Ultimate pressure diffusion pumps

Diffusion pumps operate at veiy low pressures. The ultimate vacuum attainable depends somewhat upon the vapor pressure of the pump liquid at the temperature of the condensing surfaces. By providing a cold trap between the diffusion pump and the region being evacuated, pressures as low as 10 mmHg absolute are achieved in... 

A cold cap is usually mounted on the top of the pump assembly to prevent vapour from reaching the vacuum chamber. A thermal protection switch is often used. The maximum working pressure of a diffusion pump is about 10-3 torr. The ultimate pressure of a diffusion pump can be around 10-9 torr and heavily depends on the oil vapour pressure (p < 10-8 torr at room temperature for very good oils). Pumps with very large pumping speed (up to 104l/s) are commercially available. 

DC 705 has an extremely low vapor pressure and is thus suited for use in diffusion pumps which are used to attain extremely low ultimate pressures of< 10- °mbar. 

In order to attain an ultimate pressure of 1 10 mbar within 5 minutes after starting to pump with the diffusion pump an effective pumping speed of... 

Fig. 6.4. Cross-section of a metal diffusion pump. The upper stage in this pump has a wide annular opening (A) which provides a good ultimate vacuum. The lower stage has a small annular opening (A ) so the pump will operate against a high fore pressure. (B) High-vacuum connection to the low-temperature trap and vacuum line. (C) Connection to rotary oil-sealed pump. This pump is cooled by means of water tubes (D). Air-cooled versions have fins in place of these tubes and a fan is installed to blow air over these fins. (E) Electrically heated oil reservoir.

Of the six diffusion pump oil types, none has all of the above properties. In general, lighter oils pump faster than heavier oils, but heavier oils can achieve lower ultimate pressure. When your work demands varying pump oil requirements, it sometimes is easier to have separate vacuum systems than to simply change pump oils because the pump oils are seldom compatible, and mixing may impair potential peak performance. 

To reach lower pressures, a secondary pump is used, such as a diffusion pump or a sublimation pump (both must remain connected to a primary or "roughing" pump). There are two kinds of diffusion pumps. A mercury diffusion pump can reach 10 6 Torr, but the toxicity of mercury vapor has decreased its use dramatically. A silicone oil diffusion pump can reach 10-7 Torr. For even lower pressures, a Ti sublimation pump is used it can reach about 10 11 Torr. It is usually connected to a sorption primary pump. The ultimate low pressure attained in a laboratory on earth is about 10 13Torr. 

Mechanical Pumps. Perhaps the most common form of vacuum pump is a mechanical pump that operates with some sort of rotary action, with moving parts immersed in oil to seal them against back-streaming of exhaust as well as to provide lubrication. These pumps are used as forepumps for diffusion pumps. Other common laboratory applications are the evacuation of desiccators and transfer lines and distillation under reduced pressure. These pumps have ultimate pressures ranging from 10 to 0.05 Torr, and pumping speeds from 0.16 to 150 L s or more, depending on type and intended application. 

The fluids used in oil diffusion pumps are usually hydrocarbon esters (e.g., Ai n octylphthalate) that are heated to 130 to 160°C in operation or silicone or polyphenyl ether fluids that operate at higher temperatures (180 to 280°C). The latter fluids are quite expensive but give lower ultimate vacuum levels (10 to 10 ° Torr) and greater resistance to oxidation by excessive amounts of air. Further information is given in Table 1. Warning When hot, a diffusion pump should never be exposed to pressures above about 0.1 Torr never vent an oil diffusion pump to air when it is hot. 

If the vacuum sought is close to the minimum pressure obtainable with the diffusion pump used, stopcocks and ground glass joints should not be employed on the high vacuum line. In any case the number of greased joints on the high vacuum side of any pumping system must be kept to the absolute minimum or the ultimate vacuum will not be as low as that possible and the pressure will slowly increase when the manifold is isolated from the pump. 

The planning of a vacuum system should be undertaken in consultation with the person who will use it, or with his supervisor. The type of work to be done with the apparatus and the ultimate vacuum required will determine whether a rotary oil pump or a diffusion pump must be used. The inclusion of a refrigerated trap or traps and the refrigerant to be employed will depend on the materials to be handled, and on their vapour pressures. The measurement of low pressures is not always necessary, but it is advisable to make provision for a vacuum gauge by including a side-arm to which the gauge may be attached should it prove essential at any later time. 

There are two types of diffusion pumps (a) the so called diffusion pump, which operates at a boiler pressure of about 0.5 to 1 mm. Hg and is capable of reaching pressures in the 10 6 mm. range and (b) the booster diffusion pump, which has a boiler pressure of about 3-6 mm. Hg, and has an ultimate pressure of approximately 10-4 mm. The difference in performance characteristics of these two types of diffusion pumps is illustrated in Fig. 5. 

The ultimate pressure attainable by a simple diffusion pump is not as low as the vapour pressure of the oil at the temperature of the upper parts of the pump. In the lower pressure region, these pumps emit as much contaminant gas as they remove. This happens because organic pump oils have the disadvantage that they dissolve all kinds of gases and vapours which may form metastable compounds as well as true solutions. Furthermore, thermal decompositon of the oil may result in the formation of additional volatile products. Release of these gases and vapours into the high-... 

Current facilities at the Lewis Research Center include four vacuum tanks, three of which are 5-ftin diameter and 16 ft long, and one is 3 2 diameter and 7 ft long. Two of the 5-ft tanks have internal nitrogen-cooled condensers with surface area of 730 ft. A photograph of this facility is shown in Fig. 6, Most of the tests on cesium-ion rockets discussed previously were performed in these tanks. Pressures in the 10" mm Hgrange have been readily maintained with two 32-in. diffusion pumps in addition to the condenser. As yet, the thrust level of the ion rockets has not been high enough to test the ultimate capacity of these facilities. 

The refrigeration of 0.58 milliwatts at 0.52 K corresponds to a gas throughput of 416 liters/sec. At this operating point the evaporator pressure is 200 /ll, the pressure at the top of the head is a measured 160 /u and at the inlet to the compressor stand is a measured 100 yu. From the speed specifications on the mercury pump it is estimated that the pressure at the diffusion pump inlet is about 10 A. This large line drop along the low pressure line confirms the rapid decrease in refrigeration at 0.5 K and the fact that the ultimate temperature is no lower than 0.46 K. 

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-... 

The ultimate pressure that an untrapped diffusion pump can reach is the room temperature vapor pressure of the oil being used. Different oils are available the choice requires a trade-off between expense, resistance to oxidation and cracking, and ultimate pressure. 

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