Paper Machine

a multicomponent continuous-operation aggregate on which paper and several types of cardboard are made from a highly dilute aqueous suspension of fibers.

There are two basic types of paper machine: the fourdrinier (flat-screen) machine, used in the manufacture of the basic types of paper, and the cylinder machine, on which a limited range of papers and cardboards are made. These two types have various mechanisms for the release of the paper pulp onto the wire screen and for the formation of the paper sheet, but the construction of the remaining parts and the technical process of preparing the paper are analogous (with the exception of machines for “dry preparation”).

Figure 1 is a diagram of a fourdrinier paper machine, including—in addition to the equipment of the machine itself—auxiliary equipment designed for the preparation of the paper pulp before feeding it onto the screen. Types and design of auxiliary equipment vary greatly.

The prepared paper pulp, at a concentration of approximately 3-4 percent, is transferred by means of a pump from the pulp processing compartment to the machine chest and then to the paper machine. An even degree of beating and a uniform concentration of the pulp throughout the entire volume are achieved by constant mixing of the pulp in the machine chest. The pulp is first diluted with white water (because of dehydration of the paper pulp on the wire screen of the paper machine to a concentration of 0.1-1.5 percent) and then passed through purifying apparatus (knot screens, centricleaners, centriscreens, and so on), in which various foreign substances and rough mineral and fibrous particles are removed. From the purifying apparatus the paper pulp is passed on to the flow box, which provides an outflow of pulp at a definite rate and in a stream of uniform thickness along the entire width of the screen.

The paper machine is composed of the following main parts: the fourdrinier section, in which the paper web is continuously formed from a dilute suspension and on which the first portion of excess water is removed; the press part, in which the dehydration and thickening of the web take place; the drying part, in which the moisture remaining in the paper web is removed; and the finishing end, in which the web undergoes the necessary processing to give it gloss, density, and smoothness and where it is wound on spools.

The fourdrinier section is an endless screen (woven with threads of various alloys of copper and synthetic materials), which is driven off the couch roll. On new machines that have vacuum-suction mechanisms the fourdrinier section is also driven by the screen drive shaft. End margins are mounted along the edges of the screen to prevent the paper pulp from overflowing. The dehydration of the paper pulp and the preparation of the web are accomplished through free flow and the suction of the table roll. In order to obtain a paper web that is more uniform in both length and width at a machine speed of not greater than 300 m/min, the table roll section is sometimes shaken transversally. Further dehydration is performed over the suction box in a vacuum created by special vacuum pumps. In the manufacture of high-quality papers a light dandy roller (égoutteur) is often mounted above the vacuum pumps. This roller is also used to stamp watermarks on the paper. At this point the paper web still contains a relatively high amount of moisture (88-90 percent); in order to remove this moisture, the screen—along with the web—passes over a couch roll (on low-speed machines, a couch press), which has one to three suction chambers. The couch roll is a perforated hollow cylinder made of bronze alloy or stainless steel (the perforated area is approximately 25 percent of the surface of the roll). Inside the body there is a fixed vacuum chamber with graphite seals that adhere pneumatically to the interior surface of the cylinder. The vacuum chamber is connected to a continuous-operation vacuum pump. The couch roll completes the preparation and dehydration (to a dryness of 18-22 percent) of the web on the screen of the paper machine.

Figure 1. Diagram of a fourdrinier paper machine: (1) machine chest, (2) pump, (3) constant-pressure tank, (4) conical refining mill, (5) fan pump, (6) gates, (7) purification apparatus, (8) flow box, (9) fourdrinier section, (10) breast roll, (11) couch roll, (12) table rolls, (13) suction boxes, (14) dandy roll, (15) guide roll, (16) press part, (17) roll presses, (18) wool felts, (19) drying part, (20) and (21) drying cylinders, (22) calender, (23) cooling cylinder, (24) reel, (25) slitter

Further dehydration takes place in the press part by mechanical wringing out under pressure and in a vacuum by passing the sheet over several (two to three; frequently four to five) roll presses placed in sequence (the first and second presses are often combined into a double press). In this process the bulk, durability, and transparency of the paper are increased and its porosity and absorptive capacity are decreased. Pressing takes place between press felts, which protect the still fragile paper from damage, absorb excess moisture, and at the same time transport the web. Each press has its individual felt. On all new high-speed paper machines the lower rolls of the presses are perforated (as are the couch rolls). They are covered with a special rubber that improves dehydration and extends the life of the machine. Certain paper machines have rolls with special grooved scores (gutters) instead of the lower suction rolls. On high-capacity paper machines the lower rolls of the first and second presses are involved in suction (analogous to the couch roll). In addition to presses with felts, smoothing (offset) presses without felts to thicken and smooth the paper are often used. The paper web, which is as much as 45 percent dry, then passes on to the drying part.

The drying part (the longest component of the paper machine) is composed of rotating cylinders that are heated from the inside by steam and are usually placed in two rows in staggered (checkerboard) order. The web clings to the heated surface of the cylinders because of the felts, which improve heat emission and prevent warping and wrinkling of the surface of the paper during drying. The upper and lower rows of the drying cylinders have separate felts, so that one felt envelops several cylinders (a group of drying cylinders) at once. The paper web moves from the upper cylinder to the lower, then on to the neighboring upper cylinder, and so on. In this process the paper is dried to the point where it contains 5-7 percent residual moisture. In modern paper machines there is usually a two-roll sizing press in the second half of the drying compartment for the surface sizing of paper and coating of the surface layer. In some paper machines, the drying part is equipped with automatic regulators controlling the flow of steam into the cylinders and devices for automatic threading of the paper web onto the drying cylinders. The steam collects under a hood placed above the entire drying part of the paper machine and is then removed by exhaust blowers. The heat is used in air heaters and heat exchangers.

