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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 Fig. 13. Examples of different types of com- mercial HEX that could be used for ORC ap- plications on board ships: a) Gasketed plat- and-frame HEX for marine and refrigeration applications. Model T20 Alfa Laval (by cour- tesy of Alfa Laval), b) a shell and tube HEX for marine applications. Aalborg MX, Alfa Laval (by courtesy of Alfa Laval). ORC units. This configuration offers high compactness, given the large ratio of heat transfer area-to-volume, but higher approach temperatures (i.e., temperature difference between the hot fluid outlet and the cold fluid inlet) are required compared to, e.g., plate HEXs, since the flow is a combination of co-current, counter-current and cross flow. This, in turn, may decrease the energy conversion efficiency of the ORC unit due to the lower enthalpy drop in the expander. The use of plate HEXs to recover heat from exhaust gases may be challenging since the volumetric flow rate on each side in plate HEXs is limited to 2500 m3 h− 1 [107]. Moreover, the maximum operating temperature in this type of HEX is around 250 °C [107], although the use of welded plates allows increasing the operating temperature and pressure up to 500 °C and 80 bar. Thereby, plate HEXs could be con- sidered to recuperate the exhaust gas heat on small-size vessels only. An example of a commercial plate HEX can be seen in Fig. 13a. Lastly, the possibility of using fin-plate or printed-circuit HEXs could be relevant for maritime applications owing to their high heat transfer area-to-volume ratio [108]. The weight of fin-plate and printed-circuit HEXs can be ten times lower compared to that of the shell-and-tube counterpart [109]. This equipment is easily scalable and widely adopted in the oil and gas industry. It can operate at high temperatures (800 °C) and pressures (100 MPa), if a diffusion bonding process is used to stack the plates [109]. Their purchased-equipment cost is higher compared to shell-and-tube HEXs. Moreover, cleaning is a crucial aspect to avoid clogging of the channels and to minimize the pressure drops. The regenerator in an ORC unit typically operates under a large pressure difference between the hot and the cold side. Depending on the working fluid, differential pressures may be up to 4 MPa, and the ratio between the volume flow rates of the two sides may be larger than 100 [61]. As pointed out by Angelino et al. [110], counter-flow configura- tions are difficult to achieve in ORC regenerators because of the sig- nificant difference between the volume flow rates on the sides of the regenerator. Thus, the use of a shell-and-tube HEX (with or without fins) is more common [107]. In this case, it is recommended to locate the cold high-pressure stream inside the tubes, while the superheated vapor passes through the shell. In some cases, the use of a single-tube and multiple-tube helical coil heat exchanger can be suitable to increase further the compactness and minimize the pressure drops on the vapor side [110]. An example of a commercial shell and tube HEX can be seen in Fig. 13b. A low footprint and weight can also be attained by adopting fin-plate HEXs. For the three vessels analyzed in Section 3.3, the mass flow rate of sea water required by the condenser of the ORC unit fed by the exhaust gas heat is below 2500 m3 h−1 (if a temperature increase of 5 K is as- sumed for the seawater). Thus, both plate and shell-and-tube HEXs are viable. Plate HEXs may be preferable from a performance point of view since approach temperatures as low as 1 °C can be used [107], but this would imply also a larger heat transfer area. If seawater cools the working fluid directly (no intermediate loop), expensive materials should be used to minimize the risk of corrosion in the condenser; see Section 4.1. Alaez et al. [111] proposed to tackle this problem by adopting plastic plate HEXs. They claimed that such equipment enables reducing the cost and weight of the installation, at the expense of in- creasing the HEX dimensions. The main limitations of plastic HEXs are the maximum allowable pressure (1 MPa) and temperature (140 °C) [111]. Also, the direct use of seawater could be a potential source of biofouling, i.e., the accumulation of algae, microorganisms and other marine fauna on the HEX surface. Although there is evidence that this effect could be negligible [112], a periodic monitoring of the condenser to ensure the safe operation of the unit, and the selection of HEX types with easier maintenance (i.e., plate HEX) would be recommended for this case. In the case of a simple ORC unit fed by the jacket water heat (see Fig. 10b), the temperature lift between the hot source and cold sink is typically below 70 °C, and thus the thermal efficiency of the ORC unit is bound to be low. Plate HEXs are suitable for low approach tempera- tures, which reduces the performance losses associated with the heat transfer irreversibilities. Therefore plate HEXs are arguably the best heat transfer equipment for the evaporator and condenser in this case. In addition, the use of fin-plate and printed-circuit HEXs should be considered, in order to enhance the system compactness. In conclusion, while shell and tube HEXs or once-through boilers would be more suitable for ORC units recovering the heat from exhaust gases, the use of plate HEXs, in their simpler or more complex designs (i.e., fin-plate or printed-circuit) would be recommended for applica- tions on jacket cooling water. Moreover, plate HEXs may be re- commended as condensers for ORC units on small-size vessels. 4.3.2. Expanders The selection of the expander type depends on the ORC size/power output, the thermophysical properties of the working fluid, and on the characteristics of the heat source and sink. Expanders can be classified into two categories: positive displacement machines and turbines. Examples of positive displacement expanders are scroll expanders, piston expanders, screw expanders and rotary vane expanders. Turbines can have axial, radial (inflow or outflow), or mixed-flow configurations. Fig. 14 shows images of some of them. 139PDF Image | organic Rankine cycle power systems for maritime applications
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