baghouse applications - astec inc

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BAGHOUSE APPLICATIONS by Malcolm Swanson P.E. Technical Paper T-139 T-139 BAGHOUSE APPLICATIONS

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Page 1: BAGHOUSE APPLICATIONS - Astec Inc

BAGHOUSE APPLICATIONSby Malcolm Swanson P.E.

Technical Paper T-139

T-139 BAGHOUSE APPLICATIONS

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ASTEC encourages its engineers and executives to author articles that will be of valueto members of the hot mix asphalt (HMA) industry. The company also sponsors inde-pendent research when appropriate and has coordinated joint authorship betweenindustry competitors. Information is disbursed to any interested party in the form oftechnical papers. The purpose of the technical papers is to make information availablewithin the HMA industry in order to contribute to the continued improvement processthat will benefit the industry.

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CONTENTSINTRODUCTION.................................................................................... 3TECHNOLOGY ...................................................................................... 4DESIGN ................................................................................................. 5MATERIALS ........................................................................................... 7PRE-CLEANERS ................................................................................. 10CLEANING SYSTEMS......................................................................... 13DUST REMOVAL SYSTEMS............................................................... 16OPERATION ........................................................................................ 17MAINTENANCE ................................................................................... 19PROBLEM CAUSES AND SOLUTIONS ............................................. 20TESTING.............................................................................................. 24

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INTRODUCTIONThe hot mix asphalt industry first faced particulate emissions codes in the early

seventies. The initial requirements of 0.04 grain/dscf and maximum opacity of 20percent, although routine now, were very difficult to meet at the time. Baghousesbecame the industry’s most effective answer. Today, after more than 25 yearsof refinement, baghouses are capable of much better performance than the 0.04grain/dscf standard. With the Clean Air Act Amendment of 1990 came a shift ofemphasis, on the part of the Environmental Protection Agency, from regulationof emissions to regulation of ambient air quality. States are now responsible formeeting the federally established ambient air quality standards and must often setemission limits lower than 0.04 grain/dscf in order to comply. An area which is outof compliance with ambient air quality standards is known as a non-attainmentarea. Within a non-attainment area, emission limits may be set at 0.02 grain/dscfor less. Baghouses now routinely meet these requirements without any specialprovisions. PM-10 emission control is another challenge baghouses are handlingeffectively. PM-10 refers to those particles of less than 10 microns diameter.They are of particular concern be-cause the human respiratory systemcannot filter them out. When largeamounts of PM-10 are present in theexhaust gases a baghouse has toclean, special attention must be givento ensure adequate performance.

In most cases, baghouses are theequipment of choice for particulateemissions control of hot mix asphaltplants (Figure 1). Baghouses are cer-tainly not the only solution to the par-ticulate emissions problem but areusually more cost effective and op-erationally friendly than the other op-tions. However, to utilize baghousesas effectively as possible, operatorsneed to be familiar with their specificattributes and how to use those at-tributes to maximum advantage.

One reason baghouses have becomethe air cleaning system of choice in the HMA industry it that, while providingcompliance with pollution codes, they provide economic advantages over scrub-bers. By returning dust to the mix instead of wasting it, as scrubbers do, baghousesbetter utilize the aggregate. Baghouses also require less horsepower than ven-turi scrubbers. The flexibility of not being tied to a source of water and a settlingpond is a further significant advantage because of the resulting savings in truckingcosts and increased portability of the facility.

The limitations of baghouses are seen primarily in terms of operating constraints.For instance, aramid fiber bags such as Nomex are limited to a maximum servicetemperature of 400° F. While scrubbers have no such limitations, high exhausttemperatures are not consistent with efficient plant operation anyway. Baghousesgenerally should be operated above 220°F to avoid condensation, another poten-tial problem. Moisture condensation can cause accumulation of mud on bags andbaghouse walls. This results in blinded bags and clogged dust removal equip-

HOT MIX FACILITY BAGHOUSE F1

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ment. (The term “blinded” refers to acondition of reduced bag permeabil-ity. Bags with permeability of 2 cfm orless are considered blinded.) Con-tamination of bags by condensed hy-drocarbon vapors can also causeblinding of the bags. Fuel vapors or,in parallel flow systems, asphalt va-pors can be the source of such prob-lems. Hydrocarbon contaminationcan also cause a fire in the baghousewith the subsequent loss of bags andbaghouse. Baghouses are not use-ful for removing gaseous pollutantsfrom the plant exhaust.

TECHNOLOGYA typical baghouse for an HMA op-

eration consists of a fabric filter sys-tem enclosed by a steel structure (Fig-ure 1). The basic technology of a bag-house is very simple. The exhauststream passes through the fabric fil-ter before entering the atmosphere.Series of bags make up the fabric fil-ter (Figure 2). The exhaust streamenters the bags through their felt walls.Dust does not pass through the feltwalls and accumulates on the out-side of the bags. As the dust accu-mulation increases, periodic clean-ing of the bags becomes necessary.(Although there are several types ofcleaning systems, the pulse jet sys-tem is most common in the HMA in-dustry.) The dust accumulation on thebags is referred to as the “dust cake”and is of critical importance in the per-formance of the baghouse (Figure 3).The dust cake is actually the work-ing filter, since the textile felt of thebags, without a dust cake, can onlycollect relatively large particles. Abaghouse with a good dust cake cancollect particles as small as 1.0 mi-cron with 99.99% overall efficiencyand can even collect some submi-cron particles.

