Equipment Description:Dewatering on the Forming Table
To print this FactSheet click here To return to the FactSheet home page click here
The primary function of the dewatering section on the forming table is to remove the water from the fibre / water suspension in a controlled and uniform manner. The thin stock (fibre / water suspension) is injected onto the forming fabric via the headbox. The forming fabric is designed to allow the water to drain through the fabric in an even and consistent manner, whilst forming and carrying an even and uniform fibre mat on top of the fabric. As the water suspension is drained from the fabric, the fibres are dispersed evenly across its width to form the paper web. The rate of dewatering in the early stages of the forming process can affect the formation. High activity (stock bounce) at the forming board and on the initial first few foils will affect both the drainage rate and the fibre distribution. The forming fabric design plays a significant part in the rate of drainage and the resultant ‘formation’ of the fibre mat on the wire. Modern Fourdrinier Machines have a forming board section, followed by a series of natural gravity foil units, followed by low vacuum foil units and finally high vacuum boxes. The slot width, dwell time, fabric open area and the amount of vacuum applied to the low and high vacuum sections of the dewatering table, will all influence the efficiency of the dewatering process. The design and operational performance of the vacuum system is critical to efficient dewatering. |
Forming Boardis the leading forming unit under the fabric closest to the slice. The stock jet velocity, the impingement angle and the position of the impingement onto the forming board will determine the water removal and the activity produced at this point. Modern Forming Boards are stepped to create activity at high speeds – this greatly enhances the formation. Forming Fabric (Wire)is a finely woven fabric designed to support the fibrous suspension and assist with the drainage of the stock suspension. They are classified by surface structure, often 2 or 3 layers, and by open area. They normally have a finer mesh on the top of the structure with a coarse supportive mesh on the non-paper side. Foil unitsthe blade angle will determine the amount of natural vacuum created and hence the amount of water removed from the underside of the fabric Dandy Rollsassist in the two-sided effect of dewatering on Fourdrinier Machines. |
 |
Dewatering equipment is normally made from high density polythene (HDPE) or from ceramic material. The ceramic material has high wear resistance and is recommended for high speed applications. The impingement angle can be altered by moving either or both the horizontal slice or vertical slice on the Headbox / Flowbox. Vacuum levels are increased as the sheet moisture reduces down the length of the forming table. Vacuum requirements and dewater table layout vary by the grade being produced, the furnish and machine speed. Top Formers and Dandy Rolls are fitted to higher speed machines to even out the 1-sided dewatering effect and reduce the 2-sidedness of the sheet. Retention of the fibre and other ‘solids’ on the forming fabric is determined by the amount of fines and filler being used, the drainage efficiency of the forming table and the open area of the forming fabric. |
Dewatering on the Forming Table was also mentioned in the following Fact Sheets (23):
|
(This information is from suppliers listed in the Industry Directory. Contact PITA to have your company listed here)
There are 6 other potential suppliers of relevant equipment/services. (This information is from suppliers listed in the Industry Directory Contact PITA to have your company listed here)
|