Electrostatic filters do NOT use any physical filter material such as paper, cloth,  fibre etc., to capture fine particles, but uses a high voltage electrostatic charge between a series of aluminium plates instead, to first charge floating particles as small as 0.01 micron  (1 micron is 1/25,400^th of an inch ). These positively charged particles are subsequently collected on negatively charged aluminium plates, and the clean air , devoid of particles is sent out of the filter . The efficiency is between 95% to 100% for various particulates. 

The main advantages of Electrostatic Filters are :

1.The Filters are permanant and are for the lifetime of the equipment .They can be cleaned as many times as required unlike standard filters which have to be replaced after a fixed time period, involving a replacement cost .

2.The filters can capture particles as small as 0.01 micron as compared to just 5 microns in standard filters. Thus Fumes such as cigarette smoke, welding fumes, etc can be captured, which standard filters cannot do .

3.The power consumption is upto 40% lower than when using standard filters.

4.The filters can be used for recovery of oil and coolant fumes in CNC machines, and the recovered oil or coolant may be reused giving considerable savings .

5.Pressure drop across these filters is very low .

Conventional filtration techniques just cannot compete with electrostatic filtration as highlighted through the following table.

How it Works:

A two-stage electronic precipitator is constructed in two sections - a charging (or ionizing) section, and a collecting section. The charging section consists of a series of fine wires suspended between metal plates, tubes, air foils or other shapes; the collecting section is a series of parallel flat metal plates .

In single-stage precipitators, both charging and collecting are accomplished in one section. Since the end result is much the same as the charging section of a two-stage EAF (Electronic Air Filters), an explanation of the two-stage EAF is sufficient for both types.

Note:-
In practice a small amount of collection actually occurs in the charging section of the two-stage EF, but most particles are carried through the charging section by the air stream to the collecting section where they are attracted to the charged collecting plates. Keep in mind that the airborne particles in either case are first given an electric charge. Then they are collected on plates which have an opposite charge in relation to the particles. (Like charges repel, unlike charges attract. )

To fully understand the operation of the Electronic Air Filter, it is necessary to review, briefly, the molecular structure of matter. A molecule is the smallest portion of any substance that can exist and still retain the chemical characteristics of the substance. Each molecule contains one or more atoms. An atom contains a nucleus, with a positive electrical charge and orbiting electrons, each with a negative electrical charge. The negative charges on all electrons are equal. In an electrically neutral molecule, enough electrons are present to equal the sum of the positive charges on the nucleus. If one or more of the electrons is knocked out of the molecule, the molecule is left with a surplus of positive charge and is then called a positive Ion. A positive Ion in an electrostatic field will be propelled toward the negative side of the field, while the freed electron is propelled towards the positive side of the field. The propelling force is proportional to the field intensity. A negative ion can be formed if a foreign electron is impacted into a gas molecule or into a particle.

Free electrons, those not attached to atoms, can be created almost everywhere. Within the electrostatic field of an EAF, free electrons accelerate rapidly toward the positively charged ionizing wires. The rate of acceleration becomes greater as they pass through the increasingly higher field intensity while approaching the wire. On the way to the wire, these accelerated free electrons strike air molecules and knock other electrons out of them. These liberated electrons are then free to strike other molecules. In this way a vast number of positive ions (molecules with surplus positive electrical charge) are created and move rapidly toward the grounded plates.

A pale bluish glow appears about the fine wires of an operating EAF. This is a visible indication of a corona discharge. The incoming particles receive their charge from the flow or electrical current (ions) from this discharge to the ground plates of the ionizer. The corona glow is most readily seen on the upstream side of the EAF when it is newly cleaned, operating properly and in complete darkness. Then again, you may not see a glow at all, but this does not mean that the corona discharge is not taking place.

