Design and Testing of Antistatic Filters

DESIGN AND TESTING OF ANTISTATICFILTERS FOR INDUSTRIAL AIR

FILTRATION

by

Thomas C. SavageRobert C. Carlozzi

Ole PetzoldtW. L. Gore & Associates

First Presented at Powder & Bulk SolidsConference and Exhibition

Donald E. Stephens Convention CenterRosemont, Illinois, USA

May 10, 2001

DESIGN AND TESTING OF ANTISTATIC FILTERSFOR INDUSTRIAL AIR FILTRATIONBy Thomas C. Savage, Robert C. Carlozzi, Ole Petzoldt

DESIGN AND TESTING OF ANTISTATIC FILTERSFOR INDUSTRIAL AIR FILTRATION

by

Thomas C. SavageRobert C. Carlozzi

Ole PetzoldtW. L. Gore & Associates

INTRODUCTIONIn many industrial dust collection systems,the filtration and electrical properties of thefilter media play an important role in theperformance and safety of the system. If thedust being collected is fine, difficult tocapture, or has a high intrinsic value, thefiltration effectiveness of the media isimportant. Furthermore, during processing,electric charges can develop. If the filtermedia cannot dissipate the electric charge,there is a chance of unwanted build-up andelectrostatic discharge (ESD). ESD can leadto explosions, fires, or personal injury fromshock.

This paper will examine the filtration andelectrical properties of both conventional(non-membrane) filters and filters usingexpanded polytetrafluoroethylene (ePTFE)membrane as the filtration surface. Allmaterials discussed are polyester nonwovenneedlefelts. Recent advances in membraneperformance will be discussed. Test resultsfrom the National Fire ProtectionAssociation-99 (NFPA-99) static-dissipationtest method and the Deutsches Institut fuerNormung 54345 Part 5 (DIN 543455)electrical conductivity test method will be

presented. While there are a number ofdifferent test methods available, these areamong the most commonly referenced inindustrial filtration.

ANTISTATIC FILTERS IN PULSE JETBAGHOUSES FOR INDUSTRIALFILTRATIONDuring manufacturing and conveying, fineparticulates can collect electrical charges asthey impact other particles and ductwork. Ifthese charges are not “drained” from thesystem, they can build up to such a level thata spark or ESD occurs. Under certainconditions, this could lead to an explosion orfire in the baghouse. In the paper entitled“Brace Your Plant Against a DustExplosion,”1 Thomas Godbey describesthese conditions as “…a combustible dust,air, or another oxidant, and a source ofignition.”

In baghouse applications with fineparticulates, steps should be taken to reduceor eliminate the buildup of electricalcharges. As discussed in the literature,2, 3

there are generally two approaches whenusing fabric filtration media: 1) the use ofantistatic agents, and 2) the use of specialantistatic or conductive fibers. These


antistatic fibers are added to a base materialin a low percentage (2-6%) and form anintricate network through the cross-sectionof a filter. When properly dispersed, thesefibers serve to conduct the electrical charge.As shown in Figure 1, the fibers areintricately mixed with the base fibers (e.g.,polyester) during the felting operation. Inall cases, it is important that filters of thistype use a groundwire. When properlyinstalled, this groundwire serves to groundthe cage to the tubesheet.

Figure 1: Top view of polyester felt with conductive

fibers (black)

ELECTRICAL PROPERTIESWhen testing the electrical properties of afilter media, there are two common testmethods. The test method used in theUnited States is the INDA Standard Test forElectrostatic Decay (IST 40.2-92).4 Thisstandard references the NFPA, Code 99,Chapter 12 using Method 4046 of FederalTest Method Standard 101C and measuresthe static dissipative properties of a material.In Germany, the DIN 54345 Part 55 methodis preferred. This test is used to measure theelectrical resistance of a particular material.

Both test methods identify materials that areelectrically conducting and those that areelectrically insulating.

Static Dissipation: NFPA-99 is used tomeasure the capability of a material to drainan induced electrical charge by measuringthe length of time (seconds) it takes for a5,000 volt charge to drain to 500 volts. Inorder to pass the test, a material must drainthe charge in less than 0.5 seconds. It isbelieved that this test closely duplicates theconditions in a baghouse environment. Inthese cases, it is difficult to prevent thecreation of electrostatic charges. There isalso a need for quick dissipation of anycharge.

Electrical Resistance: In Germany, the useof the DIN 54345 Part 5 is more common.In this test method, the electrical resistanceof a material is measured. A material passesthe test if the measured resistance is lowerthan 1 x 108Ω. This test determines theelectrical resistance of materials, includingthose utilizing epitropic fibers. Unlike theNFPA test method, the DIN test is based onthe assumption that a material’s electricalresistance is the important criteria inapplications subject to the development ofstatic charges.

It is not the purpose of this paper todetermine which test method is preferablefor baghouse applications. The goal is tomake the reader aware of the differentstandards being used globally so aninformed decision can be made as to theapplicability of the tests.

