Abstract
This paper presents three approaches for isentropic, energy, and exergy evaluations of a low-duty liquid natural gas (LNG) vapor turbo compressor during exploitation on a conventional LNG carrier. The evaluation was conducted on the measured performance parameters under 22 various turbo compressor operating regimes. The turbo compressor performance was evaluated in the temperature span from −69 to −105 °C and during changes in the rpm of the main propulsion turbine and, consequently, the main boiler load. The results show that the highest measured turbo compressor isentropic efficiency is in agreement with the manufacturer specifications, equaling 75.23% at a main propulsion turbine rpm of 53.5. At the highest measured loads and rpm, the turbo compressor energy and exergy efficiencies reach the highest values of 57.81% and 28.51%, respectively. In each observed operating regime, the influence of the ambient temperature change on the turbo compressor exergy efficiency was investigated. At the lowest and the highest measured loads, turbo compressor energy and exergy flow streams are presented in a Sankey diagram. Techniques for cargo temperature maintenance during the ship voyage are presented, as the results show that low suction gas temperatures influence turbo compressor efficiency.
Highlights
In marine propulsion systems, diesel engines prevail in the entire world’s fleet [1,2]
The results show that the highest measured turbo compressor isentropic efficiency is in agreement with the manufacturer specifications, equaling
The share of steam propulsion in marine propulsion is low when considering the entire world fleet, but this propulsion is still dominant on liquid natural gas (LNG) carriers due to the specificity of its operation and the transported cargo [13,14]
Summary
Diesel engines prevail in the entire world’s fleet [1,2]. Inside the propulsion systems of LNG carriers, turbo compressors are used to increase the pressure of boil off gas (BOG) from the cargo tanks and deliver it to main propulsion element inside the system (dual fuel internal combustion engines or steam generators). The low-duty BOG turbo compressor plays an important role in maintaining the operating parameters of the cargo tanks for the analyzed LNG carrier [37]. The usage of BOG as a fuel in comparison to other conventional fuels emits fewer harmful emissions; BOG emits almost no sulfur oxides (SOX ) or particulate matter (PM), while carbon dioxide (CO2 ) and nitrogen oxides (NOX ) emissions can be notably reduced [38,39] This is an additional reason why analyzing BOG turbo compressor operation is important
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