Based on the above classification and naming methods according to dust cleaning methods, the structure and performance of several commonly used bag dust collectors are introduced. Although the gas ring backflush bag type dust collector has the characteristics of high filtration wind speed and strong dust cleaning ability, it is suitable for purifying gases with high dust concentration and relatively humid air. However, due to the rapid wear of the filter bag, the gas ring box and its transmission components are easily Failure occurs and is rarely used, so it will not be introduced here.
(1) Simple dust cleaning bag dust collector
Simple dust cleaning bag dust collector includes various simple dust cleaning methods, some of which rely on the self-weight of the dust layer deposited on the surface of the filter material. Some of the ones that fall off are manually beaten, some have manual shaking mechanisms, and some use air vibration. Figure 6-31 shows two forms of simple bag dust collectors. (a) is an upper air inlet, and (b) is a lower air inlet. They are both positive pressure and internal filter structures. The clean air flows through the louvers. Or the hood is discharged, and the dust is cleaned by the dust layer falling off by its own weight and manual manual beating
The filtration wind speed of the simple dust bag dust collector is lower than other forms, generally 0.15 ~ 0.6m/min , when using cotton cloth or flannel cloth filter material, take 0.15~0.3m/min, and when using woolen filter cloth, take 0.3~0.6m/min. The pressure loss is controlled below 600~1000Pa. When designed and used properly, the dust removal efficiency can reach 99%. The diameter of the filter bag is generally 100~400mm, the length is 2~6m, the distance between the filter bags is 40~80mm, and an inspection channel with a width of not less than 800mm is left between each filter bag group.
The characteristics of the simple dust cleaning bag dust collector are simple structure, easy installation and operation, low investment, low requirements for filter materials, low maintenance and long filter bag life. The main disadvantage is that due to the low filtration wind speed, the dust collector is bulky and covers a large area. When operating under positive pressure, the working environment for manual dust cleaning is poor.
(2) Mechanical vibration bag dust collector
This type of dust collector uses mechanical transmission to vibrate the filter bag, causing the dust layer deposited on the filter bag to fall into the dust. In the fight. Figure 6-32 shows three different vibration modes. Figure 6-32(a) shows the way the filter bag vibrates in the vertical direction. Either the hanging frame of the filter bag is regularly lifted, or the eccentric wheel can be used. The way to vibrate the frame; Figure 6-32(b) is the way the filter bag vibrates in the horizontal direction, which can be divided into upper swing and waist swing. Figure 6-32(c) is to twist a certain angle so that the The dust layer breaks up and falls into the ash hopper.
The bag dust collector (see Figure 6-18) that uses vertical vibration of the eccentric wheel to clean dust has the characteristics of simple structure, good dust cleaning effect, and low power consumption for dust cleaning. It is suitable for dust with different concentrations. Dust removal from large, intermittent dust sources. When using a multi-chamber structure and setting up valves to control the opening and closing of the air path, it can also be used for dust removal from continuous dust sources.
The filtration wind speed of mechanical vibration bag dust collector is generally 0.6~1.6m/min, and the pressure loss is about 800~1200Pa.
(3) Counter air flow bag dust collector
Counter air flow cleaning means that the air flow direction during dust cleaning is opposite to that during normal filtration. Its forms include reverse blow and reverse air flow. There are two types of suction. In essence, the air ring back-blowing type and pulse injection type are also counter-air flow cleaning types.
