PRINCIPLE OF ESP:

In the electrostatic precipitator the particles are removed from the gas stream by utilizing electrical force .A charged particle in the electrical field experiences a force proportional to the size of the charge and to the strength.

The precipitation process therefore requires.

o        A method of charging the particles electrically.

o        A means of establishing an electrical field and

o        A method of removing the collected particles.

 An industrial ESP includes a large number of discharge electrodes. Pirated wires and rows of collecting electrodes plates forming passage through which the gas flows with velocity.

High voltage is applied to the discharge electrodes resulting in the high electric field near the wire and an associated corona producing gas ions .The ions collide with and held by, the dust particles and this in turn become electrically charged the particles moved towards the grounded collecting electrode plates from which the accumulated dust is dislodged by rapping the dust falls to the bottom of the precipitator casing from which it is removed by different methods.

PARTS OF THE PRECIPITATORS:

The various parts of the precipitators are divided to two groups.  Mechanical system comprising of casing, hoppers, gas distribution system, collecting and emitting system, rapping mechanisms, stair ways and galleries.

Electrical system comprising of transformer-rectifier units, electronic controllers auxiliary control panels, safety interlocks and field devices.

 

1. MECHANICAL SYSTEM: 

A. Precipitator casing:

          The precipitator casing is an all welded construction, consisting of prefabricated wall and the roof panels. The casing is provided with inspection doors for entry into the chamber. The doors are of heavy construction with machined surfaces to ensure a gas tight seal.

The roof carries the precipitator internals, insulator housing, transformers etc.The casing rests on supports, which allow for free thermal expansion of the casing during Opertion.Galleries and stairways are provided on the sides of the casing for easy access to rapping moters, inspection doors, transformers.

B. Hoppers:

The hoppers are adequately sized to hold the ash, Baffle plates are provided in each hopper to avoid gas sneak age. An inspection door is provided on each hopper. Thermostatically controlled heating elements are arranged at the bottom portion to the hopper to ensure free flow of ash. The precipitator casing is an all welded construction, consisting of prefabricated

C. Gas distribution systems:

The performance of the precipitator depends on even distribution of gas over the entire cross section of the field. Guide vanes, splitters and screens and screens are provided in the inlet funnel to direct the flue gas evenly over the entire cross section of the ESP.

D. Collecting Electrode System:

            The collecting plates are made of 1.5mm cold rolled milled steel plate and shaped in one piece by roll forming .The collecting electrode has unique profile designed to give rigidity and to contain the dust in a quiescent zone free from re-entrainment .The 400mm collecting plates are provided with hooks to their top edge for suspension .The hooks engage the slots of the supporting angles 750mm collecting plates in a row are held in position by a shock bar at the bottom. The shock bars are spaced by guides.

E. Emitting Electrode System:

The most essential part of the precipitator is emitting electrode system.4 insulators support this. The frames for holding the emitting electrodes are located centrally between collecting electrode curtains. The entire discharge frames are welded to form rigid bars

f) Rapping Systems:

Rapping systems are provided for collecting and emitting electrodes. Geared motors drive these rappers. The rapping system employs tumbling hammers, which are mounted on the horizontal shaft. As the shaft rotates slowly the hammers tumble on the shaft will clean the entire field. The rapper programmer decides the rapping frequency. The tumbling hammers disposition and the periodicity of rapping are selected in such a way that less than 2% of the collecting area is rapped at any instant. This avoids re-entrainment of dust and puffing at the stack. The rapping shaft from the gear motor drive by a shaft insulator. The space around the shaft insulator is continuously heated to avoid condensation.

g)Insulator Housing:

 The support insulators, supporting the emitting electrodes housed in insulator housings. The HVDC connection is taken through a bushing insulator mounted on the insulator housing wall.

 In order to avoid the condensation on the support insulators, each insulator is provided with one electrical heating element.  Heating elements of one pass are controlled by one thermostat.

 

2) ELECTRICAL SYSTEM:

a) High Voltage Transformer Rectifier (H.V.R) with electronic controller(E.C)

The transformer rectifier supplies the power for particulate charging and collection. The basic function of the E.C is to feed precipitator with maximum power input under constant current regulation.So,thereby any flash over between collecting and emitting electrodes, the E.C will sense the flash over and quickly react by bringing the input voltage ton zero and blocking it for a specific period. After the ionized gases are cleared and the dielectric strength restored, the control will quickly bring back the power to the present value and raise it to the original non-sparking level. Thus the E.C ensures adequate power input to the precipitator while reckoning the electrical disturbances within the precipitator. Regulated ac power from E.C is fed to the primary of the transformer, which is stepped up and rectified to give a full wave power output. The transformer rectifier is mounted on the roof of the precipitator while the E.C is located in an air-conditioned control room.

b) Auxiliary control panel (A.C.P)

The A.C.P controls the power supply to the EP auxiliary i.e. rapping motors and heating element dampers etc.The complete A.C.P. is of modular type with individual modules for each feeder. Each module houses the power and control circuits with meters, push buttons, switches and indicating lamps.

 Following are the modules for the outgoing feeders

Ø   Hopper heaters for each field

Ø  Support insulator heaters

Ø  Shaft insulator heaters

Ø  collecting electrode rapping motor for each field

The program control circuit for the sequence and timing of operation for rapping motors is included in the A.C.P.

 For continuous operation of the rapping motors, the programmer can be bypassed through a switch. Thermal overload relay is provided for overload protection to the rapping motors. Local push buttons are available for tripping the motors to meet the exigencies and for maintenance purposes.

 Ammeters with selector switches to indicate line currents of motors and heating element feeders are provided. Indicating lamps are provided “main supply on”, “overload trip”, “local push button activated”, “space meter on”, and “control supply on”.

 Potential free contacts are provided for remote indication for rapping motor trip due to overload.

c) Safety Interlock:

 A safety interlock system is incorporated to prevent accidental contact with live parts of the precipitator and enable energisation only when the ESP is boxed up. The interlock system covers all the inspection doors of casing, insulator housing and disconnecting switches.

 Warning: familiarity with this system may femon the operating personnel bypass the interlock. As this would defend the very purpose of the interlocking system, such a temptation should be resisted and the sequence of operation at every stage should be systematically followed.

d) Disconnecting switch:

 Each field is provided with one disconnecting switch for isolation of emitting system from the associated transformer .In the on position the emitting system is connected to the transformer and in the OFF position it is grounded.