Single-tier drying section tailored for compensating stretching and shrinking of paper web

A drying section for a paper making machine includes a larger number of dryer sections, fewer dryers in each dryer section, and provisions for driving many of the dryers individually. The average number of dryers per dryer section is less than five and preferably less than four. By providing a small number of dryers per dryer section and driving many of the dryers individually, it is possible to more precisely compensate for stretching of the paper web in the initial stages of the drying process and to more precisely compensate for shrinkage of the paper web during the latter stages of the drying process.

BACKGROUND OF THE INVENTION 
The present invention relates to a drying section of a paper making machine 
and, more particularly, to a drying section comprised substantially 
entirely of sequentially arranged, top-felted, single tier dryer sections 
with a, comparatively, drastically reduced number of dryers within each 
dryer section to permit compensating for stretching and shrinkage of the 
paper web. 
As used therein, the term "drying section" refers to that part of the paper 
machine which receives a web of paper emerging from the press section and 
which extends to the point where the paper web emerges from the drying 
section about 90 to 99% dry. The paper web proceeds from the drying 
section to subsequent sections of the machine, e.g. the calender, the 
coater, after dryer etc. As is well known, a drying section consists of 
several subsections comprising commonly felted drying cylinders. In the 
present patent specification, the term "dryer group" or "dryer section" 
designates a group of commonly felted dryer cylinders, except in a case of 
a double-tier dryer section where the co-extensive upper and lower dryers, 
although separately felted, are still considered by the art to be the same 
"dryer section." 
As described in the present Assignee's U.S. Pat. No. 5,311,672, a paper web 
initially stretches and later shrinks during the course of being dried in 
the drying section of the paper machine. This is because, despite 
considerable advances in paper pressing technology, the paper web still 
emerges from the press section approximately only about 45% dry. The first 
few drying cylinders merely gradually heat up the paper web to the point 
where moisture evaporation begins. During this initial stage and while the 
paper web gives up its "free water", i.e. the water located in the 
interstitial spaces between the fibers of the paper web, the paper web 
stretches as it proceeds downstream in the machine direction. Later, the 
process reverses itself as the drying process releases water molecules 
lodged within the fibers themselves, and the paper web beings to gradually 
shrink. 
The aforementioned Voith U.S. Pat. No. 5,311,672 patent proposes running 
with positive draws between the more upstream dryer sections located 
nearer the wet end of the drying section and running with negative draws, 
i.e. negative speed differentials, between those dryer sections that are 
located more downstream, i.e. closer to the very end, i.e. the dry end, of 
the drying section. 
The present inventor has discovered that the prior art does not go far 
enough to fully compensate for the stretching and shrinking of the paper 
web. The difficulty stems from the fact that the typical prior art dryer 
sections are relatively long, containing as they do 6, 7 or even 8 or more 
commonly felted dryers. These commonly felted dryers run at the same, 
identical speeds and are traversed by a single felt in the case of 
single-tier dryer sections and by a pair of felts in the case of a double 
tier dryer section. Regardless, within any given dryer section, all the 
dryer surface speeds are practically identical and it is not possible to 
compensate for stretching and shrinking of the paper web within the dryer 
sections per se. 
In this connection, it is noted that prior to 1990 all drying sections were 
constructed solely of double-tier dryer sections, each comprising a 
minimum of 6 dryers or more typically 8 or 10 dryers. This yields an 
average of approximately at least 7, and in any event, not less than 6 
dryers per dryer section. 
