Reverse Pulse Cleaning Systems are an important piece of pneumatic conveying systems because in a typical pneumatic conveying system, the conveying air is separated in the filter separator, and the filters are cleaned by a jet of compressed air in a reverse direction.
The short-duration pulse jet of compressed air helps to dislodge the fine particles attached to the filters.
The main components are:
- Compressed air reservoir which holds the amount of air required for the pulsing
- Pulse Valve with a solenoid valve which opens up to inject compressed air into the filters in reverse direction
- A timer in PLC to control the frequency of pulsing
Air Material Separators
Air Material Separators (AMS) associated with pneumatic conveying systems have two functions. The ﬁrst is to recover as much as possible of the conveyed material and the second is to minimize pollution. The design and operation of this equipment are mainly dependent on particle size and product bulk density of the product being conveyed.
How Does a Bag Filter Work?
In pneumatic conveying systems handling ﬁne or dusty material, the method of ﬁltration that has become almost universally adopted is a bag or cartridge type fabric ﬁlter, either used on its own or as a secondary or one or more cyclone separators.
They may have application as bin vents in situations where all the product to be collected is blown into a hopper, and the clean air is vented off at the top through the ﬁlter unit. In specific instances, where the powder conveyed is a fugitive dust eg. ‘Central vacuum cleaning’ the separators are called dust collectors.
What is Pulse Cleaning?
Cleaning System: Any method used by the equipment to dislodge accumulated product cake from the filter media is its cleaning system.
Reverse Pulse Cleaning by compressed air is predominantly used in almost all the filter receivers or dust collectors in operation today.
The other cleaning methods such as bag shaking, and reverse air cleaning are rarely used now.
Regardless of the style of cleaning, it is always imperative that this system function properly. Without an effective cleaning system, the product will continue to build on the filters.
The resultant will be an increased pressure drop and reduced volume of conveying air at the pick-up points. Further, airstream velocities within the ductwork will decrease and cause a drop-out of dust in the ducts.
This may choke the entire system and render it ineffective.
How does a reverse jet baghouse work?
In reverse-jet baghouses, individual bags are supported by a metal cage, which is fastened onto a cell plate at the top of the bag filter. Dirty gas enters from the bottom of the baghouse and flows from outside to inside the bags. The metal cage prevents the collapse of the bag.
Filter Concept created a video as seen below that demonstrates the process:
Cartridge collectors are another commonly used type of air material separator.
Unlike baghouse collectors, in which the filtering media is woven or felt bags, this type of collector employs perforated metal cartridges that contain a pleated, nonwoven filtering media.
Due to its pleated design, the total filtering surface area is greater than in a conventional bag of the same diameter, resulting in reduced air to media ratio, pressure drop, and overall collector size.
Modern media manufacturing and cartridge production processes have meant that polyester-based media are now a very cost-effective solution offering high strength, excellent efficiency (for example 100% down to 2 µm), and superior product release characteristics.
Interestingly, cartridges can also be effectively cleaned provided careful guidelines are adhered to. This dispels the myth that cartridges are one-time-use filters.
Over the last decade, there have been continuous advances in the development of standard and also more exotic cartridge media.
For finer or more cohesive products, PTFE membranes can be applied to the base polyester offering much greater efficiencies (100% down to 0.5 µm) and even better product release characteristics.
What is a reverse pulse jet?
Most cartridge-type solutions utilize reverse jet technology. The technique is relatively simple; a jet of high-pressure air is injected into the filter element against the direction of process airflow.
This allows particulates that have built upon the surface of the filter media to be dislodged, allowing it to fall back into the process (or in the case of a dust collector into a collection bin).
Both bag and cartridge reverse jet systems employ very much the same hardware for this operation; a compressed air manifold/reservoir, automatic diaphragm cleaning valve operated by (in the most part) electro-pneumatic and a jet tube to direct cleaning air into the core of the filter elements.
The methods of directing air into the filter element are also similar; some use a simple hole (or series of holes) drilled into the jet tube to direct air into the element, others use a venturi in order to ‘induce’ additional air into the element to improve cleaning.
How do you clean a jet dust collector?
