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Filtration and Vacuum Specialists since 1976

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A Scientific Review of Dust Collection - Part 7

Pulse Jet Baghouse Collectors

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next several months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Pulse Jet Baghouse Collectors


Blow Ring Collector

The first continuous cleaning jet collector was a blow ring collector as illustrated in Figure 7-1. In this type of collector, the dust collected on the inside of tubular bags which were typically 14 to 16 inches in diameter and 6 to 20 feet long. For cleaning, each bag had a blow ring that traveled up and down the outside of the bag. The dust laden air entered the inlet at the top of the dirty air chamber of the collector and flowed from the inside to the outside of the bag. For this type of collector, the filtering velocity (filter ratio) was commonly in the range of 18 to 22 feet per minute.

Let us further analyze a typical collector with the following applications:

Image of a Shaker collector
Figure 7-1
Bag Length 8 feet
Bag Diameter 18 inches
Bag Area 38 square feet
Filter Flow per Bag 750 CFM per bag
Number of bags 4
Total Flow per Collector 3,000 CFM
Average Pressure Drop 2 inches water column
Average Dust Penetration at 10 gr./cu.ft. load 2 inches water column 0.0002 gr./cu.ft.


The air entered the bag from the top and flowed downward at 425 feet per minute. The air traveled slow enough so there was minimum bag abrasion and formed an effective "drop out" section at the bottom of the open bag. Also, while the dusty air was traveling inside the bag, it was also traveling towards the inner circumference of the bag at a velocity of 18 to 22 feet per minute.

The cleaning of each bag was accomplished by traveling blow rings which consisted of a tubular duct with holes that faced the outside of the dust collector bag. The blow ring traveled up and down the side of the bag by mechanical power, usually chain and sprockets.

Since most collectors were built in multiples of four bags, the blower fan was able to connect to each of the four blow tubes by using flexible hoses.

The exit velocity of the blower air was generally 14,000 to 15,000 feet per minute while the cleaned width of the bag was less than half an inch. The cleaning blower flow was typically about 50 ACFM or about six percent of the filter flow which resulted in only one percent of the filter media being continuously cleaned during the operation of the collector.

The advantages of this collector were:

  • It operated at low pressure drops which were usually in the 1-1/2 to 3 inch water column pressure drop range.
  • It operated at very high dust loads with a limit of 150 grains per cubic foot.
  • It was suitable for air recirculation on most operations.
  • Bag life was only limited by abrasion which gave an average bag life of five years or greater.
  • The dust in the bag formed a very stable cake.
  • The system was inherently able to operate under a wide range of dust loadings without adjusting the speed of the blow rings.
  • Adjustment for dust loads was accomplished by either shortening the bags for heavy dust loading or lengthening the bags for the lighter dust loading.


The disadvantages of this collector were:
  • It was not suitable for operations at high temperature or in corrosive atmospheres.
  • The mechanical drives for operating the blow tubes generally required frequent maintenance.

Fabric Pulse Jet Collector Early Designs (Circa 1963)


To expand in the application area for process streams that operate at higher temperatures and corrosive conditions, an improved fabric pulse jet collector was developed. The early design is illustrated in Figure 7-2. The main changes in the collector include the collecting include the collecting of dust on the outside of the bag, the grouping of bags into rows, and the cleaning of the bags by rows. Each bag was typically 4 to 6 inches in diameter, 6 feet long, and arranged with 6 to 10 bags in a row. A pipe or purge tube was common to all of the bags in given row and it was located over the center of each bag in the row. Orifice holes were positioned in the purge tube at the center of the bags which directed the compressed air jet into the throat of the bag.

When the compressed air travels through the orifice, it becomes an air jet that expands by the Law of Conservation of Momentum until it is stopped by one of the following:


Figure 7-2
  • The opening of the bag itself.
  • A tube which inserted into the center throat of the bag and the tube diameter is calculated to generate the proper jet velocity in relation to the size of the orifice in the purge tube.
  • A so-called venturi, which serves the same purpose as the tube described above, is basically a tube with smooth transitions attempting to reduce the pressure drop as the fan air flows from the bag into the clean air chamber.
  • An orifice plate that is centered in the throat at the top of the bag and it has the same purpose as the tubular insert or venturi.
The characteristics of these early cleaning jets were as follows:
Table 7-1

Average velocity at throat of the tube, venturi or orifice 15,000 feet per minute (It should be noted that this was the same velocity as the blow ring outlet.)
Venturi throat opening 1 7/8 inches diameter
Jet Flow 290 CFM
Bag diameter and length 4 1/2 inches x 72 sq. ft.
Filter flow rating per bag 100 CFM
Nominal filter ratio 14 FPM
Average pressure drop 3 1/2 inches water column
Average Air Consumption 3/4 SCFM/1000 CFM of filtered air
Average dust penetration at 10 gr./cu.ft. load 0.0005 gr./cu.ft.

