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Filtration and Vacuum Specialists since 1976

A Scientific Review of Dust Collection - Part 8

Cartridge 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.

Table 8-1
Cartridge dust collectors were introduced around 1970's. They promised to give more power collection in a smaller space. To take advantage of the existing filters in the marketplace, manufacturers selected intake filters like those used on tractor trailer engines. Since these collectors were designed empirically, it was concluded that they would operate at low filtering velocities. It was not known at that time that the capacity of the self cleaning pulse jet collector was a function of the reverse air flow. The first major successful application came in applying them to powder coating booth vent applications.

For many years these operations were vented by shaker collectors, but as the booths were placed into continuous duty service, fabric pulse jets were also applied with mixed success. When the fabric pulse jet units were collecting very dense powder pigments, they operated at high pressure drops and consumed large amounts of compressed air. In fact, the high dust penetration into adjoining bags limited the recirculation of the exhaust air into the work areas. When the cartridge collectors were applied to venting powder paint booths, these problems disappeared

Inlet Load 1-2 Grains / Cubic Foot
Type of Collector
Outlet Dust Flow
Pulse Jet Fabric
0.0660 grains / cu. ft.
Shaker Collector
0.00035 grains / cu. ft.
Cartridge Collector
0.00005 grains / cu. ft.

Operating and Test Results

Soon it was found that these cartridge designs were more effective in many other applications where the conventional (old technology) pulse jet collectors were marginal. These included collecting products that had characteristics of high bulk densities or very fine particulate size fractions.

These new cartridge filters were able to maintain a filter cake that combines both a low pressure drop and higher filtration efficiency which was a large improvement over any other self-cleaning collector at that time. In 1978, the American Foundry Society conducted some tests on venting very dense and fine dust from a typical foundry operation and discovered some startling results as shown above in Table 8-1.

For comparison, the blow ring collector would have similar dust penetration to the adjoining bags with its felted media as the shaker collector.

Typically these cartridge filters were constructed as shown in Figure 8-1. They were constructed of rugged inner perforated or expanded metal cores and the end cap material was made from steel. A potting adhesive joined each end cap with the cellose media and inner and outer perforated cores. The cellulose media has filtering and pressure drop characteristics that are similar to the felted media in conventional pulse jet collectors. For many years, a spiral groove of hot melt adhesive was applied to the outside core of the cartridge which was developed for intake air filter cartridge used by the truck industry. The spiral groove of adhesive prevented the pleats from resonating as detrimental frequencies which are encountered in truck engine operation.

These filter elements were designed for truck service and consisted of corrugations 0.014 inch deep that were pressed into the media surface. Even though the pleats were very tightly spaced, the dirty air could penetrate to the bottom of the pleat. In this kind of intake truck filter service, the inlet loads were in the 0.0002 grains per cubic foot range, and these filters could operate for many months before any change was required. In this non-cleaning truck filter application, more media results in a longer lasting filter. Since the dust was very fine, it penetrated into the upper layer of the media and a smaller amount collected on the other media surfaces. Often the filters were manufactured with 16 pleats per inch and conventional filter media as shown in Figure 8-2.

Figure 8-1

However, there are some important differences between the operation of an intake filter on a truck engine and a dust collector.
  • Typically, in comparison, the dust loads in dust or powder collectors are 1-20 grains per cubic foot which is 5,000 to 100,000 times more than the engine intake filters.
  • The dust collector or powder collector relies on a filter cake that is on the surface of the media as well as within the media while an intake cartridge filter relies on the cake that is formed within the media.
  • The average operating pressure drop across the filter cake is two or three times larger on a typical dust collector as compared to the intake filter.
Figure 8-2

Some powders and dusts can operate with a wide range of dust filter cake thickness which significantly affects the cleaning frequency or the collection efficiency while other dusts can only operate at cake thickness of over 0.05 inches.

The pleated cartridge filter elements are suitable for applications where the dust filter cake can stabilize at cake thickness of less than 0.02 inches. Most very fine dust falls into this category. It is obvious that streams containing particles with average dimensions over 0.03 inches would not be suitable for pleated cartridge elements with narrow pleat spacing. Narrow pleat spacing is defined by more than 8 pleats per inch which is based on the inner diameter of the filter elements.

