Determine Dust Loading in Dust Collectors

Knowing how much dust will be collected in a dust collection system is very important. The method below is what is as good as any:

Let’s use the example of an 18-bag dust collector on typical woodworking dust from two table saws. We expect a large portion of the dust to be sawdust.

Each table saw requires about 660 CFM, therefore we need 1320 CFM at the dust collector.

Assume a loading of 10 grains per cu.ft. (from experience; woodworking is 10-20 grains, Material handling transfer points is 3-5 grains, fumes is 10grains per 1000 cu.ft.)

1320 CFM x 10 grains = 13200 grains/min

13200 / 7000 grains/lb = 1.89 lbs/min x 60 min = 113 lbs/hour

A 55 (45 imperial) gallon drum is 9 cu.ft. volume.

From a dust collection selection chart, density of wood is 10-30 lbs/cu.ft.

Therefore, a drum will hold 20 lbs/cu.ft. x 9 cu.ft = 180 lbs/drum

We will fill 113 lbs/hour / 180 lbs/drum = 5 drums per 8 hour shift.

For assistance with: dust collection applications and troubleshooting 

Rotary Airlock Bridging

Generally what happens is that the dust is less dense than designed for and it bridges in the hopper. Another cause can be the wrong RPM at the airlock which prevents efficient removal of the dust by the airlock.

The old way to break the bridge was with hopper rappers where there was a hammer that hit the side of the hopper wall from outside. They worked nicely except they were loud. About 40 years ago OSHA would not let them use that procedure so there were a lot of pneumatic operated gadgets that were adapted.

Carter Day sold a unit where they put a piece of fabric on the walls and inflated it to change the angle of the hopper to 90 degrees and this broke many bridges. This was primarily applied to grain collectors, which were very notorious in bridging.

In the woodworking industry the general approach was to put on bigger and bigger airlocks. They considered a 24 inch airlock as a small one on many woodworking jobs.

There are many vibrators now applied to hoppers to break the bridges. The truth is there is no guaranteed way and usually the gadgets are returned to the manufacturer if they do not do the job.

Dustex had two cheap ways of using the pulsing control from the timer. First was to drill a hole to the hopper and insert a capped pipe about two inches into the hopper wall. With a saw make a slit 1/8 inch thick to cut out a slot 30 degrees wide. The jet spreads at about a 20 degree angle. The height is the air lock size times 2.5.  A 12 inch air-lock would be using a height of 30 inches.. The other way is to put a pulse pipe with one hole in the center above the airlock. Design the velocity pressure to be 4”wc at 65 psig with a regulator if there needs to be a bigger boost. It is like designing a jet to clean a square cartridge.

For assistance with dust collector problems

Mechanics of Collection and Cleaning

Open area and permeability relationship

First we will consider the mechanics of renewable media dust collectors. The media has openings which are a small percentage of the area of a media. This type of media, which we will consider, collects dust by a sieve process. It will collect dust by the size of the holes in the media. Tortuous path filters often applied in HVAC will not be considered.   A measure of the relative areas is permeability. Permeability is a measure of the flow through a square foot of media at a 0.5 inches water column pressure drop (0.5 “ w.c.). At this pressure drop the flow has a direct relationship to the area of the openings in the filter media. How much dust the filter media can hold is a direct function of the permeability. If a media has a permeability of 20, it will hold five times as much dust holding as has a media with a permeability of 5. Also the effective open area of the media is five times as great and is proportional to the permeability

Dust Collection Process

A self cleaning collector of any type operates by collecting dust in the holes of the media until the holes get filled with dust. As they get more filled with dust the pressure drop goes up and the permeability goes down. In a disposable filter the media is replaced. In a self cleaning filter, the media is regenerated by media cleaning.

Function of Media Cleaning

The cleaning systems then remove dust from the holes, and the permeability goes up and the pressure drop goes down. The permeability does not return to its original (virgin) state. Some of the openings get smaller which increases the collection efficiency. The difference in permeability is due to formation of a filter cake on the media.