* Average filter ratio or filtering velocity was lowered by various dust and process characteristics, primarily because of the dust laden air entering into the hopper. Average filter ratios were approximately 10:1 or 10 FPM filtering velocity through the bags.

Fabric Pulse Jet Collector Later Designs (Circa 1971)


The original design was later modified by the original patent holder and the characteristics of the cleaning jet were altered, presumable to accommodate ten foot long bags. This "generic" cleaning design was then copied by the whole industry. The new characteristics were:
Table 7-2

Average jet velocity at the throat 25,000 feet per minute
Venturi throat opening 1 7/8 inches diameter
Jet flow 500 CFM
Bag diameter and length ** 4 1/2 inches x 120 inches
Bag area 12 sq. ft.
Filter flow rating per bag 90 CFM
Nominal filter ratio 8 FPM
Average pressure drop 6 inches water column
Average Air Consumption 1 1/4 SCFM/1000 CFM air flow
Average dust penetration at 10 gr,/cu.ft. load 0.008 gr./cu. ft.

** Over time there were a variety of bag diameters and lengths introduced by different suppliers. However, the jet characteristics and performance were similar.

The new design was expected to operate at the same nominal filter ratio as the early designs. However, field experience showed that the nominal filter rate actually dropped from the designed 14:1 ratio to an actual ratio of 10:1. The true reason for this reduction in performance will not be understood until much later. In reality, the nominal filter ratio for the new design was 8:1, however, most collectors actually operated between 5:1 to 6:1 ratios.

In the new 8:1 ratio design, the air consumption and pressure drop increased dramatically. Unfortunately, in the general selection of dust collectors, the air-to-cloth ratio became the dominant specification in selecting the pulse jet collectors. In time it was generally accepted that the pressure drop, air consumption, and dust penetration would be at the new higher levels. In addition, the average bag life went from 5 to 6 years for the 1963 design to 2 to 3 years for the 1971 design. In the rapidly expanding market of the early 70's, this deterioration of performance was accepted by the engineers. In fact, to solve any operational or application problems, the cure was to lower the filter ratios even further.

It is important to understand the reason for this deterioration of performance. There were two main factors: 1) upward velocity of dust entering the filter compartment form the prevalent hopper inlets (sometimes referred to as "can" velocity), and 2) the change in the velocity characteristics of the cleaning jet.

Changes in Jet Characteristics ("Generic" Baghouses)


The obvious change was that the jet velocity for cleaning had increased from 15,000 FPM to 25,000 FPM. It has been well documented that on the 1971 design, the bag inflated and formed a cylindrical shape during cleaning. This change from a concave shape between the vertical wir4es on the cage during cleaning has let many to believe that the primary cleaning mechanism was the flexing of the bag during the cleaning cycle. Like all engineering determinations, there was a certain underlying truth to these studies. The fact that when the collectors were compartmentalized and cleaned off line, this so-called "flexing" of the bag allowed the application of the pulse jet collectors to be used in many processes where no other collector, including the continuous cleaning pulse jet, was effective. However, with the development of the cartridge collector, this type of flexing could not happen during the cleaning of the media; therefore, these theories seemed to be discarded with the passing of time.

It is important to note that if the aggregate open area in the filter cake is larger than the venturi or jet area, suitable pressure will not develop from the velocity of the cleaning air. Typically, when collectors are running below 2 inches water column, whether cartridge or fabric, this indicates that the effective area of the cake and media combination is very large and flexing of the bag does not occur. When the pressure drop is over 3-1/2 inches water column, the flexing of the bags will occur on generic venturi-based fabric collectors. After the cleaning cycle, the aggregate area of the opening in the bag/cake is increased. It is in this newly opened area that the dust collects and the pressure drop is lowered until an overall pressure balance is reached.


Velocity of Dust Ejected During the Cleaning Mode


It can be concluded that the dust leaves the bag during the cleaning cycle at the velocity of the cleaning jet. The change from the 1963 design increased from 15,000 fpm to 25,000 fpm. If these velocities are converted to velocity pressure, we get 14 inches w.c., and 38 inches w.c. respectively. This indicated that the propelling force of dust from the bag has increased by 2.7 times during the cleaning mode. Refer to Figure 7-3. At the higher velocity, the dust is thrown from one row of bags in the cleaning mode towards the adjoining row of bags.

This dust at the higher velocity drives itself through the adjoining bag and its cake. The dust cake becomes increasingly denser and develops a more resistant barrier until equilibrium conditions are reached.

When examining the dust collected from the clean side of the collector during performance testing, a wide range of dust particles are noted which includes those that are in the 29 micron range and smaller.

