Patent Publication Number: US-3878620-A

Title: Method and apparatus for drying paper

Description:
United States Patent Gallahue et a1.  
 [ 1 Apr. 22, 1975 METHOD AND APPARATUS FOR DRYING PAPER Inventors: William M. Gallahue, Taunton.  
 Mass; Gilbert J. Wells, Littleton, N.H.  
 Assignee: Mount Hope Machinery Company.  
 Taunton, Mass.  
 Filed: July 17, 1972 Appl. No.: 272,354  
 US. Cl. 34/41; 226/190; 34/118; 34/116 Int. Cl. F26b 3/32 Field of Search 34/41, 116, 117, 118; 226/194,191, 190  
 References Cited UNITED STATES PATENTS 4/1940 Young 1. 34/41 2 689,392 9/1954 Robertson 226/190 X 3 151,953 10/1964 Justus 1 v 34/118 3 448,529 6/1969 Erspamer ct alm. 34/116 3 778,908 12/1975 Notbohm 34/116 Primary E.\&#39;aminerChar1es J. Myhre Assislant Examiner-William C. Anderson Attorney. Agent, 0&#34; Firml(enway &amp; Jenney [57] ABSTRACT A method and apparatus for equalizing the moisture content across the width of a paper web as it passes through the dryer section of a paper machine. A felt. which is employed to press the paper web against heated dryer drums, has the path of its central portion extended through a greater length than the path of its edge portions by means of one or more curved rolls. This serves to tighten the felt more in the center than at the edges, thereby equalizing the pressure between the paper and the drums across the width of the paper.  
 10 Claims, 8 Drawing Figures PATENTEUAPRZZMYS SHEET 1 [If 3 PATENTEDAPRZZ I975 \mNK. mo.  
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 METHOD AND APPARATUS FOR DRYING PAPER BACKGROUND OF THE INVENTION An inherent problem in conventional paper machines, equipped with banks of steam-heated dryer drums, is a tendency for the paper to remain wetter in the center than at the edges. The generally-accepted assumption has been that this is caused by poor thermal conditions in the dryer pockets. and all efforts to correct the problem have been made through thermal application by means of pocket ventilation. This requires the installation of a complex system of fans. motors, ducting. distributing pipes, and heating coils. which is very expensive to install and requires an annual operating and maintenance expenditure equal to at least percent of its initial cost. For all of this there is only a moderate degree of correction of the wet center problem. Further, this correction is not easily restored following interruptions in machine operation, and requires considerable adjustment during operation to maintain an acceptable moisture profile across the paper web. The effects are also subject to change with ambient conditions, and with changes in weather on a daily as well as seasonal basis.  
 BRIEF DESCRIPTION OF THE INVENTION The general object of this invention is the production of paper webs having a more uniform profile of moisture content across their widths. It is a further object to attain a reduction in wetness of the center of a paper web with less capital investment and operating cost than was heretofore required. Further objects and advantages of the invention will appear as the following description proceeds.  
  We have discovered that the moisture profile of paper webs can be corrected more effectively by a method which runs counter to previous understandings about the cause of this problem. Our method and apparatus involves a mechanical effect on the felts instead of a thermal effect on the paper itself. We apply differential longitudinal tension to the felt by increasing the path of its center to a length greater than that of its edges. This is preferably accomplished by means of one or more longitudinally curved rolls, which engage the felts between the dryer drums, taking the place of conventional straight rolls. The result is to tighten the felts more at the center than at the edges.  
  A number of curved rolls are preferably distributed along the banks of dryer drums. These rolls may be of a type whose curvature is either fixed or adjustable, so arranged as to establish a longer length of run for the center than the edges of the felts in the dryer run. The difference in length of run is of the general order of somewhat less than 1 percent, though this may vary, and the optimum may be determined by experiment in any given application. Modern synthetic felts take no permanent set from this amount of stretching. while older felts may be corrected by using a curved roll outside the dryer run to compensate by equalizing the total run of the felt.  
