Vacuum systems diffusion pump

As previously mentioned, a diffusion pump works in tandem with a mechanical pump. The simplified alignment of such a system should be considered vacuum line, diffusion pump, mechanical pump. However, in reality, the setup is a bit more complex. 

Two very different kinds of pump fluids have been employed in diffusion pumps. For many years, mercuiy diffusion pumps, were used in small laboratory-bench glass vacuum systems. Mercury pumps are now seldom used owing to the health hazards associated with mercury and the high probability of contamination of the vacuum system with mercury unless a cold trap is used (the vapor pressure of mercury at room temperature is —1.5 mTorr). The oil diffusion pump eliminates the safety hazard and can serve for both small glass and larger metal vacuum systems. 

Figure 13.3 shows a typical pumping station. It includes a mechanical pump, a high-vacuum oil diffusion pump, a manifold, and the associated gauges and valves. Operation of most commercially purchased vacuum systems is now at least partially automated, but older or specialized systems are operated by manual valves. There are... 

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... 

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. 

Fig. 3.20 Schematic diagram of a high vacuum pump system with optionai operation of a Roots pump or a diffusion pump...

The vacuum system must be able to attain the required pressures reliably despite these high gas loads. In the example shown, the system is evacuated with a combination of a backing and Roots pump. A diffusion pump along with a cold surface forms the high vacuum pump system. The cold surfaces pump a large portion of the vapor and volatile substances emitted by the plastic parts while the diffusion pump basically removes the non-condensable gases as well as the noble gas required for the sputter process. 

Fig. 4. Schematic vacuum system for metal atom reactions. X represents the stopcock or Teflon-in-glass valve. Satisfactory components (for a general discussion of vacuum line design see References 1 and 4) forepump, 25 L/min free air capacity diffusion pump, 2 L/sec main trap is removable and measures about 300 mm deep main manifold has a diameter of about 25 mm, stopcock or valve in manifold should be at least 10 mm substrate container is removable container with 1-2 mm Teflon-in-glass needle valve connected to bottom of container. Connection between this needle valve and the reactor may be 1/8 in. od. Teflon tubing is used. Alternatively, the substrate may be added as shown in Fig. 3.

A vacuum system composed of a combination of a rotary pump and an oil diffusion pump is frequently used. Although use of a rotary pump alone can reach low pressures of 1—10—2 Torr, use of both rotary and oil diffusion pumps is desirable because less gas remains in the reaction chamber. 

A vacuum system may consist of a diffusion pump and a backing pump, together with bailies, cold traps and isolation valves. The cold trap is essential if a mercury vapor diffusion pump is used and is best filled with liqnid nitrogen ( — 196°C) otherwise, the system will be exposed to the mercury vapor pressure, which is torr at 18CC. 

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