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Below please find the answers to the most Frequently Asked Questions (FAQ) about Donaldson Advanced Filter Media.
Industrial Filtration
Q1. What is ePTFE?
PTFE was invented in 1938 by DuPont chemist Dr. Roy Plunkett. Dr. Plunkett was searching for a refrigerant when he discovered a unique, new chemical named Polytetrafluoroethylene, or PTFE. It was soon learned that PTFE had unique properties resisting friction, temperature, chemicals, mechanical action and electrical charge.
In 1946 it was discovered that PTFE had great dielectric strength, or insulating properties against electrical charge, and PTFE is used to insulate aircraft circuitry against lightning strike damage. In 1949 PTFE's low coefficient of friction was tested. It was learned that as the load (or pressure) increases, PTFE's static friction decreases. A test at 1 to 4 kilograms (2.2 to 8.8 pounds) gave the coefficient of friction at 0.04.
PTFE belongs to the fluoropolymer family. Polymer means many parts, and these parts bond to form a "long chain" fluorocarbon. The chemical formula for the PTFE molecule is CF2, CF2 n ("n" means "repeated many times"). C represents one atom of carbon. F2 represents two atoms of fluorine. Fluorine has very low surface energy, giving PTFE its low coefficient of friction, and fluorine is chemically inert. Chemical bonds between atoms in the PTFE molecule play a critical role in PTFE properties because they provide powerful binding forces. The two forces bonding the PTFE molecular structure are the carbon bonds, which forms the backbone of the polymer chain, and the carbon to fluorine bond, which forms the shield. Both bonds are very strong and enhance PTFE's unique properties. Notice that the fluorine atoms are larger than the carbon atoms.
These powerful binding forces produce a PTFE molecule with a fluorine shield around the carbon backbone. As this graphic illustrates, the carbon backbone is completely surrounded and enclosed by large, chemically inert fluorine atoms, creating a protective shield or sheath.
Q2. What is a Needlefelt?
A non-woven material produced by combining layers (or batts) of loose fibres on a needle punch loom utilising barbed needles to interlock the fibres
Q3. What is a Scrim?
A lightweight woven base cloth used as a support for non-woven fabrics such as needlefelts
Q4. What are Epitropic® fibres?
A conductive carbon sheathed polyester fibre, which can be mixed with other fibres to reduce the electrical resistivity of the resultant material.
Epitropic® is a registered trade mark of JWH Group Ltd.
Q5. What is Hydrolysis?
A major cause of failure of filter media is through a chemical reaction known as Hydrolysis.
This is defined as a chemical process of decomposition that involves splitting of molecular bonds and the addition of elements of water. Hydrolysis only becomes active in the presence of elevated temperature, moisture and chemistry (Acidic or Alkaline)
Q6. What are Thermoplastic fibres?
Fibres that are under the influence of applied heat will melt and flow. These fibres include Polyester, Polypropylene and P.P.S
Q7. What support cage specifications does Donaldson suggest for use with Tetratex ePTFE media?
The following suggestions are intended to indicate minimum requirement guidelines to help optimise the performance of filter bags fabricated from Tetratex Filter Media.
These guidelines do not guarantee good performance, or preclude premature bag failure, which is often associated with the use or re-use of poor quality cages.
Materials of construction can be advised depending on gas conditions to be encountered in given applications.
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Standard Needlefelts
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Woven PTFE
Woven Glass
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Vertical wires spacing (mm) (Equally spaced, straight and parallel)
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40
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25
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Horizontal rings maximum spacing (mm)
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200
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150
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Vertical wire diameter (mm)
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3.0-3.2
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3.2-4.
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Horizontal ring wire diameter (mm)
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3.2-4.0
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4.0
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End cap diameter to be 2-6mm less than overall cage diameter, with vertical wires to be welded to wire ring inside end cap.
No sharp edges or burrs on vertical wires, horizontal rings, collars, end caps or welded joints
Q8. What is a Filter?
