Chemical Engineering September 2017 - 58

Edwards Vacuum
2. Isolation
1. Induction
3. Compression
4. Exhaust
Figure 6. This illustration shows the operating principle of a rotary-lobe
vacuum pump [4]
Figure 5. This drawing shows the operating principle of a rotary-vane vacuum
pump [3]
This type of pump requires lubrication oil that may
require an oil separator for exhaust. If the pump is
pulling potential condensables, a gas ballast can be
included that reduces the pressure needed to open
the outlet valve, thus reducing the likelihood of vapors
condensing in the pump cavity. A high volume of condensables,
especially solvents, can negatively impact
these pumps, as they have a tendency to mix with
the seal oil and can corrode the pump internals if not
handled properly.
Liquid-ring pumps. Liquid-ring pumps are common
vacuum pumps; they attain pressures down to 25 torr,
and are preferred from a reliability perspective because
of their low-friction design. Similar to rotary-vane pumps,
liquid-ring pumps consist of a single eccentric rotor that
pulls gas with an initially expanding and later contracting
cavity using a ring of liquid, usually water, that acts
as the boundary between the gas volumes. Solvents
such as hydrocarbons and other liquids can be used as
well, depending on the application. A liquid separator on
TABLE 1. AIR LEAKAGE RATES FOR VARIOUS SEALS,
CONNECTIONS, AND VALVES IN ROUGH VACUUM [7]
Component
Static seals
O-ring construction
Conventional gasket seals
Thermally cycled static seals
T ≤ 200°F
200 < T < 400°F
T ≥ 400°F
Motion (rotary) seals
O-ring construction
Mechanical seals
Conventional packing
Threaded connections
Access ports
Viewing windows
Valves used to isolate system
Ball
Gate
Globe
Plug-cock
0.002
0.005
0.005
0.018
0.032
0.10
0.10
0.25
0.015
0.020
0.015
Suction
0.02
0.04
0.02
0.01
Valves used to throttle control gas into vacuum system 0.25
*Note: Assumes sonic (or critical) flow across the component
56
1st stage
suction head
Steam strainer
1st stage
steam chest
High-pressure steam
inlet 1st stage
Figure 7. This cutaway diagram shows the operation of an ejector pump
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1st stage
steam nozzle
 = specific leak
rate*, lb/h/in.
the exhaust and filtration for contaminants that become
trapped in the liquid may be needed. These pumps are
quite amenable to a large condensable load; while temperature
loss due to condensing vapors can lower efficiency,
corrosion is not an issue as it would be for oilsealed
or dry pumps.
Rotary-lobe pumps. Rotary-lobe pumps, also called
Roots blowers or booster pumps, consist of two lobed
rotors that spin in opposite directions in a casing with
tight clearances (Figure 6). This type of pump is limited
by its design, in that a high pressure differential between
inlet and outlet causes a significant amount of heat generation
in the rotors, which can cause contact or seizure.
This attribute is why a rotary lobe pump does not exhaust
directly to atmosphere and is often paired with a
mechanical backing pump. Although an overflow valve
can be added to prevent this heat build-up, its addition
would further limit the maximum possible pressure differential
of the pump, making the backing pump even
more necessary.
However, the maximum pumping speed of a rotarylobe
pump is limited to pressures between about 50
and 0.1 torr, which is where other mechanical pumps
typically start to slow down and where water vaporizes at
the highest rate. This is quite convenient on both counts,
as water can add an excessive workload and increase
pumpdown time considerably on single-pump systems.
Rotary-lobe pumps are often paired with a backing pump
to accelerate startup to steady state in sensitive applications
where speed is critical. Because these pumps
are most efficient in the lower pressure regime and the
High-pressure
steam inlet 2nd
stage
2nd stage
steam
chat
Steam
strainer
1st stage
combining throat
2nd stage suction head
Discharge
2nd stage
steam
nozzle
2nd stage
combining
throat
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Chemical Engineering September 2017

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