Filter Cake

In the formation of a filter cake the efficiency of collection for a test dust improves dramatically. On a standard laboratory test, typical results are:

                Original collection efficiency                             80-85%

                Final efficiency with filter cake                        99-99.9%

                Original permeability                                          22-24

                Final overall permeability                                  8-12

Media Cleaning Systems

There are various methods to clean the media.

                a) Washing with liquids. (For tortuous path media)

                b) Mechanical cleaning (shaker systems)

                c) Reverse flow through the filter media

We will only consider mechanical and reverse air cleaning designs.

Mechanical Cleaning

Until the 1960’s, mechanical cleaning (Shaker) of the filter media were the only self cleaning fabric filters available. These were and are applied at filtering velocities of up to 8 feet per minute. Normally they were selected on applications where cleaning was required about once every three or four hours or where the process was intermittent so the collector flow could be stopped to clean. If the collector was cleaned during process flow the dust would not fall down into the collection hopper. To make it suitable for continuous processes, three or more collectors were installed in parallel. The flow was stopped with dampers to allow cleaning of the media in one module while the full flow was handled by the remaining parallel collector. The flow through the collector had to be stopped so that the mechanical action could be effective. If the cleaning was delayed until the pressure drop rose too high the dust might become imbedded in the media with enough force so that all or part dust would not be ejected during the cleaning (shaking) procedure. This raises the pressure drop and results in reduced flow at much higher pressure drop. They have an advantage on some dusts. In a pulse jet compressed air cleaning collector the dust must agglomerate into larger particles before it will fall into the collection hopper. With a mechanical collector this is not necessary.

There is a system similar to mechanical cleaning, “Plenum Pulses”, in which compressed air cannons agitate the bags in multiple compartment without reverse air. Those will be covered in a future addendum to this presentation.

Reverse Air Cleaning collectors

There are several different types of reverse air cleaning systems

1)       Compartmented Reverse Air with continuous reverse air flow fans

2)       Continuous Cleaning reverse air collectors with rotating or traveling manifolds with common dirty air plenums.

3)       Intermittent fan pulse units with common dirty air housing with traveling compartmented manifolds and common clean air plenums.

4)       High Pressure Pneumatic powered pulse jet collectors (70-100 psig)

5)       Low Pressure Pneumatic  powered pulse jet collectors  (7 to 22 psig)

Compartmented Reverse Air Cleaning.

                These are commonly referred to Reverse Air Collectors. The collector is divided into individual modules. In one of the modules a damper closes to the flow in the filtering direction.  Another fan blows air backwards through the module back through the duct inlet manifold, at approximately the same volume as the filter flow. This backwards flow, then flows back through the inlet manifold where it vents to the other compartments. This gradually pressurizes the clean side of the bags, dislodging the dust so it falls in the hopper. In a six compartment unit, the exhaust fan must be sized to handle 6/5 (1.2) times the process exhaust flow. This process continues until all the compartments are cleaned. It’s a very gentle cleaning system which was originally applied to collectors at high temperatures which required woven fiberglass bags. The mechanical shakers were too violent for the fiberglass bags. The filtering velocity was lowered enough to run at low pressure drops usually lower than 2 inches w.c. They were generally bigger and more expensive than shaker collectors and the later developed pulsed cleaning collectors. They can handle difficult to agglomerate dusts as well as standard dusts. They are able to operate on applications where the process gas stream operates at or near the dew point temperatures. This is because the reverse cleaning fans experience temperature regain in passing through the fan. Reverse air temperature is higher than the process gas temperatures. When operated at the correct filter velocities, the filter element bag life is often indefinite measured in years rather than months. For a particular volume on a continuous process, they are many times the size of shaker and pulse cleaning collectors. This type of reverse cleaning applies reverse pressure very slowly and because of the inertia of the dampers. It is a gentle cleaning action which increases with time. It also allows cleaning of woven fiberglass media which often fails as the threads bend and fracture as in mechanical cleaning and pulsed cleaning systems. .

