Air Cleaning Devices Is An Air Cleaner Needed

Air Cleaning Devices

Is An Air Cleaner Needed ? v Toxicity of materials discharged v Amount of material to be discharged v Value of material to be discharged v Local government regulations Air Cleaning Devices 2

Selection Of Dust Collection Equipment WILL MAINLY DEPEND ON v. Contaminant characteristics v. Efficiency required v. Gas stream characteristics v. Energy considerations v. Dust disposal Air Cleaning Devices 3

Air Cleaning Devices v. Dust collectors v Electrostatic v Fabric v Wet v Dry precipitators collectors centrifugal collectors v. Air filters Air Cleaning Devices 4

Dust Collector Types Electrostatic precipitators (ESP) Mechanism: v Ionizing the gas v Charging the dust particles v Transporting the particles to the collecting surface Iv) neutralizing the dust particles V) removing the dust from the collecting surface Air Cleaning Devices 5

Electrostatic Precipitators TYPES : i ) Cottrell or Single-stage - Combines ionization and collection in a single stage ii ) Penny or Two-stage - Ionization and collection in different stages ADVANTAGES : Large gas volumes & high collection efficiency for small particles needed PROBLEMS : Not suited where exhaust gas is flammable or explosive Lose of efficiency between cleaning cycles if not maintained properly l l l Air Cleaning Devices 6

Dust Collector Types Fabric Collectors l l Collection mechanism : I ) impaction Ii ) interception Iii) diffusion Design features : I ) type of fabric Ii ) fabric configuration Iii) intermittent or continuous service Iv) type of reconditioning V) housing configuration Air Cleaning Devices 7

FABRIC COLLECTORS ( Contd. ) Amount of filter area depends on: v. Release characteristics of dust v. Porosity of dust cake v. Concentration of dust in carrier gas stream v. Type of fabric and surface finish if any v. Type of reconditioning v. Reconditioning interval v. Air flow pattern within the collector v. Temperature and humidity within the gas stream Air Cleaning Devices 8

Fabric Collectors ( Contd. ) Advantages: v. Can handle varying exhaust gas flow rates & particle loading Problems : v. Bag failure v. Abrasion v. Chemical degradation of fabric v. Exceeding temperature limit of fabric Air Cleaning Devices 9

Dust Collector Types Wet Collectors Collection mechanism : – Contact particles with water or another liquid – Collect the droplets Types : I ) chamber or spray tower Ii) packed towers Iii) wet centrifugal collectors Iv) wet dynamic precipitator V) orifice type Vi) Venturi l l Air Cleaning Devices 10

Wet Collectors ( contd. ) ADVANTAGES: v Can handle high-temperature and moisture-laden gases v Dust which represent explosion or fire hazards in the dry form can be wetted and collected PROBLEMS : v Use of water may introduce corrosive conditions. v Freeze protection needed if the collectors are used outdoors in cold climates. v Plugging of nozzles due to chemical imbalances in scrubbing liquids Air Cleaning Devices 11

Dust Collector Types Dry centrifugal collectors Collection efficiency is influenced by I ) size, shape and weight of the particles Ii) size and design of the collector Iii) inlet velocity Iv) concentration of dust Types : I ) gravity separators Ii) inertial separators Iii) cyclone collectors Iv) high efficiency centrifugals Air Cleaning Devices 12

Dust Collector Types V) GASEOUS CONTAMINANT COLLECTORS v TYPES : i ) Absorbers ii) Adsorbers iii) Thermal oxidizers iv) Direct combustors v) Catalytic oxidizers VI) UNIT COLLECTORS : v Small fabric collectors having capacities in the range 200 - 2000 cfm Air Cleaning Devices 13

Gaseous Contaminant Collectors v 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. v Adsorbers: Adsorbers remove contaminants by collection on a solid. Activated carbon or molecular sieves are popular adsorbents. v 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. Air Cleaning Devices 14

Gaseous Contaminant Collectors v 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. v 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. Air Cleaning Devices 15

Dust Collecting Equipment Cost The following factors should be evaluated while estimating the equipment cost v. Price versus capacity v. Accessories v. Installation cost v. Special construction Air Cleaning Devices 16

Dust Collecting Equipment Cost v 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. v Accessories included: meticulous analysis of components of equipment included is essential. v 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. Air Cleaning Devices 17

Dust Collecting Equipment Cost v 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. v 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. Air Cleaning Devices 18

Selection Of Air Filtration Equipment The selection of air filtration equipment is based on v Efficiency v Dust holding capacity v Pressure drop Air Cleaning Devices 19

Methods Of Air Filtration Following are the five basic methods of air filtration v Straining v Impingement v Interception v Diffusion v Electrostatic Air Cleaning Devices 20

Methods Of Air Filtration v 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. v 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. Air Cleaning Devices 21

Methods Of Air Filtration v 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. v 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. v 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. Air Cleaning Devices 22

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 efficiency 2. Low maintenance 3. 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 Air Cleaning Devices 23

Radioactive And High Toxicity Operations v The need for low maintenance is of special importance when exhausting any hazardous material. v 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. v Disposal of radioactive or toxic materials by air, water or land is a serious and very difficult problem. v 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. Air Cleaning Devices 24