Filters are cleaned by a short burst of compressed air injected through a common manifold over a row of bags. The compressed air is accelerated by a venturi nozzle mounted at the top of the bag.
Since the duration of the compressed-air burst is short (0.1s), it acts as a rapidly moving air bubble, traveling through the entire length of the bag and causing the bag surfaces to flex.
This flexing of the bags breaks the dust cake, and the dislodged dust falls into a storage hopper below.
Reverse-jet dust collectors can be operated continuously and cleaned without interruption of flow because the burst of compressed air is very small compared with the total volume of dusty air through the collector.
Because of this continuous-cleaning feature, reverse-jet dust collectors are usually not compartmentalized.
The short cleaning cycle of reverse-jet collectors reduces recirculation and redeposit of dust. These collectors provide more complete cleaning and reconditioning of bags than shaker or reverse-air cleaning methods. Also, the continuous-cleaning feature allows them to operate at higher air-to-cloth ratios, so the space requirements are lower.
However, differences occur because bag type media rely on ‘depth’ type filtration which allows dust particulates to be trapped in the fibers of the filter fabric, and during the initial startup times, a cake is formed in the surface which later itself acts as a filter media.
Cartridge filters, on the other hand, utilize ‘surface’ filtration which does not allow particles to penetrate the media.
Terminology Used in Reverse Pulse Cleaning Operation
- Dirty Air Plenum: dirty side of media
- Clean Air Plenum: clean side (clean air to atmosphere or fan)
- Tube sheet: A metal plate that separates the clean side from the dirty side and holds the filters and has holes for the air to pass from the dirty side to the clean side (through the filter media).
- Hopper: collects the discharged dust
- Pulse Valve: Special diaphragm valve to introduce compressed air in reverse direction (opposite to airflow)
- Pulse Timer: Microprocessor-based timer to control ON/OFF times of pulsing.
- Top Load Filters: Filters are accessed from the top of the unit, requiring clearance at the top to remove filters.
- Bottom Load Filters: Filters are accessed from the side of the unit, requiring a side access door.
- Sequential Pulsing: To reverse pulse clean sequentially from one row of filters to the next row.
- Air to Cloth Ratio: Ratio of Air Flow (CFM) to Filter area. (This number is also the air velocity through the filter media in feet per minute (FPM). Dependent on product bulk density, particle size
- Can Velocity: Airflow velocity through the Receiver body (Airflow (CFM) divided by the free cross-sectional area of the filter receiver (FPM)
- Air Header/Pulse Bottles: Units to hold Compressed air surge/buffer to retain pressure between pulses.
- Off-time Pulsing: Use Reverse pulse only during the discharge cycle of a batch conveying system
Design/Recommended Operational Guidelines
- Keep the Can Velocity 180-250 FPM for product bulk density 30 lb./ft3.
- Air to Cloth ratio, keep higher for higher particle density and lower for lower particle density. General range 5-10 for bag filters and 2-5 for Cartridge filters
- Pulse Timer Settings:
- Do not pulse two adjacent rows at the same time.
- Properly adjust on and off times for efficient cleaning.
- Minimize on time to clean each filter, depending on the number of filters per blowpipe and filter length.
- Maximize off time based on differential pressure. (The biggest cause of filter wear is pulse cleaning.)
Application of Reverse Pulse Industries
Each Industry has specific requirements. For a conveying system in a food/dairy plant handling Sugar/Coffee beans or grounds, Maize Starch, Wheat Flour. Salt, dextrose, etc., care should be taken to see that the product quality is maintained, and external contamination is to be avoided at all costs.
Hence the compressed air used in the reverse pulse process should be oil and moisture-free.
This applies even in other industrial applications handling chemicals and plastics such as Soda Ash, Zinc Oxide, Fly Ash, Titanium Dioxide, Calcium Carbonate, Plastic powder/Pellet, where any moisture in compressed air will cause product degradation and material plugging.
The operation and maintenance of Reverse Air Filters require an eye for detail and meticulous care. Pneu-Con has many years of experience providing customers with the exact filter to meet their needs.
Contact us today to learn more about how we can help you.