Life and Reliability Characteristics of Turbo-Brayton Coolers

Abstract

Wear and internal contamination are two of the primary factors that influence reliable, long-life operation of turbo-Brayton cryocoolers. This paper describes tests that have been conducted and methods that have been developed for turbo-Brayton components and systems to assure reliable operation.

The turbomachines used in these coolers employ self-acting gas bearings to support the miniature high-speed shafts, thus providing vibration-free operation. Because the bearings are self-acting, rubbing contact occurs during start-up and shutdown of the machines. Bearings and shafts are designed to endure multiple stop/start cycles without producing particles or surface features that would impair the proper operation of the machines. Test results are presented that document extended operating life and start/stop cycling behavior for machines over a range of time and temperature scales.

Contaminants such as moisture and other residual gas impurities can be a source of degraded operation if they freeze out in sufficient quantities to block flow passages or if they mechanically affect the operation of the machines. A post-fabrication bakeout procedure has been successfully used to reduce residual internal volatile contamination to acceptable levels in closed-cycle systems. The process was developed during space qualification tests on the Near Infrared Camera/Multi-Object Spectrometer (NICMOS) Cryogenic Cooler (NCC). Moisture levels were sampled over a seven-month time interval following the initial bakeout confirming the effectiveness of the technique. A description of the bakeout procedure is presented.