High speed paper drying

An improved paper drying process and machine are described in which the paper web is supported independently of velocity-induced stresses thus permitting operation at speeds significantly in excess of the prior art for each paper grade. The process requires: (1) transporting the web on a supporting fabric that travels from the last press nip through at least the initial portion of a drying section; and (2) holding the web onto its supporting fabric by employing forces normal to the major web surfaces sufficient to overcome those forces which tend to lift the web from its supporting fabric. The web on its supporting fabric travels a serpentine path through the drying section about drying cylinders with the web alternating between direct contact with a drying cylinder followed by indirect contact with the subsequent cylinder. The principal holding forces are preferably pressure differentials created by vacuum boxes arranged to effectively hold the web to its supporting fabric on all portions of the web-fabric combination where the web is not in direct wrapping contact with the drying cylinders. The fabric supports the web at least until the web has attained sufficient strength through increased dryness to resist breaking stresses at the selected machine speed. The products made by the process of this invention possess a unique toughness or stretchability not found in conventionally prepared papers that have been strained or stretched during manufacture. Processes and machine arrangements designed to balance stretchability with certain desirable stiffness properties are disclosed. Pulp furnishes may now be selected for their contribution to product qualities, such as higher finished product tensile strength, rather than principally for wet strength. For example, chemical pulps may be significantly reduced or eliminated from newsprint furnishes where their purpose has been principally to permit economic paper machine speeds. The invention makes attainable speeds approaching twice current operating levels for each paper grade.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to paper machine productivity and means for 
attaining machine speeds significantly in excess of the prior art. The 
invention is concerned with eliminating stresses that act on the wet paper 
sheet as the web travels through the drying portions of the paper machine. 
2. Prior Art 
In papermaking, after sheet formation, the paper web, supported on one of a 
series of porous felts, passes through a series of press nips that 
mechanically express water from the sheet. The wet web at about 35-45% 
fiber content is then contacted with a series of heated drums or cylinders 
that evaporate water from the web to a finished dryness of about 90-95%. 
The web is, conventionally, unsupported at many points in the process as 
it travels through the later press nips and between the heated drums in 
the dryer section. 
Machines that are not forming or drying limited are run at increasing 
speeds to gain production. A practical limit is always reached where 
increased productivity expected by further increases in speed is nullified 
by increased production losses due to sheet breakages and product defects. 
For example, newsprint machines appear to be limited to about 3,500 
ft./min. (1070 m/min.) by current technology. This practical machine speed 
limit differs for each paper grade such as newsprint, liner, medium or 
fine paper. Further, within each grade of paper, the speed limit differs 
for differing basis weights. 
Observations of operating paper machines show that, as speed increases, 
breaks in the web generally occur at those points in the process where the 
web is: (a) transported unsupported through the process while relatively 
wet and weak, such as occurs in transferring the web from the press to the 
drying section and between drying section rolls or cylinders, or; (b) 
required to change direction quickly while in adhesive attachment to a 
supporting element, such as occurs when the web is picked up by a felt 
from the forming wire. 
When the speed of the machine is held constant, breakages increase with 
decreased paper basis weights within each grade. These breakages occur 
where the web is transferred from one machine element to another by 
pulling or peeling the web from the element to which it is adhered, such 
as occurs at transfers from forming wires to press felts and from press 
rolls to dryer sections. 
For further discussion regarding press section stresses and pressure 
differential means, see U.S. patent applications Ser. No. 091,212 now 
abandoned filed Nov. 5, 1979 and Ser. No. 091,211 now abandoned filed Nov. 
5, 1979, now continuation-in-part Ser. No. 625,313 now U.S. Pat. No. 
3,986,996, both authored by Keith Thomas of Weyerhaeuser Company, and 
incorporated herein by reference. 
Edge "flutter" in the dryer section may also be observed. Flutter tends to 
cause edge "stretch," resulting in wrinkling defects in the finished 
product. Differential stretching at the web edges also imparts instability 
or "curl" to the finished paper. 
It is well known that, as a paper web passes through the dewatering and 
drying process on the paper machine, it, in general, gradually develops 
strength with increased dryness. Practicalities determine that the overall 
speed of the paper machine be limited to make sure that stresses in the 
web do not approach, at any point, too closely to the paper web's breaking 
strength. Without a more detailed knowledge of the strength of the web and 
the stresses operating on it as it passes through the machine papermakers 
have, in the past, attempted to avoid in an empirical way the increased 
sheet breakages observed with increased speed and decreasing paper 
weights. 
These efforts include press and dryer section designs where the wet web 
travels with a porous felt or fabric during transit through at least a 
portion of either section. 
Mahoney, in U.S. Pat. No. 3,503,139, provides a fabric intended to support 
the wet sheet throughout its serpentine travel from drum to drum in the 
dryer section. What actually happens as machine speeds increase is that 
the web is lifted and separated from its supporting fabric, particularly 
at points where the web approaches and departs drying cylinders. The 
lifting forces are centrifugal forces exerted on the web at certain 
locations in the machine and air currents caused by the turning drums and 
moving belts in the dryer section. These forces are generally non-critical 
in conventional systems only because these systems operate at low speeds. 
At higher machine speeds, however, these stresses increase in magnitude to 
cause breakages. Whenever the web is lifted from its supporting fabric, it 
is subjected to velocity stresses as if the fabric were not present. 
It should be noted that the Mahoney web, as is typical of the prior art, is 
totally unsupported at the transfer from the press section to the first 
dryer cylinder. Thus, at this transfer, in addition to peeling stresses, 
the web is also subject to the velocity-related stresses noted. 
In Mahoney, the web is, alternatively, partially wrapped in direct contact 
with one drum followed by indirect contact with the next drum. Mahoney 
compensates for the loss in heating effectiveness occasioned by the 
indirect contact of the web with the heated drum surfaces on alternate 
drums by operating those drums at higher temperatures. 
In an improvement over Mahoney, Soininen et al., in U.S. Pat. No. 
3,868,780, adds a number of rolls to the Mahoney system to guide the web 
into direct contact with each of the heated drums during transit of the 
web through the dryer section. In recognition of the increased likelihood 
of "flutter" separating the web from its support on the longer runs 
between dryer drums, the Soininen guide rolls operate under vacuum that 
adheres the web to their supporting surfaces. There is also an overall 
vacuum system to help hold the web onto supporting fabrics. 
The Soininen system has a number of operating impracticalities. The guide 
rolls tend to cause a relatively large differential movement between the 
tender web and the fabric, resulting in "scuffing" damage to the web. The 
complexity of the system and extra components required introduce 
substantial capital costs. Operating costs are high because of the power 
required to drive the extra components and also since cleanout of paper 
after breakages appears to be difficult. Heat applied to only one side of 
the sheet, as in Soininen, results in paper products having different 
characteristics for each surface. These differences can cause printing 
nonuniformities when both sides must be printed. 
In sum, the prior attempts to improve paper machine productivity by 
increasing machine speeds have generally failed because their designers 
have, up until now, had only an imperfect understanding of where in the 
papermaking process stresses operating on the moving sheet become critical 
and limit speed. Also lacking has been an understanding of how paper 
machine conditions, such as those affecting sheet temperature, for 
example, affect the ability of the sheet to resist velocity stresses. 
SUMMARY OF THE INVENTION 
A principal object of the papermaking processes and machines of this 
invention is to reduce and, to the extent possible, eliminate or control 
those stresses ordinarily operating on the wet web in the drying sections 
of the paper machine that are a function of velocity of the sheet and 
which limit machine speed. These stresses limit production speeds because 
of the threat of downtime occasioned by sheet breakages and product 
quality defects which papermakers expect as speed is increased. 