  A curved roll of the adjustable-bow type is oriented with its plane of curvature aligned with a plane bisecting the angle of wrap of the felt around the roll, and with its convex face projecting into the felt.  
 A curved roll of the fixed-bow type is oriented with its plane of curvature set at an acute angle, which may fall generally in a range of about l0-45, to a plane perpendicular to the bisector of the wrap angle. The specific angle should be chosen on the basis of the degree of bow of the curved roll, the speed of the machine, and the width and weight of the felt, to achieve an appropriate difference in path length between the center and edges of the felt.  
  The application of the invention to a conventional paper machine has demonstrated a substantial improvement in the uniformity of moisture content across the width of the paper web. We attribute this effect to the correction by our method and apparatus of a cause of non-uniform drying which was not previously apprehended. That cause is that the felts tend to press the edges of the paper more firmly against the dryer drums than they do the center of the paper. There is consequently a lower rate of heat transfer to the center, proportionately reducing the rate of moisture removal from this region. The non-uniform felt pressure may arise both from gravitational action and centrifugal force, either of which tends to bow a cross-section of the felt into a catenary form.  
  Conventional felt tensioning systems increase the longitudinal tension of every portion of the felt substantially uniformly, and therefore provide no compensation for differences at various points spaced across the width of the felt.  
  By appropriately varying the path length across the width of the felt, however, we tighten the felt more at the center than at the edges. When this tightening is carried to properlengths, it tends to equalize the pressure of the felt against the paper and dryer drums. and thereby also to equalize the rate of heat transfer to improve the uniformity of drying. The degree of tightening required increases with increases in the speed of the machine, because of the rising centrifugal force; and in the lower dryer banks, also with increases in the width and weight of the felt, because of the gravitational effect.  
  According to one aspect of the invention, either adjustable-bow or fixed-bow rolls may be adjusted as to curvature or angular orientation, respectively, by automatic means responsive to moisture detectors, to control the moisture profile automatically.  
 DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out the subject matter which we regard as our invention, it is believed that a clearer understanding may be gained from the following detailed description of preferred embodiments thereof, referring to the accompanying drawings, in which:  
  FIG. 1 is a schematic view in side elevation ofa dryer section of a paper machine incorporating one embodiment of the invention;  
  FIG. 2 is a plan view illustrating the relationship of one of the rolls of FIG. 1 to a felt;  
  FIG. 3 is a fragmentary view in side elevation and on an enlarged scale of a portion of the dryer section of FIG. 1, showing a curved roll in one angular orientation;  
  FIG. 4 is a view similar to FIG. 3, but showing the curved roll in another angular orientation;  
  FIG/5 is a fragmentary view in side elevation of a portion of a dryer section incorporating a modified embodiment of the invention;  
  FIG. 6 is a graphical illustration comparing representative moisture profiles produced by the present invention and by previously known machines;  
  FIG. 7 is a schematic view of an automatic control system for adjusting the angular orientation of a fixed bow roll; and  
  FIG. 8 is a schematic view of an automatic control system for adjusting a variable-bow roll.  
  Referring first to FIG. 1, the dryer section of a paper machine includes a series of groups of heated dryer drums; a number of such groups are ordinarily employed, but only group 16-34 is shown, as this is sufficient to illustrate the invention. The paper web 10, after issuing from a Fourdrinier wire (not shown) of the papermaking machine. passes through a series of press rolls, of which two are illustrated at 12. It is then guided by a plain roll 14 to the first dryer drum 16, from whence it passes serially around the remaining dryer drums. being wrapped about an arcuate portion of each successive drum through an angle preferably in excess of 180. Issuing from the final drum 34, the paper web is passed by a guide roll 36 to further dryer banks or to other processing equipment such as a calender and windup roll. The drums in an upper and lower bank are staggered, and the paper web wraps successively around one of the drums of the upper bank 16, 20, 24, 28. 32, then around one of the drums of the lower bank 18, 22, 26, 30, and 34, and so forth to the end of the group.  