FILTRATION PROPERTIESThere are generally two types of filtrationseen in industrial baghouses:1) conventional filtration (depth) and2) membrane filtration (surface). Thefiltration efficiency of conventional felts


depends on the intricate entanglement of thefibers, and the filtration effectiveness of thesubsequent dust cake layer that builds up onthe surface. Optimum filtering occurs onlyafter sufficient dust builds up on the filter’ssurface.

Because ePTFE membrane is a continuouslayer on the filtration surface, it does notrequire the development of a surface dustlayer to achieve maximum effectiveness.Figure 2 shows the difference between thetwo types of filter media at differentmagnifications. The photos on the left showthe ePTFE membrane surface. Themembrane provides a continuous filtrationsurface which captures the dust particles.With conventional filtration, there is nocontinuous filtration surface. Filtrationefficiency depends on the intricateentanglement of the fibers. The dustparticles trapped within the felt structure areseen. Over time, the dust will build up,resulting in higher pressure drops andparticulate emissions.

Figure 2: Membrane vs. conventional filtration

The ePTFE membrane sample achievesmaximum filtration efficiency immediately.The membrane is a continuous filtrationlayer and the backing material is only usedas a support structure. Figure 3 is a cross-sectional view of an ePTFE membranelaminated to felt backer.

Figure 3: Cross-section of filtration media showingthe ePTFE membrane and backing.

The efficiency of all industrial filters can bereduced by damage during operation. In aconventional bag, the efficiency may bereduced as a result of mechanical stretchingof the felt (especially on scrimless felts).

There can be a decrease in efficiency of amembrane filter as well. This damage isusually the result of mechanical wear duringthe cleaning cycle. Recent advances inmembrane technology at W. L. Gore &Associates, Inc. have minimized thisdamage. A new series of patented6

membranes have been engineered to bestronger, while still providing the same orhigher airflow. Figure 4 shows themembrane permeability vs. strength ofseveral membrane groups. Series Arepresents some of the earliest membranesdeveloped for industrial filtration, whileseries D shows some of the newestmembranes. The new membranes havemore than double the strength of the oldestmembranes at the same permeability.


Figure 4: Comparison of permeability vs.strength for ePTFE membranes

In use, these new membranes resist damagebetter then earlier membranes. As can beseen in Figure 5, “cracks” are visible andprovide a path for particulate emissions.Damage is usually seen on or near thevertical cage wires where bag movementduring cleaning is highest.

Figure 5: Photographs showing membrane cracking(16 months service)

Figure 6 shows the backside of two usedfilters. Both have an ePTFE membrane asthe filtration surface. The filter on the rightuses the new membrane. In the photo, thedark spots represent areas where dust haspenetrated the felt due to membrane cracks.The sample on the left has significantlyfewer spots. These samples had been inservice for 9 months. While the amount ofdamage will depend on baghouse operatingconditions, it is expected that these newmembranes will result in a more durablyefficient filter.

Standard Membrane New Membrane

Figure 6: Photographs of the backside of used filters (9 months service)

ELECTRICAL PROPERTY TESTINGResults: Four different felt materials wereevaluated for electrical properties. Thesesamples included: 1) 14 oz/yd2 polyester feltwith carbon-filled ePTFE fibers with ePTFEmembrane, 2) conventional 14 oz/yd2

polyester felt with carbon-filled ePTFEfibers, 3) conventional 16 oz/yd2 polyesterfelt with epitropic polyester fibers, and 4)conventional 16 oz/yd2 polyester felt withstainless steel fibers.

Each of these materials (except #2) iscommercially available and is used forindustrial filtration applications. Sample 2is not commercially available without amembrane. It was tested to evaluate theeffect of the ePTFE membrane on bothelectrical and filtration performance.

NFPA: This test was conducted at anindependent laboratory in Canada. AsFigure 7 shows, all materials passed with adecay time of 0.01 seconds. Interestingly,even though ePTFE is considered to be aninsulator, there does not appear to be adetrimental affect from its use.Additionally, regardless of the antistaticfiber used, all materials passed the test.

Membrane Permeability vs. Strength

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

Strength (psi)

Perm

eabi

lity

(cfm

/ft2 @ 0

.5"w

.g.)

A SeriesB SeriesC SeriesD Series


Figure 7: Static decay time comparison (seconds)

When an ePTFE membrane is used, the useof either the carbon-filled ePTFE fibers orepitropic polyester fibers is preferred overstainless steel. It is believed that, due totheir high stiffness, the stainless steel fiberscan cause damage to the membrane. Thiswould detrimentally affect the filter’sefficiency. An advantage of the ePTFE fiberis their high chemical resistance and thermalproperties. These fibers can be used up to260°C and can be combined with anyfiltration fiber, including PTFE. Theepitropic polyester fibers can only beeffectively used in polyester fiber-basedmaterials.

DIN 54345 Part 5: An independent testinglaboratory in Germany tested samples of thesame materials used in the NFPA test. Thetest results are shown below in Figure 8.

