## 5 Low-Duty Turbo Compressor Isentropic, Energy, and Exergy Analysis
The isentropic efficiency and energy efficiency are compared in Figure 5. The highest isentropic efficiency was achieved in the mid-operation range at 53.5 rpm, when the turbo compressor speed was 12,338 rpm. The isentropic efficiency at that point was 75.23%, which is very close to the manufacturer's value of 77%, but for different temperature and running conditions. Although we measured the highest isentropic efficiency of the turbo compressor at that point, this point is not a significant point, as measured results were not grouped. The gap between half ahead and the full ahead command was 11.7 rpm, when the main boiler steam flow increases for the main propulsion turbine, while the dump steam line is closed. This event caused a surge in the turbo compressor delivery gas flow to the main boilers by variation in speed, IGV operation of the turbo compressor, and main boiler's gas pressure control valves. As the data were measured every two seconds, we expected the real measuring and exploitation conditions to deviate from the ideal manufacturer's values due to steam plant dynamics. More accurate measurement results could have been recorded if we had more time to observe that point, but due to commercial reasons, this was unfortunately not possible. Figure 5: Isentropic and energy efficiency. The typical parabolic shape of the isentropic efficiency curves started from 66.1 until 83 rpm, which agrees with the values in the literature [66; 67]. The average isentropic efficiency of the whole measured range was 62.73%, and the highest efficiency at the grouped measured points was 73.14%. The main difference between the isentropic and energy efficiency methods is that energy efficiency evaluates the whole unit with electric motor, gear, and seal losses, but not the turbo compressor only. Comparing Figures 4 and 5, it may be seen that the lowest energy loss is related closer to the highest isentropic efficiency of the turbo compressor but to the highest energy efficiency of the turbo compressor unit. According to the energy analysis, efficiency at 53.5 rpm is not the highest. The highest energy efficiency was found at the last measured point, 57.81%. The average energy efficiency was in the range of 21.78% to 57.81%, with a total average of 37.57%. Energy loss is shown in a Sankey diagram in Figure 6[68]. Two running points are presented here: at the beginning and the end of the measurement. Electric energy brought to the process in kW is compared with the actual compression process, electric motor loss, and energy loss. Accordingly, for the first running point, the electric motor loss was 10%, the energy loss was 69.07%, and 20.93% of the energy was usefully consumed. At the last running point, 10% was the electric motor loss, 37.97% was the energy loss, and 52.02% was usefully consumed energy. The Sankey diagram shows that the share of energy loss did not increase much with the increase in compression energy requirements, which positively affected the energy efficiency. As the measured power factor was the same throughout the measurements, we found that the electric motor loss was proportional to the increase in electric motor power. A comparison of the energy loss and exergy destruction is provided in Figure 7. Exergy destruction was more evenly distributed in the lower running zones compared to energy loss. The exergy destruction was 105 kW in that range and varied by about 10 kW, which was higher than to energy loss, which decreased from 92 kW in the beginning of the measured range to 82.4 kW in the mid-running period. As the load of the turbo compressor increased, exergy destruction constantly increased compared to the energy loss, with the highest value of about 158 kW at the last measured point. This is 58% higher than the energy loss, which peaked at 100.2 kW. The trend with increasing turbo compressor speed above 17,000 rpm was not observed for reducing the exergy destruction, which was the case for the energy losses. Figure 6: Sankey energy flow diagram for the lowest and highest measured turbo compressor load.