Gas flow in a vacuum system occurs differently than in higher-pressure environments. In vacuum systems, the behavior of gas molecules changes based on the pressure range, affecting how gas flows through the system. There are three primary flow regimes based on pressure:

1. Viscous Flow (Continuum Flow)

• Pressure Range: This occurs at higher pressures, typically above 1 mbar (750 mTorr).
• Characteristics: In viscous flow, gas molecules are close enough to interact and collide frequently with each other, creating a continuous flow similar to how gases behave at atmospheric pressure.
• Flow Nature: The flow is dominated by molecular collisions, and it behaves in a predictable way according to fluid dynamics laws, such as Poiseuille’s law.
• Applications: This regime is relevant at the beginning stages of evacuation when a system is not yet in a high vacuum.

2. Transitional Flow (Knudsen Flow)

• Pressure Range: Transitional flow occurs between about 0.01 to 1 mbar (10 mTorr to 750 mTorr).
• Characteristics: Here, the mean free path of gas molecules (average distance between collisions) becomes comparable to the size of the chamber. Both molecular and viscous flow properties are present, but molecular interactions begin to dominate.
• Flow Nature: It is a mixed regime, where neither viscous nor molecular effects dominate completely, making flow behavior more complex.

3. Molecular Flow

• Pressure Range: Molecular flow is dominant in high and ultra-high vacuum, typically below 0.01 mbar (10 mTorr).
• Characteristics: In this regime, the mean free path of molecules is much larger than the dimensions of the vacuum chamber or the tubing.
• Flow Nature: Gas molecules rarely collide with each other and instead move independently, bouncing off surfaces of the chamber and components. Flow becomes random and directional based on the probability of molecules hitting surfaces rather than colliding with each other.
• Applications: Molecular flow is crucial in high-vacuum and ultra-high vacuum processes, like semiconductor manufacturing, surface science, and space simulation.

Factors Affecting Gas Flow in a Vacuum

• Mean Free Path: As pressure decreases, the mean free path increases, and molecules travel further between collisions. This transition marks the shift from viscous to molecular flow.
• Conductance: The ability of a vacuum system’s piping to allow gas flow decreases as molecular flow dominates, since molecules randomly strike surfaces rather than moving with the bulk flow.
• Pumping Speed: In molecular flow, the pumping speed of a vacuum pump becomes less efficient, as fewer molecules reach the pump inlet per unit time. This is why high-vacuum systems are often designed with optimized geometries to maintain effective gas flow.

Practical Implications in Vacuum Systems

• Transition Between Regimes: As a vacuum pump evacuates a chamber, the system transitions through these flow regimes. Understanding each is key for optimizing vacuum system performance.
• Leak Detection: Helium leak detection often relies on molecular flow principles, as helium atoms can travel directly to the detector without molecular collisions, allowing detection of very small leaks.

In summary, gas flow in a vacuum system changes from continuous, viscous flow at higher pressures to random, molecular flow at very low pressures.