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Safety and Health Topics > Mineral Processing Dust Control > Dust Contol Handbook > Ch. 4 - Collecting and Disposing of Dust
  
Chapter 4

Collecting and Disposing of Dust
 
What is a Dust Collector?

After dust-filled air has been captured by a dry dust collection system, it must be separated, collected, and disposed of.  The dust collector separates dust particles from the airstream and discharges cleaned air either into the atmosphere or back into the workplace.
 
Necessity for Dust Collectors

Cleaning dust from the air is necessary to-
  • Reduce employee exposure to dust
  • Comply with health and air emission standards
  • Reduce nuisance and dust exposure to neighbors
  • Recover valuable products from the air

Types of Dust Collectors

Five principal types of industrial dust collectors are-
  • Inertial separators
  • Fabric collectors
  • Wet scrubbers
  • Electrostatic precipitators
  • Unit collectors

Types of Inertial Separators

Inertial separators separate dust from gas streams using a combination of forces, such as centrifugal, gravitational, and inertial.  These forces move the dust to an area where the forces exerted by the gas stream are minimal.  The separated dust is moved by gravity into a hopper, where it is temporarily stored.

The three primary types of inertial separators are-
  • Settling chambers
  • Baffle chambers
  • Centrifugal collectors
Neither settling chambers nor baffle chambers are commonly used in the minerals processing industry.  However, their principles of operation are often incorporated into the design of more efficient dust collectors.

Settling Chambers


Settling Chamber
A settling chamber consists of a large box installed in the ductwork.  The sudden expansion of size at the chamber reduces the speed of the dust-filled airstream and heavier particles settle out.

Settling chambers are simple in design and can be manufactured from almost any material.  However, they are seldom used as primary dust collectors because of their large space requirements and low efficiency.  A practical use is as precleaners for more efficient collectors.

Baffle Chambers


Baffle Chamber
Baffle chambers use a fixed baffle plate that causes the conveying gas stream to make a sudden change of direction.  Large-diameter particles do not follow the gas stream but continue into a dead air space and settle.  Baffle chambers are used as precleaners for more efficient collectors.

Centrifugal Collectors


Centrifugal collectors use cyclonic action to separate dust particles from the gas stream.  In a typical cyclone, the dust gas stream enters at an angle and is spun rapidly.  The centrifugal force created by the circular flow throws the dust particles toward the wall of the cyclone.  After striking the wall, these particles fall into a hopper located underneath.

The most common types of centrifugal, or inertial, collectors in use today are-

• Single-cyclone separators

• Multiple-cyclone separators
CycloneMulticyclone
Single-cyclone separators create a dual vortex to separate coarse from fine dust.  The main vortex spirals downward and carries most of the coarser dust particles.  The inner vortex, created near the bottom of the cyclone, spirals upward and carries finer dust particles.

Multiple-cyclone separators, also known as multiclones, consist of a number of small-diameter cyclones, operating in parallel and having a common gas inlet and outlet, as shown in the figure.  Multi-clones operate on the same principle as cyclones--creating a main downward vortex and an ascending inner vortex.

Multiclones are more efficient than single cyclones because they are longer and smaller in diameter.  The longer length provides longer residence time while the smaller diameter creates greater centrifugal force.  These two factors result in better separation of dust particulates.  The pressure drop of multiclone collectors is higher than that of single-cyclone separators.

 
Advantages and Disadvantages - Centrifugal Collectors
 
Types Advantages Disadvantages



Cyclones
  • Have no moving parts
  • Have low collection efficiency for respirable particulates
 
  • Can be used as precleaners to remove coarser particulates and reduce load on more efficient dust collectors
  • Suffer decreased efficiency if gas viscosity or gas density increases
 
  • Can be designed to remove a specific size range of particles
  • Are susceptible to erosion
   
  • Have drastically reduced efficiency due to reduction in airflow rate
   
  • Cannot process sticky dust
Multiclones
  • Have no moving parts
  • Have low collection efficiency for respirable particulates
 
  • Are more efficient than single-cyclone separators
  • Are prone to plugging due to smaller diameter tubes
 
  • Have low pressure drop when used as a precleaner
  • Improper gas distribution may result in dirty gas bypassing several tubes
   
  • Cannot process sticky dust
   
  • For a given gas volume, occupy more space than single-cyclone separators
   
  • Normally have higher pressure drop than single-cyclone separators

Common Operating Problems and Solutions - Cyclones
 
Symptom Cause Solution



  • Erosion
  • High concentrations of heavy, hard, sharp-edged particles
  • Install large-diameter "roughing" cyclone upstream of high-efficiency, small-diameter cyclone.
   
