Electrostatic Precipitator

Nobody likes to live in a city that is choking with pollution, we would rather prefer breathing fresh and clean air. The industry is an integral part of our economic system and many believe that it is their right to put up with the factory smokestacks that choke up the air. But not many know that technology has had a good solution to this for over a century in the shape of electrostatic precipitators. These greatly reduce pollution and help improve the environment. In this article, let us look closely at electrostatic precipitators.

What is Electrostatic Precipitator?

Electrostatic Precipitator Diagram

An electrostatic precipitator (ESP) is defined as a filtration device that is used to remove fine particles like smoke and fine dust from the flowing gas. It is the commonly used device for air pollution control. They are used in industries like steel plants, thermal energy plants.

In the year 1907, chemistry professor Frederick Gardner Cottrell patented the first electrostatic precipitator that was used to collect sulphuric acid mist and lead oxide fumes that were emitted from various acid-making and smelting activities.

Working principle of electrostatic precipitator

The working principle of the electrostatic precipitator is moderately simple. It consists of two sets of electrodes: positive and negative. The negative electrodes are in the form of a wire mesh and the positive electrodes are in the form of plates. These electrodes are vertically placed and are alternate to each other.

Working Principle of Electrostatic Precipitator

The gas borne particles such as ash are ionised by the high voltage discharge electrode by the corona effect. These particles are ionised to negative charge and are attracted to positively charged collector plates

The negative terminal of high voltage DC source is used to connect the negative electrodes and the positive terminal of the DC source is used to connect the positive plates. To ionize the medium between the negative and the positive electrode, a certain distance is maintained between the positive, negative electrode and the DC source resulting in a high voltage gradient.

The medium that is used between the two electrodes is air. There might be corona discharge around the electrode rods or the wire mesh due to high negativity of negative charges. The entire system is enclosed in a metallic container consisting of an inlet for flue gases and an outlet for filtered gases. There are plenty of free electrons as the electrodes are ionized which interact with the dust particles of the gas making them negatively charged. These particles move towards positive electrodes and fall off due to gravitational force. Now the flue gas is free from the dust particles as it flows through the electrostatic precipitator and discharged to the atmosphere through the chimney.

Efficiency of electrostatic precipitator

\(\eta =1-e^({\frac{-WA}{Q}})\)

where,

𝜼 is the fractional collection efficiency

W is the terminal drift velocity in ms-1

A is the total collection area in m2

Q is the volumetric air flow rate in m3s-1

Types of electrostatic precipitator

There are different electrostatic types, and here, we will study each one of them in detail. Following are the three types of ESPs:

  • Plate precipitator: This is the most basic precipitator type that consists of rows of thin vertical wires and stack of vertically arranged large flat metal plates that are placed at a distance of 1cm to 18cm apart. The air stream is passed horizontally through the vertical plates and then through the large stack of plates. In order to ionize the particles, a negative voltage is applied between the wire and the plate. These ionized particles are then diverted towards the grounded plates using electrostatic force. As the particles get collected on the collection plate, they are removed from the air stream.
  • Dry electrostatic precipitator: This precipitator is used to collect pollutants like ash or cement in a dry state. It consists of electrodes through which the ionized particles are made to flow through and a hopper through which the collected particles are extracted out. The dust particles are collected from a stream of air by hammering the electrodes.
  • Wet electrostatic precipitator: This precipitator is used to remove resin, oil, tar, paint that are wet in nature. It consists of collectors that are continuously sprayed with water making the collection of ionized particles from the sludge. They are more efficient than dry ESPs.
  • Tubular precipitator: This precipitator is a single stage unit consisting of tubes with high voltage electrodes that are arranged parallel to each other such that they are running on their axis. The arrangement of the tubes could either be circular or square or hexagonal honeycomb with gas either flowing upwards or downwards. The gas is made to pass through all the tubes. They find applications where sticky particles are to be removed.

Advantages and disadvantages of electrostatic precipitator

Advantages of electrostatic precipitator:

  • The durability of the ESP is high.
  • It can be used for the collection of both dry and wet impurities.
  • It has low operating costs.
  • The collection efficiency of the device is high even for small particles.
  • It can handle large gas volumes and heavy dust loads at low pressures.

Disadvantages of electrostatic precipitator:

  • Can’t be used for gaseous emissions.
  • Space requirement is more.
  • Capital investment is high.
  • Not adaptable to change in operating conditions.

Applications of electrostatic precipitator

A few noteworthy electrostatic precipitator applications are listed below:

  • Two-stage plate ESPs are used in the engine rooms of shipboard as the gearbox produces oil mist which is flammable in nature. The collected oil is reused in gear lubricating system.
  • Dry ESPs are used in thermal plants to clean the air in ventilation and air conditioning systems.
  • They find applications in the medical field for the removal of bacteria and fungus.
  • They are used in zirconium sand for detaching the rutile in plants.
  • They are used in metallurgical industries to clean the blast.

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