The papermaking process is essentially one of removing water. This process begins in the forming section where free water is drained from the pulp slurry using gravity and suction (vacuum) boxes.
The forming section is followed by the press section where mechanical pressing forces are used to squeeze water out of the sheet. The press section is followed by the dryer section where water is evaporated using steam to heat the surface of multiple dryer cylinders. Paper drying requires a large amount of energy and expensive equipment, so it is desirable to minimize the amount of water in the sheet entering the dryer section. This is where rotary joints play a critical role in the papermaking process. Every steam heated drying cylinder is equipped with at least one rotary joint and syphon. The rotary joint connects the piping to the rotating cylinder and allows steam into the cylinder and condensate out via a syphon inside the cylinder. The cylinder’s capacity to transfer heat depends mainly on the design and operation of the syphon and dryer bars.
Rotary joints operate with different steam pressures. Pressures vary to suit the respective grades of paper being produced, from heavy board grades to lightweight tissue, graphics, newsprint, and packaging grades. Through extensive research and development testing, all Kadant Johnson rotary joints used in the papermaking industry provide reliable and extended service for all paper grades.
Rotary joints can be found on other sections of a paper machine as well. One area is on water-cooled rolls for press and size press positions. In this application, heat is generated by nip pressure and starch temperature influences the soft rubber cover which needs cooling. The cover gets cooled by introducing water to the center of the roll to cool the metal shell and therefore the rubber cover.
Where soft nip calenders are used to give a specific finish to the paper being produced, rotary joints provide the transfer of hot water or hot oil to the calender rolls to heat and maintain an even surface temperature which gives a gloss, matte, or smooth finish to the paper web.