Design technical data
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 link with the infinitely greater size of the pore or open space. . The head loss or gass 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 = k √((D-d)/d)Vm = k √((D-d)/d)
- Vm: Highest speed.
- K:Coefficient depending on viscosity, drop size, type of wire cloth, etc., which, according to our experience and results obtained, we have set at 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 lowest speeds of the demister, setting respectively to 75% and 30% of the Vm to get aboye 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.
Section
Main types of demisters
| Type | Application | Density Kg/m3 |
Contact surface m2/m3 |
Free Volume (%) |
|---|---|---|---|---|
| CD-14 | Gas high speeds. | 80 | 145 | 99 |
| CD-18 | Thick liquids, big particles. | 100 | 180 | 98.75 |
| CD-30 | Standard, all use. | 150 | 300 | 98.00 |
| CD-39 | High efficienty. Usually, wounded type. | 200 | 390 | 97.50 |
| CD-63 | High performance. Slow speeds. | 300 | 635 | 96.25 |
| CD-175 | Distillation. | 450 | 1750 | 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.
The use of knitted wire meshes in the manufacture of demisters involves many advantages, mainly:
- Small dimensions with respect to the large contact surface this device offers.
- Increase of gas speed, which allows to improve facilities productivity.
- To avoid product loss at liquid stage, which always leads to important power losses or disturbances
in the next of final stages. - To get cleaner gas or steam, free from disturbing liquid products.
- No maintenance stages thanks to its static operation, power saving for there are no engines.
- Available in a wide range of shapes and dimensions. Simple installation, no special techniques.