When a continuous d-c voltage is applied to a fine wire suspended between grounded metal plates, a "non-uniform" electrostatic field is formed in the inter electrode space (area on both sides of the wire between the grounded plates). The field is non-uniform because it is very strong near the wire, but decreases rapidly as the distance between the plates and the wire increase. Increasing the voltage applied to the wire increases the field strength and its "distance gradation" - until, depending on the relative sizes, the shape of the wire, the distance between wire and plates, and the applied voltage - "corona-start" conditions are satisfied and the gas molecules (air) near the wire are forced to undergo an electrical change.

The disruption of the atoms in the ionization process cause energy to be radiated. Some of this energy is of the wave lengths of visible light and under some conditions produces a bluish light around the ionizing wire, giving a visible indication of corona discharge.

Ions Hitch a Ride

A dirt particle entering the deluge of gas ions is much like an airplane flying into a rain storm. "Getting wet" is inevitable. As the airborne particles pass through the electrostatic field, the ions "bombard" or collide with and attach to it. The particles then exhibit the electrical charge of the ion "hitch hikers" - the strength of the charge depends upon the number of "hitch hikers" picked up. Therefore, the larger the particles, the greater the electrical charge. This complex charging process takes place in about one-hundredth of a second. The great majority of particles charged in this way have the same charge as the corona discharge wire and are collected on the negative plates in the collector section

Charged Particles Collect on Plates 

Let’s move our attention along with the charged particles into the collecting section and see what happens there. (Incidentally, the dirt collection that occurs in the charging section or in a single stage electrostatic precipitator is very much the same as what follows). 

Recall that the collecting section is comprised of a series of flat metal plates set parallel to the air flow through the cleaner. Their spacing varies somewhat from air cleaner to air cleaner, but is usually in the neighborhood of a quater-inch apart. To make the collector work, a high voltage d-c source (+) is applied to each alternate plate and the intervening plates are grounded so there is a high voltage difference between the plates.

We can examine the collecting process by viewing what occurs on one set of two adjacent collector plates. (The following statements apply equally well to the whole series of plates). A uniform electric field of force is produced between the two plates when a voltage is applied to them. This force field cause a uniform distribution of electrons (negative charge) on the surface of one plate, and an equal and uniformly distributed deficiency of electrons (positive charge) on the other. The voltage gradation is uniform throughout this field, except at its edges and near sharp corners of the plates.

A single positively-charged particle entering this field is acted upon by a force equaling the sum of all attracting and repelling forces. These forces are due to the charge on the particle interacting with the field produced by the plates. These forces accelerate the positively-charged particles toward the negatively-charged plate. In the same manner, a negatively charged particles is forced towards the positive plates. The amount of force acting on the particle depends on the particle’s charge, the voltage applied to the collecting plates and the space between the plates. 

The uniformity of the field causes a particle to be acted upon by an equal force regardless of whether the particle is close to a negative plate, to a positive plate, or somewhere between. If no other force is acting on the particle, it moves with a constant acceleration (constant velocity increase) toward the negative plate.

Other forces also act on this little particle. These forces are the resistance of the air stream, the repelling and attracting forces between it and other particles, gravity and inertia. These forces affect the movement of the particles toward the collecting plates, causing the particle to follow an approximate diagonal path to the collector plate. (From the preceding statements you can see that the velocity is a very important factor in air cleaner performance).

The particles that are collected and are in physical contact with the charged collector plates lose their "opposite charge" and take on the charge of the respective collector plates. They remain attached to the collector plates because of molecular adhesion and due to cohesion to other particles already collected. Sometimes a prepared adhesive is applied to the collector plates in an attempt to increase their holding power. Such preparations may also make the plates easier to wash. Generally these preparations are unnecessary. Heavy loading of fine dry particles is one of the few situations where an adhesive may be useful. Most dirt contains enough oil, tar and residue that is naturally sticky so that washing the dirt away is of more concern than the chance of it blowing off while the EAF is in operation. Carefully washing the dirty cell at the correct intervals is the most effective means of effecting efficiency.