Relatively large particles, usuallyabout 200 mesh max., collect at thesurface of the felt and stack up there.Smaller particles pass through the feltuntil this happens. As particles stackup at the outer surface of the felt, the

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EXHAUST FLOW THROUGH A BAGHOUSE F2

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FILTERING FUNCTION OF DUST CAKE F3

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FELT WITHOUT A DUST CAKE FILTERS POORLY F4

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effective opening size decreasesand smaller and smaller particles arecaptured. Even though the dust cakeis normally less that 1/16 in. in thick-ness, this represents a lot of stack-ing up of particles. It takes 14.5particles of the 200 mesh size tostack up to 1/16 in. But, it takes anaverage of 1,587.5 particles of the 1micron size to accumulate to a 1/16in. cake thickness. Since the dustcake is always composed of a mix-ture of particle sizes, it is obviousthat there are many layers of par-ticles in the cake (smaller particlesbeing stacked upon larger ones).Baghouse bags without a properdust cake will pass the smaller par-ticles and, therefore, fail to meet therequirements of emission regula-tions (Figure 4).

The bags are supported by wirecages, inserted into each bag fromthe top, that prevent the bags fromcollapsing under pressure (Figure 5).The open wire construction of thecages allows air to pass through eas-ily while providing internal support tothe bags. The bag and cage assem-bly is supported by a tube sheet (Fig-ure 6). The tube sheet separates thedirty and clean air plenums so that theonly way for air to enter the clean airplenum is through the bags.

Particles much less than one micronin diameter are usually not effectivelycollected by the filter media used inHMA operations. Therefore, smoke,which is composed of particles on theorder of 0.3 microns in diameter, willnormally not be collected in a baghouse(Figure 7).

DESIGNHMA plant operating and mainte-

nance personnel do not have to bebaghouse designers. However, it canbe very helpful to understand some ofthe principles used in baghouse de-sign. Many factors must be consid-ered by design engineers, and anunderstanding of two of these may beparticularly helpful to the contractor’spersonnel. These two factors are air

WIRE CAGES SUPPORT THE BAGS F5

EMPTY TUBESHEET F6

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SMOKE PASSES THROUGH FILTER BAGS F7

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to cloth ratio and can velocity. Air tocloth ratio is gas velocity through thebag fabric. Can velocity is the upwardgas velocity between bags measuredat the bottom of the bags.

Although air to cloth ratio and canvelocity are certainly related, address-ing them separately in the design pro-cess is simple and effective. Depend-ing primarily upon dust particle size,a good choice of air to cloth ratio isusually in the range of 4 : 1 to 6 : 1(Figure 8). A good standard air tocloth ratio for HMA plants is 5.5 : 1.With typical aggregate dusts, 5.5 : 1(5.5 fpm through one square foot offelt) rarely results in stack particulateemissions greater than 0.02 grains/dscf and often results in emission lev-els below 0.01 grains/dscf. The na-tional standard for particulate emis-sions is 0.04 grains/dscf.

A very fine dust, for example a dustwith more than 80% of its particles bycount being smaller than one micron,would require a lower air to cloth ratiothan 5.5 : 1. As a gentle breeze willbarely move light dust but a tornadocan move boulders, high baghouse gasvelocity can blow particles through thefelt that would not pass through atlower gas velocities. This is why thestandard air to cloth ratio for soilremediation plants, which usuallyhave very fine baghouse dust, shouldbe a nominal 4 : 1.

Can velocity must also be chosenwith the characteristics of the dust inmind (Figure 9). Most materials usedin the HMA industry can be handledadequately in baghouses with can ve-locities in the 375 fpm range. How-ever, it is not unusual to encounter amaterial that has too many sub-mi-cron or low density particles to workwell in this velocity range. When canvelocity is too high for the particulardust, the plant’s capacity will be lim-ited by baghouse pressure drop dueto the inability of the baghouse clean-ing system to clean properly. Dustdischarged from the bags is impededin its fall into the hopper by the forceof the upward flow of exhaust gases.

AIR TO CLOTH RATIO CHOSEN ACCORDING TO PARTICLE SIZE F8

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CAN VELOCITY IN A BAGHOUSE F9

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HIGH CAN VELOCITY CAUSES MIGRATION F10

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The energy required to support thesuspended dust must be provided bythe exhaust fan. This causes the plantto appear to “run out of air.” Whendust is suspended in this manner,naturally, the lighter and smaller par-ticles are most readily suspended.These are also the particles most likelyto penetrate the felt. That, combinedwith the resulting high suction, cancause serious baghouse problems.High can velocity will ultimately causedust to penetrate all the way throughthe felt (a condition known as migra-tion) and results in visible stack emis-sions (Figures 10 and 11). HMAbaghouses should be designed forcan velocities between 265 fpm and285 fpm. Can velocities in that rangewill result in good performance evenwhen difficult situations are encoun-tered (Figure 12).

The baghouse must always be de-signed with the system in mind. Itshould never be the limiting piece ofequipment in the system. Since some-thing obviously must reach its capac-ity first, it is best that this limiting pieceof equipment be the exhaust fan. Ifthis is not the case, it will be possibleto pull more exhaust volume throughthe baghouse than it is able to handle.When this happens, dust will migratethrough the bags, eventually damag-ing them. This is an expensive way todetermine the limit of the plant’s ca-pacity. Even though it is often pos-sible to get more production capacityby increasing exhaust fan speed,baghouse problems may crop up soonafter such a speed change. Baghousecapacity must be considered beforeincreasing fan speed.

