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Heat Exchangers Design Considerations

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Heat Exchangers Key Concepts Heat Transfer Coefficients Naming Shell and Tube Exchangers Safety In Design of Exchangers Controls for Exchangers

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Heat Exchangers

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Key Concepts "Allow me to summarise: Hot stuff this side, cold stuff that side. Make the cold stuff hotter, but use inbetween stuff to not let the cold stuff actually touch the hot stuff. Cold stuff and hot stuff not allowed to destroy inbetween stuff and vice versa. Some kinds of inbetween stuff works better than others. Might need pumps or fans to make the whole shebang work a little better, too.” - Topher Gayle 4

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General Sizing Method Pick an exchanger type (S&T, Plate & Frame etc.) Choose counter or co-current flow Choose number of tube passes (for S&T) draw Temp diag, Calculate the LMTD and Q Calculate the LMTD Correction Factor (F) if more than two tube passes Choose a U value based on tables Calculate the Area, A = Q/ U LMTD F Perform rigorous rating as required (not 470)

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Key Concepts Heat LostHeat TransferHeat Absorbed Q = U A T ln Either Q = m Cp T, or Q = m H evap = Either Q = m Cp T, or Q = m H evap m Cp hot (T1-T2) = U A T ln = m Cp cold ( t1 - t2) 8-9

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Combined Equations m Cp hot (T1-T2) = U A T ln = m Cp cold ( t1 - t2) Calculate the unknowns Determine the overall heat transfer coefficient (U) value in order to calculate the Area (A) to size the exchanger 9

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Duty Considerations - Q Distillation Columns Start-up and Shut-down usually require the column to operate at “full reflux” Feed and Outlets are shut down 100% of Overhead vapour being condensed 100% of reflux being boiled Compositions can be completely different (reactor not online), therefore diff. temps Is the duty in the simulation truly the worst- case duty? For our purposes assume yes

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Mean Temp Difference T1 T2 t2 t1 Counter Current Exchanger Temp Profile 26-27 T1 T2 t1 t2 Always Draw This Graph !!

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Mean Temp Difference Correction for not strictly counter or co- current flow T2 t1 T1 t2 Hot Side temp & flow direction Cold Side temp & flow direction Exchanger Temperature Profile 26-27 200 38 30 45

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Temperature Correction Factor Form of the heat transfer equations is: The factor F is usually determined Graphically 27-28 T1 = 200 T2 = 38 t1 = 30 t2 = 45 T1 = 200 T2 = 38 t1 = 30 t2 = 45 As A Single Pass Counter Current - temp cross?

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Temperature Correction Factor Form of the heat transfer equations is: The factor F is usually determined Graphically 27-28 T1 = 200 T2 = 38 t1 = 30 t2 = 45 T1 = 200 T2 = 38 t1 = 30 t2 = 45 As a 2 Pass exch - temp cross - low F factor

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Temperature Correction Factor 28-39 R = 11.2, P = 0.0882, F = 0.471

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Condensing LMTD Divide the Exchanger into segments Evaluate U and LMTD for each segment Multicomponent, noncondensables? Arghh! 30

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Counter to the Co-current Use Counter Current maximize LMTD (minimize Area, cost etc.) minimize utility reqt’s Use Co-current minimize outlet utility temperatures during turn down - see later reduced fouling

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Determining U Tables for U values Determine U via fundamental equations note that fouling factors often overshadow much of the accuracy that the fundamental equations provide Details of exchanger configuration required Computer Programs / Vendors vendors can and will provide exch sizing Be knowledgeable enough to critique their design 9-11

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Determining U Physical configuration affects U values Tables assume certain things about the exchanger. If through poor configuration, (ie..inappropriate tube length, or number of tubes) the assumptions are invalidated, then the tables will mislead. 11-13

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U Values & Velocity 11-13

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Physical Config & U Values The following factors all affect the velocities of the fluids in the exchanger Tube Length Tube Dia Number of Tube Passes Number of Tubes / Bundle dia Baffle Spacing Note: This does not apply to condensers or boiling 11-13

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U Values & Velocity Adding a Tube Pass doubles the velocity of the liquid on the tube side Decreasing Baffle Spacing Increases Velocity Shellside 11-13

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Physical Configuration & U values Tube Layout SquareRotated Square Preferred for cleaning Triangular (high heat x-fer) 26

