The principle behind the liquid ring vacuum pump was developed as early as 1890 in the form of the “water ring pump”. Due to their simple and robust construction, liquid ring vacuum pumps (fig. 1) are suitable for vacuum generation in applications in which damp gases or vapours are evacuated or those in which condensation inside the vacuum pump tends to take place during the compression process. They are therefore ideally suited for humid processes and are thus used for generating rough vacuums in process technology, the chemical industry, during petroleum production and processing, for ventilating steam turbine capacitors, in the plastics industry, paper manufacturing, the food technology industry and many other industrial applications.

Operating principle

As an operating fluid, liquid ring vacuum pumps use water or a liquid compatible with the gas or vapour to be evacuated. Ethylene glycol, mineral oils or organic solvents are also used, as are other liquids that are already part of the process. The basic principle is the same in all sizes and versions.

An eccentrically mounted impeller wheel rotates within a cylindrical housing (fig. 2). This housing is filled with operating fluid to the extent that the vanes of the impeller are immersed in the fluid. The rotary motion of the impeller wheel and the resulting centrifugal force cause the liquid in the housing to form the so-called liquid ring. The gas is pumped in the spaces between the individual vanes and the liquid ring. Thanks to the eccentric arrangement of the impeller wheel, the volumes of these spaces change with the result that gas is sucked in, compressed and discharged again. The liquid ring seals the individual spaces down to the cylinder. This is why it is sometimes called the sealing fluid instead of the operating fluid.

Mechanism

Due to the operating fluid used, this mechanism can only be used in the rough vacuum range. The reason for this is that the achievable vacuum level depends on the vapour pressure and the viscosity of the operating fluid, which is constantly pumped through the vacuum pump. This allows a liquid ring vacuum pump to be operated at relatively low temperatures. It also predominantly operates isothermally. That means that the medium to be pumped hardly heats up during the compression process. This is why liquid ring vacuum pumps are virtually predestined for pumping vapours and gases with a high moisture proportion. The low temperatures in the vacuum pump are favourable for condensation of damp vapours and gases. To an extent, this means that the vacuum pump additionally functions as a capacitor. Because condensation already takes place when the gas enters the vacuum pump, the volume of the gas is drastically reduced. In addition to the condensation effect, this also achieves an increase in the nominal pumping speed.

The operating fluid diverts the compression heat, a process that supports condensation and an increase in pumping speed.
One significant advantage of liquid ring vacuum pumps is that the operating fluid and the materials used for the components can be adapted to fit the pumping medium. This also makes it possible to pump explosive gases and vapours. Due to the low operating temperatures, pumping explosive materials can, in any case, be considered much less problematic than with other mechanical vacuum pumps.

Construction

Basic differentiation is made between one- and two-stage liquid ring vacuum pumps. In the one-stage version, the compression process described above is performed in one compression stage. In the two-stage vacuum pump (fig. 3), the pre-compressed pumping medium from the first stage is conveyed to a second compression stage and compressed again. Ultimate pressures of 130 hPa (mbar) can be realized with one-stage liquid ring vacuum pump versions, while two-stage versions can achieve up to 33 hPa (mbar).

Fig. 3: Gas flow through a two-stage liquid ring vacuum pump (Source: Busch Dienste GmbH)

Even the sizes vary significantly. Busch Vacuum Pumps and Systems has different series and versions of