What Is a Bug Filter?
A bug filter is a type of dust collection device that uses a woven or non-woven filter cloth to collect very fine particles and dust suspended in gases and purify the gas or gases to be treated.
It is called a bug filter because the filter cloth is made into a cylindrical shape and its bag is suspended at the point where the treated gas flows in.
Although electrostatic precipitators that use electricity are also available, bug filters have the advantage of being inexpensive and easy to install compared to such precipitators.
Uses of Bug Filters
Bug filters are used at manufacturing and processing sites where soot and smoke containing large amounts of particulate matter and dust that are harmful to the human body and the environment are generated.
Typical examples are the large incinerators installed in steel plants to process waste materials.
Air purification using bug filters is also very important to keep the air clean in confined spaces or sites where a lot of dust is generated, and to control dust explosions and other hazards, thereby ensuring safe operations.
Bug Filter Material
The filter cloth used in bug filters is usually made of materials ranging from cotton to high polymer synthetic resins such as polyester, nylon, polypropylene, acrylic, Teflon, and glass fiber.
Ceramic ones are also used, depending on the temperature of the gas to be processed.
The material of the bug filter is determined based on the operating temperature, the nature of the gas, the properties of the particles, durability, and price.
For example, polyester is inexpensive but susceptible to high temperatures and alkalis. Polyimide is resistant to solvents, but weak against acids and alkalis; PTFE has good heat resistance and chemical resistance, but is very expensive. Glass fiber has excellent heat resistance and chemical resistance, but is expensive and has poor durability.
While bug filters are simple to install, the selection of filter cloth has a direct impact on running costs, because filter cloth replacement is a prerequisite. On the other hand, if the size is the same, it is easy to change the material, and the material can be changed according to the site conditions.
Principle of Bug Filters
Bug filters clean gas by trapping fine particles and dust in the gas as it passes through the filter cloth.
The bag filter has an extremely high particulate removal rate of approximately 99% and is capable of collecting even very small particles of 0.01 micrometer or less.
Although the collection capacity is very high, after a certain period of use, the pressure drop increases due to the accumulation of fine particles on the surface of the filter cloth, and the collection capacity of the bug filter declines.
Therefore, when the pressure loss reaches a set value, the accumulated particles must be swept off the filter cloth.
There are two main methods for this: the mechanical method and the pulse-jet method, in which compressed air is fed to the filter and the particles are swept off.
In addition, the method of injecting air from the opposite direction of the filtration direction is used to clean the glass fiber bug filters used in cement and steel smelting plants.
Bug Filter Pay-Off Method
Bug filters with large pressure losses need to be cleaned of accumulated particles, and mechanical vibration, reverse pressure, and pulse jets are available.
Mechanical vibration applies vibration to the bug filter, while counter-pressure removes accumulated particulate matter by blowing airflow in the opposite direction from that used for dust collection. The back-pressure method is used to clean glass-fiber bug filters used in cement and steel smelting plants.
However, mechanical vibration and back pressure require the airflow to be cut off when removing the particles, and dust collection must be interrupted. For this reason, the dust collector must be divided into multiple chambers to enable continuous operation.
In contrast, as shown in the figure below, the pulse-jet method, in which only a portion of the bugs are momentarily removed by applying a reverse jet, does not require interruption of dust collection, so continuous operation is possible without using a multi-chamber structure.
Bug Filter Trouble Cases
The basic structure of a bug filter is simpler than that of other dust collectors, allowing for space-saving installation and easy maintenance. Moreover, by appropriately selecting the material and size of the filter cloth, the fabric filter can demonstrate high dust collection performance in various environments. Because of these features, bug filters are widely used in various fields. However, this advantage is sometimes closely related to the cause of various problems.
The most common trouble is the dropping or breaking of the filter cloth. This is caused by localized concentration of gas containing particles and changes in flow velocity due to load fluctuations that cause the filter cloth to sway and contact adjacent filter cloths or casings. This can be solved by adding a rectifying plate, such as perforated metal, to regulate the gas flow. The number of filter cloths installed may be reduced and the density of the filter cloths may be lowered in areas where contact is possible, but only as a stopgap measure, since this will reduce dust collection efficiency.
The most serious problems are ignition and explosion. In general, most incinerator dust collection systems use bug filters. In incinerator bug filters, the target of dust collection is fine carbon, and because the system is often under negative pressure, outside air may be drawn in through gaps in the casing, creating the conditions for ignition. In addition, if the fine powder stays in the system due to clogging of the filter cloth, etc., the risk of dust explosion increases.
Although simple and easy to install, this equipment is also very sensitive. In order to prevent these problems, it is important to select an appropriate bug filter and perform regular maintenance.