LNG, Boil-off gas compressor, Temperature field, Thermal stress, Suction coefficient
The Boil-off Gas (BOG) compressor is used as a key facility in the liquefied natural gas (LNG) terminal, to recycle the excessive boiled gas for re-liquefaction or direct application. The low suction temperature down to -162? brings about big challenges in design of the BOG compressor. In this paper, the three-dimensional finite element model was used to simulate both the static and periodic transient temperature distribution in the cylinder of a BOG compressor, and a computational fluid dynamics (CFD) model was established to calculate the flow and heat transfer inside the compression chamber and suction/discharge pockets. A test rig was built up to validate the simulated results. The results showed that, the average temperatures in the suction and discharge pockets were about -109? and -60?, respectively, and the temperature of the compression chamber was in-between. The maximum temperature difference between outer and inner surface of the compressor cylinder reached up to 84? during start-up of the compressor, which yielded a thermal strain and stress in the cylinder much larger than those during steady operation of the compressor with only 31? of temperature difference. A variety of pre-cooling temperatures ranging from -20? to -60? were examined, and the results showed pre-cooling down to -20? before start-up was good enough. The amplitude of temperature fluctuation due to the periodic movement of the piston was less than 0.1? in the cylinder wall. The temperature coefficient tended to decrease at lower suction temperature. As the suction temperature decreased from -54.2?to -142.2?, the suction coefficient dropped drastically by 24.4%.