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Velocity Limitations Maximum Velocity is Dictated by: Vibration Erosion Hydraulic Exchanger Physical Size 13

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Velocity Limitations - Vibration Usually a Shell Side Issue Vibration Can Cause Collision Damage, Baffle Damage, Fatigue & Tubejoint Failure Causes Turbulent buffeting Fluidelastic whirling Vibration induced by flow parallel to the tubes 13-15

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Velocity Limitations - Vibration Analysis determine the natural frequency of the tubes vibration of tubes between baffles vibration on U bends account for damping (fluid properties, tube stresses etc.) determine critical flow velocity minimum cross flow velocity that the span may vibrate unacceptably large amplitudes. Analysis by Programs or TEMA Standards 13-15

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Tube Side Velocity Limitations - Erosion High Velocity causes thinning of the metal walls (erosion). It can be avoided by maintaining velocities (ft/sec) below those given by this equation. (about 12 ft/sec for water) TEMA say 2 < 6000 to eliminate tube end erosion 15

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Velocity Limitations - Hydraulics Available pressure drop will limit velocity The P rises to the square of the velocity 60 psig EXCH CV P 0 psig 15-16

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Velocity Limitations - Physical Limitations on Shipping, Floor space etc. all make a difference (don’t forget about pulling the tube bundle) 16-17

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U values of interest Condensing U values are very high (500 to 800) Reboiler U values are very high ( 700) liq / liq U values in middle (100 - 300) Cooling / heating gases (desuperheating) have very low U values (<30)

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TEMA Easier to Clean Less Costly Shell Side Fluid Leaks to Atmosphere Expensive Large Annular Space = Low U Value Cheap, Hard to clean

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Exchanger Selection Require a U-tube or Floating head, instead of fixed tube sheet, when thermal expansion between shell and tubes is an issue i.e. shell side fluid and tube side fluid temperatures differ by more than 200 °F Require a Floating Head, instead of U-tube When cleaning tubes mechanically is important (dirty fluids on tube side) When errosion may occur on tube side

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Reboilers Boiling Phenomena Nucleate boiling at shell/tube T = 20 to 50 °F Nucleate Boiling Film Boiling 52

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Reboilers Sizing Common to use “maximum heat flux” 15,000 BTU/hr sq ft Fundamental Equations can be used to determine the best T Max Flux is a function of the Number of active nucleation which is in turn affected by the materials of construction, the fluid properties and the temperature difference 53-57

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Reboilers Heat Flux can be increased with special systems (i.e. sintering, brazing, flame spraying, electrolytic deposition). Sand blasting, scoring tends not to provide stable long term enhancement. Nucleation SitesTrapped Vapour 55

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Heat Exchanger Safety What Can Fail? Control System Failure Shell & Tube Tube can rupture Tubes separate from Tube Sheet Blocked in exchanger causes cool fluid to experience temperatures of hot fluid Plate & Frame Gaskets can leak mixing hot and cold sides, or releasing either fluid to surroundings

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Heat Exchanger Safety Implications Fires, Explosions, Toxic Releases

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Controlling Exchangers Q = U A T ln A is fixed U varies slightly with velocity T ln is the controlling variable Hot In Hot Out Cold Out Cold In Design Duty Hot In Hot Out Cold Out Cold In Reduced Duty Q = m c T

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Controls Liquid / Liquid - control on cooling media C/w 58

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Controls Liquid / Liquid - control on process C/w 59

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Controls - Steam Heating Steam Pressure Control T 60

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Steam Trap

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Controls - Steam Heating Condensate Level Control 61

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Workshop - Size “Condenser” Duty: 153 x 10 6 KJ/hr T1 = 213.3 °C T2 = 35 °C t1 = 30 °C dew point: 150 °C (to be confirmed in PRO 2) U gas/water - 0.51 kW/ m 2 °C U condensing / water - 0.85 kW/ m 2 °C

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Analysis of heat exchangers: Use of the log mean temperature Difference LMTD Method: Q= (m cp ∆T) h = (m cp ∆T) c Q= U A F∆T lm A=N װ DL ∆ T lm = ∆T l.

Analysis of heat exchangers: Use of the log mean temperature Difference LMTD Method: Q= (m cp ∆T) h = (m cp ∆T) c Q= U A F∆T lm A=N װ DL ∆ T lm = ∆T l.

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