Now take the reverse air flow cleaning method as an example to illustrate the principle of reverse air flow cleaning. As shown in Figure 6-33, the counter-air flow bag dust collector is usually divided into several chambers. Each chamber has a separate ash hopper and dust-containing gas inlet pipe, clean gas outlet pipe and reverse suction pipe. And are connected to the air intake main pipe, clean air main pipe and reverse suction main pipe respectively. There is a switching valve (primary valve) in the clean air pipe, and a reverse air flow valve (secondary valve) in the reverse suction pipe. Figure 6-33(a) shows the normal filtration state, with the primary valve open and the secondary valve closed. When the dust needs to be cleaned according to a predetermined cycle (timing control) or the pressure loss of the dust collector reaches a predetermined value (constant pressure control), the controller issues an instruction and the dust cleaning mechanism starts to act. The primary valve closes and the secondary valve opens [see Figure 6— 33(b)]. Since the inside of the dust collector is under negative pressure, air is sucked in from the back suction duct, passes through the filter bag from the outside and enters the inside, causing the filter bag to deform (star-shaped) and destroy and fall off the dust layer deposited on the inner surface of the filter bag. . After the dust cleaning is completed, both valves are closed [see Figure 6-33(c)] and are in a windless state, allowing the dust suspended in the filter bag to settle naturally. After a certain period of time, the filtering state will be restored [see Figure 6-33(a)], and then the next filter chamber will be cleaned.
Generally, this dust cleaning method with three actions of Figure 6-33(a), Figure 6-33(b), and Figure 6-33(c) is called the "three-state" method, and only Figure 6-33(c) The two actions of a) and Figure 6-33(b) [without the action of Figure 6-33(c)] are called the "two-state" method. The "three-state" method can prevent dust from being adsorbed to the filter bag immediately after counter-air flow cleaning, making the cleaning effect worse.
(4) Combination of reverse air flow and mechanical vibration with bag dust collector
In order to enhance the effect of dust cleaning, the two cleaning methods can be used at the same time. For example, mechanical vibration plus back blowing air, its structure is shown in Figure 6-34. The filter bags are all hung on the support hanger 5, and the rapping mechanism can raise the support hanger to vibrate up and down. During normal filtration, dust-containing gas enters the dust collector through the air inlet pipe 1 and is distributed to each group of filter bags 9 through the distribution pipe 2. The clean gas is discharged from the main pipe 8 through the primary valve 7. Dust cleaning is carried out room by room. When a group of filter bags in a certain room needs to be cleaned, start the lifting and rapping mechanism in the upper part of the room. At the same time, close the primary valve 7 and open the back blow valve 6. During the mechanical rapping and back blowing, Under the simultaneous action, dust cleaning is achieved.
(5) Pulse injection bag dust collector
The dust filtration process of the pulse injection bag dust collector (see Figure 6-35) is roughly as follows: the dust-containing gas flows from the lower cone The body is introduced into the pulse injection bag dust collector, and the dust is retained on the outer surface of the filter bag. The clean air passing through the filter bag enters the upper box through the venturi tube and is discharged from the air outlet pipe. The dust cleaning process is: the controller triggers each exhaust valve in sequence regularly, so that the back pressure chamber of the pulse valve is connected to the atmosphere (deflates). When the pulse valve is opened, the compressed air in the air bag passes through the pulse valve and the small hole on the blow pipe. The ejection (primary wind) induces several times (about 5 to 7 times of the primary wind) of surrounding air (secondary wind) to blow into the filter bag through the Venturi tube, causing the filter bag to expand and vibrate rapidly, coupled with the opposite direction of the air flow, Remove the dust layer accumulated on the outer surface of the filter bag. This dust cleaning method has pulse characteristics, so it is called pulse dust collector. The injection pressure of compressed air is 500~700kPa, the pulse time (or injection time) is 0.1~0.2s, and the pulse period (injection period) is generally 60~180s.
The pulse injection system consists of a controller, a control valve, a pulse valve, an injection pipe and a compressed air bag.
