A mixing apparatus combines liquid oxygen and liquid nitrogen to make liquid air in small, convenient quantities. Heretofore, equipment used to make liquid air by mixing was incapable of making batches smaller than 600 gallons (2,300 L). The present mixing apparatus produces liquid air on demand in batches as small as 100 liters; a batch of this size is suitable for filling self-contained breathing apparatuses like those worn by firefighters and others working in hostile environments.
It is impractical to liquefy air directly from the atmosphere because nitrogen boils at a temperature lower than that of oxygen and thus tends to evaporate faster from the liquid mixture, leaving the mixture richer in oxygen. It is therefore more practical to make liquid air by mixing the constituent oxygen and nitrogen liquids, which are readily available. Also because of the tendency toward oxygen enrichment, it is more economical to make liquid air on demand than it is to store it in large quantities that sometimes must be dumped because they become too rich in oxygen.
In the present apparatus, a tube carrying a flow of liquid oxygen converges with a tube carrying a flow of liquid nitrogen in a Y-shaped mixing device (see figure). Within this device, the tube carrying the liquid oxygen is immersed in the flow of liquid nitrogen (which is the colder of the two constituent liquids) so that the flow of liquid air receives additional cooling on its way to the collection vessel.
Prior to mixing, the constituent liquids are stored in dewar tanks, which are pressurized by gaseous nitrogen from the liquid nitrogen supply for transfer and mixing. For safety, the pressure is limited to about 60 psi (414 kPa). The use of the same pressure for transfer of both constituent liquids helps to minimize the number of operational variables that could affect the mixing ratio. Liquid air is produced in the mixing device and collected in a receiving dewar which can then be sampled using a cryogenic sampler and used to fill respirators with no additional analysis. Sampling can also be done using the respirator as a sampler. In either method, a portable oxygen analyzer is used to determine the oxygen content.
This work was done by Robert B. Martin formerly of EG&G Florida, Inc., for Kennedy Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Mechanics category.
This invention has been patented by NASA (U.S. Patent No. 5,678,536). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Technology Programs and Commercialization Office
Kennedy Space Center
Refer to KSC-11774