Roughing, vacuum

The term vacuum means theoretically a space entirely devoid of matter. Natures closest approach to a perfect vacuum is found in free space but even there one finds a few hydrogen atoms per cubic centimetre. In technical usage, however, the term vacuum applies to any space at less than atmospheric pressure. As can be seen in Table 8, there are various degrees of vacuum rough, medium, high and ultrahigh. 

Vacuum, rough The pressure from atmospheric to about 50mTorr. See also Roughing. 

Nutsche Filter. The nutsche filter (Fig. 8) is simply an industrial-scale equivalent of the laboratory Buckner funnel. Nutsche filters consist of cylindrical or rectangular tanks divided into two compartments of roughly the same size by a horizontal medium supported by a filter plate. Vacuum is apphed to the lower compartment, into which the filtrate is collected. It is customary to use the term nutsche only for filters that have sufficient capacity to hold the filtrate from one complete charge. The cake is removed manually or sometimes by reslurrying. 

Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Radiation differs from conduction and convection not only in mathematical structure but in its much higher sensitivity to temperature. It is of dominating importance in furnaces because of their temperature, and in ciyogenic insulation because of the vacuum existing between particles. The temperature at which it accounts for roughly half of the total heat loss from a surface in air depends on such factors as surface emissivity and the convection coefficient. For pipes in free convection, this is room temperature for fine wires of low emissivity it is above red heat. Gases at combustion-chamber temperatures lose more than 90 percent of their energy by radiation from the carbon dioxide, water vapor, and particulate matter. 

Performance Data for Vacttum-Shelf Dryers The purchase price of a vacuum-shelf dryer depends upon the cabinet size and number of shelves per cabinet. For estimating purposes, typical prices (1985) and auxiliai y-emiipment requirements are given in Table 12-12. Installed cost of the equipment will be roughly 100 percent of the carbon steel purchase cost. 

First, one must estimate air or other gas leakage into the vacuum system. Of course every effort is made to keep it as tight as possible. The author is aware of possible leak points being sealed with polystyrene, which produces an excellent seal. When tests cannot be made, one must use rules of thumb. Many such rough estimating techniques exist. 

RHEED is a powerful tool for studying the surface structure of crystalline samples in vacuum. Information on the surface symmetry, atomic-row spacing, and evidence of surfece roughness are contained in the RHEED pattern. The appearance of the RHEED pattern can be understood qualitatively using simple kinematic scattering theory. When used in concert with MBE, a great deal of information on film growth can be obtained. 

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 ... 

Assume a velocity, v, ft/sec consistent with Figure 2-46. Use Table 2-21 for short, direct connected connections to the vacuum pump. Base the final specifications for the line on pump specifications. Also the diameter of the line should match the inlet connection for the pump. General good practice indicates that velocities of 100 to 200 ft/sec are used, w ith 300 to 400 ft/sec being the upper limit for the rough vacuum classification. 

Rough vacuum Medium vacuum High vacuum Ultra-high vacuum... 

Specific Air Inleakage Rates for Rough Vacuum, for Use with Equations... 

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