|Publication number||US3925906 A|
|Publication date||Dec 16, 1975|
|Filing date||Aug 14, 1972|
|Priority date||Aug 14, 1972|
|Also published as||CA1015153A, CA1015153A1|
|Publication number||US 3925906 A, US 3925906A, US-A-3925906, US3925906 A, US3925906A|
|Inventors||Chance James L, Han Shu Tang|
|Original Assignee||Beloit Corp|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (11), Referenced by (21), Classifications (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
llnite States Patet 1191 Chance et al.
[ Dec. 16, 1975 HOT WIRE DRYING  Inventors: James L. Chance, Rockford; Shu
Tang Han, South Beloit, both of 111.
 Assignee: Beloit Corporation, Beloit, Wis.
 Filed: Aug. 14, 1972  Appl. No.: 280,667
 References Cited UNITED STATES PATENTS I 1,660,640 2/1928 Asten 34/111 2,060,945 11/1936 Mellentine 2,919,495 1/1960 Underhay et al.. 3,191,312 6/1965 Allander 3,325,91 1 6/1967 Fleisher et a1. 34/159 3,368,361 2/1968 Rietdijk 62/45 3,371,427 3/1968 Thygeson, Sr. 34/155 3,504,443 4/1970 Gustafsson 34/1 17 3,696,522 10/1972 Krikorian 34/115 FOREIGN PATENTS OR APPLICATIONS 675,936 12/1963 Canada 34/116 Primary Examiner-Carroll B. Dority, Jr.
Assistant ExaminerLarry I. Schwartz Attorney, Agent, or Firm-Hill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson 57' ABSTRACT A method and apparatus for drying a continuous traveling fibrous web by passing it between a pair of looped endless wire belts and in surface contact therewith to dry the web with the belts being wrapped over cylinders and the wire belts being heated before contacting the web by directing a flow of heated gas therethrough and the gas directed onto the web after it has passed through the belts.
3,316,657 5/1967 Haywood.... 34/116 3,320,677 5/1967 Reitzel 34/116 12 Claims,5Drawing Figures 26\U ,86 2a a Q, 42
I .21 H0 1, r u u w US. Patent Dec. 16,1975 Sheet1of4 3,925,906
US. Patent D6C.16,1975 Sheet2of4 3,925,906
S 3 L11 5% :E LL1 2 AIR-GAS MIXTURE+ 25,29
1 COMBUSTION 47 I 4a zso 46 PROPORTIONAL 49 CONTROL ELECTROSTATIC FILTER 5| 3 E X -26,27 LL] HOOD ENCLOSURE Fig.2
US. Patent Dec. 16, 1975 Sheet3of4 3,925,906
HOT WIRE DRYING BACKGROUND OF THE lNVENTlON The invention relates to an improved method and mechanism for drying a continuous traveling paper web such as in a paper making machine after the web is received from the press section and more particularly to a new paper web drying concept wherein the web is carried between two loops of hot wire screens in a long nip until the web is dry andthe dry web is then taken off the wires and the wires reheated on their return runs to the desired temperature.
As will be recognized from the disclosure of the principles of the invention, the concepts may be employed in various types of web drying, but the inventive principles uniquely coact in the continuous operation of a paper making machine and will be described in that environment.
The most essential function in the drying of a paper web is to develop a bonded structure of cellulosic fibers. The fiber-to-fiber bonding process is initiated in the compression of a wet sheet as it is first formed such as between two wires in a twin wire forming machine and thereafter by compression in a press section and completed by the evaporation of water. It appears that no alternative method, mechanical or chemical is available to replace evaporative drying for the purpose of bonding.
When paper web is dried with free shrinkage being permitted, it has a cockled appearance. For flat paper processing to obtain sufficient dimensional stability, it must be dried uniformly under restraints. If fully restrained, the resulting paper web has a high elastic modulus and tensile strength, but a low stretch. Some restraint is required in the drying for most grades of paper. When paper is dried in contact with a polished surface, such as on the drum of a conventional drum dryer, its smoothness is greatly influenced by the contact pressure imposed on the paper web and reinforced by evaporation of water. Smoothness can also be imparted to paper by frictional forces in a sliding or shearing action relative to a polished surface such as in a calender. In both cases the smoothness is the result of plastic conformation of the fibrous materials to a rigid surface.
