Classification and Operation of Vacuum Pumps in Laboratory and Industrial Systems

Classification and Operation of Vacuum Pumps in Laboratory and Industrial Systems

Vacuum pumps are essential in various scientific and industrial applications, where they create and maintain a low-pressure environment by removing air and other gases from a sealed system. The exhaust pressure, defined as the pressure against which the pump can operate, is a critical parameter that influences the classification and application of vacuum pumps. Based on their exhaust pressure and operational requirements, vacuum pumps can be broadly categorized into three classes:

1. Roughing or Backing Pumps:

•    Description: These pumps exhaust to atmospheric pressure. They are used to remove atmospheric air from the system to reach an acceptable operating pressure, a process known as "roughing out the system." Additionally, they can maintain a required low pressure at the outlet of another pump, referred to as "backing."

•    Examples: Mechanical rotary pumps and ejectors are typical roughing and backing pumps. They are often used in the initial stages of creating a vacuum to reduce the system pressure to a level where high-vacuum pumps can take over.

2. Sub-Atmospheric Pumps:

•    Description: These pumps can only exhaust to pressures below atmospheric pressure and require a backing pump (in series) to exhaust to the atmosphere. They are used to achieve higher vacuum levels.

•    Examples: Diffusion pumps, Root's pumps, and molecular drag pumps fall into this category. These pumps are essential for achieving ultra-high vacuums and are commonly used in conjunction with roughing pumps.

3. Gas Immobilization Pumps:

•    Description: These pumps trap or absorb gases and vapors within the system and do not require an outlet to the atmosphere. They are used in applications where the gases must be immobilized rather than exhausted.

•    Examples: Ionization pumps and sorption pumps are typical examples. They are often used in high-vacuum applications where maintaining a very low pressure is crucial.

Example Systems

Laboratory Vacuum System

•    Configuration: A typical laboratory vacuum system, as shown in Figure 3.35, includes roughing and backing stages. The roughing pump initially reduces the system pressure to a level suitable for high-vacuum pumps to operate. The backing pump then maintains the required low pressure.

Industrial Vacuum System

•    Configuration: An industrial vacuum system, depicted in Figure 5.3, consists of three vacuum pumps in series:

•    Three-Stage Diffusion Pump: This pump is responsible for achieving the highest vacuum levels, reducing the pressure in the clean chamber to less than $10^{-6}$ Torr.

•    Root's Type Pump: This pump backs the diffusion pump, maintaining the intermediate pressure range.

•    Rotary Plunger Pump (Kinney): This pump is capable of unassisted discharge to the atmosphere and is used initially to reduce the system pressure to about $10^{-1}$ Torr. It then continues to back the diffusion pump.

Operational Considerations

•    Initial Roughing Stage: The rotary plunger pump is used to reduce the system pressure to a level where the diffusion pump can effectively operate.

•    High-Vacuum Maintenance: The diffusion pump can achieve extremely low pressures, but during processes involving outgassing or the introduction of control gases, the pressure may be maintained at around $10^{-4}$ Torr.

Conclusion

The classification and operation of vacuum pumps are crucial for achieving and maintaining the desired vacuum levels in both laboratory and industrial settings. Each type of pump serves a specific purpose, and their combined use in series can achieve the high vacuum levels required for various applications. Understanding the roles and capabilities of different vacuum pumps is essential for designing efficient and effective vacuum systems.