Hydrocyclones sizing

From Figure 12.3(b), with 5 psi a 660 mm unit will handle 1000 gpm and have a cutoff between 50 and 150 um. This corresponds to the calculated data with V about 19 vol %. For a more detailed study of hydrocyclone sizing, the article of Mular and Jull (in Mular and Bhappu, 1980, pp. 376-403) may be consulted. The pressure drop can be adjusted to compensate for changes in slurry concentration. 

To estimate the performance oF hydrocyclones, Day7 has peblished Fig. 3.4-4. This chart can be used to estimate the performance of hydiocyclones of various sizes and it can be used to predict the effect of specific gravity difference and liquid viscosity. Figure 3.4-5 allows prediction of capacities at various flows and pressure drops as a function of hydrocyclons size. 

In practice the performance of a hydrocyclone is affected by two groups of variables. These are (1) the operating variables that are related to the operating conditions but independent of hydrocyclone size and proportions and (2) the design variables that are dependent on the physical dimensions and proportions of the hydrocyclone. 

Step 8. Inlet to water treating system is at too low a pressme for a hydrocyclone. Size a skim vessel upstream of the flotation imit. 

Centrifuges. Sohd-bowl centrifuges have been proposed as an alternative classifying device to hydrocyclones for cut sizes below 10 pm. The results appear to be mixed (21). In one apphcation, where the cut size was 6.5 pm and the sharpness index 0.7, there was essentially no apparent bypass. However, in other apphcations operating at higher feed concentrations, the cut size ranged from 5—8 pm, but the sharpness index was between 0.3—0.5 and the apparent bypass between 10—30% or higher (22). Smaller cut sizes have also been reported (23). 

Classified removal of course material also can be used, as shown in Figure 16. In a crystallizer equipped with idealized classified-product removal, crystals above some size ate removed at a rate Z times the removal rate expected for a perfecdy mixed crystallizer, and crystals smaller than are not removed at all. Larger crystals can be removed selectively through the use of an elutriation leg, hydrocyclones, or screens. Using the analysis of classified-fines removal systems as a guide, it can be shown that the crystal population density within the crystallizer magma is given by the equations... 

Hydrocyclones are available in numerous sizes and types ranging from pencil-sized 10-mm diameters of plastic to the 1.2-m (48-in) diameter of rubber-protected mild or stainless steel. Porcelain units 25 to 100 mm (1 to 4 in) in diameter are becoming popular, and in the 150-mm (6-in) size the starch industry has standardized on special molded nylon types. Small units for fine-size separations are usually manifolded in multiple units in parallel with up to 480 ten-mm... 

Arterbum, R.A.,"The Sizing of Hydrocyclones, Krebs Engineers, Menlo Park, CA. (1976)... 

More elongated devices are used for slurry thickening whilst those of more squat aspect ratio devices are suitable for product classification by size, as in fines-destruction circuits. The pressure drop in a hydrocyclone varies with the feed rate raised to a power between 2 and 3.3. The cut size is a weak function of pressure drop, varying with for dilute feeds. Large pressure drops are... 

Theoretical representation of the behaviour of a hydrocyclone requires adequate analysis of three distinct physical phenomenon taking place in these devices, viz. the understanding of fluid flow, its interactions with the dispersed solid phase and the quantification of shear induced attrition of crystals. Simplified analytical solutions to conservation of mass and momentum equations derived from the Navier-Stokes equation can be used to quantify fluid flow in the hydrocyclone. For dilute slurries, once bulk flow has been quantified in terms of spatial components of velocity, crystal motion can then be traced by balancing forces on the crystals themselves to map out their trajectories. The trajectories for different sizes can then be used to develop a separation efficiency curve, which quantifies performance of the vessel (Bloor and Ingham, 1987). In principle, population balances can be included for crystal attrition in the above description for developing a thorough mathematical model. 

Because of the complexity of the flow within hydrocyclones, however, various, largely empirical, methods for prediction of the cut size have been proposed for use in practice, as reviewed by Svarovsky (2000). 

The nomographs by Zanker can be used to make a preliminary estimate of the size of cyclone needed. The specialist manufacturers of hydrocyclone equipment should be consulted to determine the best arrangements and design for a particular application. 

Estimate the size of hydrocyclone needed to separate 90 per cent of particles with a diameter greater than 20 pm, from 10 m3/h of a dilute slurry. 

The diameter of a hydrocyclone can range from 10 mm to 2.5 m, cut sizes from 2 to 250 /am, and flow rate (capacities) from 0.1 to 7200 m3/hr. Pressure drop can range from 0.3 to 6 atm (Svarovsky, 1984). For aerocy-clones, very little fluid leaves with the solids underflow, although for hydrocyclones the underflow solids content is typically 45-50% by volume. Aerocyclones can achieve effective separation for particles as small as 2-5 pm. 

Cyclone type and size of hydrocyclone Geometrical proportions Scale-up constants Running cost criterion... 

Testing of hydrocyclones for control of particle size in the feed streams to both agent and energetics electrochemical cells. 

A scoop that is positioned in the hydrocyclone vortex to obtain the desired particle size distribution in the overflow stream. 

The flow patterns in the hydrocyclone are complex, and much development work has been necessary to determine the most effective geometry, as theoretical considerations alone will not allow the accurate prediction of the size cut which will be obtained. A mathematical model has been proposed by Rhodes et alP6), and predictions of streamlines from their work are shown in Figure 1.38. Salcudean and Gartshore137 have also carried out numerical simulations of the three-dimensional flow in a hydrocyclone and have used the results to predict cut sizes. Good agreement has been obtained with experimental measurements. 

Near the top of the hydrocyclone there will be some short-circuiting of the flow between the inlet and the overflow, although the effects are reduced as a result of the formation of circulating eddies, often referred to as the mantle, which tend to act as a barrier. Within the secondary vortex the pressure is low and there is a depression in the liquid surface in the region of the axis. Frequently a gas core is formed, and any gas dispersed in the form of fine bubbles, or coming out of solution, tends to migrate to this core. In pressurised systems, the gas core may be very much reduced in size, and sometimes completely eliminated. 

Most studies of hydrocyclone performance for particle classification have been carried out at particle concentrations of about 1 per cent by volume. The simplest theory for the classification of particles is based on the concept that particles will tend to orbit at the radius at which the centrifugal force is exactly balanced by the fluid friction force on the particles. Thus, the orbits will be of increasing radius as the particle size increases. Unfortunately, there is scant information on how the radial velocity component varies with location. In general, a particle will be conveyed in the secondary vortex to the overflow, if its orbital radius is less than the radius of that vortex. Alternatively, if the orbital radius would have been greater than the diameter of the shell at a particular height, the particle will be deposited on the walls and will be drawn downwards to the bottom outlet. 

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