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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:
Equation 1 (Poiseuille’s Law) gives the expression for the flow-conductance (C) of connecting pipe in viscous flow region of evacuation process
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.
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.
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
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.
[1] https://www.everestvacuum.com/product-category/
Everest Vacuum has been providing Technical and R&D solutions for various process industries over the last 15 years. We have provided customized vacuum systems to many reputed organizations in India and overseas. We believe in empowering our customers with the best possible customized vacuum system for eco-friendly overall development.
Dr V Ravindra is a Mechanical Engineer who has done his PhD from Indian Institute of Technology, Kharagpur. He is working as R&D Manager at Everest Vacuum and he is involved in designing modern vacuum systems for process applications based on customer requirements. He is also developing training modules related to Vacuum Technology and process applications.