  • Line high-efficiency cyclone with refractor or erosion-resistant material.
  • Corrosion
  • Moisture and condensation in cyclone
  • Keep gas stream temperature above dewpoint.
   
  • Insulate cyclone.
   
  • Use corrosion-resistant material such as stainless steel or nickel alloy.
  • Dust buildup
  • Gas stream below dewpoint
  • Maintain gas temperature above dewpoint.
 
  • Very sticky material
  • Install vibrator to dislodge material.
  • Reduced efficiency or dirty discharge stack
  • Leakage in ductwork of cyclone
  • Clean cyclone routinely.
   
  • Check for pluggage and leakage and unplug or seal the ductwork.
   
  • Close all inspection ports and openings.
 
  • Reduced gas velocity in cyclone
  • Check the direction of fan rotation; if rotation is wrong, reverse two of the tree leads on motor.

Common Operating Problems and Solutions - Multiclones
 
Symptom Cause Solution



  • Erosion
  • High concentrations of heavy, hard, sharp-edged particles
  • Install cast iron tubes.
   
  • Install a wear shield to protect tubes
  • Overloaded tubes
  • Uneven gas flow and dust distribution
  • Install turning vanes in elbow, if elbow precedes inlet vane.
  • Loss of volume in tubes
   
  • Uneven pressure drop across tubes
   
  • Plugging in inlet vanes, clean gas outlet tubes, and discharge hopper
  • Low gas velocity
  • Install turning vanes in elbow inlet
 
  • Uneven flow distribution
  • Insulate multiclone.
 
  • Moisture condensation
  • Install bin-level indicator in collection hopper.
 
  • Overfilling in discharge hopper
  • Empty hopper more frequently.
  • Reduced efficiency or dirty gas stack
  • Leakage in ductwork
  • Seal all sections of ductwork and multiclone to prevent leaks
 
  • Leakage in multiclone
 

Startup/Shutdown Procedures - Centrifugal Collectors
 
Type Startup Shutdown



Cyclones 1. Check fan rotation 1. Allow exhaust fan to operate for a few minutes after process shutdown until cyclone is empty
  2. Close inspection doors, connections, and cyclone discharge. 2. If combustion process is used, allow hot, dry air to pass through cyclone for a few minutes after process shutdown to avoid condensation
  3. Turn on fan 3. Turn off exhaust fan.
  4. Check fan motor current 4. Clean discharge hopper.
  5. Check pressure drop across cyclone.  
Multiclones 1. Conduct same startup procedures as cyclones 1. Conduct same shutdown procedures as cyclones.
  2. At least once a month, measure airflow by conducting a pitot traverse across inlet to determine quantity and distribution of airflow.  
  3. Record pressure drop across multiclone.  
  4. If flow is significantly less than desired, block off rows of cyclone to maintain the necessary flow per cyclone.  

Preventative Maintenance Procedures - Centrifugal Collectors
 
Type Frequency Procedure



Cyclones Daily Record cyclone pressure drops.
    Check stack (if cyclone is only collector).
    Record fan motor amperage.
    Inspect dust discharge hopper to assure dust is removed.
  Weekly Check fan bearings.
    Check gaskets, valves, and other openings for leakage.
  Monthly Check cyclone interior for erosion, wear, corrosion, and other visible signs of deterioration.
Multiclones Daily Same as cyclones.
  Weekly Same as cyclones.
  Monthly Check multiclone interior for erosion, wear, corrosion, and improper gas and dust distribution.
    Inspect individual cyclones and ducts for cracks caused by thermal expansion or normal wear.

Fabric Collectors
BaghouseCommonly known as baghouses, fabric collectors use filtration to separate dust particulates from dusty gases.  They are one of the most efficient and cost effective types of dust collectors available and can achieve a collection efficiency of more than 99% for very fine particulates.

Dust-laden gases enter the baghouse and pass through fabric bags that act as filters.  The bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape.

The high efficiency of these collectors is due to the dust cake formed on the surfaces of the bags.  The fabric primarily provides a surface on which dust particulates collect through the following four mechanisms:
  • Inertial Collection - Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
  • Interception - Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.
  • Brownian Movement - Submicron particles are diffused, increasing the probability of contact between the particles and collecting surfaces.
  • Electrostatic Forces - The presence of an electrostatic charge on the particles and the filter can increase dust capture.

A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow.  The filter must be cleaned periodically.

