Most Advanced Technologies

The original design of most contemporary reverse pulse jet collectors was developed in the early 1970's. This design was a breakthrough in dust collector technology. In 1978, several technologies were developed to remedy operating problems of fabric pulse jet collectors also referred to as a bag house. The cause of these operating problems was a major flaw in the design.

Cause of Operating Problems

The main flaw in the contemporary design of fabric pulse jet collectors was that during the cleaning cycle dust was driven from the cleaning bags at very high velocities, then driven through the filter cake and filter media the adjoining bags. The bags became partially blinded. These velocities were typically 20,000 to 40,000 feet per minute. The permeability of the filter cake and dust imbedded in the media increased. The bag developed a coating to resist further penetration until the system stabilized. These high cleaning jet velocities resulted in short filter element life, high pressure drops and high compressed air usage.

It is and was common for fabric pulse jet collectors to run at pressures between 4 and 6.5 inches, and air consumption of over 1.4 SCFM of compressed air per 1000 CFM of filtered air at 80 psig. As might be expected the heavier density dust and powders ran at the highest pressure drops.

New Technologies

Four new technologies were developed to combat these operating problems:

1. The cartridge collector with pleated filter elements.
2. The high ratio fabric pulse jet dust collectors.
3. PTFE laminated fabric filter bags
4. Bag diffusers

Cartridge Collector

The cartridge collector was immediately widely accepted because it solved a problem with venting electrostatic powder paint systems. Previously, fabric pulse jet collectors on this application gave unsatisfactory service. Within months of their introduction, hundreds of collectors were sold and installed with spectacular results. The cartridge collector remedied some operating problems in the contemporary fabric pulse jet collectors. An example of this was the experience of Nordson Corporation of Ohio, who supplied powder pigment spray systems that sprayed powder on metal surfaces that were then cured to a hard coating. They vented the spray booths into fabric pulse jet collectors. On some pigments, they had bag lives of less than eight weeks and pressure drops of 6 to 8 inches w. g. Even at filter ratios as low as 3:1. They were desperate for a new technology and were the first to embrace it.

Read more about... different dust collectors

Laminated Filters

In 1975 the Gore Corporation introduced PTFE membrane laminated bags. This prevented the dust driven from the bag in the cleaning mode from penetrating below the surface of the media through the filter cake. Hundreds of thousands of bags were successfully installed that eliminated many operating problems with contemporary designs. The laminated bags lowered the bag permeability, which sometimes limited the filter ratio. Typically the bags cost 5-6 times more than conventional bags. Since then patents for this construction have expired.

High Ratio Pulse Jet Fabric Collectors

In 1978, Scientific Dust Collectors introduced another breakthrough in pulse jet dust collector technology. The venturies were removed from the bags and pulse jet was changed so that velocity of the jet decreased by 60 to 80%. The velocity of the dust leaving the bag was also decreased by 60 to 80%. At the same time the volume of the pulse jet was increased by more than 3 times. As an adjunct of these changes, these collectors were operated at filter ratios between 12 and 20:1, with pressure drops under 2 inches WG. In 1983 Carter Day Corp. introduced the same design with oval bags. Since then over 4,000 of these collectors operating at high filter ratios have been installed and are operating with those parameters. These collectors must have special inlets which eliminate any upward velocity of dust laden air entering the filtration section of the collector. The low velocity cleaning jets increase the collection efficiency on fine particulates. Fine particulates may have difficulty falling into the hopper if the collector has a hopper inlet. Thus the high side inlets of modern technologies.

Read more about; High-ratio pulse jet bag house or fabric dust collectors

Bag Diffusers

These diffusers consist of light gauge perforated cylinders inserted into the bags below the venturies of existing collectors. They operate to improve dust collector operation by slowing the velocity of the jet as the cleaning air exits the filter bag toward adjoining bags. Because they too increase collection efficiency on finer dust, collectors with bottom/hopper inlets can encounter limited effectiveness with this modification.