A Filter can be described as a device or material, which separates one entity from another. This process of separation in Industrial processes falls into four main categories
LIQUID from LIQUID separation
SOLID from LIQUID separation
SOLID from SOLID separation
SOLID from GAS separation
Q9. What is a Cyclone?
Relying on centrifugal forces to throw dust particles to the outside of a swirling air stream.
Q10. What is a Wet Scrubber?
Dust particles removed from an air stream by water jet sprays.
Q11. What is an Electrostatic Precipitator (E.S.P)?
Enhances the surface charge of dust particles in an air stream by passing the dust through a high voltage corona discharge, then passing the dust between plates carrying an opposite charge.
Q12. What is a Baghouse Filter?
Removes dust from an air stream by passing the dust laden stream through some kind of porous material.
Q13. What is a shaker cleaned Baghouse?
Mechanical shaking of the bags resulting in deflection and acceleration forces on the bags to remove the dust cake. Zero (or reverse) flow is normally combined with the shaking. The oldest cleaning method, but still in common use.
Dust collection surface can be inside or outside surface of the filter bag, normally off line cleaning.
Q14. What is a reverse air cleaned Baghouse?
A combination of deflection by inward collapse caused by cessation of the dust laden air stream and the introduction of airflow in the reverse direction by a dedicated fan is used to dislodge any dust cake from the bag surface. This cleaning method results in very low stresses on the bag fabric.
Dust collection surface is on the inside of the bag.
Q15. What is a pulsejet cleaned Baghouse?
During cleaning, a pulse of compressed air is directed into the bag (the reverse flow direction), inflating the bag to cause fabric / cake deflection and separation. Cleaning effectiveness and costs can be adjusted by changes to the pulse duration and interval, compressed air pressure and volume. An interval timer or differential pressure can activate the cleaning mode.
Dust Collection surface is on the outside of the bag.
Can be on line or off line cleaning.
Q16. What is Sonic cleaning?
If used, usually augments another cleaning method. Sonic energy is introduced into the baghouse by air powered horns, and the shock waves produced generate forces that tend to separate dust from the bags and the interior surfaces of the filter unit.
Q17. What is air to cloth (A/C) Ratio comparison?
Air to cloth ratio describes how much air passes through a given surface area of filter media in a given time. Described as: Cubic metre air per square metre fabric per minute M3 / M2 / MIN
Factors which will influence the choice of air / cloth ratio include:-
Filter Media – Cleaning Method – Dust Composition – Dust Density
Dust Loading – Clean Gas Residual Value Pressure Drop
Q18. Are there different types of filter bags?
Yes, Tubes, Envelopes and Multi Pockets.
Q19. Are there different types of baghouse filter systems?
Yes, Reverse Air filters, Pulse Jet filters and Shaker Filters.
Q20. What are Tubular Filter Bags?
The most widely used filter bag shape is tubular. These tubes can be circular, elliptical or square. Used in pulsejet, reverse air and shaker type filters.
Q21. What are Envelope Filter Bags?
These can be single or sub divided pockets. Used in pulse jet and reverse air type filters.
Q22. What Are Multi Pocket Filter Bags?
A series of pockets, either single or sub-divided, sewn together around a common flange. Normally used only in shaker type filters.
Q23. What are Reverse Air Filter bags?
The bags are supported externally by a series of metal ‘O’ rings with fabric over covers, to prevent inward collapse during the cleaning cycle. Most common diameters are between 20 and 30cms with lengths up to 11 meters.
The open ends of the bags are usually clamped around a thimble arrangement at the cell plate and the closed end is sewn flat with a fabric tail or sealed with a metal cap insert. The closed end is attached to a tension bar to prevent partial closure of the bags during operation.
Q24. What are Shaker Filter bags?
Tubular shaker type bags are similar to reverse air bags but without external support rings. Multi pocket bags are often internally supported to prevent internal collapse during operation.
Q25. What are Pulse Jet Filter bags?
