US3643338A - Drying system for a papermaking machine - Google Patents

Abstract

Drying of paper is accomplished by heated, dry air of relatively higher temperature than normally used in paper dryers which is directed under pressure through a restricted orifice across the paper machine width onto and through a porous woven felt and onto and against the wet paper web entrained between the felt and the periphery of a heated dryer drum, thereby creating high, locally induced vapor pressures for inducing a rapid release of the generated vapors. The web is held tightly between the felt and the face of the dryer drum thereby restricting the usual lateral contraction of the web, which contraction results in loss of web width and formation of cockles, streaks and general unevenness in the dried web.

Description

Fair

[54] DRYING SYSTEM FOR A PAPERMAKKNG MACHINE [72] Inventor: Albert Edward Harold lFair, 101 Monmouth St., Brookline, Mass. 02146 [22] Filed: July 9, 1970 [21] Appl. No.: 53,593

[52] US. Cl ..34/6, 34/116 [51] Int. Cl "F26b 3/00 [58] FieldoiSearch 34/6, 111, 116, 114

[56] References Cited UNITED STATES PATENTS 3,388,479 6/1968 Gardner ..34/1 11 X [4 lFeb.2,l972

Primary Examiner-John J. Camby Attorney-Kenwood Ross and Chester E. Flavin [5 7] ABSTRACT Drying of paper is accomplished by heated, dry air of relative ly higher temperature than normally used in paper dryers which is directed under pressure through a restricted orifice across the paper machine width onto and through a porous woven felt and onto and against the wet paper web entrained between the felt and the periphery of a heated dryer drum, thereby creating high, locally induced vapor pressures for inducing a rapid release of the generated vapors. The web is held tightly between the felt and the face of the dryer drum thereby restricting the usual lateral contraction of the web, which contraction results in loss of web width and formation of cockles, streaks and general unevenness in the dried web.

6 Claims, 1 Drawing Figure mimmnm m2 INVENTOR. ALBERT RD HAROLD FAIR y WM @714 (f CfiMU/z 1cwm ATTORNEYS.

DRYING SYSTEM FOR A IPAPERMAKIING MACHINE The invention relates generally to the art of papermaking and particularly to the drying of the wet paper web with the aid of drying cylinders or drums and a dryer felt or felts. Still more particularly, it relates to the use of such drying means in cooperation with pressurized heated air devices envisioning the forcement of airblasts at high velocity and high temperature through the dryer felt toward the drying cylinder surface for the inducement of the rapid release of generated vapors while the web is passing over the drying cylinder surface.

Important features of the invention are:

l. the optimum use of high-velocity, high-temperature dry air to scour the web surface, thereby inducing high vapor pressures and rapidly removing the vapors which are created thereby;

2. the use of the firm support afforded by the dryer surface to support the web while forcing the air against the web surface, (without support, the web would wrinkle or break); and

. the holding of the web tightly against the surface of the dryer drum, thereby restricting lateral contraction while permitting passage of vapors through the apertures of the felt, and minimizing the drying of the web while passing through the open pockets between the dryer drums, when tension while drying can cause cockles, wrinkles and streaks.

Stated in another way, the invention envisions improved ventilation means within the area of the dryer section wherewith the drying process is greatly accelerated, with markedly improved results in end products being realizable.

The invention teaches improved means for expediting the drying process through the rapid release of the vapors while the wet web is passing over the dryer peripheries, all whereby production is significantly increased.

The papermaking machine conventionally includes a dryer section in which the wet web is entrained in seriatim around a plurality of staggered heated drying cylinders for evaporating water therefrom. Normally, such cylinders are disposed in two parallel rows of tiers or banks, one above the other, with the web being entrained over the cylinders of the upper row and under the cylinders of the lower row in a system for drying the paper from both sides, passing alternately over an upper cylinder and under a lower cylinder, etc., etc.

The dryer fabric or felt, as is known, holds the wet web against the dryer surface to absorb the heat therefrom, absorbs the generated vapors which are released while the web is passing between felt and dryer, and allows the vapors to pass through the felt and to pass directionally away from the web. It is to the end of accelerating this release of vapors that this invention is especially directed.

in the prior art, after the web leaves the surface of each dryer cylinder, it in turn passes through an interdryer space where it has a chance to release the vapors which tend to leave the web due to the vapor pressures created by the temperature created in the web by the heat from the dryers. This interdryer space is normally restricted, and unless some form of ventilation is provided to absorb these vapors and to remove same from the pockets, vapor release does not readily occur.

