HT3'ppt Heat Exchanger Fouling

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HT3.pptHeat Exchanger Fouling
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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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Fouling Why Important (Motivation, Context
Relevance)

• Decreases overall heat-transfer coefficient.
• Typical decrease in overall heat-transfer
  coefficient in shell-and-tube exchanger is from
  1000 W m-2 K-1 to 200 W m-2 K-1 over 1 year in
  oil refinery service.
• Assessment of fouling use of fouling
  predictions common task for graduate engineer.

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Links to Other Courses

• Process modelling - a spreadsheet analysis may be
  helpful curve fitting techniques often necessary.

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Fouling What Can You Do About It?

• Removal.
• Prevention.
• Design to predictable fouling conditions
  (living with it).

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Removal

• Boil with water for water-soluble salts (e.g.
  Ca(OH)2, NaCl, Na2CO3
• Detergents / caustic soda (hot)
• Boil with acid (HNO3)
• High doses of water-softening agents
• Physically / mechanically (drilling,
  high-pressure water)

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Prevention

• Routine chemical dosing (water softening)
• Seed liquid with crystals
• Increase velocity

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Design to Predictable Fouling Conditions

• Either use
• constant coefficient (often only information
  available), classical approach
• variable coefficient (with time) where data are
  known as function of time from actual operation
  (Kern)

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Curve fitting for coefficients often necessary
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Conclusions

• Fouling factors come from experience some
  previous data available in texts.
• To some extent, fouling is preventable.
• Fouling can be removed chemically (in situ) or
  mechanically (off-line).