Electrostatic Precipitators for Air Cleaning

Title: Electrostatic Precipitators for Air Cleaning

Author: Oleg Tchetchel

Article:
Canadian Air Systems Co. is a designer and manufacturer of
industrial dust collection systems. Dust collecting equipment is
available in numerous designs utilizing a number of principles
and featuring wide variation in effectiveness, initial cost,
operating and maintenance expense, space, arrangements and
material of construction.

Factors influencing dust collection equipment selection include:

* Concentration and particle size of contaminant.

In most dust conveying systems, usual dust range from 0.1 to
100 micron - a rather wide range of particle sizes.

* Degree of collection required.

Evaluation will consider the need for high efficiency high cost
equipment such as electrostatic precipirators; high efficiency
moderate cost equipment such as fabric or wet collector units;
and the lower cost primary units such as the dry centrifugal
group. Degree of dust collection required can depend on plant
location; compariosn of quantities of material released to
atmoshere with different type of dust collectors; nature of
contaminant - its salvage value or its potential as a health
hazard, public nuissance or ability to damage property - and the
requirements of the local or state air pollution regulations.
* Characteristics of air or gas stream.

High temperature gas streams exceeding 180 F will prevent the
use of standard cotton media in fabric collectors; presence of
steam or condensation of water vapor will cause packling and
plugging of air or dust packages in fabric and dry centrifugal
collectors. Chemical composition can attack fabric or metal in
dry collectors and cause extremely corrosive conditions when
mixed with water in wet type collectors.

* Characteristics of contaminant.

Chemical composition can cause attack on dust collector
elements or corrosion in wet type dust collectors. Sticky
materials like metallic buffing dust impregnated with buffing
compounds can adhere to collector elements plugging dust
collector passages. Linty materials such as dust from textile
opener, picker and napper will adhere to certain types of
collector surfaces or elements. Abrasiveness of many materials
in moderate to heavy concentrations such as dust from sand
blasting will cause rapid wear particularly on dry type
centrifugal collectors. Particles size and shape will rule out
certain collector designs. The parashute shape of particles such
as "bees wings" from grain will "float" through centrifugal
collectors due to their velocity of fall being slower than much
smaller spherical particles of the same specific gravity.
Combustible nature of many finely divided materials will
influence selection of explosion proof dust collectors for such
products.

* Methods of disposal.
Methods of removal and disposal of collected materials will
vary with the material, plant process, quantity involved and the
collector design. Dry collectors can be unloaded continuously or
in batches through dump gates, trickle valves and rotary locks
to conveyors or containers. Wet collectors can be arranged for
batch removal or continual ejection of dewatered material by
flight conveyors or draining as a slurry. Material
characteristics can influence other problems, such as packing
and bridging of dry materials in dust hoppers, floating of
slurry forming characteristics in wet collectors, etc.

One of the most efficient type of dust collectors is
Electrostatic Precipitator.

The principle of dust collection relies on the ability to impart
a negative charge to particles in the gas stream causing them to
move and adhere to the grounded or positively charged collector
plates. Most precipitators are made for horizontal air flow with
velocities 100 to 600 fpm. The collecting plates or electrodes
are parallel elements, usually on 9-inch centers, and
constructed in various ways including corrugated or perforated
plates for rod curtains. The electrode rods are centered between
the collector plates. Volatge difference between electrode and
plate is 60,000 to 75,000 volts in most designs. Collector
plates of cylindrical shapes surrounding the electrode tod are
provided where water is used to wash off collected material and
where the gas stream is under high pressure or vacuum.
Removal of the collected material is obtained by rapping or
vibrating the elements either continuously or at predetermined
intervals. Vibration or unloading usually takes place without
stopping air flow through the cleaning cycle.

Pressure drop is low. Collection efficiency is high and nearly
uniform regardless of particle size including sub-micron
particles. Space is relatively large and cost is high where
small gas volumed (below 50,000 CFM) are involved due to the
cost of high voltage electrical equipment. Efficiency is
improved with increased humidity of the air stream as a change
takes palce in the dielectric properties of the dust. Heavy
concentrations, on the other hand, cause a reduction in
collection as the space chatge on numerous particles blankets
the corona effect from the electrode.

