Electrostatic precipitate/ ESP

Electrostatic dust collectors use electrostatic charges to separate dust from the dusty air stream.

Electrostatic Precipitation for Dust Collection

How Electrostatic Filtration Works

Electrostatic dust collectors use electrostatic charges to separate dust from the dusty air stream. A number of high voltage, direct current electrodes (carrying negative charge) are placed between grounded electrodes (carrying positive charge). The dust borne air stream is passed through the passage between the discharging (negative) electrodes and collecting (positive) electrodes. Dust particles receive a negative charge from the discharging electrodes (ionizing section) and are attracted to the positively charged grounded electrode (collection plates) and fasten on to it. Cleaning is done by rapping or vibrating the collecting electrode wherein dust particles fall away. Cleaning can be done without interrupting the flow.

 

For more thorough cleaning, the collection cell can be removed and washed by hand or in a parts washer with an aluminum safe detergent.  Some ESP air cleaners have automatic self washing mechanisms.

DUST COLLECTOR EXPERT TIP:  Most common filtration is best used for ambient capture of light atmospheric dust.  Unless you are using a self cleaning electrostatic precipitator, source capture or direct ducting from a heavy dust producing machine will quickly fill up the collection plates.  Heavy dust collection requires storage for a large volume of dust.  Surface area of pleated media filters, fabric bags or pleated cartridge media is much greater than surface area of electrostatic collection plates and work better for source capture and direct machine ducted than ESP would.

The basic components of an electrostatic precipitator are (i) power supply unit (to impart high voltage, uni- directional current) (ii) an ionizing section where charge is imparted to dust filled air stream (iii) cleaning system to remove dust particles and (iv) housing for the precipitator.

ESP Advantages

Electrostatic precipitators have the following advantages:

They have high efficiencies (exceeds 99.9% in some applications) Fine dust particles are collected efficiently Can function at high temperatures (as high as 700 degree F 1300 degree F) Pressure and temperature changes are small Difficult material like acid and tars can be collected They withstand extremely corrosive material Low power requirement for cleaning Dry dust is collected making recovery of lost product easy Large flow rates are possible

ESP Disadvantages

High initial cost Materials with very high or low resistivity are difficult to collect Inefficiencies could arise in the system due to variable condition of airflow (though automatic voltage control improves collector efficiency) They can be larger than baghouses (fabric collectors) and cartridge units, and can occupy greater space Material in gaseous phase cannot be removed by electrostatic method Dust loads may be needed to be reduced before precipitation process (precleaner may be needed) Efficiency is affected by the speed of the air stream. It is important to maintain even velocity distribution through precipitator from side to side and from top to bottom Care must be taken in design of distribution baffles Discharge electrodes are either hanging wires with weights or fixed frames. Hanging wires are economical. The closer the electrodes are to grounding plates, the more effective the charging forces are at lower voltage

Factors That Affect Efficiency

Some factors that could affect the efficiency of the electrostatic precipitator are as follows:

Other gases in the air stream could affect efficiency Some dusts have high charge and greater forces may be required to attract them to the collection plates Sulfur compounds in boiler gas increase collector efficiency Pressure drop usually in the range of 0.2 to 0.5 inches

Dust Discharge Considerations

Dust is removed by rapping or vibrating the collection plates with an air-powered anvil (after turning the power supply off) so dust falls into the hopper. Sometime dust particles collect on high voltage insulator (since they can migrate to any grounded or uncharged surface) forming a path for the high voltage to ground. This could cause failure of power supply and can be corrected by pressurizing insulators with a blower while maintaining a flow of outside air in collecting compartment. Charged particles do not have enough attraction by this to settle on insulators.

Tubular Precipitators

Tubular precipitators consist of collection electrodes that are tubular (similar to a pipe) with discharge electrodes placed in the center of the tube. The dust laden air stream flows through the tubes, gets negatively charged and collects on the inside wall of the collecting tube. The dust is cleaned from the bottom of the tube. This type of an electrostatic precipitator can be used with wet electrostatic precipitators by keeping the walls continuously wet or by washing the collection electrodes. Pipe collection electrodes provide highly effective gas distribution within the precipitator.

Uses of Tubular Precipitators

They are widely applied to high temperature gas streams such as boiler exhausts in power plants since they are able to adjust to expansion and contraction of parts. They are also used for mist or fog collection or for adhesive, sticky, radioactive, or extremely toxic materials. Tubular precipitators find application in operations involving mineral processing.

Two Stage Precipitator

In the Two Stage Precipitator grounded plates are placed about an inch apart with an intermediate plate that is also charged. With the two-stage system, instead of 4000-6000 DC supply, it has 13000-15000 volt supply with intermediate supply at 7500 volts. Dust load in this system is normally between 0.01 to 0.1 grains per 1000 cu.ft

Two Stage ESP Advantages

Highly efficient Self cleaning washing systems The washing system is a light duty unit designed for 250 cycles. Has longer life since cleaning is required only monthly. Air distribution is even since dust collecting filtering device operates at same velocities as heating and cooling coils

Uses of Two Stage Precipitators

Two stage precipitators are generally used in plants where welding, grinding and burning operations are performed. However since maintenance requires removal of precipitator frames and manual cleaning of delicate assemblies, the electrode wires had the problem of being broken. This caused collection efficiencies to suffer.Two stage precipitators are used in hooded and ducted automatic welding machines or welding booths when dust load has increased to 30-50 grains per cu.ft per minute

Design Considerations

Electrostatic precipitate/ ESP