362 Vacuum Systems

fire-resistant, not fireproof. Before the advent of the fluorocarbon oils, they were used for oxygen-rich environments.

The Chlorofluorocarbon Oils (CFCs).* These oils are nonreactive, do not form tars, do not break down, and are nonflammable; therefore they can be used in pumping pure oxygen. On the other hand, they have greater viscosity changes with temperature than do most other oils. In cold temperatures the viscosity can be great enough to prevent a pump from starting. In addition, if these oils (such as HF fumes) are subjected to temperatures greater than 280°C, highly toxic fluorine compounds are produced. As an extra safety precaution, there should be no smoking anywhere near pumps using CFCs. It is easy for a contaminated finger to pass a CFC onto a cigarette, which in turn will be burnt and inhaled!

The chlorofluorocarbon oils should not be used with any pump that may have aluminum components (if in doubt, check with the manufacturer). A momentary seizure of an aluminum part can cause a highly localized temperature increase, which, when in contact with a chlorofluorocarbon oil, can cause an explosion.

Incidentally, thermocouples can be affected by CFCs and display a reading three to five times greater than the real pressure. The gauge itself is not affected because the difference is due to the greater thermal conductivity of a CFCs to air. The user needs to divide the thermocouple reading by a factor of three to five to obtain actual pressure.

The Perfluorinated Polyether Oils (PFPE). These oils are the most expensive oils available. They are extremely nonreactive, do not form tars, do not break down, are nonflammable, and can therefore be used in pumping pure oxygen.

If these oils are subjected to temperatures greater than 280°C, highly toxic gases (HF) can be produced. Therefore, there should be no smoking anywhere near pumps using PFPEs. It is easy for a contaminated finger to pass a PFPE onto a cigarette, which in turn will be burnt and inhaled!

The excellent stability of these oils creates a new problem. Among the duties of pump oils is trapping vapors and particulate material from a system. Acids and particulate matter remaining within a pump oil can destroy a pump, even though the oil itself may be in fine condition. Fortunately, PFPE oils can be purified (reclaimed), which dramatically decreases the high initial costs of these fluids. Therefore, proper changing schedules, using fresh oil, do not necessarily mean a tremendous increase in cost beyond your original investment.

7.3.9 Storing Mechanical Pumps

If a mechanical pump is going to be unused for any length of time, the pump oil should be changed before storage. Although a small concentration of contaminants are not likely to affect a pump over a short period of time, damage can occur with prolonged exposure. By removing the old oil, any potential contaminants are

* The acronym "CFC" stands for all chlorofluorocarbons, including those that are specifically harmful to the environment and those that are considered benign.

Fig. 7.20 Some Schlenk apparatus items.

simply rotating the stopcock 180°,it can be refilled with dry nitrogen. Using several more repetitions of this evacuate and purge procedure, oxygen concentrations can be brought to relatively low levels.

Assuming there are no leaks, the remaining portion of atmospheric gases can be

To demonstrate the value of this system, suppose you have a mechanical pump who's general lower range is 2 torr (in an acceptable pumping time period*). By the third evacuation and purging, the amount of residual air is 2 x (2/760)2, or about 10"5 torr.23

I strongly encourage anyone seeking more information on Schlenk lines and other oxygen sensitive chemistry to examine The Manipulation of Air-sensitive Compounds by D.F. Shriver and M.A.Drezdzon (Wiley-Interscience, © 1986).

7.3.12 Diffusion Pumps

A diffusion pump (also called a vapor pump) has no moving parts, yet is considered a "fast" pump. By itself, the diffusion pump cannot move gases from one place to another. However, under specific conditions, it can compress gases to a specific region within itself. Used in tandem with an auxiliary pump (such as a mechanical pump), these compressed gases can be expelled out of the system.

*That is, prolonged evacuation could create a better vacuum, but, as is shown, the prolonged pumping isn't time-effective.

Interestingly, it is the same auxiliary pump that removes these compressed gases that creates the specific conditions for the diffusion pump to work.

The basic principle of a diffusion pump can be explained with a simple singlestage mercury diffusion pump (see Fig. 7.21). On the system side of the pump (at about 10~2 to 10~3 torr, or better), gas molecules wander around, limited by their mean free path and collisions with other molecules. The lowest section of this diffusion pump is an electric heater that brings the diffusion pump liquid up to its vapor pressure temperature.* The vapors of the diffusion pump liquid are vented up a central chimney where, at the top, they are expelled out of vapor jets at supersonic speeds (up to 1000 ft/sec). Below these jets is a constant rain of the pumping fluid (mercury or low vapor-pressure oil) on the gases within the vacuum system. Using momentum transfer/ gas molecules are physically knocked to the bottom of the pump, where they are trapped by the vapor jets from above. Finally, they are collected in a sufficient quantity to be drawn out by the auxiliary (mechanical) pump.

The upper pressure range for a diffusion pump to operate is limited by the vapor pressure of the heated oil (or mercury) at the jets. Unless the second pump can achieve this pressure (assisted by the decreased pressure within the system), the diffusion pump will not operate.

