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For problems with accessibility in using figures and illustrations in this document,
please contact the Directorate of Technical Support and Emergency Management at (202) 693-2300.

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


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.


Settling Chamber
Settling Chamber

Baffle Chambers

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.

Baffle Chamber
Baffle Chamber

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
Cyclone Multiclone
Cyclone Multiclone

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
Baghouse
Baghouse

Commonly 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.

How They Work

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
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
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.
Reverse-Jet Baghouse
Reverse-Jet Baghouse

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 the top of the bag. Since the duration of the compressed-air burst is short (0.1s), it acts as a rapidly moving air bubble, traveling through the entire length of the bag and causing the bag surfaces to flex. This flexing of the bags breaks the dust cake, and the dislodged dust falls into a storage hopper below.


Reverse-jet dust collectors can be operated continuously and cleaned without interruption of flow because the burst of compressed air is very small compared with the total volume of dusty air through the collector. Because of this continuous-cleaning feature, reverse-jet dust collectors are usually not compartmentalized.

The short cleaning cycle of reverse-jet collectors reduces recirculation and redeposit of dust. These collectors provide more complete cleaning and reconditioning of bags than shaker or reverse-air cleaning methods. Also, the continuous-cleaning feature allows them to operate at higher air-to-cloth ratios, so the space requirements are lower.

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
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
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
Wet Cyclone
Wet Cyclone

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 the bottom.


These scrubbers operated at pressure drops of 1 to 2 in. water gauge and are approximately 70% efficient on 10 m particles. Their efficiency is poor-below 10 m. 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
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
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 Scrubber
Counter-Current-Flow Scrubber
  • 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 Scrubber
Venturi Scrubber

Venturi scrubbers consist of a venturi-shaped inlet and separator. The dust-laden gases 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
  • Have no secondary dust sources
  • 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.
 
  • Maintain liquid seal.
  • 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.
 
  • Check pressure drop across mesh and baffle mist eliminators. Clean by high-pressure spraying, if necessary.
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 Precipitator
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 in the 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 Precipitator Tubular-Type Precipitator
Plate-Type Precipitator Tubular-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 1200 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
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 Collector Cyclone Collector
Fabric Collector Cyclone 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 of a dust collector should be based on the following general factors:
  • Dust Concentration and Particle Size - For minerals processing operations, the dust concentration can range from 0.1 to 5.0 grains of dust per cubic feet of air, and the particle size can vary from 0.5 to 100 m.
  • Degree of Dust Collection Required - The degree of dust collection required depends on its potential as a health hazard or public nuisance, the plant location, the allowable emission rate, the nature of the dust, its salvage value, and so forth. The selection of a collector should be based on the efficiency required and should consider the need for high-efficiency, high-cost equipment, such as electrostatic precipitators; high-efficiency, moderate-cost equipment, such as baghouses or wet scrubbers; or lower cost, primary units, such as dry centrifugal collectors.
  • Characteristics of Airstream - The characteristics of the airstream can have a significant impact on collector selection. For example, cotton fabric filters cannot be used where air temperatures exceed 180 F. Also, condensation of steam or water vapor can blind bags. Various chemicals can attach fabric or metal and cause corrosion in wet scrubbers.
  • Characteristics of Dust - Moderate to heavy concentrations of many dusts (such as dust from silica sand or metal ores) can be abrasive to dry centrifugal collectors. Hygroscopic material can blind bag collectors. Sticky material can adhere to collector elements and plug passages. Some particle sizes and shapes may rule out certain types of fabric collectors. The combustible nature of many fine materials rules out the use of electrostatic precipitators.
  • Methods of Disposal - Methods of dust removal and disposal vary with the material, plant process, volume, and type of collector used. Collectors can unload continuously or in batches. Dry materials can create secondary dust problems during unloading and disposal that do not occur with wet collectors. Disposal of wet slurry or sludge can be an additional material-handling problem; sewer or water pollution problems can result if wastewater is not treated properly.