The traveling web is urged and held against the drying cylinders by a dryer fabric or several thereof which, until recently, generally comprised a felt woven of cotton fibers or cotton fibers in combination with asbestos. More recently these felts have been made of a porous weave. The most modern socalled felts are actually rather openly woven webs of synthetic fibers made especially for the purpose. The open weave synthetic felts allow some passage of vapors through the mesh while the web is being held by the felt against the hot dryer surfaces.

These dryer fabrics are disposed on the side of the web away from the faces of the drying cylinders and are supported out of contact with the web in its travel from one cylinder to a next succeeding one. This results in the obvious formation of open ended pockets, through which the web travels, to come in contact with the adjacent drying cylinder. The greater part of the moisture evaporated from the web by the drying cylinders is expected to escape into the pockets, but has a tendency to accumulate and become entrapped therein so as to interfere with efficient and uniform web drying.

The entrapment and accumulation of this moisture frequently results in both poor drying of the web and in the formed paper being too dry in some areas and too wet in other areas. It is common, in the effort of minimizing these problems, to blow air across these pockets from one end to the other to dispose of these vapors, a practice which actually overdries the web edges.

Excess dehydration, such as commonly occurs near the web margins, results in the cockling of the edges and may reduce elasticity and strength. On the other hand, underdrying may cause blackening or excess compression of the wetter portions of the web during calendering. Too, there is a resultant tendency of such a web to lose its flatness and dimensional stability by reason of the moisture equilibrium. Nonuniform moisture content results in nonuniform conditions of bulk, which causes uneven rolls of finished paper. When a web of nonuniform bulk is compressed in a calender or supercalender v stack, differences in finish may be noticeable, leading to an uneven surface and nonuniform receptivity to coatings or inks.

Worse, nonuniformity in drying adds substantially to the cost of drying, for paper ordinarily must be dried until the wettest part of the web is dried below a certain maximum, indicating the obvious, namely that the other parts must be wastefully overdried at unnecessary and unproductive expense.

Various means of removing vapors from dryer pockets, include the cross currents mentioned and other outward flow methods.

In dryers of the type under discussion, to avoid this drying of one part of the web more than the other, past practice has been to blow the air in one direction through some of the pockets and in the opposite direction through other of the pockets. Nonetheless, nonuniformity continues to result, leading to an inferior paper, such as for example, one having a, say, 4 percent moisture content in the edge portion thereof and a, say, 8 to 10 percent moisture content in the center portion thereof.

Streaks of varying web moisture content can be caused by irregular dryer drum surfaces or by uneven tension in the warp of the dryer felts, resulting in irregular contact with the dryer surfaces. Forced hot air driving through the porous dryer felt tends to offset the uneven contact between the web and the dryer face. The reason is that a minute difference in open space between the paper web and the felt can facilitate airflow. This offsets the effect of higher temperature at points of contact caused by high spots on dryer surfaces or felt warp tension.

The effort of providing a dependably uniform humidity profile across the web being the formidable one that it is, it has seemed to be impractical, if not impossible, to attempt to control drying by way of correlating partially dryed web moisture at different stations in the dryer section with final moisture content of the dried web.

ln efficiency generally results when "the wet web is pressurized against the successive faces of the dryer cylinders of a series and is pressed through an open pocket between each contact with a dryer cylinder face, the pockets being intermittent with the faces so as to give the effect of alternating heating and releasing actions.

Although the web, when held against the dryer faces, does not shrink, it is known that, as soon as it is released between the dryers and the vapors leave the web being held under tension between the dryers, drying and contraction do occur, and simultaneously. This results in the creation of tensions in the web, with resultant contraction tending to narrow the web width. As a result, in all papers, and particularly in hydrated papers, the width of the dry web is significantly reduced. With hydrated papers, for example, greaseproof papers, such width loss can be as much as 10 percent or more. Worse, this contraction causes cockles and sheet stresses, and of course limits the width of the end product.