Types of Baghouses

As classified by cleaning method, three common types of baghouses are -
  • Mechanical shaker
  • Reverse air
  • Reverse jet
Mechanical Shaker

Mechanical-Shaker Baghouse In mechanical-shaker baghouses, tubular filter bags are fastened onto a cell plate at the bottom of the baghouse and suspended from horizontal beams at the top. Dirty gas enters the bottom of the baghouse and passes through the filter, and the dust collects on the inside surface of the bags.

Cleaning a mechanical-shaker baghouse is accomplished by shaking the top horizontal bar from which the bags are suspended.  Vibration produced by a motor-driven shaft and cam creates waves in the bags to shake off the dust cake.

Shaker baghouses range in size from small, handshaker devices to large, compartmentalized units.  They can operate intermittently or continuously.  Intermittent units can be used when processes operate on a batch basis-when a batch is completed, the baghouse can be cleaned.  Continuous processes use compartmentalized baghouses; when one compartment is being cleaned, the airflow can be diverted to other compartments.

In shaker baghouses, there must be no positive pressure inside the bags during the shake cycle.  Pressures as low as 0.02 in. wg can interfere with cleaning.

Air-to-Cloth Ratio:

The volume of gas flow passed per unit area of the bag.

The air-to-cloth ratio for shaker baghouses is relatively low, hence the space requirements are quite high.  However, because of the simplicity of design, they are popular in the minerals processing industry.

Reverse Air

Reverse-Air Baghouse In reverse-air baghouses, the bags are fastened onto a cell plate at the bottom of the baghouse and suspended from an adjustable hanger frame at the top.  Dirty gas flow normally enters the baghouse and passes through the bag from the inside, and the dust collects on the inside of the bags.

Reverse-air baghouses are compartmentalized to allow continuous operation.  Before a cleaning cycle begins, filtration is stopped in the compartment to be cleaned.  Bags are cleaned by injecting clean air into the dust collector in a reverse direction, which pressurizes the compartment.  The pressure makes the bags collapse partially, causing the dust cake to crack and fall into the hopper below.  At the end of the cleaning cycle, reverse airflow is discontinued, and the compartment is returned to the main stream.

The flow of the dirty gas helps maintain the shape of the bag.  However, to prevent total collapse and fabric chafing during the cleaning cycle, rigid rings are sewn into the bags at intervals.

Space requirements for a reverse-air baghouse are comparable to those of a shaker baghouse; however, maintenance needs are somewhat greater.

Reverse Jet

In reverse-jet baghouses, individual bags are supported by a metal cage, which is fastened onto a cell plate at the top of the baghouse.  Dirty gas enters from the bottom of the baghouse and flows from outside to inside the bags.  The metal cage prevents collapse of the bag.

Bags 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 theReverse-Jet Baghouse 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.

Cartridge Collectors

Cartridge collectors are another commonly used type of dust collector.  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.

Cartridge collectors are available in single use or continuous duty designs.  In single-use collectors, the dirty cartridges are changed while the collector is off.  In the continuous duty design, the cartridges are cleaned by the conventional pulse-jet cleaning system.

Advantages and Disadvantages - Baghouses
 
Types Advantages  Disadvantages



Mechanical-shaker baghouses
  • Have high collection efficiency for respirable dust
  • Have low air-to-cloth ratio (1.5 to 2 ft/min)
 
  • Can use strong woven bags, which can withstand intensified cleaning cycle to reduce residual dust buildup
  • Cannot be used in high temperatures
 
  • Simple to operate
  • Require large amounts of space
 
  • Have low pressure drop for equivalent collection efficiencies
  • Need large numbers of filter bags
   
  • Consist of many moving parts and require frequent maintenance
   
  • Personnel must enter baghouse to replace bags, creating potential for exposure to toxic dust
   
  • Can result in reduced cleaning efficiency if even a slight positive pressure exists inside bags
Reverse-air baghouses
  • Have high collection efficiency for respirable dust
  • Have low air-to-cloth ratio (1 to 2ft/min)
 
  • Are preferred for high temperatures due to gentle cleaning action
  • Require frequent cleaning because of gentle cleaning action
 
  • Have low pressure drop for equivalent collection efficiencies
  • Have no effective way to remove residual dust buildup
   
  • Cleaning air must be filtered
   
  • Require personnel to enter baghouse to replace bags, which creates potential for toxic dust exposure
Reverse-jet baghouses
  • Have a high collection efficiency for respirable dust
  • Require use of dry compresses air
 
  • Can have high air-to-cloth ratio (6 to 10ft/min)
  • May not be used readily in high temperatures unless special fabrics are used
 
  • Have increased efficiency and minimal residual dust buildup due to aggressive cleaning action
  • Cannot be used if high moisture content or humidity levels are present in the exhaust gases
 
