The cyclone has some obvious advantages, such as high efficiency, if it is constructed properly, easily maintained as three are no moving parts, and, furthermore, it is easy to clean, if the construction is with a fully welded centre cyclone.

The operation theory is based on a vortex motion where the centrifugal force is acting on each particle and therefore causes the particle to move away from the cyclone axis towards the inner cyclone wall. However, the movement in the radial direction is the result of two opposing forces where the centrifugal force acts to move the particle to the wall, while the drag force of the air acts to carry the particles into the axis. As the centrifugal force is preciominant, a separation takes place.

Power and air pass tangentially in to the cyclone at equal velocities. Power and air swirl in a spiral from down to the base of the cyclone separating the powder out to the cyclone wall. Powder leaves the bottom of the cyclone via a locking device. The clean air spirals upwards along the centre axis of the cyclone and passes out at the top.

The centrifugal force each particle is exposed to can be seen in this equation:

                                    C  =  m * V_t²  / r    ( 16 )

Where:

C  =  centrifugal force

m  =  mass of particle

Vc =  tangential air velocity

  r   =  redial distance to the wall from any given point             

From this equation it can be concluded that the higher particle mass, the better efficiency. The shorter way the particle has to travel the better efficiency, and the closer the particle is to the wall the better efficiency, because the velocity is highest and the radial distance is short.

However, time is required for the particles to travel to the cyclone wall, so a sufficient air residence time should be taken into consideration when designing a cyclone.

From above equation it is evident that small cyclones ( diameter less than 1 m ) will have the highest efficiency, a fact generally accepted.

However, the big tonnage dryers in the dairy industry nowadays would require many cyclones ( a cyclone battery ). As each cyclone has to have an outlet for powder in form of a rotary valve, pneumatic valve or flop valve, this means that there is a big risk of air leaks, which will reduce the cyclone efficiency. The small cyclones can also be connected to one central hopper, and only one valve is than necessary. This means however, that unless there is exactly the same pressure drop over each cyclone, air and powder will pass from one cyclone to another via the bottom outlet. This will result in decreased efficiency and increased powder loss. Cleaning the many small cyclones is a problem, as it is a time consuming job, and with the many corners there is a risk of a bacterial infection. For above reasons the cyclones have become bigger and are now constructed with diameters of 2.5-3 m, each handling 25,000-30,000 kg of air/h.  When designing a cyclone various key figures should be taken into account in order to obtain the highest efficiency. This is achieved if

                                     Cyclone diameter / exit duct diameter »3

                                    Cyclone height / exit duct diameter » 10

 
In order to know a cyclone’s efficiency the following terms have to be defined:

a)      The critical particle diameter

b)      The cut size

c)      The overall cyclone efficiency

a)      The critical particle diameter is defined as the particle size that will be com-pietely removed from the air flow ( 100% collection efficiency ). However, as there is no sharply defined point where a particle size is 100% separated or 100% lost the critical particle diameter is not very valuable.                                                                                            

b)      The cut size is defined as the size for which 50% collection is obtained and is a much better valve for stating the efficiency of cyclones. To determine a cyclone’s cut size, grade efficiency curves are worked out by systematically operating a cyclone with a uniform particle size dust.                                                                                                                                                                                                    

c)      The overall cyclone efficiency is the one obtained when handling a product of definite size distribution. Knowing the grade efficiency curve of the product size distribution of the powder passing to the cyclones, the overall efficiency can be calculated, i.e. the powder loss can be predicted.                              

Another method of learning the cyclone efficiency is by a simple powder loss

measurement after the cyclone.

 A very small fraction of the out-going air is passed through

a high-efficient mini cyclone or through micro dust filters. The amount of powder collected is directly proportional to the powder loss, which will mainly be a result of: