Fore pumps

Fig. 28. A simplified vacuum setup i—vessel being evacuated . 2—cooled traps steam-jet pump 4—vacuum valves 5—fore pump (pum p for preliminary rarefaction)...

Mechanical Pumps. Most modern fore pumps are mechanical ones with a revolving rotor and oil seal. All pump models follow the same principle, although they differ in design. 

Perform final purification of the iodine in the apparatus shown in Fig, 596. Put the iodine in vessel 4. Cool it with liquid nitrogen. Connect the apparatus to a fore pump. Evacuate the apparatus with cock 2 open (40 min), then close the cock, fill tube 1 with cold water, remove the liquid nitrogen so that vessel 4 acquires room temperature, and carefully heat it with the flame of a burner. When sublimation of the iodine is completed, cool the apparatus, carefully open cock 2,... 

The mechanical fore pump is a heavy item which sets up considerable vibration. This vibration disrupts the menisci of mercury manometers and is otherwise undesirable. To minimize the transfer of vibrations to the vacuum line, the fore pump generally is mounted on the floor (rather than on the bench of the vacuum rack) and the connection between the fore pump and the vacuum system is made with heavy-walled vacuum tubing or flexible corrugated metal tubing. 

Fig. 6.6. Pumps, traps, and bypass. (A) Mechanical fore pump (B) short length of vacuum tubing (C) stopcock for venting system when fore pump is turned off (D) diffusion pump (E) cold trap at — 196°C (F), (H) stopcock which allow the diffusion pump to be isolated and bypassed (G) stopcock which allows isolation of the main manifold from the trap and pumps.

Check pumping system to be sure it is achieving a good vacuum. If necessary, correct problem with fore pump, diffusion pump, or cold trap. 

A. Typical Systems. A simple system for the transfer of samples to an infrared gas cell or to a NMR sample tube consists of a fore pump, diffusion pump, trap, and manifold (Fig. 5.1). At the other extreme is a general-purpose chemical vacuum line, which permits the separation of volatile compounds, transfer of noncondensable gases, and storage of reactive gases and solvents (Fig. 5.2). When attack of stopcock grease is a serious problem, grease-free de-... 

To start a vacuum system such as that illustrated in Fig. 5.1, the clean main trap is fitted in place with an even coat of stopcock grease on the joint. A Dewar partially filled with liquid nitrogen is raised around this trap, and the fore pump is immediately turned on. CAUTION Oxygen from the atmosphere will condense in a trap heid at liquid nitrogen temperature therefore, it Is important never to leave a trap cooled to liquid nitrogen temperature exposed to the atmosphere for a significant length of time. When the line has pumped down to less than 1 torr, the... 

The reaction was studied in a laboratory assembled high-vacuum system (see Figures 9.8 and 9.9), consisting of two vacuum circuits, one for the evacuation of the evaporation chamber and the other for the evacuation of the gas introduction system [119,122,123], Figure 9.8 shows the fore pump, diffusion pumps, cold traps, ionization manometer, and the metal evaporation chamber [119]. The evaporation was carried out al / = 10 6 Torr, with the help of a thread helicoidal filament made of a wolfram (W) wire of 0.5mm diameter and 10cm length [119]. The whole vacuum system was coupled with a mass spectrometer, Hitachi RMU-6D, in order to follow the reaction kinetics [119,122,123] (see Figure 9.9). The procedures followed in order to study the reaction were as follows ... 

Most manufacturers suggest the size of fore pump to be used with their diffusion pump. If you do not have this information, it is possible to match a fore pump with a diffusion pump by using Eq. (7.11) ... 

Whatever you get for the fore pump speed from Eq. (7.11), it s a good idea to use a pump with at least double that figure to accommodate pressure fluctuations and make up for line impedance within the vacuum system. 

The foreline is the section of the vacuum system between the high-vacuum pump (i.e., diffusion pump) and the fore pump (i.e., mechanical pump). 

A vacuum system needs to achieve a sufficiently low pressure before the diffusion pump can begin operation. A mechanical pump that works in tandem with a diffusion pump is called a fore pump. Fore pumps must be capable of achieving 1 O 2— 1 O 4 torr and have sufficient speed to effectively work with a diffusion pump. 

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... 

Operations that require you to cycle your vacuum line back and forth from atmospheric to low pressure can put significant strain on the fore pump and its oil, so a roughing pump should again be considered. 

After shutting off the roughing and fore pumps, open Stopcocks 5 and 6 to vent the pumps (this action prevents the mechanical pump oil from accidentally being sucked into the vacuum line). 

If there is material in the cold trap near the vacuum line that needs to be removed, open Stopcock 1 to vent the trap. Then remove the trap so that it may defrost in a fume hood. It is unlikely that the cold trap near the fore pump will need to be removed for cleaning. 

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