## 7 Conclusions  
In this study, we analyzed the isentropic, energy, and exergy efficiency of a low-duty liquid natural gas (LNG) vapor turbo compressor during exploitation on a conventional LNG carrier. Isentropic analysis is widely used for the industrial estimation of turbo compressor efficiency. Isentropic efficiency only considers turbo compressor efficiency. The evaluation of the whole machinery is more complex and requires separate mechanical efficiency measurements. If calculated according to Equation (8) with an estimated mechanical efficiency of 0.98 and measured electric motor efficiency of 0.9, without complex measurements of the mechanical seal efficiency, the calculated efficiency is lower compared to the isentropic efficiency for the whole measurement range, which is about 13.38%. As Equations (7) and (8) are widely used for the evaluation of similar machines, this may lead to incorrect assumptions about the real efficiency of the analyzed machinery. The measured results were very close to the manufacturer's reported efficiency, and the best isentropic efficiency of the typical isentropic curve was 73.14%, which agrees with the manufacturer's evaluation. Energy efficiency evaluations are applicable for the analysis of the turbo compressor and the whole machinery, as they require fewer measurements and directly evaluate the whole machinery by measuring the required power consumption of the electric motor, meaning that no assumptions are required. The energy efficiency of the low-duty turbo compressor with its motor, gear box, and mechanical seal is lower than the isentropic efficiency, with the best evaluation being 57.81%. Energy efficiency evaluation is also applicable for the analysis of the turbo compressor and the whole machinery as it requires fewer measurements due to being related to power consumption and the useful exergy of the system. As the exergy efficiency considers the entropy generation of the system, it provides deeper insight into the real efficiency of the evaluated machinery. According to the exergy evaluation, the low-duty turbo compressor is not efficient machinery in the lower running zones, where it adjusts the flow rate only using inlet guide vanes. The efficiency in that range drops to 4.08%, and this operating zone is to be avoided as the consumption of power is very high compared to the given output. The best measured exergy efficiency of the turbo compressor was 28.51%. According to this short overview of the cargo temperature maintenance, it would be ideal to maintain the vessel at a speed at which the LNG vaporizer is not required and where the low-duty turbo compressor maintains the cargo tank pressure and the temperature with the mass flow rate. The energy that is recovered with the evaporated gas inside the boiler varies from 76.2% to 85.8% of the total energy delivered to the main boilers; the rest is sent to the heavy fuel oil (HFO) in dual mode burning. The electric and steam heat power are higher when the low-duty (LD) turbo compressor is  
Figure 11: Cargo equilibrium temperature maintenance for the voyage. running compared to the running mode in HFO due to the aggregate state of the gas, its transport, and heating the main boilers. The total energy consumption in the boilers is less compared to HFO-only, as LNG has a higher calorific value then HFO. No data were available from the manufacturer for the running of a turbo compressor at such low temperatures (\(-40\)\({}^{\circ}\)C only). In this study, the turbo compressor was evaluated in the temperature range from \(-69\) to \(-105\)\({}^{\circ}\)C, where the compressor was more efficient at lower running speeds due to the increased gas density. In the scientific literature, such analysis is very rare and it brings new light to the running efficiency of the low-duty turbo compressor at different loads in real running conditions. The makers provide the isentropic efficiency of the one running point, but not detailed analysis of the whole running range as presented here.