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Sunday, July 11, 2010

Main Cryogenic heat exchanger (MCHE) is one of key equipment in natural gas liquefaction (LNG) plant. MCHE used in liquefaction of natural gas having some special characteristic :
  • Intensive/excessive heat exchange (230 - 400 kW / ton LNG)
  • Complex heat transfer - Heat transfer from one (or several) very high pressure natural gas stream to one or several low pressure refrigerant streams
  • Thermal stress/shock - Very large temperature difference between inlet (40 degC) and outlet temperature (-162 degC)
  • Operate at very low temperature (-162 degC)
  • High heat transfer efficiency - Very low temperature approach (2-3 degC) to maximize heat transfer per unit area
  • Involve phase change and risk of phase separation and proper distribution
  • High risk of leakage and safety related issue
  • High risk of blockage/plugging
  • Lightweight & easy transportation



Two type of compact MCHE widely used in LNG plant. There are Plate-Fin Heat Exchanger and Coil (Spiral) Wound Heat Exchanger. Below images are typical CWHE and PFHE.


CWHE

CWHE is coils/tubes wound in spiral around a mandrel and all coils / tubes are contains within a pressure vessel. Multiple coils/tube in bundle can be flew simultaneously within the pressure vessel.



PFHE

PFHE is corrugated or serrated plate stacking on each and others to creates cross and/or counter flow paths to allow heat transfer of multiple fluids.

Although both type of HE have been widely used, CWHE and PFHE have their own special features and advantages. Below are simple comparison between both CWHE and PFHE.

FeaturesCoil Wound Heat Exchanger (CWHE) Plate-Fin Heat Exchanger (PFHE)
CompactnessCompact Extremely compact
Heat Transfer area (m²/m³) 20 - 300300 - 1400
Flow type in heat transferCross-CounterCross and/or Counter
Flow patternSingle and/or two phasesSingle and/or two phases
Flow streamsSingle or MultipleSingle or Multiple
ConfigurationSingle or multiple coil-in-vessel unitMultiple plate-fin units
Flow path8-12mm tube1-2 mm flow channel
Risk of contaminant built-upLess (smooth tube surface)More (multiple channels / cores increase crevices)
Risk of PluggingLowerHigher
Thermal Stress ResistanceHigher (tube robustness & flexibility)Lower (plate fin inflexible)
Risk of Thermal StressLowerHigher
Gas/Liquid distributionLess mal-distribution (single flow channel)Higher mal-distribution (multiple unit in parallel)
Risk of Thermal ShockLowerHigher (mal-distribution lead to imbalance heat transfer)
SafetyLower (tube contains within pressurized vessel - natural gas leaks to vessel)Higher (natural gas leaks to atmosphere)
AvailabilityHigher (production continue with some tube leaks until next shutdown)Lower (immediate production shutdown when leaks occur)
TransportationReasonable easy (with multiple bundles)Easy (Multiple units)
MaterialAluminum / Stainless Steel / Carbon Steel / Others AlloyAluminum
CostHigherLower


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posted by Webworm, 3:08 AM

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