Sketch of a reliquefaction system for LPG (propane/butane) carriers

A single-line diagram shows the cargo boil-off vapour being drawn from the tank vapour space and led to a reliquefaction plant, typical of a cascade or two-stage compressor cycle. In a single-stage direct cycle used for butane/propane, the vapour is compressed by a compressor (often driven through a clutch from a diesel engine), then cooled and condensed in a condenser (sea-water or air cooled), then returned as liquid to the cargo tank. For propane a cascade or an indirect (regenerative) two-stage system may be used because of its higher pressure and lower critical temperature: the propane vapour is compressed in a two-stage compressor with intercooling, condensed by a separate refrigerant (e.g. R-22/Dichlorodifluoromethane) cascade circuit, or cooled below its condensation temperature using a chiller.

Description of the system

Liquefied gases such as propane and butane are carried at their boiling temperatures under pressure (fully refrigerated or semi-pressurised). Heat leaking into the tanks evaporates some liquid cargo, producing boil-off vapour. If this vapour is not controlled, tank pressure rises. The reliquefaction system takes this vapour from the tank vapour space, compresses it (raising pressure and temperature), cools and condenses it back to a liquid, and returns the liquid to the tank, so there is no net loss of cargo and the tank pressure/temperature are held at set point.

The main components are:

• Cargo/boil-off compressor (often reciprocating, single or two-stage), with inlet filter and knockout pots.
• Condenser: heat exchanger, sea water or refrigerant cooled, where compressed vapour condenses to liquid.
• Expansion and liquid return: condensed liquid is returned via an expansion valve and sub-cooling arrangement to the tank, or to a holding tank.
• Cascade refrigerant circuit (for propane): a closed refrigeration loop (condensing unit = compressor, condenser, expansion valve, evaporator) where the evaporator acts as the condenser's cooling source, enabling condensation below achievable sea-water temperature.
• Control: pressure and temperature control maintains tank set point; the compressor capacity is modulated and the plant cycles on tank pressure.

The system may be operated in a "direct" mode (vapour from tank compressed and recondensed) or an "indirect/regenerative" cascade mode (a high-pressure refrigerant transfers heat). Sub-cooling improves efficiency and prevents flashing in the tank.

Note for the exam: describe the chosen cycle clearly, mention compressor, condenser, expansion and return, and explain the need for cascade for propane.