The stock leaving the hydrocyclones contains air, which may disturb the forming process. The air bubbles adhere to the fibre surfaces and obstruct the breaking up of the fibre flocs.
The air bubbles may expand and disrupt the stock jet from the headbox.The higher pressure in the box, the more the bubbles are pressed together and the more they expand in the jet.
A way to get rid of the air is to pump the stock to a deaerator. The air bubbles in the stock are fixed to the fibres. To remove them, the stock is ejected to the upper tank wall. There is an underpressure in the deaerator making the stock boil. When it boils the air bubbles disappear.
The deaerator must be placed on a high level. The outgoing stock pipe should not only overcome the underpressure in the deaerator. The stock must also have a certain pressure before the feeder pump.
Source: Tappsa
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