The pulse valve is the actuator of the control system, and its structure is shown in Figure 6-36. Room A of the pulse valve is connected to the air bag, room B is connected to the injection pipe, and room C (back pressure room) is connected to the control valve. Chambers A, B, and C are separated by corrugated diaphragm 3. Chambers A and C are connected by orifice 5. Spring 4 presses the corrugated diaphragm to block the blowing port 6. The working principle of the pulse valve is: when there is no signal from the controller, both the control valve and the pulse valve are in a closed state, and the air pressures in chambers A and C are equal. Since the pressure-bearing area of ??the corrugated diaphragm 3 in chamber C is larger than the pressure-bearing area in chamber A, plus the pressure of the return spring 4, the corrugated diaphragm seals the blowing port 6. When the controller sends a signal, the control valve and chamber C are connected to the atmosphere and quickly release the pressure. The pressure in chamber A is greater than the pressure in chamber C. The corrugated diaphragm 3 moves to chamber C, opens the blowing port, and the compressed air passes from the air bag through chamber A. and chamber B spray to the filter bag through the blow pipe. After the signal disappears, the control valve closes, chamber C stops exhausting, re-inflates and rises to the pressure of the air source, the diaphragm re-closes the blowing port, the pulse valve closes, and the blowing stops. Each pulse valve is connected to a blow pipe, which has six blow holes aligned with the axis of the venturi tube, and blows six filter bags at the same time.
The pulse controller is a device that sends pulse signals to the control valve. The injection cycle and injection time can be adjusted through the pulse controller. Therefore, the controller is the key equipment of the pulse injection bag dust collector, which directly affects the cleaning effect and normal operation of the dust collector. There are three main types of pulse controllers: non-contact electric pulse controllers (i.e. electronic control), pneumatic pulse controllers (i.e. air control) and mechanical pulse controllers (i.e. machine control). From the perspective of usage, most of them are non-contact electric pulse controllers.
The above three controllers all use timing control to clean the dust, that is, the injection cycle is fixed and the dust is blown and cleaned regularly. Although this method is superior to the manual dust cleaning method, due to the actual operation of the dust concentration at the dust collector inlet, filter wind speed, injection pressure and other factors will fluctuate with time, so when timing control is used, the dust concentration of the dust collector will The actual resistance is often different from the designed resistance (i.e. the predetermined resistance).
When the actual resistance is higher than the design resistance, the air volume of the dust removal system will be reduced, which will not only affect the dust removal effect, but also affect the dust collection effect of the dust collection hood; when the actual resistance is lower than the design resistance, the dust collector resistance will not reach the design If the resistance is high, the dust cleaning will be carried out prematurely. Too many times of dust cleaning will not only increase the consumption of compressed air, but also reduce the dust removal efficiency and affect the life of the filter bag and corrugated diaphragm.
In order to overcome this phenomenon, a fixed resistance control dust cleaning method is adopted, such as the AL-3 electric controller, which uses the design resistance of the dust collector as the working point of the controller to make the blowing cycle change with the control. Changes due to changes in dust collector resistance. The constant resistance control cleaning method can avoid the shortcomings of the timed control cleaning method, so this method is more reasonable.
The consumption of compressed air for dust cleaning in pulse injection bag dust collectors mainly depends on factors such as injection pressure, injection cycle, injection time, and the number of pulse valves. Therefore, the compressed air consumption The amount can be calculated according to the following formula:
(6-31)
Where n——number of pulse valves, pieces;
T——injection period , min;
a——Additional coefficient (including pipeline leakage loss), generally taken as 1.2;
q——The air consumption of each pulse valve for one injection, m3. When the injection pressure is (5-7)×105Pa and the injection time is 0.1~0.2s, the air consumption of each pulse valve for one injection is 0.01~0.034m3, which can be used to calculate the air consumption. 0.022m3.
In usual pulse bag dust collectors, in order to achieve the necessary dust cleaning effect, the injection pressure is required to reach (5~7)×105Pa. This not only consumes too much energy, but also generally affects the factory. The company's compressed air pipeline network often cannot reach such a high pressure, and configuring a specialized air compressor will increase equipment investment and maintenance workload. Therefore, research has been conducted on reducing the injection pressure, and the following two methods have been proposed.
