Screens. The shives and stiff long fibers are removed as rejects from below,

While the filtrate moves easily through the screen openings, causing a thickening of

The reject flow. Any pad build-up will be repeatedly destroyed by the rotor wings.

Dilution

Feed

Coarse reject

Reject

Accept P2

Accept S2

Accept T2

Pre-screen

Stage

Fine

Screening

Stages

Fig. 5.4 Left: Single-stage pressure screen (Metso TAP) [24].

Right: Multistage pressure screen (Metso MuST) [25].

The capacity and runnability of a screen, and also the screening efficiency to some

Extent, can be controlled bymeans of pulsation. Today, several designs of foil-type pulsation

Element are available. The aimis to obtain adequate pulsation by adjusting the

Number of foils and their shape, width, clearance, incident angle, and tip speed. A

Small difference between the peaks of positive and negative pulses is advantageous

For screening efficiency, but the negative pulses should be sufficiently strong to enable

The suction flow to remix flocks, fibers and contaminants resting on the edge of the

screen opening [26]. The general construction of rotors is shown in Fig. 5.5, with

A cross-sectional view on different pulsation elements of rotors in Fig. 5.6.

1116 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning

Machines and Aggregates for Screening and Cleaning

Feed Feed Feed

Accept

Reject

Semi-open closed open

f = Hydrofoil

Fig. 5.5 General construction forms of pressure screen rotors

(according to Niinimaki et al. [27]).

A Foils

B Bumps

C Radial vanes

D Tapered surface

Fig. 5.6 Rotor types with different pulsation elements (according to Bliss [28]).

Slotted screen plates have been found to have superior shives removal efficiency

Compared to holed screen plates, but the capacity of the latter is better because of

A larger open area. Contoured screen plates have further increased the capacity of

Slotted screens, and slotted screens manufactured from wedge wires (Fig. 5.7) provide

about 100% greater open surface area than screen plates with machine slots.

W S

P

W Wire width

S Slot width

P Contour height (profile height)

Fig. 5.7 Example of a wedge wire screen basket [29].

5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning

Continuous slotted screen Perforated cylinder

AFT MacroFlow™

Fig. 5.8 Pressure screen basket designs (AFT).

AFT EP Rotor™

Engineered Pulse™

AFT Gladiator™

HC Rotor

Fig. 5.9 Pressure screen rotor designs (AFT).

Some examples of industrial designs of screen baskets and rotors are shown in

Figs. 5.8 and 5.9.

Дата добавления: 2015-10-21