By Dan Herring

Fig. 1. Typical vacuum system[1]

This is the third in a series of articles in our Vacuum Heat-Treatment Series. Here we will begin to discuss vacuum pumping systems, explore the operating characteristics of mechanical pumps and blowers, and consider what we can do to make the pumps we choose work better.

In order to create a vacuum within a closed container, or vessel, we need to remove the molecules of air and other gases that reside inside by means of a pump. The vacuum vessel and pumps (mechanical, booster, diffusion, holding) together with the associated piping manifolds, valves (mechanical pump, high vacuum isolation, vacuum (brake) release, backing), vacuum measurement equipment (molecule counters) and traps comprise a typical vacuum system (Fig. 1).

The Importance of Properly Sized Vacuum Pumps

The Ideal Gas Law (Equation 1) allows us to calculate the multiplying factors that we must deal with as the pressure drops in our vacuum system (Table 1). One can immediately see the large throughput necessary in the pumping equipment in order to carry off these huge gas volumes as a consequence of lowering the overall pressure of the system.

Table 1.

1. V = nRT/P

A vacuum system is classified by its pressure range as follows:

• Rough (>1 x 10-1 Torr)
• Low (1 x 10-1 Torr to 1 x 10-3 Torr)
• High (1 x 10-3 Torr to 1 x 10-4 Torr)
• Very High (1 x 10-4 Torr to 1 x 10-6 Torr)
• Ultra High (<1 x 10-6 Torr)

The selection of the proper vacuum pumping system is application-dependent and is complicated by the wide variety of operational, process and equipment variables possible. However, each system must meet a specific set of requirements that are imposed by the process and production requirements. These requirements determine the size and type of pumps needed for the successful operation of the system.

Table 2.

Among the process variables, which should be known before attempting to size or select a pumping system, are the internal volume of the vacuum vessel, the time and pressure required and the size of the gas load that must be pumped off. In an ideal world, we know the answers to all of these questions. In the real world, we often compromise by applying safety factors to allow for the uncertainty of the information available. Here are several important formulas to help:

Equations for a Vacuum Pumping System

A. Pumping Speed:

(1) 1/St = 1/Sp + 1/Ct

where:

B. Pumpdown Time:

(2) t = 2.3 • K • [V/Sp ] • log [P1/P2]

and where K is: (Table 2)

where the molecular conductance, Ct, of an orifice of area A (in2) in a passage of cross-sectional area Ap (in2) is given above.

Fig. 2. Typical mechanical-pump cross-sectional view

Mechanical Pumps

To reach the various vacuum levels, different vacuum pumping systems are required. The foundation of any of these systems is the positive-displacement mechanical or roughing pump (Fig. 2). The roughing pump, so-called because it is used to produce a “rough” vacuum, is used in the initial pumpdown from atmospheric pressure to around 2 x 10-2 torr, depending on the type of pump. Mechanical pumps operate under the principle that they take in a large volume of air at the beginning of the cycle, compress it to a small volume and then exhaust it to the outside atmosphere. A thin layer or film of oil creates the actual seal between the moving parts in a so-called “wet” pump. The gas is exhausted against pressure applied by a spring load set to open slightly above atmospheric pressure.

The internal components of the mechanical pump (Fig. 2) help us understand its operation. Basically, it is an eccentric cylinder driven about an axis by an electric motor. During operation, the rotor turns with the shaft that causes the piston to sweep the volume between it and the stator. The piston does not turn in this case, but the vane-like extension on the piston (called the slide or slide valve) moves up and down in an oscillating seal (called the slide pin or slide-valve pin).

Fig. 3. Typical mechanical-pump operation[8]

At the start of a rotation, the ported slide valve is open. As the rotation occurs, the slide valve closes, trapping a given volume of gas. This volume is compressed as the revolution continues. Near the end of the revolution, the pressure is above atmospheric and the gas discharges through a spring-loaded poppet valve. On the completion of the revolution, the slide valve opens and another increment of gas is admitted.

A vacuum pump will remove a number of molecules with each rotation. How many molecules are actually removed will depend largely on the actual pump displacement, rotational speed and vacuum-system pressure. Each time molecules are removed, the remaining molecules spread out in the vacuum chamber to occupy the available volume. This repeats, the pump removes molecules, the pressure reduces and there are less and less molecules to expand into the pump inlet with each rotation.

Mechanical pumps can be single- or dual-stage. A single-stage design will achieve a pressure of about 1 x10-2 torr, while a dual-stage pump is capable of reaching pressures around 1 x 10-3 torr. A two-stage, or compound, pump has two pumping chambers connected in series. The exhaust of the first stage is coupled to the inlet of the second stage.

