Electrostatics

Electrostatic Precipitator

What is it?

In 1907 Frederick Cottrell invented the first machine that made use of the principles of electrostatics. (Blake.et al) "He was a chemistry teacher who wanted to remove sulfuric acid droplets from smokestacks." (Blake.et al) Smokestacks are a type of chimney, that releases the byproducts of combustion in the air. (wiseGEEK) The smokestacks usually consist of carbon dioxide and water vapour. (wiseGEEK) Also nitrogen and oxygen can be found with a number of pollutants. (wiseGEEK) The electrostatic precipitator are usually used in industries. Cottrell invented a type of cleaner that removes unwanted particles and liquid droplets from a flow of gas formally known as the electrostatic precipitator.(Blake.et al) They are effective as it reduces pollution from smokestacks. They are also known as electronic air cleaners. (Hoffman, Matt) They have an ionizer that electrically charges the particles as dirty air passes through the unit. (Hoffman, Matt) '"In general, electrostatic air cleaners are effective in removing particulates in the 0.001 to 10-micron range'. (Hoffman, Matt) Power stations burn fossil fuels like coal and oil and that is how smoke is produced. Smoke is made of tiny solid particles, like unreacted carbon, which can damage buildings and cause breathing difficulties. (BBC) That's why the smoke is removed from waste gases before they pass out of chimneys by using the electrostatic precipitator. (BBC)



How does it work?

The electrostatic precipitator uses the laws of electric charges. How it works is dust particles and liquid droplets in a polluted gas become positively charged when they come in contact with a wire that has a strong positive charge. (Blake.et al) "The positively charged particles and droplets induce a negative charge on the collection plates." (Blake.et al) "When they collide with the collection plates they are neutralized and are collected in large containers called hoppers. "(Blake.et al) The electrostatic precipitator is made out of a series of plates and charged wires. "They give the smoke an electric charge." (Kurtus, Ron)When the gas passes by electrodes of the opposite charge, most of the smoke particles cling to the electrodes thus keeping the pollution from going out into the atmosphere. (Kurtus, Ron)


The electrostatic precipitator forces dirty gas past two electrodes, which take the form of metal wires, bars, or plates inside a pipe or smokestack. (explainthatstuff) The first electrode has a very high negative charge. (explainthatstuff) As dirt particles move past it, they get a negative charge. (explain that stuff) Higher up or further along the electrostatic precipitator is a second electrode consisting of metal plates with a high positive charge. (explain that stuff) "Unlike charges attract, so the negatively charged particles are attracted to the positively charges plates and stick there." (explainthatstuff) Time to time the collecting plates have to be emptied. (explainthatstuff) That can be done manually or automatically. (explainthatstuff)




Diagram of How It Works

  1. "Smoke particles pick up a negative charge." (BBC)
  2. "Smoke particles are attracted to the collecting plates." (BBC)
  3. "Collecting plates are knocked to remove the smoke particles. " (BBC)

Advantages and Disadvantages

"advantages:
1. handles very large gas volumes and heavy dust loads with low pressure drop.
2. very high collection efficiencies, even for very small particles.
3. can handle corrosive materials, wet materials, and high temperatures.
4. low operating costs, except at very high efficiencies.
5. durable- has long service life requiring little maintenance.

disadvantages:
1. not very flexible to changes in operating conditions once installed/purchased
2. cannot control gaseous emissions
3. very dependent on the electrical resistivity of the particulate.
4. High equipment costs
5. takes up a lot of space"

(all from GlobalSpec)

Dry Electrostatic Precipitator

A dry electrostatic precipitator is a large box. (PPC) The process of separating the gases begins when gases are drawn into one side of the box. (PPC) "The negatively charged particles are attracted to a grounded collecting surface which is postively charged." (PPC) The gas which is much cleaner than it was when it entered the box then leaves the box. "This process continues inside the box, as the particles form the continuous flow build up on the collecting plates." (PPC) The particles fall into hoppers once the plates are full and then they are removed from the hoppers by a rotary screw conveyor. (PPC)


Wet Electrostatic Precipitator

The wet electrostatic precipitators control systems can be either up flow or down flow relative to where polluted air enters it during the process. (PPC) "Turning vanes and plates evenly distribute the gas flow inside of the precipitator." (PPC) The gas then enters the round collection tubes. "Above the collection tubes are two set of spray headers." (PPC) "The first spray header continually mists the collection tubes with small water droplets, which are immediately charged and flow down the tube, in the direction of the gas flow." This continually wets the tubes so it can prevent things from sticking in the tubes during this process. (PPC) This second spray header is called the flushing header, which periodically sprays a larger amount of water to flush collected particles out of the collection tubes. (PPC) The particles then move to the lower plenum. (PPC) As dust and aerosol particles enter the collection tubes, they become charged from negatively charged electrons. (PPC)"The negatively charged particles go in the wetted collection tube and are flushed out periodically into the precipitator’s lower plenum." (PPC)"The lower plenum is designed to de mist the gas stream and drain all of the collected precipitate. (to separate)" (PPC)


Citations

Electrostatic Smoke Precipitators. Explain That Stuff. Web. 12 Feb. 2013.

<http://www.explainthatstuff.com/electrostaticsmokeprecipitators.html>


Hoffman, Matt. "Choosing the Right Technology for Clean Indoor Air." Occupational Hazards. 01 May. 2004: 59. eLibrary. Web. 12 Feb. 2013.

Foster, Niki. What are Smokestacks?. WiseGEEK. Web.12 Feb. 2013.<http://www.wisegeek.com/what-are-smokestacks.htm>

How to Select Electrostatic Precipitators. GlobalSpec, 1999. Web. 12 Feb. 2013.
<http://www.globalspec.com/learnmore/manufacturing_process_equipment/air_quality/electrostatic_precipitators>


Using Static Electricity. BBC, 2013. Web. 12 Feb. 2013.
<http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/electricity/staticelectrev2.shtml>


Kurtis, Ron. Uses for Static Electricity. School for Champions. Web. 12 Feb. 2013.
<http://www.school-for-champions.com/science/static_uses.htm>


Electrostatic Precipitator. FLSmidth, 2011. Web. 12 Feb. 2013.

<http://www.flsmidth.com/en-US/Products/Product+Index/All+Products/Air+Pollution+Control/Electrostatic+Precipitator/Electrostatic+Precipitator+1>


How a Wet Electrostatic Precipitator Works. PPC Air Pollution Control. Web. 12 Feb. 2013.

<http://www.ppcbio.com/ppcwespworks.htm>


How a Dry Electrostatic Precipitator Works. PPC Air Pollution Control. Web. 12 Feb. 2013.

<http://www.ppcbio.com/ppcdespworks.htm>

ESP Industrial Air Filter for Oil Mist Elimination. Alibaba. Web. 15 Feb. 2013.

<http://www.alibaba.com/product-gs/280530729/ESP_Electrostatic_Precipitator_Industrial_Air_Filter/showimage.html>


Blake, Leesa, et al. ON Science 9. McGraw-Hill Ryerson Limited, 2009. 421-22. Print.