Continuous cleaning Collectors with traveling or rotating manifolds (Fan powered reverse air)

These were a variation of the compartmented reverse air collectors. In these units, the rotating or traveling manifolds continuously blew pressurized cleaning air backwards through either single or multiple bag sections of the collector.(Note: They were not suitable for woven fiberglass media.) These were able to operate at low pressure drop at filtering velocities in the 6-8 fpm range. The manifolds divided the compartments into 8 or more sections and the reverse air fans provided two or three times the heat regain to cleaning air. This lowered the requirements on the main exhaust fans. These were successfully applied to the grain, and food industries where operating near the dew point causes premature filter element failures for other types of collectors. The pressure drops across the filter cakes were very low. These collectors were applied to many other dusts to give excellent service. Examples of these collectors were supplied by Sly Mfg. (Dynacone), Carter Day Corp (Type RJ old style) and Pneumafil Corp. (Reverse Fan). These collectors had followers on the traveling manifolds to prevent the ejected dust from short circuiting to the bags that were previously cleaned.

Intermittent Cleaning Fan Pulse Units

These collectors were an outgrowth of the continuous cleaning collectors to operate at high filter ratios and remedy the limitations of the upward velocity component for continuous cleaning applications. These collectors also discovered the most important principle of reverse air cleaning collectors. They discovered that the capacity of a filter element depended on two factors:

                A) The open area of the openings in the bags. If a  bag permeability were doubled the dust holding capacity doubled. The capacity of a bag could be doubled by either doubling permeability or doubling the bag area. This allowed operation at high filtering velocities. It also allowed making the collectors  much smaller. Thousands of these collectors were applied at filter velocities of 10 to 20 feet per minute.

                B) The quantity air volume capacity per filter element in the filtering mode is directly related to the reverse air volume. Increasing the length of the bag or increasing permeability without increasing reverse air flow does not increase volume which the filter element can handle. There must be sufficient open area and sufficient reverse air volume to clean the open area. Doubling the filter element length or area does not increase filter element volume rating.

As the bags were made longer, the size of the reverse air fan was increased.

When introduced in the late 1960’s, they quickly replaced the mechanical shakers and pulse collectors in the grain and food industries. They were able to handle heavy and continuous loads where dew point normally ran close to the dry bulb temperatures. The reverse air fan had temperature regain so that the reverse air cleaning gas stream was 3-6 degrees F higher than the process gas streams.

Intermittent Fan Pulse units with common dirty air housing with traveling or sequencing cleaning compartments

These designs were able to operate at high filtering velocities  and pressure drops in the 1.5 -3 in.w.c. They were largely applied on trash recovery operations, on food operations and cement handling applications, where operations near the dew point were common.

High Pressure Pneumatically Pulse Jet Continuous cleaning collectors (70-100 psig)

Because the diaphragm valves that powered the reverse air valves operated in less than20 milliseconds, these collectors routinely handled loads in the 150-300 grains per cu. ft. and were applied to Pulverizers manufactured by Pulverizing Machinery Corp. (Later Renamed MikroPul  Corporation) The Pulverizes added heat to the process air and hundreds of installations were installed. These ran at filtering velocities of 6-9 fpm. In the late 60’s they were applied to other applications. Hundreds of applications were vented at filtering velocities of 12 to16 fpm. The precise selection of filter velocities was complex and was related to load, temperature, dust density and to a lesser extent to the dust load. Typical pressure drops ran between  2.5 and 3 “ w.c. Filter element life was 6 to 10 years.

Redesign of Pulse Collector cleaning System in 1969

Under competitive pressures the bag lengths were increased from 6 ft (6 sq. ft.) to 10 ft (10 sq ft.). This was accomplished by increasing cleaning jet velocity from 12,000 feet per minute to 20,000 ft per minute and the reverse air volume 240 cfm to 400 cfm. This seemed to be a consistent design. But the result was that during cleaning the dust was ejected towards adjoining rows in the filter mode at 20,000 fpm, reducing the bag Permeability by 60 to 80%. The pressure drops at the same filtering velocities went up to 6.5 to 9.5 inches w.c. To compensate for these higher pressure drops the industry lowered specifications to filtering velocities to between 5 and 6 fpm. These collectors at the reduced filtering velocities ran at pressure drops of 4.5 to 6.5 in. w.c. Filter element life was 20 to 30 months.

