Page 209 - Chemical process engineering design and economics

192 Chapters

A rotary-piston pump is an oil-sealed, positive-displacement vacuum
pump. The oil both lubricates the pump and seals the discharge from the suction
side of the pump. As the piston rotates, gas enters a chamber, as shown in
Figure 5.2. Then, the inlet port closes, and the gas is compressed in the chamber
until the discharge valve opens, exhausting the gas to the atmosphere. Possible
contamination of the oil with condensable vapors, usually water, is a problem.
One way condensation can be avoided is by reducing the partial pressure of the
condensable gases by allowing air to leak into the cylinder, which is called a gas
ballast.
A rotary-vane vacuum pump is also a positive-displacement vacuum pump.
The vanes slide in slots and are forced against the wall of a stationary cylinder
by springs for laboratory pumps or by centrifugal force for process pumps. Seal-
ing is accomplished either by oil or a dry seal using nonmetallic vanes which
continuously wear thereby forming a tight seal. As can be seen in Figure 5.2, a
gas enters the pump, is trapped between two vanes, is compressed as the volume
of the chamber is reduced, and finally exhausted at the discharge port. The ro-
tary-vane vacuum pump is sensitive to contamination, which can reduce its per-
formance rapidly.
In the rotary blower, shown in Figure 5.2, gases are trapped in between two
interlocking rotors which rotate in opposite directions. The blower requires no
seal fluid. Because of the required clearances between the rotors of 0.025
3
to 0.25 mm (9.84xlO~ 4 to 9.84xlO~ in), backfiow reduces the blower capacity
[3]. Also, overheating limits the pressure increase.
In the liquid-ring pump, shown in Figure 5.2, a seal liquid, usually water, is
thrown against the casing by a rotating impeller forming a liquid ring. Gas
drawn from an inlet port is compressed in the chamber between the rotor blades
as the impeller rotates on an axis that is offset from the casing. Some of the
seal liquid is entrained with the exhausted gas. If the gas contains a condensable
component, the pump behaves like a direct contact condenser. Provisions can be
made for separating the condensable component from the seal liquid which is
then recirculated. Because of its ability to handle condensable vapors, the liq-
uid-ring pump is ideally suited for filtering operations. Another advantage is
that the seal liquid is a heat sink, limiting the temperature rise of the compressed
gases [3]. A disadvantage of this pump is that it uses twice as much energy as an
oil-sealed- rotary-vane or a rotary-piston pump of the same capacity [4].
The performance of a vacuum pump is depicted by a plot of flow rate
against suction pressure, which is called the characteristic curve. Physically, a
vacuum pump must operate at some point on the curve, depending on the design
of the system. In Figure 5.3, the characteristic curves for a rotary piston, a liq-
uid-ring pump and steam-jet ejector are plotted. For a perfect positive-
displacement pump, the curve should be fiat over the whole pressure range. In-
stead, for the rotary-piston pump, the curve increases slightly with increasing
suction pressure because of reduced leakage. The curve for the ejector increases

Copyright © 2003 by Taylor & Francis Group LLC