It is an object of the invention to present a suitable paper machine 
equipment design that at reasonable capital cost accomplishes the 
elimination or control of velocity stresses. The new machine design 
employs familiar papermaking equipment thus permitting back fitting of 
existing machines. These improvements permit substantial reduction of pulp 
furnish costs or operation of the dryer section significantly in excess of 
prior art speeds for any particular grade. 
A paper furnish may now include a lesser amount of expensive stronger 
pulps, such as chemical pulp in newsprint grades, that heretofore have 
been added to the furnish largely to increase the speed rates at which the 
machine will operate effectively. The paper furnish may now be selected 
more for its impact on the finished paper product rather than to meet a 
processing requirement for wet strength early in the drying section. While 
some newsprint machines, for example, operate without chemical pulps as 
components of their furnish, they do so at much lower machine speeds than 
those operating at state of the art speeds where chemical pulps may 
constitute in excess of 35% of the furnish. 
The objects of this invention in eliminating velocity stress are 
accomplished by the process of: (1) transporting the web on a supporting 
means from the last press nip through at least a first portion of the drum 
or cylinder dryer section of the paper machine until the paper web has 
attained sufficient strength to be self-supporting at a given machine 
speed through increased dryness; and (2) holding the web onto the 
supporting fabric means by employing forces normal to the major web 
surfaces sufficient to overcome velocity-related stresses on all portions 
of the web during transporting at least until the web has attained 
sufficient strength to be self-supporting at the selected machine speed. 
In the process of this invention at least a portion of the holding forces 
is created by a pressure differential forcing the web against its 
supporting means. The pressure differential means operate effectively in 
holding the web to its supporting means along substantially the entire 
length of travel of the web between dryer cylinders through at least a 
first portion of the series of dryer cylinders. 
Each paper product produced by following the process of this invention 
possesses unique characteristics resulting from the reduced tensile or 
machine direction stress it experiences during its transit from the last 
press nip through the first portion of the dryer section. Reduction of 
stress results in finished paper products having retained "stretch," 
extensibility or toughness that is typically stressed out of conventional 
papers. 
The paper machine invention is a modification of the conventional machine 
design which typically consists of a series of press nips followed by a 
series of heated dryer cylinders or rolls. 
The machine improvement comprises: (1) a fabric means that receives the wet 
web from the last press nip and transports the web through the process 
until the web, through increased drying, has attained sufficient strength 
to be self-supporting at the selected machine speed; and (2) means for 
applying forces normal to the major web surfaces for holding the wet paper 
web onto its supporting fabric wherever the web would otherwise be 
subjected to the above-noted velocity stresses. 
The drying cylinders are arranged in a double row series. The web, 
supported on its fabric, is transported in a serpentine manner throughout 
the dryer section, partially wrapping each cylinder. The wet web is 
sequentially carried: into direct wrapping contact with a rotating heated 
cylinder surface; between the heated cylinder to a next heated cylinder; 
into indirect wrapping contact with that following cylinder, with the 
fabric in direct wrapping contact with its surface; and, between the 
indirect wrapped cylinder and a next heated cylinder in the sequence. This 
pattern is repeated at least until the web is dried to self-supporting 
strength, relative to machine speed. 
A portion of the holding means of the invention comprises a series of 
pressure differential zones adjacent to the path of the fabric. The zones 
extend adjacent to the web-supporting fabric substantially along its path 
wherever it is not in either direct or indirect contact with a cylinder 
surface. A differential pressure means operating on the zones forces the 
web against the fabric. 
The holding means may comprise vacuum boxes, preferably positioned adjacent 
to those dryer cylinders about which the fabric is interposed between the 
web and the surface of the cylinder. The vacuum box defines pressure 
differential zones adjacent to the surface of the fabric opposite the 
surface of the fabric in contact with the wet web. 
A process and machine of this invention for controlling the amount of 
shrinkage permitted in a paper web in the drying section to preserve 
stretchability while permitting a certain amount of strain to be exerted 
on the web to improve stiffness for curl resistance are described. 
This process involves drying the web on a first fabric wrapping a first 
group of drying cylinders, pressure differential means holding the web on 
its supporting fabric as the web travels from drying cylinder to cylinder 
and about said cylinders, followed by drying the web on a second fabric 
wrapping a second group of drying cylinders arranged similarly to the 
first group. The second group of drying cylinders operates at a rotational 
peripheral velocity less than that of the first group of drying cylinders, 
the speed being selected to attain desired shrinkage and stiffness. The 
web is transferred from the first fabric onto the second fabric by 
pressure differential means acting on the web to effect transfer without 
subjecting said web to peeling or velocity stresses. 
Three preferred arrangements and means are shown for accomplishing the 
transferring step. The first scheme involves transferring the web from the 
first fabric onto a transfer roll or cylinder by means of its pressure 
differential surface means. The transfer roll brings the web into contact 
with the second supporting dryer fabric, which is subject to a pressure 
differential means which is of sufficient strength to transfer the web 
from the transfer roll surface onto the second dryer fabric for continuing 
through the drying process. 
A second transferring process requires converging the web supported on the 
first fabric and subject to a first pressure differential means into 
contact with the second supporting fabric subject to a second pressure 
differential means. The web is momentarily sandwiched between the first 
and second fabrics with the second differential means exerting a normal 
force on the web sufficient to transfer the web from the first fabric onto 
the second fabric. 
A third transferring process is a combination of the first two with the 
transfer roll being wrapped with a transfer fabric. The web is transferred 
by pressure differential means from the first fabric onto a transfer 
fabric. The transfer fabric traveling about the transfer roll converges 
the web into contact with the second fabric. A pressure differential means 
acting on the second faric effects transfer of the web, momentarily 
sandwiched between the second fabric and the transfer fabric, from the 
transfer fabric onto the second fabric.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
1. Inherent Strength of the Paper Web 
Paper web strength, first of all, is a function of the paper furnish being 
processed. This property is a function of the species of wood making up 
the fibers. For example, papers made of softwood fibers, such as Douglas 
fir, are stronger than paper made of hardwood fibers, such as alder. 
Strength is also a function of the pulping process used in separating the 
fibers from the wood raw material. For identical wood species, groundwood, 
for example, is known to have an appreciably lower strength at a given 
moisture content than chemical pulps made by the sulfite or kraft process. 
For any pulp furnish, the strength of a paper sheet is primarily a function 
of its moisture content. Lyne and Gallay, "Measurement of Wet Web 
Strength" Tappi Vol. 37, No. 12, (December 1954). The ability of a paper 
web to resist stresses without breaking at any point in the papermaking 
process is, therefore, principally related to its moisture content. 
In general, the moisture content of the paper web decreases as it passes 
through the papermaking process, with the strength of the paper web 
increasing as the web increases in dryness. However, there is a marked 
interruption in strength gain as the web is passing through the early drum 
drying stages. 
Strength actually decreases on the first few dryer drums, after transfer 
from the last press nip, as the web experiences a rapid increase in 
temperature. At this stage, the temperature of the web is approaching the 
boiling point of the moisture present. 
This not previously recognized and quantified strength reduction is a 
temperature phenomenon. The phenomenon has remained obscured perhaps 
because strength testing, including the work conducted by Lyne and Gallay 
cited above, has been done at 70.degree. F. (21.1.degree. C.) as a matter 
of standardized testing procedure to permit comparisons between pulps. The 
temperature effect on testing has thus been known but the significance of 
the degree to which the actual strength of the sheet in the process is 
effected by processing temperature has escaped the attention of 
papermakers. 
