EngineerHub.orgWhat this simulator does
The pressure curve follows a first-order evacuation model. A constant leak or outgassing load raises the attainable pressure floor. The economics compare current and proposed pump-down time, electrical energy and the value assigned to production or test-cycle delay.
Inputs
Example systems
Chamber and pressure
Current pump
Proposed pump or upgrade
Operating economics
Effective speed: Seff = fSpump
Predicted pressure floor: p∞ = pultimate + Qgas/Seff
Pressure with time: p(t)=p∞+(p0−p∞)e−Sefft/V
Time to target: t=(V/Seff)ln[(p0−p∞)/(ptarget−p∞)]
- The target must be above the predicted pressure floor.
- Nominal speed may vary strongly with pressure; use an effective factor or pressure-segmented model for detailed design.
- Gas load is treated as constant even though real outgassing commonly decreases with time.
- Motor power is treated as constant over the run.
Results
Annual economics
Pressure versus pump-down time
Pressure is shown on a logarithmic axis. Curves approach their calculated pressure floors asymptotically.
Scroll the plot horizontally for readable labels.
How to improve pump-down performance
Shorten and enlarge the foreline, remove restrictive fittings, and verify valve and filter sizing. A large pump cannot overcome a severely restricted connection.
Use pressure-rise and leak-testing methods. A constant leak and a declining surface outgassing load require different corrective actions.
Dry the chamber, warm the pump when appropriate, use gas ballast as specified, and allow cleanup time after vapour service.
For medium or high vacuum, combine suitable backing and high-vacuum pumps and use manufacturer speed curves rather than one constant-speed estimate.
Interpretation, FAQ and references
Why is actual pump-down often longer?
Nominal pumping speed may fall with pressure, and piping conductance, thermal effects, desorption, leaks and vapours all add delay.
What does “pressure floor” mean?
It is the modelled steady pressure where pump removal balances pump ultimate pressure and the entered constant gas load.
Does a faster pump always save energy?
No. A larger motor may use more energy per cycle even when it saves production time. The economics separate electricity from delay value.
Can this be used for high-vacuum design?
Only as an early screening model. High-vacuum pump-down is often dominated by outgassing, conductance and pressure-dependent pump performance.