American Foundry Society Tests 1978

A test run by AFS on foundry dust showed that the penetration of the dust compared to a standard pulse jet was astounding:
STD Fabric Pulse jet at 4:1 filter ratio = 800x10-5 grains per cubic foot with 10 grain inlet
Cartridge Collector at 2:1 filter ratio = 3x10-5 grains per cubic foot with 10 grain inlet

Gortex laminated bags = 10x10-5 grains per cu.ft. with 10 grain inlet
Low Velocity Cleaning Jet Fabric Collectors* = 10x10-5 grains per cu. ft.
*These are New Technology collectors as described above. This test was run in 1978 prior to the introduction of the new technology low velocity jet high ratio dust collectors but other independent tests showed the performance shown above.

Later tests of new technology cartridge dust collectors, such as ULTRA-FLOW, revealed that they operate at 2x10-5 grains per cu.ft. but at 8:1 filter ratio (instead of 2:1 for conventional designs).

Determination of Flow ratings of New Technology versus Contemporary Designs

The flow capacity of a bag or cartridge in a reverse jet self cleaning collector is limited by the volume of the reverse cleaning jet. It is obvious that if I have 50 CFM flow in the filter element and the reverse jet volume is 40 cfm, the flow in the bag will not be reversed and no cleaning will take place on line. Whether the bag has 10 sq. ft. of media or 100 sq.ft. of media it will not clean on line. However, if I take this reverse jet and increase the flow to 200 CFM (50 CFM x 4) it will clean on line easily. If we temporarily accept the premise that you need four times the cleaning flow to maintain equilibrium, adding more square feet to the filter element will not increase filtering flow capacity unless you also increase the reverse flow volume. Another limitation is the operating permeability of the filter media and dust cake as covered in the first part of this article.

An important limiting factor in filtering volume through collectors is the well- known “can velocity” parameter. This effect is most pronounced on very light density dusts such as paper dust, feathers, grain, dried organic fertilizers, dust from recycling etc. Dried manure is similar to grain and is susceptible to “can velocities”. New technology collectors have special inlets where there is no upward vertical component to the dust laden gas as it enters the filter compartment of the dust collector.

If we consider the conventional designs where they have basically a reverse air volume of 250-350 CFM, and 20,000 to 40,000 FPM cleaning jet velocities, specifying a low filter ratio is excellent engineering. However, specifying the new advanced technology results in better filtration, lower operating costs and longer filter life. The better engineering approach is to specify dust collectors by considering the capacity and design of the cleaning system. This can be accomplished by taking the compressed air flow capacity of various sizes of diaphragm valves and multiplying by the number of valves, if orifices are installed in the pulse pipes. If the pulse pipes have converging-diverging nozzles installed, the cleaning jet volume is increased by 70% with the same increase of filtering flow in the collector. (This assumes that the total area orifice opening are maximum at 85% of throat area)
¾ “ valve orifices = 688 CFM;  nozzles = 1032 CFM
1 “ valve orifices = 1224 CFM;  nozzles = 1836 CFM
1 ½ “ valve orifices = 2754 CFM; nozzles = 4406 CFM

 Example of comparing two collectors:

Standard Design Collector with 81 bags, (9) ¾ inch valves, 10 sq. ft. per bag
9 x 688 CFM = 6,192 CFM, 6192/1000 sq. ft. = 6.192 (nominal filter ratio) based on cleaning volume with no regard to can velocity. For low density dusts, the ratio must be lowered for lower density dust.

Advanced Technology Collector with 81 bags, (9) 1 inch valves with nozzles, and special inlet to eliminate can velocity considerations.
9x 1836 CFM = 16,524 CFM, 16524/1000 sq. ft = 16.524 (nominal filter ratio)

The net cleaning jet velocity should be specified as no more than 9,000 fpm and gross cleaning velocity no more than 15,000 fpm for dense dusts.

Upward component flow of air entering the filter compartment should be limited to 150 fpm for lower density dusts and 350 FPM for higher density dusts.

Specifying collectors based on filter ratio alone penalizes the supplier who provides more cleaning capacity with the larger more expensive cleaning components, and better engineered cleaning systems, depriving the client of superior performance at lower cost.