The finishing end is a calender that consists of five to ten rolls made of chilled cast iron and placed one on top of another. The paper undergoes preliminary cooling and slight moistening on a cooling cylinder (with cold water passing through hollow necks) in order to give it more elasticity and softness. As the web moves between the rollers from top to bottom it becomes smoother and denser and takes on a uniform thickness. The paper is then wound in an endless strip onto spools on a reel (a driven cylinder against which the roller with the paper wound around it presses). A moisturizing apparatus is placed over the reel to moisten the paper for additional finishing on supercalenders (to obtain paper with greater smoothness, gloss, and bulk). Further on, the roll is cut to the required format on a slitter. At the same time, the paper is sorted, and tears made during its manufacture are repaired. When the paper is produced in sheets, the rolls to be cut are fed to a sheet cutter.

Paper machines also have a great deal of various equipment necessary to ensure their uninterrupted operation and automatic devices that regulate engineering parameters. The widths and operating speeds of paper machines have been established based on technical and economic consideration for each type of paper. The narrowest paper machines (with a paper web 1.6-4.2 m wide) are designed to produce the finest capacitor tissue papers, special industrial paper, and high-quality photograph and document paper. Wide paper machines (over 6 m) are used to manufacture newsprint and bag paper. The operating speed of the paper machine during production of capacitor tissue paper is 40-150 m/min; for newsprint, up to 850 m/min; and for sanitary and hygienic paper, approximately 1,000 m/min and higher. The output of a paper machine producing capacitor tissue paper of 4-12 microns thick is 1-4 tons per day; for newsprint it is 330-500 tons per day and higher. The length of the paper machine for the manufacture of newsprint can reach 115 m. Its weight is approximately 3,500 tons, the height of its separate parts is approximately 15 m, and the power of all of its electric motors (including equipment for the preparation of paper pulp) is approximately 30,000 kilowatts. The separate sections of the paper machine are driven by DC motors. Such a paper machine uses up to 45 tons of steam per hour. Automatic devices regulate the processes of stock outlet and drying of the paper at high speeds. The extensive use of automatic devices and the precision of regulation and construction of paper machines permit the use of as few as three to eight workers who service it directly.

Many new designs of paper machines, which differ mainly in the method of forming the continuous paper web, have been developed. In the Inverform type of machine (England), the paper web flows and is formed between upper and lower screens. The paper pulp coming from the flow box is fed to a grapple between the lower and upper screens, and pressure is put on the stream of liquid. A certain amount of water passes down through the deposited layer of fiber on the lower screen, but the rest is removed through the upper screen. Water is removed from the inner surface of the screen by a scraper fitted with a plastic bar and a pan for draining the water. Further dehydration is carried out in regular and “inverted” suction boxes in a vacuum not exceeding 12 kilonewtons per sq m (0.12 kilograms-force per sq cm). Beyond the suction boxes is a press, and the water that has been pressed out is drawn off by the scraper through the upper screen. In the manufacture of multilayered paper there are several upper screens (depending on the number of layers). Water is removed almost exclusively through the upper screens along the scrapers and into the “inverted” suction boxes.

In the Verti-Form paper machine the paper web is dried from both sides between two vertically moving screens by means of scrapers and suction boxes, which promotes the precipitation of fibers of an identical fraction on both sides of the paper web. In this process the short, fine fibers precipitate first, forming a surface that is best suited for printing; the large fibers are in the center of the web, which increase its strength.

In the formation of paper there is a tendency to use cylinder machines in which the formation of the paper web is carried out in cylinders covered by a screen and sometimes placed in a bath. The paper pulp is placed in these cylinders. In the rotary type the flow box and the screen are combined into one compact unit, and dehydration is accomplished by a suction chamber located inside the rotating roll. The speed of such machines reaches 300 m/min. They can function with small concentrations, which makes them important in the manufacture of paper from artificial fibers.

In the production of long-fiber paper made from cotton, asbestos, and synthetic materials, “dry formation” of the paper web is used. This process is based on the principle of precipitation on a screen of fibers that are dispersed by a jet of air. It is possible that this process will be widely applied in the manufacture of industrial and special types of paper.

Further increases in the efficiency of the paper machine depend on changes in the technology of paper production, on the perfection of the design of machines and separate units, and on an increase in productivity in terms of speed and width. A marked increase in the speed and width of the machine would be made possible by closed flow distributors and flow boxes, which permit the release of the pulp onto the wire screen at a speed matching the increased speed of the screen’s movement; table rolls of the grooved and grid types; foils; two and three-chamber suction couch rolls, which intensify the drying process; new kinds of presses (reverse suction presses, presses with wide suction chambers, multiroll presses, and hot presses); rubber-coated rolls and rolls with center mounting or grooved scoring; presses with backing wire; vacuum-suction felt cleaners; upper and lower rolls, which are mounted on the calender of an open bed frame with hinged lever mountings and which are attached in the center and are floating, not requiring camber to compensate for deflecting; and peripheral reels for winding rolls with a diameter of 2,200-2,500 mm, with pneumatic roll loading and automatic transfer of the roll from the threading supports to the operating supports. The use of higher steam pressure, new designs for steam distributors with steam circulation, removal of the condensate by siphoning, completely closed hoods above the drying part, and the introduction of drying screens instead of drying felts are among the improvements planned for the drying part of paper machines. New types of drying, which would permit a significant reduction in the operating area of the drying part and an increase in the evenness of drying, are being sought to replace the widespread and comparatively inexpensive means of drying by contact of the paper sheet with the surface of the drying cylinders of the machine. Promising new means of drying include the dielectric method (by means of high-frequency current passed through the paper sheet), infrared irradiation, blowout with hot air, and vacuum drying.