The commercial drying of paper web in a conventional paper machine is normally done by passing the web over a steam drum with a felt holding the web to the drum. Certain problems are encountered in this type of drying, and these problems are compounded and new problems appear in ever increasing machine operating speeds. Some of these problems have been resolved thus far through improved condensate siphons for the steam drums, incorporation of pocket ventilators, use of open mesh fabrics and various attempts to control the tendency of the web to flutter at high speeds. However, further increases in speed of machines are limited by limitations inherent in this type of lead to web breaks or to web stretching and damage and nonuniformity in the paper produced. Thus, the unsupported draws between dryer drums can constitute a major obstacle in further speed advances.
Nonuniform drying is another serious drawback in the conventional drum type dryer system. Even if the incoming web is uniform in every respect with respect to moisture content and basis weight, it can develop an uneven moisture profile across the machine because of variation in drying rates. Among all the contributing factors, irregular air conditions in the dryer pockets are a major cause of nonuniform moisture. The higher the speed, the more amplification occurs to the problems of nonuniformity and instability.
Over-drying as a means of correcting nonuniform moisture profiles is both inefficient and expensive. It adds an excessive burden to the massive and complicated drying system, wastes heat energy and often results in requiring the addition of controlled moisture to the sheet, or in a sheet which is more dry than desired. The capital costs of the dry end of the machines for high production units have already exceeded those of the wet end, and will rise more rapidly as new types of formers and presses become established in practice and machine speeds increase. Since heating costs cannot be reduced, the paper manufacturers must seek significant capital savings through either increased production or reduced investment or both.
Other forms of drying have been attempted to avoid the foregoing difficulties including high velocity air impingement. The mechanism for utilizing the impingement system is complicated in construction and has prevented this from wider use. Through drying by passing air through the paper web offers higher drying rates and better uniformity, but problems of air leakage and power waste present obstacles. Radiant drying has been attempted but if the heat source is gas, efficient utilization of the residual heat in the hot gas poses a problem. The means of restraining the web and the necessary precautions against fire hazards introduce further difficulties. Microwave and dielectric drying require high frequency electric power and their use is restricted to auxiliary and special application because of high capital and operating costs incurred in attempting to correct moisture profiles. With certain of these methods, the elimination of dryer felts is necessary, and this in turn requires a new means of web transfer and support introducing additional difficulties for high speed operation.
It is accordingly an object of the present invention to provide a new concept of drying a traveling paper web which is particularly capable of use in a high speed paper making machine and avoids disadvantages of methods and structures heretofore used.
In accordance with the present invention, the fragile paper web is positively controlled by sandwiching it between two permeable flexible conveyors which are heated prior to contact with the web for transferring their latent heat to the web and evaporating the moisture. These conveyors insure complete support and full control of the web at all times even in the event of a web break. The opposed permeable conveyors serve as web restraints in both machine cross directions allowing the possibility of minor shrinkage. The present invention permits drying from both sides of the web simultaneously and permits through drying to be used as an auxiliary drying means forcing air through the web either by pressure or suction. By sandwiching the web between heated premeable conveyors, heat is supplied to the web primarily by conduction, and air impingement can readily be accommodated on both-sides of the web as an auxiliary drying means. Direct conduction of the heat from the conveyors occurs continuously during contact with the permeable carriers and radiation and convection also. function to increase the temperature of the web to increase the speed of evaporation. In a preferred form, the permeable conveyors are metallic wire screens formed of a material-which has a very high thermal conductivity. We have found that in the drying of a light paper such as newsprint, the drying time can be reduced to less than one-half. In a paper such as newsprint, the temperature drop of the wires and the drying time can be considered as follows:
Basis weight of paper W 0.01 lb/ft Basis weight of wire w 0.3 lb/ft lnitial moisture of paper M l.5 lb/lb Final moisture of paper M, 0.l lb/lb Specific heat of wire o 0.09 Btu/lb/F The heat required for evaporation is:
2wc AT= 1000 W(M,- M Btu/ft where the latent heat of evaporation is taken to be 1000 Btu/lb, and the factor 2 accounts for the heat supplied by both wires. The temperature drop of the wires is, therefore, 260F. Taking the heat capacities of water and paper into consideration, plus some minor heat losses, AT will not exceed 300F.