  • Can clean continuously
 
 
  • Can use strong woven bags
 
 
  • Have lower bag wear
 
 
  • Have small size and fewer bags because of hgih air-to-cloth ratio
 
 
  • Some designs allow bag changing without entering baghouse
 
 
  • Have low pressure drop for equivalent collection efficiencies
 

Common Operating Problems and Solutions - Baghouses*
 
Symptom Cause Solution



  • High Baghouse pressure drop
  • Baghouse undersized
  • consult vendor
  • Install double bags
  • Add more compartments or modules
  • Bag cleaning mechanism not properly adjusted
  • Increase cleaning frequency
  • Clean for longer duration
  • Clean more vigorously
  • Shaking not strong enough (MS)
  • Increase shaker speed
  • Compartment isolation damper valves not operating properly (MS, RA)
  • Check linkage
  • Check valve seals
  • Check air supply of pneumatic operators
  • Compressed air pressure too low (RJ)
  • Increase pressure
  • Decrease duration and frequency
  • Check compressed-air dryer and clean it if necessary
  • Check for obstructions in piping
  • Repressurizing pressure too low (RA)
  • Speed up repressurizing fan.
  • Check for leaks
  • Check damper valve seals
  • Pulsing valves failed (RJ)
  • Check diaphragm
  • Check pilot valves
  • Bag tension too tight (RA)
  • Loosen bag tension
  • Bag tension too loose (MS)
  • Tighten bags
  • Cleaning timer failure
  • Check to see if timer is indexing to all contacts
  • Check output on all terminals
  • Not capable of removing dust from bags
  • Check for condensation on bags
  • Send dust sample and bags to manufacturer for analysis
  • Dryclean or replace bags
  • Reduce airflow
  • Excessive reentrainment of dust
  • Empty hopper continuously
  • Clean rows of bags randomly instead of sequentially (RJ)
  • Incorrect pressure-drop reading
  • Clean out pressure taps
  • Check hoses for leaks
  • Check for proper fluid level in manometer
  • Check diaphragm in gauge
  • Dirty Discharge at stack
  • Bags leaking
  • Replace bags
  • Tie off leaking bags and replace them later
  • Isolate leaking compartment or module
  • Bag clamps not sealing
  • Check and tighten clamps
  • Smooth out cloth under clamp and re-clamp
  • Failure of seals in joints at clean/dirty air connection
  • Caulk or weld seams
  • Insufficient filter cake
  • Allow more dust buildup on bags by cleaning less frequently.
  • Use precoating on bags (MS, RA).
  • Bags too porous
  • Send bag in for permaeability test and review with manufacturer
  • High compressed-air consumption (RJ)
  • Cleaning cycle too frequent
  • Reduce cleaning cycle, if possible
  • pulse too long
  • Reduce pulsing duration
  • Pressure too high
  • Reduce supply pressure, if possible
  • Diaphragm valve failure
  • Check diaphragm and springs
  • Check pilot valve
  • Reduced compressed-air pressure (RJ)
  • Compressed-air consumption too high
  • See previous solutions
  • Restrictions in compressed-air piping
  • Check compressed-air piping
  • Compressed-air dryer plugged
  • Replace dessicant in the dryer
  • Bypass dryer temporarily, if possible
  • Replace dryer
  • Compressed-air supply line too small
  • Consult design
  • Compressor worn out
  • Replace rings
  • Check for worn components
  • Rebuild compressor or consult manufacturer
  • Pulsing valves not working
  • Check pilot valves, springs, and diaphragms
  • Timer failed
  • Check terminal outputs
  • Moisture in baghouse
  • Insufficient preheating
  • Run the system with hot air only before process gas flow is introduced
  • System not purged after shutdown
  • Keep fan running for 5 to 10 min after process is shut down
  • Wall temperature below dewpoint
  • Raise gas temperature
  • Insulate unit
  • Lower dewpoint by keeping moisture out of system
  • Cold spots through insulation
  • Eliminate direct metal line through insulation
  • Water/moisture in compressed air (RJ)
  • Check automatic drains
  • Install aftercooler
  • Install dryer
  • Repressurizing air causing condensation (RJ)
  • Preheat repressurizing air
  • Use process gas as source of repressurizing air
  • Material bridging in hopper
  • Moisture in baghouse
  • See previous solutions
  • Dust stored in hoppers
  • Remove dust continuously
  • Hopper slope insufficient
  • Rework or replace hoppers
  • Screw conveyor opening too small
  • Use a wide, flare trough
  • High rate of bag failure, bags wearing out
  • Baffle plate worn out
  • Replace baffle plate
  • Too much dust
  • Install primary collector
  • Cleaning cycle too frequent
  • Slow down cleaning
  • Inlet air not properly baffled from bags
  • Consult vendor
  • Shaking too violent (MS)
  • Slow down shaking mechanism
  • Repressurizing pressure too high (RA)
  • Reduce pressure
  • Pulsing pressure too high (RJ)
  • Reduce pressure
  • Cages have barbs (RJ)
  • Remove cages and smooth out barbs

