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.
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.