INDIRECT ELECTRICAL HEAT EXCHANGERS Ap pl i ca ti on: He at i ng of C o r ro s i ve F lu id s an d H igh P r e s su re G ase s in Pot en t ia lly Exp lo sive A r ea s THE INDIRECT ELECTRICAL HEAT EXCHANGERS THE ALUMINIUM CAST HEAT EXCHANGERS Electrical Heat Exchangers are particularly interesting because they are very compact and also highly performing in terms of temperature control accuracy. In some cases however they can be penalised by the temperatures reached on the heating elements sheath, temperatures that are sometimes quite high but which represent the main factor of the abovementioned compactness and effectiveness. These are the cases in which the process fluid is corrosive and becomes even more aggressive at high temperature. In these cases, in fact, the surface of the heating elements is potentially subject to such a strong corrosion attack that even the most resistant materials can be damaged. A particular example of indirect heating system is the aluminium cast heat exchanger. In this product the coil is not sank into a liquid (which is kept at the desired temperature by an electrical heater) but it is incorporated in an aluminium casting which also includes the heating elements. The result is a monolithic construction (see Figure 5) which proves to be very safe (potential failures of the heater do not contaminate the process fluid nor generate dangerous leakages) and compact. Figure 5: aluminium cast heat exchanger In these cases an indirect heating system is adopted in which the process fluid passes through a coil that is immersed into a liquid (water, glycol or oil depending on the working temperatures). Inside the liquid, in turn, a heater is immersed which keeps it at a temperature suitable to obtain the desired process fluid heating. Figure 4 shows the functioning scheme of this system. Figure 4: functioning scheme of an electrical indirect heat exchanger This system is clearly more complex than a traditional electrical heat exchanger since it requires the adoption of specific safety systems (level control, expansion tank, safety valve). Also from a thermo-dynamic point of view this solution is less efficient since it requires larger heat exchanger surfaces. On the other hand however this can be the only viable technical solution. Main Characteristics:  compliant to all Ex standards (ATEX, IECEx, EAC, CSA, cCSAus, INMETRO, CCOE/PESO, China EX)  IP66÷IP68 terminal box in Ex-d or Ex-e construction  control and safety functions based on PT100 (also possible with thermocouples or thermostats)  Power supply divided into more stages suitable for a simple on/off control (control by thyristor possible too)  Both horizontal or vertical execution possible  Coil material: AISI316, Duplex, Inconel 625 (NACE compliance upon requested)  Design pressure up to 500 barg  Design temperature up to 350 °C  ASME U-Stamp certification available on demand Explosion Proof Heaters These products present much less severe safety and control issues since no more sensors or expansion tanks are required to control the liquid level nor a safety valve shall be installed to prevent overpressures caused by an overheating of the secondary fluid. On the other hand, it is evident that such a system does allow any type of maintenance since only the temperature sensors can be exchanged but not the electric heater. In addition it has a thermal inertia which is even higher than the traditional indirect heat exchangers and this penalises significantly the thermal control accuracy, especially if the plant is operated variable at flow rates. It is therefore suitable mainly for processes with constant operational parameters like, e.g., the heating of natural gas bled from a compressor to seal the bearings of the compressor itself (Seal Gas Heater). Thanks to its compactness this product is also suitable for installations on skids and wherever