One of the most effective ways to clean industrial gas emissions from dust is to filter them through a porous partition, in kind of which is the most often used a fabrics with different characteristics.
Filters which uses fabric as a flexible porous partition got the name “bag” due to the currently most common form of the frame in the form of a sleeve cylinder on which the fabric is stretched.
The main differences of fabric filters is in the method of regeneration, the design of filter elements (sleeves or cassettes), the number of sections in the filter, the modes of filtration and regeneration.
Bag filters are widely used in ferrous and nonferrous metallurgy, the chemical and petroleum industries, in the production of building materials and other industries.
The efficiency of dust collection in a bag filter depends on the dispersion of dust particles, the characteristics of the filter material, the method and mode of regeneration, the value of the specific gas and dust load, and the hydraulic resistance of the filter.
The physical basis of dusty gases filtration process.
The process of separation of solid particles from the gas during its filtering through a fabric septum is caused by a combination of the following mechanisms:
· inertial collision;
· gravitational sedimentation;
· Brownian motion;
· diffusion;
· electric forces.
Such factors as the roughness and hardness of the fabric’s fibers may also play a role.
If the size of dust particles exceeds the size of the pores of the filter during filtering gases, then the dust particles are simply screened out with this material. If the size of the dust particles is smaller than the pore size of the fabric, then during their movement in the pores they have to bend around all the fibers. At the same time they are pressed against the fibers and deposited on them.
Stepwise on the surface of a layer of the filtering material a dust layer with pores , the size of which does not exceed the size of dust particles is formed on. Due to this layer, dust particles are separated from the gas to be purified. Therefore, maintaining a dust layer of optimum thickness on the filter material is a necessary guarantee for highly efficient cleaning of industrial gases in this way.
As the filtering material becomes dusty and a considerable dusty layer accumulates on its surface, the hydraulic resistance of the filter increases and regeneration is necessary at a certain period of time. The regeneration process consists of removing accumulated dust from the surface of the filter material and from the inside of the pores. However, excessive removal of a layer of dust can lead to a slight decrease in the efficiency of dust collection, therefore the regeneration process must be balanced so that the decrease of hydraulic resistance of the filter does not impair its filtering ability.
Regeneration methods
There are two main ways of regeneration of bag filters, having the most different designs:
• mechanical shaking, in which dust is removed only from the surface of the filter material;
• backwash, in which the dust is removed from the surface and from the pores of the filter material.
These regeneration methods are used either alone or in combination.
The advantage of mechanical shaking of dust is the stability of this process. The main disadvantages include:
· the complexity of the shaking mechanism, requiring increased attention of staff;
· abrasion and breaks of sleeves in the same places;
· sensitivity to setting and pulling-out of hoses;
· the need to turn off the section at the time of regeneration.
The advantage of regeneration by the backwash is the smooth deformation of filtering material, and the disadvantage is a decrease in productivity due to air leaks during the regeneration of the filtering material.
A type of regeneration by reverse blowing is pulsed regeneration with compressed air through high-speed blow-off valves, the most widely used in bag filters of modern design. Pulsed blow-off filters are distinguished by the fact that there are no shaking mechanisms, chokes, and blower fans in their design.
Unloading of the collected dust from the bunkers can be done through flashers, sluice gates or through the system of pneumatic transport which can be manufactured and delivered according to the customer wish. It is also possible to install a pneumatic or vibro- crushing system of dust accumulated in the bunker to prevent its freezing.
Filtering materials
The following requirements are applied for fabrics used as filter materials:
- high dust holding capacity during filtration and the ability to retain after regeneration the amount of dust sufficient for high efficiency of gas purification from fine particles;
- preservation of optimally high air permeability in an equilibrium dusty state;
- high mechanical strength and resistance to abrasion with repeated bends, dimensional stability and properties at elevated temperatures and the aggressive effects of chemical impurities in dry or moisture-saturated gases;
- ability to easily remove accumulated dust;
- low cost.
Nowadays, nonwoven materials, needle-punched felts based on synthetic fibers, which, unlike woven materials, have a homogeneous fibrous, fine-porous structure throughout the entire volume of the material, are becoming more and more common, which makes it possible to realize much more effectively the mechanisms of particle separation.
The materials that used do not meet all of the above requirements, so each material is used in the most favorable conditions for it. Some of listed requirements depend on characteristics of the fibers used, while others depend on the structure of the finished fabrics.
Automation of the dust collection process in bag filters
Bag filters are equipped with a control system and operate in automatic mode.
Instrumentation and controls system provides measurement of all the necessary technological parameters and automatic control of such systems as regeneration, unloading of dust, reduction of the temperature of the cleaned gases at the inlet, etc.
To perform the algorithm of the regeneration system, various electronic devices are used. Depending on the wishes of the customer, it may be SIMATIC controllers from SIEMENS, logic modules Zelio Logic, specialized automatic control unit for regeneration of BAUR type programmed in-house, etc. The BAUR block is made on a modern element base using programmable microcontrollers and meets all the necessary requirements for setting up a cycle regeneration. The absence of mechanical controls significantly increases the operational reliability of the device. Setting up and operating the device does not require additional knowledge of electronics and programming.