Flow Regime. The gravitational forces which drive the separation in a hydrocyclones are generated

The gravitational forces which drive the separation in a hydrocyclones are generated

as pressure energy is converted into rotational momentum. In the very basic

design (see Fig. 6.18), the feed flow enters the cyclone tangentially at the upper

end of the cylindrical section and induces a vortex around the axis of the cyclone.

As the suspension swirls downward at the perimeter of the cylindrical and conical

sections, heavy-weight material is concentrated near the wall and is eventually

dragged to the underflow at the apex of the cone. The balance of the liquor together

with the light-weight material rotates towards the axis of the cyclone and proceeds

to the overflow at the opposite end of the cyclone. The overflow escapes

through the vortex finder, a piece of pipe extending into the body of the cyclone

which helps to limit the short-circuit flow from the feed inlet to the overflow.

Overflow (Base)

Underflow (Apex)

Feed

Fig. 6.18 Flow pattern in a hydrocyclone.

Hydrocyclones develop an air core when one of the outlets discharges into atmosphere.

Modern cyclones used in cleaning operate at a backpressure and do

not have an air core.

The geometrical form of the hydrocyclone has a major influence on the separation

efficiency. While manufacturers have developed different designs mainly based

on experience, all of them target at maintaining a largely laminar flow regime.

Unlike pressure screens, hydrocyclones cannot profit from turbulence and the

resulting fluidization. They must be operated at low consistencies in order to

minimize particle–particle interactions. Both turbulence and higher consistencies

will considerably jeopardize the cleaning efficiency.

580 6 Pulp Screening, Cleaning, and Fractionation

The tangential velocity profile observed in a cyclone starts with a forced vortex

flow at the axis. It then passes into a free vortex flow via a transition zone before

the wall effect reduces the velocity to zero again (Fig. 6.19). The axial velocity profile

shows the flow towards the apex at the perimeter of the cyclone body and

towards the base around the center. The pressure loss in the cyclone is a function

of the friction, mainly at the cyclone walls and at the inner wall of the vortex finder.

Locus of zero

axial velocity

Free vortex

v T · r = constant

Forced vortex

v T: r = constant

v tangential

r

v axial

r

Fig. 6.19 Tangential (left) and axial (right) velocity profiles in a hydrocyclone [21].

6.4.3

Дата добавления: 2015-10-21