  An upper felt 38, formed as a continuous belt and made of nylon or other conventional material. is guided by rolls 40-54 in a path which wraps the felt arcuately about a surface portion of each of the upper bank of drums I6, 20, 24, 28, and 32, overlapping the paper web and pressing it against the drums to effect heat transfer. The felt is held under an appropriate degree of longitudinal tension by means of rolls 82 and 84, in a conventional fashion.  
  A lower felt 58 of a similar type is guided by rolls 60-76 in a path which wraps the felt in overlapping relation to the paper web about an arcuate surface portion of each of the lower bank of dryer drums 18, 22, 26, 30. and 34, and thence around an additional heated drum 80, which serves to remove moisture from the felt. Rolls 85 and 88 serve to hold the felt under appropriate longitudinal tension. The apparatus thus far described is of a conventional nature well known in the art. and no further detailed description thereof is believed necessary.  
  According to the method of the present invention, we extend the path of a central portion of the felt between successive dryer drums to a greater length than the path of edge portions of the felt. This tightens the felt in the center, applying relatively less tension to the edges of the felt. The tendency is to press the paper web more firmly against the dryer drums in the center than at the edges. We have discovered that an appropriate amount of differential tightening serves to render the moisture content more uniform across the width of the paper.  
  We believe that the consistent tendency for the center of the paper to be wetter than the edges may arise from one or more causes. Among these are the tendency of the straight guiding rolls such as 42-52 and 62-74 to become elastically deflected into slightly curved forms by the tension in the felts, so that the pressure between the felts and dryer drums may be less near the center than at the edges of the paper. Another cause may be that the ends of the dryer drums may be hotter than their central portions. Still another factor may be that the central portions of the felts may in many cases be more easily stretched by centrifugal force than their edges. Whatever the cause or combination of causes may be, however, the central part of the paper tends to remain wetter than the edges. By applying increased pressure between the central parts of the felt and the drum, it is believed that we offset this harmful propensity, and bring the pressure distribution into better balance so that more uniform heat transfer can produce a more equal drying effect across the width of the paper.  
  According to the preferred mode of practice of the invention, we replace at least one, and preferably a plurality, of the conventionally straight cylindrical rolls 44-50 and 64-72 in each dryer bank with longitudinally curved rolls, illustrated by a roll 64 of fixed curvature in FIGS. 2-4 or by a roll 86 of adjustable curvature in FIG. 5. A suitable roll of the fixed-bow type is described, for example, in US. Pat. No. 2,393,191 issued Jan. 15, 1946 to J. D. Robertson; and a roll of the adjustable-bow type is described in US. Pat. No. 2,689,392 issued Sept. 21, 1954, or US. Pat. No. 2,898,662 issued Aug. 11, 1959, both to J. D. Robertson.  
  Referring first to FIGS. 2-4, a curved roll 64 is rotatably mounted on an axle 82 which is longitudinally curved in a plane P, so that the roll is&#39;free to rotate but maintains a stable curved form. The ends of the axle are mounted in clamping supports 84 of a known type, such as are illustrated in the aforementioned US. Pat. No. 2,393,191. The plane P of curvature may be set in any desired angular orientation by releasing and later retightening these clamping supports, and two such positions are illustrated in FIGS. 3 and 4.  
  The felt 58 passes arcuately about the drum 18 in overlapping relation to the paper 10, is then wrapped partially about the surface of the roll 64, and then passes over the drum 22, again pressing the paper against the drum. The roll 64 is offset as shown from a plane Y which is usually horizontal, joining the major axes of the bank of drums 18-34. A horizontal plane X parallel to this plane of centers is shown in FIG. 3 to illustrate the angular orientation of the plane P of roll curvature. The rolls 42-52 and 62-74 are each offset from the plane of centers of a corresponding bank of drums in the direction of paths of the paper between the two banks of drums.  
