Editing Paper machine (section)

=== Press section ===
[[File:Paper machine.ogv|thumb|Paper machine]]
[[File:Granite Press Roll.JPG|thumb|Granite press roll at a granite quarry site]]

The second section of the paper machine is the press section, which removes much of the remaining water via a system of nips formed by rolls pressing against each other aided by press [[felt]]s that support the sheet and absorb the pressed water. The paper web consistency leaving the press section can be above 40%.<ref name="Technical Association for the Pulp and Paper Industry">{{cite book|last1=Atkins |first1=Jim |year=2004 |title=The Paper Machine Dry End |publisher=[[TAPPI|TAPPI Press]] |series=13. Making pulp and paper |isbn=978-1-59510-031-3 |oclc=803290898 |location=Norcross |language=en |postscript=[[Compact disc|CD]] format}}</ref>  Pressing is the second most efficient method of de-watering the sheet (behind free drainage in the forming section) as only mechanical action is required.

The number of press rolls, their arrangement and the arrangement and type of felts used are influenced by the grades of paper being produced and the desired operational characteristics of the machine.<ref>{{cite book
|title= Pressing and Drying course notes
|last=Technical Association for the Pulp and Paper Industry
|author2=Various
|year= 2005|publisher= TAPPI Press}}</ref>

Press felts historically were made from wool. However, today they are nearly 100% synthetic. They are made up of a [[polyamide]] woven fabric with thick batt applied in a specific design to maximise water absorption.

Presses can be single or double felted. A single felted press has a felt on one side and a smooth roll on the other. A double felted press has both sides of the sheet in contact with a press felt. Single felted nips are useful when mated against a smooth roll (usually in the top position), which adds a two-sidedness—making the top side appear smoother than the bottom. Double felted nips impart roughness on both sides of the sheet. Double felted presses are desirable for the first press section of heavy paperboard.

Simple press rolls can be rolls with grooved or [[Blind hole|blind drilled]] surface. More advanced press rolls are suction rolls. These are rolls with perforated shell and cover. The shell made of metal material such as bronze or stainless steel is covered with rubber or a synthetic material. Both shell and cover are drilled throughout the surface. A stationary suction box is fitted in the core of the suction roll to support the shell being pressed. [[End face mechanical seal]]s are used for the interface between the inside surface of the shell and the suction box. For the smooth rolls, they are typically made of granite rolls.<ref>{{cite journal|title=Papermaking: Papermachine – Pressing|pages=2, 3, 12, 13|url=http://www.fibrelab.ubc.ca/files/2013/01/Topic-14-Papermaking-Pressing-text.pdf|access-date=25 August 2014|publisher=UBC Fibre Lab}}</ref> The granite rolls can be up to {{Convert|30|ft|adj=on}} long and {{Convert|6|ft}} in diameter.<ref>{{cite journal| last1=Richter| first1=Dorothy A.|title=Barre granite quarries, Barre, Vermont|journal=Geological Society of America Centennial Field Guide—Northeastern Section|date=1987| volume=5| pages=239–242| doi=10.1130/0-8137-5405-4.239| isbn=0-8137-5405-4}}</ref>

Conventional roll presses are configured with one of the press rolls is in a fixed position, with a mating roll being loaded against this fixed roll. The felts run through the nips of the press rolls and continues around a felt run, normally consisting of several felt rolls. During the dwell time in the nip, the moisture from the sheet is transferred to the press felt. When the press felt exits the nip and continues around, a vacuum box known as an Uhle Box applies vacuum (normally -60 kPa) to the press felt to remove the moisture so that when the felt returns to the nip on the next cycle, it does not add moisture to the sheet.

Some grades of paper use suction pick up rolls that use vacuum to transfer the sheet from the couch to a lead in felt on the first press or between press sections.  Pickup roll presses normally have a vacuum box that has two vacuum zones (low vacuum and high vacuum). These rolls have a large number of drilled holes in the cover to allow the vacuum to pass from the stationary vacuum box through the rotating roll covering. The low vacuum zone picks up the sheet and transfers, while the high vacuum zone attempts to remove moisture. Unfortunately, at high enough speed [[centrifugal force]] flings out vacuumed water, making this less effective for dewatering. Pickup presses also have standard felt runs with Uhle boxes. However, pickup press design is quite different, as air movement is important for the pickup and dewatering facets of its role.

Crown Controlled Rolls (also known as CC Rolls) are usually the mating roll in a press arrangement. They have [[hydraulics|hydraulic]] cylinders in the press rolls that ensure that the roll does not bow. The cylinders connect to a shoe or multiple shoes to keep the crown on the roll flat, to counteract the natural "bend" in the roll shape due to applying load to the edges.

Extended Nip Presses (or ENP) are a relatively modern alternative to conventional roll presses. The top roll is usually a standard roll, while the bottom roll is actually a large CC roll with an extended shoe curved to the shape of the top roll, surrounded by a rotating rubber belt rather than a standard roll cover. The goal of the ENP is to extend the dwell time of the sheet between the two rolls thereby maximising the de-watering. Compared to a standard roll press that achieves up to 35% solids after pressing, an ENP brings this up to 45% and higher—delivering significant steam savings or speed increases.  ENPs densify the sheet, thus increasing tensile strength and some other physical properties.