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For dry processes in which a non-condensable gas mixture (air for example,) is to be pumped, the pump to be used is clearly characterized by the required working pressure and the quantity of gas to be pumped away. The choice of the required working pressure is considered in this section. The choice of the required pump is dealt with on the page Choosing a pump size.
Each of the various pumps has a characteristic working range in which it has a particularly high efficiency. Therefore, the most suitable pumps for use in the following individual pressure regions are described. For every dry vacuum process, the vessel must first be evacuated. It is quite possible that the pumps used for this may be different from those that are the optimum choices for a process that is undertaken at definite working pressures. In every case the choice should be made with particular consideration for the pressure region in which the working process predominantly occurs.
The usual working region of the rotary pumps lies below 80 mbar. At higher pressures these pumps have a very high-power consumption (see Fig. 2.11) and a high oil consumption. Therefore, if gases are to be pumped above 80 mbar over long periods, one should use, particularly on economic grounds, jet pumps, water ring pumps or dry running, multi-vane pumps. Rotary vane and rotary piston pumps are especially suitable for pumping down vessels from atmospheric pressure to pressures below 80 mbar, so that they can work continuously at low pressures. If large quantities of gas arise at inlet pressures below 40 mbar, the connection in series of a Roots pump is recommended. Then, for the backing pump speed required for the process concerned, a much smaller rotary vane or piston pump can be used.
1 Operating temp. curve 1 - 89°F (32°C)
2 Operating temp. curve 2 - 104°F (40°C)
3 Operating temp. curve 3 - 140°F (60°C)
4 Operating temp. curve 4 - 194°F (90°C)
5 Theoretic curve for adiabatic compression
6 Theoretic curve for isotherm compression
If a vacuum vessel is merely to be evacuated to pressures in the medium vacuum region, perhaps to that of the required backing pressure for diffusion or sputter-ion pumps, single and two-stage rotary pumps are adequate for pressures down to 10-1 and 10-3 mbar, respectively. It is essentially more difficult to select the suitable type of pump if medium vacuum processes are concerned in which gases or vapors are evolved continuously and must be pumped away. An important hint may be given at this point. Close to the attainable ultimate pressure, the pumping speed of all rotary pumps falls off rapidly. Therefore, the lowest limit for the normal working pressure region of these pumps should be that at which the pumping speed still amounts to about 50% of the nominal pumping speed.
Between 1 and 10-2 mbar at the onset of large quantities of gas, Roots pumps with rotary pumps as backing pumps have optimum pumping properties. For this pressure range, a single-stage rotary pump is sufficient, if the chief working region lies above 10-1 mbar. If it lies between 10-1 and 10-2 mbar, a two-stage backing pump is recommended. Below 10-2 mbar the pumping speed of single-stage Roots pumps in combination with two-stage rotary pumps as backing pumps decreases. However, between 10-2 and 10-4 mbar, two-stage Roots pumps (or two single-stage Roots pumps in series) with two-stage rotary pumps as backing pumps still have a very high pumping speed. Conversely, this pressure region is the usual working region for vapor ejector pumps. For work in this pressure region, they are the most economical pumps to purchase. As backing pumps, single-stage rotary positive displacement pumps are suitable. If very little maintenance and valveless operation are convenient (i.e., small vessels in short operation cycles are to be pumped to about 10-4 mbar or large vessels are to be maintained at this pressure unattended for weeks), the previously mentioned two-stage Roots pumps with two-stage rotary pumps as backing pumps are the suitable combinations. Although such a combination does not work as economically as the corresponding vapor ejector pump, it can operate for a much longer time without maintenance.
Diffusion, sputter-ion, and turbomolecular pumps typically operate in the pressure region below 10-3 mbar. If the working region varies during a process, different pumping systems must be fitted to the vessel. There are also special diffusion pumps that combine the typical properties of a diffusion pump (low ultimate pressure, high pumping speed in the high vacuum region) with the outstanding properties of a vapor ejector pump (high throughput in the medium vacuum region, high critical backing pressure). If the working region lies between 10-2 and 10-6 mbar, these diffusion pumps are, in general, specially recommended.
For the production of pressures in the ultrahigh vacuum region, sputter-ion, and sublimation pumps, as well as turbomolecular pumps and cryopumps, are used in combination with suitable forepumps. The pump best suited to a particular UHV process depends on various conditions (for further details, see the page on oil free vacuum).