 The path of the center of the felt 58 over the center of the roll 64 is shown at C, and is somewhat longer than the path E of the edges of the felt because of the angular orientation of the roll. The effect is to apply different degrees of longitudinal tension to the felt at various points across its width. Because the roll 64 is smoothly curved, the variation in tension increases gradually and continuously from a minimum at either edge to a maximum at the center. The effect is to apply increasing pressure of the felt against the paper and drums toward the center of the machine. This action is intended to be carried to the point at which it will balance and offset the aforementioned tendency of the paper to remain wetter in the center than in its edge portions. The pressure gradient is intended to be sufficient to obtain substantially uniform rates of heat transfer and drying action.  
  The required amount of difference in path lengths and tension increases in the lower bank of dryer drums 18, 22, 26, 30, 34, as the width and weight of the felt rises, because of the increasing gravitational effect; and also increases in both banks as the speed of the paper machine rises. because of the increasing centrifugal force. In general. an angle between the planes P and X in the range of approximately l0-45 may be found suitable.  
  FIG. 4 illustrates the roll 64 oriented with its plane of curvature P at an angle of 0, that is, coincident with the horizontal plane X. The plane P is now perpendicular to a plane shown at Z. bisecting the angle of wrap W of the felt around the roll 64. In this position the path lengths C of the center, and E of the edges. of the felt are identical. and the roll is ineffective to produce any difference in longitudinal tension in the felt. Consequently, the roll acts only to expand the width of the felt; if reversed 180, it would tend to contract the felt laterally. It will be seen, therefore, that it is necessary to turn the plane P of curvature at an angle to the plane X, i.e., the plane P must not lie at 90 to a plane bisecting the wrap angle W. Further, the plane P must be turned in such an angular direction from the plane X as to press the center of the roll 64 into the felt, rather than away from it.  
  A number of the rolls 44-50 and 64-70 may be curved rolls as shown in FIGS. 2 and 3, while the re maining ones of these rolls may be straight cylindrical perforated rolls of a conventional kind, supplied with dry heated air to aid in drying the felt, or these rolls may be straight cylindrical rolls. For example, the rolls 44, 46, 50, 64, 66, and 70 might be curved rolls; the rolls 48, 52, 68 and 72 could then be of the perforated straight type; while the remaining rolls 42, 54, 82, 84, 40, 60, 62, 74, 76, 86, and 88 would be plain cylindrical rolls. However, the best judgment as to the optimum number and location of the curved rolls must be based upon experience with various applications, as well as the controlling variables previously discussed.  
  The stretching action of the apparatus has no permanent effect on a modern felt of synthetic material, e.g., nylon, which assumes no permanent set when stretched as moderately as required in the practice of this invention. Any set in older felts of cotton or the like can be corrected merely by placing additional curved rolls at 54 and 76, oriented to give the centers of the felts a shorter path length than the edges through the return run to the rolls and 60, thereby correcting any bow distortion of the felts by equalizing their paths through their total lengths.  
  It has been noted that a curved roll set in the position of FIG. 4, contrary to this invention, would act to expand or contract the felt laterally. At an angle of 90 between the planes P and X, that is with the plane P coinciding with a perpendicular bisector of the wrap angle W, there would be no such action. However, in any intermediate angular position, such as that shown in FIG. 3, there will be an intermediate degree of expanding or contracting action. Which of these occurs will depend on whether the roll has its convex face directed downstream or upstream with respect to the movement of the felt. A beneficial side-effect can be obtained by facing alternate curved rolls in opposite directions, in that the pores of the felt will be alternately opened and closed, tending to squeeze out moisture and solids and thereby to clean the felt.  
  Referring now to FIG. 5, curved rolls 86 of the adjustable-bow type, for example as shown by the afore mentioned US. Pat. Nos. 2,689,392 or 2,898,662, may be used in place of the fixed-bow type shown in FIGS. 2-4. In this case, the plane P of curvature may coincide with a perpendicular bisector of the wrap angle W, since the amount of curvature may be adjusted appropriately without moving the axle. This affords a more facile means for controlling the differential stretching of the felt, since it is unnecessary to stop the paper machine in order to adjust the roll. In the illustrated posi tion of the roll 86, it has no expanding or contracting effect on the felt.  