(1) Use a straight-through pulse valve instead of a right-angle pulse valve (see Figure 6-37). It is compared with a right-angle (pressure inlet and outlet are at an angle of 90°) single-diaphragm or double-diaphragm pulse valves. , the resistance is greatly reduced, the injection pressure can be reduced by about 50kPa, and the filtration speed can be increased by about 10% at high pressure.
(2) Use a low-pressure injection system. The following measures are mainly taken to reduce the injection pressure: use a straight-through pulse valve; appropriately increase the diameter of the injection pipe; use a special nozzle to replace the injection hole. The test results show that under the same injection time, the compressed air injection volume when the injection pressure is 3×105Pa is the same as the injection volume of the pulse injection bag dust collector using a right-angle pulse valve at 6×105Pa. That is, the injection pressure can be reduced by 1/2. Due to the reduced injection pressure, the life of the diaphragm can be extended and the maintenance workload can be reduced.
In the late 1970s, my country introduced an annular-injection pulse bag dust collector from Germany, which uses an annular ejector tube for pulse-injection dust cleaning, as shown in Figure 6-38 , consisting of an upper body with connecting sleeves and annular channels, and a lower body that functions as a spray pipe. There is a narrow annular gap between the upper and lower bodies. Each ejector tube is connected to the compressed air distribution pipe by means of a quick-detachable plug-in pipe, and the filter bag and its frame are all embedded and hung on the annular ejector tube. This annular blowing structure is simple, convenient and reliable to install and maintain. Compared with the ordinary nozzle hole-Venturi tube pulse bag dust collector, the blowing dust cleaning effect is good and can increase the filtration wind speed by more than 66%. . But compressed air consumes about 25% more. In addition, the pulse valve adopts a double diaphragm structure, which improves reliability and anti-interference ability.
In addition, pulse bag dust collectors also have structural forms such as forward spray and counter spray, which are not listed here.
Rotary back-blowing flat bag dust collector
In addition to the wedge-shaped flat bag form shown in Figure 6-38, there are also rotary back-blowing flat bag dust collectors, such as As shown in Figure 6-39. The shell of this dust collector is cylindrical, and the flat bags are arranged in a radial shape inside the cylinder. According to the required filtration area, the filter bags can be arranged in 1, 2 or even 4 circles. The cross-section of the filter bag is trapezoidal, with the long side being 320mm, the two short sides being 40mm and 80mm respectively, and the bag length being 3 to 6m.
Dust-containing gas is introduced along the tangential direction of the cylinder, and the coarse dust is separated by centrifugal force, and then enters the filter bag for filtration (external filter type), and the clean gas is led out from the upper box.
The dust cleaning of the filter bag adopts the rotary arm back-blowing method. The back-blowing air volume accounts for about 15% of the filtered air volume. The back-blowing fan air pressure is about 5kPa. The rotary arm is driven by the motor and reducer installed on the top cover of the dust collector. This dust collector has the following features.
(1) The inlet of the dust collector is designed according to the cyclone dust collector, which can act as a local cyclone to reduce the dust load of the filter bag.
(2) The dust collector is equipped with a back-blowing fan, which is not limited by the compressed air source of the use site. It has few wearing parts and a large back-blowing distance. Long filter bags can be used to make full use of space and floor area. Small.
(3) The trapezoidal filter bags are arranged in the cylinder and have a compact structure. According to calculations, in the same simplified space, trapezoidal flat bags have 32% more filtration area than round bags.
(4) The upper cover of the dust collector is equipped with a rotating cover and a bag-changing manhole. There is no need to remove the cover when changing bags.
(5) The cylindrical shell is evenly stressed and can be used to purify explosive flue gas (such as electric arc furnace flue gas) to prevent deformation.
The main problem is that the backflush time of the inner and outer filter bags is different, the filter bags are easily damaged, and the resistance and load of each filter bag are different.
(7) Pre-coated bag dust collector
A dust collector that adds pre-coating (filtration aid) to the filter bag of the bag dust collector to capture pollutants It is called a pre-coated baghouse.