Electrostatic precipitators have been extensively used in high
temperature gas cleaning from equipment such as blast furnaces,
open heart furnaces and central station pulverized fuel boilers.
The chemical industry has many applications including sulfuric
acids plants, carbon black, cement kilns and soda sh from paper
mill black liquor furnaces. As voltage setting is close to the
spark-over potential, application is limited to materials that
are not explosive or combustible in nature unless the carrier
gas stream is an inert gas.

For additional information please refer to
http://www.nis-co.com/dustcollection/Index.html.

Oleg Tchetchel, Ph.D Process Engineer Canadian Air Systems
http://www.nis-co.com
http://www.nis-co.com/fumeextractor/Index.html

About the author:
Oleg Tchetchel, Ph.D Process Engineer Canadian Air Systems
http://www.nis-co.com
http://www.nis-co.com/fumeextractor/Index.html

DISTURBANCE IN BOILER DRUM LEVEL

DISTURBANCE IN BOILER DRUM WATER LEVEL-IF THERE IS SUDDEN LOAD THROW OFF THE WATER LEVEL WILL GO DOWN AND PRESSURE WILL RISE.HENCE WHENEVER THERE IS POSSIBILITY OF LOAD DROP,OPERATOR SHOULD BE CAREFUL TO CONTROL WATER LEVEL.DUE TO SUDDEN LOAD PICK UP WATER LEVEL WOULD RISE,IF TOOMUCH COLD WATER IS FED TO THE BOILER WATER WULD RISE.IF THERE IS ANY DOUBT ABOUT WATERLEVEL THEN CHECK THE WATER LEVEL AT DRUM GAUGE GLASS.IF NONE OF YHE GAUGE GLASSES ARE IN SERVICE IT IS PREFERABLE TO WITHDRAW BOILER FROM SERVICE.IF WATER LEVEL VASHES OUT OF SIGHT AND FEED FLOW IS NOT AVAILABLE FOR CONSIDERABLE TIME IT IS PREFERABLE NOT TO FILL IN WATER UNTIL THE BOILER HAS COOLED DOWN NATURALLY.THIS IS TO AVOID QUENCHING OF HOT METAL PARTS.

POSSIBLE CAUSES OF WATER LEVEL DROPPING IS FEED PUMP FAILURE,FEED WATER PRESSURE REGULATING VALVE MALFUNCTION,ECONOMISER LEAKAGE,BOILER OR WATER WALL TUBE FAILURE,EXTRAORDINARY CHANGE IN LOAD.

ABNORMALLY HIGH WATER LEVEL SHOULD BE AVOIDED AS IT MAY LEAD TO CARRY OVER AND EVEN PRIMING.USE EMERGENCY BLOWDOWN VALVE TO REDUCE LEVEL.

CARRY OVER-IF CARRY OVER OCCURS,AS INDICATED BY LOWER THAN NORMAL STEAM TEMPERATURE OR SUDDEN DROP IN STEAM TEMPERATURE THEN LOWER DRUM LEVEL,CHECK BOILER CONCENTRATION,IF IT IS HIGH REDUCE THE SAME BY BLOWING DOWN THROUGH CBD,INVESTIGATE DRUM INTERNALS ON THE EARLIEST OPPORTUNITY.

REHEATER PROTECTION UPON LOSS OF LOAD

STOPAGGE OF STEAM FLOW THROUGH THE REHEATER WHEN THE ENTERING GAS TEMPERATURE IS HIGH CAN RESULT IN OVERHEATING AND DAMAGE TO THE REHEATER TUBING.TO PREVENT THIS CONDITION,MEASURES TO BE TAKEN.TURBINE VALVE CLOSURE-IF LOSS OF LOAD SHOULD CAUSE THE TURBINE VALVES TO CLOSE.ALL FUEL SHOULD BE TRIPPED AUTOMATICALLY.UNDER A MFT RESULTING FROM TURBINE CONTROL VALVE CLOSURE AN IMMEDIATE RESTART IS IS FEASIBLE IN SOME INSTANCES,AS THE TURBINE IS STILL ROLLING AND WILL BE AVAILABLE TO TAKE STEAM AS SSON AS THE GOVERNER HAS THE TURBINE SPEED UNDER CONTROL.

