Pumps diffusion systems

Diffusion Pump System. After the pump line and trap have been shut off, a large valve is opened slowly enough that the mass flow of gas from the chamber through the valve into the od-diffusion pump system does not dismpt the top jet of the diffusion pump (DP) (Fig. 4). When the Hquid nitrogen is replenished after the trap has been operated for some time, release of previously trapped gas must be avoided. The so-caded ionization-gauge response pips at the start of the Hquid-nitrogen replenishment are an indication of trap ineffectiveness. 

The diagram below shows a conventional diffusion-pumped system. 

What typically happens with a glass vacuum system is that first a mechanical pump removes a great deal of the loose, or free, gas particles. Then, greater vacuum is achieved with the combination of a diffusion pump (or similarly fastpumping unit) and traps that remove or bind up the various vapors within the system (for example, oil, mercury, and water). The only way a system can achieve a vacuum lower than 10 6 to 10 7 torr is if the pump can remove water vapor faster than the water vapor can leave the walls. Most diffusion pumping systems cannot achieve this goal, but even if they could, there is such a substantial amount of water vapor within the glass that, unless the walls are baked, a better vacuum cannot be obtained. 

Fig. 7.14 The proper orientation of a mechanical pump s exhaust condensate trap. Reprinted from N.S. Harris, Practical Aspects of Constructing, Operating and Maintaining Rotary Vane and Diffusion-Pumped Systems, Vacuum, Vol. 31, 1981, p. 176, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Unless otherwise indicated the catalysts studied are degassed at 350°C. for sixteen hours by means of a mercury diffusion pump system. The helium used for dead space measurements is 99.9% pure and is obtained in special Pyrex flasks from the Ohio Chemical Company. The nitrogen which is over 99 % pure as received from the Ohio Chemical Company is purified further by passage over copper gauze at 500°C. and phosphorous pentoxide with a final distillation at liquid nitrogen... 

As seen in this example, low-pressure pumping costs by the two-phase method are appreciably less than with the conventional diffusion pump system in some... 

A schematic diagram of the apparatus used in this preparation is shown in Fig. 2. The vacuum chamber was evacuated with an oil diffusion pump system, and the ultimate pressure was 2 X 10 torr. Sapphire and silicon were used as the substrates. The substrate was fixed on the tungsten sheet heater (0.05 mm thick) and was heated from 600 to 1200°C (Fig. 3). A tantalum sheet was also used as a substrate and was heated between 350 and 700°C by direct resistance heating. The sapphire substrate temperature was estimated from the heater temperature, which was measured with an optical pyrometer. The relation between the substrate and heater tempera-... 

Figure 3.14 Fail-safe Designs for Use With (a) A Cryopumped System, (b) A Diffusion-Pumped System...

The hydrocarbon lubricating and sealing oils used in mechanical pumps must not be allowed to backstream or creep to the DP and contaminate the DP oil Power failure, cooling failure, or mistakes in operating a diffusion-pumped system can result in pump oil contaminating the processing chamber. In some applications, cryopumps or turbopumps are used instead of DPs to avoid the possibility of oil contamination. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

The prepared flask is supported so that the tube B can be sealed to a vacuum system. If a suitable furnace is available the flask is surrounded by this, with the constriction A still within reach of the hand torch flame otherwise the heating must be done by flame. A liquid air trap is included between the flask and diffusion pump. With a vacuuih of below 10 mm of mercury the temperature of the flask is raised to near the softening point—the heating may be rapid... 

The membranes used are typically composed of cross-linked silicones and are suitable for on-line monitoring of volatile organic and inorganic compounds [93-94]. An alternative material is microporous PTFE, which has more rapid responses as well as lower selectivities and higher fluxes of the mobile phase compared to nonporous silicone membranes. More recently, developments in membrane introduction systems include the use of liquid membranes composed, for example, of a polyphenyl ether diffusion pump fluid [95-96]. This membrane has the advantage that it can take any desirable analyte and the selectivity can be modified using appropriate reagents. 

In addition to the vacuum valves, which perform solely an isolation function (fully open - fully closed position), special valves are needed for special functions. Typical are variable leak valves, which cover the leakage range from 10" ° cm /s (NTP) up to 1.6 10 cm /s (NTP). These valves are usually motor driven and suitable for remote control and when they are connected to a pressure gauge, the process pressures can be set and maintained. Other special valves fulfill safety functions, such as rapid, automatic cut-off of diffusion pumps or vacuum systems in the event of a power failure. For example, SECUVAC valves belong to this group. In the event of a power failure, they cut off the vacuum system from the pumping system and vent the forevacuum system. The vacuum system is enabled only after a certain minimum pressure (about 200 mbar) has been attained once the power has been restored. 

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