The Knudsen Compressor As an Energy Efficient Micro-Scale Vacuum Pump

Abstract The low-pressure pumping limit of the MEMS Knudsen Compressor, a thermal transpiration pump, is identified and discussed. The practical low-pressure limit is due to a requirement for an approach to continuum flow in the connector section, it is roughly 10mTorr. A previously developed transitional flow model was used to size several Knudsen Compressor cascades that operate down to the low-pressure limit. Designs based on previous experimental Knudsen Compressors do not provide the necessary pumping efficiency. A design, employing carefully sized capillaries etched in aerogel transpiration membranes, is shown to result in a viable device. A cascade incorporating this design provides a gas flow rate of 3E16 mol/sec, while pumping from a pressure of 10mTorr to 1 atm. It requires a volume of 73 cm3 and 2.0 W. Design considerations are outlined for MEMS Knudsen Compressors operating at their lower pressure limit. A primary concern, efficiently transitioning from the capillary section to the connector at constant temperature is discussed.