TURBINE TRIP-IF LOSS OF LOAD OCCURS IN WHICH THE TURBINE IS TRIPPED THE REHEATER SHOULD BE AUTOMATICALLY AND INSTANTANEOUSLY TRIPPED.
REHEATER SAFETY VAVLES WILL PROTECT AGAINST OVERPRESSURE IN THE REHEATER.THE REHEATER CONDENSER DRAINS SHOULD BE OPENED TO VACUATE THE STEAM IN THE REHEATER.

STATOR WATER COOLING SYSTEM OF TURBO-GENERATOR

THE STATOR WINDING OF THE TURBO-GENERATOR IS COOLED BY IRCULATING DEMINERALISED WATER THROUGH HOLLOW CONDUCTORS OF STATOR WINDING BARS IN A CLOSED LOOP.

THE D.M WATER IS FED TO THE FEED HEADER MOUNTED INSIDE THE GENERATOR CASING ON THE TURBINE END.FROM THE FEED HEADER WATER FLOWS THROUTH TEFLON HOSES WHICH CONNECT FEED HEADER TO THE INDIVIDUAL LOWER BARS.WATER PASSES THROUGH LOWER ALONG THE LENGTH TO THE OTHER END AND RETURNS THROUGH THE UPPER BARS OF ANOTHER SLOT AND DRAINS INTO THE DRAIN HEADER ALSO MOUNTED ON TURBINE SIDE AND CONNECTED WITH TEFLON HOUSES TO THE UPPER BARS.THE WATER FLOWING THROUGH THE TERMINAL BAR CIRCULATES THROUGH THE CORRESPONDING TERMINAL BUSHING AND COOLS IT ALSO.

THE PUMP DRIVES THE D.M WATER THROUGH THE COOLERS FILTERS AND WINDINGS AND DISCHARGE INTO A SEPERATE COMPARTMENT OF THE HERMETICALLY SEALED EXPANSION TANK MOUNTED MOUNTED 5M HIGHER FROM CENTRE LINE OF GENERATOR AND MAINTAINED AT VACCUM OF 250-300mm.OF Hg.THE WATER FROM CENTRE THE EXPANSION TANK IS AGAIN DRAWN BY THE PUMP COOLED AND RECIRCULATED.

IF THE PRESSURE OF D.M WATER FALLS IN THE SYSTEM BELOW PARTICULAR VALUE,THE OTHER PUMP AUTOMATICALLY STARTS.THE CLOSED CIRCUIT D,M WATER IS IN TURN COOLED BY D.M WATER SUPPLIED FROM THE STATION D.M PLANT.THE USE OF D.M WATER ON SECONDARY SIDE ELIMINATES ANY ACCIDENTAL CONTAIMINATION OF THE CLOSED CIRCUIT D.M WATER WHICH CALLS FOR TRIPPING AND SHUT DOWN OF THE MACHINE.THE MECHANICAL FILTERS REMOVE FOREIGN PARTICLE IN THE WATER THUS AVOIDING CHOCKING AND EROSEN OF FILTERS OF HOLLOW CONDUCTORS.THESE FILTERS SHOULD BE PERIODICALLY CLEANED ONE BY ONE.THE CHOCKING OF FILTER IS INDICATED BY DIFFERENCE IN INLET AND OUTLET PRESSURES.MAGNETIC FILTERS CHECK MAGNETIC PARTICLES FROM ENTERING HOLLOW CONDUCTORS.THE EXPANSION TANK PROVIDES CONSTANT LEVEL OF WATER DURING NORMAL RUNNING AND HAS MAKE UP PROVISION FROM TURBINE CONDENSATE SYSTEM OR D.M PLANT.A WATER JET EJECTORS CREATES NECESSARY VACCUM IN EXPANSION TANK.

AN ELECTRONIC LEVEL SIGNALLING DEVICE GIVES ALARMS FOR HIGH AND LOW LEVEL IN THE EXPANSION TANK.ONLY STAINLESS STEEL CONNECTING PIPES ARE USED IN THIS SYSTEM.THE WATER FLOW IS MEASURED THROUGH AN ORRIFICE PLATE ALONG 3NO OF DIFFERENTIAL PRESSURE TRANSDUCERS.THESE TRANSDUCERS SEND IMPULSE TO THE FLOW METERS ON SIGNALLING AND ANNUNCIATION PANNELS AND FLOW RECORDER IN CONTROL ROOM.THERE IS PROVISION FOR ALARM AND TRIPPING FOR AND EMERGENCY FLOW AND RESISTIVITY WHEN TWO OUT OF THREE METERS/RECORDERS SHOW EMERGENCY FLOW.

