1. Requirements of Good Boiler
• Simple construction, Excellent
workmanship
Low maintenance cost, High availability
• Accommodate expansion and contraction
• Adequate steam and water space
• Good water circulation
• Furnace ensures efficient combustion and maximum
rate of heat transfer
2. contd
• Responsiveness to sudden load demands
and upset conditions.
• Easy access for inspection and repairs.
• A factor of safety that meets code
requirements
3. Heat Transfer in Boiler
• Heat transfers to the water by
– Conduction
– Convection
– Radiation
• Boiler design will determine which mode is
more dominant.
• Radiant boilers are more robust against
sudden load variations.
4. Based on flow of flue gas
Smoke/Fire Tube boilers
•Low steaming capacity
•Low pressure (12-15 bar)
•Low temperature ( 200-300 bar)
• Bigger in size due to large tube bore
• Less flexibility due to large bore tubes
• Poor heat transfer due to lack of circulation
Types of Boiler
5. Water Tube Boilers
•Better suited for power generation/marine
applications
•More compact hence saving in weight
•Can withstand high pressure and temperature
•Much higher thermal efficeiency
•Mechanically more flexible due to small bore, bend
tupes
•Greater heating surface area due to small bore tubes
•Quick response to sudden load variation
Types of Boiler
6. •Greater mechanical flexibility, can accommodate bigger
value of tube expansion.
• Saving in space in consideration of steam production
compared to smoke tube boiler.
•Better water circulation due to natural convection
•Easy of boiler tube renewal.
• Tubes may act as risers or down comers depending on
operating conditions of the boiler.
•Large diameter external down comers ensure efficient and
reliable water circulation.
Water Tube Boiler
11. Characteristics of D-Type
Boilers
• Advantages
– Good natural water circulation because temperature
difference between screen tubes and generating
tubes is large due to location of super-heater tubes
between these tubes.
– Quick steam generation following sudden load
variation. This is because the super-heater tubes are
located in high temperature region of the furnace.
• Disadvantages
– Frequent failure of super-heater tubes due to tube
burning
12. contd
• Super-heater tubes burn out quickly because these are
located in very high temperature region of the furnace and
steam is a poor cooling medium compared to water.
– The super-heater support tubes in particular suffer the
most, because these are made from solid steel angle
bars and no coolant flow through.
– Due to high metal temperature super-heater tubes are
more susceptible to slagging and deposit of molten
glass like substance.
– Use large amount of fire bricks and refractories.
13. General Construction of D-Type
• Upto about 52 tons/hr steam generation.
• At 60 bar pressure.
• At maximum supreheat temp of 515 deg C.
• Consists of two drums, steam drum located
above the water drum.
• Main tube bank consists of 3 rows of fire
row tubes and many rows of small diameter
generating tubes.
14. contd
• Furnace has side and rear water walls.
• Roof tubes are part of side water walls and are
connected to steam drum.
• The bottom ends of water walls connect to a
header which also accommodate floor tubes to
the water drum.
• Rear water walls connect the water and steam
drums through a top and bottom header.
• The top header is connected to steam drum
through risers.
15. Contd
• Super-heater consists of U-bend tubes arranged
between fire rows and generating tubes running
at right angle to the boiler tubes.
• The super-heater tubes are supported by special
alloy heat resisting steel plates.
• Super-heater tubes are connected through
headers which carry internal baffle plates to
provide longer flow path to the wet steam to
absorb required amount of heat.
• Baffle plates also regulate flow velocity of the
steam in the super-heater tubes.
17. Contd
• Furnace floor and front wall are refractory
lined and oil burners are fitted in front wall.
• Economiser and air heaters are located
high up in the gas uptake.
• Adequate number of soot blowers are
provided.
20. Heating Surface
According to ASME code Heating surface of
water tube boilers comprise of following
• 2/3 of shell area
• Plus all tube outer surface area
• Plus 2/3 area of both heads
• Minus area of the tube holes
21. ESD Design
• Since superheat temperature required is only
around 515 C therefore it is not necessary to
place the super-heater in side the furnace where
temperature is 1200-1400 deg C.
• Effect of slow boiler response could be
addressed by providing super-heat temperature
controller.
• This concept led to development of boilers with
superheater tubes located outside the furnace,
in low temperature region of gas uptake.
22.
23.
24.
25. ESD I Boiler
•Super Htr located in
low temp region
exhaust gas path
•Both Primary and
Second have contra
flow flow heating
•Metal temp of
secondary high
•Air attemperator less
efficient
•Burner front
fired
•Flame
impingement
reduced not
eliminated
•Response to
sudden load is
slow.
26. ESD-II
• Poor response of ESD I was mainly due to air
attemperation.
• This was eliminated by installing damper
operated attemperator which could regulate the
flow of hot gases over the super-heater tubes.
• A feed water temperature control element was
also incorporated to absorb heat from the
exhaust gas when the damper was in the by-
pass mode at low loads.
• Feed water control element is basically an
extension economiser .
27.
28. contd
• The problem of superheater metal temperature
continued to exist resulting in slagging.
• Burner remained front fired so all problems of
flame impingement and non-uniform heat flux in
the furnace remained.
• Frequent failure of dampers due to corrosion
and poor lubrication.
• Heat transfer between gas and steam poor due
poor thermal conductivity of hot gases. This
affected boiler response to sudden load
demands.
29. ESD III
• Following changes were made to eliminate
drawback of ESD II.
– Water Attemperation, more effective because water is
better coolant.
– Secondary Super Heater banks receive heat through
parallel flow, hence metal temperature low reducing
slag problem.
– Roof firing to eliminate flame impingement and
uniform heat flux distribution.
– Mono/membrane wall construction, fully water cooled
furnace resulting in less heat loss.
– Due to monowall construction, refractory use has
been reduced almost negligible levels.
31. Disadvantages
• Two additional opening in steam drum, weakens
the drum and also danger of water leakage.
• High risk of furnace explosion due to roof firing
which forms gas pocket in the upper part of
furnace.
• Poor purging of furnace due to gas pocket
formation.
• Explosion more severe due to gas tight
construction.
32.
33.
34. Roof Firing
• Advantages :
– Increased efficiency due to the longer length
allowed for the flame giving more time for
complete combustion. This also allows more heat
to be released as radiant rather than convective
cutting down the required number of screen wall
generating tubes
• The longer period allowed for complete combustion
means that less excess air is required, this has the
knock on effect of lowering the Dew Point of the flue
gasses.
• Equal length flames
– Better gas flow.
35. Reheat Boilers
Advantages :
• Efficiency of steam cycle increases with rise in average
temperature at which heat is supplied.
• By reheating the steam from some appropriate temperature
in the cycle to a higher temperature, effectively means
raising the average temperature at which heat is added to
steam.
• This results in the increase in cycle efficiency and hence
saving in fuel consumption.
• Typical value of gain in specific fuel consumption due to
reheating for a 18500 tons tanker is about 5-7 % of total
fuel consumption.
36. Contd
• Since the work output per kg of steam with
reheat increases, the plant size reduces
considerably for the same maximum
power demand.
• Since, the steam condition is less wet due
to reheating, erosion of blades
considerably reduces.
