2.1 Cyclone performance

Cyclones are basically centrifugal separators. They simply transform the inertia force of gas particle to a centrifugal force by means of a vortex generated in the cyclone body. The particle laden gas enters tangentially at the upper part and passes through the body describing the vortex. Particles are driven to the walls by centrifugal forces (an expression for this force is given blow eq. 5.1), losing its momentum and falling down to the cyclone leg. In the lower section, the gas begins to flow radially inwards to the axis and spins upwards to the gas outlet duct.

............................................(5.1)

ρ_p = particle density, (kg/m³)

d_p = particle diameter, inches (µm)

v_p = particle tangential velocity (m/s)

r = radius of the circular path, (m)

The main variables describing the cyclone performance are pressure drop, efficiency and cut diameter. Equations involving each of these parameters are provided in this section.

2.1.1 Cut diameter

The cut diameter of the cyclone is defined as the size of the particles collected with 50% collection efficiency. It is an indicator of the size range of particles that can be collected. It is a convenient way of defining as it provides information on the effectiveness for a particle size range. A frequently used expression for cut off diameter is

............................(5.2)

μ = viscosity (Pa.s); B_c = inlet width (m)

N = effective number of turns (5-10 for common cyclone)

v_i = inlet gas velocity (m/s)

ρ_p = particle density (kg/m³ ); ρ = gas density (kg/m³ )

 

A value of N, number of turns, must be known in order to solve equation (5.2) for d_pc. Given the volumetric flow rate, inlet velocity, and dimension of the cyclone, N can be easily calculated. Values of N can vary from 1 to 10, with typical values in the 4-5 range.