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

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 k cp  Pr Temperature of SC Steam

Isobaric Variation of Prandl Number SC Steam

Local Heat Transfer Coefficient of A SC Steam

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: Water walls Economizer Furnace

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