Referring to FIG. 1, the significant decrease in sheet strength resulting 
from increasing temperature for a typical newsprint furnish is shown. The 
family of curves 1, 2, 3 and 4 shows the temperature effect on strength 
for a newsprint furnish. The curves are for 70.degree., 100.degree., 
150.degree. and 200.degree. F. (21.1, 37.7, 65.6, and 93.5.degree. C.,) 
respectively. The data used in plotting FIG. 1 where derived from samples 
of a newsprint, comprising a combination of groundwood and chemical pulp. 
In FIG. 1, paper web strength, in terms of "breaking length" is shown as a 
function of "sheet dryness", in weight percent fiber. Breaking length, 
expressed in meters, is the length of a strip of paper which would break 
of its own weight if suspended vertically. Breaking length is related to 
tensile strength which is the force, parallel with the plane of the paper, 
required to produce failure of a specimen of specified width and length 
under specified conditions of loading. 
Curve 5 shows sheet temperature as the sheet proceeds through the 
papermaking process on a typical machine. The temperature of the sheet 
remains relatively unchanged from the head box through the last press nip, 
indicated at point 6, on curve 5. As the web contacts the first few dryer 
drums, the temperature rises extremely rapidly. Thereafter the temperature 
remains relatively constant as drying continues. 
The strength of the newsprint paper sheet as it passes through the machine 
is shown by dashed curve 7. There is an increase in strength initially as 
the sheet is dewatered on the forming wire. There is a relatively lower 
rate of increase through the press section. A substantial decrease in 
strength follows as the web contacts the first several drum dryers where 
the water and web are heated with little change in dryness. While some 
water is driven off, the drying effect is more than offset by a decrease 
in web strength due to the temperature effect, previously demonstrated by 
curves 1-4, resulting in a significant net decrease in strength. Thus the 
discontinuity in increasing strength as the sheet increases in dryness, at 
the first few drums in the dryer section, is the result of the sudden 
increase in temperature of the web. 
Curve 7 of FIG. 1, a composite of the strength curves 1-4 and process 
temperature curves, shows the strength of the sheet of the particular 
newsprint furnish examined, as a function of dryness and temperature, as 
it travels through the papermaking process. If velocity stresses, for 
example, exceed the strength of the sheet, related to the dryness, 
temperature and paper furnish, sheet breakage will occur. 
2. Identification and Elimination of Productivity Limiting Stresses 
Prior art paper machine operating speeds are limited or bottlenecked by web 
breakages of the weak, wet fiber web. As previously noted, the critical 
stress points in the process, observed most frequently, are: 
(a) where the web experiences large angular changes; 
(b) where the web is allowed to run unsupported and is thus subjected to 
velocity stresses; and 
(c) where the web is pulled or peeled from a machine element to which the 
web is adhered, e.g., a press roll. 
The relationship between velocity and the ability of a web, made of a given 
material, to survive without breaking, as the machine speed is increased 
can be analyzed mathematically. The quantified results are confirmed by 
actual observations of the critical locations in the process. 
Inspection of the paper sheet as it passes unsupported through the 
conventional paper machine shows that the paper web does not travel 
without a certain amount of slack building up in the web, particularly as 
it travels between drying cylinders. This is so because only a limited 
tension can be exerted on a relatively weak paper web in pulling or 
"drawing" it through the process without causing a breakage. Bulges and 
series of standing waves tend to build up in the slack web, their form or 
frequency dependent upon sheet velocity, the distance the web travels 
unsupported and air currents generated by operating machinery. 
The forces exerted on the web as it moves through a standing wave, as 
described above, or about a roll may be viewed in terms of conventional 
centrifugal force analysis. The minimum loads or stresses parallel with 
the plane of the paper that a fiber paper web experiences as it travels 
through the machine can then be calculated in terms of tensile stress. 
For an element of a dry paper web, the tensile stress due to the 
centrifugal forces exerted on the web as it passes through, for example, a 
standing wave, generally circular or sinusoidal in profile, may be 
expressed as: 
##EQU1## 
where 
T.sub.s =tensile stress=the tensile force in the sheet resisting 
centrifugal forces acting on the element per unit thickness of the sheet, 
v=liner speed of the sheet, g=gravitational acceleration, and 
.sigma.=density of the sheet=basis weight.div.thickness of the sheet, 
wherein basis weight is the weight of fiber in a standard area of paper. 
The tensile stress, T.sub.s, can be expresed in terms of "equivalent 
breaking length" (EBL.sub.v) as follows: 
##EQU2## 
This expression is based on dry density. For wet webs, a dryness factor, 
d=(100/% dryness,) must be introduced. 
Thus 
##EQU3## 
This analysis shows that web stresses T.sub.s and EBL.sub.v are independent 
of the radius traveled by the sheet, its basis weight and its dry density. 
These stresses are inversely proportional to sheet dryness and constant 
for any velocity and dryness. 
Referring to FIG. 1, curves 101, 102, 103, 104 and 105 show velocity stress 
expressed as equivalent breaking lengths versus dryness for machine speeds 
of 2,000, 3,000, 3,300, 4,000 and 5,000 ft./min. (610, 915, 1006, 1220 and 
1525 m/min.), respectively. The curves show the minimum strength the web 
must have in order to travel unsupported at the selected speed. 
These calculated stresses are minimum stress loads because generally there 
are additional stresses caused by local flapping or fluttering, both 
longitudinal and cross machine, particularly at the edges of the web. Air 
currents, generated by the rapidly turing rolls, fabrics and other 
machinery, typically cause these stresses on the moving sheet. 
Stresses calculated by the above analysis are valid even for those cases 
where prior workers have attempted to support the paper web on a fabric or 
felt as, for example, Mahoney, cited above. This is so because air 
currents tend to penetrate a porous fabric and "bulge" or lift the paper 
sheet from its supporting contact with the fabric. These bulges cause the 
web to be subjected to the above-described velocity stresses. Also, on 
those dryer rolls where the fabric directly wraps the drum with the web on 
the outside, the web tends to separate from the fabric under the 
centrifugal stresses resulting from passing about the rotating roll. In 
situations suh as Soininen, et al., cited above, describes, the web 
leaving the last press nip adheres to the solid press roll and must be 
peeled therefrom. In the gap between the surface of the press roll and 
initial contact with a supporting fabric, the web is unsupported and thus 
subjected to web breaking velocity stresses. 
A conclusion to be drawn from the analysis of the velocity stresses acting 
on the web is: the web web must be transported on a supporting means 
wherever it would be, if not supported, subjected to speed related 
stresses that are likely to exceed the breaking strength of the web. FIG. 
1 indicates, for a particular paper, that the web must be supported 
whenever "breaking length" stresses, for example the velocity stress 
levels indicated by curves 101-105, are above the strength curve 7 levels 
at any point in the process. 
Analysis of the failure of past attempts at supporting the web leads to a 
further conclusion that a means must be provided to ensure that the paper 
web is held onto its supporting means, in order for the web to remain 
independent of the velocity forces that tend to act on the separated web. 
Failure to recognize this need for a means to hold the web onto its 
supporting fabric characterizes, in general, the prior art designs. 
