For high-speed machines such as air compressors, blowers and steam turbines, dust particles and dust are one of the main reasons that affect the long-term operation of equipment. Dust particles (≥4μm) in the high-speed air stream cause wear on the surface of the main parts of the machine. Dust fouling inside the equipment will affect the airflow movement, destroying the dynamic balance of the impeller, causing the shaft vibration of the rotor to exacerbate the standard, causing the surge curve to drift and even being shut down for maintenance. Corrosive dust can also cause chemical corrosion of the equipment. Therefore, various types of air filters are generally provided in front of the intake pipes of these devices.
1. Classification of air filters
Common air filters can be classified into dry and wet according to the filter dust removal mechanism. The wet filter relies on the oil film to adhere to the dust particles and dust in the atmosphere to achieve the purpose of dust removal, but the filtration precision is low and the gas has oil content; there are chain belt type and Laxi ring filter. The dry filter is when the air passes through the surface or inner layer of the fabric of various animals, plants or chemical fibers, and the dust particles and dust in the air are retained on the outside of the filter due to gravity, inertial diffusion, contact retention and electrostatic adsorption. (Dust film is built inside) to achieve the purpose of dust removal; there are dry belt, cloth bag, frame plate and barrel (filter paper) filters.
2, self-cleaning air filter introduction
The self-cleaning air filter commonly used in air separation equipment is a type of dry filter. After filtration, the particle size is ≤ 2μm, the efficiency is >99 9%, the filter elements are mostly cylindrical, and the internal filter material is synthetic filter paper. The self-cleaning backflushing system is equipped with self-cleaning nozzles, back-blowing tubes, venturi tubes, air reservoirs and filter cartridges. The control system includes accessories such as a programming controller (including PLC), a diaphragm solenoid valve, a differential pressure transmitter, a differential pressure sensor, and a liquid crystal display. Self-cleaning backflushing is a self-cleaning back-blow mode command issued by the computer programming controller according to the preset pressure difference of more than about 300 Pa. There is a time self-cleaning mode (can be set by itself) or a constant pressure difference self-cleaning mode. (can be set by yourself). When the current pressure difference exceeds the alarm value, it will automatically alarm, and the computer will control the pulse width of the backflushing, continuous automatic backflushing at intervals, or enter the manual mode for self-cleaning backflush until the pressure difference remains within the specified range.
The normal resistance range of the self-cleaning air filter is generally set between 400 and 700 Pa, the alarm resistance is generally set at 800 to 1000 Pa, and the termination resistance is set at 1200 to 1500 Pa (can be set according to the air compressor and production requirements) . Setting the resistance value too low will consume more backflushing self-cleaning gas, making the diaphragm solenoid valve operate frequently, shortening the service life of the valve and shortening the service life of the filter cartridge, greatly increasing the running cost; and destroying the dust of the filter layer Membrane, gas filtration accuracy and efficiency will decrease.
3. Causes and solutions for excessive resistance of self-cleaning air filters under special weather conditions
　　 In the case of continuous rain, heavy fog or frequent dust storms, self-cleaning air filters often have a resistance of more than 1200 Pa, or even severely exceeded the standard. The prolonged occurrence of such conditions will endanger the safety of the equipment and deviate from the normal operating conditions of the air compressor, reduce the amount of suction air, increase the operating cost, and severely cause the air compressor to surge. The following is an analysis of the causes and countermeasures for the high resistance of self-cleaning air filters under special weather conditions.
3.1, continuous rain or continuous foggy weather
The filter material of the self-cleaning air filter is mostly made of wood pulp fiber paper filter paper. The filter paper is made of multi-layer thin paper. The average micropores of each layer of paper are about 30-70μm, and the micro-holes are staggered. Arranged with a certain degree of air permeability. The air permeable filter paper has a low resistance value at a very low filtration speed (the initial resistance is generally below 100 Pa). In continuous rainy and foggy weather, the relative humidity of the air is very large (96% to 100%), and the filter paper expands after the fiber is damp, causing some micropores to shrink or block. If the filter paper has been used for a period of time, there is dust on the outer surface of the filter paper and the paper core layer. After damp, it will also expand and block the micropores, so that the air permeability of the filter paper is greatly reduced, and the dust is cleaned at a low back pressure. The effect is also greatly reduced. In order to solve the problem of high self-cleaning air filter resistance under such weather conditions, the following countermeasures can be adopted:
(1) In the southern coastal areas of China or in foggy areas (such as Chongqing), the relative humidity is relatively large (>80%), and the amount of actual processing gas is generally increased in the selection of filters (it should be increased by 1 to 15). Double). For filter cartridge filter paper, use 20% or more waterproof filter paper with polyester, such as FA-6279 or other waterproof imported filter paper with high tensile strength and high stiffness.
(2) The air blowback equipment generally provides a low backflush gas source pressure (0 47 MPa), especially when the relative humidity is high, the effect of backflushing self-cleaning should be increased to increase the backflushing pressure. The ideal backflushing pressure can be increased to 0 75 ~ 0 8MPa, and backflushing with dry nitrogen or air, the self-cleaning effect is more than doubled when the pressure is lower.