To a degree, these adverse characteristics may be limited by using porous woven webs on the dryers instead of the older and better known dryer felts for the reason that part of the generated vapors are thus enabled to pass through the open mesh of the fabrics.

It is a feature of this invention to accelerate this release through penetrating the pores of the felts and forcing the vapors away from the paper web surface. Drying on paper machine dryers is vastly accelerated through the inducement of the rapid release of the vapors while the wet web is passing over the dryer surfaces.

Also hereby, lateral contraction of the web, during drying, is radically reduced, wherefore internal stresses, cockles and streaks, and web unevenness are accordingly reduced. In addition, a wider dry paper web can be made, thereby producing greater trim or additional web area at the same paper machine speed.

Additionally, the use of higher dryer temperatures is allowed through the means of the forced evaporation which creates high vapor pressures through locally induced surface velocities, resulting in surface cooling.

To obtain a more unifonn condition of dryness, controlled blasts of pressurized heated air are forced through narrow restricted orifices against and through the dryer fabric, and against the web thereunder, which web is supported by the cylinder or drum transversely across the entire width of the machine, the force of the air being such as to induce the rapid release of the vapors from the immediate area.

Other objects and advantages of the invention will become apparent from the following description, when taken in conjunction with the supporting drawing in which:

The FIGURE is a fragmentary view, in side elevation, of a typical dryer section of a papermaking machine having upper and lower rows or tiers of drying cylinders and illustrates the drying technique in accordance with the preferred form of the invention.

The FIGURE shows an upper row or tier or bank of drying cylinder or drums 10, 12, and a lower row or tier or bank of drying cylinders, 16, which cylinders rotate in directions as indicated to advance a wet paper web W in entrained manner over successive cylinders, the web being alternately in contact with the drying cylinders of the respective rows.

An upper dryer fabric or felt is supported above the web by a plurality of guide rolls 22, 24, 26 and their disposition is such as to urge the upper dryer fabric against the upper parts of the upper drying cylinders with web W therebetween, the web being held against the upper part of the respective drying cylinders l0, 12.

Similarly, a lower dryer fabric or felt 30 is supported below the web by a plurality of guide rolls 32, 34, 36 and the lower dryer fabric holds the web against the lower parts of the respective drying cylinders, 14, 16.

The upper dryer fabric is supported out of engagement with the web while the web passes under lower dryer cylinders 14, 16 and the lower dryer fabric is out of contact with the web while the web passes over the upper dryer cylinders 10, 12 so as to define and confine a plurality of open ended upper pockets 41, 42, and a plurality of open ended lower pockets 51, 52, a pocket, by definition in paper machine dryer practice, being the space bounded by a dryer cylinder on one side and by the web flowing to and from this cylinder on the two sides thereof.

That is, the web passes alternatingly between rows, with the web being exposed in its travel from tangency with a cylinder of a first row to tangency with a cylinder of a second row and thence back from the last named cylinder to the next cylinder of the said first row.

The upper and lower drying cylinders are steam or otherwise heated and the web is initially heated by the drying cylinders to evaporate water therefrom.

Stationary manifold means are provided for serving the web and fabric across the width of each drying cylinder or drum for the production of more uniform drying.

There may be one or several of such manifold means disposed in confronting relationship with fabric web and the face of each drum.

Each manifold or plenum 66 is charged with heated dry air delivered from a source 60 by way of suitable ducting 62 along which a pressurizing means 64 may be disposed. From the manifold or plenum, the air is discharged via an elongated orifree 68 in the form of a slot so that the air is discharged radially toward the run of the fabric at a point at which the web is entrained around the cylinder and between the fabric and cylinder.

To best achieve the effect, an open-mesh dryer fabric, many times more air permeable than the conventional dryer felt, is employed so that air may be directed toward the permeable fabric in the form of a jet at relatively high linear velocity with the kinetic energy of the jet driving it through the fabric and to the surface of the web.

The air is given a substantial component of velocity so as to purge the moisture from the paper web and the open spaces in the fabric.

Under such conditions, most of the air in the jet passes into and through the fabric, relatively little thereof being lost and wasted.

The air, having been driven through the fabric and onto the web, is reflected outwardly by the cylinder face, carrying moisture outwardly therewith, thus purging the web of moisture, and inducing a further flow of vapors from within the web.