When to specify Fabric or Pleated filter elements

The other decision is between pleated filters and unpleated filters. The selection for pleated filters is confined to dust that has a thin filter cake. If the cake is deep, bridging will take place in the valleys of the pleats rendering the media below the bridges uncleanable. A new advanced technology bag house/fabric dust collector can be specified on virtually any dry particulate dust application, even those traditionally reserved for cartridge dust collectors.

Read more about; Dust Collection

Bag House Dust Collector Technology

History

The first pulse jet collector was developed by Pulverizing Machinery of Summit New Jersey in the early 1960’s, to collect dust from their Pulverizers. They had tried to use the Blow Ring design but it could not handle the dust (powder) loads as Pulverizers became bigger. The typical load to the collectors were between 50 and 300 grains per cubic foot. The collector design was based on the same blow ring velocity and the cages were based on available designs from hipping pulverizer shafts. The pulse valves selected were diaphragm valves that were fast and the lowest cost valve available. This happened to be a ¾ inch valve. They decided to use several valves in a collector and pulse them with an electronic timer. It was found that the hole sizes and venturi were an air ejector design that had the same jet velocity that the blow ring collector was using. But the big breakthrough came with the realization that the dust was ejected from the bag during the first 4 or 5 milliseconds of the valve opening. It became apparent that the frequency of cleaning was a function of the load to the collector. For instance for loadings of 300 grains the collectors would operate at a filtering velocity of between 7 and 9 feet per minute. At material handling facilities such as a quarry would operate at a filtering velocity of 14 to16 feet per minute. The typical pressure drop in these collector designs were about 2 to 3.5 inches WG pressure. The typical compressed air usage on the high loads were 1 to 2 SCFM at 80 psig per 1000 CFM of filtered air. For loads under 10 grains per cubic foot, the air usage was 0.2 to 0.8 SCFM per 1000 CFM of filtered air. Determining the filter velocity (then referred to as filter ratio) became a rather complicated procedure. The ratio presumably was determined by dust load, fineness of the dust, temperature of process gas stream, and other factors.

The hopper inlet was a carry over design from both the blow ring collector and the previous mechanical shaker collectors.

By 1969, there were over 10,000 collectors in operation. Almost all of them were installed on process equipment or in Foundries. Pulverzing Machinery changed their name to Mikropul and licensed FlexKleen to build and Market collectors. The collectors for MikroPul had 4 ½ inch diameter bags and the FlexKleen units had 5 inch bags. The Mikro units had six foot and on occasion a collector with 8 ft long bags (to compete with FlexKleen on some projects) and the flex units had nominally eight foot long bags. Bag life was usually 4 to 7 years.

Engineering Disaster 1971

In 1971, the patent was challenged and the Pulverizing Machinery patent was declared invalid. The market changed radically because Air Pollution Control Regulations became effective. Many new suppliers entered the market. In order to compete Mikropul changed their design. They went from 6 foot to 10 foot bags. They increased their pulse pipe holes by the same ratio. The whole industry followed and copied the new design for hole size and venturi throat diameter. At the time, Mikropul had 40,000 venturies in stock and kept the same venturi sizes. This increased the jet velocity of the cleaning jet by 66 per cent.

This was when the dust collector market was growing at a 20% annual rate. With the new designs pressure drop increased to 4 ½ - 6 ½ inches WC. Compressed air consumption increased by over 50% for similar applications. Bag life was reduced by over 50%. In reaction to these problems the filter ratios were reduced to between 4-6 on almost all applications.

Reasons for Disaster

What happened was no one at that time realized a rather obvious truth, that the velocity with which the dust is ejected from the bag during cleaning is proportional to the velocity of the cleaning jet. At the new velocities, dust is driven toward adjacent rows of bags in the filter mode. Depending on the dust density, the dust will be driven through the adjoining cake into the clean side of the bags. The cake becomes more dense and the pressure drop increases until the process stabilizes which takes 16-100 hours. Even after the equilibrium, the dust still penetrates and bag wear is high. With low filter ratios it takes longer for the bag to wear out and require replacement.

