Coiled Tube Air Heater Test Loop Design

Abstract

Coiled tube air heaters (CTAH) are heat exchangers currently under investigation at the University of California, Berkeley for application to Reheat Air-Brayton Combined Cycles (RACC). In a CTAH, molten fluoride salt on the tube side heats air on the shell side in a cross-counterflow arrangement, leading to high heat exchange effectiveness, while the coiled geometry provides good transient mechanical performance during thermal transients. This paper describes the important phenomenology involved in CTAH design and operation, such as salt freezing, flow induced vibration, start-up and shut-down control, and air circulating power requirements. The THEEM simulation tool used in the design of CTAHs and its validation against simple water to air test sub-bundles is presented. Finally, an initial design for a demonstration 370 kWth CTAH closed air test loop is presented. This test loop design, when constructed, would enable demonstration of salt-to-air heating under conditions relevant to RACC power conversion (air pressures from 4 to 20 atm and air temperatures from 400 to 650°C).