Fig. 7.10 The total conductance is the addition of the reciprocal conductance of all the parts (see Eq. (7.9).

7.3Pumps

7.3.1The Purpose of Pumps

A pump's function (when creating a vacuum) is to remove gases and condensable vapors from the chamber where you want the vacuum. There are two ways to remove gases and vapors. The one that typically comes to mind is gas transfer, which physically removes gases and vapors from one area and release them into another. The other approach for removing air is to trap the offending gases and vapors. Once trapped, the vapors are still in the region where you are working but are bound up in such a fashion so as not to impede the work in progress. The two approaches of gas removal, pumps, and corresponding pumping ranges are shown in Table 7.5.

The most common level of vacuum required to do most laboratory procedures is, in reality, not a very significant vacuum. In fact, a high (=10~3 torr) to very high (=10~6 torr) vacuum will satisfy some 95% of most laboratory vacuum needs. The levels of ultrahigh vacuum (whose demands are usually reserved to surface studies, ultraclean vapor deposition, space simulation, and particle accelerators) are not that common in many research laboratories. Thus, in the chemical research laboratory, the pumps that are usually found are the gas transfer type. More specifically, one is likely to see aspirators, diffusion pumps, and rotary vane and/or piston pumps. Although turbomolecular pumps (turbopumps) and cryopumps may also be found in the laboratory, their high initial cost (along with their higher level of technical support), excludes them from being the first pump of choice unless unique requirements demand their use. The other pumps mentioned in Table 7.5 are usually used in specific industrial or highly specialized research. With all due respect to these fine pumping alternatives, discussions within this book are limited to aspirators, rotary (vane and piston) pumps, and diffusion pumps. For further information on the other pumps mentioned, refer to Appendix D.

The concept of "you get what you pay for" can be altered for vacuum pumps to "the less you want, the more you pay." With that thought in mind, you do not want to pay for vacuum you do not need. Note however, that the stated limit of a pump

cally trapped and removed from the aspirator body. In addition, the faster air moves, the less its density. As moving (and therefore less dense) air is removed, nonmoving (and therefore more dense) air is drawn in.

7.3.3 Types and Features of Mechanical Pumps

There are many mechanical pump designs, the most common of which is the rotary pump. It is named for its use of rotating internal parts that collect, compress, and expel gas from a system. Despite the simplicity in concept, there are very interesting mechanisms that pump manufacturers have developed to overcome the problem of mechanical pumps, namely, that they are inherently a slow pump. It is their simplicity, however, that has made them the general workhorse for creating a vacuum all over the world.

There are two different types of rotary pumps: vane (both singleand doublevane, see Fig. 7.12) and piston (see Fig. 7.13). The vane pump, by definition, uses a spring-backed vane both to separate the atmospheric gases from an evacuated system and to sufficiently compress the gases to a pressure greater than atmospheric so they can be expelled out of the pump. Regardless of whether the vanes sweep in motion (as in the double-vane pump) or remain still and let an eccentric drum roll past (as in the single-vane pump), the vanes permit motion within the pump while maintaining a pressure difference of up to (typically) 10"3 torr. One of the obvious differences between the doubleand single-vane pumps is that the sin- gle-vane pump's main rotor spins eccentrically. This eccentric spinning creates a vibration, and if the pump is placed or mounted too close to sensitive instruments, this vibration can affect their operation. The double-vane rotary pump rotates evenly and quietly with little vibration. Pump vibration is important if the pump is mounted in a location that could affect sensitive instruments.

The piston pump, like the single-vane pump, also has an eccentrically rotating drum. However, rather than a vane pressing against the drum, there is a free rotating sleeve around the drum that rolls around the pump's inner cavity as the drum rotates. By rolling around the pump's inner cavity, there is no sliding of moving parts as the vane pumps. Connected to the rotating drum is a sliding shaft with a hole in it that (a) allows air into the pump only when the drum is in the correct position and (b) prevents air coming in at all other times. Properly made piston rotary pumps are counterbalanced so as to have very minimal amounts of vibration.

