The medium frequency furnaces are each equipped with a fume collection device. A rotating top suction hood is installed at the furnace mouth, which can rotate freely. It opens during charging and tapping and closes during melting. Electric butterfly valves are installed on the main pipes of the two furnaces. One of the two furnaces is used for melting, and the other for holding.
1.2 The selection of pulse valves, the blowing distance of the injection pipes, and the injection pressure are precisely calculated.
1.3High-quality electromagnetic pulse valves are selected to ensure the effectiveness of pulse reverse cleaning for dust removal.
1.4High-temperature resistant, waterproof, and oil-resistant needle-punched felt material is selected. This material features high-temperature resistance, good air permeability, small filtration particle size, and easy dust cleaning performance.
1.5The equipment has a compact structure and a small footprint.
1.6All steel structural components of the equipment are cleaned by shot blasting or sandblasting, which greatly improves the appearance quality and extends the service life of the equipment.
1.7The ash discharge mechanism uses a hopper and a rotary discharge valve. During ash discharge, the discharge valve ensures the airtightness of the entire system, maintaining a seal so that the equipment does not leak air, prevents dust re-entrainment, and keeps the site clean.
1.8The equipment is easy to maintain, avoiding the inconvenience of removing filter bags from the top as in old-style dust collectors. Filter bags can be directly removed by opening the maintenance door on the side of the dust collector, making maintenance very simple.
II. Functional Overview
The development of modern industry has an increasingly significant impact on the atmospheric environment, especially in heavy industry, metallurgy, smelting, infrastructure such as cement kilns, power plants, etc. The pollution to the environment has reached an unbearable level.
Currently, China places great emphasis on environmental protection, stressing that while the economy continues to develop, it is necessary to protect the environment scientifically, utilize resources rationally, and strive to prevent pollution and hazards from the "three wastes" (waste gas, wastewater, and solid waste). To adapt to the growing demands of environmental protection, we should pay attention to protecting our environment while developing industry.
Using high-efficiency pulse baghouse dust collectors to treat dust-laden flue gas is currently the most effective method for dealing with various types of air pollution. The dust collector filters out various harmful dust-laden flue gases, so that the clean air discharged into the atmosphere is harmless to the environment and residents' lives. This not only reduces pollution but also saves enterprises significant costs related to emissions.
In the dust collector, high-pressure compressed air is used for pulse cleaning of the filter bags, with the cleaning time and interval controlled by a pulse controller. The working principle of the cleaning system is as follows: The pulse controller installed on the dust collector performs timed cleaning. That is, the pulse length and interval of the pulse valves are set to carry out pulse cleaning of the filter bags. The action of the electromagnetic pulse valves is controlled by the pulse controller, and the sequence of valve operation is from left to right, row by row.
The dust collector adopts an upward air inlet, downward air outlet, and external filtration structure. This aligns the direction of dust settling with the airflow direction, which is conducive to dust settling and improves dust collection efficiency.
In this solution, based on the provided working status, flue gas volume, and flue gas temperature of the induction furnace, our company's latestFD-type pulse bag filter is used for dust removal from two sets of medium-frequency furnaces.
This type of dust collector features a small footprint, high cleaning efficiency, and easy maintenance. (The structural type of the dust collector is shown in the figure below.) The filter bags are rectangular and arranged in a matrix pattern inside the steel structure housing of the dust collector. Dust-laden gas enters from the top of the dust collector, passes through the bag filtration, and clean gas is discharged from the bottom. The filter bags can be easily pulled out from the side on the maintenance platform for inspection or replacement, avoiding the inconvenience of maintaining old-style dust collectors from the top.
The working principle of the dust collector is downward flow and external filtration. After the dust-laden gas is filtered, dust particles are trapped on the outer surface of the filter bags, while clean gas is discharged into the atmosphere from inside the bags under the action of the fan. The filtration efficiency of this dust collector can reach99.9%, and the emission concentration of the dust collector is <50mg/m3.
III. Equipment Composition and Structural Features
3.1 Flue Gas Collection Hood
The function of the flue gas collection hood is to collect the escaping smoke and dust for transportation through pipes to the dust removal equipment for purification. This solution rationally arranges the position and designs the structural shape of the collection hood based on the actual location and structure of the flue gas generation, ensuring that the hood can completely collect the dust-laden flue gas and transport it to the next dust removal process via pipes. The hood opening velocity is designed according to actual conditions. Two sets of3-ton medium-frequency furnaces share one flue gas collection system, with each furnace set having one in operation and one as backup. Each dust removal pipe is equipped with an automatic switching butterfly valve to switch furnace usage. The flue gas collection hood is made into two parts: a rotating top suction hood, based on the diameter and shape of the furnace body. When the furnace is charging or pouring, the rotating hood moves to the other side; during normal operation, the hood rotates to cover the top of the furnace.