* MS = Mechanical shaker
   RA = Reverse air
   RJ  = Reverse jet
 
Startup/Shutdown Procedures - Baghouses

Startup Shutdown


1. For processes generating hot, moist gases, preheat baghouse to prevent moisture condensation, even if baghouse is insulated.  (Ensure that all compartments of shaker or reverse-air baghouses are open.)
 
1. Continue operation of dust-removal conveyor and cleaning of bags for 10 to 20 minutes to ensure good removal of collected dust
2. Activate baghouse fan and dust-removal conveyor.  
3. Measure baghouse temperature and check that it is high enough to prevent moisture condensation  

Preventive Maintenance Procedures - Baghouses
 
Frequency Procedure


Daily
  • Check pressure drop
 
  • Observe stack (visually or with opacity meter)
 
  • Walk through system, listening for proper operation
 
  • Check for unusual occurrences in process
 
  • Observe control panel indicators
 
  • Check compressed-air pressure
 
  • Assure that dust is being removed from system
   
Weekly
  • Inspect screw-conveyor bearings for lubrication
 
  • Check packing glands
 
  • Operate damper valves
 
  • Check compressed-air lines, including line filters and dryers
 
  • Check that valves are opening and closing properly in bag-cleaning sequence.
 
  • Spot-check bag tension
 
  • Verify accuracy of temperature-indicating equipment
 
  • Check pressure-drop-indicating equipment for plugged lines
   
Monthly
  • Check all moving parts in shaker mechanism
 
  • Inspect fans for corrosion and material buildup
 
  • Check drive belts for wear and tension
 
  • Inspect and lubricate appropriate items
 
  • Spot check for bag leaks
 
  • Check hoses and clamps
 
  • Check accuracy of indicating equipment
 
  • Inspect housing for corrosion
   
Quarterly
  • Inspect baffle plate for wear
 
  • Inspect bags thoroughly
 
  • Check duct for dust buildup
 
  • Observe damper valves for proper seating
 
  • Check gaskets on doors
 
  • Inspect paint, insulation, etc.
 
  • Check screw conveyor for wear or abrasion
   
Annually
  • Check fan belts
 
  • Check welds
 
  • Inspect hopper for wear

Wet Scrubbers
Wet Scrubber
Dust collectors that use liquid are commonly known as wet scrubbers.  In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles.  The greater the contact of the gas and liquid streams, the higher the dust removal efficiency.

There is a large variety of wet scrubbers; however, all have of three basic operations:
  • Gas-Humidification - The gas-humidification process conditions fine particles to increase their size so they can be collected more easily.
     
  • Gas-Liquid Contact - This is one of the most important factors affecting collection efficiency.  The particle and droplet come into contact by four primary mechanisms:
      
    • - Inertial Impaction - When water droplets placed in the path of a dust-laden gas stream, the stream separates and flows around them.  Due to inertial, the larger dust particles will continue on in a straight path, hit the droplets, and become encapsulated.

      - Interception - Finer particles moving within a gas stream do not hit droplets directly but brush against them and adhere to them.

      - Diffusion - When liquid droplets are scattered among dust particles, the particles are deposited on the droplet surfaces by Brownian movement, or diffusion.  This is the principal mechanism in the collection of submicron dust particles.

      - Condensation Nucleation - If a gas passing through a scrubber is cooled below the dewpoint, condensation of moisture occurs on the dust particles.  This increase in particle size makes collection easier.
  • Gas-Liquid Separation - Regardless of the contact mechanism used, as much liquid and dust as possible must be removed.  Once contact is made, dust particulates and water droplets combine to form agglomerates.  As the agglomerates grow larger, they settle into a collector.
The "cleaned" gases are normally passed through a mist eliminator (demister pads) to remove water droplets from the gas stream.  The dirty water from the scrubber system is either cleaned and discharged or recycled to the scrubber.  Dust is removed from the scrubber in a clarification unit or a drag chain tank.  In both systems solid material settles on the bottom of the tank.  A drag chain system removes the sludge and deposits in into a dumpster or stockpile.

