AVS2001 Session VST-TuA: Vacuum Gas Dynamics

Tuesday, October 30, 2001 2:00 PM in Room 125

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		VST-TuA-1 Calculated Energy Transfer in an Accommodation Pump J.P. Hobson (National Vacuum Technologies Inc., Canada) In an accommodation pump gas is transferred from one point to another at the same temperature without the application of any external mechanical or electrical forces to the gas - only temperature gradients are used.footnote1 If there is no continuous gas flow a static and unchanging pressure difference develops between the two points, requiring no additional input of energy. Little attention has been paid to the energy transfer, i.e. the number of calories per mole of gas pumped, in this process. A simple model is presented in which this energy transfer is analytically calculated for a single-stage Pyrex accommodation pump,upper temperature room (295 K), lower temperature liquid nitrogen (77.4 K), with gas helium, compression ratio 1.2, and pressure in the free molecular range. Under these conditions the physical adsorption of helium on pump surfaces is quantitatively negligible. The model calculates the energy differences between a true accommodation pump with smooth and rough (leached) surfaces and a non-pump of exactly the same geometrical dimensions, but with all surfaces rough. The energy differences are assigned to accommodation pumping. It is found that an energy in the range of 150 calories per mole, is required, independent of the overall size of the pump, but becoming smaller as the cold volume is decreased relative to the warm volume. The results are extended to multi-stage pumps, which are ponderous to calculate analytically, but which bear a simple relationship to a single-stage pump. ¹ footnote1 J.P.Hobson and D.B.Salzman, J. Vac. Sci. Technol. A 18(4), 1758-1765, 2000
2:20 PM		VST-TuA-2 Rapid Modelling of Molecular Flow in Steady-State Arbitrary Geometries M.A. Galtry, R.G. Livesey (BOC Edwards, UK); A. Ghobadian (Computational Dynamics Ltd.) Analysis of molecular flow in simple geometries is commonly approached using methods such as Monte Carlo or view factor analysis. These methods are applicable to a generalised geometry but require significant rework for even small geometry changes, rendering them impractical for rapid engineering design analysis. A method is developed, drawing on the strong analogy between Knudsen cosine reflection for molecules and Lambert diffuse reflection for radiation, where analysis is conducted using the heat transfer facilities of a commercial code. Test cases establish the accuracy of the method, and practical examples illustrate its use in arbitrary topologies.
4:20 PM		VST-TuA-8 Edison's Vacuum Technology Patents R.K. Waits (Consultant) In 1879 Edison's laboratory had developed the means to evacuate glass lamp globes to less than a mTorr in twenty minutes.¹ Among Edison's nearly 1100 patents are five for vacuum pump improvements and at least nine others that are vacuum-related; most were applied for in 1880-1881. Edison had hired Ludwig Boehm, who had worked for Geissler, to construct Geissler pumps and "mercury drop" pumps based on those developed by Crookes and Sprengler and used in experiments described by de la Rue and Muller (France) in 1878. The Sprengler pump required a continuous gravity-fed stream of mercury droplets to trap, compress, and exhaust the air. Edison patents describe means to automate the delivery of the mercury. Other patents described the vacuum technology that Edison investigated and employed. Various means were used to remove residual gases: driving adsorbed water from glass by direct heating, and using iron particles or incandescent iron "to decompose moisture," phosphorous pentoxide and charcoal to absorb moisture, halogen or a halide to remove mercury vapor, metals such as copper or zinc to remove chlorine, and "magnesia, charcoal, and other inert substances which absorb gases in their pores." Pressure was measured with a McLeod gauge, and a high (hard) vacuum was indicated when the glow discharge in a Geissler tube was extinguished. Edison also patented a method for preserving food by sealing in an evacuated glass globe (but omitted the essential bacteria-killing heating step). A vacuum was also required for the evaporation and sputter-coating processes that later were used to produce molds for the manufacture of phonograph cylinders.² ¹R. Friedel, P. Israel and B.S. Flinn, "Edison's Electric Light: Biography of an Invention," Rutgers Univ. Press (1987), pp. 61-62. ² R.K. Waits, J. Vac. Sci. Technol. A 19(4), Jul/Aug 2001.