It has long been the experience of papermakers that the productivity of a 
paper machine is reduced when there is a significant reduction in the 
basis weight of the grade being manufactured. This production rate penalty 
is accepted because lightweight papers often command a price premium in 
the market. Machines making lightweight paper grades are conventionally of 
the type that utilize a single felt in the last press nip, pressing the 
web against a smooth hard-surfaced roll. The sheet adheres to these rolls 
requiring a peeling or tensile stress to be exerted on the web to pull the 
web free of the roll surface. 
The forces in the sheet required to pull it from a press roll have been 
defined by Mardon and others. See Mardon, "The Release of Wet Paper Webs 
from Various Papermaking Surfaces," APPITA Vol. 15, No. 1 (July 1961). 
Peeling stress is the primary speed-limiting factor in conventional paper 
machines when basis weights are reduced, aside from velocity stress 
considerations. The peeling force per inch of width required to remove the 
web from a smooth press roll is independent of the sheet weight. However, 
reducing the basis weight by reducing the thickness of the sheet increases 
the peeling stress experienced by the sheet. If the basis weight is 
reduced by one-half, the stress exerted in the web is doubled. Peeling 
stresses are discussed in more detail in the above-identifed concurrently 
filed U.S. application, Ser. No. 091,212. 
Other factors affect the operating speed of a given machine, including 
limitations imposed by forming, pressing, drying and sheet treatments such 
as coating, sizing, calendering and the like. Factors other than those 
imposing stresses on the sheet during pressing and drying are outside the 
scope of the invention and, for discussion, are assumed to be met by the 
strength of the sheet. In other words, the prior art machine is speed 
limited by the velocity stresses imposed on the sheet where it is 
unsupported in the press and dryer sections. 
3. Detailed Description of the Invention 
The above analysis clarifies the velocity stress, peeling stress and basis 
weight interactions which place speed and paper furnish restrictions on 
the prior art processes and machines. Generally, the velocity stresses are 
speed limiting for heavier weight sheets. These stresss have been shown 
earlier to be independent of the basis weight. As basis weight is reduced, 
peeling stresses increases until they become the predominant speed 
limitating factor. 
The elements of this invention eliminate velocity stresses which currently 
limit machine speeds and productivity. Thus, a major advantage of the 
present invention is that since the papermaking process is made 
independent of velocity stresses the machine may be run at speeds limited 
only by drying rates, assuming peeling stresses are insignificant or 
controlled. Additionally, weaker furnishes may be substituted for 
expensive chemical pulps. 
Referring now to FIGS. 2 and 3, a preferred embodiment of the invention is 
shown. Paper web W is formed on wire 10. Pick-up roll 11 transfers the web 
onto press felt 12. The web W progresses, supported on the felt 12, 
through the first two press nips 13, 14. The web W is transferred to a 
belt 15 at the nip 14 for subsequent travel through the last two nips 16, 
17 of the press section. Felts 52 carry away water absorbed from the web 
at the nips 13, 16 and 17. After the last pressing nip 17, transfer roll 
18, with directional roll 51 in cooperation, effects a transfer of the web 
W from the belt 15 onto the dryer section fabric 19 for transport through 
the dryer section 20. 
The web travels on fabric 19 thereafter in a serpentine path through the 
dryer section 20 about each of the dryer drums successively. The web is in 
indirect wrapping contact with the initial drum 21, with the fabric in 
direct contact with the heated surface of the drum. The web is then 
transported into direct heat transfer contact with the upper drum 22. 
Thereafter the web is transported into indirect or direct contact with the 
cylinders in sequence through the dryer system. 
The characteristics of the web and machine conditions determine what 
holding forces adhere the web to its supporting means during transit 
through the machine. The sheet leaving the forming wire 10 is wet and 
adheres well to the pickup press felt 12 and press belt 15. Adherence of 
the web to the press belt 15, independent of velocity stresss, depends 
upon belt characteristics, such as low permeability and porosity, more 
fully discussed in the above-identified concurrently filed applications. 
The sheet after the press section will not, in general, adhere to the 
typical dryer fabric 19; in part, because the sheet, in passing through 
the dryer, becomes drier and more permeable, and; in part, because the 
dryer fabric 19 is much more permeable than press felts 12 and press belt 
15. Adherence forces, dependent upon surface tension forces between the 
web and a fabric become weaker and weaker and eventually ineffective as 
the web and fabric become drier and more permeable. 
Referring to FIGS. 2 and 3, a preferred means for applying pressure 
differential holding forces to the web to positively hold it to its 
supporting fabric 19 comprises a contoured vacuum box 30 (vacuum source 
not shown). The vacuum box 30, in general, fills dryer section "pockets" 
existing between cylinder rows and the traveling fabric 19. A vacuum box 
30 is positioned adjacent to each drum 21, 23, 25, etc., in the dryer drum 
section 20 where the fabric 19 wraps the drum surface directly with the 
web traveling on the fabric about the drum. A vacuum box between the 
pickup vacuum roll 18 which removes the web from press belt 15 and the 
first drying cylinder 21 will generally be necessary, depending upon 
actual physical layout of the drying section. None is required here 
because the first vacuum box 30 has been extended to bear upon transfer 
roll 18 to exert holding forces on the web. 
The suction box 30 is provided with four pressure differential surface 
zones or suction surfaces 31, 32, 33 and 34. Three of the suction zones 
31, 33 and 34 are adjacent the web-supporting fabric 19 as the fabric 
travels to and from a fabric-wrapped cylinder, for example, cylinder 23 of 
FIGS. 2 and 3. These suction zones 31, 33 and 34 extend, at least in 
effect, to create a pressure differential force acting through the fabric 
19 to hold the relatively impervious wet web W to the fabric surface, 
independent of any velocity stresses such as stray air currents or 
centrifugal forces. 
Referring to FIGS. 2 and 3, the suction zone 32, adjacent the portion of 
the drum 23 not wrapped by the fabric 19 ensures that a pressure 
differential force holds the fabric 19 and web W to the surface of the 
drum 23, overcoming centrifugal stresses that are exerted on the web as it 
travels about the drum. 
In a preferred embodiment, each bottom cylinder 21, 23, 25 is provided with 
a plurality of shallow circumferential grooves cut into the cylinder's 
outer surface, spaced across the face or length of the drum. These grooves 
40 are indicated at the periphery of each lower drum 21, 23, 25. The 
resulting pressure differential induced in the drum grooves 40 by suction 
zone 32 holds the fabric-web combination in supporting contact with the 
drum surface. 
Referring to FIG. 3, in a preferred embodiment of the invention, it is 
desirable to divide the vacuum box 30 internally into relatively high and 
low pressure differential zones depending upon what forces must be exerted 
on the web to hold it to its supporting fabric 19. FIG. 3 shows the vacuum 
box divided into four zones by walls 41 and seals 42, 43. Vacuum zone 32 
must operate at a relatively high vacuum in order to hold the web and 
fabric to the dryer drum 23 as they are subjected to centrifugal stresses 
during travel about the drum. Vacuum zone 34 must also operate at a 
relatively high vacuum in order for the zone forces to capture and to hold 
the web onto the supporting fabric as it departs direct contact with the 
dryer drum 22. Zones 31 and 33 may be operated at significantly lower 
vacuum values as they need only keep the web adhered to the fabric as it 
travels between the dryer drums where otherwise the web would be subjected 
to speed limiting stray air currents and minor centrifugal forces. 