The jet is distributed in the cross machine direction so as to achieve a desired moisture content along substantially the en tire machine width.

The air is heated to high temperatures of a degree greater than those heretofore used on such general type of dryer, temperatures in the general range of 250320 F. being practical. Such hot air has a moisture-carrying capacity many times that of air in the normally used dryer range of l-220 F.

Significantly, the high temperature is not damaging either to the web or to the fabric, due in large measure to the realized cooling effect of the forced evaporation.

The heated air is delivered under a pressure adequate to force the air through the felt and is directed from its manifold at an angle generally perpendicular to the felt and web at the point of tangency to the drying cylinder.

The manifold is spaced from the felt with sufficient clearance to avoid contact in the event of any irregularities in the felt or fabric structure. The evaporation of vapors from the unsupported web, while passing from one dryer to another, is thereby greatly reduced.

As earlier explained, a normally substantial amount of the air evaporates and accumulates in the pockets because there is a continued evaporation from the web after it leaves actual physical contact with each drying cylinder. By evaporating moisture while the web is in contact with the dryers a reduction in accumulation in the pockets can be accomplished.

Most webs encountered in the drying of paper are not only thin but also relatively weak due to the weight of water therein contained and because the interfiber bonding, giving the paper its strength, is not fully developed until the paper is dry. For this reason, I avoid the use of such airblasts or jetstreams as issue from the usual types of ventilating devices and directed toward the comparatively long and unsupported draws of weak web in passing from one dryer to the next.

Changes in drying rates can be realized in a matter of seconds. Therefore, the final moisture content of the paper responds rapidly to control and defines a more satisfactory procedure than conventional attempts at moisture control by varying steam pressure in the drying cylinder, where a substantial time interval generally elapses after steam pressure is changed before the effect is perceptible in the paper.

The air flowing from the manifold is ltilled of all transverse velocity by virtue of the narrowness of the orifice so as to allow uniformity of delivery.

Vanes may be used, if desired, on opposite sides of the orifice to provide a better distribution of the air onto the fabric as an air jet.

operationally, the blower provides a supply of heated relatively dry air from the duct in sufficient volume to penetrate the fabric and scour the web surface.

It may be preferential to confine these vapor release mechanisms to the intermediate part of the dryer section as they may not be as useful and/or practical in the first percent of the web travel through the dryer section for the reason that relatively little evaporation from the web accrues as it is first being heated up to drying temperatures. Similarly, the last 5 or l0 percent of the web travel in the dryer section demonstrates little gain because evaporation rates have fallen so low in this range.

Although there is considerable turbulence, intentionally so, the air has little remaining lateral velocity, but ample pressure for the discharge of a thin jet through the delivery slot.

It will be understood that particulars of dimensions and pressures are merely by way of exemplification and not by way of limitation, a wide variety of changes being conceivable.

Typically, the hot air may be passed into a manifold of approximately 2 inches to 8 inches in inside diameter, depending upon paper machine width, and the slot or orifice may be approximately one-sixteenth inch to one-fourth inch in width for the full transverse machine width. With such an orifice, I have used air velocities of approximately 4,000 f.p.m. Contrariwise, experimentation indicates that orifices ranging from 0.10 inch to 1.00 inch are practicable, with velocities ranging from 2,000 f.p.m. to 12,000 f.p.m.

Through use of the invention, the drying capacity of a paper machine is markedly increased. The useful life of a dryer felt is noticeably augmented. Operating problems such as web streaking are reduced. The adverse random effects of ambient air movements are substantially eliminated. The steam consumption required for drying is reduced. And all of these ad vantages are available at low capital cost and low operating cost.

I claim:

i. In a drying system in papermaking, the steps of radially directing a jet of high-temperature high-velocity air under pressure through a restricted orifice across the machine width onto and through a porous fabric toward the wet paper web entrained between the fabric and the periphery of a heated dryer drum with the web being held against the drum surface for forced impingement on and through the fabric with the creation of high locally induced vapor pressures and the inducement of a rapid release of such generated vapors with the reflected air outwardly from the surface of the drum.

2. in the drying system as set forth in claim l, the jet of heated dry air having a component of velocity sufiicient to purge the vapor created by the heat of the drum for reducing the amount released in the pockets between adjacent drums and to reduce web tension and lateral web contraction.

