Effects of Low Suction Temperature on the Boil-off Gas compressor

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°C 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°C and -60°C, 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°C during startup 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°C of temperature difference. A variety of pre-cooling temperatures ranging from -20°C to -60°C were examined, and the results showed pre-cooling down to -20°C before start-up was good enough. The amplitude of temperature fluctuation due to the periodic movement of the piston was less than 0.1°C in the cylinder wall. The temperature coefficient tended to decrease at lower suction temperature. As the suction temperature decreased from -54.2°C to -142.2°C, the suction coefficient dropped drastically by 24.4%.