Reliability Evaluation for an Oxygen Generator in an Air-conditioner by Accelerated Life Testing
The oxygen generators in air-conditioners separate oxygen gas from air under high pressure by PSA (Pressure Swing Adsorption) method. The equipment was newly developed and there were not enough reliability test methods for it. Since we had to bring the product into market timely, we applied accelerated life tests by which we could detect weak points and obtain life and failure rate data in a short time. Zeolite beds and compressors which are the most important items to the equipment were selected as test subject items. They were evaluated under step stress margin tests first in order to verify their operating limit in certain environmental stresses and then accelerated life tests were carried out. Additionally we had to decide the material of compressor cylinders which one material was honing and the other was graphited-coated. So two types of compressors were simultaneously tested. Stress factors for zeolite beds were temperature, humidity and oxygen density. Also, temperature, humidity and output pressure stresses were selected for compressors.
We tested two items for over 500 hours and obtained failure data. As results, we calculated the life and failure rate using exponential distribution, Arrhenius equation and Eyring relationship. It was revealed about a zeolite bed that MTTF is 169, 567 hours but its temperature operating margin was not wide. Therefore we improved the cover material of zeolite beds for its higher reliability. Also we found that the MTTF of compressors is 20, 380 hours for honing cylinder and 66, 770 hours for graphited-coated one. So we chose graphite-coated compressor cylinder because it showed much longer life than honing. Finally we established test methods for both zeolite beds and compressors that fixed number of units must survive for 500 hours to assure 1% annual failure rate in 95% confidence level.
KeywordsTest Plan Pressure Swing Adsorption Output Pressure Oxygen Generator Oxygen Density
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