A B C D E F
1/f FLUCTUATIONS IN BOILING CRISIS F-FACTOR METHOD FACHINFORMATIONSZENTRUM KARLSRUHE, FIZ FACILITIES WITH A MOVING PISTON FAHRENHEIT TEMPERATURE SCALE FALKNER-SKAN EQUATION FALLING DROPLET EFFECT FALLING FILM EVAPORATORS Falling film flow Falling film heat transfer Falling film mass transfer Falling film, combined heat and mass transfer FALLING RATE PERIOD, DRYING CURVE FANNING EQUATION FANNING FRICTION FACTOR FANNO FLOW FARADAY MHD GENERATOR FARADAY'S LAWS FAST DEFLATION METHODS FAST NEUTRONS FAST REACTORS FATIGUE LIFE PREDICTION FAULT TREES FAXEN FORCES FEEDBACK CONTROL FEEDBACK, DYNAMIC FEEDFORWARD CONTROL FEEDING SYSTEM FOR PLASMA REACTOR FEEDWATER HEATERS FERMI-DIRAC DISTRIBUTION FERROELECTRICITY FERROMAGNETIC FLUID FERTILE ISOTOPES FERTILE MATERIAL FIBER COMPOSITE MATERIALS Fibers FIBRE FILTERS FIBRE OPTICS FIBRE SATURATION POINT FIBROPOROUS MEDIA FIBROUS MEDIA FICK'S LAW FICK'S LAW OF DIFFUSION FICK'S LAW, GENERALIZED FIGURE OF MERIT FOR NUCLEAR REACTOR COOLANTS FILIPPOV EQUATION, FOR THERMAL CONDUCTIVITY OF SOLUTIONS FILM BOILING FILM BOILING COLLAPSE FILM CONDENSATION FILM CONDUCTANCE METHOD, FOR FILM THICKNESS MEASUREMENT FILM COOLING FILM FLOW RATE MEASUREMENT FILM FLOW REGIMES FILM INVERSION FILM METHOD FILM THICKNESS FILM THICKNESS DETERMINATION BY OPTICAL METHODS FILM THICKNESS IN ANNULAR FLOW Film thickness measurements FILMWISE CONDENSATION FILTERING CENTRIFUGES FILTERS FILTRATE FILTRATION FIN EFFICIENCY FIN EFFICIENCY FACTOR FIN TEMPERATURE EFFECTIVENESS FINITE DIFFERENCE METHODS FINITE ELEMENT METHODS FINITE VOLUME FOR SINGLE-PHASE FLOW Finite-element method for radiation diffusion in nonisothermal and nonhomogeneous media FINNED TUBES FINS FINS, CIRCUMFERENTIAL OR HELICAL FIRE BALLS FIRE POINT FIRE SPRINKLERS FIRE TUBE BOILERS FIRES FIRES SUPPRESSANTS First approach engineering models and useful data FIRST LAW OF THERMODYNAMICS FIRST NORMAL STRESS DIFFERENCE COEFFICIENT FISSILE MATERIAL FISSION FISSION PRODUCTS FISSION REACTION FIXED BED REGENERATORS FIXED BEDS FIXED TUBE SHEET EXCHANGERS FIZ FLAME SPEED, OR VELOCITY FLAME TEMPERATURES FLAMEOUT FLAMES FLAMES, TURBULENT FLAMMABILITY FLAMMABILITY LIMIT FLANGES FLASH DISTILLATION FLASH EVAPORATORS FLASH POINT FLASH SMELTING FLASHING FLOW FLAT PLATE COLLECTOR FLAT PLATE, BOUNDARY LAYER ON FLAT PLATE, COMBINED RADIATION AND CONVECTIVE HEAT TRANSFER TO FLIP-FLOP FLOW IN PARALLEL FLOATING HEADER EXCHANGERS FLOCCULATION Flooding and flow reversal FLOTATION FLOW ACROSS CYLINDERS, TUBES FLOW AND SOLUTE TRANSPORT IN SATURATED POROUS MEDIA FLOW BOILING OF ISO-OCTANE FLOW ENHANCEMENT FLOW EXCURSION INSTABILITIES FLOW IN A CURVED POROUS FLOW INSTABILITIES FLOW LAWS IN METALLIC FOAMS FLOW MALDISTRIBUTION Flow Metering