MATERIALSBag material is the primary consider-

ation in baghouse material selection.The “standard” filtration media in theindustry is aramid fiber felt (Figure13). Nomex, a trade name of DuPontwhich has become synonymous witharamid, is one of the fibers used inthis application but certainly not theonly one. Many materials are avail-

BAGHOUSE WITH CLEAR STACK F12

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DUSTING STACK F11

BAGHOUSE FELT FOR FILTER F13

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able for the manufacture of bags forbaghouses. A few of these materialsare: polyester, fiberglass, Ryton, andP-84. These and others are excellentmaterials; but, all things considered,aramid has been found to be the mostsuitable material for HMA plants. Sev-eral factors have gone into makingthis determination. Among them are:

• Filtration performance

• Chemical resistance

• Tensile strength

• Durability

• Cost

• Temperature resistance

• Combustibility

The commonly used felt in the indus-try is 14 oz. aramid felt made of fibersof 2 denier size. The 14 oz. specifica-tion denotes the weight of one squareyard of the fabric. The term “denier”relates to fiber diameter but more cor-rectly describes how many grams ofmaterial it takes to make a certainlength of the fiber. Even so, there aremajor differences in bags. The mostcritical difference is density. Felt den-sity is achieved by having enough fi-ber material to start with and then bysufficient needling. Calendering orheat setting is sometimes used toachieve density. These methods in-volve pressing the felt between hotsurfaces to make it dense. However,because nothing holds the fibers to-gether, they will eventually fluff out

again. Nominal 14 oz. aramid may vary down to as little as 10 oz. in some areas.It is impossible to achieve sufficient density with so little fiber. To assure sufficientdensity, felt specifications should assure that weights will be a minimum of 14 oz.at any given location on the bag.

The term “scrim” refers to a woven fabric layer sandwiched between layers of feltin the cloth used to make bags (Figure 14). For 14 oz. bags, the scrim shouldweigh 2 oz. per square yard (+/- 0.2 oz.). The scrim is included in the bags purelyfor support. It contributes nothing to filtration. Therefore, extra scrim means lessfelt for filtration in a bag of a given total weight. Scrimless bags may soon becommon in the HMA industry. They rely upon interlocking directional layers offiber in the felt to make it self-supporting. Theoretically, scrimless bags weighingonly 12 oz. per square yard would provide filtration performance equivalent to the14 oz. scrim supported bags.

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CONSTRUCTION OF SCRIM SUPPORTED BAGS F14

COMPARISON OF FELT F15

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P-84 is usually “overkill” for HMA ap-plications. However, it is often stan-dard for soil remediation plants and issometimes used in hot mix plants tosolve emission problems or to addressparticularly challenging applications.P-84 is superior to aramid with respectto filtration performance and is suit-able for service temperature up to500°F as compared to 400°F for ara-mid, but its cost is about 1-1/2 timesthat of aramid. Without this cost dif-ferential, P-84 would be the fiber ofchoice for HMA plants. This is whythere are such products as P-84“capped” bags, which take advantageof the filtration capability of P-84 andthe cost advantage of aramid by mak-ing a composite of the two. The supe-rior filtration performance of P-84comes from the shape of the fibers(Figure 15). Unlike aramid, which hasa more or less round cross section, P-84 is very irregular in shape. Thischaracteristic provides places for fineparticles to become caught.

More and better materials are becom-ing available regularly. They are notbeing used much in hot mix plants atthis time because of cost and the factthat present regulations can be easilymet with 2 denier aramid felt. Muchwork is being done now with very finefibers call “micro-deniers" (Figure 16).This is an area to watch as workprogresses.

Bag shapes, other than the traditionalcylindrical shape, are often tried, somewith a fair amount of success (Figure17). Alternate shapes are generally experimented with in an effort to get morecloth into service in a given volume. This is the purpose of pleated bags. Specialbag shapes reduce air to cloth ratio for a given number of bags. However, this willbe of no benefit if the baghouse performance is limited by can velocity, as is oftenthe case. One should always keep in mind that there is no bag shape that permitsthe violation of the fundamental principles of baghouse design. Control of air tocloth ratio and can velocity are necessary with all bag shapes.

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SIZE OF FIBERS AVAILABLE IN ARAMID FELT F16

AVAILABLE ARAMID BAG SHAPES F17

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Little consideration is usually givento cage material. Cages are com-monly made of galvanized carbonsteel wire (Figure 5). Basically, all acage has to do is keep the bag fromcollapsing under pressure. Any wirecage that fulfills that function is usu-ally acceptable. Sometimes, wherespecial materials dictate, a 20-wirecage may be necessary to limit flex-ing and associated fatigue failure ofthe felt. Stainless steel wire is usedwhere chemical attack is anticipated.

The plate and structural membersused in HMA plant baghouses arealmost always made of structural typecarbon steel. An internal coating ofepoxy paint is used to provide corro-sion resistance. This can be impor-tant since fuel oils, even no. 2, oftencontain sulfur. The sulfur in the fuelcan cause sulfuric acid formation inthe baghouse.