There are inherent weaknesses in the design of vane pumps that are overcome with piston pumps. For example, on each sweep of a vane, oil is brought from the atmospheric side to the vacuum side, and needs to be re-outgassed on each sweep. A piston pump's roller does not sweep oil around. Rather, it rolls over the oil without pushing it aside, thus significantly reducing the outgassing problem. In addition, the slide shaft (within the piston pump) maintains the internal sections of the pump as two separate sections. This separation further prevents the oils from mixing. Within the pump, one section is maintained at the vacuum of the chamber

Gas ballast

compression

compression

Fig. 7.12 This series of drawings displays how gas is drawn into single-and dou- ble-rotary vane mechanical pumps, compressed, and expelled into the atmosphere.

being evacuated, and the other section is maintained at the pressure for expelling gas into the atmosphere. Because vanes are constantly scraping the internal walls of the pump, extra heat is generated. This heat produces extra wear on the pump, its parts, and the oil. On the other hand, the extra heat helps to outgas the oil to some degree.

Finally, because the piston does not need to make actual contact with the internal sections of the pump (only contact with the oil is required), there is less wear than with vane pumps. Because the piston pump parts do not make physical contact, they can tolerate larger-sized paniculate matter (which may accidentally enter the pump) and are also more forgiving of internal scratches.

Rotary piston pump studies made by Sadler12 demonstrated that the above attributes of piston pump design helped them outperform vane design pumps both in speed and vacuum achieved (Sadler compared pumps of comparable size). On the other hand, piston pumps had greater vibration.

Aside from the two types of rotary pumps mentioned, you will often see the terms single-stage and double-stage mentioned in the context of pumps. A twostage pump (also called a compound pump) simply refers to a mechanical pump that has one or two pumps connected in line together. The exhaust of the first one

346 Vacuum Systems

pump can go beyond the physical limits of a single-stage pump. In addition, twostage pumps fractionate their oil as they work. The oil on the inlet (vacuum) side maintains its purity while the oil on the atmospheric side tends to collect the contaminants and products of oil degradation. This design helps maintain the capability of the pump between oil changes.

The design of two-stage pumps makes them poor choices for pumping air at high pressures (i.e., atmospheric) for an extended amount of time. The excess air pressure tends to cause the oil in both sections of the pump to froth and foam, which in turn may cause the bearings and/or the seals to overheat. Thus, two-stage pumps make poor roughing pumps. It is for this reason that some systems require two mechanical pumps. One pump is a single-stage roughing pump to get a large system (or a system with large quantities of moisture) down to a pressure that a diffusion pump can begin to operate. This system is then joined by a two-stage pump that can deliver the proper fore pressure for the diffusion pump (or other ultrahigh-vacuum pump). The second pump is called the fore pump. If the system is small and can be pumped down to about 1 torr within a minute or two, then the roughing pump and the fore pump can be the same pump. However, if your work tends to cycle between atmospheric and vacuum on a repeated and continual basis, the addition of a roughing pump is recommended.

Pumps can also be divided into direct-drive or belt-driven models. Direct-drive models run faster and quieter, and they are smaller and lighter for the same level of performance. For many years they were not considered able to withstand the workloads of belt-driven pumps because they tended to wear out prematurely. Fortunately, direct-drive pumps have improved considerably over the years.

In their early design, direct-drive pumps simply had the shaft of the motor extended, and a pump was placed on this shaft. Unfortunately, any minor variation in alignment caused excessive wear and tear on the pump bearings and vanes. Inexpensive direct-drive pumps that are in production today still have this inferior design.

Currently, the motor section of the direct-drive pump is connected to the pump section by a coupling. This coupling allows acceptable variations in alignment between the motor and pump, but each component independently maintains their own tight tolerances.

Another holdover from belt-driven pumps that was applied to direct-drive pumps was that the oil was initially gravity fed. Unfortunately, this was not sufficient for the greater speeds of direct-drive pumps, and early designs often suffered premature bearing failure. (Direct-drive pumps typically spin at 1800 rpm while the pump on a belt-drive pump is typically spinning at about 600 rpm.) A small set of blades ws added to direct-drive pumps that forced the oil into the pumping region, thereby eliminating the problem.