Pumping speed within a diffusion pump is proportional to the area being pumped. When diffusion pump vapors are traveling their fastest, they can make the pump pump faster. Unfortunately at these greater speeds, they have the least compression ratio, which means that the collected gases may not have sufficient pressure to be drawn out by the fore pump. By shaping the insides of the diffusion pump accordingly (i.e., a larger space around the first stage and succeedingly less annular space around subsequent stages) engineers can manipulate the vapors of the pump to get the best of both worlds. The design for a typical metal three-stage pump can be adapted to glass as seen in the unique Wheeler pump in Fig. 7.23. The more commonly seen "old style multistage glass diffusion pump" can be seen in the pumps shown in Fig. 7.22.

The diffusion pump in Fig. 7.21 has only one jet.* By definition, this design would be called a single-stage pump. Metal diffusion pumps can have from one to six stages, whereas glass diffusion pumps can have one to three stages. Pumps with several stages allow each stage to be aided by the work of the preceding stage. The first stage (at the inlet of the pump) can offer a high pumping speed (but by design cannot have a high compression ratio). Because the gas flow is constant, the next stage does not require the pumping speed that the first was capable of achieving and can therefore provide a higher compression ratio. This process

Relative to the pressure within the system.

+ Momentum transfer is possible because momentum = (mass) x (velocity), and the velocity of the diffusion pump fluid (when it leaves the pump jets) is typically at supersonic speeds. Thus, it is easy for the diffusion pump liquid to knock around any size molecule.

*For comparison, the diffusion pump in Fig. 7.23 is a three-stage pump.

368 Vacuum Systems

some attachment decisions). Therefore, be sure you know what type of glass you need before ordering a glass-to-metal flange.

For years, glass diffusion pumps provided (relatively) easy attachment to a glass vacuum system (but rather difficult removal), are mostly free from attack by corrosive substances, provide easy observation of the materials inside the pump, and can be cleaned (with some difficulty).

Figure 7.22 displays three different traditional designs of glass diffusion pumps (that use low-vapor-pressure oil). These three designs demonstrate the addition of stages. They are also among the most commonly seen diffusion pump designs. Although the three pumps shown are all air-cooled, it is possible to obtain most small diffusion pumps as airor water-cooled. The advantage of an air-cooled diffusion pump is that there is no water hose connection to slip off the pump and no overheating occurs if the building water is shut off without warning. However, in a warm, nonventilated room, air-cooled diffusion pumps can lose their efficiency. Water-cooled pumps work efficiently regardless of room conditions.

The data in Table 7.8 show that the addition of stages dramatically increases the speed of a pump while only moderately affecting the net vacuum potential.24 However, the speeds listed were determined in 1965 and were ostensibly derived from a vacuum gauge placed at the inlet end of the pump. For one thing, this location provides the best potential vacuum that can be expected from any system and is not likely to represent what would be found on any vacuum system. However, more significantly, new research initiated by Strattman (and performed at Varian) has created a significant controversy on the merits of the single-stage glass diffusion pump of the design shown at the top of Fig. 7.22. Varian found that this design had no pumping capabilities whatsoever because it had a pumping speed of essentially zero. With that in mind, all the other speeds presented in Table 7.8 are suspect.

Strattman quotes Hablanian, stating that this single-stage pump design was created by Gaede in 1915 and later shown to be inefficient by Donouyer in 1922. A more viable design was created by Langmuir in 1928 by implementing jets into the design. Although popular (probably by its simplicity in design), it is at best assisting the mechanical pump by limiting backflow of mechanical pump oil vapors. A diffusion pump requires a boiler that can obtain a pressure of at least 1 torr and a jet to accelerate the vapors. If this pump had a jet arrangement added, it could theoretically obtain a pump speed of approximately 1 liter/m.

On the other hand, Snyder, using a different independent testing lab, came up with results comparable to the results presented in Table 7.8. There are a variety of possible explanations for this, including measuring different parameters, one pump design is better than the other, improper fluids, or improper pump instillation. It appears that despite the simplicity of design, more analysis needs to be done to fully appreciate the value (or lack thereof) of the single-stage glass diffusion pump.

One of the problems in analyzing the speeds of any glass diffusion pump is the fact that all diffusion pump construction is handmade by a glassblower. There is

Distillation

pot

Fig. 7.22 Various forms of glass diffusion pumps. The potential speeds and ranges of glass diffusion pumps are shown in Table 7.8. From Journal of Chemical Education, 42, pp.A445-A459. (1965), reproduced with permission.

Table 7.8 Glass Diffusion Pump Capabilities" b

" For more information on the speed of single-stage diffusion pumps, see page 368.

b From Journal of Chemical Education, 42, pp.A445-A459. (1965), reproduced with permis-

no question that the vast majority of diffusion pumps one may purchase are of excellent quality. It is also true that construction can vary from one pump to another by the same glassblower and can vary considerably from glassblower to glassblower. Intentional alterations such as a design tweak here, an alteration there can subtly or significantly affect the potential quality of the pump. Likewise, unintentional shape alterations can equally affect the potential of a glass diffusion pump.