least 5 inches of water at a fiow rate of 50 gallons per minute per inch of width in said nozzle, said unit having openings therethrough and being mounted in said nozzle across substantially the entire depth and width thereof between said walls, said walls toward the upstream end of said nozzle diverging downstream at an angle of at least 70 to define an expansion chamber, the depth of said expansion chamber at the downstream extremity thereof being at least twice the depth thereof at the upstream extremity, said upstream extremity of said expansion chamber being formed by an inlet opening and the downstream extremity of said expansion chamber being defined by a plane extending across said nozzle normal to the mean flow path and passing through the center-line of said mechanical unit, and said walls converging free from divergence in a downstream direction, downstream of said expansion chamber to a discharge opening of the nozzle.

8. The stock inlet of claim 7 in which the depth of said expansion chamber at the downstream extremity thereof is five times the depth thereof at the upstream extremity formed by said inlet opening.

9. The stock inlet of claim 7 in which said mechanical unit is of generally cylindrical form.

10. The stock inlet of claim 7 in which said mechanical unit at its upstream extremity is spaced from said inlet opening a distance not greater than the depth of said expansion chamber at the downstream extremity thereof.

11. The stock inlet of claim 10 in which said nozzle is at a maximum depth at said downstream extremity of said expansion chamber.

12. A stock inlet for a paper making machine generally uniform in vertical cross section across a width corresponding to the width of paper to be formed comprising a pair of transversely extending opposed Walls defining a nozzle, said walls toward the upstream end of said nozzle being relatively close together to define a narrow stock inlet opening and diverging immediately downstream thereof at an angle of at least 70 to form an expansion chamber of increased cross sectional area at least twice the depth of said narrow inlet opening, a mechanical unit of generally cylindrical outer form and having openings therethrough to cause a head loss in said nozzle, said cylindrical unit mounted at the downstream boundary of the expansion chamber and extending through substantially the entire width and depth of said nozzle, and a pair of successively smaller diameter cylindrical head-loss-causing units of similar nature mounted downstream of said first-mentioned cylindrical unit and also extending through substantially the entire width and depth of said nozzle, the depth of said nozzle being a maximum at the downstream boundary of the expansion chamber, which is along a plane normal to the mean flow path and passing through the axis of the first-mentioned cylindrical unit, and said nozzle converging continuously therefrom to a discharge opening at the slice end of said nozzle, each of said cylindrical units having a resistance sulficient to produce at least a five inch loss of head in stock flowing at fifty gallons per minute per inch of nozzle width.

13. The stock inlet of claim 12 in which the axis of said first-mentioned cylindrical unit is vertically above the center-line of said narrow inlet opening.

14. The stock inlet of claim 13 in which said opposed walls define a narrow vertical passage upstream of and downwardly extending from said narrow inlet opening.

15. The stock inlet of claim 13 in which said firstmentioned cylindrical unit has a resistance sufficient to produce about twelve inches of water loss of head in stock flowing at fifty gallons per minute per inch of nozzle width.

16. The inlet of claim 12 in which said walls are symmetrically disposed about a plane extending through the center line of said narrow opening and the axis of said first-mentioned cylindrical unit throughout said expansion chamber.

17. The inlet of claim 12 in which the mean flow path extends upwardly throughout substantially the entire length thereof.

18. The inlet of claim 12 in which said cylindrical units are perforated tubular rolls.

19. The inlet of claim 12 in which at least one of said cylindrical units includes a multiplicity of circumferentially mounted bars.

20. A stock inlet for a paper making machine generally uniform in vertical cross section across a width corresponding to the width of paper to be formed comprising a pair of transversely extending opposed walls and a multiplicity of mechanical units having openings therethrough mounted therebetween having suflicient resistance to cause a head loss of at least inches of water at a flow rate of 50 gallons per minute per inch of width in said stock inlet, each said mechanical unit extending substantially throughout the width and depth of its respective portion of said stock inlet, said inlet being of maximum depth at the downstream end of an expansion chamber toward the upstream end of said inlet, the downstream end of said expansion chamber being defined by a plane extending across said stock inlet normal to the mean flow path of stock therethrough and passing through the center of the unit farthest upstream, and said inlet being of minimum depth at a narrow inlet opening at the upstream end of said expansion chamber, said expansion chamber being at least twice the depth of said narrow inlet opening, said walls defining upstream of said narrow inlet opening a narrow stock inlet passage, and downstream of said expansion chamber converging continuously in a downstream direction to a slice opening.

21. The inlet of claim 20 in which the plane bisecting said narrow stock inlet passage is at an angle to the vertical.

22. The inlet of claim 21 in which said angle isacute.

23. The inlet of claim 20 which includes an air pressure dome, said inlet being free therebeneath from any mechanical unit to cause a head loss.

24. The inlet of claim 23 in which a distributor roll extends through said inlet beneath said dome.

25. The stock inlet of claim 20 in which said mechanical unit farthest upstream is of generally cylindrical outer form, and the upstream extremity of said cylindrical unit is directly adjacent to said narrow inlet opening, whereby eddy currents are avoided.

26. A head box for a paper making machine comprising a nozzle generally uniform in vertical cross section across a width corresponding to the width of the paper to be formed, and defined across the width thereof by a pair of transversely extending opposed walls, said nozzle having an inlet opening therein at its upstream end, an expansion chamber immediately downstream of said inlet opening and a slice discharge opening at the downstream end of the nozzle, said walls diverging sharply downstream of said inlet opening to provide in said expansion chamber a flow depth at least twice the flow depth of said inlet opening and consequent head loss, a mechanical flow impedance unit having sufiicient resistance to cause a head loss of at least 5 inches of water at a flow rate of 50 gallons per inch of width in said nozzle, said unit being mounted between the walls at said expansion chamber adjacent said inlet opening and extending across substantially the width of said nozzle, the downstream end of said expansion chamber being defined by a plane ex tending across said head box normal to the mean flow path of stock therethrough and passing through the center of said mechanical flow impedance unit, said walls converging toward each other from the downstream end of said expansion chamber to said slice discharge opening.

References Cited in the file of this patent UNITED STATES PATENTS 1,629,088 Aldrich May 17, 1927 1,707,375 Upson Apr. 2, 1929 2,281,293 Lang Apr. 28, 1942 2,329,799 Thorsen Sept. 21, 1943 2,347,130 Seaborne Apr. 18, 1944 2,664,033 Hornboste et al. Dec. 29, 1953 2,756,651 Lee July 31, 1956 2,764,917 Niks Oct. 2, 1956 2,847,913 Cirrito Aug. 19, 1958 2,870,690 Corbin et a1 Jan. 27, 1959 2,892,499 Logan et a1 June 30, 1959 2,929,449 Mardon et al. Mar. 22, 1960 OTHER REFERENCES McGraw-Hill, Pulp and Paper M-anufacture; vol. 3, 1953, page 106.

Mardon et al.: Paper Trade Journal, pages 28-29, June 25, 1956.