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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
electrostatic precipitator 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 dust capture 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)
The efficiency of electrostatic precipitators can be increased by:
(i) larger collection surface areas and lower air flow rates give more time and
area for dust particles to collect
(ii) increased speed of dust particles towards collection electrodes
Variation in Electrostatic Dust Collection Technology
Electrostatic precipitators are either single stage
precipitators (high voltage)
or two stage precipitators (low voltage).
Single Stage Precipitator
Single stage precipitators are of two types (i) Plate precipitators or (ii)
Tubular precipitators.
 Plate
Precipitators
Single stage plate precipitators consist of a number of grounded plates
(collecting plates), suspended parallel to one another with equal spacing
between them (4-6 inch) and high voltage discharge (4000-6000 volts) electrodes
that are suspended vertically between the plates from an insulated mounting
frame. The discharge voltage causes the air stream to ionize and dust particles
are negatively charged. As the negatively charged dust particles pass along the
positively charged collection plates, they adhere to them. The collecting
electrode is cleaned, and dust particles are collected in a hopper.
Design Considerations
- 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
- Plates have to be maintained at precise distances. This calls for special
tooling.
- Installing insulators in compartments through which air is blown or drawn from
outside, reduces dust particles collection on insulators. This increases life of
insulator in heavy load conditions as well.
- Since even flow distribution is critical to achieve 99% precipitation, care
must be taken such that charged particles quickly lose static charge. In poor
velocity distribution circumstances under atmospheric conditions of low
humidity, particles may not lose charge, sometime indefinitely. In such cases
all surfaces become collecting surfaces and this phenomenon is called plating.
- Plating is a major disadvantage in Two Stage Precipitators and can be avoided
by having even flow, lower speed air stream flow. Plating can also be countered
by applying an alternating current to the high voltage power supply that
effectively removes charge from particles coming through the collector. This
method can be used effectively even at fairly high speeds velocities.
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