The basic geometry or shape of thebaghouse is straightforward: it is abox. Beyond that, specific dimen-sions are determined based on per-formance factors such as gas veloci-ties, dust removal requirements, baglengths and quantities, and allowableshipping dimensions and weights(Figure 18).

PRE-CLEANERSPre-cleaners come in several differ-

ent designs and are important to theperformance of the baghouse andentire plant. The total dust load exit-ing the dryer of the typical HMA plantis a lot for a baghouse to handle alone.Not only is the quantity of dust a prob-lem, but also the size distribution.There are usually a lot of relativelylarge particles in the exhaust streambefore it enters a pre-cleaner. Theseparticles tend to abrade the bags andcause premature failure. Also, largeparticles tend to form a porous dustcake (Figure 19). Since the dust cakeis the filter for very small particles, itis clear that the absence of a pre-cleaner can cripple the filtration per-formance of the baghouse.

INSIDE OF BAGHOUSE F18

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WITHOUT PRE-CLEANER THE DUST CAKE FILTERS POORLY F19

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A cyclone is the most commonly usedpre-cleaning device, with a knock-outbox being the second most common(Figure 20). A few plants are equippedwith multi-clones.

A cyclone, as the name suggests, isconfigured to produce a cyclonic airflow pattern inside. As the air spiralsthrough the cyclone, the centrifugalforce causes the dust, which is heavierthan the exhaust gases, to move tothe outside of the flow toward the platewalls. The gas flow is then turnedinward away from the walls leavingthe dust to slide down into the cone orhopper. At a particular flow rate andtemperature, a cyclone can collecteverything larger than a certain sizeparticle and practically no particlessmaller than that particular size. Thissharp division is called the “cut,” andenables a design engineer to accu-rately predict the efficiency of a cy-clone under specific conditions. Cy-clones perform equally well in verticaland horizontal orientations. Horizon-tal cyclones are usually preferable inHMA applications because they workwell, are compact, require minimalsupport and elevation, and can beeasily accessed for inspection ormaintenance (Figures 21 and 22).

A knock-out box is simply an en-larged section of ductwork with a hop-per underneath (Figure 23). A knock-out box slows the exhaust gas streamand allows some of the dust to settle.However, it is difficult to get even asufficiently large knock-out box to bereally functional. The knock-out boxis a pretty turbulent place. One wayto improve its effectiveness is to makeit a two pass device with gases enter-ing and exiting at the top, making a U-turn at the bottom. The inertial effectof the U-turn aids the settling. In anycase, the velocity in the knock-out boxhas to be reduced to the range of 300to 500 fpm. This means that it will bea relatively large device.

ACCESS INTO HORIZONTAL CYCLONE F22

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DRYER WITH SINGLE CYCLONE & BAGHOUSE F20

BAGHOUSE WITH HORIZONTAL CYCLONE F21

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A certain amount of pre-cleaningoccurs in the mixing zone of parallelflow drum mixers (Figure 24). How-ever, the pre-cleaning that occursthere, by capture of dust in the ex-posed asphalt, is often less than ad-equate. Even parallel flow drum mix-ers, in general, need a separate pre-cleaning device.

Pre-cleaning can also be done toowell (Figure 25). A high-efficiencycyclone or multiclone can collect thelarger particles needed in the bag-house to serve as the foundation ofthe dust cake. The result is the sameas with a porous cake caused by nopre-cleaning. The very small particlesare not collected. The best pre-clean-ing device for a baghouse is a cy-clone of moderate efficiency. Its col-lection efficiency should be in therange of 85% to 90%, at the ratedcapacity of the exhaust system. Thecollection efficiency of a cyclone isvariable depending upon the condi-tions under which it operates. Cy-clones can be designed for a particu-lar efficiency only under one specificset of operating conditions. Cycloneefficiency changes on any cyclone inservice as the exhaust volumechanges. For all practical purposes,the harder a cyclone is pushed, interms of exhaust volume, the moreefficient it gets. Exactly the oppo-site is true of a knock-out box. Whenyou need a knock-out box most isprecisely the time when it does theleast for you.

It is important to remember that anypre-cleaning device will add a pres-sure drop to the exhaust system.Because it does more work and istherefore more effective, a cyclonewill cause a larger pressure drop thanother pre-cleaners. The drop acrossthe cyclone is usually about 3 to 4inches water column. For this rea-son, you may not be able to just adda cyclone to an existing system with-out doing anything else and get theresults you are after. In a new plant,this pressure drop is known and ex-

ENLARGED EXHAUST BREECHING (KNOCK-OUT BOX) F23

KNOCK-OUT BOX

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DRYER WITH MULTICONE COLLECTOR & BAGHOUSE F25

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PARALLEL-FLOW DRUM MIXER F24

VIRGINAGGREGATE

HOT MIX

EXHAUST

BURNER

LIQUID ASPHALT

Some dust is captured in themixing zone of a parallel flow mixer.

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haust fan capability is provided for it.With an existing plant, a system ap-proach to the application must betaken to ensure success (Figure 26).

CLEANING SYSTEMSBy far the most commonly used clean-

ing system in HMA plant baghouses isthe pulse jet system (Figure 27).There are also quite a few older re-verse air cleaning systems still in ser-vice (Figure 28). The advantage ofthe pulse jet system over the reverseair system is that no bags have to beremoved from service for cleaning.Another system, which is seldom orever used in the asphalt industry isthe shaker system. Regardless ofwhich system is employed, it is im-portant that the dust cake on all bagsbe kept as nearly uniform as possible.More will be said about this in thesection on operation.

