Flow Regime

As the pulp suspension passes through the screening zone, the flow pattern near

the screen can be broken down into an axial flow vector from the feed side to the

reject side, a tangential flow vector induced by the rotor, and a radial flow vector

through the apertures to the accept side of the screen (Fig. 6.4).

The rotor plays a most essential part in influencing the flow regime. Its motion

fluidizes the pulp suspension, provides the tangential fluid velocity along the

screen plate, and backflushes the screen apertures. Fluidization suppresses the

particle–particle interactions and provides for quickly changing particle orienta-

564 6 Pulp Screening, Cleaning, and Fractionation

6.2 Screening Theory

v vtangential axial

vradial

Feed

Reject

Accept

Fig. 6.4 Flow vectors near the screen basket.

tion relative to the screen aperture, thus increasing the probability for acceptable

particles to pass. As the rotor element passes cyclically over the aperture, it generates

a backflush through the aperture every time it passes by. The backflush removes

particles trapped in the narrow screen apertures and thus keeps them clear.

While the screen performance is influenced by all the three flow vectors, the

radial accept flow through the apertures is most critical for continuous operation.

When the accept flow becomes limited or even completely disrupted by fibers

blocking the screen apertures, the situation is referred to as “blinding” or “plugging”

of the screen. Blinding leads to the formation of a fiber mat on the screen

surface, and it can affect only part of the screen or the total screen. In the latter

case, it may take several minutes for the screen to be blinded, but typically it takes

only a few seconds. The triggering factor of blinding – that is, the build-up of

fibers at the edge of a screen aperture – occurs very rapidly, within several thousands

of a second [3]. Consequently, very frequent backflush is required to avoid

plugging. The typical pulse frequency provoked by the rotor in a pressure screen

is above 30 Hz [4].

The aperture velocity, or passing velocity, v, is often regarded as a fundamental

design parameter for a pressure screen. v (m s–1) is calculated from the accept

flow rate QA (m3 s–1) and the open area of the screen basket, AO (m2):

v _

QA

AO _1_

It is important to realize that the true flow velocity through the screen apertures

is considerably higher than the passing velocity calculated from Eq. (1). On the

one hand, pressure pulsation as induced by the rotor action leads to a backflow

from the accept side to the feed side, thus both increasing the volume to be transferred

from the feed side to the accept side and reducing the time for this transfer.

On the other hand, fiber accumulation at the screen apertures reduces the open

area [5].

6 Pulp Screening, Cleaning, and Fractionation

6.2.3

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