Initiation of the Cleaning Pulse Cycle

Even when the dust can be collected by using very thin filter cakes, thicker filter cakes can usually be tolerated. In cylindrical filter elements (not pleated such as filter bags), thicker filter cakes can save compressed air in many applications. In fact, it can be effective and is becoming popular on many coarse dusts to initiate the pulse cleaning cycle with a pressure switch (photolytic gage). If more dust can be ejected during each cleaning cycle, then the frequency of the cleaning cycles can be reduced. A common pressure switch setting is 3 1/2 inches water gage. The best way to set a pressure actuated cleaning system is to operate the collector at a setting of 1/4 inch water column above the initial pressure drop. This is accomplished by reducing the "off" time cleaning interval which will allow the pressure drop to stabilize.

This lower pulsing frequency has one other advantage in the operation of cylindrical bag collectors. For reasons beyond the scope of this discussion, the overwhelming dust penetration occurs during each cleaning cycle and the dust penetration into the adjoining filter media is directly related to the pulsing frequency. This relationship is not the same with the pleated filter elements. While pleated filter elements in pulse cleaning collectors can operate with pressure switch actuation, the pressure drop setting is normally limited by the pleat spacing. In fume dusts and welding operations, the dust loads approach the rate of intake truck filters and may require only in frequent cleaning such as one or two cleaning cycles per day.

Figure 8-3 demonstrates the effect of different pressure with actuation settings for a dust that can effectively filter at a wide range of pressure settings.

The initial pressure drop across the filter media is 0.1 inches water gage in the example that is being considered. If the dust bridges across the pleat, the cleaning air will not flow below the dust bridge. The cleaning air takes the path of least resistance and all the media below the bridge is rendered ineffective when the bridge forms. In the example in Figure 8-3, the dust load is assumed at 2 grains per cubic foot. The dust is mineral dust from a miming application. Starting from the right side of Figure 8=3, the pressure switch can be set to start at 1 1/2, 2 1/2 or 3 1/2 inches W.C.

The shaded portion between the pleats is the cleanable filter cake. The darkened section between the pleats is the location and size of the dust bridge. The results are taken from Figure 8-3 and tabulated in Table 8-2 below.

Pressure Switch Cleaning Cycle Time Typical Compressed Air Usage at 85 psig

1 1/2 inch w.c. 5 minutes 0.7 scfm per 1000 acfm filtered air
2 inch w.c. 5 minutes 0.6 scfm per 1000 acfm filtered air
2 1/2 inch w.c. 4 minutes 0.9 scfm per 1000 acfm filtered air
3 1/2 inch w.c. 3 minutes 1.2 scfm per 1000 acfm filtered air

Figure 8-3

In the above example, if the pleats are placed closer together, an increase in compressed air consumption would also be expected. Two effects must be considered. A deeper filter cake can allow more powder to be collected between each cleaning pulse, however, a deeper cake will form a higher dust bridge in the valley of the pleat. The result is that there is less filter media to be cleaned since the air travels the path of least resistance and tends to go around the dust bridge. In the table, the air consumption starts with larger air usage due to the very shallow cake and then the air consumption is reduced even though some bridging occurs. Soon the effect of the bridging becomes more pronounced and the cleaning frequency increases and the compressed air consumption also increases.

Cartridge Pulse Jet Cleaning

It is important to note that the cleaning configuration for a cartridge collector is identical to a fabric jet collector. Figure 8-4 shows a standard cartridge arrangement and Figure 8-5 illustrates a standard fabric bag arrangement, At "A", the compressed air orifice is energized and, as the air leaves the orifice, the flow of air drags or "induces" more clean air from the clean air plenum.

Figure's 8-4 and 8-5

The jet will grow with a cone angle form 14-16 degrees and the air flow will follow the Law of Conservation of Momentum. As it enters the opening in the top of the filter element, the filter media and the existing filter cake have enough resistance so that no more fan air can be drawn into the cleaning air jet. The air jet then compresses all of the fan air that is inside the filter element. The force of the air jet is transmitted through the opening at the top of the cartridge by compressing the column of fan air that is inside the filter. The pressure wave on the column of fan air beneath the jet travels toward the bottom of the filter element at the speed of sound since forces travel through any material solid, liquid or gas at the speed of sound. When this pressure wave reaches the bottom of the cylindrical column, t5he cleaning air jet displaces out the fan air that is inside the filter element and generates a positive pressure inside the filter element. This positive pressure causes a uniform outward velocity that ejects the agglomerated dust that rests on the outside surface of the filter media in the dirty air chamber of the collector. The dust is ejected perpendicularly from the surface of the media and it is propelled at the velocity of the cleaning air jet. The ejected dust does not strike the adjoining cartridges/filter cakes since the direction of the ejected dust is perpendicular to the media surface and towards an adjacent pleat that is also angles and ejecting dust. This pleated configuration eliminates the entrainment of dust from active cleaning cartridge rows to the passive filter rows. In cylindrical and envelop bag pulse collectors, the dust can get reentrained from the active cleaning row to the adjoining passive rows. In fact, the improvement in collection efficiency in Table 8-1 of the cartridge collector is explained over the other types of collectors by this phenomenon of lack of dust being transferred onto the neighboring cartridges during the cleaning process.