The basic action of the pulse jet clean-ing system is to direct a burst of com-pressed air into each bag at its opentop. This air burst, or pulse, is admit-ted by the timed opening and closingof a solenoid valve. The solenoidadmits air into the blowpipes, one ofwhich is positioned above each rowof bags. The blowpipes have in themsmall holes positioned over and di-rected towards each bag top. A ven-turi is built into the top of each cage.The venturi is used to encourage theentrainment of air by the pulsed air sothat a larger volume of air is used incleaning than can be supplied by thecompressed air system alone. (Apolished spot inside the top of a ven-turi indicates that the blowhole for thatbag is not properly positioned. If thepulse is not directed straight down thethroat of the venturi, the bag will notbe cleaned properly.) At each pulse,air is discharged from each of theblowpipes. (Usually two rows arecleaned simultaneously.) The shockand momentary back-flow producedby the compressed air pulse releases

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CONSIDER IMPACT OF PRE-CLEANER ON WHOLE SYSTEM F26

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PULSE-JET BAGHOUSE F27

REVERSE AIR BAGHOUSE F28

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some of the dust from the bags allow-ing it to drop into the hopper (Figure29). Because the bags are not takenout of service for cleaning, dischargeddust must fall counter to the rising gasstream. For this reason, the tendencyof fine dust to cling together is a char-acteristic without which the pulse jetsystem will not work well. It is impor-tant for a plant operator to know thisbecause severe cleaning efforts tendsto break the agglomerated dust par-ticles into individual particles whichthen become easily re-entrained inthe rising exhaust gas and depositedback on the bag (Figure 30). In gen-eral, it is best to clean frequently withthe least possible effort. If a baghouseis not overloaded or otherwise dis-tressed, compressed air pressure of60 to 70 psig will be adequate. Pres-sures in the 90 to 100 psig range willcause more problems than they solve.While such high pressure may enableto operator to keep the plant runningat high rates, it will very likely causedust migration and damage to thebags by abrading them near their tops.It is also possible to rip seams out atthe bottom of the bags (Figure 31).

Control systems for pulse jet clean-ing systems should be designed toallow the operator to keep the clean-ing system in operation at all timeswhen the plant is running. Turningthe cleaning system on and off tocontrol baghouse differential pressureis to be avoided. When the system isturned off, it resets to row one in thecleaning sequence. Therefore, if thesystem is turned on and off routinelyas a means of control, it is very likelythat those rows that happen to be earlyin the sequence will be over-cleanedwhile those which are late in the se-quence may never get cleaned. Thishas the effect of reducing the effec-tive size of the baghouse. Cleaningsystems should have adjustablecleaning speeds. With three adjust-able speeds, the system can shift fromone speed to another based upon

GENTLE PULSING BAG CLEANING F29

SEVERE PULSING BAG CLEANING F30

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PULSING WITH HIGH AIR PRESSURE DAMAGES BAGS F31

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baghouse differential pressure (Fig-ure 32). When differential pressurerises above the operator’s chosen setpoint for the upper end of the pres-sure range, the system reduces thepulse off-time thereby increasing sys-tem cleaning speed. Similarly, whenthe differential pressure falls below thelower set point of the range, off-time isincreased, which causes the systemto clean more slowly. If these threerates are set and used properly, theoperator has to do little to keep thesystem working. Some good initialsettings for the system are as follow:

• Differential Pressure Range - 3 inwg to 5 inwg

• Pulse “on-time” - 0.15 sec.

• Low Pulse Rate - 20 sec. Interval

• Normal Pulse Rate - 15 sec. Interval

• High Pulse Rate - 10 sec. Interval

Both over-cleaning and under-clean-ing can cause problems. The opera-tor must be aware that a dust cakehas to be developed and maintainedon the bags to provide good filteringperformance. Keeping the differen-tial pressure in the 3 to 5 inwg rangeis a reliable indicator of a proper dustcake. However, it is possible, by clean-ing slowly with high compressed airpressure, to have the 3 to 5 inwg dif-ferential pressure and still have a prob-lem. This is true because under theseconditions some bags will be over-cleaned while others will be under-cleaned. If the pulse interval is to large,dirty gas takes the path of least resistance and rushes to the most recentlycleaned row of bags (Figure 33). This inrush may pull particles through the bag.As long as the pulse intervals are not extended excessively, the 3 to 5 inwgdifferential pressure is a reliable indicator of a proper dust cake. As a general rule,it is good practice to clean with the lowest effective pressure. Some pulse jetbaghouses clean well with as little as 35 psig compressed air pressure. Theoperator should find the lowest effective pressure by adjusting pressure down-ward until the cleaning effort begins to be ineffective and then increase the pres-sure by about 5 psig. This procedure should be performed when the facility isrunning at a high production rate; otherwise, the adjustment procedure will haveto be repeated when high demand is encountered.

