First, the dust removal principle of wet dust collector
In a wet dust collector, there are roughly three forms of contact between water and dusty airflow:
1. water drop
Table 8- 10 specifications and main technical performance of bag filter
Because of mechanical spraying or other means, water forms water droplets of different sizes, which are dispersed in the airflow to become dust collectors, such as spray towers. The dust catching mechanism conforms to the filtering mechanism in principle, but water droplets are used as the dust catching body.
2. Water film
This is a water film formed on the dust catching surface, and the dust in the airflow collides with the water film due to inertia and centrifugal force, such as a cyclone water film dust collector. Its separation principle is the same as that of dry cyclone separator. However, due to the existence of water film, the probability of catching dust is increased, and the secondary dust is effectively prevented. Thereby greatly improving the dust removal efficiency.
3. Bubbles
The contact between water and gas in the form of bubbles is mainly produced in foam dust collector. As the airflow passes through the water layer, bubbles of different sizes are generated according to the velocity of airflow, surface tension of water and other factors. The deposition of dust in bubbles is mainly due to inertia, gravity and diffusion.
In the actual wet dust collector, there may be two or even three contact forms.
Second, the spray tower
Spray tower is a kind of hollow tower, which collects dust with the help of water mist sprayed from nozzles. The typical structure of spray tower is shown in Figure 8-8. The gas enters from the lower part of the tower, and the airflow is evenly distributed in the tower through the airflow distribution plate. One or several rows of nozzles are arranged in the tower (the spraying pressure is not lower than (1.5 ~ 2) × 106 Pa), and the water mist flows downward under the action of gravity, which is opposite to the dusty airflow. The dusty airflow is purified by water mist and discharged from the upper part. The dusty air flow rises at a speed of 0.6 ~ 1.2m/s to avoid the fog droplets from being lifted up and taken away.
The main mechanism of dust removal in spray tower is to use water droplets as dust catcher to collect dust under the action of inertia, interception and diffusion, in which inertia is the main role. In order to improve the efficiency of dust collection, especially the collision mechanism of inertial dust collection, it is necessary to increase the relative speed of water droplets and airflow and reduce the size of water droplets. Figure 8-9 shows the relationship between water droplets and dust removal efficiency in the spray tower. As can be seen from the figure, the efficiency is the highest when the diameter of water droplets is 500 ~ 1000 micron, and the efficiency of reducing the diameter of water droplets decreases. Therefore, it can be considered that when the dust is less than 5μm and the diameter of water droplets is 800 μ m, the theoretical efficiency is the highest. Therefore, it is most appropriate that the water droplets generated by colliding with the nozzle are slightly less than1mm.
Figure 8-8 Structure Diagram of Spray Tower
1- water inlet; 2- water filter; 3- water pipe; 4- Water deflector; 5- nozzle; 6- air distribution board; 7- Sewage outlet
Three, centrifugal spray dust collector
Centrifugal spray dust collector is a wet dust collector using centrifugal force. This kind of dust collector (scrubber) can be roughly divided into two types. One is to strengthen the collision between droplets and dust particles by centrifugal force. Therefore, as mentioned above, the relative velocity (inertial collision) and spray density (interception) of dust particles and droplets are two important factors that affect the collection efficiency of droplets. If the relative velocity of droplets and dust particles increases, the collision efficiency will increase. This can be achieved by applying centrifugal force. For example, the gas rotates at the radius of 0.3m at the tangential speed of 17m/s, and the centrifugal force is 100 times that of gravity. We don't use gravity, but use such centrifugal force to act on droplets, and the effect of catching dust particles can of course be greatly improved (it can be calculated that the efficiency of catching dust particles due to inertial collision of droplets of various sizes under the action of force of 0. 1N is shown in Figure 8- 10. The curve in the figure shows that the droplet size is in the range of 40 ~ 200 microns, and the droplet size around 100μm is the most effective. The structure of this dust collector is that a water inlet pipe with several nozzles is arranged in the center of the lower part of the dust collector to spray water into the rotating gas, which is called the central water jet cyclone dust collector; Several rows of nozzles are arranged around the cylinder of the dust collector, and water is injected into the rotating airflow at the same angle or tangential direction, which is called circumferential water jet cyclone dust collector; Others spray water like an ordinary spray tower, but the airflow rotates in the tower. There are also two ways for these dusters to make the gas rotate: one is to let the air flow enter the duster tangentially at the speed of 15 ~ 45m/s, as shown in Figure 8- 1 1(a), to generate rotating motion; The other is to rotate the airflow with fixed guide vanes as shown in Figure 8- 1 1(b).
Figure 8-9 Relationship between Droplet Diameter and Collision Efficiency
Fig. 8- 10 η-d relation curve of centrifugal spray scrubber
In addition to using centrifugal force to strengthen the collision between droplets and dust particles, there is also a centrifugal dust collector that uses centrifugal force to make dust particles reach the wall wetted by water and be captured. It belongs to this cyclone with water film (Figure 8-12); And a horizontal cyclone water film dust collector.