On many applications, "puffing" can be observed from the exhaust of collectors immediately after the pulsing of each cleaning valve. This phenomenon is dependent on the effective density of the dust. The lower density dusts tend to penetrate the adjoining bags more than the higher density dusts. Very low density dust such as paper and many fibrous dusts can also operate at low pressure drops, low air consumption, and extremely low penetrations.


Figure 7-3
Effect on Media Selection

The phenomenon of driving dust through adjoining bags has led bag suppliers to offer a wide array of bag media formulations. If we ignore the requirements imposed by temperature and chemical attack, the main consideration in selecting filter media is its ability to resist the penetration of the propelled dust that traveled thought he bag and its associated cake. There are several approaches.

The most effective approach is to use bags with laminated construction where PTFE media is laminated to the felted or woven bag. This laminate has such fine openings that the coating can hold water, yet allows air to pass through the laminate freely. Its original application was to make waterproof fabrics that prevent water from entering the fabrics yet allows the vapor and air to pass through unimpeded. Unfortunately, PTFE bags are expensive when compared to the standard media and therefore are usually used only in special applications.

Another approach is to fabricate the filter cloth with finer threads, especially near the filter surface, to provide a more complex serpentine path so that the dust penetration is reduced. Dual dernier felts and woven felts are examples of materials that have a layer of fine threads on the filter surface and coarser threads below the surface.

Bag Modifications

Used pleated filter elements. When a pleated filter is cleaned, the dust can be driven against adjoining elements at high jet velocities, but since the dust is directed at another dust collecting surface that is also blowing dust in the opposite direction, penetration does not occur. This will be explained further in later chapters. There are some limitations and principles that must be applied to selecting and applying pleated filter elements that are beyond the scope of this discussion.

Insert bag diffusers. These proprietary inserts reduce the velocity of the jet cleaning forces as the bats are cleaned. The inserts consist of perforated cylinders that fit into the cage but around the outside of the venturi.

Baffles. Baffles have been inserted between the rows of bags to prevent the dust form impacting the adjoining rows.


Pulse Jet Collector Technological Breakthrough (1979) by Scientific Dust Collectors Company
Noting that the blow ring collector was able to operate at very low pressure drops and filter ratios of 18 to 22:1, the engineers at Scientific Dust Collectors launched a research and development effort to determine if they could develop a pulse jet collector that had the same characteristics. They made some important discoveries and a number of patents were issued.

A key principle was identified to be that filter flow of air depends on the cleaning capability which in turn depends on the flow of reverse air in the cleaning jet, whether the filter element is a bag, cylindrical, pleated or envelope configuration. In other words, the better the media can be cleaned, the more airflow can be tolerated.

By reducing the jet velocity, the operating pressure drop is reduced even to the equivalent of the blow ring collector. This is actually 50 percent below the old technology designs. In addition, the reduction of the jet velocity reduces the dust penetration by over 80 percent and accomplishes a gain in bag life in the 200 percent range.

Since its introduction, a great many "high ratio" collectors of this design have been installed with the following operating characteristics:

Table 7-3

Average velocity at bag opening 10,000 feet per minute
Bag opening (no venturi) 4 1/2 " diameter
Jet Flow 740 CFM
Bag diameter and length 4 1/2 inches x 96 inches
Bag Area 10 sq. feet
Filter flow rating per bag 190 CFM
Nominal filter ratio 20 FPM
Average pressure drop 2 1/2 inches water column `
Average air consumption 1/2 SCFM/1000 CFM or flow
Average dust penetration at 10 gr./cu.ft. load 0.0005 gr./cu.ft.

In achieving the high performance of these "High Ratio" collectors (see Figure 7-4), there were some additional modifications that had to be developed:

Special Inlet Configurations. The inlets were moved from the hoppers to the upper section of the baghouse. This "high side inlet" created a naturally downward air flow pattern. The new cleaning system can now collect very fine dust that previously was driven out of the exhaust. Typically, these fine dust particles do not agglomerate as well with the use of hopper inlets. These inlets also changed the direction of the airflow which caused larger particulate to simply drop out of the airstream.

Special Baffles: The use of perforated vertical baffles directs the horizontal air and dust distribution into predetermined dust flow patterns in the filter compartment. In addition, a wider bag spacing was introduced.

Applications: These fabric collectors can be applied everywhere other old technology collector designs were applied whether it consisted of fabric or cartridge filters. This includes the collection of submicron fume dusts such as in smelting, welding, or combustion processes.

