boilers and_condensers

Chemical Engineering Design

Heat Transfer EquipmentHeat Transfer Equipment2. Boiling and Condensing2. Boiling and Condensing

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy


Boiling Heat TransferBoiling Heat Transfer

• Important for design of reboilers, vaporizersImportant for design of reboilers, vaporizers

• Generally carry out boiling in separate exchangersGenerally carry out boiling in separate exchangers

• Design of steam boilers is covered in next lectureDesign of steam boilers is covered in next lecture



Boiling Heat TransferBoiling Heat Transfer

Pool BoilingPool Boiling

• Agitation by bubbles and Agitation by bubbles and natural convectionnatural convection

• Occurs in kettle reboilersOccurs in kettle reboilers

Flow BoilingFlow Boiling

• Agitation by bubbles and Agitation by bubbles and forced convectionforced convection

• Occurs in thermosiphon Occurs in thermosiphon reboilersreboilers

• High fluid velocityHigh fluid velocity



Thermosiphon ReboilersThermosiphon Reboilers• If we set the reboiler below the liquid If we set the reboiler below the liquid

level in the column sump then the static level in the column sump then the static head drives liquid into the reboilerhead drives liquid into the reboiler

• The difference in density caused by The difference in density caused by vaporization then sets up a circulation, vaporization then sets up a circulation, limited by pressure droplimited by pressure drop

• Typically design for about 25 to 33% Typically design for about 25 to 33% vaporization per passvaporization per pass

• Thermosiphon orientation can be Thermosiphon orientation can be vertical (tubeside flow) or horizontal vertical (tubeside flow) or horizontal (shellside flow)(shellside flow)

• Horizontal is usually cheaper, but Horizontal is usually cheaper, but vertical handles dirty fluids bettervertical handles dirty fluids betterSaturated liquid

Two-phasereturn

HotUtility

Saturated liquid

Two-phasereturn

HotUtility



Thermosiphon ReboilersThermosiphon Reboilers

• Double reboilers in each caseDouble reboilers in each case

• Note large vapor return pipesNote large vapor return pipes

Source: UOP



Flow Regimes in Thermosiphon TubesFlow Regimes in Thermosiphon Tubes

• Different flow regimes occur Different flow regimes occur as vapor/liquid ratio increasesas vapor/liquid ratio increases

• Slug flow is undesirable as it Slug flow is undesirable as it causes noise and vibration, causes noise and vibration, but is also unavoidable in but is also unavoidable in vertical thermosiphonsvertical thermosiphons

• Annular flow is avoided by Annular flow is avoided by designing for < 33% designing for < 33% vaporizationvaporization

• See section on hydraulics for See section on hydraulics for calculation of pressure drop in calculation of pressure drop in two-phase flowtwo-phase flow



Kettle ReboilersKettle Reboilers

heating

medium in

bubble

point

liquid in liquid

outheating

medium

out

vapor

disengaging

spacevapor out

weir

• More expensive than More expensive than horizontal thermosiphonhorizontal thermosiphon

• Larger diameter shell for Larger diameter shell for same dutysame duty

• Additional liquid outlet nozzle Additional liquid outlet nozzle allows for blowdownallows for blowdown

• TEMA types are (A or B) K (T TEMA types are (A or B) K (T or U)or U)

• Often used as steam Often used as steam generator because of generator because of built in separator for built in separator for vapor and allowance for vapor and allowance for blowdownblowdown



Stab-in ReboilersStab-in Reboilers

• The tube bundle can The tube bundle can sometimes be fitted inside sometimes be fitted inside the column sump: this saves the column sump: this saves a shella shell

• The behavior is similar to The behavior is similar to kettle reboilerskettle reboilers

• The designer has to make The designer has to make sure there is enough sump sure there is enough sump height to give good level height to give good level control and pump NPSH control and pump NPSH without exposing tubeswithout exposing tubes



Boiling Heat Transfer CoefficientBoiling Heat Transfer Coefficient• As As ΔΔT between wall and T between wall and

fluid increases, h fluid increases, h increases at first due to increases at first due to bubbles, but then vapor bubbles, but then vapor blankets the surface and blankets the surface and the heat transfer the heat transfer coefficient fallscoefficient falls

• For fired boilers this can For fired boilers this can lead to tube failurelead to tube failure

• Avoid film boiling by Avoid film boiling by limiting design to limiting design to maximum “critical flux”maximum “critical flux”

• See, Chapter 19, Perry’s See, Chapter 19, Perry’s Handbook or good heat Handbook or good heat transfer references for transfer references for correlation of critical flux correlation of critical flux

0.1 1.0 10 100 1000

h

ΔT = Twall - Tfluid

I II III IV V

I: Natural convection heat transferII: Nucleate boiling with agitation by bubblesIII: Nucleate boiling with unstable filmIV: Stable film boilingV: Radiant heat transfer



Approximate h Values for BoilingApproximate h Values for Boiling

Liquid Boiling (Shell Side or Tube Side) h (Btu/(hr.ft2.F))

Water 1500Water solutions, 50% water or more 600Light Hydrocarbons 300Medium Hydrocarbons 200Freon 400Ammonia 700Propane 400Butane 400Amines 300Alcohols 300Glycols 200Benzene, Toluene 200

Note: Coefficients are based on 3/4 inch diameter tubes. For Tube side flows, correct by multiplying by 0.75/Actual OD.



