Shoe Press, more like Shoe…, YES
The Science Behind the Shoe Press on a Paper Machine
- Improves dewatering in the wet press section
- Reduces strain on evaporation drying (expensive and inefficient)
- Works by extending pressing time
Beloit took about 13 years to develop the technology. The shoe press didn’t become popular until 1994 when the wet presses became limited by rising machine speeds.
Background; Pulp and Paper
The Pulp and Paper industry is a capital-intensive industry where the value of paper cycles through fluctuations in price and slowly increases. Money is made from continuous operation and technology that improves the yield of the pulp and paper process.
The three major steps of the process are furnish preparation, paper machining, and end operations. During furnish preparation the consistency of % dry solids in the pulp (furnish) is about 1% and can vary +5% at different storage tanks/processes. Once the furnish is fed into the paper machine the furnish will drain on the forming section and go to about 20% consistency. Then at the press section, the paper is run through a number of press nips and goes to about 45% dryness. Finally, the sheet is run through the evaporator dryers and will go to about 90–95% consistency or dryness.
The drying section requires the most amount of energy per kilogram water removed, so if we can remove more water at the press section we will have significant energy savings.
The Shoe Press will improve the dewatering capability of the press section so less drying can take place in the dryers. This is done by extending the nip residence time or the time that the sheet remains in the press nip. The pressure and nip residence time can be adjusted for various grades and control.
A Shoe Press can be 4–5 times longer than a Roll Press and handle a linear load 3 times as much around 1,000–1,500 kN/m.
The Shoe Press is a stationary shoe (roll with a concave bit) pressed against a press roll. A belt forms a shell that runs between the mechanical press and the bottom felt.
Drying can increase 5–10% compared to a conventional pressing. Higher dryness means increased production of about 10–20%. This reduces the need for steam in the drying section saving 0.5 GJ to 2.0 GJ per ton paper which can be around $3 per ton paper.
The paper characteristics can improve as well. Most physical and surface characteristics are related to the density of the sheet. Pressing causes densification. This causes an increase in strength. This can lead to fewer strength additives. Also with higher dryness without reducing the thickness of the sheet, fiber savings can be made.