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Reverse Jet / Reverse Pulse Dust Collectors

Reverse Pulse Jet Dust Collectors Information The term reverse pulse or jet dust collector refers to the air cleaner's cleaning cycle.  To clean the filter, a quick burst of compressed air is shot through the filter media in the opposite direction of the dusty air stream.  During normal dust collection, dusty air flows through the filter, and a solid layer of dust forms on the filter surface.  This dust build-up is called a dust cake or filter cake.  A quick burst of air shot through the filter in the opposite direction knocks the dust cake off the filter, breaking it apart.  The dust cake crumbles and falls into a collection bin or barrel.

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Some dust collectors must stop in order to clean, while other can pulse clean while running.  Reverse pulse cleaning was first used in fabric bag filters.  Later the reverse jet dust collector cleaning technology was adapted to cartridge filters.

Variations of Continuous Pulse Jet Cartridge Collectors

Portable Reverse Jet Dust Collectors:

Portable cartridge jet dust collectors range in size of a vacuum cleaner to a refrigerator.  A moveable source capture arm can be positioned to directly suck up the dust before it can disperse into the air.  The beauty of a portable reverse jet dust collector is serious dust collection power you can carry or wheel around from workstation to workstation. 

Stationary Intermittent Duty Cartridge Collectors:

Using one or two cartridge filters in a stationary cabinet, can offer highly efficient dust filtration.  As the name implies, intermittent duty dust collectors are appropriate for workstations or tools that do not run continuously, yet produce a considerable amount of dust.  A stationary cartridge dust collector can be used for several workstations or dedicated to one workstation, depending on the application.  These dust collectors can use either flexible source capture arms or permanent ducting.

Modular Cartridge Dust Collectors:

Multiple rows of cartridge filters can be joined side by side for limitless filtration. These systems are custom engineered for each installation, providing heavy duty dust collection for continuously running production facilities.  For noise and space concerns, a large multiplex of cartridge reverse jet dust collector can be installed outside of a building.  A carefully engineered system of air ducts transport dusty air to the filtration area from workstations and tools throughout the plant.

Designs for Continuous Fabric Jet Dust Collectors

Blow Ring Collectors

Blow ring collectors are the first version of continuous cleaning pulse jet fabric collectors. They consist of tubular bags (14-16 inch in diameter and 6 to 20 feet long) with an inlet for dusty air provided at the top of the collector. This is a common inlet to all the bags in the system (normally in multiples of four).
Each bag is fitted with a blow ring on the outside. The rings move vertically along the length of the bags in a continuous up-down motion (powered by a chain and sprocket arrangement) and provide cleaning action. The rings have outlets facing the outside of the collector bags through which clean air is ‘blown’ out. A blower fan is connected to the bags by hoses.

How They Work

Dusty air travels from the inlet at the top and enters the bags in a downward flow at 425 feet per minute. The slow air speed aids efficient dust collection at the bottom of the bags and protects the surface of the bag from abrasion as well. Cleaning is continuous. Clean air flows out from the blow ring outlets to the outside of the bags while dust is collected in the hopper.

The filtering velocity (filter ratio) is about 18-22 feet/minute while exit velocity of blower air (cleaning air) is normally at 14000 to 15000 feet/minute. With a cleaning blower flow of about 50 ACFM (6% of the filter flow), 1% of the filter media was cleaned during continuous cleaning operation. Cleaned width of the bag was about half an inch.

Advantages of Blow Ring Dust Collectors

• They can operate at low pressure drops (1-1 ½ to 3 inch w.c.)
• They can handle high dust loads (upper limit of 150 grains per sft)
• Air can be re-cycled (mostly)
• Long bag life with an average of 5 years or greater
• Forms stable dust cake
• Versatile system. Can operate under different types of dust loading without adjusting speed of blow rings.
• Flexible system that easily adapts for different dust loads. Shortening of bags for heavy dust loading and lengthening bags lighter dust loading.