FLOW OF FLUIDS FLOW OSCILLATIONS FLOW PATTERN ANALYSIS OF FLOW BOILING IN MICROGRAVITY FLOW PATTERN MAPS IN MINICHANNELS FLOW PATTERNS FLOW REGIME INDUCED INSTABILITIES, TWO-PHASE SYSTEMS FLOW REGIME RELAXATION INSTABILITIES FLOW REGIMES IN BUBBLE FLOW FLOW REVERSAL FLOW SEPARATION FLOW SPLITTING FLOW VISUALIZATION FLUE GASES Fluid FLUID DYNAMICS FLUID FILLED THERMOMETERS FLUID MECHANICS FLUID-SATURATED POROUS MEDIUM FLUIDICS FLUIDIZATION Fluidized bed FLUIDIZED BED GASIFICATION FLUIDS FLUMES FLUORESCEIN FLUORESCENCE FLUORESCENCE METHOD, FOR FILM THICKNESS MEASUREMENT FLUORESCENCE PHOTOGRAPHY FLUORINE FLUTED TUBES FLUX METHODS FLUX PARAMETER FLYING VEHICLES, AERODYNAMICS OF FMEA, FAILURE MODES AND EFFECTS ANALYSIS FOAM FRACTIONATION FOAM METALLIC FOGGING FOGGING IN CONDENSERS FOKKER-PLANK EQUATION FORCED CONVECTION FORCED CONVECTION BOILING FORCED DRAFT AIR COOLED HEAT EXCHANGERS FORCED VORTEX FOREST FIRES FORM DRAG FORWARD SWEPT AXIAL COMPRESSOR ROTORS FOSSIL FUEL FIRED BOILERS FOSSIL FUELS FOULING FOULING FACTORS FOULING RESISTANCE FOUR STROKE CYCLE FOURIER ANALYSIS FOURIER EQUATION FOURIER FLUID FOURIER INTEGRAL FOURIER LAW, GENERALIZED FOURIER NUMBER FOURIER SERIES FOURIER SPECTRAL SOLVER Fourier spectroscopy and experimental techniques FOURIER TRANSFORMATION FOURIER'S LAW FOURIER, BARON JEAN BAPTISTE JOSEPH (1768-1830) FOURIER-BESSEL SERIES FRACTAL ATTRACTOR FRACTAL GEOMETRY FRACTIONAL JEFFREY FLUID FRACTIONATION FRACTURE MECHANICS FRACTURE OF SOLID MATERIALS FRACTURE TOUGHNESS FRANCIS TURBINES FRAUNHOFER DIFRACTION FREDHOLM INTEGRAL EQUATIONS FREE BOUNDARY FREE CONVECTION FREE CONVECTION BOUNDARY LAYERS FREE ENERGY FREE JETS Free Molecule Flow FREE SETTLING FREE SURFACE FLOW FREE TURBULENT FLOW FREE VORTEX FREEZE DRYING FREEZING FREEZING FOULING FREEZING POINT DEPRESSION FREONS FRESNEL'S FORMULAS FREYN CHEQUERWORK REFRACTORIES FRICTION COEFFICIENT FRICTION FACTOR Friction factor. Skin- friction coefficient FRICTION FACTORS FOR SINGLE PHASE FLOW IN SMOOTH AND ROUGH TUBES Friction Velocity FRICTIONAL PRESSURE DROP FRIEDEL CORRELATION FRINGE MODEL, FOR LDA FROSSLING MARSHALL EQUATION FROST FORMATION FROSTING FROTH FROTH FLOTATION FROTH FLOW FROUDE NUMBER FROUDE NUMBER, EFFECT ON JET IMPINGEMENT FUEL ASSEMBLIES FUEL CELLS FUEL RODS FUEL TO AIR RATIO FUEL-COOLANT INTERACTION, FCI FUELS FUELS, PROPERTIES OF FUGACITY FUGACITY COEFFICIENT FULLER-SCHETTER-GITTINGS EQUATION, FOR DIFFUSION COEFFICIENT IN GASES Fully developed flow (stabilized flow) FUNCTIONS Fundamental unit of measurement FURNACE FIRED BOILER FURNACE MODELS FURNACES FUSED SILICA OPEN TUBULAR COLUMNS, FSOT FUSION REACTORS FUSION, NUCLEAR FUSION REACTORS
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FEEDWATER HEATERS