    Comparison of Dust Collector Characteristics
     
    Device
    To Control Particulates Greater Than (μm)
    Pressure Drop
    (in. wg)

    Water Usage (gal/min per 1,000 ft3/min)
    Humid Air Influence
    Space Requirements
    Maximum Temperature (1)(°F)
    Costs (ft3/min)(2)
    Cyclone 20-40 0.75-1.5 - May cause condensation and plugging
     
    Large 750 5¢-25
    Multiclone 10-30 3-6 - May cause condensation and plugging
     
    Moderate 750 5-25
    Shaker baghouse 0.25 3-6 - May make bag cleaning difficult
     
    Large 180 (3) 30-$2.50
    Reverse-air baghouse 0.25 3-8 -   Moderate 550(4)  
    Reverse-jet baghouse 0.25 3-8 - May cause bag to blind Large 180(3)  
    Low-energy scrubber (e.g., spray tower) 25 0.5-2.5 5 None Large Unlimited  
    Low- to medium- energy scrubber (e.g., centrifugal collector) 1-5 2.5-6 3-5 None Moderate Unlimited 25-75
    Medium- to high-energy scrubber (e.g., packed bed) 1-5 6-15 5-10 None Large Unlimited  
    High-energy scrubber (e.g., Venturi) 0.5-2 15 and greater 5-15 None Moderate Unlimited  
    Precipitator (single- or double-stage) 0.25 0.5 - Improves efficiency Large 500 50-$1.00

    Notes:  (1)  Based on standard construction.
               (2)  Cost based on collector section only.
                     Does not include ducting, water, and power requirements.
                     Cost figures should be used for comparison only. Actual costs may vary.
               (3)  180° F based on cotton fabric. Synthetic fabrics may be used to 275° F.
               (4)  550° F based on glass-fiber bags.



Fan and Motor

The fan and motor system supplies mechanical energy to move contaminated air from the dust-producing source to a dust collector.

Types of Fans

There are two main kinds of industrial fans:
  • Centrifugal fans
  • Axial-flow fans
Centrifugal Fans

Centrifugal fans consist of a wheel or a rotor mounted on shaft that rotates in a scroll-shaped housing. Air enters at the eye of the rotor, makes a right-angle turn, and is forced through the blades of the rotor by centrifugal force into the scroll-shaped housing. The centrifugal force imparts static pressure to the air. The diverging shape of the scroll also converts a portion of the velocity pressure into static pressure.

There are three main types of centrifugal fans:
  • Radial-Blade Fans - Radial-blade fans are used for heavy dust loads. Their straight, radial blades do not get clogged with material, and they withstand considerable abrasion. These fans have medium tip speeds and medium noise factors.

  • Radial Blades
    Radial Blades

  • Backward-Blade Fans - Backward-blade fans operate at higher tip speeds and thus are more efficient. Since material may build up on the blades, these fans should be used after a dust collector. Although they are noisier than radial-blade fans, backward-blade fans are commonly used for large-volume dust collection systems because of their higher efficiency.

  • Backward Blades
    Backward Blades

  • Forward-Curved-Blade Fans - These fans have curved blades that are tipped in the direction of rotation. They have low space requirements, low tip speeds, and a low noise factor. They are usually used against low to moderate static pressures.

  • Forward-Curved Blades
    Forward-Curved Blades
Axial-Flow Fans

Axial-flow fans are used in systems that have low resistance levels. These fans move the air parallel to the fan's axis of rotation. The screw-like action of the propellers moves the air in a straight-through parallel path, causing a helical flow pattern.

The three main kinds of axial fans are-
  • Propeller Fans - These fans are used to move large quantities of air against very low static pressures. They are usually used for general ventilation or dilution ventilation and are good in developing up to 0.5 in. wg.

  • Propeller Fan
    Propeller Fan

  • Tube-Axial Fans - Tube-axial fans are similar to propeller fans except they are mounted in a tube or cylinder. Therefore, they are more efficient than propeller fans and can develop up to 3 to 4 in. wg. They are best suited for moving air containing substances such as condensable fumes or pigments.

