Hydrocyclone bodies

Another interesting design for oil-water separation and mineral flotation involves the use of a porous wall through which air passes into the hydrocyclone body. This is... 

It can be demonstrated (Svarovsky, 1984) that, similar to the case of cyclones, the most appropriate characteristic velocity to describe hydrocyclone operation is the superficial velocity in the hydrocyclone body described by Equation 10.38. 

Characteristic cyclone number Hydrocyclone body diameter Inlet equivalent diameter (by area)... 

The pattern of fluid flow within the hydrocyclone body is best described as a spiral within a spiral with circular symmetry. A schematic view of the spiral flow inside a hydrocyclone is shown in Fig. 23b. The entering fluid flows down the outer regions of the hydrocyclone body. This combined with the rotational motion creates the outer spiral. At the same time, because of the wall effect, some of the downward moving fluid begins to feed across toward the center. The amount of inward motion of fluid increases as the fluid approaches the cone apex, and fluid that flows in this inward stream ultimately reverses its direction and flows upward to the cyclone overflow outlet via the vortex finder. This reversal applies only to the vertical component of velocity, and the spirals still rotate in the same circular direction. In the meantime, the downward flow near the wall carries solid particles to the apex opening (bottom outlet). 

Typical target diameter is 5 to 100 pm and is the diameter that 50% reports to the overflow and 50% reports to the underflow. Typically three times target diameter is the diameter below which all particles in disttibution are removed. The standard hydrocyclone has an inlet diameter of 0.28 D the overflow exit diameter = 0.34 D the vortex finder length is 0.4 D cylindrical body of height of 0.4 D, vertical length of cone = 5 D or cone angle about 10°. Underflow diameter adjustable to adjust the volume split between the overflow and underflow. 

The Reynolds number defines flow features of the system and, in the case of hydrocyclones, the characteristic dimension may be taken as the cyclone body diameter D ... 

Combining the definitions of Re and Stkso (r) from Equations 10.55 and 10.56, with the definition of superficial velocity in the cyclone body (Equation 10.81), and considering the density of the suspension due to its high concentration, the following relations for the hydrocyclone diameter and the cut size are obtained ... 

This relationship was then used to calculate the tangential velocities from static pressure measurements in different places within hydrocyclones run with clean liquids. Driessen and many others following him thus deduced the general expression for tangential velocity profiles in the outer vortex given previously in equation 6.1, where n is an empirical exponent, usually from 0.6 to 0.9. Note that for a free vortex in inviscid flow n =, while in a forced vortex (solid body rotation) = 1. 

A full understanding of the hydrocyclone requires a detailed analysis of the flow pattern within its body. A number of reviews on this subject may be found in the literature (Bradley and Pulling, 1959 Fontein, 1951 Kelsall, 1952). Only a brief qualitative description will be presented in this section. 

As discussed earlier, there are two spiral flow patterns existing in the hydrocyclone. Only particles existing in the outer spiral flow will be separated by the centrifugal force. Any particles in the inner spiral flow will pass upward to the overflow outlet. It should be noted that there are two important stages in the process of particle separation. One is the separation of the solids from the main body of the flow into the boundary layer adjacent to the inner wall of the hydrocyclone by centrifugal forces. The other is the removal of the separated solids from the boundary layer by downward fluid flow (not by gravity) to the apex of the cone and out of the hydrocyclone. 

Separation criteria are fundamentally different depending on whether the flnid flow is of sohd-body rotation type or vortex type. By way of example, the qnality of separation is affected when the flow rate through a centrifugal separator is increased, while the inverse property is obtained in a hydrocyclone. 

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