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, it improves dramatically. Typical results:
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 agglomerated 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 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.

A new concept Universal Application Dust collector was introduced in October 2008 by Ultra-Flow.
This is a great improvement on the 1978 designs and makes it easy to select advanced technology. This collector handles all applications generally reserved for either baghouse or cartridge dust collectors.

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.

Read more about ... Dust collection

DUST BEHAVIOR

There are several principles in the distribution of dust inside a pulse jet dust collector that are important to understand.

1) The cleaning frequency is related to the pressure drop across the filter media and filter cake by the square of the pressure drop. This is because the quantity of dust that the media can hold between cleanings is related to the pressure drop by the same relationship.
At a one inch average pressure drop the media can hold 16 times the amount of dust than it can hold at a 4 inch average pressure drop. Operating at an average pressure drop of four inches will require16 times the pulsing frequency to maintain equilibrium.
The dust penetration through a dust collector is related to this pulsing frequency, when there are no leaks and an efficient filter cake is formed. In the example above the penetration at 4 inch operating pressure drop is at least 16 times more than operating at one inch pressure drop.
Collectors should be operated at the minimum pulsing frequency to maintain equilibrium in pressure drops. Over cleaning wastes compressed air, lowers collection efficiency and shortens bag or cartridge life.

Pulsing Frequency is affected by the following:
Porosity of the dust. The higher the porosity the lower the cleaning frequency. Paper dust has low porosity and metallic fumes have high porosity.
Coarseness of the dust. Coarse dusts have lower cleaning frequency than the same load of fine dust.
Dust Load. If the dust load is doubled the pulsing frequency is doubled (Assuming identical particle size distributions)
Liquids. Liquid droplets in the exhaust stream or compressed air supply may radically affect filter cake characteristics. A hard crust may form, i.e. ciment dust.
Other Factors that affect pulsing frequency
• Permeability of the basic filter media
• Ineffective seal design at the dust wall
• Improper filter element installation. In snap band filters not seated properly and insufficient tension in clamped systems.
• Impingement of the cleaning jet against the internal surfaces of the cleaning element. The impingement can be induced by the shape of the bottom of the filter element. Concave bottoms are effective and convex shapes are not.

2) Distribution of dust in the filter compartments.
Fine dust behaves like gas in that it follows the path of least resistance. For long and narrow collectors with inlets on the narrow wall, the fine dust will be drawn to the bags closer to the inlet and there will be very little fine dust furthest from the inlet. Multiple inlets on the long side wall distribute the fine dust among the bags. In some cases of existing collectors, multiple timers are applied to the rows of bags closest to the inlet which are cleaned more often to distribute the flow and (filter wear) in the collector.
Coarse dust with high inertia acts like billiard balls independent of the exhaust gas flow in the collector. In a collector with pyramidal hoppers and the inlet entering the hopper, the high inertial dust will strike the hopper walls and ricochet up to the bottom of the filter elements and cause wear and premature failure of the filter elements. There are many clever designs of baffles to absorb the dust impact and deflect these high inertia particles toward the hopper outlet. These baffles are shaped or constructed to absorb or resist the impact of high inertia, sometimes abrasive dust particles. Some are made of hard abrasive resistant metals and others are coated with rubber to resist the wear. In some instances like in pneumatic conveyors and raw mills in the cement manufacturing process, the inlets are directed down and into the hoppers.

For more information on Dust Behavior ...  Dust Collector Selection Guide and Correspondence Course.

Moisture and Freezing in Pulse Jet Dust Collectors

The root of the problem comes from the fact that as you compress air the moisture holding capacity decreases. Compressors have after coolers in which most of the condensed water is removed before the compressed air enters the distribution system. The compressed air is usually a bit higher than the ambient temperature. As it flows from the compressor to the machines some additional water will condense. Usually any large droplets will be collected in the air line filters before they reach the machines. There is still some moisture but it does not affect the operation of most machines. On other machines where this remaining moisture is undesirable or harmful, dryers are installed between the machinery and the compressor. On compressed air powered pulse jet collectors, the presence of liquid moisture in the pneumatic lines can have serious effects:
1) The water can collect in the compressed air manifold. When sufficient water is collector, it may “squirt” into the filter elements during a cleaning cycle. The drenching of the filter elements is intermittent, but the long term effect is higher pressure drop, more frequent cleaning and premature filter element replacement. Often the filter will dry itself from the exhaust flow through the collector. But residual effects from this wet dry cycling are cumulative. Cellulose cartridge filter elements are especially vulnerable as each wet cycle cause the permeability to increase and harmful effects are much faster.
2) If a collector is installed outdoors in below freezing conditions, even very small amounts of moisture droplets can condense on the diaphragms of air valves. The diaphragms will then stick to the seats of the valves and will not close. This will discharge all the air from the system since typically 150 to 400 SCFM can be discharged through the valve. Since these valves operate with internal pilot ports, the valve will not close until the supply pressure reaches 25 psig. There needs to be an external shutoff to get the collector (and sometimes associated compressed air supply) back to an operating mode.

QAM has a two different products to address these problems:
A) Manifold Tank Automatic Drain Valve System
B) Thermostatically control compressed air manifold heater. This system will allow the collector to operate even when the compressed air dryer is malfunctioning by turning any liquid moisture to water vapor.

QAM also provides dust collector retrofit and consulting services to help resolve common problems that no one else seems to have any solutions.