Paper Machine Vacuum Suction Box and Couch Roll Vacuum Problems

In the forming section of a paper machine, vacuum components such as vacuum suction boxes and couch roll vacuum chambers play an important role in paper sheet dewatering and transfer.

In particular, the vacuum suction box is one of the key dewatering elements in the forming section. It gradually extracts moisture from the paper sheet and the forming fabric during forming.

The press roll vacuum chamber, located at the end of the forming section, primarily dewaters the paper sheet’s trailing edge and transfers the wet paper sheet.

What Problems Can Occur When a Vacuum Suction Box Malfunctions

Vacuum Suction Box Issues Causing High Web Moisture Despite Normal Vacuum Level

Possible causes include:

• Blockage or contamination of the vacuum suction box panel.

• Reduced air permeability of the forming fabric.

• Air leaks in the vacuum box seal.

• Uneven lateral distribution of the suction trough.

• Water accumulation in the vacuum lines.

• Abnormal operating conditions of the vacuum pump’s cooling water.

• Poor white water drainages.

High Couch Roll Vacuum but Edge Drop Occurs

When couch roll vacuum is too high, the edges of the paper sheets may experience intense localized suction, damaging the fiber structure at the edges.

Common symptoms include:

Thinning of the paper sheet’s edges on both sides;

Edge loss at the press roll;

• Unstable paper pickup by the felt;

• Localized wear on the wire and felt;

• Paper sheet tears before entering the press section;

• Fluctuations in moisture content at the edges of the paper web.

If the vacuum on the couch roll is increased at this point, the problem of paper shedding at the edges may worsen.

Worn Vacuum Suction Box Seals Causing Frequent Vacuum Fluctuations

After prolonged operation, the vacuum sealing strips may become worn, deformed, broken, or develop abnormal local gaps.

Poor sealing can result in:

• Fluctuations in vacuum level.

• Increased load on the vacuum pump.

• Cross-contamination between different water-absorption zones.

• Uneven transverse dewatering of the forming fabric.

• Rising temperatures in the seals.

• Localized wear on the forming fabric.

When some workshops experience a drop in vacuum, their first instinct is to increase the vacuum pump's speed, but the root cause may actually be a leak in the water trap seal.

Functions of Vacuum Suction Box and Couch Roll Vacuum Chambers in Papermaking

Vacuum Suction Box Continuous Dewatering in the Forming Section

Vacuum suction boxes are typically installed below the forming fabric and serve as the main vacuum dewatering equipment in the rear section of the forming section.

It uses negative pressure to extract moisture from the paper sheet, the forming fabric, and the space beneath the fabric.

The vacuum suction box is primarily used to:

• Enhance dewatering in the rear section of the forming fabric.

• Improve the dryness of the paper sheet as it exits the fabric.

• Stabilize the negative pressure conditions beneath the forming fabric.

• Control the dewatering curve of the forming section.

• Support paper sheet formation and fiber retention.

• Provide the appropriate paper sheet condition for subsequent transfer to the press roll.

Vacuum dewatering boxes typically do not remove all the moisture from the paper sheet at once. Instead, they gradually increase the dewatering intensity through multiple vacuum zones.

A reasonable dewatering sequence generally follows this pattern:

Gentle dewatering in the initial stage, steady dewatering in the middle stage, and intensive dewatering in the final stage.

Applying excessive vacuum from the start may cause structural damage to the paper sheet, loss of fine fibers, and deterioration of sheet formation.

Couch Roll Vacuum Chamber: Final Dewatering and Web Transfer

The vacuum chamber of the couch roll is located inside the roll and is connected to the perforated area on the roll shell and to the external vacuum system.

As the couch roll rotates, the vacuum chamber creates a suction zone within a set angular range. As the paper sheet passes through the couch roll area, vacuum is applied to the sheet through the perforated area of the roll shell, ensuring a stable transfer of the sheet between the forming fabric and the felt.

The couch roll vacuum chamber is primarily used for:

• Dewatering the edges of the paper sheet.

• Increasing the dryness of the wet paper sheet before it enters the press section.

• Transferring the paper sheet from the forming fabric to the felt.

• Stabilizing the edges of the paper sheet and the web run.

• Reducing paper drag and fluttering during high-speed operation.

Compared to vacuum suction boxes, the vacuum chamber with couch roll places greater emphasis on:

The localized suction and stable transfer of the paper sheet at specific locations and at specific times.

Therefore, the vacuum chamber with couch rolls must consider not only the vacuum level but also the length and position of the suction zone, the sealing condition, and the contact between the paper sheet and the press felt.