  • Tube-Axial Fan
    Tube-Axial Fan

  • Vane-Axial Fans - Vane-axial fans are similar to tube-axial fans except air-straightening vanes are installed on the suction or discharge side of the rotor. They are easily adapted to multistaging and can develop static pressures as high as 14 to 16 in. wg. They are normally used for clean air only.
Vane-Axial Fan
Vane-Axial Fan

Direct Driven Belt Driven


  • Less space requirements
  • Greater space requirements
  • Assure constant fan speed
  • Fan speeds easily changed (a vital factor in many applications)
  • Fan speeds limited to available motor speeds
 

When selecting a fan, the following points should be considered:
  • Volume required
  • Fan static pressure
  • Type of material to be handled through the fan (For example, a radial-blade fan should be used with fibrous material or heavy dust loads, and nonsparking construction must be used with explosive or inflammable materials.)
  • Type of drive arrangement, such as direct drive or belt drive
  • Space requirements
  • Noise levels
  • Operating temperature (For example, sleeve bearings are suitable to 250 F; ball bearings to 550 F.)
  • Sufficient size to handle the required volume and pressure with minimum horsepower
  • Need for special coatings or construction when operating in corrosive atmospheres
  • Ability of fan to accommodate small changes in total pressure while maintaining the necessary air volume
  • Need for an outlet damper to control airflow during cold starts (If necessary, the damper may be interlocked with the fan for a gradual start until steady-state conditions are reached.)
Fan Rating Tables

After the above information is collected, the actual selection of fan size and speed is usually made from a rating table published by the fan manufacturer. This table is known as a multirating table, and it shows the complete range of capacities for a particular size of fan.

Points to Note:
  • The multirating table shows the range of pressures and speeds possible within the limits of the fan's construction.
  • A particular fan may be available in different construction classes (identified as class I through IV) relating to its capabilities and limits.
  • For a given pressure, the highest mechanical efficiency is usually found in the middle third of the volume column.
  • A fan operating at a given speed can have an infinite number of ratings (pressure and volume) along the length of its characteristic curve. However, when the fan is installed in a dust collection system, the point of rating can only be at the point at which the system resistance curve intersects the fan characteristic curve.
  • In a given system, a fan at a fixed speed or at a fixed blade setting can have a single rating only. This rating can be changed only be changing the fan speed, blade setting, or the system resistance.
  • For a given system, an increase in exhaust volume will result in increases in static and total pressures. For example, for a 20% increase in exhaust volume in a system with 5 in. pressure loss, the new pressure loss will be 5 x (1.20)2 = 7.2 in.
  • For rapid estimates of probable exhaust volumes available for a given motor size, the equation for brake horsepower, as illustrated, can be useful.

Brake Horsepower Equation
 
bhp = cfm x TP

6356 x ME of fan

where:
 
   
cfm = air volume, ft3/min
TP = total pressure, inches of water
ME = mechanical efficiency of fan (Operating points will be 0.50 to 0.65 for most centrifugal fans.)

Fan Installation
Typical Fan Discharge Conditions

Fan ratings for volume and static pressure, as described in the multirating tables, are based on the tests conducted under ideal conditions. Often, field installation creates airflow problems that reduce the fan's air delivery. The following points should be considered when installing the fan:
  • Avoid installation of elbows or bends at the fan discharge, which will lower fan performance by increasing the system's resistance.
  • Typical Fan Discharge Conditions
    Typical Fan Discharge Conditions
  • Avoid installing fittings that may cause nonuniform flow, such as an elbow, mitred elbow, or square duct.
  • Check that the fan impeller is rotating in the proper direction-clockwise or counterclockwise.
  • For belt-driven fans-

    - Check that the motor sheave and fan sheave are aligned properly.

    - Check for proper belt tension.

  • Check the passages between inlets, impeller blades, and inside of housing for buildup of dirt, obstructions, or trapped foreign matter.
Electric Motors

Electric motors are used to supply the necessary energy to drive the fan. They are normally classified in two groups:
  • Integrals
  • Fractionals
Integral-horsepower electric motors are normally three-phase, alternating-current motors. Fractional-horsepower electric motors are normally single-phase, alternating-current motors and are used when less than 1 hp is required. Since most dust collection systems require motors with more than 1 hp, only integral-horsepower motors are discussed here.