Bag dust collector is a kind of efficient dust collector, but traditional bag dust collector is difficult to deal with dust with strong adhesiveness and fixation, and cannot remove tar and oil components in dust-containing gas at the same time. , sulfuric acid mist and other pollutants, otherwise hard-shell-like agglomerates will appear on the filter bag, causing the filter bag to become clogged and making the bag dust collector ineffective. When it is used to deal with low-concentration dusty gases, the dust removal efficiency is not high. In 1962, a company in the United States successfully added a pre-coating (the filter aid was calcined dolomite) to glass fiber to capture condensed SO3 droplets (H2SO4) in the boiler flue gas. In 1973, Girard proposed to use it in the aluminum industry. There are reports on the use of pre-coated filter media to capture oil mist. This fully demonstrates that adding appropriate filter aids as pre-coatings to the filter bags of bag dust collectors can simultaneously remove solid, liquid, and gas three-phase pollutants in the gas, creating a new era for the application of bag dust collectors. way.
The dust removal system of the pre-coating bag dust collector is shown in Figure 6-40. It consists of a pre-dust collector, an automatic filter aid feeding device, and a pre-coating bag dust collector (the filter bag is Cylindrical open type, installed on the upper and lower ceilings), exhaust fan and muffler device. The pre-dust collector is equipped with a metal fiber filling layer to remove coarse dust and function as a flame arrester. When the initial dust concentration is low and there is no spark entering the pre-coating bag dust collector, the pre-dust collector does not need to be set up.
During filtration, the dusty gas with gaseous and liquid phase pollutants first enters the pre-dust collector to remove the coarse dust. The uncaptured dust (including gaseous and liquid phase pollutants) flows out with the air flow. When the top of the pre-coated bag dust collector enters the filter bag chamber and forms a cylindrical filter bag, the dust is trapped on the pre-coating on the inner surface of the filter bag, and the purified gas is discharged into the atmosphere through the fan. As dust accumulates on the filter bag, the dust attachment layer gradually thickens, and the dust collector resistance also increases accordingly. When the resistance reaches the specified value, the back-blowing mechanism and the vibrator (not shown in the figure) act simultaneously to back-blow and clean the filter bag, removing the dust adhesion layer and the filter layer of the filter inhibitor together. After cleaning, the filter aid automatic feeding device resumes the adding operation, and the adding time can be controlled by a timer. Since the dust collector is a multi-chamber structure, each chamber can perform adding operations and implement filtration and dust cleaning processes according to determined procedures.
The filter aid used in pre-coated bag dust collectors has not yet been finalized and is still in the development stage. Generally speaking, fine powders that have a large specific surface area, will not increase the filtration resistance too much after being applied to the filter bag, and can adsorb, absorb or neutralize gas and liquid phase pollutants are suitable as filter aids. Selecting the appropriate filter aid is the key to improving the collection efficiency of pre-coated baghouses. For example, when alumina powder with a specific surface area greater than 45m2/g is used to absorb the gas containing fluorine compounds generated from the aluminum electrolysis furnace in the reactor before the bag dust collector, the purification efficiency can reach more than 99%.
The pre-coating bag dust collector has the following characteristics.
(1) Due to the role of filter aids, pre-coated bag dust collectors can purify tar components, oil components, sulfuric acid mist, fluoride and dew points that traditional bag dust collectors cannot purify. The following dust-containing gases are also relatively easy to handle for dust with strong adhesiveness and fixation.
(2) Since the filter aid plays a role in protecting the surface of the filter material, the service life of the filter bag can be extended.
(3) It can be used as an air filter to purify low-concentration dusty air at the entrance of precision machine assembly workshops, electrical rooms, pharmaceutical factories, clean rooms, and large air compressors.
Although pre-coated bag dust collectors and filter aids have been confirmed to be effective in capturing certain gas and liquid phase pollutants, they are all obtained for specific pollutants and specific processes. Based on practical experience, whether it is applicable to other pollutants and processes remains to be further studied and discussed.