HT4'ppt Heat Exchanger Selection

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HT4.pptHeat Exchanger Selection
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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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Outline

• Motivation
• Basic philosophy
• General procedure
• General considerations - what you need to think
  about
• Typical questions and answers
• Examples

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Motivation Specification Design

• Most common design situation is where you need to
  specify type of new exchanger (always first
  step), estimate cost and assess suitability.
• Links to other courses process economics,
  materials, design.

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Basic Philosophy

• Rule out inapplicable exchangers (pressure,
  temperature, fluid compatibility, size range)
• Rank on economic basis

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General Procedure

• Must calculate heat duty
• Minimise cost subject to constraints
• fluid inlet and outlet temperatures
• allowable pressure drops
• compatibility of materials (corrosion) and fluids
  (direct/indirect contact)
• maintenance (repairs)
• availability (can we get it easily?)
• sensitivity to other conditions

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General Considerations

• Design pressures
• Design temperatures
• Heat duty / size range
• Fluid type / compatibility
• Boiling/condensation (quality)
• Temperature driving forces
• Allowable pressure drops
• Fouling tendency
• Space limitations

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Typical Questions to be Answered

• Although their construction is quite complicated,
  large shell-and-tube heat exchangers are often
  used in industry (eg oil refineries) why ?
• For a small heat duty (100 kW) and two
  non-corrosive liquids, what types of exchangers
  should be considered, and what are some of the
  pros and cons of each?

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Parts of the Answers

• Shell-and-tube exchangers are common, because
  construction rules and operating practice are
  well established.
• Double-pipe (concentric pipe) exchangers are
  cheap simple in small sizes (lt20 kW), can be
  constructed by non-specialised labour. It would
  be worthwhile to inquire about small plate
  exchangers.

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General Aspects

• Specialised labour plate, shell-and-tube
• Non-specialised double-pipe

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Plate Spiral Designs

• Plate not for gt 250oC or gt 20 bar (sealing,
  gasket materials)
• Spiral
• like plate, rolled up
• compact, high htcs like plate
• not so flexible cannot add area easily
• have better sealing than plate

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• Not so common for two-phase flow
• narrower gaps in plate, spiral (5 mm typical)
  than the tube diameters (gt19 mm typical) in shell
  tube
• shell tube get blocked less easily
• gasket materials frequently difficult to find for
  plate

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Conclusions

• General heat exchanger selection situation
  involves minimising cost subject to a long list
  of possible constraints
• In general, robustness is a very important factor
  - shell-and-tube exchangers may not be the most
  efficient, but they score highly in this category