Pressure loss, through cyclones

Pressure Loss through Cyclone—Lissman (1930) has shown how pressure-drop through a cyclone may be determined. The total drop in pressure consists of the following losses (a) Entrance losses including... 

Pressure loss through the cyclone is also a key performance parameter, and this depends mainly on the design of the cyclone. In general, the pressure drop across the cyclone collector is small compared with most other dust collectors, but the higher the collection efficiency required, the larger the pressure drop and hence the energy consumption required. 

It should be noted that most of these theories for the prediction of the pressure losses in cyclones ultimately require the assignment of certain experimentally determined quantities in order to produce reasonable agreement between theory and experiment. The involvement of these empirical constants almost certainly restrains the use of the theories to the limited group of cyclones that the experiment has covered in order to produce good predictions of pressure drops through the cyclone. Therefore, these empirical theories may be used only as a preliminary estimate of the energy consumption in cyclones. Prototype cyclone experiments may well be required in order to obtain an accurate value of the pressure loss for a newly designed cyclone. 

Several attempts have been made to calculate the frictional loss or AP of a cyclone, although none has been very satisfactory. These are because assumptions made have not considered entrance compression, wall friction, and exit contraction, all of which have a major effect. Consequently, no general correlation of cyclone AP has been adopted. Pressure drop in a cyclone with collection efficiency is important in evaluating its cost. Correlations for the pressure drop have been empirical and are acceptable up to AP = 10 in. HjO. The pressure drop (AP) or the frictional loss is expressed in terms of the velocity head based on the cyclone inlet area. The frictional loss through cyclones is from 1 to 20 inlet velocity heads and depends on the geometric ratios. AP through a cyclone is given by... 

A similar case may be made for the use of density in Stokes law, the buoyancy of particles in the separation zone must be taken into account. The fine particles displace the continuous phase and hence it is the density of the liquid that is used in the model. In any case, the suspension density in the zone is not known but is likely to be much less than that of the feed. The second use of fluid density is in the resistance coefficient, Eu. The density to be used there depends on how we define the Euler number the dynamic pressure in the denominator (equation 6.9) is simply a yardstick against which we measure the pressure loss through a cyclone. We have used the clean liquid density in the dynamic pressure alternatively, the feed suspension density may be used. It is immaterial which of the two densities is used (they are both equally unrealistic) provided the case is clearly defined conversion from one to the other is a simple matter. 

Equation (3.3.2) is especially useful to the plant engineer who wishes to estimate the pressure loss through his or her cyclone system at conditions other than design conditions, or at some fiow rate other than one for which the pressure loss is known. We shall see how in Chap. 8, where also the derivation of this and other dimensionless numbers characterizing cyclones and swirl tubes can be found. 

The pressure drop and the friction loss through a cyclone are most conveniently expressed in terms of the velocity head based on the immediate inlet area. The inlet velocity head, h i, which is expressed in inches of water, is related to the average inlet-gas velocity and density by ... 

Fev = Friction loss (inlet-velocity heads) A pev = Pressure drop through the cyclone (inlet-velocity heads)... 

Rotational flow in the forced vortex within the cyclone body gives rise to a radial pressure gradient. This pressure gradient, combined with the frictional pressure losses at the gas inlet and outlet and losses due to changes in flow direction, make up the total pressure drop. This pressure drop, measured between the inlet and the gas outlet, is usually proportional to the square of gas flow rate through the cyclone. A resistance coefficient, the Euler number Eu, relates the cyclone pressure drop Ap to a characteristic velocity v ... 

Vacuum sublimation is a development of simple sublimation, which is particularly useful if the pressure at the triple-point is lower than atmospheric, where the transfer of vapour from the vaporiser to the condenser is enhanced by the increased driving force attributable to the lower pressure in the condenser. Iodine, pyrogallol, and many metals have been purified by vacuum sublimation processes in which the exit gases from the condenser are usually passed through a cyclone or scrubber to protect the vacuum equipment and to minimise product loss. 

---