  FIG. 6 gives a graphical comparison of typical mois ture profiles of paper webs made on the same machine with and without the use of this invention. The abscissa represents various points spaced across the width of the paper web, while the ordinate shows actual percentages of moisture content at these points relative toanominal standard.  
  Curve A shows a typical profile a paper made with rolls of the conventional perforated steam-supplied type in all of the dryer banks, e.g., at all of the positions 44-50 and 64-70 in FIG. I, and with no curved rolls in the felt runs. Curve B shows a typical profile using three curved rolls in the lower dryer bank, e.g., at the positions 64, 68, 72 in FIG. I, and with the perforated rolls left in place at the remaining positions. In curve A, the maximum difference in percentage moisture between the wettest location near the center, and the dryest near the left or back edge, was about 1.6 percent; while in Curve B, the maximum difference was reduced to about 1.0 percent.  
  In another test all curved rolls in the lower dryer bank were first placed in the inoperative positions shown in FIG. 4, that is, with the plane of curvature P at 0 to the plane X. Later the three curved rolls in the lower bank were adjusted to angles of 45 between the planes P and X. The following were the average percentages of moisture at spaced locations across the paper web:  
  The maximum difference in moisture content between any two parts of the paper in the first case, without the employment of our invention, was 1.20 percent; while the result of applying our method was a reduction to a maximum difference of 0.67 percent. Thus an improvement of about 44 percent in uniformity, as between the extremes of the moisture profile, was found. While our method produced a reduction of 0.93 percent in the moisture content at the front edge, and 0.71 percent at the back edge, it brought about a substantially greater moisture reduction of 1.24 percent in the center.  
  While we have illustrated preferred modes of practice of our improved method, and preferred embodiments of our apparatus, it should be understood that many variations may be made within the scope of the broader aspects of our invention, such as will readily occur to those skilled in the art.  
  Another alternative within the scope of the broader aspects of the invention is to employ known types of barrel-shaped rolls in place of the preferred types of curved rolls. However, this does not permit a convenient or practical means of adjustment of the amount of differential longitudinal tension applied at different points across the width of the felt. It would be necessary to replace such barrel rolls, which have a fixed curvature, with other differently curved barrel rolls in order to bring about such adjustments.  
  As previously explained, paper production results are subject to numerous variables many of which are not controllable, so that adjustments must be made frequently, as dictated by the judgment of the machine operator, to maintain proper specifications for the various grades of product. For this reason, the last-mentioned alternative expedient is not believed as desirable as the preferred use of curved rolls, although it is considered feasible.  
  It may also be feasible to employ longitudinally curved stationary bars of a known type in place of curved rolls. although the friction between the bars and a felt placed under longitudinal tension may be objectionable in this case.  
  Referring now to FIG. 7, a control system is shown for automatically adjusting the angular orientation of a curved roll 64 of the fixed-bow type shown in FIG. 3. The paper web 10 passes before a moisture scanner or detector 114 ofa conventional and commercially available type, which may operate on the principle of measurement of transmitted nuclear radiation, or that of measurement of admittance of radio-frequency radiation, or otherwise, to determine the moisture content of the paper. Suitable scanners are available, for example, from the Measurex Company of Cupertino, Calif, or from the Industrial Nucleonics Corporation of Columbus, Ohio. The scanner 114 is traversed back and forth across the width of the web 10, as shown by the arrows, by suitable mechanism (not shown), and is supported by a stationary frame 116, 118.  
