A method of designing purging flows of clean, dry gases to maintain acceptably low levels of contamination in enclosed volumes has been developed. The method is applicable to diverse enclosures that must be kept clean, including housings of precise optical instruments, clinical facilities, facilities for manufacturing microelectronic devices, and clean rooms in general.

In the simplest case, the purging flow of clean, dry gas is required to limit the entrance of external contaminants into the purged volume through a single purge vent (see figure). External contaminants include gases (e.g., water vapor) and particles (e.g., microbes and dust). Also, typically, the purging flow is required to limit the concentration of contaminants generated internally by outgassing and to sweep out these contaminants. The present design method, based on equations developed in a theoretical and experimental study of bulk and diffusional flow, enables one to select the pressure, flow speed, and volumetric flow rate of the purge gas to satisfy these requirements.

The purging volumetric flow rate needed to limit potentially contaminating external air to an acceptably low partial pressure is a function of its partial pressure, of the ambient pressure, and of the characteristic time for entry of air or of the contaminant(s) of interest into the volume through the purge vent. This characteristic time can be determined experimentally. The purge-gas pressure needed to maintain the required volumetric flow through the purge vent can be calculated as a function of the ambient temperature and pressure and the cross-sectional area of the purge vent. The speed of flow through the vent is, of course, directly related to the volumetric flow rate and the cross-sectional area of the vent. The size of the largest airborne particle that can be prevented from entering against the purging flow can be calculated as a function of the flow speed and of the viscosity and mass density of the vented gas.

This work was done by John J. Scialdone of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Physical Sciences category. GSC-14241