Enhancement of Boiling Heat Enhancement of Boiling Heat Transfer: UOP High Flux TubingTransfer: UOP High Flux Tubing

Porous metal coating applied to ID or ODPorous metal coating applied to ID or OD

PorousCoating

Tube Wall

• Porous boiling surfacePorous boiling surface• Coating thickness 0.127mm - 0.381 mmCoating thickness 0.127mm - 0.381 mm• Strong metallurgical bondStrong metallurgical bond• Interconnecting Channels or “Re-entrant Sites”Interconnecting Channels or “Re-entrant Sites”• Boiling performance ~ 10x greater than bare tubeBoiling performance ~ 10x greater than bare tube• Overall performance ~ 2-5x greater than bare tubeOverall performance ~ 2-5x greater than bare tube

Source: UOP



Magnification of High Flux Tube SurfaceMagnification of High Flux Tube Surface

500x Mag500x Mag

Cavity Coating

Source: UOP



Experimental Pool Boiling CurvesExperimental Pool Boiling Curves

High Flux TubeHigh Flux Tube Bare TubeBare Tube

Hea

t F

lux

Hea

t F

lux

(W/m

²)(W

/m²)

T T (°C)(°C)

Propylene

Water Water

Propylene

10000

100000

1000000

0.1 1 10 100

• Enables closer temperature approachesEnables closer temperature approaches© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy


High Flux Tube ProductsHigh Flux Tube Products

• ID CoatedID Coated– OD Bare or FlutedOD Bare or Fluted

• OD CoatedOD Coated– ID Bare or FinnedID Bare or Finned

Source: UOP



Applications of High FluxApplications of High Flux• ReboilersReboilers

– Thermosyphons (TEMA type H, J or X)Thermosyphons (TEMA type H, J or X)– KettlesKettles– Stab-in BundlesStab-in Bundles

• Condensers (Kettles)Condensers (Kettles)– Boiling RefrigerantBoiling Refrigerant

Source: UOP



Condensing Heat TransferCondensing Heat Transfer

• Important for condensers and heaters that use Important for condensers and heaters that use condensing steamcondensing steam

• Condensing behavior also occurs in many heat Condensing behavior also occurs in many heat exchangers if a vapor is cooled below the dew pointexchangers if a vapor is cooled below the dew point



Two Types of CondenserTwo Types of CondenserTotal CondenserTotal Condenser

• No incondensible materials No incondensible materials in vapor phasein vapor phase

• Heat transfer coefficient Heat transfer coefficient determined by thermal determined by thermal resistances onlyresistances only

Partial CondenserPartial Condenser

• Some vapor components don’t Some vapor components don’t condensecondense

• Heat transfer coefficient determined Heat transfer coefficient determined by thermal and mass transfer by thermal and mass transfer resistancesresistances

• Therefore much lower h.t.c.Therefore much lower h.t.c.

Concentrationprofile

Liquid film

Temperatureprofile

Concentration profile of condensing component

Liquid film

Temperatureprofile

Vapor film with high concentration of non-condensibles



Partial CondensersPartial Condensers• Condensing molecules (e.g. water) have to diffuse through noncondensibles Condensing molecules (e.g. water) have to diffuse through noncondensibles

(e.g. air)(e.g. air)

• Diffusion resistance decreases coefficientDiffusion resistance decreases coefficient• Big difference to size of exchangerBig difference to size of exchanger

• Calculation of condensing coefficients is complex, particularly when coupled Calculation of condensing coefficients is complex, particularly when coupled with diffusion resistances, with diffusion resistances, usually done using HX design programs (HTRI, usually done using HX design programs (HTRI, HTFS, BJAC)HTFS, BJAC)

• Need to design for accumulation of noncondensibles even in total condensersNeed to design for accumulation of noncondensibles even in total condensers• Provide a vent at the top of the exchangerProvide a vent at the top of the exchanger• Manually vent as often as required by experienceManually vent as often as required by experience

• To keep noncondensibles out of steam system, boiler feed water is degassed To keep noncondensibles out of steam system, boiler feed water is degassed by steam strippingby steam stripping



Condensate draining from tubes above increases condensate level on tubes below which decreases condensing coefficient.

Main Resistance to Heat Transfer on the Main Resistance to Heat Transfer on the Condensing Side is the Liquid Film.Condensing Side is the Liquid Film.

Liquid Condensate Film

Condensate draining from tubes above creates constant rippling and turbulence which improves condensing coefficient

Bundle Effects On Heat TransferBundle Effects On Heat Transfer

Tubes in bundle



Approximate h Values for Approximate h Values for CondensationCondensation

Vapor Condensing (Shell Side or Tube Side) h (Btu/(hr.ft2.F))

Steam 1500Steam, 10% non-condensable 600Steam, 20% non-condensable 400Steam, 40% non-condensable 220Pure Light Hydrocarbons 250-300Mixed Light Hydrocarbons 175-250Gasoline 150-220Gasoline-steam mixtures 200Medium Hydrocarbons 100Medium hydrocarbons with steam 125Pure Organic solvents 250Ammonia 600

Note: Coefficients are based on 3/4 inch diameter tubes. For Tube side flows, correct by multiplying by 0.75/Actual OD.


boilers and_condensers

Engineering

Transcript of boilers and_condensers