Panasonic PT-DZ12000U Operating Instructions Manual page 136

Since these differences exist, pump selection should be
based on actual free air capacity rather than on displacement.
In short, the air removal rate is a measure of vacuum pump
capacity and the capacity of standard machines must be
determined from the manufacturers' tables or curves showing
cfm of free air delivered at rated speed for vacuum levels
ranging from 0 "Hg (open capacity) to the maximum vacuum
rating. Free air capacity at different speeds for a given
vacuum also may be included in the manufacturers'
performance curves. The rated capacity of any pump is
highest at 0 "Hg and will drop rapidly as the vacuum level
increases. This reflects a drop in both volumetric efficiency
and the volume of air that can be drawn into the pumping
chamber. To repeat, a basic characteristic of positive
displacement pumps is that capacity drops as the vacuum
level increases. The same principle holds for diaphragm
pumps.
Effects of Temperature Rise
Vacuum pump performance is significantly affected by
heating of the pump itself. At higher vacuum levels, there is
very little air flow through the pump. There is thus very little
transfer of internal heat to this remaining air. Much of the
heat generated by friction must be absorbed and dissipated
by the pump casing. Since some pumps generate heat faster
than it can be dissipated, a gradual rise in pump temperature
results, drastically reducing service life. One solution is to
give careful consideration to pump ratings. For example, a
continuous-duty pump should have a high maximum vacuum
rating.
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