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.
| Features | Coil Wound Heat Exchanger (CWHE) | Plate-Fin Heat Exchanger (PFHE) |
| Compactness | Compact | Extremely compact |
| Heat Transfer area (m²/m³) | 20 - 300 | 300 - 1400 |
| Flow type in heat transfer | Cross-Counter | Cross and/or Counter |
| Flow pattern | Single and/or two phases | Single and/or two phases |
| Flow streams | Single or Multiple | Single or Multiple |
| Configuration | Single or multiple coil-in-vessel unit | Multiple plate-fin units |
| Flow path | 8-12mm tube | 1-2 mm flow channel |
| Risk of contaminant built-up | Less (smooth tube surface) | More (multiple channels / cores increase crevices) |
| Risk of Plugging | Lower | Higher |
| Thermal Stress Resistance | Higher (tube robustness & flexibility) | Lower (plate fin inflexible) |
| Risk of Thermal Stress | Lower | Higher |
| Gas/Liquid distribution | Less mal-distribution (single flow channel) | Higher mal-distribution (multiple unit in parallel) |
| Risk of Thermal Shock | Lower | Higher (mal-distribution lead to imbalance heat transfer) |
| Safety | Lower (tube contains within pressurized vessel - natural gas leaks to vessel) | Higher (natural gas leaks to atmosphere) |
| Availability | Higher (production continue with some tube leaks until next shutdown) | Lower (immediate production shutdown when leaks occur) |
| Transportation | Reasonable easy (with multiple bundles) | Easy (Multiple units) |
| Material | Aluminum / Stainless Steel / Carbon Steel / Others Alloy | Aluminum |
| Cost | Higher | Lower |
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