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Vacuum conductance and effective pumping speed

How piping conductance and pump speed combine to determine the effective pumping speed seen by the chamber.

By Semiconductor Tools Editorial Team · Last updated: 2026-06-14

In molecular-flow vacuum piping, each geometric element—tube, bend, valve opening, orifice—presents a conductance C (L/s) that limits gas transmission. The pump and piping together determine Seff, the pumping speed actually experienced by the process chamber. Misestimating Seff is a leading cause of incorrect pumpdown predictions and base-pressure surprises.

Molecular-flow conductance formulas

For air near room temperature, widely used approximations (D, L, A in cm) are:

  • Long circular tube: Ctube ≈ 12.1 · D³ / L [L/s]
  • Thin orifice: Chole ≈ 11.6 · A [L/s], A = πD²/4

The D³ scaling means doubling tube diameter increases conductance eightfold—often the most cost-effective pumpdown improvement.

For other gases and temperatures, C scales approximately as √(T/M). Our conductance tool applies this factor relative to air at ~293 K.

Combining elements and Seff

Conductances in parallel add directly. In series, reciprocals add: 1/Ctotal = Σ 1/Ci. With pump speed S at the pump inlet:

1/Seff = 1/S + 1/Ctotal

Worked example — Chamber Seff with Vacuum Conductance & Effective Pumping Speed

A CVD chamber connects to a 2000 L/s turbo through: (1) a 10 cm ID, 120 cm foreline tube, then (2) a partially open gate valve modeled as a 4 cm orifice. Gas: N₂, T = 300 K. Pump catalog speed: 2000 L/s.

  1. Add Tube: D = 10 cm, L = 120 cm, series. Tool computes Ctube ≈ 101 L/s (air basis, scaled for N₂).
  2. Add Orifice: D = 4 cm (series after tube). Corifice ≈ 146 L/s.
  3. Combined Ctotal (series): 1/(1/101 + 1/146) ≈ 60 L/s.
  4. Enter pump speed 2000 L/s. Tool reports Seff ≈ 58 L/s at the chamber—only ~2.9% of catalog speed.
  5. Conclusion: Replacing the turbo with a 4000 L/s unit barely helps; enlarging the gate valve opening or foreline diameter is the correct lever.

After estimating Seff, use Outgassing Load Estimator for base-pressure prediction, or Pumping Speed Calculator to check if Seff supports your process gas load at target pressure.