If the pollution source releases only a small quantity of the contaminants then pollution is not noticeable as these pollutants easily diffuse into the atmosphere but if the quantity of air contaminants is beyond the limited capacity of the environment to absorb the contaminants then pollution is caused.
(c) Diffusion of Pollutants in Air:ĭilution of the contaminants in the atmosphere is another approach to the control of air pollution. The centrifugal force throws these particles towards the wall from where they are drained downwards to the bottom of the scrubber. The high pressure spray nozzles generate a fine spray that intercepts the small particles in the polluted gas. The dry cyclone chamber can be converted into a wet cyclone scrubber by inserting high pressure spray nozzles at various places within the dry chamber (Fig.
To separate the droplets carrying the particulate matter from the gas stream, this gas-liquid mixture in the Venturi Scrubber is then directed into a separation device such as a cyclone separator. Due to the atomisation of water there is proper contact between the liquid and the gas increasing the efficiency of the Venturi Scrubber (their power cost is high because of the high inlet gas velocity). Venturi Scrubbers can also remove soluble gaseous contaminants. The particles get entrained in the droplets and fall down to be removed later on. The liquid droplets collide with the particulates in the polluted gas stream. breaks the water into droplets) due to the impact of high velocity of the gas. The polluted gas passes downwards through the throat at the velocity of 60 to 180 m/sec.Ī coarse water stream is injected upwards into the throat where it gets atomised (i.e. As shown in Fig 5.6 a Venturi Scrubber has a Venturi shaped throat section. Submicron particulates (size 0.5 to 5 µn) associated with smoke and fumes are very effectively removed by the highly efficient Venturi Scrubbers. (a) A positively charged collecting surface (grounded). electrically charged particulates present in the polluted gas are separated from the gas stream under the influence of the electrical field.Ī typical wire and pipe precipitator consists of: 5.4) works on the principle of electrostatic precipitation i.e. For efficient filtration and a longer life the filter bags must be cleaned occasionally by a mechanical shaker to prevent too many particulate layers from building up on the inside surfaces of the bag. This dust mat acts as a filtering medium for further removal of particulates increasing the efficiency of the filter bag to sieve more sub micron particles (0.5 µm).Ī typical filter (Fig 5.3) is a tubular bag which is closed at the upper end and has a hopper attached at the lower end to collect the particles when they are dislodged from the fabric. The particulate matter is left in the form of a thin dust mat on the insides of the bag. In a fabric filter system, a stream of the polluted gas is made to pass through a fabric that filters out the particulate pollutant and allows the clear gas to pass through. walls of the cone and are discharged from the outlet. flow reverses to form an inner vortex which leaves flow through the outlet). The dust laden gas enters tangentially, receives a rotating motion and generates a centrifugal force due to which the particulates are thrown to the cyclone walls as the gas spirals upwards inside the cone (i.e. A tangential inlet discharging near the top and an outlet for discharging the particulates is present at the base of the cone. Centrifugal force, several times greater than gravitational force, can be generated by a spinning gas stream and this quality makes cyclone separators more effective in removing much smaller particulates than can possibly be removed by gravitational settling chambers.Ī simple cyclone separator (Fig 5.2) consists of a cylinder with a conical base. Instead of gravitational force, centrifugal force is utilized by cyclone separators, to separate the particulate matter from the polluted gas. (2) Cyclone Separators (Reverse flow Cyclone): The several horizontal shelves or trays improve the collection efficiency by shortening the settling path of the particles. The particulates having higher density obey Stoke’s law and settle at the bottom of the chamber from where they are removed ultimately.