If we treat the paper as a heat sink, the wire temperature would decrease exponentially with time in a manner described by Let the wire temperatures, T,- and T,, be 550 and 250F, respectively, and the paper temperature, T, be 200F. Then the drying time, t, would be inversely proportional to the heat transfer coefficient, h, as
h sec Ill where the air conductivity is taken to be 0.016 Btu/hrft-F and the fiber conductivity assumed to be 0.14. If the void fraction, 6, of the paper is about 0.6, the paper conductivity is roughly 0.07. Uncalendered newsprint has a caliper about 0.004 in. Its equivalent heat transfer coefficient for half of the thickness would be nearly 400. The overall heat transfer coefficient from the wire to the mid-plane of the paper would then be about 130,
and the drying time 3 sec. In a conventional newsprint machine the drying time is 7 8 sec.
Heat conduction from the wires to the web improves with contact pressure. We have discovered that excess pressure tends to enhance wire impressions or marks on the surface of the web, especially in the wet state, and a limited pressure is employed for optimum heat transfer, but one which will not create excessive wire impressions. It has been found, however, that the normal wire impressions which are encountered will be erased in passing the web through a calender after it leaves the dryer section. The characteristics of paper are affected by contact pressure. The four major characteristics of newprint or ground wood publication grades, tensile, stretch, bulk and smoothness are affected by contact pressure and a compromise of pressure is employed to enhance these characteristics.
In determining the maximum pressure which can be employed to create wire impressions that will be removed by calendering, the other characteristics bulk, tensile, and stretch may be adjusted to some extent by the duration of restraint on the paper web by the wire belts between which the web is carried for drying. If a web is dried unrestrained, its stretchability will be increased, but its ultimate tensile strength will be less than if the web is dried fully restrained. In the fully restrained drying, the tensile strength will be greatest, but the stretch will be reduced. By drying to approximately 30% moisture, fully restrained and then completing the drying unrestrained, a good tensile strength is obtained with good stretch properties. The present invention contemplates, in addition to drying between traveling wire belts, partial drying in a restrained manner between such wire belts and completion of drying in an unrestrained manner with the belts separated to release the web, finishing the drying using air impingement.
With current developments in paper making machines, two-sided uniformity has been greatly improved. This has been because of the contributions of developments in twin wire forming and developments in presses. The features of the present invention are wholly compatible with two-sided uniformity in that this characteristic is maintained by the uniform application of drying heat from both sides of the web by sandwiching the web between the heated wires.
Paper uniformity is also enhanced. Because of the high conductivity of metallic wires, heat tends to flow along the wire strands in short distances as soon as temperature differences develop due to variations of moisture in the web. Such lateral heat flow from a dry spot to a wet neighboring location is helpful in lessening localized nonuniformity. Because the web is carried between open wires, through drying is possible, and through drying serves best as a final correction of moisture profile. If a sufficient amount of heated gas at an appropriate temperature and humidity is forced through a nearly dry web, the web tends to approach equilibrium with the gas resulting in uniform moisture. This system is more effective than dielectric or microwave drying in improving moisture uniformity. In the instance where persistent wet streaks occur in the web due to defects in wet end operations, an adjustable gas impingement structure may be used for initial correction. Both the gas impingement cap and through drying systems may be used for increasing drying rates if gas conditions are properly adjusted.
The wet web received from the press section of a machine is passed between the traveling heated wire belts which are kept at an elevated temperature and are cyclically heated to the maximum temperature tolerable by the sheet. The sheet is readily removed from the wires eliminating the problem of picking which occurs with a drum dryer system at elevated temperatures and high speeds. The cost of felts is eliminated since the carrier is the heat transfer element itself.
