## 3 Results and Discussion
### \(O_{2}\) Production System Design Process Results
#### 3.2.2 Heat Exchanger System  
The heat exchanger network was conceived on the basis of previous studies and adapted to the marine engine conditions, as explained in the Method section. Its optimization process was oriented to take advantage of the residual energy from the fluids circulating in the O\({}_{2}\) production system by using as few regenerators as viable. In this way,  
Figure 14: Two\(-\)stage air compression configuration evaluated at all engine operating points: (a) MIEC operating conditions and burner fuel consumption; (**b**) turbine map of TCH-1 NA; (**c**) compressor map of TCH-1 NA; (**d**) turbine map of TCH-2 NA; (**e**) compressor map of TCH-2 NA.  
the heat exchanged by each regenerator is the criteria utilized to discard the less necessary ones and thereby obtain the most simplified heat exchanger network.  
Figure 15a first shows a bar chart of the heat exchanged in each regenerator as the ratio over the total heat exchanged in the network, for the most optimal turbocharging system selected and for the full engine operating point; this chart also represents the hot and cold fluids passing through each regenerator. Secondly, Figure 15b shows the same as the previous one but for all engine operating points, so a typical line graph is used instead of a bar chart. Both are in logarithmic scale for representation purposes due to the different orders of magnitude in heat exchange.  
As reflected in the first graph, N\({}_{2}\) (RG-4) is the fluid that contributes the most to air warming, followed by engine exhaust gases (RG-7); both account for almost three-quarters of the total energy transferred to the air. On the other hand, the O\({}_{2}\) line (RG-14) provides the least energy to the air; however, its heat input helps more to reduce the fuel consumption necessary to heat up the EGR at low loads, and thus add the missing energy to the air in order to meet the MIEC requirements. Although the regenerators that heat the EGR before it transfers energy to the air (RG-13 and RG-14) present much less heat transferred than the other regenerators, they contribute more at low loads to the reduction in fuel consumption in the burners, in addition to cooling the O\({}_{2}\) and exhaust gases to facilitate the following processes involving these fluids, as explained in the Method section.  
When evaluating the heat exchanger network at the other engine operating points (Figure 15b), it is observed that as engine load decreases, the heat transferred to the air by the exhaust gases also decreases because the mass flow and the energy available in this fluid is proportional to the engine load. Since the O\({}_{2}\) demand by the engine decreases with the engine speed, the operating requirements from the MIEC change and the heat exchanged between the gases leaving the MIEC and the intake air declines. As a result of the loss of energy available in these flow lines, the EGR needs to transfer a larger share of heat to the air to meet the MIEC requirements. As a result, the heat transferred by the EGR (RG-12) increases for the operating points with the lowest speed, which translates into an increased fuel consumption that burns with the excess O\({}_{2}\) from the EGR.  
Figure 15: Heat exchanged in the regenerators of the two\(-\)stage air compression configuration. The bars are arranged following the heat transfer order of the flows. (**a**) Full load engine operating point. The color coding of the fluids is the same as in the O\({}_{2}\) production system diagram of Figure 6. (**b**) All engine operating points.