Disadvantages of Blow Ring Dust Collectors

• They are not designed to withstand high temperature or corrosive action
• Regular maintenance is required for the mechanical drives that operate blow tubes

Improvements of Fabric Jet Dust Collectors

The next generation fabric collectors were designed to handle process air streams with high temperatures and corrosive conditions - a limitation in early continuous cleaning pulse jet fabric collectors such as blow ring collectors. In these improved design, bags (diameter of 4-6 inch and length of 6 feet) are arranged in rows of 6 to 10 bags each. Cleaning action is done by directing a continuous stream of compressed air jet into each bag through holes in a compressed air pipe that is placed atop the row of bags. The compressed air jet holes were located at the throat of each bag.

The cleaning action is based on the principle that ‘compressed air jet expands until stopped’ (Law of Conservation of momentum). The expansion of the air jet can be limited by

1. The bag opening (size)
2. By inserting a tube in the bag’s throat. The tube is designed with an appropriate diameter to generate appropriate jet speed, given the size of the hole in the pipe
3. By a venturi, to reduce pressure drop when air flows from the bag into the clean air chamber
4. By installing an orifice plate at the center of the throat of the bag.

This design of fabric pulse jet collector could handle high temperature and corrosive conditions effectively.

The older designs were modified in 1971. The specifications of the early designs and ‘generic’ collectors are compared in the following table:

Comparison between specifications of older design and modified designs of fabric pulse jet collectors:

The effects of the modifications are discussed below.

Effects of Lower Filter Ratio

The new design had lower filter ratio (filtering velocity) which caused air utilization, pressure drop and dust penetration to increase while bag life reduced by 2-3 years. The problems of the modified version can be attributed to two main considerations (i) speed of dust moving upward from hopper inlets to filter compartment (ii) change in cleaning jet attributes

Effects of Change in Jet Speed From 15000 to 25000 ft/min

With increased jet speed, the bags ballooned into a cylindrical shape in the new design during the cleaning operation. This ‘flexing’ movement of the bag helped in providing an effective cleaning action. (When cleaning process was not on, the bags were normally in a concave shape between the vertical wires on the cage).
When the collective open area of dust cake is larger than the jet area, pressure does not build up enough to dislodge the bag from the wires and the ‘flexing’ does not occur (when pressure drop is below 2 inch water column, w.c.). At 3 ½ inches w.c., flexing occurs on generic jet based fabric collectors.
After cleaning, total area of the opening in the bag/ cake is increased and facilitates more efficient dust collection. Pressure equilibrium can be achieved by lowering the pressure drop.

Effect of Increase in Dust Flow Speed During Cleaning

A feature of the new design was the increase in jet flow speeds from 15000 fpm to 25000 fpm which when converted into velocity pressure translates to 14 inches w.c. and 38 inches w.c. respectively. From the above we can see that dust is now being expelled from the bag at 2.7 times the original force during cleaning.
With increase in speed, the dust that is expelled normally landed on adjacent bag and cake. The cake grows denser and becomes an efficient filter until pressure balance is reached. This cake can collect fine dusts that are less than 20 microns.
In certain cases, depending on the density of dust, outlet of collecting bags expanded after a compressed air jet burst. This is known as ‘puffing’. Lower density dusts thrust themselves onto adjacent bags more easily and can operate with low pressure drop, air utilization and low penetration.

Effect on Selection of Filter Media

The prime consideration for filter media selection in this case is to restrict dust that is pushed through the bag and its cake. To counter this problem the following options could be considered:

• Laminated construction: By laminating PTFE media to the fabric, filter media develops excellent filtering characteristics. The fine openings of the laminate obstruct water while letting air pass through them since the laminate has waterproofing qualities. This type of a filter is expensive.
• Fabricate filter cloth: By providing finer threads at the filter surface and coarser threads below the surface, the fabric becomes a mangled path, which is not easy to infiltrate by dust. This type of a fabric is a characteristic of dual dernier felts and woven felts.

