IV4

Air Cleaning Devices


Is An Air Cleaner Needed ?Toxicity of materials dischargedAmount of material to be dischargedValue of material to be dischargedLocal government regulations


Selection Of Dust Collection EquipmentWILL MAINLY DEPEND ON Contaminant characteristicsEfficiency requiredGas stream characteristicsEnergy considerationsDust disposal


Air Cleaning DevicesDust collectorsElectrostatic precipitatorsFabric collectorsWet collectorsDry centrifugal collectorsAir filters


Dust Collector Types Electrostatic precipitators (ESP)Mechanism:Ionizing the gasCharging the dust particlesTransporting the particles to the collecting surfaceIv) neutralizing the dust particlesV) removing the dust from the collecting surface


Electrostatic Precipitators TYPES : i ) Cottrell or Single-stage - Combines ionization and collection in a single stageii ) Penny or Two-stage - Ionization and collection in different stages ADVANTAGES :Large gas volumes & high collection efficiency for small particles neededPROBLEMS :Not suited where exhaust gas is flammable or explosiveLose of efficiency between cleaning cycles if not maintained properly


Dust Collector TypesFabric Collectors

Collection mechanism :I ) impactionIi ) interceptionIii) diffusion

Design features :I ) type of fabricIi ) fabric configurationIii) intermittent or continuous serviceIv) type of reconditioningV) housing configuration


FABRIC COLLECTORS ( Contd.)Amount of filter area depends on:Release characteristics of dustPorosity of dust cakeConcentration of dust in carrier gas streamType of fabric and surface finish if anyType of reconditioningReconditioning intervalAir flow pattern within the collectorTemperature and humidity within the gas stream


Fabric Collectors ( Contd.)Advantages:Can handle varying exhaust gas flow rates & particle loadingProblems :Bag failureAbrasionChemical degradation of fabricExceeding temperature limit of fabric


Dust Collector TypesWet Collectors Collection mechanism :Contact particles with water or another liquidCollect the dropletsTypes :I ) chamber or spray towerIi) packed towersIii) wet centrifugal collectorsIv) wet dynamic precipitator V) orifice typeVi) Venturi


Wet Collectors ( contd.)ADVANTAGES: Can handle high-temperature and moisture-laden gasesDust which represent explosion or fire hazards in the dry form can be wetted and collected

PROBLEMS :Use of water may introduce corrosive conditions.Freeze protection needed if the collectors are used outdoors in cold climates.Plugging of nozzles due to chemical imbalances in scrubbing liquids


Dust Collector TypesDry centrifugal collectors Collection efficiency is influenced by I ) size, shape and weight of the particlesIi) size and design of the collectorIii) inlet velocityIv) concentration of dustTypes :I ) gravity separatorsIi) inertial separators Iii) cyclone collectorsIv) high efficiency centrifugals


Dust Collector TypesV) GASEOUS CONTAMINANT COLLECTORS TYPES :i ) Absorbersii) Adsorbersiii) Thermal oxidizersiv) Direct combustorsv) Catalytic oxidizers

VI) UNIT COLLECTORS :Small fabric collectors having capacities in the range 200 - 2000cfm


Gaseous Contaminant CollectorsAbsorbers: these remove soluble or chemically reactive gases from an air stream by contact with a suitable liquid. Water is the most frequently used absorbent. Packed towers are typical absorbers.

Adsorbers: Adsorbers remove contaminants by collection on a solid. Activated carbon or molecular sieves are popular adsorbents.

Thermal oxidizers: thermal oxidizers, or afterburners, may be used where the contaminant is combustible. Most combustible contaminants can be oxidized at temperatures between 1000 and 1500 F.


Gaseous Contaminant CollectorsDirect Combustors: Direct Combustors differ from thermal oxidizers by introducing the contaminated gases and auxiliary air directly into the burner as fuel. Auxiliary fuel, usually natural gas or oil, is generally required for ignition.

Catalytic Oxidizers: These may be used where the contaminant is combustible. The contaminated gas stream is preheated and then passed through a catalyst bed which promotes oxidation of the combustibles to carbon dioxide and water vapor.


Dust Collecting Equipment CostThe following factors should be evaluated while estimating the equipment cost Price versus capacityAccessoriesInstallation costSpecial construction


Dust Collecting Equipment CostPrice versus capacity: prices per cfm of gas will vary with the gas flow rate for all the dust collectors. The smaller the flow rate, the higher the cost per cfm.Accessories included: meticulous analysis of components of equipment included is essential.Some of the collector designs include exhaust fan, motor, drive, and starter. Dust connections between elements may be included or omitted. Recirculating water pumps or setting tanks may be required but not included in the equipment price.


Dust Collecting Equipment CostInstallation cost: installation cost can equal or exceed the cost of the collector. Factory installed media will reduce the installation cost. The cost can also be greatly influenced by the need for water and drain connections, special or extensive electrical work, and expensive material handling equipment for collection material disposal.

Special construction: prices shown in any tabulation must necessarily assume standard or basic construction. The increase in cost for corrosion resisting material, special high-temperature fabrics, insulation or weather protection for outdoor installations can introduce a multiplier of one to four times the standard cost.


Selection Of Air Filtration EquipmentThe selection of air filtration equipment is based onEfficiencyDust holding capacityPressure drop


Methods Of Air FiltrationFollowing are the five basic methods of air filtration StrainingImpingementInterceptionDiffusionElectrostatic


Methods Of Air FiltrationStraining: straining occurs when a particle is larger than the opening between fibers and cannot pass through. It is a very ineffective method of filtration because the vast majority of particles are far smaller than the spaces between fibers.

Impingement: when air flows through a filter, it changes direction as it passes around each fiber. Larger dust particles, however, cannot follow the abrupt changes in direction because of their inertia. As a result, they do not follow the air stream and collide with a fiber. Filters using this method are often coated with an adhesive to help fibers retain the dust particles that impinge on them.


Methods Of Air FiltrationInterception: interception is a special case of impingement where a particle is small enough to move with the air stream but, because its size is very small in relation to the fiber, makes contact with a fiber while following the tortuous air flow path of the fiber.Diffusion: diffusion takes place on particles so small that their direction and velocity are influenced by molecular collisions. These particles do not follow the air stream, but behave more like gases than particulate. Diffusion is the primary mechanism used by most extremely efficient filters.Electrostatic: A charged dust particle will be attracted to a surface of opposite electrical polarity. Most dust particles are not electrically neutral, therefore, electrostatic attraction between dust particle and filter fiber aids the collection of efficiency of all barrier type air filters.


Radioactive And High Toxicity Operations:There are three major requirements for air cleaning equipment to be utilized for radioactive or high toxicity applications:

High efficiencyLow maintenanceSafe disposal

High efficiency is essential because of extremely low tolerances for the quantity and concentration of stack effluent and the high cost of the materials handled


Radioactive And High Toxicity OperationsThe need for low maintenance is of special importance when exhausting any hazardous material. For many radioactive processes, the changing of bags in a conventional fabric collector may expend the daily radiation tolerances of 20 or more persons, so infrequent, simple and rapid maintenance requirements are vital.Disposal of radioactive or toxic materials by air, water or land is a serious and very difficult problem. For example, scalping filters loaded with radioactive dust are usually incinerated to reduce the quantity of material that must be disposed of in special burial grounds.


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IV4