Since 1990 there has been a concerted switch over from all double-tier 
drying sections to drying sections comprised of either entirely 
single-tier dryer sections and/or of a mix of single tier and double-tier 
dryer sections. To date, most of the installed single tier dryer sections 
contain at least 5, 6 or even 7, but most typically 6 dryers per dryer 
section. In any event, the average number of dryers per dryer section is 
still greater than 5 dryers per section. Reflecting the state of the prior 
art is U.S. Pat. No. 4,934,067 which shows in FIG. 1 thereof 6 dryer 
sections each containing 6 dryers for a total of 36 dryers. This patent 
reflects the design of a machine that has been actually constructed, and 
scores of similar machines installed around the world. Similarly, the 
Voith Company's U.S. Pat. No. 5,050,317 shows a plurality of single felted 
dryer sections in which the number of dryers per dryer section is 8, or 
even as many as 10 dryers per section. See also, Voith's U.S. Pat. No. 
5,177,880. 
In drying sections containing a mix of single tier dryer sections and 
double tier dryer sections, the number of dryers is still on the order of 
about 6 dryers per dryer section, as reflected in U.S. Pat. No. 5,269,074. 
This patent again generally reflects the dryer layout of an actual 
operating drying section. 
Although some prior patents have depicted dryer sections with fewer dryers, 
those patents are illustrative and do not reflect actual feasible machine 
designs which could dry a paper web from about 43% dry to 90-99% dry as 
required in an actual paper machine. See e.g. U.S. Pat. No. 4,982,513. 
Thus, in present day drying cylinders measuring some 6 to 7 feet in 
diameter and having intermediate vacuum guiding rolls, a paper web length 
measuring well over 100 feet within any given dryer section cannot be 
compensated for the stretching or shrinking thereof within the dryer 
section. The only points of stretch/shrinkage compensation is at the web 
transfer zones between dryer sections where, by speeding up or slowing 
down a down stream dryer section, one is able to pickup the slack i.e. 
stretch, or compensate for the shrinkage of the paper web. Here too, there 
are limitations, as it is not desirable to pull a paper web too hard in 
the early stages of drying, because of the risk of increasing the number 
of web breakages. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a drying 
section which is capable of compensating for stretching and shrinking of a 
paper web within the dryer sections themselves. 
It is another object of the present invention to provide a drying section 
constructed of proven drying section technology such as by using 
top-felted, or bottom-felted or combinations of single-tier dryer 
sections, which are inexpensively and simply reconfigured to provide 
compensation for stretching and shrinking of the paper web. 
The foregoing and other objects of the present invention are realized in 
accordance with the present invention with a drying section which 
preferably comprises only top felted, single tier dryer sections, wherein 
each dryer section includes a considerably lesser number of drying 
cylinders. This flies in the face of conventional wisdom which has been 
striving to felt together increasing numbers of drying cylinders for 
reasons of cost, both in constructing the drying section and reducing the 
number of felts which have to be periodically replaced. 
Contrary to the prior art, the present invention provides a drying section 
in which most of the dryer groups contain 3 or at most 4 drying cylinders. 
Each dryer section has a respective felt which traverses the drying 
cylinders as well as vacuum rolls located between each pair of adjacent 
dryers, below the dryers, to provide successively arranged, top felted, 
single tier dryer sections, in which the paper web is conveyed from 
section to section substantially without open draw via lick down 
transfers. 
Preferably, the average number of drying cylinders in the dryer sections is 
5 or less, but the number could be as low as below 4 and even below 3 
drying cylinders per dryer section. 
The vacuum guide rolls of the present invention can be located very close, 
i.e. in close proximity, to the drying cylinders, e.g. only about 2-5 
inches away from the surfaces of the drying cylinders. Alternatively, they 
can be located at a distance of a foot or more from the drying cylinders 
and a vacuum box should then be located in the pocket defined by the 
drying cylinders and the vacuum roll. Moreover, the vacuum rolls may be 
gutless, perforated vacuum rolls in which the vacuum within is generated 
through a suction effect provided by the vacuum boxes located above these 
gutless, vacuum rolls. 
