National Bureau of Standards Special Publication 403 (Issued June 1976) Summary POTENTIAL ENERGY SAVINGS IN THE FORMING OF PAPER by T. Speidel, Thermo Electron Corp. Sword Street Auburn, Massachusetts 01501 D. Kallmes, M/K Systems The United States produced 59 million tons of paper and paper products Key Words: Energy; industrial; paper In 1972, the United States produced 59 million tons of paper and paper products. This paper production required 320 x 10- BTU, exclusive of the energy required for first producing the necessary wood pulp. The conventional paper forming process is described in brief. A new method of forming paper, as developed by Thermo Electron Corporation's Lodding Engineering Division, has the potential of saving 40% of the energy conventionally required to form paper. This new method, as embodied in the Lodding K-Former, and the energy saving potential are described in detail. Conventional Paper Forming Process Today, most wood pulp that is made into paper is formed on a machine whose basic principle was invented in 1799. This machine is called a Fourdrinier after its inventors. Figure 1 shows a schematic of such a machine. The pulp mixture, or stock (199 lbs. of water per 1 lb. of wood fiber), is pumped into a head box which feeds the water-pulp mixture from the slice opening, as a jet, onto an endless, rapidly-moving screen known as the Fourdrinier wire. The large water to fiber ratio (0.5% solids content; termed 0.5% consistency of the stock) is presently required to assure a uniform dispersion of the fiber free from fiber flocculation. As the stock mixture is deposited onto the wire, the excess water is drained through the wire by the forming board, foils, and flat boxes. By the time the wire reaches the couch roll where the web is transferred from the Fourdrinier wire to the press felts, a thin, wet web, now 4 lbs. of water per 1 lb. of fiber (20% consistency), is formed. The press felts (endless belts of heavy cloth) carry the weak paper web through the successive sets of press rolls. These rolls remove further water in much the same manner as huge wringers. The web now begins to resemble paper and can support its own weight. After the web of paper goes through the presses there is 1.5 lbs. of water per 1 lb. of fiber, or a 40% consistency. From the presses the web of paper moves into the dryers, a series of rotating cylinders continually fed with high pressure steam. The web is alternately wrapped around top and bottom dryer cylinders so that the remaining water is evenly evaporated from both sides of the web. The dried paper web contains 0.05 to 0.07 lbs. of water per pound of fiber. This small amount is necessary to prevent brittleness. As a final step, the paper is passed through a series of heavy revolving steel rollers, known as calenders. The calenders give the sheet of paper the finished surface. The paper is wound into jumbo rolls, 8 feet in diameter. These rolls are moved to winding machines where they are cut into smaller rolls of various widths and diameters as required by the users. Thus, the present forming of 1 lb. of paper starts with stock of 1 lb. of fiber and 199 lbs. of water, from which 195 lbs. of water is drained on the Fourdrinier wire, 2.5 lbs. of water is mechanically removed in the presses, and 1.5 lbs. of water is evaporated in the dryers. Problems with Present Day Headboxes As every papermaker knows only too well, pulp fibers flocculate extremely rapidly and the flocs are difficult to break up. The rate at which a suspension flocculates is a function of several characteristics, in particular the length of its fibers, their concentration or consistency, and the amount of microturbulence which the suspension contains. By microturbulence, we mean energy supplied over extremely small distances, of the order of millimeters or less. This energy is best supplied by creating pressure drops, changing the direction of flow, etc. It has only recently been demonstrated that following the removal of energy input, microturbulence dissipates and excessive flocculation (i.e., over and above the amount in a random suspension) sets in within a matter of milliseconds. Table I shows the order-ofmagnitude of the times required for incipient flocculation at various consistencies. It should be evident that microturbulence must be supplied to the stock throughout its passage through the headbox, or the stock residence time must be kept well below a second. Every headbox that has essentially a box-like structure meets neither of these requirements, i.e., none of them supply any true microturbulence, and the residence time in them is three seconds or over. Thus, it is impossible for a headbox to deliver anything but a flocculated suspension to a wire, which has to be redispersed on the Fourdrinier before it can be formed into a saleable sheet. Conventional headboxes even out the incoming flows by providing a cavernous volume, or dead space, in which they intermingle. Throughout most of its passage through the headbox, the flow of the stock is laminar. A very small amount of low energy, large scale (i.e., operating over a large distance) turbulence is introduced by the slowly turning rectifier rolls. The low level and the large scale of this turbulence is dramatized by the "holey |