1. Heat Transfer analysis of Supercritical SG
P M V Subbarao
Professor
Mechanical Engineering Department
Generation of Entropy to Generate Most Eligible Steam …..

2. Religious to Secular Attitude of Water

3. Constant Pressure Heating of Supercritical Fluids

4. Isobaric Divergence of Specific Heat

5. Specific heat of Supercritical Water

6. Pseudo Critical Line

7. Extended p-T Diagram

8. Divergence of Thermal Conductivity

9. Divergence of Volume Expansivity

10. Isobaric Variation of Fluid Viscosity

11. Constant Pressure Supercritical Steam Generationk cp  Pr
Temperature of SC Steam

12. Isobaric Variation of Prandl Number SC Steam

13. Local Heat Transfer Coefficient of A SC Steam

14. Actual Heat Transfer Coefficient of SC Water

15. Heating of Ultra Supercritical Flow

16. Impact of Surface Area of Heating

17. Variation of Tube Wall Temperature : Control of Thermal Stresses and Circumferential Cracking

18. Variation of Tube Wall Temperature : Control of Thermal Stresses and Circumferential Cracking

19. 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.

20. 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.

21. Tube to Tube Variation of Sub-Critical Water/Steam Heating
Tangential fired furnace*

22. Solutions to Heterogeneous Heating

23. Spiral Wall : Justice to All

24. 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.

25. 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.

26. HT Performance of Riffled Tubes

27. Furnace Design Vs Ash Content
130%
160%
10%
10%
Ash Content

28. 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.

29. 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.

30. 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.

31. Furnace Energy Balance
Enthalpy to be lost by hot gases:
Water walls
Economizer
Furnace

32. Capacity of Flue Gas Total Thermal Power available with flue gas:
Rate of steam production: