Cyclones performance curves

Practically all cyclone performance data have been related to a present cyclone set of geometric ratios. One model for cyclone grade-efficiency curves has been tested against reported commercial cyclone efficiencies (159). A good fit was obtained. 

Although performance curves are valuable in assessing classifier performance, frequently the cyclone overflow size analysis is used more than the d Q of the cyclone. In practice, clusters of cyclones (in parallel) are used to handle large capacities. Cyclones are manufactured in sizes ranging from 0.01 to 1.2 m in cyclone diameter, ie, the cylindrical section at the top (2,10). Capacities mn from 75 to 23,000 L/min. Materials of constmction vary widely. Rubber-lined or aH-polyurethane cyclones are used when abrasion is a problem. 

Figure 10.45. Performance curves, standard conditions (a) High efficiency cyclone...

An alternative method of using the scaling factor, that does not require redrawing the performance curve, is used in Example 10.4. The cyclone should be designed to give an inlet velocity of between 9 and 27 m/s (30 to 90 ft/s) the optimum inlet velocity has been found to be 15 m/s (50 ft/s). 

Several classification functions, C(L), are given in Figure 4.20. Here, the fraction of particles by mass reporting to the recycle stream is given as a function of particle size, L, for a screen and a cyclone. Several authors have used empirical classification functions instead of dassifier performance curves with reasonable results for the overall comminution-classification circuit control. The steady state (i.e., dmidt = 0) macroscopic population balance on a discrete mass basis over the grind-... 

Two characteristics are used to define cyclone performance. They are the critical particle diameter (particle size that is completely removed from the air stream) and the cut size (the particle diameter for which 50% collection efficiency is achieved). A typical example of theoretically and experimentally obtained efficiency curves is shown in Figure 9.24. It is evident from this diagram that particles above 15 pm are removed with high efficiency in the cyclone. The pressure drop across the cyclone unit ranges between 700 and 2000 Pa. 

As was pointed out at the beginning of this section, there are several other models available for predicting cyclone performance, but those were obtained from curve-fitting exercises and, as such, do not represent a true physical model of the separation process in hydrocyclones. These models are reported in the next section. 

Geometric standard deviation of the reduced grade efficiency curve of the cyclone Performance criteria Total coarse recovery by mass, r Total reduced mass recovery, r Actual cut size, xso Reduced cut size,... 

Knowlton and Bachovchin (1978) studied the effect of pressure on cyclone performance and found little change in overall cyclone efficiency with pressure over a pressure range from 0 to 55 barg. However, fractional efficiency curves for the same study (Fig. 14) showed that cyclone efficiency decreased with pressure for particle sizes less than about 20 to 25 microns. For particle... 

Ceramics tend to have a much smoother surface than refractory liners and this can result in improved cyclone efficiency. This statement is qualified somewhat, however, by the fact that some ceramic cyclone installations consist of fiat pieces of tile that produces a surface finish which only approximates a smooth, continuous surface. From a cyclone design point of view, a surface that only approximates a smooth cylindrical or conical inner surface is difficult to simulate in any computer model of separation performance. The difficulty lies in establishing a reasonable estimate of the effective wall roughness of the tiled surface. This effect, however, is less of a concern as the cyclone diameter increases. This is because cyclone performance is affected by the relative roughness (roughness divided by the radius or diameter of the cyclone) and not the absolute roughness, per se. The tile effect is also of less concern in units greater than about 2 meters in diameter. If the tile surface is curved... 

The collection efficiency curve is usually employed to demonstrate the performance of a cyclone. Figure 13.3 shows a typical collection efficiency curve for a cyclone at a particular airflow rate. The size of particles that have a collection efficiency of 50% is usually employed as a simple indication of the separation efficiency of the cyclone, and is known as the cut-off particle size... 

For the separation of gas-solid mixtures, preliminary design of inertial separators, cyclones, electrostatic precipitators, scrubbers and some types of filters can be carried out on the basis of collection efficiency curves derived from experimental performance. 

Example 7.1 In a test of the performance of a cyclone, a dust-laden gas with the particle mass flow rate of 600 kg/h is used. Given that the dust escape rate is 30 kg/h, what is the overall collection efficiency Plot the fractional efficiency curve to identify the cut-off size of this cyclone. The results of particle size analysis (by weight percentage [wt%]) at the inlet and outlet of the cyclone are given in Table E7.1. 

The proportion of each particle size fraction which is precipitated in a cyclone of particular shape and rating is called the fractional dust collection efficiency and is plotted as a percentage against the particle size diameter (in microns). The curve thus obtained is called a separation curve. It is an important criterion for comparing different cyclones with one another. Table6 indicates, by way of example, the overall collecting performance of a cyclone for dust with known particle size distribution. 

The performance of the hydrocyclone is calculated by using a partition curve similar to a screen curve. This gives the d o size, or 50% probability at the cut point. This cut point is defined as the condition for which 50% of the feed will be discharged as coarse particles in the cyclone underflow and 50% as fines or cyclone overflow. For every cyclone design, there is a base d oc or cut-off for the recovery (Figure 7-27). 

In this section we will compute the grade-efficiency curve and overall separation efficiency at low inlet solids (classification only) loadings (co < Cql)- The grade-efficiency curve for a given cyclone expresses the functional dependence of separation performance upon particle diameter. As discussed in Chap. 3, it is normally an s-shaped function satisfying the limits ... 

When one is designing a new cyclone system, it is customary to perform the calculations indicated above to find 50, choose a slope—based on experience or otherwise - and then use Eq. (6.3.2) to compute points on the cyclone s grade-efficiency curve, r]i as a function of Xi, using the known x o and m. 

The model predictions of the cyclone s grade-efficiency curve are shown in Fig. 6.A.6. The model is seen to predict actual performance quite well for particle sizes greater than about 2 pm but overpredicts actual performance for smaller sized particles. In this case, the model predicts a mass loading effect for the smaller sized particles that is larger than what was observed. 

In this chapter we shall derive and present relationships or formulae that will allow us to predict a cyclone s cut-point diameter, grade-efficiency curve, overall or gross efficiency, and pressure drop on the basis of measurements taken on a geometrically similar cyclone. These formulae should also allow us to evaluate the performance of an operating cyclone and, if necessary, assist us in troubleshooting its design, mechanical condition, or mode of operation. 

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