The Dynamics of a Roll Press Nip Albany International

The press section of a paper machine is designed to remove by mechanical means the maximum possible amount of water from a just-formed paper sheet. The paper sheet is carried by a textile fabric conveyer belt through a series of roll nips, which cause water to be transferred from the sheet to the fabric. In the most general case the problem to be considered at each press nip involves transient fluid flow at high speed in two contacting, partially saturated, anisotropically porous media undergoing non-linear compression. Though a great deal of empirical information concerning press performance has been accumulated over the years, there is no generally accepted theoretical treatment available, and new insight into the physics of the water movement could lead to valuable process improvements. THE PAPERMAKING PROCESS OVERVIEW The modern papermaking process consists of three distinct coupled stages: forming, pressing, and drying. In the forming stage a dilute suspension of individual wood fibers in water, typically at a concentration of around 1%, is evenly distributed onto a porous conveyer belt known as a forming fabric, and a substantial fraction of the water is removed by the action of various forces. In earlier times, most of the water removal relied on gravity drainage, but in modern highÐspeed machines the pressure gradients are enhanced by a variety of mechanisms according to the detailed configuration of the paper machine. In all cases, however, water is filtered through the forming fabric as it moves through the machine, and wood fibers are retained at the surface of the fabric to form the paper sheet. At the end of the forming section the paper sheet typically has a consistency of approximately 20% fiber and 80% water, and has just enough mechanical integrity to permit its transfer to the next stage of papermaking, the press section. The press section continues the process of water removal that was started in the forming section. The just-formed sheet is transferred to a second conveyer belt known as a press fabric, which is both porous and compressible, and is carried by this belt through the nip formed by two press rolls. The rolls apply considerable pressure at the nip zone, and water is expressed from the saturated sheet and is transferred to the porous press fabric. At the exit side of the nip the sheet, now at a typical consistency of 40% fiber and 60% water, is separated from the press fabric and moves onto the dryer. The expressed water is carried off by the press felt and is removed by vacuum before the fabric reenters the nip. Modern press sections are capable of working close to the limit of water removal by purely mechanical means and the final stage of papermaking relies on thermal energy to produce a fully dried sheet. In the dryer section the sheet is carried over a series of heated cylinders by another porous belt known as a dryer fabric and the remaining water is removed by evaporation. At the end of the dryer section the sheet consists of about 95% fiber and 5% water, which is the typical consistency of the paper of commerce.