Vacuum range

Separate all noncondensables before feeding to WEE or SPE unit (even a small amount of noncondensables overloads vacuum system, especially at ultrahigh vacuum ranges) most low molecular weight compounds do not condense at cooling water temperatures under high vacuum. 

The majority of industrial chemical and petrochemical plants vacuum operations are in the range of 100 microns to 760 torn This is practically speaking the rough vacuum range noted above. For reference ... 

Let us also anticipate that, if a vacuum chamber initially contains air at the atmospheric pressure (with a typical composition like that reported in Table 1.2), during the pumping process the composition remains approximately the same in the low-vacuum range. Then, the composition changes, becoming usually richer in light molecules. 

When a gas is removed from a container through a tube, the type of gas flow depends on pressure. In the low vacuum range, the gas flow is controlled by the collisions among molecules (viscous flow). If some molecules are removed from a region (by a pump), other molecules will refdl the empty region. In this situation, the diameter of the pumping line... 

A precision aneroid manometer is used for measurements in the 760— 1 torr range. Thermocouple gauges are used in the 1 — 1 x 10 3 range. A cold cathode ionization gauge is used in the high vacuum range down to 10-6 torr. 

This will be found almost exclusively in the rough vacuum range. The character of this type of flow is determined by the interaction of the molecules. Consequently Internal friction, the viscosity of the flowing substance. Is a major factor. If vortex motion appears In the streaming process, one speaks of turbulent flow. If various layers of the flowing medium slide one over the other, then the term laminar flow or layer flux may be applied. 

Molecular flow prevails In the high and ultrahigh vacuum ranges. In these regimes the molecules can move freely, without any mutual Interference. Molecular flow Is present where the mean free path length for a particle Is very much larger than the diameter of the pipe X d. 

The transitional range between viscous flow and molecular flow is known as Knudsen flow. It is prevalent in the medium vacuum range = d. 

In the high and ultrahigh vacuum ranges the properties of the vacuum container wall will be of decisive importance since below 10 mbar there will be more gas molecules on the surfaces than in the chamber itself. If one assumes a monomolecular adsorbed layer on the inside wall of an evacuated sphere with 1 I volume, then the ratio of the number of adsorbed particles to the number of free molecules in the space will be as follows ... 

Thus, not only will the pumps needed to achieve these pressures in the various vacuum ranges differ, but also different vacuum gauges will be required. A clear arrangement of pumps and measurement instruments for the individual pressure ranges is shown in Figures 9.16 and 9.16a in Chapter 9. 

Measurement of pressures in the rough vacuum range can be carried out relatively precisely by means of vacuum gauges with direct pressure measurement. Measurement of lower pressures, on the other hand, is almost always subject to a number of fundamental errors that limit the measuring accuracy right from the start so that it is not comparable at all to... 

U-tube vacuum gauges fiiied with mercury are the simpiest and most exact instruments for measuring pressure in the rough vacuum range (1013 to a... 

Classical physics teaches and provides experimental confirmation that the thermal conductivity of a static gas is independent of the pressure at higher pressures (particle number density), p > 1 mbar. At lower pressures, p < 1 mbar, however, the thermal conductivity is pressure-dependent (approximately proportional 1 / iU). It decreases in the medium vacuum range starting from approx. 1 mbar proportionally to the pressure and reaches a value of zero in the high vacuum range. This pressure dependence is utilized in the thermal conductivity vacuum gauge and enables precise measurement (dependent on the type of gas) of pressures in the medium vacuum range. 

In addition to the conventional ionization gauge, whose electrode structure resembles that of a common triode, there are various ionization vacuum gauge systems (Bayard-Alpert system, Bayard-Alpert system with modulator, extractor system) which more or less suppress the two effects, depending on the design, and are therefore used for measurement in the high and ultrahigh vacuum range. Today the Bayard-Alpert system is usually the standard system. 

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