Advantages of Scientific's High Technology ("High Ratio") Fabric Collectors

  • Most compact collector available. Normal operation at 14 to 18:1 filter ratios or typically twice the filter ratio of "generic" baghouses.
  • Bag life increased by over 200% using fewer bags.
  • Compressed air usage decreased by at least 50%


Figure 7-4
Compressed Air Actuated Pulse Jet Considerations

When compressed air leaves an orifice drilled in a pipe, the air increases in velocity to the speed of sound. This sonic velocity is developed when the pressure in front of the orifice is approximately 13 psig. If this pressure is further increases, more air will flow through the orifice but the velocity will stay the same. The pressure in the orifice throat remains at 0.528 times the absolute pressure in the pipe. The difference between the throat pressure and atmospheric pressure is wasted. Table 7-4 shows the conversion efficiency for the orifices and advanced nozzle designs at various pressures in the compressed air pulse pipe.



Col 1
Pulse Pipe Air
Pressure in PSIG
Col 2
Orifice Exit
Pressure
Col 3
Conversion
Efficiency
Col 4
Nozzle * Exit
Pressure
Col 5
Nozzle *
Efficiency

13 psig
0 psig
100%
N/A
N/A
25 psig
6.5 psig
74%
0 psig
95%
50 psig
19.3 psig
61%
0 psig
95%
75 psig
32.5 psig
57%
0 psig
95%
90 psig
40.0 psig
55%
0 psig
95%
* Converging Diverging Nozzle used in Scientific Dust Collectors. When comparing these results, one can see that the efficiency of the nozzle (Col. 5) is much greater than the orifice (Col. 3)

Converging Diverging Nozzles

Nozzles mounted on pulse pipes were developed as part of a proprietary cleaning system by Scientific Dust Collectors. Nozzles process the air at orifice pressure to allow further conversion of pressure energy to velocity energy. In the orifice throat, the velocity is sonic or nominally 1,000 feet per second (60,000 ft./min.). When converging diverging nozzles are mounted on the pulse pipe, the exact velocity from the nozzle will increase to 1,750 ft./sec. (105,000 fpm), or 1.7 times sonic velocity with 90 psig in the pulse pipe.

For the dust collector's throat orifice in both the nozzle and the standard diameter hole orifice, Scientific takes advantage of the higher velocity to induce more air into the cleaning jet as determined from the momentum equation. This key accomplishment results in better bag cleaning during everyday operation.

Like the generic cleaning system, Scientific Dust Collectors also limits the expansion of the air jet by stopping the induction of the induced air. However, instead of using a flow restricting venturi, Scientific Dust Collectors uses the whole open area of the bag mouth to limit the secondary air induction. A lower air jet velocity can be used because the filtered fan air velocity through the bag opening is also lower.

Generic Pulse Jet Cleaning

The 1 3/4" diameter venturi stops the expansion of the induced secondary jet air. However, the jet velocity stays high, thereby allowing the air jet to overcome the filtering fan air and reach the bottom of the bag. Then, the jet cleaning air reflects off the bottom of the bag and expands to fill the interior of the bag with cleaning air. Also, the filtered fan air flow is reversed by the oncoming jet air, and the built up layer of dust cake is blown off the outside surface of the bag material by the jet air so that it can fall into the hopper.

Comparison of Generic vs. Scientific Dust Collectors Cleaning Systems

The comparison which follows assumes the only difference between dust collectors is the method of pulse jet cleaning and the air-to-cloth ratio used. A Scientific Dust Collector is operating at twice the air-to-cloth of the generic system which is typical of actual field practice.

Generic System
Scientific Dust Collectors
Bag Length
8'
8'
Bag Diameter
4 1/2"
4 1/2"
Bag Fabric Area
9.46 ft2
9.46 ft2
Air-to-Cloth Ratio
5:1
10:1
Filtered Air Volume per Bag
(5)(9.46) = 47.3 CFM
(10)(9.46) = 94.6 CFM
Bag / venturi Throat Area
1 3/4" at venturi
4 1/2" at bag opening
Bag / Venturi Throat Area

π (1-3/4)2 = 0.0167 ft2

π(4-1/2)2 = 0.1104 ft2
(4)(144)
(4)(144)
Filtered Air Velocity at Bag / Venturi Throat Opening
47.3 = 2,832 fpm fan air
94.6 = 857 fpm fan air
0.0167
0.1104
Cleaning Air Jet Velocity at Bag / Venturi Throat Opening
Higher
Lower
As these calculations indicate, in the generic system the cleaning air jet must overcome 2,832 fpm, a much higher filtered air velocity, even though the air volume flow per bag is only half that of the volume flow run through the Scientific Dust Collector bag. The energy required to overcome the high filtered air velocity in the generic system is not available to effectively clean the dust cake from the fabric bags.

Special Configurations Are Available on Pulse Jet Baghouse Collectors:

  • Walk-In Plenum with Top Bag Access
  • Roof Doors with Top Bag Access
  • Bottom Door Bag Access
  • Horizontal Bag Configurations with End Door Bag Access



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