Low pressure compressed air cleaning systems were introduced in 1970, but the jet velocities were the same as the high pressure cleaning systems introduced in the late 60’s. They had the same issues in performance.

Advanced Technology Design 1978 Continuous Cleaning Pulse Jet with 70-100 psig

These were introduced using the principles outlined above.

AA)       The reverse air volume was increased by 4-5 times

BB)       Reverse Air Jet velocity was decreased back to under 12,000 fpm.

                CC) The inlets we redesigned to allow the finer dust components to fall into the hopper unimpeded by the upward velocities of the dirty air entering the filter compartment. These were widely applied on many different applications from cement to paper dust over the next 27 years.

The low cleaning jet velocities allowed the filter media to maintain permeability so that they ran at  filtering velocities in the15 to19 fpm range. Pressure drops were lowered to 3.5 inches. And filter element lives were over 48 months. In the late 1980’s the same technology was introduced to the low pneumatic pressure powered units

Consideration  from Refrigeration Cycle in Compressed Gases  for pneumatically powered pulse jet collectors

In Thermodynamics, it can be noted that as a compressed air expands it cools. This principle is used in designing air conditioning and refrigeration designs. The net effect is, that on all pneumatically powered collectors, the jet temperature of the jet/air mixture, when compressed air is mixed with air from the clean air plenum, is 4-8 degrees F cooler than the air in the clean air and dirty air plenums. One approach, to counteract this when process gas streams are near the dew point, is to heat the compressed air in the pulse pipes high enough so the jet temperature is higher than the process temperature.

For more on the evolution of baghouses 
Read more about ... Advanced Technology Baghouses

Filter Ratio (or air-to-cloth), A serious Myth

New Advanced Technology eliminates design flaws; allows for High Ratio Operation.
The History of Reverse Jet and Pulse Jet Design and Development must be reviewed to determine proper selection of collectors.
The first pulse jet collector was developed by Pulverizing Machinery of Summit New Jersey in the early 60’s, to collect dust from their Pulverizers. They had tried to use the Blow-ring design but they could not handle the dust (powder ) loads as their grinder Pulverizers became bigger. The typical load to the collectors from the Pulverizers were between 150 and 300 grains per cubic foot. The collector design was based on the same blow-ring filtering velocities at these loads. The cages were based on available designs from shipping pulverizer shafts. The pulse valves selected were diaphragm valves that were the fastest and the lowest cost valve available. This valve happened to be a ¾ inch diaphragm pilot operated valve. They decided to use several valves in a collector and pulse them with an electronic timer. It was found the hole sizes and venturi formed 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. The valves were operated as fast as the mechanical design allowed. The operation was completed in less than 0.10 seconds. 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 ft per minute. At material handling facilities such as quarries, the collector would run at velocities of 14 to16 feet per minute. The typical pressure drop in these collector designs were about 3.5 inches water gauge pressure for the high loads and 2.0 for the lower dust loads. The typical compressed air usage on the high loads were 1 to 2 SCFM 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 exhaust from Pulverizers or in foundries. Pulverzing Machinery changed their name to Mikropul and licensed FlexKleen to also build and Market collectors. The collectors for MikroPul had 4 ½ inch diameter bags 72 inch long and the FlexKleen units had 5 inch bags 102 inches long. Bag life was 3-5 years on Pulverizer applications and over eight years on low loading applications.
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 also became effective at the same time. 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:
(1) pressure drop increased to 4 ½ to 6 ½ inches w.c..
(2) Compressed air consumption increased by over 50% for similar applications.
(3) Bag life was reduced by over 50%.
(4) 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 what might have been 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 longer times between replacements.

