ZJN Bag Filter Series（5）-Reverse Gas Bag Filter

Note: Bag filter is a dry dust remover,suitable for collecting dry non-fabric fine dust. The filter-bag is made of woven filter cloth or non-woven felt, to filter the dust-bearing gas by filtration effect of the fiber fabrics. When dusty gas passes through the bag filter, larger and heavier particles deposit in the ash bucket while smaller particles are blocked when passing the filter material, thus the gas is purified.

The bag dust collector is mainly composed of upper chamber, middle chamber,lower chamber(ash bucket), dedusting system, ash discharging gear, etc. Compared with similar products, bag filter has 1) Higher efficiency,usually over 99%; 2)Wider range of air volume handling capacity, widely used in various industrial furnaces and drying equipment; 3) Simpler structure; 4) Lower production cost; 5) Easier operation and maintenance.

At present, the bag filter has become the main auxiliary product of ZJN Multiloop Three Cylinders Rotary Harrow Dryer and is the first choice of many customers with its remarkable efficiency and economic benefits.

This article describes the main cleaning method of bag filter --- reverse flow cleaning.

Main cleaning method of bag filter-- Reverse flow cleaning

Reverse flow cleaning is also called as reverse blowing flow or reverse-flow. In this way, compartmentalization working system is mainly used. With the automatic adjustment of the valve, the airflow in the direction of the filtered airflow is generated chamber by chamber. Reverse flow cleaning commonly uses internal filter type. The cylindrical filter bag is compressed into a star-shaped cross section by the action of the reverse air flow to cause reverse air speed and vibration thus the deposited powder layer being fallen off. Since it is an internal filter type, it is necessary to appropriately adjust the tension of the filter bag so that the deformation shrinkage is in a reasonable range. A metal ring is added at a certain distance in the longitudinal direction of the filter bag to control the deformation of the filter bag, so that the cleaning effect is evenly distributed on the entire bag. During cleaning, more than two times of reverse flow are carried out.

Woven cloth filter materials like 729 filter cloth is mostly used in reverse flow cleaning. Filter felt materials can also be used for dust that is relatively easy to clean.

For the negative pressure type, the generation of reverse airflow is to close the outlet side valve and open the reverse blow valve. The reverse airflow is formed from the atmosphere or the inhalation air from the fan discharge pipe. For the positive pressure type, it is to close the dust hopper inlet side valve, open the reverse blow valve. The reverse airflow is formed from the inhalation air from the inlet pipe to the fan. In order to increase the anti-blowing effect, a special type of small fan is also installed.

The effect of cleaning by the method of reverse blowing is relatively weak, and the damage to the filter cloth is smaller than that of the vibration cleaning method. So glass fiber filter cloth is mostly used in this way.

In production, when the dust cannot be fully shaken off by only reverse flow cleaning, the combined ash cleaning method with weak vibration is often adopted, which is called reverse flow vibration combined ash cleaning method. High temperature glass fiber filter bag is mostly adopted in this combined ash cleaning method.

Pulse Jet Ash Cleaning

A perforated plate (flower plate) for fixing filter bags is arranged at the upper part of the dust collector box body. There is an injection pipe above each row of filter bags, a compressed air injection hole (nozzle) is opened towards the center of each filter bag on the injection pipe, and the other end of the injection pipe is connected with the pulse control system (consists of a pulse valve, a control valve, etc) and compress air storage tank. And then pulse injection and ash removal are carried out according to a specified time or resistance value and on basis of an automatic control program.

The filter bag adopts an external filtration type, and the inner side is provided with a supporting framework, and dust is deposited on the inner surface of the filter bag. At the moment of ash removal, when the high-speed jet air flows through the top of the filter bag, it can induce air several times of the jet air volume to inject into the filter bag together to form a air waves, which causes the filter bag to generate rapid expansion and impact vibration from top to bottom, and generates strong air flow to remove the dust. Generally, the pulse period can be adjusted from 1 minute to several minutes.

According to the flow direction of the pulse jet flow and the purified flow, there are three injection modes: forward jet type, reverse jet type and opposite jet type. The forward jet type means that the two airflow directions are the same, and the cleaned air is discharged from the bottom of the filter bag. The opposite type actually divides the filter bag into two parts: half are opposite-jet and the other are forward-jet.

During injection, the filter bags to be cleaned do not have dust catching effect. Because the injection time is very short and the filter bags clean dust for the first time row by row, the dust catching operation can almost be regarded as continuous processing, the dust cleaning can be off-line and carried out by adopting a chamber structure, or be on-line and carried out without chamber structure.

The pulse ash cleaning has strong and good effects of ash removal, which can improve the filtering air speed. Its intensity and frequency can be adjusted. The ash removal effect is related to the structure of the venturi atmospheric pressure and the consistency between the centerline of the jet flow and the centerline of the filter bag . When the filter bag is longer, good ash removal effect can also be obtained by using a better injection device.

Due to its strong ash removal effect, it is also effective for viscous felt filter materials. The use of felt filter materials has also begun to spread. Felt filter material can be used for effective filtration as a whole from a microscopic point of view, so its apparent filtration speed can be higher than that of woven cloth, thus making the device miniaturized.

The pulse injection device needs compressed air as energy source. In the cold zone, since the water vapor in the compressed air is easy to condense and affect the injection effect, it is not suitable to be placed outdoors, and corresponding technical measures should be taken to prevent condensation or freezing.

Air Ring Reverse Flow Cleaning

This kind of cleaning method is to place a hollow slotted ring close to the surface of the filter bag on the outer side of the inner filter type round filter bag. The ring can move up and down to connect the compressed air and high-pressure fan pipes. The high-speed air flow from the slit nozzle on the ring clears the dust layer deposited on the inside of the filter bag.

The working principle of the air ring back-blowing is to fix a group of adjacent gas rings on a frame, and driven by a chain to move up and down along the guide rail. The structure is relatively complicated and the filter bag is easily damaged. Due to the application of the pulse jet cleaning method, it has been rarely used except for special purposes.

Gas Box Pulse Jet Cleaning

It is also called forced pulse mode, which is characterized in that the filter bag is divided into several chambers, and the clean air box above the filter bag is separated by a partition to form a compartment, and the upper end of the filter bag is not provided with a venturi tube. The cleaning is carried out in a sequence-by-chamber manner. The exhaust valve is closed, and the pulsed airflow is sprayed from the upper part of the lateral sub-chamber, and enters each bag through the sub-chamber, and is cleaned by the impact and expansion.

Pulse Reverse Flow Cleaning

It has a strong cleaning effect, but it should have a mechanical structure that can produce pulsation action. Due to the strong effect , there is no need to compartment the bag house if some filter bags are to be cleaned successively.