Heat Transfer Equipment 2 Boiling and Condensing 2012

Heat Transfer Equipment 2. Boiling and Condensing © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Boiling Heat Transfer • Important for design of reboilers, vaporizers • Generally carry out boiling in separate exchangers • Design of steam boilers is covered in next lecture © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Boiling Heat Transfer Pool Boiling Flow Boiling • Agitation by bubbles and natural convection • Agitation by bubbles and forced convection • Occurs in kettle reboilers • Occurs in thermosiphon reboilers • High fluid velocity © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Thermosiphon Reboilers © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy • If we set the reboiler below the liquid level in the column sump then the static head drives liquid into the reboiler • The difference in density caused by vaporization then sets up a circulation, limited by pressure drop • Typically design for about 25 to 33% vaporization per pass • Thermosiphon orientation can be vertical (tubeside flow) or horizontal (shellside flow) • Horizontal is usually cheaper, but vertical handles dirty fluids better Chemical Engineering Design

Thermosiphon Reboilers Source: UOP • Double reboilers in each case • Note large vapor return pipes © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Flow Regimes in Thermosiphon Tubes • Different flow regimes occur as vapor/liquid ratio increases • Slug flow is undesirable as it causes noise and vibration, but is also unavoidable in vertical thermosiphons • Annular flow is avoided by designing for < 33% vaporization • See section on hydraulics for calculation of pressure drop in two-phase flow © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Kettle Reboilers heating medium in heating medium out vapor disengaging space • More expensive than horizontal thermosiphon vapor out • Larger diameter shell for weir bubble point liquid in © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy same duty • Additional liquid outlet nozzle allows for blowdown • TEMA types are (A or B) K (T or U) • Often used as steam liquid out generator because of built in separator for vapor and allowance for blowdown Chemical Engineering Design

Stab-in Reboilers • The tube bundle can sometimes be fitted inside the column sump: this saves a shell • The behavior is similar to kettle reboilers • The designer has to make sure there is enough sump height to give good level control and pump NPSH without exposing tubes © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Boiling Heat Transfer Coefficient I II IV V h • As ΔT between wall and fluid increases, h increases at first due to bubbles, but then vapor blankets the surface and the heat transfer coefficient falls • For fired boilers this can lead to tube failure 0. 1 1. 0 10 1000 ΔT = Twall - Tfluid I: Natural convection heat transfer II: Nucleate boiling with agitation by bubbles III: Nucleate boiling with unstable film IV: Stable film boiling V: Radiant heat transfer © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy • Avoid film boiling by limiting design to maximum “critical flux” • See, Chapter 19, Perry’s Handbook or good heat transfer references for correlation of critical flux Chemical Engineering Design

Approximate h Values for Boiling Liquid Boiling (Shell Side or Tube Side) h (Btu/(hr. ft 2. F)) Water solutions, 50% water or more Light Hydrocarbons Medium Hydrocarbons Freon Ammonia Propane Butane Amines Alcohols Glycols Benzene, Toluene 1500 600 300 200 400 700 400 300 200 Note: Coefficients are based on 3/4 inch diameter tubes. For Tube side flows, correct by multiplying by 0. 75/Actual OD. © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Enhancement of Boiling Heat Transfer: UOP High Flux Tubing Porous metal coating applied to ID or OD Porous Coating Tube Wall Source: UOP • • • Porous boiling surface Coating thickness 0. 127 mm - 0. 381 mm Strong metallurgical bond Interconnecting Channels or “Re-entrant Sites” Boiling performance ~ 10 x greater than bare tube Overall performance ~ 2 -5 x greater than bare tube © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Magnification of High Flux Tube Surface Cavity Coating Source: UOP 500 x Mag © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Experimental Pool Boiling Curves Heat Flux (W/m²) 1000000 Bare Tube High Flux Tube 100000 Water Propylene 10000 0. 1 1 10 100 DT (°C) • Enables closer temperature approaches © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

High Flux Tube Products • ID Coated – OD Bare or Fluted • OD Coated – ID Bare or Finned Source: UOP © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Applications of High Flux • Reboilers – – – • Thermosyphons (TEMA type H, J or X) Kettles Stab-in Bundles Condensers (Kettles) – Boiling Refrigerant Source: UOP © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Condensing Heat Transfer • Important for condensers and heaters that use condensing steam • Condensing behavior also occurs in many heat exchangers if a vapor is cooled below the dew point © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Two Types of Condenser Total Condenser Partial Condenser • No incondensible materials in vapor phase • Some vapor components don’t condense • Heat transfer coefficient determined by thermal resistances only • Heat transfer coefficient determined by thermal and mass transfer resistances • Therefore much lower h. t. c. Liquid film Concentration profile Temperature profile Concentration profile of condensing component Temperature profile Vapor film with high concentration of non-condensibles © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Partial Condensers • Condensing molecules (e. g. water) have to diffuse through noncondensibles (e. g. air) • Diffusion resistance decreases coefficient • Big difference to size of exchanger • Calculation of condensing coefficients is complex, particularly when coupled with diffusion resistances, usually done using HX design programs (HTRI, HTFS, BJAC) • Need to design for accumulation of noncondensibles even in total condensers • • • Provide a vent at the top of the exchanger Manually vent as often as required by experience To keep noncondensibles out of steam system, boiler feed water is degassed by steam stripping © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Bundle Effects On Heat Transfer Liquid Condensate Film Tubes in bundle Condensate draining from tubes above creates constant rippling and turbulence which improves condensing coefficient Condensate draining from tubes above increases condensate level on tubes below which decreases condensing coefficient. Main Resistance to Heat Transfer on the Condensing Side is the Liquid Film. © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design

Approximate h Values for Condensation Vapor Condensing (Shell Side or Tube Side) h (Btu/(hr. ft 2. F)) Steam, 10% non-condensable Steam, 20% non-condensable Steam, 40% non-condensable Pure Light Hydrocarbons Mixed Light Hydrocarbons Gasoline-steam mixtures Medium Hydrocarbons Medium hydrocarbons with steam Pure Organic solvents Ammonia 1500 600 400 220 250 -300 175 -250 150 -220 200 125 250 600 Note: Coefficients are based on 3/4 inch diameter tubes. For Tube side flows, correct by multiplying by 0. 75/Actual OD. © 2012 G. P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy Chemical Engineering Design