Figure 8: DIN test results (DIN 54345 Part 5)

The results for the ePTFE membrane sampleindicate that although ePTFE is an insulator,it does not significantly affect the ability ofthe filter to pass the test. This is likely the

result of the fact that the membrane is thinand has a good bond to the conductive fiberswithin the backing material.

While not tested for this paper, W. L. Gore& Associates, Inc. has recentlycommercialized filter media7 thatincorporates a patented conductivemembrane. This membrane uses specialconductive carbon particles to enhance itselectrical properties. Filtration productsbased on these membranes will be discussedin upcoming papers.

The DIN testing results show an expected,but interesting, phenomenon. The electricalresistance changes depending on the originalorientation of the test sample. The reason isthat during the felting process, the fibers arepreferentially oriented in one direction. Thisis not an indication of a problem in felting.It is characteristic of felted materials.

EFFICIENCY TESTINGResults: All comparative efficiency testingwas done on new samples (tests wereconducted at W. L. Gore & Associates,Inc.). These efficiencies were measuredusing a TSI, Inc., AFT 8160 AutomatedFilter Tester. As shown in Figure 9, initialefficiency of the non-membrane materials issignificantly lower than the sample withePTFE membrane. Conventional filters donot reach maximum filtration efficiencyuntil both the primary cake (within the felt)and the secondary cake (on the surface) havedeveloped.

Efficiency must be considered in two ways:1) initial efficiency, and 2) long-termefficiency. In industrial filtration

Electrical Resistance (Ω)Specimen(All are polyester felt) Machine Direction Cross-Machine

Direction

Electrode Spacing(mm)

Epitropic PolyesterFibers

1.2 x 107 5.4 x 106 30

Stainless SteelFibers

< 1 x 104 < 1 x 104 30

Carbon-FilledePTFE Fibers

3.1 x 107 1.4 x 106 30

Carbon-FilledePTFE Fibers andePTFE Membrane

1.2 x 107 1.2 x 106 30

Filtration Side(average)

Back Side(average)

Specimen(All are polyester felt)

+5kV -5kV +5kV -5kVEpitropic Polyester Fibers 0.01 0.01 0.01 0.01

Stainless Steel Fibers 0.01 0.01 0.01 0.01Carbon-Filled ePTFE

Fibers0.01 0.01 0.01 0.01

Carbon-Filled ePTFEFibers and ePTFE

Membrane0.01 0.01 0.01 0.01


applications, efficiency changes over timedue to seasoning effects and filter durability.

ParticleSize(µm)

ConventionalPolyester

Felt with SSfibers

ConventionalPolyesterFelt withEpitropicPolyesterFibers

ConventionalPolyesterFelt withCarbon-

Filled ePTFEFibers

Polyester Feltwith Carbon-Filled ePTFEFibers andePTFEMembrane

0.03 73.60 71.53 74.11 97.920.04 66.65 64.18 66.86 96.980.05 58.81 57.02 59.18 96.020.07 50.88 48.21 49.60 94.740.1 40.30 39.02 41.41 94.550.15 32.32 29.84 32.90 95.290.2 23.92 22.86 25.70 96.430.3 18.87 19.53 20.41 98.12

Figure 9: Filtration efficiency (count basis, facevelocity = 5.0 cm/sec): comparison ofantistatic polyester felts.

CONCLUSIONAs dry chemical producers continue todevelop finer products, the need for highefficiency, static dissipating filters increases.The demands placed on filters require higherperformance materials providing longservice life. The data presented shows thatchemical producers have a number of viablefiltration products from which to choose.

The chemical producer should consider afilter media’s:

1) electrical properties2) filtration properties

Prior to selecting a filter for yourapplication, all relevant regulatory andinsurance requirements should beconsidered.

Today’s filtration products companies areassisting chemical producers in meetingtheir filtration needs. The benefits of thesefilters include improved filtration efficiencyand static electricity dissipation to minimizethe chance of electrostatic discharge.

REFERENCES1. “Brace Your Plant Against a Dust

Explosion,” Chemical Engineering,Thomas Godbey, Sr., April 1999,pp.130-138.

2. “Conductive Fabrics in Dust Collectors,”Staub Reinhaltung der Luft 53, LutzBergmann, 1993, pp.307-309.

3. Kirk-Othman Encyclopedia of ChemicalTechnology, Vol. 3, Antistatic Agents,Seev Gur-Arieh, pp. 149-183.

4. “Electrostatic Decay,” IST 40.2-92,INDA Standard Test Methods,Association of Nonwoven FabricsIndustry, 1993, pp. 77-82.

5. DIN 54345 Part 5, “Testing of Textiles:Electrostatic Behaviour; Determinationof Electrostatic Charge of TextileFabric,” Deutsches Institut fuerNormung, translated by BritishStandards Institutions.

6. “Strong, Air Permeable Membranes ofPTFE,” US Patent No. 5,814,405, issued9/29/98, Branca et al.

7. “Conductive Filter Laminate,” US PatentNo. 5,527,569, issued 6/18/96, Hobsonet al.

Copyright © 2001 W. L. Gore & Associates

Design and Testing of Antistatic Filters

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