Other objects and advantages will become more apparent with the teaching of the principles of the invention, as will equivalent structures which are intended to be covered hereby, in the disclosure of the preferred embodiments in the specification, claims and drawings in which:
DESCRIPTION OF THE DRAININGS FIG. 1 is a schematic illustration of a structure operating in accordance with the principles and method of the present invention;
FIG. 2 is a schematic illustration showing the gas flow system;
FIG. 3 is a fragmentary plan view of a portion of the wire for carrying and drying the web;
FIG. 4 is another schematic view illustrating a form wherein a portion of the total drying is by heated air flow; and
FIG. 5 is a schematic view illustrating a preferred form of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a wet web is continuously fed into surface-to-surface contact with a traveling flexible permeable heating member. This member is formed by two traveling flexible hot wire screens and 11 which form a nip N therebetween. The web passes through an opening 21 in an enclosing housing 22, coming from a press section. The wires or screens 10 and 11 are each formed in loops supported on guide rolls with nip entry rolls shown at 12 and 13. The web passes into the nip between the wires which is trained in a sinuous path over a plurality of cylindrical drums 15, l6, 17, 18, 19 and 20 which may be formed with porous surfaces to accommodate the escape of moisture. The heated wires immediately conduct heat to the moisture in the web by conduction, convection and radiation so that the moisture vaporizes and escapes through the openings in the wires.
The wires are cyclically heated with heat being applied before they receive the web such as by passing a flow of heated gas through the wires. Because of the amount of metal exposed in the woven wires, they will heat very rapidly.
High temperature gas is supplied through inlet ducts 26 and 27. Heaters such as burners 28 and 29 heat the entering gas.
Ducts and deflectors 30 direct the gas passing through the wire into a path back and forth through the wires to heat them, as shown by the additional baffles 31 and 33. After the heated gases have made a number of passes through the wire, they are guided into a low velocity air impingement cap 34, 35, respectively for the wires 10 and 11, so that the gases directly engage the web to heat the web carried between the wires. The heated gas passes out of the housing 22 through ducts 36 and 37 which are provided with exhaust fans, not
The heated air passing through the low velocity impingement caps 34 and 35 thus additionally heats the web and contributes its heat to the web as it flow through the housing carrying away evaporating moisture.
The wires are cleaned as they travel through their path, in advance of passing back onto the web by jets 38 and 40 for the upper wire and 39 and 41 for the lower wire.
To aid in containing the heat within the inner housing 22 an outer housing 42 is provided as an enclosure. An exhausting fan 43 is provided to reduce the pressure between the housings 22 and 42 and with a partial vacuum existing in the space between the housings, heat transfer will be reduced.
The wires may be of a woven or perforate construction, and as illustrated in FIG. 3, are preferably of a highly heat conductive woven wire such as known to the trade as Invar. In another form the belts which carry the web between them may be metal sheets each having a planar surface so as to be in very good heat conducting engagement with the web, with distributed openings cut through the belts. These openings are preferably in the form of small rectangles or squares.
The belts are preferably of a material which is capable of withstanding flexural fatigue and thermal stress for continued high speed operation. The wire is a more efficient heating exchanger than a flat belt in that it has more area per square foot for heat exchange, so that it can be more rapidly heated by the heated gasses and can convey its heat more rapidly to the web. The planar surface material above referred to is also known to the trade as Electro-mesh which is a wire screen with a flat surface and which holes are electrostatically perforated.
While in a preferred form the wires are supported on a series of cylinders, it is also contemplated to use an air floating system wherein pressurized air is blown through perforate surfaces to support the wire belts with the sheet sandwiched therebetween.
The wires are heated to a temperature of about 45 OF to 550F (depending upon the wire mass per unit area). There is sufficient thermal energy in the wires to completely dry the web, and in the typical machine, the wires are capable of drying a newsprint weight sheet with a moisture content of High surface contact pressures between the web and wires is to be avoided, but this can be adjusted to a force to cause only marking which can be removed by the calender. The wire force is controlled by adjustment of tension rolls 21 and 22 which have a tension mechanism shown schematically at 21a and 22a.
Even at the relatively elevated temperatures above referred to, picking of the sheet does not occur because of the limited contact area between the sheet and the wire belts.
The web is restrained between the wires for its drying process, and a relative smoothness is imparted to the web because of the relative movement between the wires and the web as they are threaded back and forth in the sinuous path between the drums.
FIG. 4 illustrates an arrangement wherein the structure of FIG. 1 is arranged so that partial drying is completed by air caps with the web carried on the lower wire solely after it leaves the housing. This final drying by the air caps in section 54 is done with the web unrestrained to attain the improved stretchability hereinabove referred to. Following the air cap drying, the web 7 passes through a calender stack 56 to be wound on a reel 58.
