Page 375 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Chapter

6

Ejectors and Mechanical Vacuum

Systems

Ejectors unmaintainable. A vacuum system indicates a system that
can be a matter of the degree to which the system
Industry has grouped pressures below atmospheric approaches absolute zero pressure. To create a vacuum in
into [24]: a fixed system, it is necessary to draw out or pump out the
air in the volume. When part of the air is removed, the sys-
Range of Pressure tem has a partial vacuum. For example, when 15-inch Hg
Rcugh vacuum 760 torr to l torr vacuum is referenced to a 29-inch Hg barometer, then the
Medium vacuum 1 torr to 0.001 torr absolute pressure is 29 - 15 = 14-inch Hg absolute. The
High vacuum 0.001 torr to 10- torr 15-inch Hg vacuum can be considered a negative gauge
7
7
Ultra-high vacuum 10- torr and below reading. See Chapter 2 for a diagrammatic relationship of
pressures.
Rough vacuum is used in about 90% of the chemical For quick reference, the listing below presents the most
and petrochemical and other processing industries. This commonly used types of vacuum pumping equipment:
range generally includes vacuum distillation, filtration,
crystallization, drying, reaction, and others. Medium vac- • liquid piston/ring
uum is most applicable to molten metals degassing, mole- • centrifugal
cules distillation, freeze drying, and others. High and • axial
ultra-high ranges are most useful for thin films, research, • two-impeller straight lobe
and space simulation. • helical lobe
For reference, note that unit of vacuum measurement
is the torr. • reciprocating
• sliding-vane rotary
l.00 torr= 1 mm mercury (mm I-Ig), abs • ejector
25.400 torr = 1 in. mercury (in. Hg) • rotary oil-sealed
750.1 torr = 1 bar rotary piston type
i.868 torr= 1 in. water at 4°C (in H20) vane type
760.0 torr= 1 standard atmosphere (atm) (at sea level) • diffusion ( not used for industrial/ commercial appli-
Absolute pressure = barometric pressure at location- cation)
vacuum
Vacuum Safety
Statements about vacuum can be misleading when a
clarification is not included. Vacuum refers to the "degree Safety around mechanical vacuum pumps is possibly
of emptiness" of a process system. A perfect vacuum rep- no different than that for other process mechanical rotat-
resents an absolute zero of pressure, which is technically ing machinery. However, there is a decided danger of an

343