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Dry Screw Vacuum Pumping System for Large UHV Chambers

Discover why our Dry Screw Vacuum Pumping System is suitable in Large UHV Chambers

Dry Screw Vacuum pumps of Everest Vacuum offer very large pumping speeds in rough and medium vacuum regions (i.e from 1000 mbar to 1 mbar). Hence, these pumps are used for faster initial hogging (before high vacuum pumps are switched on) of very large chambers used in modern critical applications related to space/nuclear/chemical/metallurgical/semiconductor sectors. A typical schematic of connecting the dry screw vacuum pump with a large vacuum chamber using a suitable/convenient interconnecting pipe. The gas flow region in the connecting pipe in this initial pump- down operation lies in the laminar-viscous region,  where 99.9 % of the gas is removed from the chamber.

This article helps the customer in predicting the pump down time to reach a specific vacuum level for a given combination of chamber volume, connecting pipe dimensions and the pumping speed of chosen dry screw pump. Alternatively, it also helps the customer in selecting a suitable screw pump for a prefixed pump down time to reach the required vacuum level for starting the high vacuum Pumps.

The procedure to predict pump down characteristics in viscous flow region, for a given combination of chamber volume, pumping speed and pipe dimensions involves three steps:

  • Calculation of flow conductance of connecting pipe.
  • Estimation of effective pumping speed at chamber site.
  • Evaluation of Pump-down time

Equation 1 (Poiseuille’s Law) gives the expression for the flow-conductance (C) of connecting pipe in viscous flow region of evacuation process

Visualization of pump-down time for dry screw vacuum pump in UHV chambers

Here, D & L are hydraulic diameter and length of pipe, η is the viscosity of the flowing gas and P is the mean pressure

Equation 2 gives the expression for the effective pumping speed (Seff) at the chamber port, in terms of manufacturers maximum pumping speed (Smax) and the pipe conductance.

Dry Screw Pumps pump-down time for hogging UHV chambers IR

Applying the mass conservation for the gas flow through the pipe from the vacuum chamber into the vacuum pump, we obtain Eq 3 governing the pump-down process.

Dry Screw Pumps pump-down time for hogging UHV chambers IR variant

Here ‘V’ is the volume of the chamber, ‘P’ is the instantaneous pressure in the chamber (mbar) and ‘t’ is the time lapsed from the starting of the pump. In the above equation, ‘-V(dp/dt)’ represents gas flow out of the chamber, while ‘Seff*P’ represents gas flow into the vacuum pump.

Integrating both sides of Eq 3 and after rearranging the terms, we get

Another pump-down time visualization for UHV chambers
Visualization of pump-down time in UHV chambers

For example, let us calculate the time required to attain a preset vacuum level (P1=1 mbar) using our Dry Screw vacuum pump (Superscrew series) of 3000 m³ /hr capacity in laminar viscous flow region with air as the medium. Initial pressure for hogging is taken as atmospheric pressure (i.e. P2=1000 mbar) and volume of the chamber is taken as 200 m³. From Eq 5, we arrive at a pump down time of 27 minutes (neglecting conductance effect for short connecting pipes) for reaching 1 mbar. In case the length of the connecting pipe is long or the diameter of the pipe is small,  the effect of conductance needs to be taken into account using Eqs 1,2 and 3.

In conclusion, we recommend our Dry screw vacuum pumps of various capacities to pre-evacuate (hogging) very large chambers used in various process applications before starting the high vacuum pumps. A matching combination of the modern turbomolecular/cryo pumps backed by our Superscrew dry vacuum pumps greatly reduces the total pump-down time for various high tech applications (semiconductor/metallurgy/space/nuclear/chemical), thereby saving electrical energy for the evacuation process. Studies have shown that for similar nominal displacements, our Dry screw vacuum pumps result in lower power consumption (about 20 % less) as against the traditional oil lubricated rotary vane systems [1]. Further, our dry screw pumps create an oil-free fore vacuum needed for contamination-free UHV systems.

 

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