Vacuum pumping speed

After having reached the operating pressure it is therefore recommended to limit the vacuum pumping speed using a throttle valve, controlled gas ballast and/or in case of rotating pumps like roots blowers a frequency controller. 

Vacuum flow is usually described with flow variables different from those used for normal pressures, which often leads to confusion. Pumping speed S is the actual volumetric flow rate of gas through a flow cross section. Throughput Q is the product of pumping speed and absolute pressure. In the SI system, Q has units of Pa m vs. 

For a vacuum pump of speed withdrawing from a vacuum vessel through a connecting line of conductance C, the pumping speed at the vessel is... 

Cold finger refrigerant traps are often used in vacuum lines as a substitute for the more efficient total immersion traps which, however, tend to cut down the pumping speed of the system. In constructing a cold finger trap a Dewar seal is first made, then, before... 

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. 

The motor (usually working in vacuum) is moved by a special current power supply. The rotor turns at 104-105rpm, usually a multiple of the line frequency. The pumping speed of a turbo pump unit depends on its rotational speed. High-speed turbo pumps need more frequent maintenance interventions. In some turbo pumps, a low-speed mode allows operation up to 10 1 torr. However, full rotational speed is achieved at pressures... 

As in the molecular beam experiment, the reactor volume, pumping speed, and rate of introduction of reactants have values which lead to a flux of reactants well defined in time. Strozier, however, simply doses gas into the vacuum system (reactor) rather than using a molecular beam. He studied CO oxidation, which has nonlinearities in the surface rate equation, so that computer rather than analytic solutions are necessary. The results are represented at constant frequency and varying temperature as shown in Fig. 8, which is a computer simulation (37). 

In conclusion, TDS of adsorbates on single crystal surfaces measured in ultrahigh vacuum systems with sufficiently high pumping speeds provides information on adsorbate coverage, the adsorption energy, the existence of lateral interactions between the adsorbates, and the preexponential factor of desorption, which in turn depends on the desorption mechanism. Analysis of spectra should be done with care, as simplified analysis procedures may easily give erroneous results. 

The lowest pressure which can be achieved in a vacuum vessel. The so-called ultimate pressure Pgi j depends not only on the pump s suction speed but also upon the vapor pressure p j for the lubricants, sealants and propellants used in the pump. If a container is evacuated simply with an oil-sealed rotary (positive displacement) vacuum pump, then the ultimate pressure which can be attained will be determined primarily by the vapor pressure of the pump oil being used and, depending on the cleanliness of the vessel, also on the vapors released from the vessel walls and, of course, on the leak tightness of the vacuum vessel itself. 

The effective pumping speed required to evacuate a vessel or to carry out a process inside a vacuum system will correspond to the inlet speed of a particular pump (or the pump system) only if the pump is joined directly to the vessel or system. Practically speaking, this is possible only in rare situations. It is almost always necessary to include an intermediate piping system comprising valves, separators, cold traps and the like. All this... 

Normally a Roots pump will be operated in connection with a dovmstream rough vacuum pump having a nominal pumping speed Sy. The continuity equation gives ... 

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