During each cleaning cycle, some dust bleeds through the filter media until the dust cake develops to its most effective configuration. In a short time period, the bleeding of dust through the media is not noticeable, but in a long time period, the dust on the clean side causes problems by imbedding itself into the clean side of the media during the cleaning pulse cycle. This causes a high pressure drop, increases the need to clean more frequently, and lowers the life of the filter element. To combat this tendency, some suppliers have provided pleated filter elements with surface treatments that minimize but do not eliminate the filter cake damage that is caused by this dust bleeding phenomenon.

Filter Mounting Arrangements

The original cartridge filter element pulse jet collectors had vertically mounted filter elements that were mounted in the dirty air section of the collectors. Changing the filter elements exposed the maintenance and operating personnel to the dust when the cartridges were replaced. Then collectors were introduced to the industry that allowed the replacement of filters from the outside of the collector. These types of collectors became popular because of the ease of cartridge changeover. The most common of these arrangements is the horizontal mounted filter collectors as shown in Figure 8-6. These pleated cartridge filter elements were arranged in vertical rows. Most often the cartridges were cleaned so that the top filter was cleaned first, then the one below it was cleaned next in sequence until the bottom one was cleaned.

The general steps for the mechanism of collection of dust and powders are as follows:

  • The fine dust collects on the surface of the filter media.
  • The fine dust agglomerates with other fine dust particles on the surface of the filter media.
  • The agglomerated dust is removed by the cleaning process and is collected in the hopper only if the agglomerated dust is large enough to fall into the hopper while it is being unaffected by the uplifting air currents.
Figure 8-6

In some collectors with horizontal cartridge filter elements where the cartridges are cleaned from the top row to the bottom row, some impediment can occur to prevent the cleaning process form being as effective as vertically mounted cartridge filters. When a cartridge is cleaned in the top row #1, the dust is propelled radially outward from the horizontal center of the cylindrical axis of cartridge. The ejected dust from the bottom of the cartridge filter in row #1 falls radially downward and towards the upward portion of the top of the cartridge filter in row # 2 which will tend to catch the dust. Soon these pleats on the top of filter # 2 can bridge and that bridged portion of the filter will be rendered affected by the cleaning of the cartridge that is directly above it. Figure 8-7 illustrates a configuration that will reduce or eliminate the dust deposits on the top of the filters in the lower rows by the use of horizontal deflectors that are placed at the inlet of the collector.

Figure 8-7

Internal Inlet Baffles

When the dust laden air enters through the inlet of a collector, it is channeled through an initial opening such as a rectangular or round inlet. Upstream from the inlet, the dusty air has traveled through various arrangements of elbows, transitions, or other abrupt obstructions. As a result, the dirty air enters the collector with existing turbulence and unbalanced air flows that reduce the efficiency and capacity of the collector. The goal inside the dirty air chamber is to provide an equal distribution of fan air to all of the cartridges, to promote a downward air flow with little or no updrafts, and to reduce the inlet velocity of the incoming air stream into the dirty air chamber.

Scientific Dust Collectors has addressed these issues and has a patented baffle arrangement as illustrated in Figure 8-7 above. In general, the process consists of the dirty air entering the inside of the collector as the inlet and encountering the baffle arrangement. The baffles cause the dust laden air to be divided into multiple flows in order to evenly redistribute the dirty air throughout the dirty air chamber and to promote a general downflow of process air. In face, some of the air flows are redirected enough to cause counter flows that hit into each other so that their initial velcoities are greatly reduced. This process tends to reduce the internal swirling around the walls of the collector by the process dirty air and to reduce the updrafts that can cause dust reentrainment on the cartridges.