In the prefered vacuum box 30, the divider walls 41 are apertured with 
adjustable orifices 44 which permit communication between vacuum zones 31, 
32, 33 and 34. The orifices 44 are typically adjusted so that evacuating 
zones 32 and 34 to create a high vacuum in thos zones causes evacuation of 
zones 31 and 33 at a lower rate. As a result, zones 31 and 33 operate at 
lower relative vacuum than zones 32 and 34, but sufficient to ensure that 
the web is held to supporting fabric 19 opposite zones 31 and 33. 
The vacuum box suction zones are designed to effectively provide sufficient 
pressure differential forces acting perpendicular to the major surface of 
the web to ensure that the web is held onto its supporting fabric 19 
regardless of machine environmental conditions, web characteristics or 
specific fabric or machinery factors which would otherwise operate to 
cause the web to separate from its supporting fabric. These factors, of 
course, influence the exact operational shape of box 30. It was discovered 
experimentally that vacuum zone 34, which initially operates on the web as 
it leaves direct contact with dryer drum 22 must exert its pressure 
differential forces on the web-fabric combination significantly prior to 
the expected line of departure of the web-fabric combination from the drum 
22. The zone 34 must operate on the web and fabric sufficiently in advance 
of the tangent line of departure in order to have sufficient time for the 
vacuum to remove air from the dryer fabric and establish forces sufficient 
to hold the web to the fabric. 
As a practical matter, a doctor blade may be provided to ensure complete 
removal of the web from web wrapped cylinders 22, 24 etc. In general, 
however, the web will travel adhered to the fabric at departure from the 
web wrapped cylinder as there is a layer of vapor between the hot cylinder 
surface and the web which prevents the web from adhering to the cylinder 
surface. This is a very different condition from that existing at the 
smooth press roll where the web is pressed into adherence with the roll 
surface and must subsequently be peeled from that surface at departure. 
The vacuum zone 31 need only operate from the line of departure of the web 
from the drum 23 up to direct contact of the web with the next drying drum 
24. 
A key practical feature of the vacuum box 30 of this invention is that 
contact between the rapidly moving fabric supporting means and other 
machine elements is minimized. Fabric wear and damage will inherently 
occur whenever the web comes into contact with a stationary, rigid 
surface. The most significant damging conditions occur in typical paper 
mill arrangements when a wad of paper comes between a fabric and the dryer 
drum and the resulting bulge contacts a rigid machinery surface. Such 
contact can destroy the fabric and, of course, cause a machine shut-down. 
As a solution to this problem the vacuum box 30 is provided with flexible 
seals 42 extending across the width of the machine. The seals are made of 
any resilient flexible material that will cause minimal damage to the 
fabric if the fabric, traveling at high speed, inadvertently contacts a 
seal. The seals extend, perpendicular to the fabric surface, as close as 
practical to the surface of the fabric without bearing against it. The 
seal 42 bends when a paper wad 100 bulges out fabric 19 in passing about 
the drum surface 22. 
The seals 42 must approach the fabric where the fabric-web combination is 
in contact with a solid surface, such as a dryer roll. Otherwise, air 
currents traveling with a moving fabric or roll will penetrate the fabric 
and lift the web from its supporting means exposing it to velocity 
stresses. 
Seals 43 may be made of more rigid materials since there is no wad damage 
problem. These seals 43 bear directly on the surface of the drum 23. 
End seals for the vacuum box 30 are shown in FIG. 4. The function of these 
seals is to preserve the vacuum in the box 30 while accommodating the 
passage of wads of paper through the system without damage to the fabric 
or box. The end wall 46 of the vacuum box is dimensioned to conform 
closely to the adjacent drum 23 where there is no danger of paper waste 
blockages. The portions of the end wall 46 adjacent the traveling fabric 
19 are fitted to allow a generous space between its edges and the 
traveling fabric to accommodate waste. The end seals 45 are attached to 
end wall 46 at pivot 47, near adjacent drum 23. At the upper end of the 
seal 45 springs 48 urge the seal leading edge 49 into close proximity to 
traveling fabric 19. An adjusting screw 80 attached to the end seal 45 
through nut 80a and stop 81 fixed to the wall 46 permits adjustment of the 
clearance between the seal leading edge and the fabric. The leading edge 
49 may be contoured to reasonably conform to the path that the fabric-web 
actually travels between the dryer drums. 
A wad of waste paper passing about the drum between the cylinder surface of 
drum 22 and the fabric forces the end seal 45 to pivot away from its 
normal position. After the wad passes, the spring 48 urges the seal back 
into its original position. The wad, upon issuing from between the drying 
cylinder and fabric, drops clear. Wads are not a problem at the bottom 
cylinders as the sheet is on the outside of the fabric where it wraps the 
bottom cylinders. 
As noted previously, air currents are created by the moving cylinders and 
flow adjacent to the moving equipment. In conventional designs, "bulges," 
wherein the web is slightly separated from its fabric, tend to occur at 
certain locations, as for example, where the web-fabric combination 
approaches and departs a drying drum. While static deflectors in the dryer 
cylinder "pockets" may reduce this problem, the vacuum box design of this 
invention is more positive and controllable 
It is advantageous to shape certain portions of the vacuum box 30 to 
deflect some of the air flow. The top surface 35 of the box 30 is formed 
into a curved surface to assist in deflecting air from entering the pocket 
area between the drums. Reduction in the amount of air that enters the 
pocket area reduces the amount of vacuum required and, hence, energy that 
must be provided to create the differential pressure necessary to hold the 
web onto its supporting fabric. 
To eliminate the chance of the fiber sheet wrapping an upper cylinder and 
causing high tensions in the fabric and on the cylinder bearings and 
gears, a doctor (not shown) is fitted to each upper cylinder. 
Cylinders on conventional machines are all typically gear driven. The dryer 
fabric is strong and can impose heavy, varying loads on the cylinder gears 
and bearings. To reduce capital costs and these loads, some cylinders may 
be free running, that is, driven by the fabric 19. 
The drying rate of drum dryers is dependent upon the arc of contact or 
degree of wrap of the paper web about the heat transfer surface of the 
drum. In the conventional paper machine, where the paper web is 
unsupported between drums, the actual arc of contact is considerably less 
than suggested by the geometry of the layout. Air bulges, as noted above, 
at the approach and departure of the web from the drum tend to separate 
the web from heat transfer contact with the drum surfaces. The 
introduction of a supporting means for the web during drying increases the 
arc of contact at the top cylinders, but results in interposing the fabric 
between the cylinder and web on the bottom cylinders. The air currents and 
centrifugal forces operating on the system in this lower drying region 
tend to separate the web from its fabric where it nears and passes around 
the bottom cylinders, greatly reducing the drying achieved by the bottom 
cylinders. Bringman and Jamil, "Engineering Considerations for Lightweight 
Paper Drying in High Speed Machines," Paper Technology & Industry--UK Vol. 
6, pp. 198-200 (July-August 1978). The pressure differential surface zones 
at suction box surfaces 31, 32, 33 and 34 of the present invention cause 
the web W to engage in greater contact, with the lower drums 21, 23 and 
25, for example, than possible with previous conventional supporting 
systems. This permits more heat to be transferred to the web through the 
fabric. 
The proximity of the sheet to these lower pressure zones increases the 
thermodynamic forces driving water vapor from the sheet into the low 
pressure adjacent areas. The combination of a greater arc of contact on 
the top cylinders, more effective ontact at the lower cylinders and lower 
pressures adjacent the sheet in the vacuum boxes and grooved lower 
cylinders results in drying rates above those obtainable with present 
conventional or serpentine fabric arrangements. Thus, this invention 
avoids the solution of Mahoney, which adds extra heat to the lower rolls 
which is less energy effective. Also, the advantageous solution of this 
invention is attained without the more complex solution shown in the prior 
art, for example, Soininen. 
