## 3 Plant Arrangements and the Loading and Unloading Processes  
The schematics of the kind of CNG plant considered in this study are shown in Figure 1, in both the loading (a), and unloading (b), configurations. The compressor, with its control valves and CNG cooler, is installed on board the ship. The two valves mounted before and after the compressor introduce a variable pressure drop, allowing the machine to operate in non-flooding conditions. The cooler always presents after the compressor to maximize the compressed gas density. During the loading phase, the suction is connected to the storage terminal and delivers to the pressure vessels. During the unloading phase, the suction is connected to the pressure vessels, with the interposition of a heater, while the delivery is connected to the pipeline. The heater is used only in the unloading configuration, because the temperature of the pressure vessels can reach process values as low as \(-60\)\({}^{\circ}\)C, due to the expansion of the gas contained in it. In both configurations, a by-pass allows the compressor to be excluded from the circuit when the flow occurs spontaneously due to the pressure difference. In fact, the loading and unloading processes are divided into two subsequent free and forced phases. During the free phases, the gas flow is due to the pressure difference between the pressure vessels and the storage terminal or the pipeline, respectively. During the forced phases, the compressor provides the pressure vessels with complete filling in the loading process and the most complete as possible emptying in the unloading one. By acting on the lamination valve placed on the intake, the compressor can operate even when the pressure of the upstream tanks is higher than that of the downstream pipeline. For example, this is necessary when the flow in the free discharge phase falls below 20 kg/s; therefore, it is more convenient to operate the compressors to speed up the process. Since the required volume flow rates and compression ratios are not excessively low or high, they can be obtained with both centrifugal and volumetric compressors. In a centrifugal compressor, the maximum flow is limited by choking, the minimum flow is limited by surge, and the compression ratio is a function, other than of stages number and geometry, of gas molecular weight and compressibility. In a volumetric compressor with automatic valves, the flow rate is proportional to the speed of rotation, while the stage compression ratio is independent from it and less sensitive, with respect to the centrifugal ones, to changes of the composition of the fluid. On the other hand, the composition of NG depends on the field of origin, but the variation of the mean molecular weight, density, and other physical properties is not so large as to prefer the use of volumetric compressors for this reason. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Pressure Vessel Main Specifications:**} \\ \hline gas cylinder outer diameter & 3.4 m \\ gas cylinder length & 22.5 m \\ volume (water) & 175 m\({}^{3}\) \\ vessel mass capacity, referred to CH\({}_{4}\) @300 bar, 20 \({}^{\circ}\)C & 38 ton \\ design pressure & 300 bar \\ \hline \multicolumn{2}{c}{CNG ship design:} \\ \hline length, overall & 205 m \\ max breadth & 36 m \\ height to main deck & 22.0 m \\ design draft & 7.5 m \\ n\({}^{\circ}\) vessels in the CNG ship & 272 \\ total vessels volume (water) & 44800 m\({}^{3}\) \\ total gas mass @ 300 bar 20 \({}^{\circ}\)C (\({}^{\bullet}\)) & 9771 ton \\ total gas volume & 13.762 MNm\({}^{3}\) \\ cargo gas/container weight ratio & 1.5 \\ \hline \hline \end{tabular} \({}^{\ast}\) referred to pure methane.  
\end{table} 
Table 2: GASVESSEL project [4]: CNG vessel and ship design main specifications. Reciprocating compressors have very high adiabatic efficiency, but as compression ratio drops, adiabatic efficiency drops. However, with NG, the drop is limited, and adiabatic efficiency can be 93% at a compression ratio of 3 and 91% at a compression ratio of 1.4 [13].