Panasonic PT-DZ12000U Operating Instructions Manual page 133

Rotary Vane Pumps -Most rotary vane pumps have lower
vacuum ratings than can be obtained with the piston design:
only 20 to 28"Hg maximum. However, some two stage oil-
lubricated designs have vacuum capabilities up to 29.5"Hg.
The rotary vane design offers significant advantages:
compactness; larger flow capacities for a given size; lower
cost (about 50 percent less for a given displacement and
vacuum level); lower starting and running torques; and quiet,
smooth, vibration free, continuous air evacuation without a
receiver tank.
Rotary Screw and Lobed Rotor Pumps - Vacuum
capabilities of rotary screw pumps are similar to those of
piston pumps, but evacuation is nearly pulse-free. Lobed
rotor vacuum pumps, like the corresponding compressors,
bridge the gap between positive and non-positive
displacement units. Air flow is high but vacuum capabilities
are limited to about 15"Hg. Capabilities can be improved with
staging.
Non-positive Displacement Vacuum Pumps
Non-positive displacement vacuum pumps use changes in
kinetic energy to remove air from a system. The most
significant advantage of this design is its ability to provide
very high volume flow rates, much higher than possible with
any of the positive displacement designs. But because of
their inherent leakage, these machines may not be practical
for applications requiring higher vacuum levels and low flow
rates. The principle types of non-positive displacement
vacuum pumps are the centrifugal, axial-flow, and
regenerative designs. Single-stage regenerative blowers
can provide vacuums up to 7"Hg with flows to several
hundred cfm. Vacuum capabilities of these designs are
improved when they are multi-stage.