An alternative to the circumferential grooves cut into the drying cylinder 
is to employ a special dryer fabric having longitudinal, with respect to 
the machine, ridges built into its structure on the side opposite to that 
carrying the paper web. The spaces between the ridges serve the same 
function as the grooves in the cylinders. The fabric must be permeable in 
order for the vacuum to communicate through the fabric and hold the web or 
sheet to it. 
The grooved, heated lower cylinders may, as an alternative, be replaced 
with cylinders having foraminous major surfaces. For example, the bottom 
of the grooves 40 of the cylinders may be apertured about their 
circumference. A vacuum on the cylinder interior then evacuates the 
grooves thereby holding the web and fabric combination together onto the 
cylinder outer surface, independent of centrifugal or other velocity 
stresses. The formainous cylinder may be of relatively light weight 
construction since it does not have to withstand conventional steam 
pressures. 
In the conventional paper machine, peeling and velocity related stresses 
result in tensioning or stretching stresses being imparted to a web at 
many locations in the papermaking operation. A paper web is typically 
intermittently stressed below its breaking level by being longitudinally 
stretched or tensioned between drying cylinders, for example. Each paper 
has a certain limited ability, represented by its breaking length at each 
point in the processing, to resist these stresses. Each stress is 
translated into a strain from which the paper does not totally elastically 
recover. These retained strains are cumulative as each subsequent stress 
occurs. The amount of inelastic strain that a paper has accumulated during 
processing determines the remaining extensibility or toughness of the 
finished sheet. 
In the process of this invention, the web, since it is all times supported, 
does not experience the tensioning stresses that typically exist in 
transfers and transport through the drying process. The dryer fabric and 
its holding means ensures that the web is always supported while drying, 
thus avoiding velocity-related stretching in the dryer section. Thus the 
paper product at the reel has a retained toughness or stretchability not 
found in conventionally prepared papers. Improved toughness is beneficial 
for many paper grades. For example, a tougher newsprint reduces the number 
of breaks on printing presses. As another example, bag papers are less 
prone to bursting or tearing. 
The ratio of cross to machine direction strength of the web is improved 
since the web is held onto a supporting fabric at all times. The resultant 
change in internal strain in the web in the cross direction improves 
desired properties for some grades, such as liner and medium. 
The sheet, as it passes through the drum dryer section, tends to shrink. It 
is desirable to allow this to occur when the sheet is to retain its 
stretchability. This retention may be encouraged by dividing the 
supporting means function in the dryer between a number of supporting 
fabric means and driving each successive fabric at successively lower 
speeds. Sometimes it is advantageous to stretch the sheet slightly as it 
is drying. This sacrifices some stretchability but enhances sheet 
properties such as stiffness making the sheet less prone to curl. Some 
stretching or "draw" may be useful in preventing wrinkle defects from 
developing at transfer of the web from thep ress felt onto the first dryer 
fabric. This stretching may be controlled by driving the succession of 
dryer fabrics at speeds differential to those matching the shrinkage rate. 
Transfers of the web between drying fabrics are accomplished by methods 
that ensure positive web support at all times. FIG. 5 illustrates a 
transfer between a dryer fabric 110 and a subsequent fabric 200. 
Transfer is effected by vacuum transfer roll 111 which removes the web W 
from fabric 110 just as the web-fabric combination leaves the influence of 
suction box 112. The web adheres to transfer roll 111 which rotates the 
web into contact with fabric 200. The roll 111 may rotate at any 
convenient speed to enhance the desired result. 
The suction box 113, adjacent fabric 200, then causes the web W to leave 
suction roll 111 and adhere to fabric 200. 
Referring to FIG. 2, an arrangement similar to that of FIG. 5 may be used 
in the initial transfer of the web from belt 15 by transfer roll 18 onto 
dryer fabric 19. A speed or velocity differential of roll 18 and fabric 19 
with respect to the belt 15 of up to 2.5% (preferably 1-2%) will prevent 
wrinkles from forming in the paper sheet. 
As an alternative to the FIG. 5 arrangement, a transfer between dryer 
fabrics can be effected using the pressure differential means operating on 
the web fabric combination during transit between cylinders. FIG. 6 
illustrates this scheme. 
The web W travels about the dryer drum 23" supported on the fabric 110'. As 
the web leaves the drum 23", fabric 200' is brought into contact with it. 
The web, now sandwiched between fabrics 110' and 200', travels across 
suction boxes 112' and 114. The pressure differential between the two 
suction zones of the boxes is adjusted so that the web transfers from 
adherence to fabric 110' onto fabric 200'. 
Yet another transfer scheme is shown in FIG. 7. The transfer is made from a 
first dryer fabric 300 to second fabric 301, about dryer cylinder 302. 
Cylinder 302 is provided with circumferential grooves 303 and is wrapped 
with a permeable belt or fabric 304. Vacuum box 305 evacuates cylinder 
grooves 303 only to adhere web W and belt or fabric 304 onto the cylinder 
as it passes about the cylinder. 
Transfer from the first fabric 300 onto belt 304 occurs as previously 
described, without stress, at point 306 where the influence of vacuum box 
308 just ends and the evacuated grooves 303 just begin. A similar transfer 
occurs at point 307 from belt 304 onto second fabric 301 under the 
influence of vacuum box 309. Cylinder 302 may rotate at any convenient 
speed, generally between that of the first and second fabrics 300, 301, 
that enhances the desired result. 
Of course, after the web has attained sufficient strength, through drying, 
to reliably exist independently of stresses in the machine environment, 
the web no longer requires a supporting fabric. At that point, which is 
determined by the inherent strength of the web's furnish, the speed of the 
machine and web dryness for a given web, the web may continue through the 
machine unsupported without great risk of breakage. A key feature of this 
invention is this understanding which permits determining where in the 
machine the expense and costs of supporting and holding is required. 
Likewise, it establishes where these costs are not necessary. 
Other combinations of suction rolls, vented rolls, solid rolls, felts, 
belts and fabrics, etc. will be evident to those skilled in the art. Any 
combination may take advantage of the invention which requires reduction 
or elimination of peeling or velocity stresses on the web. 
EXAMPLE 1: Mill Economics 
A review of the economics of the design of the invention depicted in FIG. 
2, compared with those of a current, conventional process, shows the 
advantages of the new design. Here the advantage highlighted is the choice 
of a furnish containing a reduced amount of the more expensive bleached 
kraft chemical pulp, typically included to improve wet processing strength 
of the web. 
The following table of costs for a 750-ton-per-day operation for making 
newsprint shows a $27/ton improvement over conventional technology as a 
result of reducing the chemical pulp fiber content of a finished newsprint 
from 15% by weight to 5%. The machine speed remains the same for both the 
process of the invention and the conventional technology. The reduced 
chemical pulp furnish results in a weaker sheet during initial drying, but 
the supporting and holding process of this invention permit it to be 
processed at the same speed as if it were a stronger sheet or even faster 
if desired and the machine has the required drying capability. The 
following table illustrates the savings resulting only from reduced 
chemical pulp demand. 
