Rotor Tip Velocity

As described above, the rotor is responsible for creating turbulence, providing the

tangential speed of the pulp along the screen plate, and for backflushing the

screen by pulsation. A higher tip velocity means a higher turbulence and a more

intense pressure pulse at basically unchanged pressure pulse profile. The intensity

of the pressure pulse increases with the square of the rotor tip velocity [13]. The

recommended operating range of rotors is varying significantly between rotor designs

and equipment manufacturers. Common tip speeds are between 10 and 40 m s–1.

6 Pulp Screening, Cleaning, and Fractionation

Increasing the rotor tip speed improves the screen capacity and allows higher feed

consistencies, while increasing the power demand of the screen. The power requirement

was found to be proportional to the cylinder area and to the tip speed cubed [20].

Within the ranges of velocities recommended by rotor suppliers for their products,

the screening and fractionation efficiencies are not notably affected [10,12,15].

6.3.3

Furnish Parameters

Pulp Fibers

With respect to screening, pulp fibers are characterized by a number of physical

properties such as fiber length, fiber flexibility, freeness and disruptive shear

stress of the fiber network. Together with the consistency, these properties determine

the performance of the furnish in a pressure screen.

The influence of fiber flexibility on passage ratio is secondary to the influence of

fiber length. Flexibility plays no role as long as the fibers are shorter than the

width of the slot or the diameter of the hole. As the fibers become longer, however,

the flexible fibers’ passage ratios are higher than those of stiff fibers [2,18]. Note

that fiber stiffness is a function of the temperature, with fibers becoming more

flexible as the temperature rises. Figure 6.16 exemplifies the fiber passage ratio as

a function of the fiber length and hole size.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Fiber passage ratio

Fiber length [mm]

1.00 mm holes

1.75 mm holes

Fig. 6.16 Example of fiber passage ratio as a function of the

fiber length and hole size;smooth hole screen, bump rotor,

softwood thermomechanical pulp (TMP) [8].

Regarding the pulpwood raw material, a distinct difference can be observed between

the long softwood and the short hardwood fibers. The capacity of a given

slotted pressure screen with hardwood pulp is 20–30% higher than its capacity

with softwood pulp.

6.4 Centrifugal Cleaning Theory

Дата добавления: 2015-10-21