Screen Basket

The screen basket is fundamentally characterized by aperture size, aperture shape

and aperture spacing, as well as the character of its surface. There is a basic distinction

between perforated, or holed, screen plates and slotted screen plates.

Both types can be furnished with contours on the side of the screen surface which

faces the feed. Such contoured, or profiled, screens increase the turbulence near

the screen aperture and allow the screen to be operated at a higher capacity.

6.3Screening Parameters

Only profiled screens in combination with the wedge-wire design have made

today’s narrow screen slots practical and widely accepted. Slotted screens with a

slot width around 0.15 mm have become state of the art for applications targeted

at the removal of smaller contaminants. Wedge-wire screens consist of solid bars

placed aside each other, forming long slots over the complete length of the screen

basket, while machined slots are milled out of a solid screen basket. Wedge-wire

slotted screens have considerable capacity advantages over machine slotted

screens due to their larger open area.

Holed screens have been traditionally preferred for their high capacity and reliable

operation and easy control under varying conditions. Their robustness makes

them first choice for the removal of larger contaminants. The advantages of holed

screens for fractionation have been discussed above. Typical hole sizes are

4–10 mm for larger contaminant removal, and about 1 mm for fractionation.

The aperture size is the most critical design variable of a screen. Holes of small

diameter and slots of narrow width have advantages with regard to the screening

efficiency. Their size actually determines whether a particle is rejected on the principle

of barrier separation, or whether it is subject to probability separation. On

the other hand, smaller apertures mean lower capacity at a given screen surface

area.

Similarly, the profile depth of the screen surface causes divergent screen performance.

By tendency, the additional turbulence created by a higher contour provides

a greater capacity but reduces the screening efficiency. If in turn the aperture

size is reduced to regain lost efficiency, the capacity of the contoured screen

still remains higher [12].

It has been shown that slot spacing is important, and that longer fibers require

wider slot spacing than shorter fibers. If the slots are too close, then stapling of

fibers occurs as the two ends of individual fibers enter adjacent slots at the same

time. Similar conclusions have been drawn for holed screens.

Note that the performance of a screen will deteriorate over time if the pulp furnish

contains an abrasive material such as sand. Especially with heavily contoured

screens, wear will significantly decrease both the capacity and the screening efficiency.

Rotor

There is a variety of different rotors available, with special shapes and sophisticated

local arrangements of bumps or foils. All of these are deemed to have their

individual advantages regarding screen capacity, screening efficiency or power

consumption.

The characteristic shape of the pressure pulse generated by a rotor depends on

the design of the pulsation element, for example on the shape, length and angle

of incidence of the foil, or on the shape and length of the bump. The intensity of

the pulse is determined again by the rotor shape, as well as by the rotor tip velocity,

the clearance between the pulsation element and the screen basket, as well as

the pulp consistency and pulp furnish parameters.

6 Pulp Screening, Cleaning, and Fractionation

-3

-2

-1

0 10 20 30 40 50 60

Dynamic pressure [bar]

Time [10-3 s]

Fig. 6.12 Example of pressure pulse profile for a short foil rotor [5].

-3

-2

-1

0 10 20 30 40 50 60

Dynamic pressure [bar]

Time [10-3 s]

Fig. 6.13 Example of pressure pulse profile for a contoured-drum rotor (S-rotor) [5].

Figures 6.12 and 6.13 show typical pressure pulses caused by the movement of

a foil rotor and a step rotor, respectively. At a random point on the screen surface,

there is in general a positive pressure pulse upstream of the rotor element, and a

negative pressure pulse right after the smallest clearance between the rotor tip

and the screen basket has passed by. The negative pressure is responsible for the

backflush through the screen apertures.

It is evident that the profile of the pressure pulse is very different between rotors.

Short negative-pressure pulses, as created by bump rotors and rotors with short foils,

keep the backflush flow low. At the same time, they ensure comparatively low true

6.3Screening Parameters

aperture velocity and low overall screen resistance. Longer negative-pressure

pulses, as created by rotors with long foils and step rotors, reduce reject thickening

by intensified backflushing. Higher feed consistencies require longer negative-

pressure pulses to keep the consistency at the reject end of the screening zone

low enough to avoid blinding. Note that the screen capacity decreases with the

magnitude and duration of the negative pressure pulse.

The clearance between the pulsation element and the screen basket is quite different

between rotor designs. Common clearances are between 3 and 10 mm. Reducing

the clearance between the pulsation element and the screen basket leads

to some increase of the pressure pulse intensity [13,14].

6.3.2

Дата добавления: 2015-10-21