Prior art drying cylinders of the same dryer group rotate at the same 
speeds because they are driven together due to being commonly felted or by 
being mechanically coupled to one or two driven cylinders. In marked 
departure from the prior art, the present invention provides separate 
direct drives to many of the drying cylinders within the dryer sections, 
so as to enable adjusting the paper web speed not only between dryer 
sections, but also within dryer sections from one dryer to the next. 
Another concept introduced by the present invention is increasing the 
number of drying cylinders in each dryer section from the wet end toward 
the center of the drying section and thereafter decreasing the number of 
drying cylinders as the drying section extends toward the dry end thereof. 
This feature concentrates most of the capability of controlling 
dryer-to-dryer speed differentials at the early stages and at the end 
stages of the drying process where they are needed most, and to a lesser 
extent at the center region of the drying section. 
Other features and advantages of the present invention will become apparent 
from the following description of the invention which refers to the 
accompanying drawings.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
FIGS. 1A and 1B illustrate a drying section 10 which comprises nine dryer 
sections numbered I-IX. In a well known manner, each dryer section I-IX 
comprises drying cylinders 12, with vacuum rolls 14 interposed between and 
below the drying cylinders 12, defining top-felted, single-tier dryer 
sections. Each dryer section I-IX is traversed by a respective felt 16, 
whereby a paper web 18 has its bottom side 20 pressed against the drying 
cylinders 12 by the felt 16 while being guided between the dryers 12 and 
around the vacuum rolls 14, again in well known manner. The paper web 18 
emerges from the last dryer 22 of the last dryer section IX to be 
thereafter conducted to other sections of the machine such as the 
calender, coater, after dryer, reelers etc. (not shown) as the case may 
be. 
Note that the number of dryers in the dryer section I is 2; and that the 
number of dryers 12 in the remaining sections II-IX is respectively 3, 4, 
5, 6, 5, 4, 3, and 2. In other words, the average number of dryers is 
3.25. Second, the number of dryers is smallest at the point where the 
paper first emerges from the press section, where it engages the first 
dryer 12 of the first dryer section I. The number of dryers increases 
toward the center of the drying section 10 and then gradually decreases, 
so that the number of dryers adjacent the dry end of the drying section is 
2 or perhaps 3 dryers. 
As already noted, this contrasts sharply with the conventional wisdom which 
has advocated including more rather than less dryers within a dryer 
section, to reduce the number of felts 16, felt rolls 24 and other 
components of the drying section. 
In addition to comprising only an average of 3.25 dryers per dryer section, 
the drying section 10 of FIGS. 1A and 1B is distinguished from the prior 
art in that it comprises 9 dryer sections which is well over the typical 4 
or at most 5 dryer sections associated with double tier drying sections 
and the 6 dryer sections or at most 7 dryer sections which have been 
provided in prior art, all single tier drying sections. It should be noted 
that although the drying section 10 of FIGS. 1A and 1B provides an average 
of 3.25 dryers per dryer section, the concept of the present invention 
extends generically to dryer sections which have on average below about 
5.5 dryers per dryer section. 
In the drying section 10 of the present invention the paper web 18 is 
initially heated and the evaporation process begins in the more upstream 
dryer section I-IV, which are located nearer the wet end of the drying 
section. It is there that the paper web may initially stretch and thus sag 
so that it does not properly and firmly adhere to the exterior face of the 
felt 16 as that felt traverses the vacuum rolls 14. This can cause web 
breakage, web wrinkling and otherwise adversely affect the final paper 
quality. 
In contrast, during the later stages of the drying process, e.g. while the 
paper web is being dried at the dryer sections VI-IX, the paper web may 
begin to shrink. That shrinkage has to be accommodated to prevent the 
paper web from breaking. 
To compensate for both the stretching and/or wrinkling of the paper web 
during the early drying stages and the possible web bursting in the later 
stages, the present invention also provides individual drives for many or 
even all of the dryers 12. This feature is illustrated as respective drive 
mechanisms 30 coupled to the individual dryers 12, which enable providing 
relatively small dryer-to-dryer speed variations within the same dryer 
section. The dryer drive mechanisms 30 are per se known in the art as can 
be appreciated from the Voith Company's U.S. Pat. Nos. 5,311,672 and 
4,820,947, the contents of which are incorporated by reference herein. 