Effects on Bag and Suggested Modifications Therein

• Pleated filter elements: Dust penetration during cleaning can be eliminated by using pleated filter elements. In pleated filter elements dust can be propelled at high speeds against adjacent bags but since that collecting surface is also blowing dust in the reverse direction, dust penetration does not occur or is reduced.
• Bag diffusers: By inserting bag diffusers such as perforated cylinders that fit into the cage outside the venturi, the cleaning jet speed can be slowed down.
  Baffles: By inserting baffles between rows of bags, dust is prevented from being propelled against adjacent bags.

Pulse Jet Collector Variations and Improvements

The next variation developed a pulse jet collector that can operate at low pressure drops and high filter ratios (18 to 22:1). The principle was that the better the cleaning capability of the filter media, greater was the air flow.

By slowing down compressed air jet speed, the following were achieved in these dust collectors:

• pressure drop was reduced to levels used in the blow ring collector
• dust penetration came down by 80%
• bag life increased by 200%.

 
Adaptations made to achieve greater efficiencies in High Ratio collectors:

Changes to Inlet:

Inlets were provided at the top of the collector to create a downward flow of air enabling the system to collect finer dusts that were not collected by hopper inlets in the earlier design (since fine dusts do not accumulate properly, they have the tendency to be carried away by the upward air streams prevailing near the hopper inlet and may not collect in the hopper). The efficiency of the new inlet design increased due to a change in direction of the airflow that facilitated easy removal of heavier dust particles from the air stream.

Baffles and Bag Spacing:

Perforated vertical baffles controlled direction of air stream and dust in the filter compartment. Wider bag spacing was provided.

Uses:

Highly versatile, the high ratio dust collectors can be used in all the applications of fabric and cartridge filters. They are effective even in efficient collection of very fine sub micron dusts that are normally associated with smelting, welding and combustion processes.

Advantages of High Ratio Fabric Collectors:

• They require the least space among all other dust collectors.
• They can operate at 14-18 % filter ratio (twice the filter ratio of ‘generic’ collectors)
• 200% increase in bag life
• 50% reduction in compressed air requirement

Pulse Jet Considerations With Regard to Compressed Air:

Compressed air at the pipe outlet reaches the speed of sound (sonic speed) when the pressure in front of the outlet is about 13 psig. At this speed, by increasing pressure, airflow increases but the speed does not increase anymore. Pressure at the pipe outlet is constant at 0.528 times absolute pressure in the pipe, and the difference in pressure goes unused.

Comparison of efficiency of orifices (in pipe) and nozzles at various pressures is shown in the table below:

The results indicate higher efficiencies in nozzles.

Nozzles

Nozzles help in conversion of pressure energy to velocity energy. Airflow at the orifice is normally at sonic velocity or 1000 fps. but by using a converging diverging nozzle, air flow speed increases to 1750 fps at nozzle with 90 psig in the pipe. Cleaning efficiency is thus increased sharply by taking advantage of the higher jet speed and using it in the cleaning jet.

Expansion of air jet can be limited during cleaning process as done in the generic cleaning system. Instead of a tube or a venturi, the open area of the bag mouth can be used to prevent expansion of air jet. Since filtering fan speed is low, the compressed air jet can be reduced as well.

Generic Pulse Jet Cleaning

In this type, a 1 ¾ inch diameter venturi is used to prevent expansion of air jet. Jet speed being high, it sweeps through the opposing filtering fan air by compressing it, until it reaches the bottom of the bag. The air jet bounces off the bottom, expands and fills the bag with cleaning air. The jet air reverses the direction of filtered air flow and dislodges the dust cake from the outer surface of the bag by the jet air.

A comparison between Generic dust collectors and Pulse jet collectors characteristics:

Cleaning air jet in generic collectors is lower than the filtering fan air, which is the reason why the cleaning efficiencies are low when compared to pulse jet collectors.