Types of Scrubbers

Spray-Tower Scrubber Wet scrubbers may be categorized by pressure drop (in inches water gauge) as follows:
  • Low-energy scrubbers (0.5 to 2.5)
  • Low- to medium-energy scrubbers (2.5 to 6)
  • Medium- to high-energy scrubbers (6 to 15)
  • High-energy scrubbers (greater than 15)
Due to the large number of commercial scrubbers availabe, it is not possible to describe each individual type here.  However, the following sections provide examples of typical scrubbers in each category.

Low-Energy scrubbers

In the simple, gravity-spray-tower scrubber, liquid droplets formed by liquid atomized in spray nozzles fall through rising exhaust gases.  Dirty water is drained at theWet Cyclone bottom.

These scrubbers operated at pressure drops of 1 to 2 in. water gauge  and are approximately 70% efficient on 10 µm particles.  Their efficiency is poor-below 10 µm.  However, they are capable of treating relatively high dust concentrations without becoming plugged.

Low- to Medium-Energy Scrubbers

Wet cyclones use centrifugal force to spin the dust particles (similar to a cyclone), and throw the particulates upon the collector's wetted walls. Water introduced from the top to wet the cyclone walls carries these particles away.  The wetted walls also prevent dust reentrainment.Cross-Flow Scrubber

Pressure drops for these collectors range from 2 to 8 in. water, and the collection efficiency is good for 5 um particles and above.



Medium- to High-Energy Scrubbers
Co-Current-Flow Scrubber



Packed-bed scrubbers consist of beds of packing elements, such as coke, broken rock, rings, saddles, or other manufactured elements.  The packing breaks down the liquid flow into a high-surface-area film so that the dusty gas streams passing through the bed achieve maximum contact with the liquid film and become deposited on the surfaces of the packing elements.  These scrubbers have a good collection efficiency for respirable dust.



Three types of packed-bed scrubbers are-

Counter-Current-Flow Scrubbers
  • Cross-flow scrubbers
  • Co-current flow scrubbers
  • Counter-current flow scrubbers
Efficiency can be greatly increased by minimizing target size, ie., using .003 in. diameter stainless steel wire and increasing gas velocity to more than 1,800 ft/min.

High-Energy Scrubbers

Venturi scrubbers consist of a venturi-shaped inlet and separator.  The dust-laden gasesVenturi Scrubber enter through the venturi and are accelerated to speeds between 12,000 and 36,000 ft/min.  These high-gas velocities immediately atomize the coarse water spray, which is injected radially into the venturi throat, into fine droplets.  High energy and extreme turbulence promote collision between water droplets and dust particulates in the the throat.  The agglomeration process between particle and droplet continues in the diverging section of the venturi.  The large agglomerates formed in the venturi are then removed by an inertial separator.

Venturi scrubbers acheive very high collection efficiencies for respirable dust.  Since efficiency of a venturi scrubber depends on pressure drop, some manufacturers supply a variable-throat venturi to maintain pressure drop with varying gas flows.

 
Advantages and Disadvantages - Wet Scrubbers
 
Advantages Disadvantages


  • Have low capital costs and small space requirements
  • Have high operating and maintenance costs
  • Can treat high-temperature and high-humidity gas streams
  • Require corrosion-resistant materials if used with acidic gases
  • Are able to collect gases as well as particulates (especially "sticky" particulates)
  • Require a precleaner for heavy dust loadings
 
  • Cause water pollution; require further water treatment
 
  • Are susceptible to erosion at high velocities
 
  • Collect wet products
 
  • Require freeze protection

Common Operating Problems and Solutions - Wet Scrubbers

Problem Solution


  • Wet/dry buildup
  • Keep all areas dry or all areas flooded
 
  • Use inclined ducts to a liquid drain vessel
 
  • Ensure that scrubber is installed vertically
  • Dust buildup in fan
  • Install clean water spray at fan inlet
  • Excessive fan vibration
  • Clean fan housing and blades regularly
  • Liquid pump failure
  • Divert some of the recycle slurry to a thickener, settling pond, or waste disposal area and supply clean water as makeup
 
  • Increase the water bleed rate
  • Worn valves
  • Use wear-resistant orifice plates to reduce erosion on valve components
  • Jammed valves
  • Provide continuous purge between valves and operating manifold to prevent material buildup
  • Erosion of slurry piping
  • Maintain pumping velocity of 4 to 6 ft/s to minimize abrasion and prevent sedimentation and settling
  • Plugged nozzles
  • Replace nozzles or rebuild heads
 
  • Change source of scrubbing liquid
 
  • Supply filtered scrubbing liquid
  • Buildup on mist eliminators
  • For vane-type demisters, spray the center and periphery intermittently to clean components.
 