3.2 Pulse Reverse-Jet Bag Filter
The pulse bag filter is currently the most common and efficient bag filter internationally. Its feature is that a blowpipe is installed at the outlet of each pulse valve, responsible for efficient pulse cleaning of the filter bags located beneath the blow holes. TheFD series pulse bag filter produced by our company mainly consists of the following parts:
3.2.1 Dust Collector Housing
The fabrication of the dust collector housing strictly ensures no air leakage at any connection point. If the housing leaks, dust will escape, rendering the dust collector ineffective. Therefore, all connections are sealed with sealing materials during installation. The thickness of the steel plate used for the housing fully meets strength and rigidity requirements, ensuring no deformation under working pressure.The dust collector housing is entirely made of high-quality steel plate and is treated with shot blasting or sandblasting to improve the surface strength and smoothness of the steel plate..
3.2.2 Cleaning System
The quality of the cleaning system directly affects the overall operation of the dust removal system. Selecting the appropriate cleaning pressure and pulse valve is key to a cleaning system. The efficient pulse cleaning system of this dust collector mainly includes:
3.2.2.1 Compressed Air Filtration System
A compressed air purification system is installed on the compressed air supply pipe of the dust collector to remove oil and water from the compressed air, preventing damage to the pulse valves. Additionally, a standard oil-water filter triple unit is installed at the air inlet of the air reservoir, with a secondary filter mesh aperture ofless than 5 microns. The triple unit should preferably be equipped with a pressure gauge; otherwise, a separate pressure gauge must be installed on the air reservoir.
3.2.2.2 Compressed Air Reservoir
A. 
When designing the air reservoir, considering safety and quality requirements, our company follows the JB/T10191 "Safety Requirements for Baghouse Dust Collectors - Pulse Baghouse Dust Collector Air Manifold" standard. This ensures the reservoir can operate normally under working pressure.
B. The air reservoir is designed with sufficient capacity to meet the injection air volume. After pulse injection, the pressure drop inside the reservoir does not exceed 30% of the original storage pressure. The installation method of the right-angle valve is shown in the figure.
C. The air inlet pipe of the reservoir is selected with a large diameter to ensure adequate air replenishment speed.
D. The valve is installed on the side of the reservoir to prevent oil and moisture inside the reservoir from being injected into the filter bags through the pulse valve.
E. Each reservoir bottom is equipped with a manual oil-water drain valve, which periodically discharges sediment and impurities from the container.
F. After the reservoir is manufactured, it is continuously blown with compressed air to clean internal welding slag before installing the valve.
3.2.2.3 Injection Pipe
A. The length of the injection pipe is determined based on the number of filter bags. Ensure that the injection pipe can align with each filter bag for pulse cleaning.
B. The wall thickness of the injection pipe is determined by its length and material (hardness) to ensure no bending deformation occurs due to its own weight.
C. Each injection pipe has corresponding injection holes for each filter bag, with each hole responsible for backwashing one filter bag, improving the overall cleaning efficiency of the system.
D. To ensure the difference in pulse airflow entering the first and last filter bags is within ±10%.
E. Based on factors such as reservoir pressure, pulse valve resistance, injection pipe dimensions, and number of injection holes, combined with filter bag diameter and design experience and experimental data, we determine the optimal distance between the injection pipe and the tube sheet to ensure the injection airflow can cover the entire length of the filter bag.
The pulse jet dust collector primarily uses compressed air from a pressure air tank as the cleaning energy source, forming a pulse airflow that vibrates in reverse from the filter bag opening to the bottom when the pulse valve is activated. The purpose is to shake the dust cake attached to the outer surface of the filter bag into the dust collector hopper through pulse vibration. If the pressure or flow rate is insufficient, the airflow is too weak, and the cleaning force cannot reach the bottom of the filter bag, preventing the dust cake from peeling off. This leads to localized dust accumulation, increased equipment resistance, uneven filter material load, and shortened filter bag life. Conversely, if the cleaning force is too strong, fine particles that have penetrated the surface layer of the filter material may be expelled, causing a "secondary dusting" phenomenon. Additionally, excessive vibration force may cause the filter bag to rub excessively against the cage, leading to bag rupture. Therefore, regardless of whether high, medium, or low-pressure compressed air sources are used, the cleaning force and flow rate of our equipment are rationally configured based on the process, flue gas, and filter material properties.
3.2.2.4 Filter Bags and Cages
In the entire dust collector, the main component for filtering dust is the filter bag. The selection of the filter bag must be determined based on the properties of the dust to be treated, such as temperature, moisture content, viscosity, and particle size.