The web enters an enclosing housing 59 through an opening 66 to pass between twin looped wires with the upper looped wire shown at 71 and the lower looped wire at 72. The upper wire is heated by an in-flow of hot air through a duct 67, and the lower wire is heated by an in-flow of hot air through a duct 68. The wires are wrapped in serpentine fashion over a series of drums shown at 62 and 63 with the last drum sequence being shown at 64. During the travel of the web W, a predetermined portion of the moisture is evaporated from the web. The web leaves the housing through an opening 65 to pass through the air cap section 54. The air cap section includes a plurality of drums such as 69 over which the web is wrapped, and on the outer surface of each of the drums is an air cap 70 which directs an atmosphere of heated drying air over the web to complete the drying.
Preferably in the first stage of drying, the wires sandwich the web therebetween until 75% to 80% of the moisture is evaporated. This drying interval will take a period of time for approximately 2 seconds or less. After the sheet is separated from the wires, it passes through a breaker stack 73, and the final drying is accomplished by the air caps 70. The drying which occurs in the air caps continues until the web is bone dry. In some circumstances it may be more desirable to use steam cylinders rather than the air cap arrangement. This system is advantageous inasmuch as once the wire temperatures drop to a point where the drying rate begins to diminish, the drying energy is supplied by another means. A system of this type has the further advantage that the sheet properties can be controlled to some extent. It is well-known that restraining a wet web during drying results in a sheet with improved tensile properties, but also causes a large reduction in stretch of the sheet. Of course, in printing operations involving newsprint it is desirable to have the added tensile strength, but a certain amount of stretch is desirable also. It has been found that by restraining a sheet and drying it to to moisture and then releasing it and drying it unrestrained, can obtain a sheet with superior tensile properties with little loss in stretch. Therefore, these desirable sheet properties can be obtained by using a twin wire drying system to evaporate 70% to 80% of the moisture while keeping the web restrained. Finally, the Web is then dried to final dryness, essentially unrestrained to obtain the needed stretch. In a 3000 feet per minute machine, a drying length of only approximately 50 feet will be required to remove three-fourths of the moisture from a newsprint weight sheet. Then the final drying is provided by the air cap arrangement shown, or even by through drying. The through drying would offer the advantage of large drying rates even at relatively low moisture contents, and would require smaller fan horsepower requirements for the dry web.
FIG. 5 illustrates the preferred form of the hot wire drying arrangement and includes an insulated outer housing 80. The moisture carrying web W enters through a slot 89 into the housing and passes up to be sandwiched between a pair of looped traveling drying wires 81 and 82. The wires are porous and formed of a high heat content material to impart heat to the moisture in the web and evaporate it by conductive and convection drying while the web is sandwiched between the wires. The wires are threaded in serpentine fashion with the web therebetween over a series of carrying drums 83, 84, 85 and 86. While the wire is hottest, the web is sandwiched therebetween and carried over the first two drums 83 and 84. As the wires begin to cool somewhat, a series of air cap hoods 87 are positioned to direct a flow of heated air against the web between the wires as it is carried over the third drum 85. Additional air cap hoods 88 are provided to direct heated air over the web while it is carried on the last drum 86. The web then passes out of the housing through a slot 90.
The air cap impingement boxes 87 and 88 are side fed with heated air. The heated air evaporates moisture from the web, and the heat also serves to increase the temperature of the wire. It will be noted that on the drum 85, one surface of the web, that adjacent to wire 81 is exposed to the air caps, and on the other drum 86, the other wire 82 and the other surface of the web exposed.