Fig. 4. Typical dry-pump operation

Lower pressure, fewer molecules and increased movement in the same volume results in less pumping efficiency. Why mechanical pumps start off with high efficiency and fall off at these pressure ranges can be explained as follows. Consider 1 cubic foot of volume at atmospheric pressure (760 torr). If we were to put this volume of gas in a container that was twice as large, the pressure would be exactly half, or 380 torr. That is, if we double the volume, we halve the pressure. Thus, doubling the volume again to 4 cubic feet results in a pressure of 190 torr. So, to evacuate a chamber to 1 x 10-3 torr theoretically requires that we remove a volume of 760,000 cubic feet. In everyday operation, a mechanical roughing pump will have great difficulty achieving this ultimate pressure (lowest attainable pressure) since its efficiency begins to fall off at 1 x10-1 to 8 x 10-2 torr. In other words, mechanical pumps will reach “blank-off” pressure – where they are no longer capable of pumping gas. However, mechanical pumps will become inefficient at pressures considerably higher than blank-off pressures.

An alternative to “wet” mechanical pumps (those that use mechanical pump oil) are the so-called “dry” mechanical pumps. These pumps are used in applications where pumping efficiency and process contamination concerns are important. They have positive environmental impact (due to reduced oil consumption and minimal disposal issues) and operate with less noise and vibration.

Dry pumps operate on the compressor principle (Fig. 4). As the two rotors rotate, gas is drawn in through an inlet slot aligned with the cavity in one of the rotors. Further rotation closes the inlet while the lobs, or claws, compress the trapped volume of gas until the cavity in the second rotor exposes the outlet or exhaust slot. A small volume of gas remains trapped and is carried over into the next pumping cycle. These designs produce high compression ratios and operate at high efficiency.

Fig. 5. Typical roots blower[2]

Blowers (Booster Pumps)

The booster pump (Fig. 5), or blower, is a different type of mechanical pump that is placed in series with the roughing pump and designed to “cut in,” or start, at around 700 torr. It is designed to provide higher speeds in the pressure range of 100 torr to 1x10-3 torr. In this intermediate pressure range, the roughing pump is losing efficiency while the diffusion (vapor) pump has yet to start to reach full efficiency.

The operation of the booster (Fig. 6) is as follows. Two impellers are mounted on parallel shafts and rotate in opposite directions. They are geared together so that the correct relative position of each impeller to the other can be maintained. The impellers do not touch each other, and no sealing fluid is used. The back leakage is small compared to the total speed of the pump in its useful range.

During operation, gas from the inlet side is trapped between the impeller and housing. No compression takes place as this gas is moved from the inlet to the discharge port. When the leading lobe of the impeller passes the discharge port, gas from the discharge area (which is at higher pressure) enters but is swept away by the trailing lobe.

Mechanical booster pumps have a useful compression ratio of 10:1, so they must be backed by a mechanical roughing pump in order to reach their maximum efficiency.

The mechanical booster pump is highly efficient in reducing the time required to evacuate a large or “gassy” system to the operating pressure at which the diffusion pump is efficient.

Fig. 6. Booster-pump operation[1]

Ways to Help Pumping Efficiency

Individuals familiar with vacuum furnaces know the importance of having absolutely leak-tight vacuum chambers, doors, feed-throughs and penetrations. For critical applications, such as processing of superalloys or reactive metals, a leak rate of less than 5 microns/hour is mandatory. For normal vacuum applications, the leak rate should not exceed 10 microns/hour. Even a slight air or water leak will overwhelm the vacuum pumps and cause the vacuum level to rise several decades in pressure. Proper attention, therefore, must be given to the entire vacuum system. Leak detection of all joints, welds, seals, valves and pumps as well as the vessel itself is critical to success.

An important operational consideration is to limit the amount of time a furnace chamber is exposed to room air, either during loading/unloading or when the unit is not in production. The effects of humidity (water vapor) are often devastating to the pumping system, decreasing its efficiency and creating an oil/water mixture in the pump that requires it to be ballasted. When not in production, vacuum furnaces should be pumped down to several hundred microns and then turned off.

Finally, moisture trapped in the hot zone or heat-exchanger tube bundles (if internal) is extremely difficult to overcome by pumping alone. Oftentimes backfilling with nitrogen or, if available, argon, will help minimize this effect.

Next time: Part four of this series continues to talk about vacuum pumping systems by discussing diffusion pumps as well as offering troubleshooting tips for all types of vacuum pumping systems.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. Herring, D. H., "The Molecule Movers," Industrial Heating, November 2007.
2. Brunner Jr., William F. and Batzer, Thomas H., Practical Vacuum Techniques, Robert E. Krieger Publishing Company, 1974.
3. Jones, William R., "Pumping and the vacuum furnace," Heat Treating, July 1986.
4. "Understanding the Effects of Air Exposure," R&D Magazine, April, 2005.
5. BOC Edwards - Stokes Vacuum, Mr. Dave Morris, private correspondence.
6. Tuthill Corporation, Kinney Vacuum Division, Mr. Brad Weber, private correspondence.
7. Steel Heat Treatment Handbook, edited by Totten, George E. and Howes, Maurice .A. H., Chapter 7: Vacuum Heat Treating, Marcel-Dekker, 1997.
8. Kimball, William H., Vacuum…is it really nothing? C. I. Hayes Inc., 1977.
9. Dresser Corporation, Roots Blower, Mr. Jim Wolford, private correspondence.