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Superheating of Steam. P M V Subbarao Professor Mechanical Engineering Department. Recognition and Adaptation of Efficient Mode of Heat Transfer …. Furnace Energy Balance. Enthalpy to be lost by hot gases:. Water walls. Economizer. Furnace. Capacity of Flue Gas. - PowerPoint PPT Presentation
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Superheating of Steam
P M V Subbarao
Professor
Mechanical Engineering Department
Recognition and Adaptation of Efficient Mode of Heat Transfer …..
Furnace Energy Balance
Water
walls
Economizer
Furnace
Enthalpy to be lost by hot gases:
FEGTadgaspgas TTcm ,
Capacity of Flue Gas
Total Thermal Power available with flue gas:
chimneyadgaspgas TTcm ,
Rate of steam production:
steam
chimneyadgaspgassteam h
TTcmm
,
s
1
2
3
4
5
6
2f
2s
4523 hhmhhm mreheatsteafmainsteam
Paths of Steam and Gas
Water walls
Drum
Economizer
DPNL SH
Platen SHT
R
RHTR
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCWpump
Bottom ash
stack
screentubes
Thermal Structure of A Boiler Furnace
Convective Superheater (Pendant)
• Convective super heaters are vertical type (Pendant ) or horizontal types.
• The Pendant SH is always arranged in the horizontal crossover duct.
• Pendant SH tubes are widely spaced due to high temperature and ash is soft.
• Transverse pitch : S1/d > 4.5
• Longitudinal pitch : S2/d > 3.5.
• The outside tube diameter : 32 – 51mm
• Tube thickness : 3 – 7mm
S1
S2
Convective Superheater (Horizontal)• The horizontal SH are located in the back pass.• The tubes are arranged in the in-line configuration.• The outer diameter of the tube is 32 – 51 mm.• The tube thickness of the tube is 3 – 7 mm.• The transverse pitch : S1/d = 2 – 3.• The longitudinal pitch :S2/d = 1.6 – 2.5.• The tubes are arranged in multiple parallel sets.• The desired velocity depends on the type of SH and operating steam
pressures.• The outside tube diameter : 32 – 51mm• Tube thickness : 3 – 7mm
S1
S2
Thermal Balance in Convective SH.
• The energy absorbed by steam
• The convective heat lost by flue gas
• Overall Coefficient of Heat Transfer, K
)( sup,sup,, inoutsteamconabs hhmQ
TFKAQ SHlosscon
,
ssc
sc
me
me
as
as
g hh
K11
1
Mean Temperature Difference
• The average temperature difference for parallel flow and counter flow is expressed as
min
max
minmax
lnTT
TTT
2minmax TT
T
• It is also called log mean temperature difference• When tmax /tmin > 1.7, the average temperature may be expressed
as:
• Generally, the flow direction of the flue gas is perpendicular to the axes of tubes.• If number of bends are more than four, the flow can be treated as counter or parallel
flow.
S1
S2
S1
S2
Inline
Staggered
Effectiveness Factor
• The ratio of the heat transfer between fouled and clean tubes.
oK
K
sg
O
hh
kK11
1
Thermal Balance in Super Heater.
• The energy absorbed by steam
• The convective heat lost by flue gas
• Overall Coefficient of Heat Transfer, U
)( sup,sup,, inoutsteamconabs hhmQ
TUAQ SHlosscon
,
Platen SH, U (W/m2 K) 120 – 140
Pendent SH, U (W/m2 K) 120 – 140
Convective SH, U (W/m2 K) 60 – 80
Reheater
• The pressure drop inside reheater tubes has an important adverse effect on the efficiency of turbine.
• Pressure drop through the reheater should be kept as low as possible.• The tube diameter : 42 – 60mm.• The design is similar to convective superheaters.
• Overall Heat Transfer Coefficient : 90 – 110 W/m2 K.