3. In the drying system as set forth in claim 1, the use of high temperature high velocity air through the fabric serving as a means for eliminating streaks of uneven moisture in the paper web.

t. In a method for drying a web, the steps of:

moving the web towards and downwards in alternating manner between and around a plurality of rotating heated drying cylinders arranged in two tiers, one above the other,

pressing the web into sequential contact with the surfaces of the drying cylinders by means of an endless permeable dryer felt,

blowing heated dry air radially inwardly through a restricted orifice onto and through the dryer felt: for forced impingement to one side of the web adjacent the dryer felt for releasing and blowing away the vapor while the web is being held against the hot dr er surface. 5. In apparatus for drying a we comprising:

an upper tier of heated and rotatably mounted dryer cylinders,

a lower tier of heated and rotatably mounted dryer cylinders,

at least one endless porous fabric for pressing the web sequentially and alternatingly against the dryer cylinders in each of the upper and lower tiers,

supply means for supplying a heated drying medium,

a slotted manifold means disposed adjacent each dryer cylinder for blowing at high velocity the heated drying medium supplied thereto from the supply means radially outwardly to and through the fabric and to the outer side of the web entrained around the respective dryer cylinder for the creation of high locally induced vapor pressures and the inducement of a rapid release of the vapors while the web is passing over the respective dryer cylinder.

6. In a method of drying a web as it is passed through an entrained position around a heated drying cylinder with a dryer fabric holding the web against the surface of the drying cylinder for absorbing the heat therefrom, the step of directing a blast of high-temperature high-velocity air radially toward the surface of the drying cylinder and through the dryer fabric with the force of the air being sufficient for inducing the rapid release of the vapor while the web is being held against the cylinder surface and for eliminating streaks of uneven moisture in the web by facilitating selective water removal therefrom.

Claims (6)

1. In a drying system in papermaking, the steps of radially directing a jet of high-temperature high-velocity air under pressure through a restricted orifice across the machine width onto and through a porous fabric towaRd the wet paper web entrained between the fabric and the periphery of a heated dryer drum with the web being held against the drum surface for forced impingement on and through the fabric with the creation of high locally induced vapor pressures and the inducement of a rapid release of such generated vapors with the reflected air outwardly from the surface of the drum.

2. In the drying system as set forth in claim 1, the jet of heated dry air having a component of velocity sufficient to purge the vapor created by the heat of the drum for reducing the amount released in the pockets between adjacent drums and to reduce web tension and lateral web contraction.

3. In the drying system as set forth in claim 1, the use of high temperature high velocity air through the fabric serving as a means for eliminating streaks of uneven moisture in the paper web.

4. In a method for drying a web, the steps of: moving the web towards and downwards in alternating manner between and around a plurality of rotating heated drying cylinders arranged in two tiers, one above the other, pressing the web into sequential contact with the surfaces of the drying cylinders by means of an endless permeable dryer felt, blowing heated dry air radially inwardly through a restricted orifice onto and through the dryer felt for forced impingement to one side of the web adjacent the dryer felt for releasing and blowing away the vapor while the web is being held against the hot dryer surface.

5. In apparatus for drying a web comprising: an upper tier of heated and rotatably mounted dryer cylinders, a lower tier of heated and rotatably mounted dryer cylinders, at least one endless porous fabric for pressing the web sequentially and alternatingly against the dryer cylinders in each of the upper and lower tiers, supply means for supplying a heated drying medium, a slotted manifold means disposed adjacent each dryer cylinder for blowing at high velocity the heated drying medium supplied thereto from the supply means radially outwardly to and through the fabric and to the outer side of the web entrained around the respective dryer cylinder for the creation of high locally induced vapor pressures and the inducement of a rapid release of the vapors while the web is passing over the respective dryer cylinder.

6. In a method of drying a web as it is passed through an entrained position around a heated drying cylinder with a dryer fabric holding the web against the surface of the drying cylinder for absorbing the heat therefrom, the step of directing a blast of high-temperature high-velocity air radially toward the surface of the drying cylinder and through the dryer fabric with the force of the air being sufficient for inducing the rapid release of the vapor while the web is being held against the cylinder surface and for eliminating streaks of uneven moisture in the web by facilitating selective water removal therefrom.

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