Although FIGS. 1A and 1B illustrate a respective drive for each of the 
dryers 12, it is not necessary that each dryer be provided with a separate 
drive. Rather, it is important to provide the drives 30 where it is deemed 
that the stretching or shrinking will take place, to accommodate and 
compensate such stretching and/or shrinking. In this connection note that 
if the felts 16 grip the drying cylinders very tightly it is possible to 
stretch the fabric somewhat by speeding up a downstream dryer, thus 
picking up the slack, i.e. the stretch of the paper web. Conversely, 
toward the dry end of the drying section, because the felt tightly grips 
the dryers it is normally somewhat stretched whereby slowing down a 
downstream dryer has the potential of relieving or relaxing somewhat the 
stresses in the paper web which, in conjunction with the use of very small 
dryer groups and negative draws between the groups, can fully compensate 
for the web shrinkage. This realizes substantial and improved control of 
the paper web stretching/shrinking properties throughout the drying 
section. 
Throughout the instant description, references to driving of the dryers is 
intended to include the vacuum rolls. Thus, either the dryers, or the 
vacuum rolls or combinations thereof may be driven to obtain the 
above-described control over shrinking and/or stretching of the paper web. 
FIG. 5 shows a drying section 50 which, like the drying section 10 of FIGS. 
1A and 1B, includes drying cylinders 12, vacuum rolls 14, individual dryer 
drivers 30 and felts 16. Although a actual paper machine drying section 
would require more than the illustrated 15 dryers, this drawing 
nevertheless shows a drying section variant of the present invention in 
which the number of dryers per dryer section is either 2 or 3, for an 
average of about 2.5 dryers. Also note the individual lines 52 which 
couple the drivers 30 to a central speed controller or governor 54, by 
which the speeds of the individual dryers may be controlled in accordance 
with the concept of the present invention. 
Similarly, FIG. 2 illustrates a drying section 60 with 11 dryer sections 
numbered I-XI which contain felts 16, dryers 12, vacuum rolls 14, felt 
rolls 24, drivers 30, etc. The eleven dryer sections comprising the dryer 
section 60 is about twice or even as much as almost four times the number 
of dryer sections found in conventional drying sections. 
FIG. 3 illustrates that the present invention as illustrated in FIGS. 1A, 
1B, 2 and 5 is not limited to vacuum rolls 14 which are located in close 
proximity to the dryers 12. That is, the concepts of the invention are 
applicable to and may be realized by using vacuum rolls 62, which may be 
gutless vacuum rolls that are located at a considerable distance, e.g. one 
foot to 18 inches away from the surfaces of the drying cylinders 12. The 
vacuum here is supplied from vacuum boxes 64 which provide a vacuum for 
holding the paper web against the felt 16 over the joint run of web-felt 
between dryers and around the vacuum roll 62. 
In the same vein, the present invention is not limited to the all top 
felted, single tier dryer sections illustrated in FIGS. 1A, 1B, 2 and 5. 
Rather, as shown in FIG. 4, successive ones of these dryer sections can be 
alternatingly inverted, so that the dryers 12 in the boxed area 66 in FIG. 
2 can be oriented so that the dryers in the third dryer group III are 
arranged at a lower plane, below the dryers of the second section II and 
the vacuum rolls 14 are located above the dryers as shown. Here the felt 
16 completes its loop around the dryers by traveling below the dryers, 
resulting in a bottom-felted dryer section in which the dryers 12 rotate 
counterclockwise, opposite to the clockwise rotating cylinders of the top 
felted dryer section. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. It is preferred, 
therefore, that the present invention be limited not by the specific 
disclosure herein, but only by the appended claims.