Types of Motors

The two most common types of integral-horsepower motors used in dust collection systems are-
  • Squirrel-Cage Motors - These motors have a constant speed and are of a nonsynchronous, induction type.
  • Wound-Rotor Motors - These motors are also known as slip-ring motors. They are general-purpose or continuous-rated motors and are chiefly used when an adjustable-speed motor is desired.
Squirrel-cage and would-rotor motors are further classified according to the type of enclosure they use to protect their interior windings. These enclosures fall into two broad categories:
  • Open
  • Totally enclosed
Drip-proof and splash-proof motors are open motors. They provide varying degrees of protection; however, they should not be used where the air contains substances that might be harmful to the interior of the motor.

Totally enclosed motors are weather-protected with the windings enclosed. These enclosures prevent free exchange of air between the inside and the outside, but they are not airtight.

Totally enclosed, fan-cooled (TEFC) motors are another kind of totally enclosed motor. These motors are the most commonly used motors in dust collection systems. They have an integral-cooling fan outside the enclosure, but within the protective shield, that directs air over the enclosure.

Both open and totally-enclosed motors are available in explosion-proof and dust-ignition-proof models to protect against explosion and fire in hazardous environments.

Selection of Motor

When selecting a motor, the following points should be considered:
  • Required brake horsepower and revolutions per minute
  • Characteristics of power supply, such as line voltage (110, 220, 440 volts), single-phase or three-phase alternating current, and frequency
  • Environmental conditions under which motor would be operated (for example, temperature, humidity, corrosive atmospheres, or open flames)
  • Characteristics of the load (i.e., the fan and drive elements) and power company restrictions on starting current
  • Necessary overload protection for the motor
  • Ability to supply adequate power under "cold" starts
Fan Troubleshooting Chart
 
Symptom Probable Cause Solution



Insufficient airflow, low ft3/min Fan  
 
  • Forward curved impeller installed backwards
  • Reinstall impeller
 
  • Fan running backwards
  • Change fan rotation by reversing two of the three leads on the motor
 
  • Impeller not centered with inlet collar(s)
  • Make impeller and inlet collar(s) concentric
 
  • Fan speed too low
  • Increase fan speed by installing smaller diameter pulley
 
  • Elbows or other obstructions restricting airflow
  • Redesign ductwork
   
  • Install turning vanes in elbow
   
  • Remove obstruction in ductwork
 
  • No straight duct at fan inlet
  • Install straight length of ductwork, at least 4 to 6 duct diameters long, where possible
   
  • Increase fan speed to overcome this pressure loss
 
  • Obstruction near fan outlet
  • Remove obstruction or redesign ductwork near fan outlet
 
  • Sharp elbows near fan outlet
  • Install a long radius elbow, if possible
   
  • Install turning vanes in elbow
 
  • Improperly designed turning vanes
  • Redesign turning vanes
 
  • Projections, dampers, or other obstructions near fan outlet
  • Remove all obstructions
  Duct System  
 
  • Actual system more restrictive (more resistant to flow) than expected
  • Decrease system's resistance by redesigning ductwork
 
  • Dampers closed
  • Open or adjust all dampers according to the design
 
  • Leaks in supply ducts
  • Repair all leaks in supply duct
Too much airflow, high ft3/min Fan  
 
  • Backward inclined impeller installed backwards (high horsepower)
  • Install impeller as recommended by manufacturer
 
  • Fan speed too fast
  • Reduce fan speed
   
  • Install larger diameter pulley on fan
  Duct System  
 
  • Oversized ductwork; less resistance
  • Redesign ductwork or add restrictions to increase resistance
 
  • Access door open
  • Close all access and inspection doors
Low static pressure, high ft3/min Fan  
 
  • Backward inclined impeller installed backwards (high horsepower)
  • Install impeller as recommended by manufacturer
 
  • Fan speed too high
  • Reduce fan speed
   
  • Install larger diameter pulley on fan
  Duct System  
 
  • System has less resistance to flow than expected
  • Reduce fan speed to obtain desired flow rate
  Gas Density  
 
  • Gas Density lower than anticipated (due to high temperature gases or high altitudes)
  • Calculate gas flow rate at desired operating conditions by applying appropriate correction factors for high temperature or altitude conditions
Low static pressure, low ft3/min Duct System  
 
