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Demisters |
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These
devices are used to separate in the most efficient manner the very fine drops
carried away by a gas or steam, either at a boiling stage, vacuum evaporation or
bubbling of a liquid. In fact there are two phenomena, one mechanical and one
physical, but no filtration at all since the size of the held drops has no
relation with the infinitely greater size of the pore or open space. The head
loss or gas passage resistance is practically imperceptible, however demisting
can reach up to 99% efficiency.
A demister
reaches the highest efficiency when the speed of a gas or steam ranges between
certain limits:
Vm Highest
speed.
k Coefficient
depending on viscosity, drop size, type of wire cloth, etc., wich, according to
our experience and results obtained, we have set an average of
0.109.
D
Density of the liquid (kg/m3) under operation
conditions.
d
Density of the gas (kg/m3) under operation
conditions.
Once the highest speed has been
reached it is possible to set both the best and the slowest speeds of the
demister, setting respectively to 75% and 30% of the Vm to get above mentioned
speeds.
Low speeds
rather than high speeds affect the efficiency of a demister so, if necessary, it
is always advisable to operate at high speeds.
| Type |
Application |
Density
(kg/m3) |
Contact surface
m2/m3 |
Free Volume (%) |
| CD-14 |
Gas high speeds |
80 |
145 |
99,00 |
| CD-18 |
Thick liquids, big
particles |
100 |
180 |
98,75 |
| CD-30 |
Standard, all use |
150 |
300 |
98,00 |
| CD-39 |
High efficiency. Usually, wounded
type |
200 |
390 |
97,50 |
| CD-63 |
High performance. Slow
speeds |
300 |
635 |
96,25 |
| CD-175 |
Distillation |
450 |
1.750 |
94,00 |
When the very fine
drops of a liquid are carried away upward by a gas at a determined speed through
the labyrinth of a porous structure there are sudden changes of the sense of
direction which send out, by inertia, these drops against the meshes. This
causes a coalescence phenomenon that makes them gather in bigger drops and, by
gravity, fall down to the bottom of the vessel or separation tower.
Efficiency and head
loss
The use of knitted wire meshes in
the manufacture of demisters involves many advantages, mainly:
1 Small dimensions with respect to the large contact surface
this device offers.
2 Increase of gas speed,
which allows to improve facilities productivity.
3 To avoid product loss at liquid stage, which always leads to
important power losses or disturbances in the next of final
stages.
4 To get cleaner gas or steam, free
from disturbing liquid products.
5 No
maintenance stages thanks to its static operation, power saving for there are no
engines.
6 Available in a wide range of shapes
and dimensions. Simple installation, no special techniques.
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