Low Pressure pulsed air cleaning systems

Reverse Air Fan induced pulsed air collectors

In the mid 70’s, it was discovered that the compressed air cleaning pulse jet collectors were encountering high pressure drops when applied to grains and to a lesser extent on woodworking applications. The reason was that the compressed air as it left the pulse pipes was subject to refrigeration cycle as the compressed air expanded. The first approach was to apply reverse air blowers to the cleaning system. The blowers were mounted on the roof of the collectors and the reverse flow was pulsed with mechanical dampers. The reverse air jet actually was higher in temperature than the process gas stream because of heat regain from the cleaning air as it passed through the fan. The downside of these collectors was that the fan on top of the roof of the collectors were difficult to service and as collector systems expanded the weight of the fan was significant. These collectors pioneered some arrangements that allowed them to operate with grain dust with densities under10 pounds per cubic foot. They introduced the high cyclonic inlets of cylindrical shaped collectors. They also featured a rotating reverse air manifold. There are thousands of these collectors, some we serviced for over 20 years.

Air Pump Pulse Jet collectors with 8-10 psi operation

This was an outgrowth of the reverse air induced fan pulse jet designs. They used the technique of a cylindrical housing and a rotating pulsing arm but reduced the effects of the refrigerant cycle and loss of energy when the compressed air expanded. Also, in the advanced technology concepts, they reduced the velocity and the effect of the air leaving the bags propelled to adjoining bags during cleaning. The oval shaped bags reduced the re-entrainment effect. The collectors usually had the effect of a high inlet as the air entered the bags mostly through the hollow cylinder in the middle of the cylindrical housing. Some of the advanced technology designs used a high cylindrical inlet similar to the F4 fan cleaning units. These collectors with bottom inlets were also applied to boilers at conservative issues.

Today’s Conditions

The disastrous design, mentioned above, continues to be employed by most of the pulse jet collector suppliers in the world, especially for boilers. The market has become one in which it is a commodity and the equipment is built by the lowest cost suppliers.

New Technology

Early 1980s, one of our associates worked for a company called Scientific Dust Collectors and developed a new pulse jet collector that basically changed the cleaning system design. The key to this design was that he changed the jet velocity to a fraction of the existing designs. This eliminated the penetration of dusts from the row of cleaning bags to the adjoining row in a filtering mode.

This allowed pulse jet collectors to operate at lower pressure drops (2-3 inches w.c.), lower air consumption (50-75% less), 3 to 4 times more bag life and filter ratios of over 12 : 1 on any application while decreasing dust penetration by up to 90%.

There are many different considerations of using this new technology more effectively, which we can teach the client to apply. These techniques were developed over last 30 years for Ultra-Flow, Carter Day (now Donaldson), Dustex and with several smaller companies. These include air distribution baffles and special inlet and outlet configurations. Now our has joined our group as a consultant. With the help of this system designer, having over 60 years of experience, QAM has brought this advanced technology to the Canadian market and carry on the legacy during the last 10 years. We do not provide a design which is relatively simple but provide a technology which will enable our clients to develop new radical designs and systems. Specifically, there are many little details such as protecting the inside of pulse pipes from corrosion, techniques for reducing corrosion when burning coals containing sulphur, special techniques to start up and shutdown when faced with unusual conditions, techniques for adjusting cleaning systems to operate at higher elevations, inspection techniques for qualifying critical components etc. This technology is an on going process. We have seen radical changes in periods of less than six months as new components and new manufacturing procedures are developed.

This technology allows the client to adapt to different field conditions and to burn different fuels. These collectors have been applied to incinerators which operate under the most difficult conditions. By comparison a coal fired boiler is relatively simple. Sometimes the operating techniques may change as the source of fuel is changed. There are some advanced innovations that are especially significant to operating costs. When air expands from 90-100 psi absolute, in an orifice blowpipe it accelerates the air to sonic velocity. Then the pressure in the blowpipe is 100 psia, the pressure in the throat is 52.8 psia absolute or 38 psia. The energy below that down to atmospheric is dissipated. There are techniques to design nozzles to install on pulse pipes which will increase the velocity to around 1690 feet per second and lower air consumption by 30%.

Read more on ... New Technology Bag House Dust Collectors