11. Explain the principle of the working of smoke
precipitators.
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Smoke might look like a gas but it's actually an aerosol. Most aerosols (such as air fresheners and polishes that come out of aerosol cans) are made of liquid droplets dispersed through gases, but smoke is a bit different: it's a solid dispersed in a gas. Smoke consists of microscopically tiny particles of soot (unburned carbon) dispersed through hot, rising air. Imagine if you could "rub" all the tiny smoke particles as they entered the bottom of a smokestack to give them a tiny electrical charge... and you wrapped something like a sweater round the inside of the smokestack at the top. In theory, the electrically charged smoke particles would cling to the sweater, removing the dirt from the smoke and leaving clean hot air to emerge by itself. Strange as it sounds, that's the basic idea behind electrostatic smoke precipitators!
How an electrostatic smoke precipitator works?
Electrostatic smoke precipitators work by forcing dirty flue gas (the gas escaping from a smokestack) past two electrodes (electrical terminals), which take the form of metal wires, bars, or plates inside a pipe or smokestack. The first electrode is charged to a very high negative voltage. As the dirt particles move past it, they pick up a negative charge. Higher up the pipe (or further along, if it's a horizontal pipe), there's a second electrode consisting of metal plates charged to a high positive voltage (50,000–00,000 volts is typical). Since unlike charges attract, the negatively charged soot particles are attracted to the positively charged plates and stick there. From time to time, the collecting plates have to be shaken to empty away the soot; that can be done either manually (by someone brushing them clean) or automatically (by some kind of automated shaking or brushing mechanism in a process called rapping).
How effective are they?
As air pollution controls have become increasingly stringent, so smoke precipitator technology has had to evolve to keep pace. State-of-the-art precipitators remove well over 99 percent of the particulates in flue gas moving at a typical speeds of 20cm (6–7in) per second or so.
Who invented electrostatic smoke precipitators?
According to the Archives of the Research Corporation for Science Advancement [PDF], we owe the basic idea of electrostatic precipitation to Dr Frederick Cottrell (1877–1948), who carried out the earliest experiments with the technology in 1906. In US Patent 895,729: Art of separating suspended particles from gaseous bodies, filed in 1907, Cottrell described how high-voltage electrodes could be used to clean all kinds of factory fumes and dust. Intriguingly, he also suggested the same basic technology could be used for "the problem of destroying fog and mists in the open air, both on land and water". The same year, he formed the International Precipitation Company to license his invention to other companies, including Western Precipitation of California. The first large-scale precipitator was a dust-busting device built by Western Precipitation for the Riverside Cement Company in 1911. Cottrell later worked with British electricity pioneer Dr Oliver Lodge and German Dr Erwin Moeller to develop and market the technology worldwide.
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