  Signals indicating the moisture content at various transverse locations of the scanner are transmitted on a continuous basis through leads 112 to a control circuit 110, which compares these signals to determine the difference in moisture content across the width of the paper. The control circuit compares this difference with a predetermined acceptable value, and supplies output signals through leads 108 to control a reversible servomotor 106, which has a driving connection 104 with a gear 102, in turn meshed with a gear 100 mounted onthe shaft 82 of the roll 64. The control circuit and servomotor are so arranged that an increasing difference between the moisture content in the center and edges of the paper, in excess of a predetermined acceptable difference, turns the axle 82 clockwise as seen in FIG. 7, thereby increasing the path length of the center of the felt (see FIG. 3) to tighten it against the drums 18, 22, etc., and thus diminish the relative moisture content in the center of the paper. Conversely, a decreasing difference in the moisture as between the two regions to a value less than a predetermined acceptable difference, turns the axle 82 counterclockwise to reduce the pressure between the felt and the drums, and thereby restore the difference in moisture toward the reference value.  
  An automatic control system for an adjustable-bow curved roll 86, of the type shown in FIG. 5, is illustrated in FIG. 8. The roll is of the same general type described and claimed by US. Pat. Nos. 2,689,392, issued Sept. 2 l 1954; and 2,898,662, issued Aug. 11, 1959, both to John Douglas Robertson, and assigned to the assignee of this application.  
  The roll 86 has an axle assembly 82 which includes an elongated axle member 128. This member, initially a cylindrical bar, is longitudinally slit at 131 on an axial plane from the right end as seen in FIG. 8, through a substantial part of its length, but the left end is left integral. The slit 131 forms substantially hemicylindrical elements 130 and 132. The right end of the element 132 is secured in a cup member 152, as by means of screws 154. The right end of the element 130 is left free, and is axially abutted by a piston 136 which is conformably received in a cylinder 134 formed in the cup 152. By supplying pressurized fluid from a conduit 126 to the cylinder 134, a controllable amount of longitudinal compression may be applied to the element 130, resulting in a corresponding application of longitudinal tension to the element 132. This causes the axle assembly to bend to an extent which is proportional to the applied fluid pressure, according to principles which are described by the aforementioned patents, are now well understood in the art,-and therefore need not be further elaborated.  
  The axle assembly 82 is incorporated in a roll assembly 86 which has a flexible hollow surface sleeve 156 of rubber or other suitable material. This sleeve is received over an axially-spaced array of annular cylindrical spools 140, which are in turn rotatably mounted on the axle assembly by means of a series of axially spaced bearings 138. The sleeve 156 is located with respect to the end-most spools by means of discs 158 attached thereto. The bearings 138 are axially located by a ring 150 secured to the axle near one end, annular cylindrical spacers 148, 142, and 144, and the cup member 152. A series of the spacers 142 and 144 have their mating ends chamfered as at 146 to allow realignment as required by changes in the degree of curvature of the axle. Spring means such as Belleville washers (not shown) may be interposed in the stack of bearings and spacers to facilitate realignment of the parts as the axle curvature is changed, while maintaining axial preloading sufficient to insure accurate positioning of the bearings. The sleeve 156 is thus supported for rotation in a stable curved form which follows the curvature of the axle assembly 82.  
  The control system of FIG. 8 includes a control circuit like that of FIG. 7, which is supplied by leads 112 with signals received from a similar scanner 114&#39; (not shown) indicating the moisture content of the paper web at various transverse locations. The circuit 110 supplies output signals through leads 108 to control a three-way electrically operated valve 120, which is supplied with pressurized fluid through a conduit 122. The valve is operable by the output signals to supply pressurized fluid to the cylinder 134 through the conduit 126, or to drain fluid from the cylinder through an exhaust conduit 124, thereby to control the pressure acting on the piston 136.  
  The control circuit 110 and valve are so arranged that an increasing difference between the moisture content in the center and the edges of the paper, in excess of a predetermined acceptable difference, applies increased pressure to the piston 136 and thereby increases the curvature of the roll 86. This lengthens the path of the center of the felt 58 (see FIG. to tighten it against the drums 18, 22, etc.. and thereby reduce the excess moisture content in the middle of the paper. A decrease in the relative moisture of the center to less than a predetermined acceptable value, conversely, results in a reduction in the pressure on the piston 136 and a correspondingly decreased curvature of the roll 86. The pressure of the felt against the center of the drums is consequently relieved to restore the moisture differential to an acceptable value.  