A multi wire cage to prevent inward collapse during operation supports the bags internally. Most common diameters for tubular bags are between 10 & 15cm, and lengths up to 8 meters are in use. The open ends of the bags are usually sealed at the cell plate by a cuff arrangement containing a steel snap band, coiled spring, a metal ‘0’ ring or a square section felt ring. At the base, a fabric disc, with or without a reinforcement cuff, usually closes the bag.
Envelope filter bags can be up to 3 metres wide and 3 metres long.
Q26. I didn’t know that Donaldson manufactured filter Media. Isn’t Donaldson Company primarily a filter systems manufacturer?
Yes, but … Filter media is the heart of any filtration system. Donaldson maintains a leadership position in filtration technology research and development. In cases like nanofiber products and Tetratex® PTFE membranes, we have developed proprietary filter media for some of the world’s most demanding filtration applications. This filter media technology differentiates Donaldson’s filtration systems from others that use commodity filter media. We also sell our proprietary nanofiber media and Tetratex® PTFE membranes for other applications where Donaldson doesn’t provide complete filtration systems
The selection of a filter media for a given filtration application is determined in the main by the thermal and gas conditions in which the filtration will take place.
Important Factors to consider
- Temperature: Continuous and surge –timescale
- Adverse gas and chemical conditions: Ability of the filter media to resist degradation from advised levels of acids, alkalis, solvents and oxidising agents at the operational temperatures of the filtration application.
- Humidity: Resistance to Hydrolysis at operational temperatures and humidity levels
- Dimensional Stability: The Filter media must be expected to resist shrinkage and stretching in the advised operational conditions
Q27. Is advised cleaning pressure the same for glass fabrics and needlefelts?
No, advised maximum for glass fabrics is 3.5 – 4.0 bar and for needlefelts, 5 – 7 bar is acceptable, although the lower the cleaning pressure, the less the stress on the filter media leading to longer bag life possibility and reduced operating costs.
Q30. Why is a membrane superior to dust cake formation as a filter aid?
Building a dust cake on the surface of a filter bag in order to increase the fine particle collection efficiency of the fabric allows the ingress of some particulate into the filter media which increasingly reduces the fabric permeability, allows for particle migration and emission through the media and decreases the effectiveness of the cleaning mechanism.
Tetratex membrane filter media prevents particle ingress, preserves the integrity of the substrate, maintains reduced constant pressure drop, which allows for increased airflow at the same or reduced energy consumption.
Q31. Is Tetratex the same as other ePTFE membranes?
Tetratex ePTFE membranes are manufactured from 100% PTFE resin by a unique patented process of Donaldson Company Inc. and are marketed by Donaldson Filter Components Limited.
Q32. Are Tetratex membranes suitable for use at low temperatures?
Tetratex ePTFE membranes will withstand temperatures in the range of minus 273°C up to plus 287°C and are only restricted in use by the ability of any substrate employed to withstand temperatures and chemical conditions within this range.
Q33. What are the differences between Tetratex ePTFE membrane and a PTFE finish?
Tetratex ePTFE membrane is produced in roll form in a bi-axial construction with a controlled pore size structure and for industrial filtration uses can be permanently laminated to substrates by a bonding process.
A PTFE finish is commonly a liquid dispersion containing a percentage of solid PTFE particulate, which is applied to a substrate either by complete immersion or by sprayed surface coating. Depending on technique of application and finishing, some PTFE solids are retained on the substrate fibres or on the surface of the fabric. This is often claimed to enhance, in part, chemical attack resistance, dust cake release and particle collection efficiency, or a combination of all three.
Q34. Does the membrane affect the conductivity of Tetratex antistatic media?
Tests carried out by an independent and approved test house show that the electrical resistivity of conductive substrates when bonded to Tetratex e PTFE membranes, satisfy the requirements of the DIN 54345 test standards.
Q35. Is the membrane suitable to be used in foodstuff processes?
Tetratex ePTFE membranes are manufactured from 100% PTFE resin, a substance approved by The Food and Drugs Administration.
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