## 5 Sizing of the Heat Exchangers for the ORC Regenerated Cycle  
The specific design of shell and tube heat exchangers has been performed by using the software Aspen Tasc(r), to address in detail the performance of the regenerative cycle and to perform the economic analysis of the investment. The used software incorporates an extensive library of organic fluids and therefore it has been possible to define in detail the geometry of heat exchangers, according to the prescriptions of the Tubular Exchanger Manufacturers Association (TEMA) [44], and to calculate accurately the phase transition and the pressure drop in the shell and in the tubes  
The heat exchangers in the regenerated cycle are the exhaust gas/thermal oil heat exchanger, the evaporator, the regenerator and the condenser (Figure 8). During sizing and geometry selection, it has been paid attention to limit the size, in view of a possible installation in a room next to the engine room. The main data obtained for the different heat exchangers are summarized in Table 4.  
To ensure the proper operation of the engine, the pressure drop of the gas stream through the exhaust gas circuit must be below 3 kPa [4], therefore the maximum loss through the exhaust gas/thermal oil heat exchanger has been set to 1.2 kPa. In this way, an acceptable backpressure is guaranteed for the engine turbocharger, which is just upstream this heat exchanger.  
The exhaust gas/thermal oil heat exchanger is a vertical tube bundle (containing the thermal oil), with the direct flow of gases in the shell without support baffles, for reducing losses.  
For the evaporator it was used a shell and tube heat exchanger, with geometry type CFU (channel Integral with tube-sheet, Two-pass shell with longitudinal baffle, U-tube bundle [44]) and two shells in parallel. Taking the toluene pressure (20 bar), it is better to locate it inside the tubes [45], while the flow  
Figure 11: Increase in percentage points of the combined propulsion system efficiency, according to the load, for the different cycle configurations studied.  
of hot thermal oil circulates outside. The losses are not a limit for this heat exchanger, as a sensitivity analysis has shown that fluctuations in the maximum cycle pressure of about few bars lead to negligible power losses. The pinch-point obtained in the oil/toluene exchanger is equal to 5.3 \({}^{\circ}\)C, a value that demonstrates a high heat transfer efficiency.  
The chosen geometry for the regenerator is a shell-type DFU (Special high pressure closure, Two-Pass shell with longitudinal baffle, U-tube bundle [44]), with a rinsed tube bundle in order to obtain a more compact design.  
One possible concern is the pressure drop affecting the vapor of toluene coming from the turbine. This loss not only increases the size of the exchanger, but also penalizes the turbine, reducing the available enthalpy drop. By limiting this loss in 0.025 bar, a power penalty of approximately 30 kW only has been obtained for the turbine.  
The condenser is cooled by the demineralized water of an intermediate ring, cooled by the sea. The same ring can cool in series the central cooler of the engine HT cooling circuit (see Figure 2).  
It has been verified that a condenser of BIM type (integral cover, split input flow-single output flow, fixed tube-sheet) is the most suitable, allowing better performance while limiting the size and the final cost. In this case, it is also required that the pressure drop in the vapor side is the lowest as possible. In fact, each loss means a pressure increase at the end of the expansion, limiting the power of the turbine. Comprising such loss in 0.013 bar only, it has limited the penalty for the turbine power in about 20 kW.