MAN B&W ME-C series for LNG Carriers. Boil-off cycle
The cargo cycle consists of an LD compressor, a plate-fin cryogenic exchanger, a separator and an LNG return pump. Boil-off is evacuated from the LNG tanks by means of a conventional centrifugal low duty compressor. The vapour is compressed to 4.5 bar and cooled at this pressure to approximately -160oC in a plate-fin cryogenic heat exchanger. This ensures condensation of hydrocarbons to LNG. The fraction of nitrogen present in the boil-off that cannot be condensed at this condition remains as gas bubbles in the LNG. Phase separation takes place in the liquid separator. From the separator, the LNG is dumped back to the storage tanks, while the nitrogen-rich gas phase is discharged (to atmosphere or burnt in an oxidizer).