The air that presses out of the air caps and over the web which escapes into the housing is then used to heat the oncoming wire before it engages the web. The air is directed through exhaust passages 92 and 93 for the wires 81 and 82 respectively. The exhaust passages are arranged so that the hot air makes numerous passes over the wire. As shown for the wire 81, the air first flows over the wire in a duct space 92a and then flows over the wire through another duct space 92b and subsequently in another duct space 92c to thereafter be exhausted from the housing. This same air can be dried to be again used in the air caps or it can be used to preheat additional air which is fed to the air caps. It will be noted that the air in leaving the air caps 87 and 88 picks up moisture from the web, but the heat is utilized in the air thereafter being passed over the wires. The fact that the air has then become humid does not detract from its heating ability in heating the wire. The air flows into the air caps at 600F to 800F, and drops in temperature to approximately 450F in evaporating moisture from the web. The 450 air passes out through the exhaust ducts 92 and 93 to heat the wire. The wire, after it leaves the last drum 86, has a temperature of about 300F and is heated to 425 to 450F as it passes through the duct passages 92 and 93. These temperatures can, of course, be varied in accordance with the moisture content of the web and in accordance with other factors such as the speed of the web travel through the dryer mechanism.
We claim as our invention:
1. An apparatus for drying a traveling fibrous moisture containing web which comprises in combination:
first and second flexible pervious looped wire belts formed of highly heat conductive metal;
means for guiding a fibrous moisture containing web between the wire belts for heating the web by conduction and convection from the wires of the belts;
a plurality of dryer drums over which the wire belts are trained to travel in a serpentine path for carrying the web through a drying path sandwiched firmly between the wire belts insuring heat conducting drying contact between the wire belts and the web;
a first heating chamber positioned in advance of the drying path with said first wire belt passing therethrough;
a second heating chamber positioned in advance of the drying path with said second wire belt passing therethrough;
a first heating means for said first chamber;
a second heating means for said second chamber; each of said heating means heating each of said wires to a high temperature sufficient to heat the web by contact with the wire belt to evaporate the moisture and dry the web;
and an enclosure surrounding the web and wire belts in said dryihg path for receiving moisture evaporating from the web.
2. An apparatus for drying a traveling fibrous moisture containing web constructed in accordance with claim 1:
wherein each of the wire belts is a flexible sheet form member having a planar surface area engaging the web with a plurality of openings formed through said belts for the escape of evaporating moisture.
3. An apparatus for drying a traveling fibrous moisture containing web constructed in accordance with claim 2:
wherein the openings through each of the wire belts are rectangular in shape.
4. An apparatus for drying a traveling fibrous moisture containing web constructed in accordance with claim 1:
wherein said first and second heating means heat the wire belts to a temperature in the range of 420F to 450F.
5. An apparatus for drying a traveling fibrous moisture containing web constructed in accordance with claim 1:
wherein said first and second heating means include means for providing a flow of heated air through the belts passing the air through the belts from both directions.
6. An apparatus for drying a traveling fibrous moisture containing web constructed in accordance with claim 5:
wherein the air is heated to a temperature in the range of 600F to 800F.
7. An apparatus for drying a traveling fibrous moisture containing web constructed in accordance with claim 5:
10 including means for directing the air from the heating means into said enclosure after passing through said wire belts. 8. The method of drying a traveling fibrous moisture containing web comprising the steps:
tightly sandwiching the web between a pair of looped traveling flexible pervious wire belts of highly heat conductive metal for a drying run so that the web will be heated and moisture evaporated therefrom due to conduction and convection from the belts; directing the belts through a sinuous path so that the tension in the belts firmly holds the web in heat transferral intimate contact therewith during the drying path; heating each of the belts in advance of the drying path and in advance of contact with the web to a temperature sufficient to heat the web by contact therewith to evaporate moisture and substantially completely dry the web. 9. The method of drying a traveling fibrous moisture containing web in accordance with the steps of claim 8: wherein the wire belts are heated in advance of contact with the webs by passing heated air through the wire belts from a plurality of directions until the belts are thoroughly heated. 10. The method of drying a traveling fibrous moisture containing web in accordance with the steps of claim 9: wherein the wires are heated to a temperature in the range of 425F to 450F. 1 l. The method of drying a traveling fibrous moisture containing web in accordance with the steps of claim 9. wherein the air which is passed through the wire belts is heated to a temperature in the range of 600F to 800F. 12. The method of drying a traveling fibrous moisture containing web in accordance with the steps of claim 9: wherein the air is directed into engagement with the wires at a location where the wires are in contact with the web after the air has passed through the wires in advance of contact with the web.
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|U.S. Classification||34/422, 34/123|
|International Classification||D21F5/00, D21F5/02|