At present, the centrifugal dust collector widely used in China is the water film cyclone dust collector. Its structure is that several nozzles are installed around the upper part of the cylinder to spray water on the inner wall of the outer cylinder, so that the inner wall of the cylinder is always covered with a thin water film flowing downwards. The dusty gas enters tangentially from the lower part of the cylinder, then spirals up and is discharged from the upper part of the cylinder. The dust separated by centrifugal force is thrown to the wall, adhered by the water film, and then discharged through the sewage outlet with the sewage, and the purified gas is discharged from the upper part of the cylinder.
Figure 8- 1 1 Two kinds of rotary centrifugal scrubbers
(a) tangential inlet rotation; (b) the guide vane rotates.
Fourth, the impact dust collector
The working principle of the impact dust collector is that the dusty airflow rushes into the water from the nozzle at a certain speed, and then turns to 180 to change its flow direction. Under the action of inertia, part of the dust is separated, and the airflow is further purified by the spray and water mist splashed by the airflow, and the purified airflow is dehydrated and discharged by the water baffle, as shown in Figure 8- 13. The dusty airflow is sprayed into the water from the air inlet pipe, and there is a cone at the outlet of the air inlet pipe. An annular gap is formed between the bottom of the cone and the pipe wall. The airflow is ejected from this gap (the nozzle is 2 ~ 3 mm high from the water surface), and the water mist is stirred up, and then becomes 180, which is discharged from the air outlet after water and gas separation by the baffle.
The air velocity in the annular gap, that is, the impact velocity, is an important design factor that dominates the dust removal efficiency. Usually this speed is 45 m/s, and the pressure drop of the whole dust collector is about 2 times of the dynamic pressure calculated according to the impact speed. When the impact velocity is 39m/s or higher, there is a very high degree of turbulence in the dust collector with appropriate size, and the water and gas are in a state of strong disturbance, and the turbulent disturbance area extends to the position directly below the gas outlet.
Fig. 8- 14 shows the Roto-clone self-excited dust collector. After the dusty airflow enters from the middle, it first hits the surface of the washing liquid, and some coarse particles settle down, and then it is forced to pass through the S-shaped channels on both sides to increase its speed to about15m/s. The S-shaped channel consists of two curved blades, and the lower parts of the blades are submerged in water. Because the air velocity in the channel is relatively high, a chaotic water curtain is aroused, and then it is broken into many water droplets, and dust particles collide with the water droplets and are captured. The purpose of designing S-shaped channel is to quickly change the direction of air flow, increase centrifugal force and improve the degree of liquid turbulence. When the airflow leaves the S-shaped channel, it turns downward due to the restriction of the upper blade, and then rises again. At this time, due to inertia, some water droplets and dust are separated from the gas and fall into the water. The residual water droplets and dust in the updraft are removed by the eaves board dehydration device and then flow out of the dust collector.
Fig. 8- 12 CLS cyclone water film dust collector
Figure 8- 13 Impact Dust Collector
Fig. 8- 14 rotary self-excited dust collector
Large-scale self-excited dust collectors are generally equipped with a mechanical dust removal device at the lower part, and a drain valve is installed at the bottom of the dust collector to discharge sludge regularly.
The main advantage of this kind of dust collector is that it does not use the nozzle with small orifice to spray water, and there is no small gap in each part of the dust collector, which is not easy to be blocked, so it can be used to treat large flow gas with high dust concentration. It uses less water and is only used for evaporation and sludge discharge.
The efficiency of self-excited dust collector depends largely on the water level, so in order to ensure the normal operation of the equipment, it is necessary to set up an automatic water level control device.
In addition, the design of self-excited dust collector should also consider:
1. In order to make the gas entering the dust collector evenly distributed at the inlet of the whole blade, the distance between the lower edge of the air inlet and the water surface should not be less than 0.5 ~ 1.0m, and the air inlet speed should not be too high, generally less than18m/s. ..
2. In order to make the air flow uniform, prevent water from being carried, and facilitate the control of water level, the spacing between the two sides of the blade should be roughly equal, and the section width is generally not less than 0.5m.
3. The fog separation room should have enough space to prevent excessive water droplets from being brought into the water baffle, and the rising speed of air flow should generally not be greater than 2.7m/s. ..
4. The treated air volume per unit blade length is 5800m3/h, so the energy consumption is small. When the air volume exceeds 6000m3/h, the resistance increases obviously.
5. Blade, the key component of dust collector, must meet the design requirements when making, and its installation must be horizontal, otherwise it will directly affect the dust removal efficiency. The blade length is generally not more than 5m.
6. The folding baffle is easy to block, and it is best to use double-layer shielding type, which has good dehydration effect and is not easy to block.
The actual application effect of the dust collector is listed in Table 8- 1 1.
Table 8- 1 1 Dust removal efficiency of rotary self-excited dust collector