Forming-section vacuum suction box for paper machines

01

Step the vacuum from box to box

In a laboratory study at Karlstad University, published in BioResources, Björn Sjöstrand dewatered 20 g/m² tissue sheets on a forming fabric at -10, -20 and -30 kPa, and then in succession through all three levels, to imitate three suction boxes with rising vacuum. He reports that progressively higher vacuum levels improved both outgoing dryness and vacuum pump energy, and that for 20 g/m² sheets with 5 ms at each level the triple setup was particularly efficient. [1]

A follow-up study by Öman and Sjöstrand on 86 °SR grease-proof paper simulated a four-step box at 10, 20, 30 and 40 kPa. For 50 g/m² paper the four steps reached 11.7% dryness, against roughly 10% after 40 ms at a constant 20 kPa, and the authors report a 14% increase in dryness over a single box at equal energy, measured as air drawn through the sheet. For 100 g/m² paper the four-step run reached about 10.3%. [2]

The same study warns that a multi-slit box applied too early in the forming section should be avoided, to limit wire wear and protect formation, and that the highly refined pulp clogged the fabric: almost 50% of the air pulled through the 100 g/m² sheet passed at the 10 kPa step. [2] Editorial note: ask the box supplier for the vacuum setting of each box in the sequence and how it is controlled, not one figure for the whole table.

Give the supplier the grade and basis weight, freeness, machine speed and width, forming fabric, existing box layout, vacuum available per box, target dryness after the forming section, and the cover and slot pattern you use now.

02

Set dwell time from slot length and speed

Sjöstrand measured dwell times of 0, 1, 2.5, 5, 10 and 20 ms, designed to capture the dewatering of a suction box with a total dwell time of 20 ms, and found that dryness rose quickly at first and then reached a plateau, where the pumps keep drawing air through the sheet and fabric for little further gain in dryness. [1]

For grease-proof paper, Öman and Sjöstrand report that dryness rose readily between 0 and 10 ms and that the 10 and 20 kPa steps plateaued after about 10 ms each, so a longer dwell there would waste energy; at 40 kPa no plateau appeared within the times tested. [2] Editorial calculation: dwell time equals open slot length divided by fabric speed, so at 600 m/min (10 m/s) a 10 ms dwell needs 100 mm of open slot in the machine direction, and a 20 ms dwell needs 200 mm.

Rewetting after the box also counts: Sjöstrand cites laboratory rewetting of 3 to 6% in dryness for 50 and 100 g/m² sheets and pilot-scale rewetting of 0 to 4% for 20 and 100 g/m² sheets, and his study used a forming fabric of 0.645 mm caliper, 380 cfm air permeance and 59% void volume. [1]

03

Choose the cover, slots and housing

Qinyang Aotian describes its vacuum suction box as a traditional dewatering element of the paper machine, with 2 to 11 fitted, located at the rear of the fourdrinier table between the vacuum couch roll and the wet suction box. [3]

Aotian lists ceramic panels in 95% and 99% alumina and in zirconia on an ultra-high-molecular-weight polyethylene and glass-fibre substrate, in a stainless steel housing, with single, double and triple slit covers and a seat that can be installed rotated through 360 degrees. [4] [5]

Huatao lists high-vacuum boxes with adjusting blocks at both ends of the panel to set the suction opening, lubricated with clean water to reduce friction on the wire, and offers dewatering elements in aluminium oxide, zirconium oxide or silicon nitride ceramic. [6] Huadong Paper Machinery states that it relieves residual stress in dewatering-element housings, with minimal welding seams, so that they keep their flatness in use. [7]

04

Cover material, slot pattern, open area and flatness to request from box makers

Ask each supplier for the cover material, slot pattern, open area, housing material and flatness tolerance, and for the vacuum and speed range the box is designed for.

05

Ceramic cover, substrate, sealed housing, adjusting blocks, vacuum and drain connections

06

Vacuum per box, air flow, cover length and flatness for a fourdrinier suction box

Cover length and machine speed together set the dwell time, so give both; the Karlstad tests found dryness levels off within a 20 ms total dwell. [1]

07

Send the grade, vacuum and cover data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machine: [width; speed; former type]
Grade: [basis weight; freeness; target dryness]
Box layout: [number of boxes; position on table]
Vacuum: [kPa per box; pump capacity]
Cover: [ceramic; slot count and width; open length]
Housing: [stainless grade; flatness]
Connections: [vacuum; drain]

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