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Tuesday, 20 December 2011

THERMODYNAMICS MANUAL / B- TECH MECHANICAL/ BABCOCK & WILCOX BOILER


AIM: – TO STUDY THE MODEL OF BABCOCK &WILCOX BOILER.

APPARATUS: - A MODEL OF BABCOCK & WILCOX.




THEORY: -
THIS IS A HORIZONTAL, EXTERNALLY FIRED, WATER TUBE, NATURAL CIRCULATION TYPE STATIONARY BOILER. THIS BOILER IS THE MOST COMMON TYPE USED IN THERMAL POWER PLANTS FOR GENERATION OF STEAM IN LARGE QUANTITIES. IT CONSISTS OF A HIGH PRESSURE STEEL DRUM MOUNTED AT THE TOP .FROM EACH END OF THE DRUM, CONNECTIONS ARE MADE WITH THE UPTAKE HEADER AND A DOWN TAKE HEADER. THE HEADERS ARE JOINED TO EACH OTHER BY A LARGE NUMBER OF WATER TUBES WHICH ARE KEPT INCLINED AT AN ANGLE OF ABOUT 15DEGREE TO THE HORIZONTAL. THE WATER TUBES ARE STRAIGHT, SOLID DRAWN STEEL TUBES ABOUT 10 CM IN DIAMETER AND ARE EXPANDED INTO THE BORED HOLES OF THE HEADERS.
THE FURNACE IS LOCATED BELOW THE UPTAKE HEADER. THE COAL IS FED TO THE CHAIN GRATE STOKER THROUGH THE FIRE DOOR BAFFLES ARE PROVIDED ACROSS THE WATER TUBES TO ACT AS DEFLECTORS TO THE FLUE GASES. THE HOT GASES RISE UPWARDS, GO DOWN AND THEN RISE UP AGAIN AND FINALLY ESCAPE TO CHIMNEY THROUGH THE SMOKE CHAMBER. TO MAINTAIN A UNIFORM VELOCITY OF FLUE GASES THROUGH OUT THEIR TRAVEL, THE PASSAGE OF THE GASES IS DECREASED FROM FURNACE TO EXIT.
THE CIRCULATION OF WATER IS MAINTAINED BY CONVECTIVE CURRENTS. THE COLD WATER FLOWS FROM THE DRUM TO THE REAR HEADER AND THUS CYCLE IS REPEATED. FOR GETTING SUPERHEATED STEAM, THE STEAM ACCUMULATED IN THE STEAM SPACE IS SENT TO THE SUPERHEATED TUBES WHICH ARE ARRANGED ABOVE THE WATER TUBES. THE SUPER HEATED STEAM IS FINALLY SUPPLIED THROUGH A STEAM STOP VALVE AND STEAM PIPE.
EVAPORATIVE CAPACITY OF SUCH BOILER RAGES FROM20, 000 TO 40,000 KG/HOUR AND OPERATING PRESSURES OF 11.5 TO 17.5 BAR ARE QUITE COMMON. THE INSPECTION OF BOILER CAN BE CARRIED OUT EVEN WHEN THE BOILER IS IN OPERATION, DRAUGHT LOSS IS MINIMUM AND REPLACEMENT OF DEFECTIVES TUBES CAN BE MADE READILY.





CONSTRUCTIONAL DETAILS OF BABCOCK &WILCOX BOILER.

BABCOCK AND WILCOX HAVE FOLLOWING PARTS:-
1. A HORIZONTAL STEAM & WATER DRUM: -
THIS IS MAIN PART OF BOILER. IT IS SUPPORTED BY STEEL STRUCTURE AT CERTAIN HEIGHT & IS INDEPENDENT OF BRICK WORKS. THE SIZE OF THE BOILER DRUM IS SMALL AS COMPARED TO BOILER DRUM OF FIRE TUBE BOILER OF SAME CAPACITY. IT CONTAINS WATER & STEAM. ALL SAFETY & CONTROL DEVICES ARE MOUNTED OVER THE BOILER DRUM.

2. A BUNDLE OF STEEL TUBES: -
THE FRONT END OF THE BOILER DRUM CONNECTED TO THE UPTAKE HEADER BY SHORT TUBE &REAR END IS CONNECTED TO DOWN TAKE HEADER BY ALONG TUBE. IN BETWEEN THE HEADER A NO. OF SMALL DIAMETER STEEL TUBES ARE FITTED AT AN ANGLE 5O DEGREE TO 15 DEGREE WITH HORIZONTAL TO PROMOTE THE WATER CIRCULATION. THE STEEL TUBES ARE ARRANGED IN COMBUSTION CHAMBER IN ZIG ZAG MANNER SO THAT MORE SURFACE AREA OF THE TUBE IS EXPOSED TO HOT GAS.

3. COMBUSTION CHAMBER: -
IT IS A SPACE ABOVE THE GRATE BELOW THE FRONT END OF THE DRUM WHERE COMBUSTION OF FUEL TAKES. THIS COMBUSTION IS ENCLOSED BY BRICK WORKS & IT IS LINED FROM INSIDE BY FIRE BRICKS & DOOR ARE PROVIDED TO GIVE ACCESS OF CLEANING, INSPECTION & REPAIR PURPOSE. COMBUSTION CHAMBER IS DIVIDED INTO THREE SEPARATE COMPARTMENTS BY BAFFLED. THIS MAKES THE LONGER PATH OF HOT GASES BEFORE LEAVING THE BOILER THROUGH THE CHIMNEY. DAMPERS PROVIDED AT REAR END OF CHAMBER TO REGULATE THE FRESH AIR SUPPLY FOR MAINTAINING PROPER COMBUSTION OF FUEL.

4. SAFETY & CONTROL DEVICE: -
SAFETY & CONTROL DEVICE ARE CALLED MOUNTINGS BECAUSE BASICALLY THESE DEVICES ARE MOUNTED OVER THE BOILER DRUM. THESE ARE SAFETY VALVE, PRESSURE GAUGE, WATER LEVEL INDICATOR, FEED CHECK VALVE, STEAM VALVE, BLOWN OFF COCK, FUSIBLE PLUG, AND MAN HOLE.

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