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Oil sealed rotary vane pumps generally come in either 2 or 1 stage
design. In most cases a 2 stage pump will go down below 1 micron
.001mm mercury vacuum. A single stage pump is generally specified
out at less than 15 micron .015mm mercury vacuum. In both cases the
pumps exceed 29.9 inches of mercury vacuum. In selecting either 1 or
2 stage designed pumps, consideration needs to be addressed by the user as to what
vacuum pressure they want to achieve on their system. If the answer
is the best vacuum without knowledge of the specific vacuum value, many times a single stage pump can do the job and for less
money than a 2 stage pump. If on the other hand, a specific number
like 10-15 micron is desired then the 2 stage pump is generally the pump
of choice.
2 stage pumps are more expensive than a 1 stage pump because they have
a high vacuum stage what we refer to as an exhaust stage. This 2nd
stage serves to help speed the pumps performance curve from about 150
micron or .150mm down to below 1 micron (.001mm). Single stage pumps will
take longer to get to 15 micron than a 2 stage pump will because the 2
stage pump has the high vacuum stage which boosts performance on the pump
down curve under 150 micron. Some users, like in the neon production
market need the 2 stage pumps. Users performing degassing of RTV
rubbers probably can get by with a single stage pump. The price
difference is not huge so often users decide in favor of the 2 stage pumps
just to make sure.
Above, about 300, micron gas flows are pretty much laminar in action and
similar to liquid flow characteristics and can be measured speed wise in basically a straight line method. As
an example, a 30L pump takes 5 minutes to get the chamber down then a 60L
pump will basically take half the time at least to 300 micron. On
going to say 15 micron though a 60L 1 stage pump that takes 2.5 minutes
may be outperformed by a 30L 2 stage pump that takes 1.5 minutes for
evacuating the same space. The 2 stage pumps strut their stuff
when gas flows or pumping speeds start to get into the molecular flow range. Actually
the pressure range they cover in this range is really kind of a transition
phase from laminar to molecular flow. You physics majors and chemistry
people may understand what I am saying.
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