(3) Increasing the diameter and capacity of the air reservoir and the back-blowing tube to stabilize the back-blow pressure during back-blowing and reduce the pressure fluctuation. The air reservoir is increased to 159mm × 6mm or more (a 3 to 5m3 buffer tank can also be added), the diameter of the backflush pipe and the diaphragm valve is increased to 15in, and the number of backflushing cartridges per valve should be 5 Below, to increase the strength and effect of backflushing.
(4) Adjust the programming controller to increase the pulse width from 0 1 to 0 15s to 0 25 to 0 4s; shorten the pulse gap (controlled at 10 to 15 s) to shorten the cycle of the backflush cycle.
(5) Replace some filter paper with severe moisture (the removed filter cartridge can be used again after drying).
(6) The air intake mode can be changed, that is, from the open air in the surrounding air to the air in the compressor building (as shown in Figure 2). In continuous foggy weather, the self-cleaning air filter is surrounded by colored strips, non-woven fabrics (also available for catching fog mesh, but at a higher cost), and an additional suction channel is built from the factory. Gas (flow rate controlled at 5 ~ 7m / s). Due to the high temperature inside the plant, the air does not contain tiny droplets and is relatively dry. If the above measures are taken, the effect will be more ideal. For small self-cleaning air filters, it is also considered to be indoors. This method is applied in areas with relatively high humidity and continuous foggy weather, and the effect of reducing the filtration resistance is obvious. Such methods as the air separation plant of Handan Iron and Steel Power Plant and Jinan Bald Gas Company have greatly reduced the running resistance, ensured the normal operation of the air compressor, and achieved good use and economic benefits.
3.2. Dust storm weather The climate in Northwest China, Inner Mongolia, Gansu and Henan is dry and the relative humidity is low. Due to the destruction of the ecological environment, the dusty weather in the area is increasing day by day, and the harsh sandstorms often occur in spring and autumn. The dust in the air is large and lasts for a long time. The high resistance of the self-cleaning air filter often occurs. In order to reduce the running resistance and extend the life cycle of the filter cartridge, Hangyang proposed the following countermeasures for the self-cleaning air filter of Datang International KOD56 air compressor.
(1) The air intake of the air compressor is 361000m3/h, and the processing capacity of the conventional optional filter is about 550,000m3/h. Taking into account the local sandstorm weather, the dust in the air is large, and the margin is further increased in the selection and matching, with 756 DH32100 filter cartridges, the actual treatment volume is ≥860000m3/h (87 5kPa, 25°C). The actual filter area of ​​the filter cartridge reaches 22,680 m2, which increases the dust holding capacity of the filter cartridge and reduces the actual filtration speed. Use imported filter paper to increase the strength and air permeability of the filter paper.
(2) Using a high self-cleaning backflushing pressure (0 8 MPa) and dry nitrogen as a backflush gas source to enhance the strength and effect of backflushing and self-cleaning. Use heavy-duty air filter paper, such as FA-7180A, FA-7406, and other imported filter paper with high strength and high stiffness.
(3) Increase the diameter and capacity of the gas cylinder, the diameter of the gas cylinder is expanded to 159mm×6mm, the back-blowing tube is made of 15in galvanized steel pipe, the inlet diaphragm solenoid valve of 15in and the air source of the back-blowing source are separately led to stabilize The pressure of self-cleaning.
(4) Add a PSB/275SP300 type (or PSB/290SP400 type) non-woven chemical fiber filter material to the inside of the filter filter (see Table 1 for some non-woven fabric technical parameters), which can increase by more than 120kg when dust storms come. The amount of dust. Thanks to this filtration, the load of the filter cartridge backflushing is reduced, the thickness of the filter dust film is reduced, and the life cycle of the filter cartridge is increased. When the sandstorm is eliminated, remove the non-woven fabric and clean it for later use. This measure can stabilize the resistance and save the operating cost of the filter.
(5) In the system back-flushing self-cleaning control, 3 solenoid valves can be connected in parallel with each contact to ensure that the filter (three-layer structure) has 1 row (5 per row) filter cartridges at the same time.
(6) Adjustable programming controller to reduce backflush interval in sandstorm weather, control backflush interval of each row of filter cartridges in 10~15s1 times, all filter cartridges can be self-cleaning and backflushing once in 9~13min to make surface dust Get cleared immediately.
(7) In the case of continuous sandstorm weather, the non-woven fabric that has reached the dust holding capacity can be removed, and new non-woven fabrics can be installed (or some new filter cartridges can be replaced) to meet the requirements for reducing the resistance.
The self-cleaning air filter used above to reduce the resistance is suitable for use in special harsh weather conditions, and some methods and measures have achieved significant practical results, and some are still under further exploration.
4 Conclusion
The high-efficiency self-cleaning air filter has high filtration precision, automatic blowback cleaning, easy management, and can be used to replace the filter cartridge without stopping. It has been widely used in steel, petrochemical, air separation equipment, fertilizer and medicine industries. For areas with frequent special weather such as continuous rain, continuous fog and dust storms, according to different weather characteristics, sufficient self-cleaning air filter should be added to handle the rich air volume, and the appropriate filter cartridge should be selected. Improve and stabilize the pressure of self-cleaning and self-cleaning, and adopt reasonable control and adjustment measures to avoid the situation that the self-cleaning air filter resistance increases too fast or continuously high, and prolong the life cycle of the filter cartridge and reduce the running cost. Ensure the normal operation of the air compressor.
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