Heat Transfer analysis of Supercritical SG
P M V Subbarao
Professor
Mechanical Engineering Department
Generation of Entropy to
Generate Most Eligible Steam …..
Religious to Secular Attitude of Water
Constant Pressure Heating of Supercritical Fluids
dh Q s hATww T fluid
0.8 kf
1/ 3
h 0.023 Re pr
d
Isobaric Divergence of Specific Heat
Specific heat of Supercritical Water
Pseudo Critical Line
Extended p-T Diagram
Divergence of Thermal Conductivity
Divergence of Volume Expansivity
Isobaric Variation of Fluid Viscosity
Constant Pressure Supercritical Steam Generation
cp
Pr k
Temperature of SC Steam
Isobaric Variation of Prandl Number SC Steam
Local Heat Transfer Coefficient of A SC Steam
hd 0.8
k
0.4 f
0.023 Re pr
kf d
Actual Heat Transfer Coefficient of SC Water
Heating of Ultra Supercritical Flow
Impact of Surface Area of Heating
Variation of Tube Wall Temperature : Control of Thermal
Stresses and Circumferential Cracking
Variation of Tube Wall Temperature : Control of
Thermal Stresses and Circumferential Cracking
Thermo Physics of Supercritical Fluids
• A fluid is in a supercritical state when its temperature and
pressure exceed their critical points Tc , pc.
• As the critical point is approached, several thermophysical
properties of the fluid show strong divergence.
• The isothermal compressibility and isobaric thermal expansion
tend to infinity.
• The thermal diffusivity tends to zero.
• Due to these specific material properties, a new adiabatic
process, often called the ‘‘piston effect’’ can play an important
role in heat transfer problems near the critical point.
The Piston Effect
• When the wall of a tube filled with a near-critical fluid is
heated, a thin thermal boundary layer forms at the wall.
• Due to the high expansion coefficient of the fluid, the layer
can expand very rapidly and, like a piston, it can compress
the rest of the highly compressible fluid.
• The compression results in a homogeneous temperature
rise in the fluid .
• It is worth noting that material properties also change
abruptly far above the critical pressure and around pseudo
critical temperature.
Tube to Tube Variation of Sub-Critical
Water/Steam Heating
Tangential fired furnace*
Solutions to Heterogeneous Heating
Spiral Wall : Justice to All
Spiral Tube Furnace
• The spiral design, utilizes fewer tubes to obtain the desired flow
per tube by wrapping them around the furnace to create the
enclosure.
• This also has the benefit of passing all tubes through all heat
zones to maintain a nearly even fluid temperature at the outlet of
the lower portion of the furnace.
• Because the tubes are “wrapped” around the furnace to form the
enclosure, fabrication and erection are considerably more
complicated and costly.
Riffled Tubes
• The advanced Vertical technology is characterized by low fluid
mass flow rates.
• Normally, low fluid mass flow rates do not provide adequate tube
cooling when used with smooth tubing.
• Unique to the Vertical technology is the use of optimized rifled
tubes in high heat flux areas to eliminate this concern.
• Rifled in the lower furnace, smooth-bore in the upper furnace.
• The greatest concern for tube overheating occurs when the
evaporator operating pressure approaches the critical pressure.
• In the range 210 to 220 bar pressure range the tube wall
temperature required to cause film boiling (departure from
nucleate boiling – DNB) quickly approaches the fluid saturation
temperature.
HT Performance of Riffled Tubes
Furnace Design Vs Ash Content
130% 160%
10% 10%
Ash Content
Issues with High Ash Coals
• Severe slagging and/or fouling troubles that had occurred in
early installed coal fired utility boilers are one of the main
reasons that led to their low availability.
• Furnace dimensions are determined based on the properties of
coals to be burned.
• Some coals are known to produce ash with specific
characteristics, which is optically reflective and can
significantly hinder the heat absorption.
• Therefore an adequate furnace plan area and height must be
provided to minimize the slagging of furnace walls and platen
superheater sections.
• The furnace using high ash coal need to be designed such that the
exit gas temperature entering the convection pass tube coils
would be sufficiently lower than the ash fusion temperatures of
the fuel.
• For furnace cleaning, wall blowers will be provided in a suitable
arrangement.
• In some cases as deemed necessary, high-pressure water-cleaning
devices can be installed.
• As for fouling, the traverse pitches of the tubes are to be fixed
based on the ash content/properties.
• An appropriate number and arrangement of steam soot blowers
shall be provided for surface cleaning.
Countermeasures for Circumferential
Cracking
• There have been cases of waterwall tube failures caused by
circumferential cracking in older coal-fired boilers.
• It is believed that this cracking is caused by the
combination of a number of phenomena,
• the metal temperature rise due to inner scale deposits,
• the thermal fatigue shocks caused by sudden waterwall
soot-blowing, and
• the tube wastage or deep penetration caused by sulfidation.
• Metal temperature rise due to inner scale deposits can be
prevented by the application of an OWT water chemistry
regime.
Furnace Energy Balance
Enthalpy to be lost by hot gases:
m gas c p , gas Tad TFEGT
walls
Water
Economizer
Furnace
Capacity of Flue Gas
Total Thermal Power available with flue gas:
m gas c p , gas Tad Tchimney
Rate of steam production:
m gas c p , gas Tad Tchimney
m steam
hsteam