All of the aforementioned pumps require oil for operation. One of the drawbacks to standard mechanical pumps is their propensity toward backstreaming of their oils, causing the potential contamination of the system. Once the pump has achieved its greatest vacuum, its efficiency drops to zero. At that point there is lit-

tie to prevent vapors and gases going toward or away from the pump, and backstreaming can occur.

To decrease or limit the potential for backstreaming, traps and baffles are used. An alternative is to use a pump that doesn't require oil; these dry pumps have no grease or liquids (oils) in the pumping region. One type of a small dry pump design is the scroll pump. A scroll pump can achieve speeds of about 8 liters/sec and have ultimate total pressures of about 10"3 torr. The value of these pumps is that because there are no oils in the pumping region, there is no concern about any backstreamed oils getting into the vacuum system.* This means that for any processes that must be kept (oil) contamination-free and whereone wishes not to deal with traps and all the accompanying hardware and maintenance, scroll pumps seems like a wonderful opportunity. Be forewarned that these pumps are very expensive and are intended to be cost-effective when compared to the potential costs of trap and baffle maintenance over the life of the pump.

For all pump types or designs, there are three different types of motor housings, each providing different levels of safety. Therefore, the environment that the pump will be used in should also be considered when selecting a pump:

1.Open motors provide an inexpensive housing that is to be used in a clean dry environment (no protection necessary).

2.Totally enclosed fan-cooled motors are used where moisture or dirt is prevalent (protects the motor).

3.Explosion-proof motors are required to meet the National Electrical Code Standards for use in hazardous conditions. These are used where gas, dust, or vapors may be in hazardous concentrations and a spark from the motor could cause a fire or explosion.

The final pump characteristic is pump size. To obtain a pump that is too large and/or too great, a pumping power for a given need can be a waste of money. However, if a pump is not large or powerful enough, there is a further waste of extra money spent when the proper size pump must be purchased. In general, it is better to err too large than too small. A simple formula can be used to aid in the selection of a pump size14:

Among the duties of oils in standard mechanical pumps are those of protection. Since scroll pumps do not have oil, they are dependent upon traps and baffles for protection against gaseous and paniculate contamination. Although backstreaming of pump oils is not a concern from a scroll pump, the need for traps still remains. Ironically, the demands of maintenance on traps for scroll pumps is greater than for regular pumps due to the total lack of protection the pump has for itself.

" From "Vacuum Products" by Welch Vacuum, Thomas Industries, Inc., p. 125, © 1988 by Welch Vacuum Technology, Inc, reproduced with permission.

t is the time of evacuation desired (minutes)

F is the pump down factor,

and is (2.3 x log (760 / desired chamber pressure in torr)), or see Table 7.6.

For example, if you have a vacuum line of about 20 liters that you wish to bring to 10'2 torr in 10 minutes, then

This determination is assuming that you have a perfectlydesigned vacuum system with no narrow passageways, no extra chambers, no bends, no contamination, and no traps, baffles, or stopcocks. Not much of a vacuum system, but it sure would be fast. Because you are working in the real world, the real pumping time will be slower. To compensate for this slower time, it can be helpful to multiply the above result by 2 or 3 to ensure that the pump will be adequate to handle your system's needs. However, as a tool for approximating the minimum size needed, the above formula is usually sufficient. The final decision of what pump to purchase, as in all selections, will depend on what you are willing to settle for and/or what you can afford. (More information on pump speeds is provided in Sec. 7.3.10.)

In many laboratories, the demands on a standard mechanical pump are less critical than in industrial use. A lab is generally more interested in obtaining a low pressure than in the speed and/or volume pumped. Typically, the primary consideration in the laboratory is that the mechanical pump achieves the vacuum required (for example: for diffusion pump operation) in a reasonable amount of time.

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):

(7.11)