The cartridge dust collector design was in response to the flawed design of the fabric pulse jet collector…
The cartridge element life was approximately four to eight months but this was acceptable for the industry in that element change was not difficult… With the performance realized on the powder spray, acceptance in many industries followed quickly. The filter elements were a popular size employed as intake filters on tractor trailer engines.
The initial specifications were as follows: Outer diameter” 13.37 inches Inner Diameter: 8.25 inches Length: 26 inches Number of pleats: 265 Filter media area: 185 sq. ft Pleats per inch: 10 Media:: safety media of cellulose Inner core: usually expanded metal.
Cartridge Dust Collectors Referring to the Figure 1:
is the open end cap generally is spun and has a thin coating of zinc to allow the spinning to be smooth during the spinning process. It also has a gasket to prevent dust leakage.
is the outer core of expanded metal rolled and spot welded so there are no burrs at the joint
is the media enclosed between the inner and outer cores
is the inner core, this is generally an expanded metal cylinder
is the closed end cap. The cone built into the end cap spinning was to make room for the wing nut and spring washer with integral gasket.
is the adhesive to bind the media and cores to the end caps.
… forces developed during the pulse cleaning process gave the gaskets a permanent set which shortened the filter element life dramatically.
… Many times the cleaning forces would break the bond between the lower end cap and the media. …causing premature failures…
Engineering Disaster (a flawed cartridge dust collector design)
…erroneous conclusions that have dogged the industry for over 25 years: That cartridge collectors can only operate at filter ratios of 2:1 or 2 fpm The assumption was that they could increase the capacity of this cartridge collector by squeezing more pleats in the cartridge…
The flaw in this reasoning is there are two factors which determine the filtering capacity of a pulse jet collector. The square foot of the media that are cleanable and the volume and pressure characteristics developed by the cleaning jet. It can only clean continuously on line when the volume is at least three times the filtering volume for each cartridge and the pressure developed by the jet is at least three times the operating pressure drop. The offline cleaning characteristics are not considered in this chapter. This applies to all pulse jet collectors regardless of the shape of the filter elements.
Other limitations of early cartridge dust collector design
Referring to the above figures.
On the left side we see that the jet grows at an angle … until it is stopped … As it enters the cavity inside a filter element it forms a compression wave… The action cleans the element evenly…
…early designs the element had a truncated cone built into the closed end cap… On many dusts this would partially damage the cake… some dust would leak into the clean air side … which led to premature filter element replacement…
…if the pulse pipe was too low, the jet would strike the media below the opening in the filter element. The cleaning action above this intersection had limited effectiveness… because of this incorrect height, the added media contributed little to the filter’s performance, but did raise the cost of constructing and operating the filters.
Effect of Excess Filter Media and Pressure actuated pulse Controls
… once a bridge forms in the valley of the pleat, it renders the media below the bridge un-cleanable by on line cleaning.
If a cleaning system is actuated by a pressure control, the setting must be determined by experimentation since it is virtually impossible to predict the best pressure setting before the collector is installed. If it is set at a higher pressure this will affect operating pressure, frequency of cleaning (air consumption) and dust penetration through the filter. Typically a collector pressure switch is set at 3 1/2 w.c. when the proper setting might be 2 “ w.c. Below are typical operating parameters at the correct and incorrect settings for a typical operation venting a material handling operation:
Operating at Correct setting for 4,000 CFM system with 3 grains per cubic foot load:
Pressure Drop 2 inch w.c
Air Consumption at 85 psig 1.5 SCFM
Dust Penetration 0.00004grains/cuft.
Cartridge life 18 months
Operating at Incorrect setting for4,000 CFM system with 3 grains per cu. ft. load:
Pressure drop 3.5 inch water column
Air consumption at 85psig 4.5 SCFM
Dust Penetration 0.00012 grains/cu.ft.
Cartridge life 6 months.
Notice that operating power is 1.32 higher, air consumption and dust emissions are increased 3 times, cartridge life is only 1/3.