TABLE 
______________________________________ 
Relative Costs Per Ton of Newsprint Produced 
Process Conven- Benefit 
of the tional of 
Invention Process Invention 
Costs ($/ton) ($/ton) ($/ton) 
______________________________________ 
Power, $0.01/ 
57 51 -6 
KWN 
Chemical Pulp 
22 67 +45 
@ $450/ton 
(5% of furnish) 
(15% of furnish) 
Chips, TM 114 102 -12 
@ 120/ton (95% of furnish) 
(85% of furnish) 
Total $ 193 220 27 
______________________________________ 
At an operating rate of 750 tons/day, 350 days/year, the savings amount to 
$7.1 million per year using typical costs of power, chemical pulp and 
chips. 
Alternatively, of course, the speed of the drying section may be increased, 
the other components of the papermaking process permitting. Every 100 
ft./min. (30.5 m/min.) increase in effective speed is equivalent to an 
increase in production benefit of about $1 million per year for a large 
size newsprint machine. 
Salable newsprint is presently being made from 100% thermomechanical pulp 
but at low production speeds by today's standards. The fastest newsprint 
machine today achieves an average operating speed of 3650 ft./min (1122 
m/min.) using 38% chemical pulp. The process of this invention should be 
able to attain 5,000 ft./min (1525 m/min) without the necessity of using 
substantial amounts of chemical pulps. 
A combination of reduced chemical pulp requirement and speed increases has 
the potential to increase the return of the largest newsprint machines by 
in excess of $45 million per year at current pulp and energy costs. 
EXAMPLE 2: Pilot Machine Trials 
The pilot machine comprises a complete one meter wide paper machine using a 
Sym-Former producing a paper sheet about 600 mm in width. The web is 
formed and pressed in an arrangement similar to that shown in FIG. 2. The 
machine is provided with eleven cylinders in the dryer section arranged as 
shown in FIG. 2. The solid surfaced cylinders are electrically heated 
rather than conventionally steam heated. The bottom cylinders have grooved 
surfaces. A vacuum box (not shown in FIG. 2) holds the sheet onto its 
supporting fabric during transport of the web from the transfer roll 
(which transfers the web from the press belt onto the dryer fabric) up to 
where the web is brought into direct wrapping contact with the first 
heated drying cylinder. Vacuum boxes similar to that depicted in FIG. 3 
occupy the dryer "pockets" as shown in FIG. 2. 
The following tables and observations are pilot trial results using various 
strength furnishes to produce certain typical paper products at varying 
machine speeds. 
Trial A Corrugating Medium 
The target paper was corrugating medium at 127 grams per square meter 
(g/m.sup.2) basis weight. 
The furnishes tested were 100% hardwood pulp made by a conventional green 
liquor semichemical pulping process. At 37.degree. C., this pulp furnish 
had a wet web strength, at 35% solids, of 20 BLM (breaking length, 
meters). In a second group of trials the hardwood pulp was blended with a 
strong chemical kraft pulp consisting of a bleached sulphate process pulp 
made from a long fiber softwood. The furnish containing 80% hardwood and 
20% kraft pulp had, at 37.degree. C. and 35% solids, a wet web strength of 
40 BLM. 
In the corrugating medium trials, the press belt 15 shown in FIG. 2 was 
used to transport the web from the last nip until transfer by suction roll 
18 onto dryer fabric 19. During trials of the process and equipment of 
this invention a pressure differential was established at vacuum boxes 30, 
including the additional box operating between the point of transfer of 
the web onto a supporting dryer fabric and its contact with the first 
drying cylinder. Referring to FIG. 3, the second suction box in the pilot 
machine was equipped with vacuum gauges located at points a-d. Table I 
shows vacuum at points a-d for a drying fabric having a permeability of 
500 m.sup.3 /m.sup.2 h (at .DELTA.P=100 Pa). 
TABLE I 
______________________________________ 
VACUUM BOX PRESSURES 
Pressure Differential 
Speed (see FIG. 3, Points of Measurement) 
Trial m/s a b c d e 
______________________________________ 
Without paper web 
on machine 12.5 460 160 210 50 30 
Same as above 
15.0 430 160 180 40 20 
With paper web 
on machine 12.5 720 310 400 400 400 
______________________________________ 
A tension was exerted on the fabric to prevent rubbing between the fabric 
and the vacum boxes. A tension of about 3 kN/m was sufficient when vacuum 
box pressures were on the order of 500 Pa. At speeds above 15 m/s, suction 
in the vacuum boxes had to be increased to 700-800 Pa. This vacuum caused 
some fabric rubbing at the seals until seals were readjusted. 
The necessity of using the vacuum boxes was demonstrated by shutting them 
off during a number of trials. When the boxes were shut down, conditions 
similar to conventional paper machine environments were quickly 
established resulting, in general, in sheet breakages. Table II presents 
the results of these trials at increasing speeds for both the 100% 
hardwood and 20% kraft furnishes. 
TABLE II 
______________________________________ 
CORRUGATING MEDIUM, 127 g/m.sup.2 
Ma- 
Furnish chine Process & Equipment of Invention 
(Species mix, 
Speed Vacuum System 
Vacuum System 
wt. %) (m/s) Operating Shut Down 
______________________________________ 
100% hardwood 
7.5 Satisfactory 
Sheet break-machine 
Run down 
100% hardwood 
10.0 Satisfactory 
Sheet break-machine 
Run down 
100% hardwood 
12.5 Satisfactory 
Sheet break-machine 
Run down 
20% kraft and 
80% hardwood 
12.5 Satisfactory 
Satisfactory Run.sup.1,2 
Run 
20% kraft and 
80% hardwood 
15.0 Satisfactory 
Sheet break-machine 
Run.sup.3 down 
______________________________________ 
Notes: 
.sup.1 Transfer suction roll off. 
.sup.2 Sheet separated slightly from fabric on last three bottom cylinder 
even though draw increased to 2.8%. 
.sup.3 Transfer suction roll off. 
Transfer of the web from the press belt onto the dryer fabric was generally 
without difficulty. In some cases is was possible to shut down transfer 
roll vacuum without adversely affecting transfer. The suction in the 
vacuum transfer roll ranged from 0 to 100 Pa. If a good transfer off the 
press belt could be obtained, then no suction was used at the transfer 
point. At 100 Pa in the box some rubbing of fabric on the box surfaces was 
experienced. 
A slight longitudinal stress or "draw" was exerted on the web at the point 
of transfer from the press belt. The draw was established by operating the 
transfer roll and dryer fabric combination at a higher speed than the 
press belt speed. The amount of draw exerted on the web is expressed as a 
percentage representing the speed differential between the press and dryer 
sections. The draw differentials were 0.5-2.3%, and preferably 1-2%. Too 
low a draw resulted in wrinkle defects in the paper product. Too high a 
draw resulted in web breaks and machine shutdowns. A 1.5-2% draw was 
applied, except where noted, in the pilot trials. 
In general, runnability was good when the vacuum boxes of the invention 
were operating. This is indicated in Table II by the "Satisfactory Run" 
observation. Shut-down of the boxes resulted in the web separating from 
its supporting fabric at all speeds, leading in all but one case to 
failure of the sheet. The time between suction shutdown and web breaks was 
about 0.5-1.0 minute. 
Table II demonstrates that weak hardwood furnishes can be run where the 
paper machine uses the supporting and holding process and equipment of 
this invention. When the holding systems were shut down, this furnish 
could not be run at the test speeds. For a 20% kraft furnish, speeds of 
15.0 m/s were attained for the process and equipment of the invention. The 
furnish could be run without the vacuum box holding means operating at 
12.5 m/s. However, at this speed the web had separated from its supporting 
fabric on the last three bottom drying cylinders. The separated web was 
thus subject to machine velocity stresses and susceptible to breakage 
should, for example, inherent wet web strength decrease or speed be 
increased. Increasing machine speed to 15.0 m/s did, in fact, result in 
web failure when the vacuum holding forces were cut off. 