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AUTOMATIC PULSE RATE CONTROL F32

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PULSE INTERVAL TOO LARGE F33

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DUST REMOVAL SYSTEMSDust that has been discharged from

the bags by the cleaning system fallsinto the hopper. It is then removedfrom the hopper by one or more screwconveyors. The screw conveyors maybe arranged so that they either col-lect the cyclone dust together with thebaghouse dust or separately. It is notunusual to collect cyclone and bag-house dust separately so that bag-house dust can be easily wasted (Fig-ure 34). This is often necessary withaggregates that have a high percent-age of fines. Superpave designs areresulting in the need to waste somedust that was previously used in themix. For these reasons dust removalsystems are normally combined withdust return or dust disposal systems.These systems can be set up to con-trol the amount of dust wasted or tocontrol the amount of dust returnedto the process. The most common ap-proach, at the time of this writing, is toreturn all dust to the process by directscrew conveyor return without mea-surement. It is apparent that this prac-tice is about to change with the imple-mentation of the Superpave mix de-sign system. Since Superpave mixestend to be relatively low in dust con-tent, effective means of wasting dustand controlling the return of dust tothe process will be essential.

Another frequently used method ofremoving dust is to pneumaticallyconvey it to a storage silo. Once inthe silo, the dust can be returned tothe process by volumetric or massflow control, or it can be wasted (Fig-ure 35). This approach satisfies re-quirements for baghouse operationand of Superpave.

For the successful utilization of thebaghouse it makes little differencewhich of the many available meansof dust removal may be employed. Itis important to baghouse operation,however, that an effective means ofcontrolling air leakage at dust dis-

BAGHOUSE WITH HORIZONTAL CYCLONE F34

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CONTINUOUS DUST WEIGH SYSTEM WITH DOUBLE BARREL® MIXER F35

ROTARY AIR LOCK F36

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charge points is provided. Rotary airlocks or double tipping valves are goodchoices for this service (Figures 36and 37). Highly abrasive materialsmay cause a rotary air lock to becomea maintenance problem. When a ro-tary airlock is used with very abrasivematerials (i.e. very coarse materials),special care should be taken to keepthe air lock in good condition to avoidexcessive air leakage into the system.Air leakage reduces plant productioncapacity and baghouse temperature.

For more information about manag-ing baghouse dust refer to Astec’sTechnical Paper T-121 titled Bag-house Fines.

OPERATIONOperating the baghouse should not

be the main focus of running an HMAfacility. The baghouse should requireas little attention as possible so thatplant personnel can concentrate onmaking mix. Sometimes, not clearlyunderstanding the factors that affectfacility performance ends up causingan undue investment of effort into thebaghouse. This section is intended tohelp plant personnel minimize timeand money spent on the baghouse. Itcould be argued that this is a “pay menow or pay me later” situation. How-ever, “pay me a little now or pay me alot later” is a more accurate perspec-tive (Figure 38).

At start-up, the baghouse should al-ways be preheated before beginningto feed aggregate into the dryer. Thisis true even when starting up from amidstream stop, in which case drumrotation will not be restarted until thebaghouse has been preheated. If anadequate preheat is not performed,moisture will condense on the metalsurfaces of the baghouse and prob-ably even on the bags. Besides form-ing mud, which will quickly plug thebags, moisture will promote chemi-cal attack of the bags, cages, andbaghouse structure (Figure 39).

MOTORIZED PNEUMATIC TIPPING VALVE F37

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PAY ME A LITTLE NOW OR A LOT LATER F38

PRE-HEAT THE BAGHOUSE F39

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Chemical compounds typically foundin HMA plant exhaust gases, eventhough they may be low in concentra-tion, can form acids when they con-tact water. Sulfur dioxide is a primeexample. It will form sulfuric acid,which attacks both steel and bags.Sulfur is often present in plant exhaustbecause most fuel oils have at leasta small percentage of sulfur in them,making preheating essential. Ad-equate preheating may vary accord-ing to the situation. The basic prin-ciple to observe is that the colder thebaghouse prior to start-up the longerit should be preheated. Rememberthat the purpose of the preheat is toavoid condensation on baghousesurfaces. That means that it is notsufficient just to allow the exhaust gastemperature to get up to normal oper-ating temperature. The bags and platestructures need to get hot. A fairlytypical preheat condition is 350°Fbaghouse inlet temperature for 20minutes, when starting up for the firsttime for the day. Cold and/or wetambient conditions may requirelonger preheats. Restarting while thebaghouse is still warm will require lesspreheating. Shorter preheats will alsobe acceptable in hot dry conditions,even on first starts.

Operating temperature may varyquite a bit as conditions vary. Thepreferred baghouse gas inlet tem-perature range is 240-250°F. Thisrange provides good efficiency andusually keeps the bags dry. How-ever, cold ambient temperature, pro-duction rate, RAP percentage in-cluded in the mix, aggregate mois-ture content, mix gradation, etc. willcause exhaust temperature tochange. The operator has to knowhow various conditions affect operat-ing temperature and what to do aboutit. He also should know how plantproblems may affect exhaust tem-perature. With aramid felts, such asNomex, the baghouse inlet tempera-ture should never exceed 400°F.

HANGER BEARING F42

DON'T OVERHEAT ARAMID BAGS F40

DRAFT CONTROL SYSTEM F41

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Operating above this temperature forany significant period of time will de-stroy the bags (Figure 40). Briefperiods over 400°F may cause littledamage, but accumulated damagedue to repeated brief periods at el-evated temperatures can be costly.