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Introduction

A feedwater heater is used in a conventional power plant to preheat boiler feed water. The source of heat is steam bled from the turbines, and the objective is to improve the thermodynamic efficiency of the cycle. The most common configuration of feedwater heater is a shell and tube heat exchanger with the feedwater flowing inside the tubes and steam condensing outside. (See Boilers and Shell and Tube Heat Exchangers.)

Temperature Profiles

Figure 1 depicts the temperature profiles for a high-pressure feedwater heater which receives superheated steam extracted from a high-pressure turbine.

Temperature profiles for a high pressure feedwater heater.

Figure 1. Temperature profiles for a high pressure feedwater heater.

If sufficient superheat is available, it is possible to make use of the large temperature difference by specifying a separate section within the heater in which desuperheating occurs with a dry wall. This gives a higher heat flux than if condensation occurs, and also allows the possibility of raising the feedwater outlet temperature above that of the steam saturation temperature. The steam condenses almost isothermally, and the condensate is subcooled below the saturation temperature.

In the subcooling zone heater surface is assigned to extract heat from the condensate (drains) from the condensing zone.

A heater may have neither a desuperheating zone nor a drain cooling zone.

Feedwater Heater Geometries

Figure 2 shows, in schematic form, the general arrangement of a three-zone heater. The shell contains a bundle of tubes (normally U-tubes). Two tube passes are almost always used. The feedwater inlet and outlet nozzles are connected to a channel on one side of the tube plate.

Typical airangemcnt of a three zone feedwater heater. (From Process Heat Transfer, 1994, CRC Press.)

Figure 2. Typical airangemcnt of a three zone feedwater heater. (From Process Heat Transfer, 1994, CRC Press.)

In the condensing zone, the tubes are supported by plates or grids of rods. The desuperheating and drain-cooling zones are contained within the shell by a shroud or wrapper, and are usually well baffled to both support the tubes and promote a satisfactorily high shellside heat transfer coefficient. Sometimes other types of a baffle support, based on some form of grid or array of rods, are used to minimize the risk of tube vibration.

High pressure units are sometimes of the "header-type" construction. This is a specialized design in which the feedwater inlet and outlet headers take the form of separate cylindrical vessels which penetrate into the heater shell. Each tube is individually welded onto the headers, and the headers are welded to the shell. There are usually four tube passes.

Feedwater heaters can be located either horizontally or vertically. The horizontal orientation is more common, but vertical heaters are sometimes preferred.

A feedwater heater must be equipped with a vent to allow removal of non-condensing gases.

Thermal Design Considerations

Thermal design of a feedwater heater requires an economic optimization of many factors, including material and operating costs.

Two publications which describe feedwater heaters, and their design, in some detail are those of BEAMA (1968) and HEI (1984). These documents provide performance charts which can be used to estimate the surface area requirement. However, a computer program is required to achieve an optimized design. The paper by Clemmer and Lemezis (1965) presents a design logic which is suitable for implementation in a computer program. Further background information can be found in the publication by EPRI (1984).

Special attention must be paid to avoidance of (a) wet-wall conditions in the desuperheating section, in order to avoid erosion/corrosion problems and (b) excessive pressure drop in the drain cooler, which could cause flashing, and consequent tube damage.

Pressure loss in the desuperheating zone causes a reduction in the saturation temperature of the steam condensing zone. This in turn causes a reduction in the temperature difference in the condensing zone. Design of the two zones is therefore a compromise between the need to maintain a high heat transfer coefficient in the desuperheating zone, while avoiding an excessive reduction in the overall mean temperature difference.

REFERENCES

BEAMA (1968) Guide to Design of Feedwater Heating Plant, The British Electrical and Allied Manufacturers' Association Ltd., London.

EPRI (1984) Symposium on State-of-the-art Feedwater Heater Technology, Report No. CS/NP-3743, EPRI, Palo Alto, California.

Clemmer, A. B. and Lemezis, S. (1965) Selection and Design of Closed Feedwater Heaters, ASME Paper 65-WA/PTC-5, ASME. Winter Annual Meeting, Chicago, November 7-11, (1965) ASME, Vol. 79, No. 7, 1494-1500.

HEI (1984) Standards for Closed Feedwater Heaters, 4th edition. Heat Exchange Institute, Cleveland, Ohio.

Hewitt, G. F, Shires, G. L., and Bott, T. R. (1994) Process Heat Transfer, CRC Press.

References

  1. BEAMA (1968) Guide to Design of Feedwater Heating Plant, The British Electrical and Allied Manufacturers' Association Ltd., London.
  2. EPRI (1984) Symposium on State-of-the-art Feedwater Heater Technology, Report No. CS/NP-3743, EPRI, Palo Alto, California.
  3. Clemmer, A. B. and Lemezis, S. (1965) Selection and Design of Closed Feedwater Heaters, ASME Paper 65-WA/PTC-5, ASME. Winter Annual Meeting, Chicago, November 7-11, (1965) ASME, Vol. 79, No. 7, 1494-1500.
  4. HEI (1984) Standards for Closed Feedwater Heaters, 4th edition. Heat Exchange Institute, Cleveland, Ohio.
  5. Hewitt, G. F, Shires, G. L., and Bott, T. R. (1994) Process Heat Transfer, CRC Press.

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