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CHAPTER 2 (BOILER AND THERMIC FLUID HEATER), Position of water, Position…
CHAPTER 2 (BOILER AND THERMIC FLUID HEATER)
Basic concept
Consists of:
Heat source
Water inlet
Steam outlet
Water containing drum
Effector
Density
Pressure
Temperature
Water level
Types of boilers
7 types
Fire tube boiler
Small steam capacity
Medium steam pressure low
Water tube boiler
High steam & pressure requirement
Tolerance for water quality low
Packaged boiler
Fluidized bed combustion boiler
Particle in high velocity air stream
Benefits:
Compactness
Fuel flexibility
Higher combustion efficiency
Reduces SOx & NOx
Stoker fire boiler
Speader stokers
Chain-grate or traveling-grate stoker
Waste heat boiler
Thermic fluid heater
Thermic Fluid Heater
Advantages:
Automatic control settings, which offer operational flexibility
Good thermal effeciencies
Non-pressurized steam operation
Closed cycle operation with min losses as compared to steam boilers
Thermal fluid is heat transfer medium
Modern oil fire thermic fluid heater consists of:
Three pass construction
Fitted a with modulated pressure jet system
Double coil
Heater operates on low / high depends on temperature
Assessment of boilers
Boiler blow down
Intermittent blow down
Requires large short-term increase in the amount of feed water
Substantial heat loss
Manual operated valve
Continuous blow down
Steady and constant dispatch
There is no need for regular operator invention once a blow down valve is set
Benefit
Lower pretreatment costs
Less make-up water consumption
Reduced maintenance downtime
Increased boiler life
Lower consumption of treatment chemicals
Process to control total dissolved solid in the boiler
Boiler feed water treatment
Deposit control
Internal water treatment
Chemicals add to prevent the formation of scale
Conditions:
High TDS content in boiler water is tolerated
Small water quantities is required to be treated
Feed water is low in hardness salts
Low pressure
Not recommended if alone
External water treatment
De-aeration
i) Mechanical De-aeration
ii) Chemical De-aeration
Reverse osmosis
Ion exchange
i) Softening
ii) Demineralization
Boiler performances
Causes of poor boiler performance
Poor operation & maintance
Deteriorating fuel & water quality
Poor combustion
Heat transfer surface fouling
Heat balance
Energy losses
Unavoidable
Avoidable losses
To improve efficiency, the following losses can be avoided/ reduced:
Stack gas losses
Losses by unburnt fuel in stack or ash
Blow down losses
Condensate losses
Convection & radiation losses
Boiler efficiency
Direct method
Advantages
Few parameters for computation
Few monitoring instruments
Quick evaluation
Easy to compare evaporation ratios with benchmark figures
Complete mass & energy balance for each steam
Makes it easier to identify options to improve boiler efficiency
Disadvantages
No explanation of low effieciency
Time consuming
Requires lab facilities for analysis
Various losses not calculated
Indirect method
(Not covered in syllabus for this part)
Fouling : deposition of unwanted material
Types of fouling:
Particulate fouling
Chemical reaction fouling
Precipitation fouling
Corrosion fouling
Biological fouling
Solidification fouling
Water Tube Boiler (WTB)
Most common type of large boiler
Consists of:
Upper drum
Connected by tube
Lower drum
Main Components
2) Tubes
Upper drum
Connected by steam generating tube.
Lower drum
3) Drums
Classified as:
Upper (steam) drum.
Lower (mud) drum.
Always liquid full
1) Boiler furnace
To reduce the loss of heat & enhance the heat energy.
4) Gas and oil burners
Burners inject air & fuel through a distribution system that mixes them into the correct concentration so combustion can occur easily.
Key component of the combustion apparatus.
Spuds
Impeller
Ignitor
Dampers
Most plant use forced-draft fans to supply combustion air.
5) Economizer section
To increase boiler efficiency by preheating the water.
6) Boiler functions
Process occurs:
Furnace, which contains cool water in drums and tubes, starts to heat up.
When burners lit, hot combustion gases begin to flow over the generating tubes, riser tubes, downcomer tubes and drums.
Radiant, convective and conductive heat transfer begin to take place.
Hot gases flow out of the firebox, into economizer section & out of stack.
Water temperature increase, pressure increase.
Steam maybe initially vented to the atmosphere.
Density of water decrease & initial circulation is established.
Bubbles begin to form and rise in water, circulation & pressure increase
7) Steam
Steam & water are in contact with each other.
Steam is saturated
Temperature of water, a corresponding pressure of steam exists.
During the desuperheating process, part pf the superheated steam is returned to the steam drum.
Fire Tube Boiler (FTB)
Most complicated boiler
Modified shell & tube exchanger
Effector
Pressure
Temperature
Density
Water level
Function:
To maintain above the tube to protect them from overheating.
Term tube boiler consists of:
Burner
Feed water inlet
Combustion tube
Steam Outlet
Fire tubes
Tube sheets
Boiler shell with feed inlet and outlet connections.
Comparison FTB & WTB
FTB
Outside tube
Inside tube
Internally fired
Lower
Not suitable
Lesser risk
WTB
Inside tube
Outside tube
Externally fired
Higher
Suitable
More risk
Position of water
Position of hot gases
Mode of firing
Rate of steam production
Suitability for large plant
Risk of bursting