  • Fan inlet and/or outlet conditions not same as tested
  • Increase fan speed
   
  • Install smaller diameter pulley on fan
   
  • Redesign ductwork
High static pressure, low ft3/min Duct System  
 
  • Obstructions in system
  • Remove obstructions
 
  • Duct system too restricted
  • Redesign ductwork
   
  • Install larger diameter ducts
High horsepower Fan  
 
  • Backward inclined impeller installed backwards
  • Install impeller as recommended by manufacturer
 
  • Fan speed too high
  • Reduce fan speed
   
  • Install larger diameter pulley on fan
  Duct System  
 
  • Oversized ductwork
  • Redesign ductwork
 
  • Access door open
  • Close all access/inspection doors
  Gas Density  
 
  • Calculated horsepower requirements based on light gas (e.g., high temperature or high altitude) but actual gas is heavy (eg.,cold startup)
  • Replace motor
   
  • Install outlet damper, which will open gradually until fan comes to its operating speed
  Fan Selection  
 
  • Fan not operating at efficient point of rating
  • Redesign system
   
  • Change fan
   
  • Change motor
   
  • Consult fan manufacturer
Fan does not operate Electrical  
 
  • Blown fuses
  • Replace blown fuses
 
  • Electricity turned off
  • Turn on electricity
 
  • Wrong voltage
  • Check for proper voltage on fan
 
  • Motor too small and overload protector has broken circuit
  • Change motor to a larger size
  Mechanical  
 
  • Broken belts
  • Replace belts
 
  • Loose pulleys
  • Tighten or reinstall pulleys
 
  • Impeller touching scroll
  • Reinstall impeller properly



Disposal of Collected Dust

After dust-laden exhaust gases are cleaned, the collected dust must be disposed of properly. Ideally, dust can be returned to the product stream and sold; if this is not possible, disposal of dust may become a problem. For example, when dry dust collectors are used, secondary dust problems may arise during unloading and disposal of collected dust; for wet dust collectors, the disposal of wet slurry or sludge can be a problem.

Proper disposal of collected dust can be accomplished in four steps:
  1. Removing dust from the hopper of the dust collector
  2. Conveying the dust
  3. Storing the dust
  4. Treating the dust for final disposal
Removing Dust from Hopper

Collected dust must be removed continuously (while the dust collector is operating), rotary air locks or tipping valves should be used to maintain a positive air seal. If the material in the hopper has a bridging tendency, equipment such as bin vibrators, rappers, or air jets should be used.

Conveying the Dust

After the dust has been removed from the collector, it must be transported to a central point for accumulation and ultimate disposal. Conveying of dust can be accomplished by the following methods:
  • Use of screw conveyors
  • Use of air conveyors (pneumatic conveying)
  • Use of air slides (low-pressure pneumatic conveying)
  • Use of pumps and piping systems to convey slurry
Screw conveyors have been used with a great deal of success. However, trouble areas to be considered are maintenance access, worn-out bearings and casings due to abrasive materials, and air leaks. For wet dust collectors, inclined conveyors can be used to convey the slurry to a settling pond.

Pneumatic conveyors are often selected to convey dry dust because they have few moving parts and can convey either horizontally or vertically. They operated on a high-velocity, low-air-volume principle. Trouble areas include excessive wear and abrasion in the piping and high capital and operating costs.

Air slides are commonly used for nonabrasive, light dust. They work on the principle of air fluidization of dust particles and are useful for heavy horizontal conveying. Trouble areas include ability to maintain a certain downward slope and greater maintenance requirements.

Pumps and piping systems are used to convey the slurry to a settling pond. However, care must be taken in this method to prevent water-pollution.

Storing the Dust

After the material has been removed and transported from the dust collector, a storage facility must be used to permit disposal in efficient quantities. Elevated storage tanks or silos are normally used to permit loading of dry dust into enclosed trucks underneath.

For wet dust collectors, the accumulation area is a settling pond. A settling pond may require considerable space. Since the storage area can only be decanted and dried out during the dry season, two settling ponds are usually needed. Also, most collected materials have very fine components that may seal the pond and prevent percolation.

Treating the Dust for Final Disposal

In most cases, the disposal of fine dust requires great care to prevent recirculation by the wind. Several final dust disposal methods commonly used are-
  • Landfilling
  • Recycling
  • Pelletizing
  • Byproduct utilization
  • Backfilling mines and quarries