Rough-Vacuum Systems

This chapter is primarily devoted to pumps for high vacuum-operation (10 3-10 5 torr), which is the vacuum range of greatest interest in chemical vacuum lines. In addition, rough-vacuum systems (760-0.1 torr) are discussed in connection with their use in manipulating mercury-filled apparatus, such as Toepler pumps and McLeod gauges. 

B. Design Of Equipment. Since the rough-vacuum system may have a practical vacuum limit of 1 torr to several tens of torr, equipment must be designed and filled with mercury to take into account the disparity in pressures between the high-vacuum side and the mercury reservoir. The reservoir must be... 

Fig. 6.1. Rough-vacuum system. Frequently used items such as the Toepler pump and constant-volume manometer are often connected permanently into the rough-vacuum system. Also, one or two outlets, with vacuum tubing attached, are included for general use. The manifold generally is constructed from rigid plastic or metal tubing.

The tilting McLeod gauge (Fig. 7.8) is a simple, inexpensive, and portable gauge which may be used to measure pressures down to about 10 3 torr. These gauges are very useful for checking rough vacuum systems, Schlenk systems, and for the calibration of thermal conductivity vacuum gauges. 

It is more costly to remove air from a high-vacuum system than from a rough-vacuum system. 

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. 

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. 

Over a 30 m length of a 150 mm vacuum line carrying air at 295 K, the pressure falls from 0.4 kN/rrr to 0.13 kN/m2. If the relative roughness e/d is 0.003 what is the approximate flowrate It may be noted that flow of gases in vacuum systems is discussed fully by Griffiths 11. 

Almost all the high vacuum systems with which we will be concerned have at one end a rough pump, usually a rotary oil pump, capable of attaining ca. 10 Torr. This is followed by a high vacuum pump which can attain ca. 10 Torr, which is followed by a cold trap, the purpose of which is to condense out any volatile matter to prevent it entering, and possibly... 

Stator blades. However, one advantage of these small turbopumps is that they do not need to be bypasssed for roughing, which simplihes the design of the vacuum system. 

Vapors can be transferred into or out of the apparatus by applying a rough vacuum to the reservoir which draws mercury out of the U and into the reservoir. After the volatile materials are condensed into the apparatus, they are isolated from the vacuum system by bringing the mercury reservoir to atmospheric pressure and slowly bleeding mercury into the U. If it is important to know the gas volume in the apparatus, the mercury level is adjusted to some reference point, such as A in Fig 9.2, for which the volume of the apparatus has previously been determined. 

Many vacuum pumps are supplied with a gas ballast valve which is designed to prevent the condensation of moisture and similar condensable materials. This valve bleeds a small amount of air into the pump and as a result the ultimate vacuum and pumping speed of the pump suffer. There is no occasion to use this feature on a pump on a typical chemical high-vacuum line, and it rarely, if ever, is used in typical laboratory rough-pumping systems. 

The preparation of rough silver films by vapor deposition results in reproducible and stable surfaces for SERS. For example, deposition of 20-nm Ag films onto Teflon, polystyrene, or latex spheres [29,30] has been performed. These substrates produced strong SERS intensities for various organic adsorbates and good reproducibility between multiple rims. However, vapor deposition can be slow and needs access to a vacuum system. There are also some variables that need to be controlled, such as the film thickness, deposition temperature, and use of annealing procedures. Moreover, unless the experiment is performed under vacuum, the film is exposed to the atmosphere after deposition. Even a brief exposure to the atmosphere results in contamination of the surface and the formation of an inactive oxide layer. 

The 328 nm light is then coupled into a linear enhancement cavity placed inside the vacuum system. The metastable ions will ultimately be focused through the centre of the resonant mode of the cavity where they will interact with the light. The waist size of the fundamental mode of the cavity is around 100 /rm, chosen to make the transit-time broadening roughly equal to the natural width of the transition. A cavity is a convenient way of providing the counter-propagating beams required for the Doppler-free excitation of the two-photon transition,... 

Fortunately, vacuum systems in most laboratories are small enough that the fore pump can double as the roughing pump with no problems. For the few times that one needs to go from atmospheric to vacuum conditions, all that is necessary is to close off... 

All manipulations of volatile reactants and products are accomplished with a standard Pyrex vacuum system. Kel-F stopcock grease is inert to the species to be handled and is recommended though it is somewhat thin. Apiezon(N) tends to darken rapidly on contact with phosphorus(V) fluorides, but can be used successfully for short periods of time and for manipulations necessary for rough transfer and purification. 

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