## 5 Low-Duty Turbo Compressor Isentropic, Energy, and Exergy Analysis  
For the low-duty turbo compressor analysis, we compared three different approaches to the efficiency calculation and analyzed their results. Two running points of the low-duty turbo compressor are given in the \(h\)-s diagram in Figure 3. The actual compression of the gas in the low-duty turbo compressor is 1-2 and the isentropic compression is 1-2 IS. The first compression was recorded at dead slow ahead and the second running point of the low-duty turbo compressor was 3-4 IS, which was taken as the last measured point. Figure 3 shows that the angle at first compression from 1-2 related to 1-2 IS is wider than the isentropic turbo compression in the case of compression from 3-4 related to 3-4 IS. The wider angle distance from the isentropic compression line is related to the higher entropy generation at lower loads of the low-duty turbo compressor compared to the higher turbo compressor load. Figure 3 also shows that the second running point of the low-duty turbo compressor is not in the saturated area, but is located in the two-phase zone where \(x\approx 0.992\). As flying LNG drops may damage the turbo compressor, so a moisture separator is fixed before the low-duty turbo compressor in which LNG droplets are separated from the gas phase of the LNG. Entropy generation in the actual process occurs, but is not incorporated in the isentropic efficiency of the low-duty turbo compressor. Isentropic efficiency was evaluated as per Equation (7). Figure 4 compares the analysis of the energy loss with the low-duty turbo compressor speed. Turbo compressor speed is directly related to boiler load with its output at the main propulsion shaft. The energy loss in the lower running zones ranged from 93 to 95 kW and then decreased until 70.4 rpm, reaching 83 kW. In that range, the turbo compressor speed is almost at its minimum of around 12,000 rpm. Increasing the turbo compressor speed increases the energy loss, which peaks at 80.4 rpm, at up to 100 kW. After that point, increasing the turbo compressor speed to more than 17,000 rpm reduces the energy loss of the turbo compressor. Figure 4: Energy loss vs. turbo compressor speed. Figure 3: Low-duty turbo compressor, two running points, the lowest measured load (1-2), and the highest measured load (3-4). The turbo compressor operation is led by the main boilers' gas management control. The gas delivery to the main boilers is controlled by the turbo compressor inlet guide vanes' (IGV) position, turbo compressor speed, and the main boiler gas pressure control valves. The main boiler's gas pressure control valves are calibrated with normal delivery of the turbo compressor and are opened 75% of their full extent. If the main boilers change load operation, the gas pressure control valves open more to ensure the main boilers receive enough gas with changes in turbo compressor IGV position and speed. If the load on the main boilers drops down, the gas pressure control valves close to less than 75% and reduce amount of the gas to the main boilers. At the same time, the turbo compressor reduces speed and changes the IGV position if required. Figure 4 shows that the main boiler does not require a significant speed change in the turbo compressor up to 70 rpm; gas delivery is controlled to the main boilers with the IGV operation and the position of the gas pressure control valves. The burning mode at that time is a dual fuel mode with fuel oil at a minimum and the rest of the power provided by gas burning. The highest measured rpm of the turbo compressor was recorded at the two last measured points, which was about 73% of the turbo compressor's full speed. The turbo compressor speed and delivery are also related to the gas temperature. At lower temperatures, a lower speed of the turbo compressor wheel is required for pressurizing the higher density gas.