  What we claim is: l. The method of drying a paper web of indefinite length issuing from a papermaking machine, by means of at least two heated dryer drums and a continuous felt which comprises the steps of:  
 arranging said drums to form a path for the paper web to be wrapped successively about a substantial arcuate portion of each said drum. and to form a path for said felt to be wrapped about a substantial arcuate surface portion of each said drum in overlapping relation to said path for the paper web about each said drum; while extending the paths between said drums of various laterally spaced, longitudinally extending portions of said felt to different lengths changing continuously from the edges of the felt toward its center; detecting differences in the moisture content of the paper web at central and edge portions thereof;  
 said extending step being carried out by varying the relative path lengths of said various felt portions in a manner to apply different degrees of tension to said various felt portions and thereby to maintain the detected moisture differences within predetermined limits.  
  2. The method recited in claim 1, in which the step of extending said paths is carried out in a manner to provide path lengths and degrees of felt tension increasing continuously from a minimum at the edges of said felt to a maximum at its center.  
  3. The method of drying a paper web of indefinite length issuing from a papermaking machine, by means of at least two heated dryer drums, a continuous felt, and at least one roll having longitudinal curvature parallel to an axial plane of curvature thereof, which comprises the steps of:  
 arranging said drums to form a path for the paper web to be wrapped about a substantial arcuate portion of each drum successively; arranging said longitudinally curved roll to support said felt for movement in a path in which said felt is wrapped about a substantial arcuate surface portion of each said drum, in overlapping relation to said path for the paper web about each said drum;  
 while angularly orienting said roll to provide different path lengths between said drums for various laterally-spaced longitudinally extending portions of said felt, said path lengths changing continuously from the edges of said felt toward its center;  
 detecting differences in the moisture content of the paper web in central and edge portions thereof, and comparing the detected differences with predetermined standards;  
 while adjusting said roll to vary the difference between the path lengths of said various felt portions between said drums, and thereby to apply different degrees of tension to said various felt portions, as  
 required to maintain the detected moisture differences within said predetermined standards.  
  4. The method recited in claim 3, in which the step of angularly orienting said roll is carried out in a manner to provide path lengths and degrees of felt tension increasing continuously from a minimum at the edges of said felt to a maximum at its center.  
  5. The method recited in claim 3, said roll having convex and concave surface portions;  
 said step of orienting said roll being carried out by extending said convex surface portion of said roll into said central portion of said felt;  
 said step of adjusting said roll being carried out by increasing the extension of said convex surface portion into said felt in response to detection of an increasing difference in moisture content between the central and edge portions of the paper web in excess of a predetermined standard, and by decreasing the extension of said convex surface into said felt in response to detection of a decreasing difference in moisture content between the central and edge portions of the web to less than a predetermined standard.  
  6. Apparatus for drying a paper web of indefinite length issuing from a papermaking machine, said apparatus comprising in combination:  
 an array of at least two heated drums arranged to guide the paper web in a path of longitudinal motion to wrap the web about an arcuate portion of each of said drums in a serial order;  
 a continuous unitary felt extending across the entire width of said web;  
 means for guiding said felt about an arcuate portion of each of said drums in serial order and in overlapping relation to said paper web;  
 and at least one longitudinally-curved roll engaging said felt between said drums, said roll having a fixed curvature in planes parallel to an axial plane of curvature thereof; said roll being oriented to extend various laterallyspaced longitudinally extending portions of said felt through different path lengths between said drums, said path lengths changing continuously from the edges of said felt toward its center; thereby to apply different degrees of tension to said various portions of said felt in a manner to increase the uniformity of moisture distribution in said paper web;  
 means supporting said roll for angular adjustment of the position os said plane of curvature thereof with respect to the path of said felt;  
 control means for detecting differences in the moisture content of the paper web at central and edge portions thereof, and for comparing the differences with predetermined standards;  
 and adjusting means constructed and arranged for angularly adjusting the position of the plane of curvature of said roll;  
 said control means being operatively connected to control said adjusting means to position said plane of curvature of said roll in a manner tending to reduce said moisture differences to no more than said predetermined standards, by means of resulting adjustments of the relative path lengths of central and edge portions of said felt.  