THE SPECIFIC RESISTIVITY IS CONTINUOUSLY MEASURED BY DEVICE WHICH PASSES ELECTRICAL IMPULSE TO INDICATING AND SIGNALLING DEVICE MOUNTED IN U.C.B.THERMOMETERS AND PRESSURE GAUGES ARE MOUNTED FOR MONITORING TEMPERATURES AND PRESSURES AT VARIOUS POINTS.A STARTING PANEL FOR MOTORS AND ANNUNCIATION PANEL FOR CONTROLS HAVE BEEN PROVIDED.

GOVERNING AND PROTECTION SYSTEM OF TURBINE

THE TURBINE IS EQUIPPED WITH HYDROMECHANICAL SYSTEM OF AUTOMATIC GOVERNING WHICH ENSURES SMOOTH AND STABLE OPERATION OF THE TURBOSET UNDER ALL CONDITIONS OF OPERATION.THE SYSTEM IS CAPABLE OF SUSTAINING FULL LOAD DUMP FROM THE GENERATOR WITHOUT THE OPERATION OF PROTECTION SYSTEM AND THUS ENABLES QUICK RELOADING OF THE TURBOSET.SPEED GOVERNOR-THE PRINCIPAL SENSING ORGAN IS DIRECTLY COUPLED TO THE TURBINE ROTOR THROUGH A GEAR COUPLING AND HAS BEEN SO DESIGNED TO MAINTAIN AUTOMATICALLY THE SPEED OF THE TURBOSET WITHIN THE STATIC SPEED REGULATION OF 4.1PERCENT.THE SPEED GOVERNER IS HYDRAULICALLY LOCATED WITHIN THE FRONT BEARING PEDESTAL.

BOILER FEED PUMP

DURING NORMAL RUNNING OF UNIT THE FEED PUMP HAS TO HANDLE HOT WATER RECEIVED FROM FEED WATER STORAGE TANK OF DEARATOR THROUGH ITS BOOSTER PUMP ENHANCING THE POSITIVE SUCTION PRESSURE AND HAS TO DELIVER THE HOT WATER UNDER HIGH PRESSURE TO BOILER DRUM THROUGH HIGH PRESSURE HEATERS AND ECONOMISER FROM STEAM GENERATION.THE FEED PUMP MAY BE TREATED AS THE HEART OF THE UNIT AND THEREFORE FULL ATTENTION IS TO BE PAID DURING ITS STARTING UP SHUTTING DOWN AND NORMAL RUNNING.FEED WATER DELIVERED BY THE FEED PUMP IS ALSO UTILISED IN THE PLANT FOR THE TEMPERATURE CONTROL OF AUXILIARY PRDS STEAM,FOR TEMPERATURE CONTROL OF SUPERHEATED STEAM AND FOR TEMPERATURE CONTROL OF REHEAT STEAM.

DEARATOR

THE CONDENSATE WATER FROM L.P.HEATER OUTLET IS ADMITTED AT THE TOP OF THE DEARATING COLUMN AND FLOWS DOWNWARDS THROUGH THE SPRAY VALVES AND TRAYS.THE TRAYS ARE DESINED TO EXPOSE TO THE MAXIMUM WATER SURFACES FOR EFFICIENT SCRUBBING TO EFFECT THE LIBERATION OF THE ASSOCIATED GASES.STEAM IS ALLOWED TO ENTER FROM THE UNDERNEATH OF THE TRAY AND FLOWS IN THE COUNTER DIRECTION OF CONDENSATE WATER.WHILE FLOWING UPWARDS THROUGH THE TYAYS SCRUBBING AND HEATING IS DONE.THUS THE LIBRATED GASES MOVE UPWARDS ALONG WITH THE STEAM.STEAM GETS CONDENSED ABOVE THE TYAYS AND IN TURN HEAT THE CONDENSATE WATER.LIBRATED GASES ESCAPE TO ATMOSPHERE FROM THE ORRIFICE OPENING MEANT FOR IT.THIS OPENING IS PROVIDED WITH NUMBER OF DEFLECTOR TO MINIMISE THE LOSS OF STEAM.