Today’s Conditions

This disastrous design continues to be employed by most of the pulse jet collector suppliers in the world.

New Technology eliminates design flaws
In 1979, a new technology was developed, a new pulse jet collector that basically changed the cleaning system design. The key to this design was to change the jet velocity to a fraction of the existing designs. New Technology 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:
(1) lower pressure drops (1- 3 inches w.c.),
(2) lower air consumption (50-75% less)
(3) 3 to 4 times longer bag life
(4) filter ratios of over 14 : 1 on any application
(5) decrease dust penetration by up to 90%.
There have been several suppliers building and selling these New Technology collectors since 1982. In fact the patents have now expired. There are over 4000 installations worldwide.

WHY IS THIS NEW TECHNOLOGY NOT ACCEPTED BY ALL THE MAJOR SUPPLIERS?
1) If you produced 40,000 collectors after the development of the new technology was published over 20 years ago, you might be subject to legal action for poor judgment and causing the public to be overcharged for their dust collection.
2) They do not have the engineering expertise to build these new technology collectors.
3) People using the old obsolete technology control over 90% of the market world-wide.
4) The suppliers of valves and filter elements would have their markets cut in half.
5) Air compressor sales and service for pulse jet collectors would be cut by 60%

MODIFYING EXISTING COLLECTORS WITH ALMOST NO RISK TO THE PURCHASER.
We can supply new bags, pulse pipes and bag plugs to alter performance to high technology low pressure drop, reduced air consumption, lower penetration (immediately noticeable) and long bag life (it takes some time to verify that but it should be obvious from the other indications). The modifications take only a few hours and if a customer is not satisfied, he can return pipes and cages for credit and re-install the old components. If this was not an absolute certainty customers would not pay for the equipment.

For more information: Retrofit existing baghouse, or, Retrofit cartridge dust collector

Powder Spray Booths

4 Service Reports; inefficiency, filter clogging/blinding, filter cake destruction, oil on filters.

Project in Michigan The system involved coating parts with a PVC Powder. The complaint was that the dust was bleeding through the filters even after 89 hours of operation. We diagnosed the problem that during the pulse cleaning action the cake was also being blown off the media. Our solution was to build a filter cake below the surface that would not be blown off. We fed 1 ounce of baking soda per 100 square feet of filter media which required two hours of hand feeding. This stabilized the pressure drop at 2-3inches and five years later the filters were still operating perfectly with no further bleeding
Project in Northern Ohio Customer has an electrostatic powder paint coating. The booth supplier built his own collector by copying existing designs. The pressure drop was 7 inches water gauge and the volume was insufficient to keep dust from entering the room. We modified the cleaning system to increase flow through the cartridge filters. The pressure drop went to only 2 inches water gauge. Then they installed a shut off damper at the exhaust fan to lower flow and reduce horsepower requirements at sufficient flow.
Project in Antwerp Holland They were using electrostatic booths to coat grating and a bag dust collector. There was serious leakage through the filter-bags. We stopped the leakage by using duct tape on the bottom of the bag cages. The pulse-jet must clean the whole bag evenly. If a solid impermeable stop is not placed at the bottom of the cage the filter cake will be destroyed and dust will bleed through the bottom after each pulse. We remedied the situation in less than an afternoon. The manufacturer changed his design to prevent the leakage.
Project in Cleveland Ohio Machine was designed to remove rust and add coating to wire hangers provided to dry cleaners to hang cleaned clothes. Dust went right through the filter bags even though the pressure drop was less than 3 inches water column. Much of the dust consisted of pieces of scale that were bigger than fifty microns. This was much bigger than the biggest holes in the media.  After investigating we found that the trouble was because of an improper installation of the rotary screw compressor. The installer left off an oil trap that recirculated the oil through the screw. This oil coated the bags and lubricated the fibers so that dust larger than the openings in the bags was able to slide through. We had them install the oil trap and install new bags. They paid for the service call including installing new filter bags. The collector ran with the new bags for several years.

For more information on system design and troubleshooting

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%.

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