  7. Apparatus as recited in claim 6, said roll having a convex surface portion and having said plane of curvature thereof oriented to turn said convex portion into said felt;  
 said control means being constructed and arranged to actuate said adjusting means to adjust said plane of curvature in such an angular direction as to extend said convex portion further into said felt in response to detection of an increasing difference in moisture content between said central and edge portions of the paper web in excess of a predetermined standard, and to actuate said adjusting means in an opposite angular direction to partially retract said convex portion from said felt in response to detection of a decreasing difference in moisture content between said central and edge portions of the paper web to less than a predetermined standard.  
  8. Apparatus for drying a paper web of indefinite length issuing from a papermaking machine. said apparatus comprising in combination:  
 an array of at least two heated drums arranged to guide the paper web in a path of longitudinal motion to wrap the web about an arcuate portion of each of said drums in serial order;  
 a continuous unitary felt extending across the entire width of said web;  
 means for guiding said felt about an arcuate portion of each of said drums in serial order and in overlapping relation to said paper web;  
 and at least one longitudinally curved roll engaging said felt between said drums. said roll having an adjustable degree of curvature in planes parallel to an axial plane of curvature thereof; said roll being oriented to extend various laterallyspaced longitudinally extending portions of said felt through different path lengths between said drums, said path lengths changing continuously from the edges of said felt toward its center; thereby to apply different degrees of tension to said various portions of said felt in a manner to increase the uniformity of moisture distribution in said paper web;  
 control means for detecting differences in the moisture content of the paper web at central and edge portions thereof, and for comparing the differences with predetermined standards;  
 and adjusting means constructed and arranged for adjusting the degree of curvature of said roll;  
 said control means being operatively connected to control said adjusting means to adjust the curvature of said roll in a manner tending to reduce said moisture differences to no more than said predetermined standards, by means of resulting adjustments of the relative path lengths of central and edge portions of said felt.  
 9. Apparatus as recited in claim 8, said roll having a convex surface portion and having said plane of curvature thereof oriented to turn said convex portion into said felt;  
 said control means being constructed and arranged to actuate said adjusting means to increase the curvature of said roll. thereby to increase the path of a central portion of said felt, in response to detection of an increasing difference in moisture content between said central and edge portions of the paper web in excess of a predetermined standard, and to actuate said adjusting means to decrease the curvature of said roll, thereby to reduce the path of a central portion of said felt, in response to detection of a decreasing difference in moisture content between said central and edge portions of the paper web to less than a predetermined standard. 10. Apparatus for drying a paper web of indefinite length issuing from a papermaking machine, said apparatus comprising, in combination:  
 an array of at least two heated drums arranged to guide the paper web in a path of longitudinal motion to wrap the web about an arcuate portion of each of said drums in serial order a felt of indefinite length;  
 means for guiding said felt about an arcuate portion of each of said drums in serial order and in overlapping relation to said paper web;  
 means constructed and arranged to extend the path of a central portion of said felt between said drums to a greater length than the edge portions thereof, thereby to draw a central portion of said felt at least as tightly against said paper web and said drums as edge portions thereof;  
 control means for detecting differences in the moisture content of the paper web at central and edge portions thereof, and for comparing the differences with a predetermined standard;  
 said path-extending means being adjustable to vary the differential between the path lengths of central and edge portions of said felt;  
 and adjusting means operatively connected with said path-extending means and with said control means to adjust the differential path lengths of said portions of said felt in response to comparison of moisture differences with said standard by said control means;  
 said control means and said adjusting means being constructed and arranged to control said pathextending means in a manner tending to reduce said moisture differences to no more than said predetermined standard.