Accept Flow Rate

A pressure screen’s capacity is given by the accept flow rate, and is often expressed

in terms of the aperture velocity defined by Eq. (1). A higher aperture velocity

leads to a reduction in screening efficiency [15,16]. Figure 6.14 illustrates graphically

that a decreasing aperture velocity reduces the fiber passage ratio. This

means that reject thickening becomes more critical at lower aperture velocities,

and hence screen capacities. Note that the operation of a pressure screen below its

nominal capacity may soon lead to severe operating problems caused by reject

thickening (Fig. 6.15). The dependence of the fiber passage ratio on the screen

capacity of holed screens is less pronounced than that of slotted screens.

As mentioned in Section 6.2.2, the gross aperture velocity calculated from Eq. (1) is

a parameter of limited significance. Since the actual flow rate through the apertures

depends on many factors, the meaningfulness of the aperture velocity as a parameter

for evaluation of screen capacity or screening efficiency is restricted to systems

of similar mechanical design, pulp furnish and operating conditions.

For a given screen geometry, pulp furnish and pulp consistency, the pressure

drop across a screen is linearly related to the square of the accept flow rate, with

the slope determined by the hydraulic resistance of the screen plate [17].

6 Pulp Screening, Cleaning, and Fractionation

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6

Passage ratio, P

Aperture velocity [m/s]

Slotted plate

Holed plate

Fig. 6.14 Example of fiber passage ratio as a function of the aperture

velocity;experimenta l data, bleached softwood kraft pulp [6].

0.5

1.0

1.5

2.0

2.5

3.0

0% 10% 20% 30% 40% 50%

Reject thickening factor, T

Volumetric reject ratio, Rv

2.000 l min-1

3.000 l min-1

4.000 l min-1

Fig. 6.15 Example of reject thickening in a pressure screen as a

function of the accept flow rate;3000 L min–1 nominal screen capacity,

experimental data, bleached softwood kraft pulp [6].

Optimum levels of fractionation in slotted screens occur at low aperture velocities

where the passage ratio of long fibers remains low, but that of short fibers is

significant [18]. In contrast, the fractionation performance of holed screens is

widely independent of the aperture velocity at the hole sizes of interest for fractionation

[9].

6.3Screening Parameters