SHELLBELL.ppt Shell-side Calculations Using Bell

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SHELLBELL.pptShell-side Calculations Using
Bells Method
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Components of Course What Stage are We Up To?

• Types of exchangers, revision of OHTCs, fouling
  factors.
• Heat exchanger selection.
• Thermal performance analysis (NTUs) for co-
  counter-current exchangers.
• Multi-pass exchangers (ST).
• Condensation boiling.
• Radiation.

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Reference

• This is an advanced topic more details can be
  found in the book by Hewitt et al
• Hewitt et al, ch. 6.3.2, pp. 275-285
• (Bells method for shell tube exchangers)
• This lecture will focus on the concepts general
  design considerations.

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Basis of Bells Method Motivation for Use of
Method

• Consider flow patterns (first) effects on heat
  transfer pressure drop (not done by older
  methods).
• Still simple enough to be done by hand.
• Can be set up on spreadsheet.

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Flow Patterns, Basic Geometry

• Why shell--tube exchangers are built this way.
  What effects do these manufacturing details have
  on performance (heat transfer pressure drop)?

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Manufacturing Details
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Manufacturing Details (cont.)
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So What?

• These details mean bypass leakage virtually
  inevitable

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Leakage Bypass Diagram
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Leakage Bypass
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Flow Patterns

• Want flow to go across tubes (stream B).
• However, some flow goes through
• gap between tubes baffles (stream A)
• tube bundle shell (stream C)
• baffle shell (stream E)
• between tube passes (stream F)
• Bypass leakage streams degrade heat-transfer
  performance some also reduce pressure drop.

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Leakage (cont.)
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Bypassing
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Why Tube-Lane Partition?
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Reduce Bypassing?
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Common Calculations

• Some calculations common to both heat transfer
  pressure drop estimation
• Shell-side Reynolds number
• free cross-stream flow area
• maximum inter-tube velocity, Vmax
• Re(shell)
• Bypass leakage Good design Combined effect
  decreases actual h to no less than 40-50 of
  ideal value.

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Design Considerations HT Performance Bought with
Pressure Drop

• Reducing pressure drop
• Double, multi-segmental, disk doughnut baffles
• Shell type to split-flow
• Decreasing L
• Increasing tube pitch
• Removing sealing strips
• Removing baffles

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Pressure-Drop Limited Design

• Tend to increase size to reduce pressure drop
• Heat-transfer requirements easily satisfied
• Common example low-viscosity fluids (water)

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Heat-Transfer Limited Design

• Opposite case to pressure-drop limited design
• Common example high-viscosity fluids
• Worth accepting higher running costs

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Design Considerations Velocities

• Too low excessive fouling
• Too high tube vibration, erosion
• Liquids
• tube-side 1-2 ms-1, up to 4 ms-1 if fouling
• shell-side 0.3-1 ms-1
• Gases
• atm pressure 10-30 ms-1

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Design Considerations Temperatures

• Very good heat recovery justified if value of
  value of heat is high
• series 1-2 or 2-4 shell--tube
• plate
• compact
• 1-2 shell--tube guidelines
• coolers max dTgt20oC, min dTgt5oC
• heat recovery min dTgt20oC

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Design Considerations Fouling

• Put fouling fluids inside tubes
• Inside of tubes easier to clean than outside
• Velocities easier to control
• No dead zones

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Design Considerations Fluid Property Effects

• High pressure into tubes (containment)
• High temperatures, corrosive fluids, into tubes
• special alloys may be cheaper in tubes
• reduces heat losses
• High viscosity, low flowrate
• easier to get turbulence on shell-side (Re 100)
• if turbulence not possible on shell-side, better
  to have laminar flow in tubes (prediction better)

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Design Considerations Tube Diameter

• Small tubes give improved heat transfer
• BUT
• Tube cleaning practices require d gt 20mm (approx.)

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Design Considerations Tube Length

• Longer tubes mean
• fewer tubes (fewer pieces to handle)
• smaller shell diameter
• both cheaper
• Usually (tube length)/(shell diameter) from 5-101

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Design Considerations Tube Layout, Pitch

• Pitch 1.25 x tube diameter
• 30o triangular best for clean fluids
• Square with large spacing for fouling fluids

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Design Considerations Baffle Type, Spacing, Cut

• 25 segmental is standard
• Low pressure drop segmental, disk doughnut
• Baffle spacing window size is important should
  be about 1 with 25 cut, this gives spacing
  diameter/4

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Design Considerations Baffle Type, Spacing, Cut
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Design Considerations Baffle Type, Spacing, Cut
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Pressure Drop Heat Transfer

• Pressure drop proportional to (velocity)1.8
• Heat transfer proportional to (velocity)0.6
• Pressure drop harder to predict than heat
  transfer because it is very sensitive to flow
  maldistribution flow-pattern details

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Conclusions

• Bells method considers flow patterns in more
  detail than that of Kern
• Hence generally more accurate
• Predicts pressure drop heat transfer (heat
  transfer predicted more accurately)