The
settling mechanism of dust particles, such as inertial collision, interception,
diffusion, gravity and electrostatic force, is the theoretical basis for
analyzing the mechanism of filter dust.
The
filtration process of the filter is very complicated.
Generally
speaking, the sedimentation of dust particles on the trap, i.e. separation and
filtration, is not only the result of one sedimentation and filtration
mechanism, but also the result of the combined action of various sedimentation
and separation and filtration mechanisms.
According
to the different mechanical characteristics of the movement of dust with
different particle sizes in fluids, the mechanism of filtration and dust
removal involves the following aspects:
1.1.1
Screening
The mesh
of filter material is usually 5-50 micron. When the particle size of dust is larger
than mesh or pore diameter or when the dust deposits in the inter space between
the particles, the dust will be blocked.
For the
new fabric filter material, because the pore size between the fibers is much
larger than the particle size of the dust, the screening effect is very small,
but when a large amount of dust deposits on the surface of the filter material
to form a dust layer, the screening effect is significantly enhanced.
1.1.2
Inertial Collision
Generally,
the dust with larger particle size is mainly trapped by inertial collision.
When the dust-laden airflow approaches the filter material, the airflow will
bypass the fiber, and the larger particles (larger than 1 micron) will deviate
from the airflow line due to inertia, continue to move along the original
direction of motion and impact on the fiber and be trapped.
All the
large dust particles in the critical line of dust trajectory can reach the
surface of the fiber and be trapped.
The
inertial collision effect increases with the increase of particle size and
airflow velocity.
Therefore,
the inertial collision can be improved by increasing the flow rate through the
filter material.
1.1.3
Interception
When the
dust-laden airflow is close to the filter material fiber, the finer dust
particles flow with the airflow. If the radius of the dust particles is larger
than the distance between the center of the dust particles and the edge of the
fiber, the dust particles will be intercepted because of the contact with the
fiber.
1.1.4
Diffusion
For dust
particles less than 1 micron, especially sub-micron particles less than 0.2
micron, they break away from streamline under the impact of gas molecules and
make Brownian motion like gas molecules. If they are in contact with fibers
during the movement, they can be separated from the airflow. This effect is
called diffusion, which increases with the decrease of flow rate and the
diameter of fibers and dust.
1.1.5
Electrostatic Action
Many fiber-woven
filter materials, when air flows through, will produce static electricity due
to friction, while dust will be charged due to friction and other reasons in
the transport process, which will form a potential difference between the
filter material and dust particles. When dust tends to filter material with air
flow, due to the Coulomb force, dust and filter material fibers collide and
enhance the adsorption of dust by the filter material. Force is captured to
improve the efficiency of capture.
1.1.6
GRAVITY SETTLEMENT
When the
slowly moving dust-laden airflow enters the dust collector, the dust particles
with large particle size and density may naturally settle due to gravity (see follow
Table1-1).
Table1-1
Particle
size range of various trapping mechanisms
|
Serious
No.
|
Mechanism
|
particle
size range
|
Effect
of Wind Speed Increase on Mechanism Efficiency
|
|
1
|
Interception
|
>1μm
|
reduce
|
|
2
|
Inertial
collision
|
>1μm
|
improve
|
|
3
|
diffusion
|
<0.01～0.5μm
|
reduce
|
|
4
|
Electrostatic
Action
|
<0.01～5μm
|
reduce
|
|
5
|
Screening
|
>
Micro pore size of filter layer
|
reduce
|
Generally
speaking, various dust removal mechanisms are not effective at the same time,
but one or several combined functions.
Moreover,
with the change of the void, airflow velocity, dust particle size and other
reasons, the effects of various mechanisms on the filtration performance of
different filters are also different.
In fact,
when the new filter material starts to filter dust, the efficiency of dust
removal is very low. After using for a period of time, the coarse dust will
form a layer of dust on the surface of the filter cloth.
The
effects of various mechanisms on the filtration performance of different
filters are also different due to the changes of the voids, airflow velocity,
dust particle size and other reasons. In fact, when the new filter material
starts to filter dust, the efficiency of dust removal is very low. After using
for a period of time, the coarse dust will form a layer of dust on the surface
of the filter cloth. Because of the dust filtering effect of the initial layer
of dust and the dust layer gradually accumulated on it, the filtering
efficiency of the filter
material is continuously improved, but the resistance is also
correspondingly enhanced.
When
cleaning ash, the primary layer should not be destroyed, otherwise the
efficiency will decrease. The structure of dust initial layer plays a very
important role in the efficiency, resistance and ash removal effect of bag
filter.