The fastest machine making corrugating medium today operates at a maximum 
speed of 10.7 m/s (2100 ft./min.) and average 9.9 m/s (1950 ft./min.). 
These speeds are only attainable when the furnish includes about 30% 
expensive chemical pulp to improve wet strength. The pilot machine trial 
results demonstrate a 40% speed increase. A 16.8% speed increase was 
attained with the furnish from which all chemical pulp had been excluded. 
Trial B-Fine Paper 
The target paper in this group of pilot machine trials was a fine paper of 
74 g/m.sup.2, having a filler content of 12%. 
The furnishes tested ranged from 100% hardwood to furnishes containing 30% 
kraft. The hardwood pulp for this trial was a bleached sulphite pulp made 
from a 1 to 1 mixture of mixed northern dense hardwood and aspen. At 
39.degree. C., 35% solids, this pulp has a wet web strength of 39 BLM. The 
strong chemical pulp used to improve wet web strength of the hardwood 
furnish for these trials was a bleached sulphate kraft pulp made from a 
long fiber softwood. A 30% kraft, 70% hardwood furnish has a wet web 
strength of 59 BLM at 39.degree. C., 35% solids. 
In the fine paper trials, the paper machine arrangement was as described 
above. Vacuum box suctions were increased to 1000-1500 Pa, which caused 
some rubbing between the fabric and box surfaces. Table III shows how this 
pressure was distributed in the vacuum box for two different fabric 
permeabilities. 
TABLE III 
______________________________________ 
FINE PAPER TRIAL VACUUM BOX PRESSURES 
Fabric 
Permeability 
Con- Pressure Differential 
(m.sup.3 /m.sup.2 h, 
ditions Speed (See FIG. 3) 
.DELTA.P=100 Pa) 
of Trial (m/s) a b c d e 
______________________________________ 
100 without 12.5 1,150 
12 950 850 650 
paper 
web 
100 with 12.5 1,300 
96 1,220 
1,310 
1,420 
paper 
web 
500 without 15.0 510 
100 230 40 40 
paper 
web 
500 with 15.0 860 
140 510 500 530 
paper 
web 
______________________________________ 
A draw of about 1.5-2.0% was used to keep the sheet wrinkle free on the 
dryer. 
Table IV presents the results of pilot trials for the various furnishes at 
increasing machine speed. 
TABLE IV 
______________________________________ 
FINE PAPER, 74 g/m.sup.2 
Furnish Machine Process & Equipment of Invention 
Species Speed Vacuum System 
Vacuum System 
mix, wt. %) 
(m/s) Operating Shut Down 
______________________________________ 
100% hardwood 
10 Satisfactory Run 
Satisfactory Run.sup.1 
100% hardwood 
12.5 Satisfactory Run 
Sheet break, 
machine down 
100% hardwood 
15 Satisfactory Run 
Sheet break, 
machine down 
5% kraft 
95% hardwood 
10 Satisfactory Run 
Satisfactory Run 
5% kraft 
95% hardwood 
12.5 Satisfactory Run 
Sheet break, 
machine down 
5% kraft 
95% hardwood 
15 Satisfactory Run 
Sheet break, 
machine down 
30% kraft 
70% hardwood 
12.5 Satisfactory Run 
Satisfactory Run.sup.2 
30% kraft 
70% hardwood 
15 Satisfactory Run 
Sheet break, 
machine down 
30% kraft 
70% hardwood 
17.5 Satisfactory Run 
Sheet break, 
machine down 
______________________________________ 
Notes: 
.sup.1 With increased draw. 
.sup.2 Sheet separated slightly from fabric on last three bottom 
cylinders, even though draw increased. 
The fine paper furnishes were somewhat more difficult to transfer from the 
press belt onto the dryer fabric. A 30 kPA (maximum) suction at the 
transfer roll was required to affect transfer, in contrast to the 
corrugating furnishes which could often be transferred without any suction 
on at the transfer roll at all. A somewhat stronger draw on the paper web, 
on the order of 2.5%, was sometimes required with the fine furnish. 
Dryer section runnability with the fine paper furnish was worse that with 
the corrugating furnish. There was a strong tendency for the fine paper 
furnish web to adhere to the drying cylinders because of the 
characteristics of the pilot machinery. As noted earlier, vacuum box 
suction had to be increased considerably. 
Referring to the Table IV results, the 100% hardwood furnish trials show 
the greater inherent strength of the furnish. Thus, the furnish would run, 
without the vacuum boxes exerting holding forces on the web, at 10 m/s. 
However, when the speed was increased to 12.5 m/s, web breakage was 
experienced when the vacuum boxes were shut down. With the boxes 
operating, the web ran satisfactorily at 12.5 m/s and also at 15 m/s (the 
highest speed attempted). When the vacuum boxes were shut down, the sheet 
broke at 12.5 m/s. 
At this point in the trial the furnish was modified to improve its wet web 
strength to determine how much kraft chemical pulp would be needed to 
allow the machine to operate without the vacuum box holding means of the 
invention. Not until the kraft pulp content had reached 30% was the web 
able to run at 12.5 m/s without the holding means of the invention. 
However, when the speed was increased to 15 m/s, the sheet broke when the 
vacuum boxes were shut down. With the vacuum box holding force operating 
on the web to hold the web onto its supporting fabric, the web was run 
satisfactorily at 15 m/s and even at 17.5 m/s. 
Trial C--Newsprint 50 g/m.sup.2 
The objective of this trial was to produce newsprint at 50 g/m.sup.2 at 
high production speeds. 
The furnish comprised 44% groundwood pulp, 44% thermomechanical pulp and 
12% kraft chemical pulp. 
The identical arrangement described above was used in the trials. It was 
found that the following "draw" was necessary to obtain satisfactory 
newsprint. 
TABLE V 
______________________________________ 
Speed Difference Between 
Press Section and Dryer Section 
Speed m/s Speed Difference at Transfer Point 
______________________________________ 
15.0 1.5 .+-. 0.5% 
17.5 2.0 .+-. 0.5% 
20.0 2.6 .+-. 0.6% 
______________________________________ 
The highest speed attainable, where the sheet could be reliably produced 
was, 20 m/s (3937 ft/min). Speeds of 22 m/s/(4331 ft/min) could 
occasionally be established but tended to break at transfer from the press 
to the dryer section. The speed improvement over conventional speeds was 
limited by the lack of suitability of the press belt (FIG. 2, element 15) 
for effecting a relatively tensionless transfer of the newsprint furnish 
used into the dryer section. 
In sum, the pilot trial results demonstrate the operation of the processes 
and equipment of the invention. The results show that the invention 
operates largely independent of the inherent strength of the furnish being 
processed. The trial results show that this advantage is in distinct 
contrast to prior art processes, represented by trials in which the vacuum 
box holding forces were shut off. 
The speed increasing benefits of the process and equipment of the invention 
were likewise demonstrated by the pilot trials. The upper limits of the 
speed improvements contemplated were not attained in these trials because 
of equipment limitations described above. The speed improvements 
contemplated are limited only by process and equipment limitations that 
are unrelated to velocity stresses. 
The improvement of this invention may also be translated into several other 
productivity advantages. For example, the capital cost for a new machine 
may be reduced for a given capacity since all elements of the machine 
might be reduced in width because of the higher production speed of the 
new machine. The advantages of this invention are readily retrofitted onto 
existing conventional paper machines.