working of ESP | Features of Electrostatic precipitators | effect of the size of the particle and voltage supplied on the performance of ESP
working of ESP
Working of ESP
DISCHARGE ELCTRODE (DE)
o UNDER THE APPLICATION OF VERY HIGH DC VOLTAGE,
o DISCHARGES FREE ELECTRONS WHICH IN TURN IONIZES THE GAS AND DUST PARTICLES.
• COLLECTING ELECTRODE (CE)
o GENERALLY AT THE GROUND POTENTIAL,
o COLLECTS THE CHARGED DUST PARTICLES AND NEUTRALIZES THEIR CHARGE.
• CORONA DISCHARGE
WHEN THE HIGH DC NEGATIVE POTENTIAL, APPLIED TO THE DISCHARGE ELCTRODE W.R.T. THE GROUNDED COLLECTING ELECTRODE, IS INCREASED, AT A PARTICULAR VOLTAGE THE BREAKDOWN OF THE MEDIUM TAKES PLACE, RESULTING IN THE DISCHARGE OF FREE ELECTRONS FROM THE DISCHARGE ELECTRODES. THIS PHENOMENON IS KNOWN AS ‘CORONA DISCHARGE’.
• DEW POINT
THE TEMP. AT WHICH THE MOISTURE PRESENT IN THE GAS/ AIR CONDENSES AND APPEARS AS “DEW”.
THE ELECTRICAL RESISTANCE OF ONE CUBIC CENTIMETER DUST PARTICLE, EXPRESSED IN UNITS “OHM-CENTIMETER”. IT IS THE TYPICAL ELECTRICAL CHARACTERISTICS OF DUST AND THE PERFORMANCE OF ESP IS VERY SENSITIVE TO THIS PARAMETER.
High DC negative potential is applied to DE w.r.t. grounded collecting electrode.
• Above dielectric strength, the breakdown of medium takes place.
• Discharge of free electrons from DE which move towards CE.
• Ionizes gas molecules & charges dust particles negatively.
• These negatively charged dust particles move towards CE, gets collected & loose their charge.
Features of Electrostatic precipitators :
1. High installation cost but low operation cost
2. Low pressure drop.
3. Less maintenance required.
4. No physical obstacle (unlike bag filters) to capture the dust
5. Unique method of dozing out the dust, perpendicular to the flow of electrostatic Charging.
6. Performance depends on synchronization of various functional aspects of different fields (like electrical, Mechanical and process).
7. Efficiency as high as 99.9%
8. Efficiency fluctuates with process conditions (like temp. & % of water)
9. Performance increases with increase in Dew point (Min. Dew point 50°C).
10. Not suitable for High resistivity dust.
11. Hot ESP can handle gas temp. as high as 350°C.
12. Well suited to treat alkali rich raw materials.
Major changes in dust concentration affect the ESP performance as this parameter affects the development of voltage and current in precipitator.
For a precipitator to perform well, both voltage and current of precipitator are important. While the voltage is responsible for pushing the dust particles towards the CE, the current is responsible for keeping them there.
The performance of ESP is related to the product of voltage and current i.e. electrical work.
A well designed precipitator under ideal operating conditions, will go to the maximum value of precipitator voltage and current of the transformer rectifier set values.
Voltage and Current:
In ESPs, more than one electrical fields are created through separate source of voltage.
These fields are usually numbered as 1,2,3…. from inlet side.
Voltage is the pushing source for the dust particle accelerating them towards the collecting plates.
Higher acceleration towards the collecting plates can result in increased efficiency.
The voltage pushes the dust particle, and the corona current holds the particle charging. The higher particle size charging results in higher Migration velocity and therefore it also improves the efficiency.
ESPs develop different voltage and current level in different fields. The efficiency of dust separation of each field is proportional to the electrical works i.e. the product of voltage and current for that particular field.