Dryer burner suction should alwaysbe controlled at the value specifiedby the burner manufacturer (usually0.2 inwg) (Figure 41). If burner suc-tion is elevated from 0.2 inwg to 0.4inwg, which is easily done, the inflowof air at an open-fired burner is in-creased by 41%. The baghouse musthandle the extra air, which can cre-ate a number of problems (i.e. blind-ing, migration, dust carry-out from thedrum, excessive fuel consumption,diminished drying capacity, abrasionof bags and structures).

MAINTENANCEWhen applied and operated properly,

baghouses are relatively low mainte-nance items. Routine maintenanceconsists of keeping the hanger bear-ings for the hopper screw conveyorslubricated (Figures 42 and 43). Withhard iron bushings, which are typical,lubrication with grease should bedone four times daily.

Fan belts should be checked for wearand tightness equalized (for two mo-tor drive arrangements) periodically(Figure 44). Motor amps will beequalized by proper tightening ofbelts. Belts should not squeal whenthe exhaust fan is starting.

Solenoid valves for the pulse jetcleaning system require no attentionexcept for replacement of failedvalves (Figure 45).

Water should be drained from thecompressed air accumulator at leastdaily. More frequent draining may benecessary during high humidity and/or cold periods. The typical HMA plantcompressed air system does not havea dryer. A dryer would be a good

GREASE FITTING ON HOPPER AT HANGER BEARING F43

TWIN MOTOR FAN & DRIVE F44

SOLENOID VALVES F45

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addition to most plants because itwould eliminate several problems.

Flexible couplings between the so-lenoid valves and the blow pipesshould be checked for deteriorationat least twice annually.

The clean air plenum should bechecked twice annually for corrosionand dust accumulation (Figure 46).

The hopper should be checked dailyfor dust accumulation, which may becaused by dampness (Figure 47).

Keep a daily visual check on the stackplume (Figure 48). Any visible emis-sions, other than moisture, should beinvestigated.

The following data should be re-corded in an operations log:

• Differential pressure

• Inlet temperature

• Outlet temperature

With a data log, deviations from nor-mal can be quickly noticed and shouldbe promptly investigated.

PROBLEM CAUSES ANDSOLUTIONSThere are many different potential

problem and solution combinationsconcerning baghouses. A few of themore common ones follow:

PROBLEM:High pressure drop across the bags

combined with reduced capacityPOSSIBLE CAUSE NO. 1:Hydrocarbon contamination of bags (Figure 49).

SOLUTION:The first step is to determine the source of the hydrocarbons contaminating the

bags. There are only three possible sources. The most likely is the fuel, if the fuelis oil, particularly a heavy or waste oil. In this case, high oil viscosity due to lowfuel temperature may prevent the fuel from burning completely. If this is thecause, the problem can be solved by preheating the oil until its viscosity is 90 SSUor less. It is also possible that the asphalt cement is the source of the contami-nating hydrocarbons. This may be due to the particular AC having been cut withsome volatile component to adjust viscosity. If so, a change of AC sources maybe in order. Another possibility is that virgin AC is being introduced into the systemonto very hot aggregate. This is most likely to occur when the mix being madeincludes a significant amount of RAP. In this case, the solution would be to

INSIDE CLEAN AIR PLENUM F46

INSIDE HOPPER WITH DUST SCREW F47

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change the AC injection point so thatmore time is allowed for mixing of RAPand virgin aggregate before AC is ap-plied. The hydrocarbon source mayalso be the RAP. This is likely withvery high RAP percentages or whenRAP is used in a parallel flow drummixer where it is directly exposed tohot gases. There is no good solutionto this problem. In any of these situa-tions, elevating the baghouse tem-perature will tend to help reduce hy-drocarbon condensation in thebaghouse. The operator should beaware that hydrocarbon contaminationis not only restricts capacity but alsorepresents a dangerous fire hazard.

POSSIBLE CAUSE NO. 2:Ineffective cleaning.

SOLUTION:Adjust pulse pressure and/or times.

Typical initial settings are 0.25 sec.pulse duration, 15 sec. pulse interval,and 80 psig. air pressure. Somenewer systems have the capability ofbeing set up with three different pulseintervals. Typical settings are 10 sec.,15 sec., and 20 sec. It may be neces-sary to decrease the pulse intervalsand/or increase air pressure. De-crease the pulse interval as the firststep of cleaning system adjustment.If it is necessary to increase air pres-sure, do not set it above 100 psig.Doing so only causes abrasion of thebags. Make sure blow pipes are prop-erly positioned (Figure 50).

POSSIBLE CAUSE NO. 3:Re-entrainment of collected dust

SOLUTION:Re-entrainment is caused by verti-

cal velocity (can velocity) being toohigh to allow the dust to drop into thebaghouse following a pulse of clean-ing air (Figure 51). This can be due toexcessively high can velocity or verylight dust particles. Enlarging thebaghouse is not a practical option, sothe solution for both cases is to de-crease the volume of exhaust gases.This can be accomplished by elimi-nating air leaks, reducing aggregate

STACK WITH STEAM PLUME (MOISTURE) F48

BAGS WITH HYDROCARBON CONTAMINATION F49

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MAKE SURE BLOW PIPES ARE PROPERLY POSITIONED F50

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moisture content, correcting improperburner adjustment, or reducing pro-duction. A condition of locally high canvelocity can occur when the pulseinterval is set too high. It may be pos-sible to partially compensate by us-ing a high air pressure setting, butthis practice is likely to result in dustemissions.

