Pressure Screens

In the Impco HI-Q Fine Screen, the pulp feed enters the unit tangentially in the

upper section. Heavy contaminants separate centrifugally into the junk trap.

Then, as the pulp suspension enters the screening zone, prerotation vanes

increase its tangential velocity to improve screening efficiency. The accept passes

through the screen apertures, which are kept clean by pulsation provoked by the

special bump elements attached to the closed rotor. The reject proceeds to the bottom

of the screen where it is diluted and discharged through the reject nozzle [30].

Fig. 6.33 The GL&V Impco HI-Q Fine Screen [30].

In Metso’s DeltaScreen, the pulp suspension is fed tangentially into the bottom

part of the unit, where the heavy debris is trapped and can be removed through

the junk nozzle. The pulp proceeds upwards through the rotor to the screening

zone. The screening process takes place as the pulp moves downwards between

the foil rotor and the fixed screen basket. As with some other pressure screens,

the DeltaScreen can be equipped with a cyclone-type separator on top of the

machine, which offers the possibility for removal of light-weight contaminants

[31].

6.9 Screening and Cleaning Equipment 601

Fig. 6.34 The Metso DeltaScreen [31].

In the Impco HI-Q Knotter, the pulp feed enters the unit tangentially in the

upper section. Heavy contaminants are separated centrifugally into the junk trap.

The accept passes from the outside of the screen through the screen apertures to

the inside and proceeds to the accept nozzle at the bottom of the screen. Hydrofoils

rotating at the accept side provide the pressure pulses which keep the screen

apertures open. At the same time, the rotor action does not break up the knots.

On the reject side outside the screen basket, the knots are diluted and washed as

they proceed downwards to the reject nozzle [32].

Fig. 6.35 The GL&V Impco HI-Q knotter [32].

602 6 Pulp Screening, Cleaning, and Fractionation

Metso’s DeltaCombi is a combined knotter and fine screen that operates in similar

fashion to the DeltaScreen. In the lower section, the screen is equipped with

an additional rotating screen basket with holes for knotting. The feed pulp must

first pass this coarse screen basket from the outside to the inside. The pulse-generating

stationary foils of the knotting section are located on the accept side of the

screen basket. The coarse reject is taken out from the bottom part. The accept

which has passed the coarse screen basket is led upwards through the rotor of the

fine screen, and then enters the fine screening section between the foil rotor and

stationary fine screen basket [33].

Fig. 6.36 The Metso DeltaCombi [33].

The Noss Radiscreen features a different design, with the pulp feed entering

the unit in an axial direction. Accepted fibers pass through the two conical screens

plates fixed in the housing, while the reject proceeds to the reject nozzle at the

housing perimeter. As the rotor vanes pass along the screen plates, their peripheral

velocity increases by the radius towards the reject end of the screen plates, and

this leads to increased turbulence in the critical zone of higher consistency.

Radiscreens are available with perforated screens for both knotting and screening

applications. Their design does not require internal dilution, and features a comparatively

small pressure drop and low power consumption [34].

The abundance of screen designs makes it impossible to present all variations

offered by screen suppliers in this book. Tailor-made solutions are available for

special applications, with recent developments including, for example, screen baskets

with intermediate dilution [35] or intermediate deflocculation [36] half-way

down the screen basket to reduce the effects of reject thickening, or different surface

profiling along the length of the screen basket providing increased turbulence

towards the reject end of the screening zone [37].

6.9 Screening and Cleaning Equipment 603

Fig. 6.37 The Noss Radiscreen [34].

6.9.2

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