Filter Ratio Considerations

It is important to note that when using a cartridge with a significant amount of media, a low face velocity in a cartridge will not be acceptable if the proper amount of cleaning air is not achieved. Likewise, a higher face velocity in a cartridge such as that obtained when using a cartridge with a minimal amount of media can still be effective, providing a good cleaning system si used and the correct amount of media is chosen for the particular application. In order to clean the filter element, the cleaning air-to-process air ratio should be between 4 and 6. Therefore, a cartridge filter rated fro 400 CFM of process air would require a cleaning flow between 1600 and 2400 CFM. Even at this high cleaning air flow rate, a portion of the cartridge remains uncleaned but collects some dust. In time, the active filter media becomes plugged and the previously uncleaned filter media becomes active.

As long as there is excessive dormant filter media, the operating pressure drop across the filter media will remain constant. Over time, when the active effective media area drops due to excessive plugging and when there is no more dormant filter media available to be converted into an active filter media, the pressure drop at the magnahelic pressure gage will start to rise. It is important to note that the cleaning frequency must be increased to stabilize the collector since there is less effective filter media available to clean the dust from the cartridge. Eventually, the pressure drop becomes too high to maintain the required fan air flow through the system and the filter cartridges must be remove and replaced. In a cartridge filter element, the uncleaned pleats collect dust and the dust quickly bridges across the pleats, which renders the bridged pleats useless. As the amount of dust builds in the uncleaned portion so the cartridge, the mechanical stresses are also increasing on the seals and mounting structure.

Cartridge Cleaning System Improvements by Scientific Dust Collectors

Scientific Dust Collectors' cartridge cleaning system optimizes the use of dry compressed air and nozzle technology. The orifice of the nozzle is sized to supple the necessary amount of compressed air, and the nozzle section efficiently increases the velocity of the compressed air from a standard orifice velocity of 1030 feet per minute to a nozzle velocity of approximately 1750 feet per minute. The extra velocity allows more induces air to be merged into Scientific Dust Collectors' cleaning air jet than the generic clean air jet which helps to clean more filter media. As the expanding cleaning air jet enlarges and travels outward from the exit of the nozzle, its shape remains conical and the approximate angle between the diverging elements is 14 degrees to 15 degrees. When the cleaning air jet action on the filter media is both robust yet gentle.

The ventrui / evase is located before the entrance into the filter media in the clean air plenum in order to maximize the total cleaning of the cartridge. In addition, it provides the structure to seal the fan air from leaving the cartridge when the cleaning air jet reaches the orifice of the venture / evase. Also, when the cleaning air jet is not flowing, the venturi / evase helps the fan air to exit the cartridge area with less turbulence and, therefore, at a lower loss in velocity pressure. In addition, to help provide the best opportunity to clean the cartridge and to not have the dust reentrained back onto the cartridges, the distance between the cartridges is generous and controlled to give an optimum downward flow rate.

It is an ongoing common and prevalent misunderstanding throughout the industry that the more media we can put into a cartridge, the better the system will function and the linger the cartridge will last. In face, this is totally and sole dependent upon how ell you can clean the cartridge. If the cleaning system is unable to remove the dust particles captured in the pleats, any original system gain in airflow and pressure drop is short lived and ineffective due to the plugging of the cartridges over time. It is not always in the end users best interest to install cartridges with the greatest amount of media. The correct amount and type of media used is really dependant upon the nature and size of the dust particles being filtered.

Proper Seal Design

Seals play an important role in separating air spaces and relative pressures such as:

  • Isolation of process air from the cleaned filter air.
  • Maintain separate differential working pressures such as isolating the outside atmospheric air from the clean air plenum and the dirty air chamber.

In many generic cartridge collectors, seal failure is frequently the cause of plugged cartridges, dusty effluent air, and overall loss of collection efficiency. Scientific Dust Collectors' patented cartridge door design has multiple positive advantages that ensure long lasting sealing service. Refer to Figure 8-8. The main advantage is the use of compression spring that automatically adjusts for seal deflection on both inner cover door as well as front and back cartridges. Each seal is generously sized and made from quality gasket material so that the seals do not crush with time. The door assembly consists of an inner door cover that seals the back cartridge opening, front outer door plate that seals the internal dusty air chamber from the outside atmospheric pressure environment, and washers to seal the outside locking door handle form the internal dirty air chamber. Installation and / or removal of the door assembly is easy since all of the interconnecting parts are attached to each other.

Figure 8-8

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