POSSIBLE CAUSE NO. 4:High air to cloth ratio (Figure 52).

SOLUTION:This usually occurs along with re-

entrainment (above), and the solu-tion is the same.

PROBLEM:Dust emission with each cleaning

pulse (Figure 53).

POSSIBLE CAUSE NO. 1:Over-cleaning.

SOLUTION:Effective filtration with felt media re-

quires a dust cake on the bags. Over-cleaning will prevent the accumula-tion of an adequate dust cake. Re-duce cleaning effort to obtain pres-sure drop in the range of 3 to 5 inwg.

POSSIBLE CAUSE NO. 2:Bag deterioration.

SOLUTION:Bags can become deteriorated for

any of several reasons. Over-heat-ing, chemical attack, and abrasionand fiber fatigue at the bag tops dueto high air pressure are all possiblecauses. Whatever the cause, once abag has deteriorated, it must be re-placed. Bags suspected of deteriora-tion should be sent to a reputable labfor testing. ASTM has establishedstandardized tests for bag materials.The lab should be certified to performtests to the ASTM methods. Testingis especially important if a large num-ber of bags are suspect. Testing canidentify a loss of strength, loss of bagweight, and penetration of dust intoor through the felt and prevent un-necessary replacements. Another im-portant reason for testing is to iden-

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HIGH CAN VELOCITY MAY REINTRAIN DISCHARGED DUST F51

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HIGH AIR TO CLOTH RATION PULLS PARTICLES THROUGH BAGS F52

DUST EMISSION WITH EACH CLEANING PULSE F53

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tify the cause of the deterioration. Itwill probably happen again unless thecause is found and corrected.

PROBLEM:Continuous dust emission (may be

more severe at certain pulses).

POSSIBLE CAUSE NO. 1:A particular bag may have a hole in

it due to wear or an unsewn or rippedseam.

SOLUTION:Replace the damaged bag. The hard

part is finding it. Taking a look at thetube sheet may reveal a pattern ofdust around a particular bag. A blacklight test is usually a pretty sure wayof locating a damaged bag.

POSSIBLE CAUSE NO. 2:One or more bags may not be sealed

properly in the tube sheet.

SOLUTION:Finding the poorly seated bag(s) is

the problem. The same methods asthose used for a damaged bag (above)will work. Once found, the solution isusually just to make sure that the bagseal snaps firmly into place in the tubesheet (Figure 54).

POSSIBLE CAUSE NO. 3:An incomplete or cracked weld in the

tube sheet or between the tube sheetand a wall can be the source of a con-tinuous dust leak.

SOLUTION:It will probably be necessary to run a

black light test to locate this problem.The solution is usually to weld repairthe leaking area (Figure 55).

PROBLEM:Corrosion (Figure 56).

POSSIBLE CAUSE NO. 1:Sulfur in the fuel.

SOLUTION:Any fuel oil is likely to contain some

sulfur. Small amounts are usually nota problem. The best solution is to finda low sulfur fuel supply. Elevating thebaghouse temperature to avoid mois-

MAKE SURE THAT THE BAG SEAL SNAPS FIRMLY INTO PLACE F54

REPAIR INCOMPLETE OR CRACKED WELDS F55

ACID CORROSION CAN DESTROY STEEL PLATE AND BAGS F56

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ture condensation will help, since thesulfur dioxide formed by the burnerwill combine with condensed mois-ture to form sulfuric acid. Proper pre-heating and purging with fresh air atshutdown will also help.

POSSIBLE CAUSE NO. 2:Low baghouse temperature.

SOLUTION:Low baghouse temperature will very

likely cause condensation in thebaghouse, which will contribute torusting even if no corrosive chemi-cals are present. Any corrosives willprobably be more active when con-densed moisture is present.

PROBLEM:Inconsistent differential pressure

readings (Figure 57).

POSSIBLE CAUSE NO. 1:Water in the sensing lines.

SOLUTION:A tiny leak in an underground sens-

ing line will cause water to be suckedinto the line. The best solution, is touse solid lengths of tubing for each ofthe two sensing lines, since leaks aremost likely to occur at connections.

POSSIBLE CAUSE NO. 2:Malfunctioning gauge (Figure 58).

SOLUTION:Replace the gauge.

TESTINGCompliance testing for the purpose

of obtaining the pollution permit canonly be conducted by a certified testing firm. Application of specific test methodsproving attainment of applicable performance standards are required by theresponsible agency before a permit will be issued. The Clean Air Act Amendmentof 1990 makes the states responsible for meeting federally established ambientair quality standards. Where those standards are being met, even though thestate can require performance to a more demanding standard, they will normallyonly require that the federally mandated emission standard of 0.04 grains/dscf bemet. In Non-attainment Areas a lower emission standard of 0.02 grains/dscf isusually imposed. The performance is usually verified by means of an EPA method5 test, which is an isokinetic particulate test. Limits may also be set for PM-10.PM-10 are those particles smaller than 10 microns in diameter. Many statesrequire periodic particulate testing for the life of the equipment to keep the permitin effect.

It can be very beneficial for plant personnel to run an annual black light test, eventhough no dust emission is visible. Such a practice can catch a small problembefore it becomes a crisis that ends up with the state agency forcing the shutdownof the plant until the problem is corrected.

INCONSISTENT DIFFERENTIAL PRESSURE READINGS F57

MALFUNCTIONING GAUGE F58

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