  • For chevron-type demisters, spray the water from above to clean the buildup

Startup/Shutdown Procedures - Wet Scrubbers

Prestart Checkout Shutdown


1. Start fans and pumps to check their rotation. 1. Shut down fan and fan spray.  Insulate scrubber from operation.
2. Disconnect pump suction piping and flush it with water from an external source 2. Allow liquid system to operate as long as possible to cool and reduce liquid slurry concentrations.
3. Install temporary strainers in pump suction line and begin liquid recycle. 3. Shut off makeup water and allow to bleed normally.
4. With recycle flow on, set valves to determine operating conditions for desired flow rates. Record the valve positions as a future baseline. 4. When pump cavitation noise is heard, turn off pump and pump gland water.
5. Record all system pressure drops under clean conditions. 5. Open system manholes, bleeds, and other drains.
6. Perform all recommended lubrications.
7. Shut down fan, drain the system, and remove temporary strainers.
 
Startup
 
1. Allow vessels to fill with liquid through normal level controls. Fill large-volume basins from external sources.
2. Start liquid flow to all pump glands and fan sprays.
3. Start recycle pumps with liquid bleed closed.
4. Check insulation dampers and place scrubber in series with primary operation.
5. Start fan and fan inlet spray.  Leave inlet control damper closed for 2 min to allow fan to reach speed.
6. Check gas saturation, liquid flows, liquid levels, fan pressure drop, duct pressure drops, and scrubber pressure drop.
7. Open bleed to pond, thickener, or other drain systems so slurry concentration can build slowly.  Check final concentration as cross-check on bleed rate.

Preventative Maintenance Procedures - Wet Scrubbers
 
Frequency Procedure


Daily
  • Check recycle flow
 
  • Check bleed flow
 
  • Measure temperature rise across motor
 
  • Check fan and pump bearings every 8 hours for oil level, oil color, oil temperature, and vibration.
 
  • Check scrubber pressure drop.
 
  • Check pump discharge pressure
 
  • Check fan inlet and outlet pressure
 
  • Check slurry bleed concentration
 
  • Check vibration of fan for buildup or bleeds
 
  • Record inlet and saturation temperature of gas stream
 
  • Use motor current readings to detect flow decreases.  Use fan current to indicate gas flow
Weekly
  • Check wet/dry line areas for material buildup. Clean, if necessary
 
  • Check liquid spray quantity and manifold pressure on mist eliminator automatic washdown
 
  • Inspect fans on dirty applications for corrosion, abrasion, and particulate buildup
 
  • Check bearings, drive mechanisms, temperature rise, sprocket alignment, sprocket wear, chain tension, oil level, and clarifier rakes.
 
  • Check ductwork for leakage and excessive flexing, Line or replace as necessary
 
  • Clean and dry pneumatic lines associated with monitoring instrumentation
Semiannually
  • Verify accuracy of instruments and calibrate
 
  • Inspect orifice plates
 
  • Clean electrical equipment, including contacts, transformer insulation, and cooling fans
 
  • Check and repair wear zones in scrubbers, valves, piping, and ductwork
 
  • Lubricate damper drive mechanisms and bearings.  Verify proper operation of dampers and inspect for leakage



Electrostatic Precipitators

Electrostatic Precipitators

Electrostatic Precipitator Electrostatic Precipitators use electrostatic forces to separate dust particles from exhaust gases.  A number of high-voltage, direct-current discharge electrodes are placed between grounded collecting electrodes.  The contaminated gases flow through the passage formed by the discharge and collecting electrodes.

The airborne particles receive a negative charge as they pass through the ionized field between the electrodes.  These charged particles are then attracted to a grounded or positively charged electrode and adhere to it.

The collected material on the electrodes is removed by rapping or vibrating the collecting electrodes either continuously or at a predetermined interval.  Cleaning a precipitator can usually be done without interrupting the airflow.


The four main components of all electrostatic precipitators are-
  • Power supply unit, to provide high-voltage, unidirectional current
  • Ionizing section, to impart a charge to particulates int he gas stream
  • A means of removing the collected particulates
  • A housing to enclose the precipitator zone

The following factors affect the efficiency of electrostatic precipitators:
  • Larger collection-surface areas and lower gas-flow rates increase efficiency because of the increased time available for electrical activity to treat the dust particles.
  • An increase in the dust-particle migration velocity to the collecting electrodes increases efficiency.  The migration velocity can be increased by-
-   Decreasing the gas viscosity

-   Increasing the gas temperature

-   Increasing the voltage field
Types of Precipitators

There are two main types of precipitators:
  • High-Voltage, Single-Stage - Single-stage precipitators combine an ionization and a collection step.  They are commonly referred to as Cottrell precipitators.
  • Low-Voltage, Two-Stage - precipitators use a similar principle; however, the ionizing section is followed by collection plates.
Described below is the high-voltage, single-stage precipitator, which is widely used in minerals processing operations.  The low-voltage, two-stage precipitator is generally used for filtration in air-conditioning systems.

