Air Cleaning Devices

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Is An Air Cleaner Needed ?

Toxicity of materials discharged

Amount of material to be discharged

Value of material to be discharged

Local government regulations

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Selection Of Dust Collection Equipment

WILL MAINLY DEPEND ON Contaminant characteristics Efficiency required Gas stream characteristics Energy considerations Dust disposal

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Air Cleaning Devices

Dust collectorsElectrostatic precipitatorsFabric collectorsWet collectorsDry centrifugal collectors

Air filters

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Dust Collector Types Electrostatic precipitators (ESP)

Mechanism: Ionizing the gas Charging the dust particles Transporting the particles to the collecting

surfaceIv) neutralizing the dust particlesV) removing the dust from the collecting surface

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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

explosive Lose of efficiency between cleaning cycles if not

maintained properly

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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

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FABRIC COLLECTORS ( Contd.)

Amount of filter area depends on: Release characteristics of dust Porosity of dust cake Concentration of dust in carrier gas

stream Type of fabric and surface finish if any Type of reconditioning Reconditioning interval Air flow pattern within the collector Temperature and humidity within the gas

stream

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Advantages: Can handle varying exhaust gas flow

rates & particle loadingProblems :

Bag failure Abrasion Chemical degradation of fabric Exceeding temperature limit of fabric

Fabric Collectors ( Contd.)

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Dust Collector TypesWet Collectors Collection mechanism :

– Contact particles with water or another liquid– Collect the droplets

Types :I ) chamber or spray towerIi) packed towersIii) wet centrifugal collectorsIv) wet dynamic precipitator V) orifice typeVi) Venturi

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ADVANTAGES: Can handle high-temperature and moisture-

laden gases Dust 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

Wet Collectors ( contd.)

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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 dust

Types :I ) gravity separatorsIi) inertial separators Iii) cyclone collectorsIv) high efficiency centrifugals

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Dust Collector Types

V) 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

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Gaseous Contaminant Collectors

Absorbers: 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.

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Gaseous Contaminant Collectors

Direct 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.

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Dust Collecting Equipment Cost

The following factors should be evaluated while estimating the equipment cost

Price versus capacity Accessories Installation cost Special construction

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Dust Collecting Equipment Cost

Price 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.

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Dust Collecting Equipment Cost

Installation 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.

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Selection Of Air Filtration Equipment

The selection of air filtration equipment is

based on Efficiency Dust holding capacity Pressure drop

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Methods Of Air Filtration

Following are the five basic methods of air filtration

Straining Impingement Interception Diffusion Electrostatic

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Methods Of Air Filtration Straining: 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.

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Methods Of Air Filtration Interception: 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.

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Radioactive And High Toxicity Operations:There are three major requirements for air cleaning

equipment to be utilized for radioactive or high toxicity applications:

1. High efficiency2. Low maintenance3. Safe 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

 

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Radioactive And High Toxicity Operations

The 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.