SHELL.ppt Shell

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SHELL.ppt Shell Tube Heat Exchangers
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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).
• Boiling condensation.
• Radiation.

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Content

• Shell tube heat exchangers are most common in
  industry.
• What are they why are they built like this?
• What considerations are involved in analysing
  their performance?

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Outcomes

• Understand the basic layout of these exchangers.
• Appreciate the significance of this layout in
  terms of the implications for analysis.

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USEFUL REFERENCES Design of Shell Tube Heat
Exchangers

• Hewitt et al pp. 263-269.
• Heat Exchanger Design Handbook (HEDH), 6 volumes,
  Library reference 621.4025

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Double-Pipe Exchangers
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Development of Shell Tube Heat Exchangers

• Put shell around tubes (shell tube)
• Simple, more or less countercurrent
• BUT
• poor flow distribution (short circuiting)
• poor heat transfer (along tube) on shell side
• hard to get high enough shell-side flows

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Shell Around Tubes
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Development of Shell Tube Heat Exchangers

• Baffled Shells (HEDH 4.2.3-3 to 4.2.3-6)
• Now, combination of
• crossflow, and
• co- or counterflow
• Not easy to analyse.
• BUT
• have turbulent heat transfer to tubes even at low
  Reynolds number

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Baffles in Shells to give high enough shell-side
velocities
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Baffles Tubes

• Many different kinds of baffle
• 25 cut segmental baffle standard (HEDH 4.2.5-6)
• What about tubes?
• Packed in on square or triangular pitch

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Segmental Baffles
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Disk Doughnut Baffles
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Tube Pitch
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Tube Pitch Arrangements
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Tubes

• Tube pitch distance between tube centres.
• Typically, tube pitch 1.25 x tube O.D.
• Tubes held in tubesheet
• welded
• rolled
• packed
• Tube pitch HEDH 4.2.5-2

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Tube Counts

• HEDH 4.2.5-13, Table 2a
• How many tubes are there for a given
• shell I.D.
• tube O.D.
• pitch (square or triangular)
• pitch size
• number of tube passes

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Tubesheet Arrangements

• HEDH 4.2.6-6
• Tubesheet integrity is very important
• corrosion possible
• leakage
• stream mixing may be hazardous

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Tubesheet Layouts
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Thermal Expansion

• Shell tube exchangers can be 10 m long
• thermal expansion problems are possible
• allow one end (or head) to float
• this is difficult with a single tube pass
• hence get 1-2 (etc) exchangers (one shell pass
  shellside fluid goes one way through shell two
  tube passes)

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Flow Velocity

• Advantages of increased flow velocity
• better heat transfer
• less fouling
• BUT
• worse pressure drop
• Can optimise, but usually dont.

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Conclusions

• Shell tube heat exchangers are a development of
  co- counterflow exchangers to give greater
  area, but...
• They involve co-, cross- and counterflow,
  sometimes within the same exchanger.
• We can account for this.