Chapter 4 - Collecting and Disposing of Dust

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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

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 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
Cyclone

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

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.

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

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

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 the Reverse-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

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:

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 the Wet 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 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
	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 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 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.

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 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