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The liquid ring vacuum pump can be operated as
a simple continuous flow system, or as a partial or
total recirculation system.

Over many years, these vacuum pumps have pro-
ven themselves to be robust and reliable vacuum
generators in chemical processes. The operating
fluid in the compression chamber continuously
dissipates the compression heat, so the vacuum
pump operates nearly isothermally. This means
that the process gas does not heat up to a notable
degree and the vacuum pump operates at relatively
low temperatures. This significantly reduces the Fig. 2 - Two-stage liquid ring vacuum pump operating principle
risk of unwanted reactions or an explosion. Low
operating temperatures also facilitate condensation
of vapours and gases, which increases the nominal
pumping speed of the vacuum pump.

Water is usually used to create the liquid ring.
Ethylene glycol, mineral oils or organic solvents are
also often used in practice. The ultimate pressure
of the vacuum pump depends on the vapour pres-
sure and viscosity of the liquid. The viscosity of the
operating fluid will impact the power consumption
of the vacuum pump.

Liquid ring vacuum pumps are available on the
market in different versions, materials and shaft
seals. Fig. 3 - COBRA NC screw vacuum pump

Advantages of liquid ring vacuum pumps:
• Not sensitive at all to vapours or liquids ente-
ring the system
• The different material versions enable them to
be tailored to the process gas

Disadvantages:
• Possible contamination of the operating fluid
with condensate from the process gas, which
makes it necessary to subsequently treat the
operating fluid before its disposal
• High energy consumption
• Ultimate pressure depends on the vapour
pressure of the operating fluid
Fig. 4 - Modern screw vacuum pump operating principle

Dry screw vacuum pumps the gas outlet. During the compression process the
Dry screw vacuum technology is also very widely screw rotors do not come into contact with each
used in the chemical and pharmaceutical industri- other or the cylinder. Precise manufacturing and
es. However, this is relatively new compared to li- minimal clearance between the moving parts ena-
quid ring technology. ble this operating principle and, in addition, guaran-
In the 1990s, Busch launched the first dry screw tees a low ultimate pressure of <0.1 mbar.
vacuum pump on the market, the COBRA AC. The
major difference compared to the liquid ring vacu- Screw vacuum pumps operate using water coo-
um pump described above is that screw vacuum ling, which ensures even temperature distribution
pumps (Fig. 3) do not require operating fluid to throughout the pump body and thus thermal stabi-
compress the process gas. This is why it is called a lity in the entire process.
“dry” screw vacuum pump.
Modern screw vacuum pumps feature variable
In a screw vacuum pump, two screw-shaped ro- pitch screws, which results in pre-compression of
tors rotate in opposite directions (Fig. 4). The pum- the process gases. The advantage is that both gas
ped medium is trapped between the cylinder and temperatures and power consumption of the vacu-
screw chambers, compressed, and transported to um pump can be reduced significantly. In older ge-

Impiantistica Italiana - Settembre-Ottobre 2020 93 93