Improvements in Cartridge Filter Design
The encapsulating compounds were replaced by adhesives. Epoxies and various thermo-setting adhesives were supplied. This resulted in a stronger stiffer cartridge. Cartridges were able to be constructed without the expanded outer core. To keep the pleats from inverting during the cleaning process, plastic bands or even strings were applied with great success. These stronger and more open cartridges opened up new applications for cartridge collectors.
To go along with the changes in adhesives stronger cellulose media was developed. They were reinforced with polyester and other fibers. This increased the tensile strength by three or four times and allowed the introduction of more powerful cleaning systems to increase flow per filter element. Another important development was incorporation of pleat spacers, by upsetting the tips of the pleats. This solved a problem in constructing cartridges by eliminating wide variations in pleat spacing. A section of pleats that were too narrow would bridge too easily.
In the quest to prevent any leakage from the dirty to the clean side of the cartridge, resilient seals were added. The new seals could maintain an even pressure on the sealing surfaces. In order to maintain a good seal the pressure must remain within a narrow range of pressure. They can be tightened so much that sealing compounds will go beyond their plastic limit and produce uneven pressure and actually leak through the joint. The natural tendency for many maintenance people is to over tighten gaskets and seals. Some gaskets are designed to be squeezed beyond their elastic limit and flow along the joint. This is similar to the gaskets applied in gasoline engines. .However the valve cover gasket is really an elastic seal. In cartridge mounting systems, the gaskets would need so much pressure that the cartridge would be damaged. For this reason elastic seals must be applied. Stops are often built into the seals themselves. Refer to the diagram below
L M A resilient seal, labelled L is placed between the sealing service presumably round, and at 120 degree intervals the seal is slit and a ball bearing, labelled M, is dropped into the slot. This maintains sufficient sealing pressure but does not allow over compression of the seal. Other stops can be incorporated into the mounting system.
Un-cleaned media considerations
The area of filter media that can be cleaned depends on the reverse air volume and pressure of the reverse air jet. If we consider different orifices and valves and assuming a permeability of 18 -20 CFM at 0.5 inches of pressure drop for a sq. ft of the base media, the area of media cleaned is:
1/4 “orifice 7-10 sq. ft./li>
3/8 “orifice 15-20 sq.ft.
7/16” orifice 22-26 sq. ft.
1/2 inch orifice 28-37 sq. ft.
3/4 inch valve 75 to 78 sq.ft
1 inch valve 210 to 220 sq. ft.
If we consider a typical tandem design with two cartridges, each being cleaned by a single 3/4 inch valve. Only about 80 square feet of media are cleaned. We find that the media can hold 0.1 to 0.3 lbs of mineral dust if it is not cleaned. We can compute the weight of dust on the un-cleaned media.
450 sq. ft. total less 80 sq ft. cleaned by pulse jet = 370 sq. ft.
370 sq. ft. x (0.1 to 0.3 lb.) per sq. ft. = 37 to 110 lb per 2 cartridge tandem cleaning sets.
On some of the later designs the mounting bolts on the cartridge support tripods had to be reinforced to prevent mounting failures.
Cartridge Construction Improvements
Some other innovations to improve cartridge construction were the following.
Many new designs included a flat bottom on the closed end cap.
Some designs incorporated a flat disc above the truncated cone
Spun bond and other medias were introduced to counter the tendency of cellulose medias to expand and shrink with changes in humidity. Other cartridges were supplied with pleated medias even woven or felted media and were quite successful. With these new constructions pleated elements with wide pleat spacing operated at filter ratios of over 10.
Flexible Media pleated filter elements One limitation of some spun bond and felted medias was that they would have poor operation if the pleats were too narrow or lacked mechanical support of the media itself.
Flexible Media pleated filter elements
Support Media squeezed so it touches Initial final squeezed shape The support can be a laminate that is stiff but a very open media. This allows the use of virtually any media in pleated construction..
Conclusion about cartridge dust collector design
The users and designers of dust collectors have the possibility of virtually unlimited filter element life at negligible dust penetration. If the seals are intact and no dust penetration occurs to the clean side, cartridge filters can be restored to ‘as new’ condition.
More about … Dust Collection