Water Side of Power Plant Steam Generators By P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Maximum, Efficient & Safe Production of the Working Steam ……

Flow Boiling in Subcritical Systems Flow boiling occurs when all the phases are in bulk flow together in a channel; e.g., vapor and liquid flow in a pipe. This is known as diabatic multiphase flow with heat addition at the channel wall. In these cases the flow patterns would change as the inlet mass flow rates of the gas or liquid are altered. The velocity and void distributions develop along the tube height.

Flow in Tubes demands A Pressure Drop The pressure drop through a tube comprise several components: friction, entrance loss, exit loss, fitting loss and hydrostatic. Pressure drop decides the total allowable quality ….

Modes of Boiling Figure shows a standard form of the flow patterns and the variation of the surface and liquid temperatures in the regions.

Variation Wall Temperature during Sub-critical Boiling process in vertical tube

Single-Phase Liquid Heat Transfer Under steady state one-dimensional conditions the tube surface temperature in region A (convective heat transfer to single-phase liquid), is given by: Newtons Law of Cooling h is the heat transfer coefficient to single-phase liquid under forced convection.

Dittus-Boelter Equation Heat transfer in a flow thru a circular tube can be estimated by the well-known Dittus-Boelter equation.

The Onset of Nucleate Boiling If the wall temperature rises sufficiently above the local saturation temperature pre-existing vapor in wall sites can nucleate and grow. This temperature, TONB, marks the onset of nucleate boiling for this flow boiling situation. From the standpoint of an energy balance this occurs at a particular axial location along the tube length, ZONB. Once again for a uniform flux condition, We can arrange this to emphasize the necessary superheat above saturation for the onset of nucleate boiling

Water Wall Arrangement Reliability of circulation of steam-water mixture. Grouping of water wall tubes. Each group will have tubes of similar geometry & heating conditions. The ratio of flow area of down-comer to flow are of riser is an important factor, RA. It is a measure of resistance to flow.

Design Rules for Steam Generation Capacity For high capacity Steam Generators, the steam generation per unit cross section is kept within the range. High pressure (>9.5 Mpa) use a distributed down-comer system. The water velocity in the down-comer is chosen with care. For controlled circulation or assisted circulation it is necessary to install throttling orifices at the entrance of riser tubes.

Flow Velocity in Riser Tubes The riser tubes are divided into several groups to reduce variation in heat absorption levels among them.

Once Through SGs

Circulation Ratio The circulation ratio is defined as the ratio of mixture passing through the riser and the steam generated in it. The circulation rate of a circuit is not known in advance. The calculations are carried out with a number of assumed values of mixture flow rate. The corresponding resistance in riser and down comer and motive head are calculated. The flow rate at steady state is calculated.

Density Variation during Constant Pressure Heating : Subcritical Vs Supercritical Fluids

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:

Steam Generator : Convective Heating Surfaces HT thru Licking of tubes by Flue gas……..

Distribution of Steam Generation

Paths of Steam and Gas Drum Water walls Economizer

Furnace Wall

Capacity of Super heaters Super heater heats the high-pressure steam from its saturation temperature to a higher specified temperature. Super heaters are often divided into more than one stage. The enthalpy rise of steam in a given section should not exceed 250 – 420 kJ/kg for High pressure. > 17 MPa < 280 kJ/kg for medium pressure. 7 Mpa – 17 MPa < 170 kJ/kg for low pressure. < 7 MPa

Platen Superheaters

Thermal Balance Equation for PSH Energy given out by flue gas: Energy absorption for a Platen SH:

Mechanism of Heat Transfer : Generalized Newton’s Law of Cooling Rate of heat transfer from hot gas to cold steam is proportional to: Surface area of heat transfer Mean Temperature difference between Hot Gas and Cold Steam. Thot gas,in Tcold steam,in Thot gas,out Tcold steam,out

Thermal Profiles of Fluids in A HX Thot gas,out Thot gas,in Tcold steam,out Tcold steam,in Thot gas,in Thot gas,out Tcold steam,out Tcold steam,in

Log Mean Temperature Difference Rate of Heat Transfer U Overall Heat Transfer Coefficient, kW/m2.K

Platen Superheater Platen Superheater : Flat panels of tubes located in the upper part of the furnace, where the gas temperature is high. The tubes of the platen SH receive very high radiation as well as a heavy dust burden. Mechanism of HT : High Radiation & Low convection Thermal Structure: No. of platens No. of tubes in a platen Dia of a tube Length of a tube

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 Super Heater The energy absorbed by steam The convective heat lost by flue gas Overall Coefficient of Heat Transfer, U Platen SH, U (W/m2 K) 120 – 140 Pendent SH, U (W/m2 K) 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.

Economizer The economizer preheats the feed water by utilizing the residual heat of the flue gas. It reduces the exhaust gas temperature and saves the fuel. Modern power plants use steel-tube-type economizers. Design Configuration: divided into several sections : 0.6 – 0.8 m gap

Thermal Structure of Economizer Out side diameter : 25 – 38 mm. Tube thinckness: 3 – 5 mm Transverse spacing : 2.5 – 3.0 Longitudinal spacing : 1.5 – 2.0 The water flow velocity : 600 – 800 kg/m2 s The waterside resistance should not exceed 5 – 8 %. Of drum pressure. Flue gas velocity : 7 – 13 m/s.

Thermal Balance in Economizer. The energy absorbed by steam The convective heat lost by flue gas Overall Coefficient of Heat Transfer, U

Air Pre-Heater An air pre-heater heats the combustion air where it is economically feasible. The pre-heating helps the following: Igniting the fuel. Improving combustion. Drying the pulverized coal in pulverizer. Reducing the stack gas temperature and increasing the boiler efficiency. There are three types of air heaters: Recuperative Rotary regenerative Heat pipe

Combustion Losses C & R losses Hot Exhaust Gas losses APH Economizer CSH Pendent SH Reheater Platen SH Furnace absorption