High-Voltage, Single-Stage Precipitators

The two major types of high-voltage precipitators currently used are-

  • Plate
  • Tubular
Plate-Type PrecipitatorTubular-Type Precipitator
Plate Precipitators - The majority of electrostatic precipitators installed are the plate type.  Particles are collected on flat, parallel surfaces that are 8 to 12 in. apart, with a series of discharge electrodes spaced along the centerline of two adjacent  plates.  The contaminated gases pass through the passage between the plates, and the particles become charged and adhere to the collection plates.  Collected particles are usually removed by rapping the plates and deposited in bins or hoppers at the base of the precipitator.

Tubular Precipitators - Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes located on the axis of the cylinder.  The contaminated gases flow around the discharge electrode and up through the inside of the cylinders.  The charged particles are collected on the grounded walls of the cylinder.  The collected dust is removed from the bottom of the cylinder.

Tubular precipitators are often used for mist or fog collection or for adhesive, sticky, radioactive, or extremely toxic materials.

 
Advantages and Disadvantages - Electrostatic Precipitators
 
Advantages Disadvantages


  • Have collection efficiencies in excess of 99% for all particulates, including sub-micron-sized particles
  • Have high initial investment costs
  • Usually collect dust by dry methods
  • Do not respond well to process changes such as changes in gas temperature, gas pressure, gas flow rate, gaseous or chemical composition, dust loading, particulate size distribution, or electrical conductivity of the dust
  • Have lower pressure drop and therefore lower operating costs
  • Have a risk of explosion when gas stream contains combustibles
  • Can operate at high temperatures (up to 1200º F) and in colder climates
  • Product ozone during gas ionization
  • Can remove acids and tars (sticky dust) as well as corrosive materials
  • Require large space for high efficiency, and even larger space for dust with low or high resistivity characteristics
  • Allow increase in collection efficiency by increasing precipitator size
  • Require special precautions to protect personnel from exposure to high-voltage
  • Require little power
  • Require highly skilled maintenance personnel
  • Can effectively handle relatively large gas flows (up to 2,000,000 ft3/min)
 

 

Unit Collectors

Unlike central collectors, unit collectors control contamination at its source.  They are small and self-contained, consisting of a fan and some form of dust collector.  They are suitable for isolated, portable, or frequently moved dust-producing operations, such as bins and silos or remote belt-conveyor transfer points.  Advantages of unit collectors include small space requirements, the return of collected dust to main material flow, and low initial cost.  However, their dust-holding and storage capacities, servicing facilities, and maintenance periods have been sacrificed.
Unit Collector
A number of designs are available, with capacities ranging from 200 to 2,000 ft3/min. There are two main types of unit collectors:
  • Fabric collectors, with manual shaking or pulse-jet cleaning - normally used for find dust
  • Cyclone collectors - normally used for coarse dust
Fabric CollectorCyclone Collector

Fabric collectors are frequently used in minerals processing operations because they provide high collection efficiency and uninterrupted exhaust airflow between cleaning cycles.  Cyclone  collectors are used when coarser dust is generated, as in woodworking, metal grinding, or machining.

The following points should be considered when selecting a unit collector:
  • Cleaning efficiency must comply will all applicable regulations.
  • The unit should maintain its rated capacity while accumulating large amounts of dust between cleanings.
  • The cleaning operations should be simple and should not increase the surrounding dust concentration.
  • The unit should be capable of operating unattended for extended periods of time (for example, 8 hours).
  • The unit should have an automatic discharge or sufficient dust storage space to hold at least 1 week's accumulation.
  • If renewable filters are used, they should not have to be replaced more than once a month.
  • The unit should be durable.
  • The unit should be quiet.
Use of unit collectors may not be appropriate if the dust-producing operations are located in an area where central exhaust systems would be practical.  Dust removal and servicing requirements are expensive for many unit collectors and are more likely to be neglected than those for a single, large collector.


Selecting a Dust Collector

Dust collectors vary widely in design, operation, effectiveness, space requirements, construction, and capital, operating, and maintenance costs.  Each type has advantages and disadvantages.  However, the selection