In this solution, since the induction furnace generates high-temperature and corrosive gases containing dust when melting raw materials such as scrap steel, to prevent damage to the dust collector filter bags, equipment failure, or combustion and explosion, needle felt with high-temperature resistance is adopted to ensure the normal operation of the entire dust collector.
Our company prioritizes the use of high-quality needle felt filter materials produced by manufacturers such as Shanghai Borg, Dagong, or Wuxi BWF (Germany) to ensure the dust collector's performance.
The filter bag cages are all made of high-quality cold-drawn steel wire spot-welded and fully galvanized, ensuring not only the rigidity of the entire cage but also a smooth surface without burrs or other welding defects that could tear the filter bags.
3.3 Dust Discharge System
To ensure the dust collector's sealing performance during operation and prevent dust from escaping through gaps, the dust discharge system of this dust collector consists of a hopper and a rotary discharge valve. After dust is filtered by the filter bags, large particles fall directly into the bottom hopper, while small particles adhere to the outer surface of the filter bags. After the pulse reverse cleaning system operates, the dust attached to the filter bag surface is blown into the bottom hopper, completing the cleaning process. The dust in the bottom hopper is collected and discharged through the rotary discharge valve. During operation, the rotary discharge valve not only allows continuous dust discharge but also maintains the airtightness of the entire dust collector. After treatment by the discharge valve, the dust not only avoids dusting phenomena but also facilitates bagging, preventing secondary pollution.
3.4 Clean Air Discharge System
The gas filtered by the dust collector is clean and meets national emission standards. The clean gas is discharged into the atmosphere through a centrifugal induced draft fan and chimney. Since high-speed airflow generates noise during operation, an F-type airflow silencer is installed on the dust collector chimney to reduce airflow noise, keeping the entire flue gas discharge system noise below 90 decibels. The fan is selected based on the required airflow and pressure loss of the entire system, ensuring the dust collection system operates under negative pressure. The opening and closing of the fan valve can be controlled by a manual butterfly valve, enabling control during startup and shutdown.
3.5 Dust-Laden Gas Duct
The dust-laden gas duct connects the fume collection hood and the dust collector. Based on the material properties of the dust (such as material suspension velocity) and dust concentration, the filtration velocity is reasonably selected, and the diameter of the dust-laden gas duct is designed accordingly.
3.6 Electrical System
The operation of the electrical system can be monitored through control elements set at various control points. After collecting feedback signals, the information is processed via automatically set programs. All electrical components are from joint venture or domestic high-quality brands to ensure normal operation of the electrical system. The control system can achieve remote control.
IV. Working Principle
Since the two medium-frequency furnaces are used alternately, with one working and the other holding, each furnace is equipped with a fume collection hood and an automatic switching butterfly valve to switch and absorb furnace fumes. The fume collection hood is designed as a rotating top suction hood in two parts based on the diameter and shape of the furnace body. When the electric furnace is charging or pouring, the rotating hood moves to the other side, and the annular hood is made to rotate integrally with the furnace. During normal operation, the rotating cover hood rotates to the furnace annular hood for fume collection.
The working principle of the pulse jet dust collector is to draw high-concentration gas through the rotating fume collection hood and dust collection duct at the dust collection point into the dust collector housing. After filtration through filter bags, most dust particles adhere to the filter bags, while clean gas is discharged into the atmosphere via the induced draft fan. A temperature automatic monitoring device is installed at the gas inlet of the dust collector and can send signals. A fresh air valve is installed on the dust-laden gas conveying duct. When the fume temperature exceeds the temperature tolerance of the dust collector filter bags, the temperature monitoring device sends a signal, and the fresh air valve automatically activates to supplement cold air, mixing with the hot air in the duct to lower the temperature of the dust-laden gas, ensuring that the gas temperature entering the dust collector remains below the tolerance of the filter bags, preventing the dust collector from burning.
The FD series dust collector is a pulse cleaning, external filtration type dust collector that uses online cleaning filtration to improve working efficiency. It uses flat filter bags, making the entire system smaller in volume and footprint compared to round bag dust collectors. It is also easy to maintain, allowing convenient side access for filter bag maintenance.
V. Technical Parameters
Technical Parameters of 2*2-3 Ton Medium-Frequency Furnace Dust Removal and Purification System
1 Dust removal air volume 40000m3/h
2 Dust collector model FD621
3 Filtration area 432m2
4 Filtration velocity 1.54m/min
5 Number of filter bags 288 pieces
6 Filter bag material Fumex high-temperature resistant needle felt
7 Fan Model 4-72-10D
8 Fan Power 55KW
9 Air Volume 40441～56605m3/h
10 Air Pressure 3202～2532Pa
11 Pulse Valve Working Pressure 0.5～0.6MPa
12 Emission Concentration <50mg/m3