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The cyclone has some obvious
advantages, such as high efficiency, if it is constructed properly, it
is easily maintained as there are no moving parts, and, furthermore, it is easy
to clean, if the construction is with a fully welded centre cyclone, see Fig.
51.
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 predominant, a separation takes place.
Powder and air pass tangentially into the cyclone at equal
velocities. Powder and air swirl in a spiral form 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. See Fig. 52.
The centrifugal force each particle
is exposed to can be seen in this equation:
C = m × Vt2 / r (16)
Where:
C =
centrifugal force
m = mass of particle
Vt = tangential air velocity
r = radial
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 operation 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 flap 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
then necessary, see Fig. 53. 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 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
Air through-put (velocity Vt) and increased
pressure drops will also increase the efficiency, but the energy requirement
will increase simultaneously, so in general the upper limit is 175-200 mm WG
for skim milk powder. 140-160 mm WG is the maxi-mum for whole milk in order to
avoid deposits and final blocking.
In
most cases rotary valves are used as air lock and product discharge at the
bottom of the cyclone. The conical type allowing for easy adaption of the gap
between the housing and the rotor should be preferred as the powder loss may be
reduced.
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-pletely 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 value 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
handl ing a product of definite size distribution. Knowing the grade efficiency
curve of the cyclone and 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:
-
Feed with low solids or feed containing air
-
High outlet air temperature
-
Low particle density (as a result of the above, for
example)
-
Leaking product outlet from old non-adjusted rotary
valves
-
Blocked cyclone
-
Change in drying parameter resulting in decrease of mean
particle size
-
Old cyclones, dented due to heavy beating to avoid
blockings