Development of C - SiC ceramic compact plate heat exchangers for high temperature heat transfer applications

This paper investigates the use of polymer and liquid silicon infiltrated carbon/siliconcarbide composite (C-SiC) materials for the development of inexpensive compact heat exchangers, as part of efforts for thermochemical hydrogen production. These heat exchangers will be capable of operating in the temperature range of 500 to 1400°C with highpressure helium, liquid fluoride salts (a potential intermediate heat transfer fluid), or other corrosive gases such as SO3 and HI. C-SiC composites have several potentially attractive features, including ability to maintain nearly full mechanical strength to temperatures approaching 1400°C, inexpensive and commercially available fabrication materials, and the capability for simple forming, machining and joining of carbon-carbon performs, allowing the fabrication of highly complex component geometries. To meet cost goal, candidate materials must have relatively low bulk costs, and fabrication methods must extrapolate to low-cost mass manufacturing. Composite compact offset fin plate heat exchangers concept has been developed to meet the above functional and cost goals, which will serve as the intermediate heat exchanger (IHX) to transfer high temperature heat from a helium-cooled high temperature nuclear reactor to a liquid salt intermediate loop which couples to hydrogen production loops. The IHX uses offset fin structures with fin width and height at 1 mm scale. The detailed local and global thermal mechanical stress analyses show that the designed composite plate heat exchanger can tolerate pressure difference up to 9 MPa and large temperature difference from two fluid sides. Two potential low cost methods to fabricate C-SiC are liquid silicon (melt) infiltration (MI) and Polymer Infiltration and Pyrolization (PIP). Mechanical strength tests on MI coupons show above 200 MPa failure stress. Leak-tight pyrolytic carbon coatings have been successfully applied on MI C-SiC coupons and excellent helium hermeticity were obtained under high pressure and stress after coating. PIP plates with high-quality millimeter-scale fins formed using teflon molds have been successfully demonstrated. The teflon molds were proven to be reusable, so that the process could be extrapolated to inexpensive mass fabrication of compact ceramic heat exchangers. Prototype test heat exchangers are being fabricated basing on both MI and PIP methods.