Oil diffusion pumps are still the most popular type of high-vacuum pump because they are relatively inexpensive, highly reli-able and very durable. Yet oil diffusion pumps require careful maintenance in order to sustain a high level of performance and maintain the integrity of the vacuum chamber. This article will explain practical steps that can be taken to ensure optimal per-formance of oil diffusion pumps in a production environment.

Fig. 1. Oil diffusion pump cut-away

How Oil Diffusion Pumps Work

Diffusion pumps differ from conventional mechanical pumps in that they use a high-speed jet of fluid to direct gas molecules in the pump throat, down to the bottom of the pump and out the exhaust, which is usually called the foreline. The high-speed jet is produced by boiling the fluid (such as silicone oil) and then directing the resultant vapor through a series of vertically stacked cone-shaped jet assemblies. Typically there are three jet stages of diminishing size with the largest at the bottom and the smallest at the top (Fig. 1).

The boiling oil moves upward through the jet assemblies and is expelled in a downward direction at the top of each assem-bly. Exiting from the jets, the high-energy oil droplets travel downward in the periphery of the chamber at speeds up to 335 meters per second. The vapor jets entrain gas molecules and direct them towards the exhaust port, where they are pumped away by a roughing pump. The effect of removing molecules is to create a high vacuum in the upper portion of the chamber, which is connected to the vacuum chamber.

The roughing pump lowers the pressure inside the diffusion chamber to approximately 10-2 mbar or lower. This makes it possible for the diffusion pump to create a vacuum ranging from 10-3 to 10-9 mbar (10-1 to 10-7 Pa). Pumps and jet designs have evolved over the years and improved synthetic fluids have been developed, enabling the high levels of vacuum to be achieved with this pump technology. The pump external housing (or shell) is generally water-cooled to prevent thermal run-away.

In all diffusion pumps, a small amount of backstreaming, or movement of oil molecules into the vacuum chamber, occurs. In many applications this is acceptable. In more critical applications an optically dense baffle is typically provided to deflect parti-cles before they can reach the process stream. A trap is placed above the baffle and cooled to a low temperature to condense hydrocarbons and water vapor from the pump chamber. Due to the possibility of backstreaming, diffusion pumps may not be suitable for use with the most sensitive analytical equipment.

Operating Diffusion Pumps

A large amount of energy is stored in an oil diffusion pump, so careful attention must be paid to ensure safe operation.
  • Oil diffusion pumps should never be used for roughing because exposure to atmospheric pressure levels can cause hot hydrocarbon fluids to ignite or explode.
  • Air leaks into the system also present safety concerns for the same reason.
  • Avoid contaminating the pumping fluid with foreign matter because it can change its viscosity and obstruct flow passages.
  • Operation of the diffusion pump without circulating cooling water to the main body cooling coils is also dangerous and will lead to unstable operation.
More basic operating rules:
  • Do not turn on the heater without fluid in the pump. This may damage the heaters and pump assembly.
  • Do not air-release the pump while the boiler is hot. Most diffusion-pump fluids break down under these conditions.
  • Do not power the pump heaters unless cooling water is circulating. This can cause the pump and fluid to overheat and will lead to unstable operation.
  • Do not operate the pump without a foreline baffle. This may cause a greater than normal fluid loss.


    • Fig. 2. Two Varian HS-16 Diffusion Pumps provide the high vacuum needed to operate this Physical Vapor Deposition (PVD) coating system (Photo courtesy of Mill Lane Engineering)

      Diffusion Pump Maintenance

      It is recommended to keep record logs for diffusion pumps to track maintenance history and operational manuals to ensure maintenance is done per the manufacturer’s recommendations.

      During maintenance activity, the surfaces inside the vacuum diffusion pump need to be kept very clean in order to avoid contaminating the vacuum chamber. Even a single fingerprint can outgas water vapor and other molecules. When a pump is disassembled for routine maintenance and cleaning, it should be purged with dry nitrogen just prior to use. When a diffusion pump is pumping at a slower than normal rate, look for outgassing of moisture on the surfaces of plastics or other volatile sub-stances or a leak.

      Check the condition and level of the fluid when the pump is cold. Withdraw a sample through the drain and visually check the level of fluid through the sight glass. Use new o-ring gaskets when replacing fill and drain plugs. During maintenance intervals, if there is diffusion-pump oil loss, log the amount. This loss of fluid can be caused by:
      • Air leaking into the pump
      • Inadequate water cooling
      • Continuous operation in the overload inlet pressure range, also referred to as the knee of the curve
      • Failure to insert the foreline baffle in the pump assembly
      The recommended frequency at which the diffusion-pump fluid should be changed depends on the specific process. Slight discoloration of the fluid does not affect pump performance but material build-up can lead to inefficient heat trans-fer. For a dedicated manufacturing process, it is possible to establish color-based standards to help provide guidance for fluid replacement.

      It’s important to note that new fluid is subject to outgassing. This can cause periods of foreline pressure fluctuations. These will quickly resolve themselves as the fluid becomes fully degassed.

      If multiple processes are run on the same pump or vacuum chamber, there is the risk of cross-contamination of by-products through the fluid. It may become necessary to change the fluid more often in this situation.

      After maintenance, the pump-down characteristics of the system should be monitored. The best approach is to keep a log that compares the pump-down curves after maintenance over time. This approach will help to provide an early warning of sys-tem leakage. When a leak is identified, the first step should be to check the following items:
      • The integrity of the inlet and foreline connections
      • The tightness of the drain and fill plugs
      • The other compression fittings, such as those that connect the high-vacuum gauges
      • Threaded connections, such as the foreline gauge
      • The performance and accuracy of the vacuum gauges used in the system
      In some cases cooling coils may become obstructed due to solid-deposit buildup. Check the flow meters used to measure the cooling-water flow to ensure that water can flow freely through the coils and that the flow rate does not fall below the manufacturer’s specifications.

      Heater elements may burn out from time to time and need to be replaced. As a preventive precaution, when the pump is cold, check that the heaters are bolted snugly to the boilerplate and that all heater terminal connections are fastened tightly inside the junction box. Check the total heater power input and load balance.

      The following items should also be checked on a regular basis:
      • The position of the system valves before repositioning
      • The level of the diffusion-pump fluid
      • Confirm that water is flowing through the cooling water lines


      Diffusion Pump Troubleshooting Guide

      Table 1 lists general faults, probable causes and suggests corrective actions to take if a problem with a diffusion pump is sus-pected.

      Conclusion

      Diffusion-pumped vacuum systems often provide the most efficient and cost-effective environments for production proc-esses. To secure these benefits, it is extremely important to follow the simple maintenance practices described here or as recommended by the pump manufacturer. A small investment in planned maintenance can yield a high return in the ongo-ing high-level performance of your vacuum system. IH

      For more information: Contact: Varian, Inc., Vacuum Technologies, 121 Hartwell Ave., Lexington, MA 02421-3133 USA; ph: 1.781.861.7200, 1.800.882.7426; fax: 1.781.860.5437; Web site: www.varianinc.com

      Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: vacuum pumps, oil diffusion pump, roughing pump, outgas